ISSN:0003-2654    

DOI:10.1039/AN99116FX045

出版商:RSC    

年代:1991

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN99116BX047

出版商:RSC    

年代:1991

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN99116BP050

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, DECEMBER 1991, VOL. 116 1203 Editorial-Fou r Anniversaries Sesquicentenary This special issue of The Analyst serves to commemorate four anniversaries. The first of these is the 150th anniversary of The Royal Society of Chemistry, which was established in 1841 under its first president, Professor Thomas Graham, as ‘The Chemical Society of London’. The oldest surviving Chemical Society, it initiated a fundamental change in science. The society brought together diverse groups from all four corners of Britain (and later, the world) for professional self-enhance- ment; its strength lay in the way that it could pool the efforts and unite the voices of its members. The Society was granted its first Royal Charter seven years later in 1848. Over the next 50 years, various other specialist organizations came into being: The Society of Public Analysts, later known as The Society for Analytical Chemistry (1874); the Institute of Chemistry of Great Britain and Ireland, later known as the Royal Institute of Chemistry (1877); and the Faraday Society (1903).Two of these earlier groups, Ana- lytical and Faraday, are now Divisions of The Royal Society of Chemistry. The Chemical Society and the Royal Institute of Chemistry were unified in 1980 and a new Royal Charter was granted marking the formation of The Royal Society of Chemistry, which is the heir to all these earlier bodies. Today the RSC undertakes and promotes a variety of activities on behalf of chemists and chemistry throughout the world including: Professional Affairs; Employment Counselling; Parliamen- tary Work; Professional Qualifications and Examinations; Education and Training; and, of course, public awareness of chemistry.Its aims, however, remain essentially the same, i.e., the advancement of chemistry as a science, the dissemina- tion of chemical knowledge, and the development of chemical applications. The RSC staged several events to mark its sesquicentenary including a fashion show at London’s Savoy Hotel entitled ‘Chemically Chic’, which highlighted how chemistry has influenced fashion from the production of dyes to the development of synthetic materials. The thermochromic textile ink Licritherm (developed by Merck) and its use to produce textiles that change colour according to body temperature attracted international media attention. An Open Day was held at Thomas Graham House (named after the first president of the Chemical Society) in Cambridge which was attended by about 400 chemists, both from industry and academia.The day allowed visitors not only to view the building but also to see how the various journals are produced, and to ask questions about the intricacies of scientific publishing. To mark the 150th anniversary of the Society, Her Majesty the Queen as patron of the RSC, and HRH the Duke of Edinburgh (an Honorary Fellow) paid a private visit to the RSC headquarters in Burlington House. The Queen had particularly requested to see information on the contribution of chemistry to solving environmental problems and to crime detection. An exhibition stand from the Metropolitan Police Forensic Laboratory on its DNA fingerprinting service revealed how chemistry has helped to revolutionize methods of detecting crime and criminals.On the environmental theme, ICI manned displays on how they are helping to solve the problems of the atmosphere, for example, through developing alternatives to CFCs and producing poly- urethane insulation to reduce fossil fuel consumption and global warming. The Sesquicentenary celebrations culminated in the 150th Annual Congress at Imperial College, London, which attrac- Four distinguished participants at the RSCs 150th Anniversary Congress. L to R: Professor J . N. Miller; Professor S. Greenfield; Mr. D. C. M. Squirrell; and Professor D. W. Wilson ted over 2000 participants, among whom were eight Nobel Laureates including the guest of honour, Lord Porter.The Analytical Division ran two parallel sessions at the Congress, one on ‘Luminescence and Optical Sensors’, the other jointly with the Faraday Division on ‘New Electrochem- ical Sensors’. These sessions included the Theophilus Red- wood Lecture given by Professor J . N. Miller (Loughborough) and the Robert Boyle Anniversary Lecture by Professor A. M. Bond (La Trobe University, Melbourne, Australia) which will appear together with other lectures in Analytical Proceedings. Four of the invited lectures given at the Congress at the aforementioned symposia appear in this issue to mark the occasion. Michael Faraday The second anniversary marked by this issue is the 200th anniversary of the birth of Michael Faraday (22nd September, 1791) who, through his extraordinary talents as experimenter and philosopher, changed not only the shape of science but also of society.Among his many achievements are: the discovery of new chemical substances (including benzene); the laws of electrolysis; he invented the electric motor and discovered the principle of electromagnetic induction. He also established the highly entertaining and informative Royal Institution Christmas Lectures, which have helped to eluci- date and de-mystify science for young and old alike. The recently knighted Director of the Royal Institution, Professor Sir John Meurig Thomas, gave an excellent lecture about Michael Faraday at the RSCs 150th Anniversary Congress, part of which is reflected in a special article in this issue of The Analyst entitled ‘Faraday the Man, Faraday the Genius’.Chromatography The third and fourth anniversaries are both associated with Martin and Synge and the birth of ‘modern chromatography’. It is 50 years since the fundamental paper on partition chromatography was presented (and later published in Bio- chem. J . , 1941, 35, 1358) at the 214th Meeting of the Biochemical Society, June 7th, 1941, at the National Institute of Medical Research, Hampstead, London. Finally, it is 401204 ANALYST, DECEMBER 1991, VOL. 116 years since Martin and Synge received the Nobel Prize for their pioneering work and almost 40 years since the first paper on gas-liquid partition chromatography by James and Martin appeared in The Analyst (Analyst, 1952, 77, 915).Dr. Les Ettre has written a special article on the Golden Decade of chromatography (1941-51) to commemorate these two anniversaries and his article is followed and comple- mented by an article by Dr. Lloyd Snyder who writes about the future of chromatography with particular reference to Computer Assisted HPLC Method Development. In addition to the above anniversaries there is good reason to highlight chromatography. Chromatographic and related equipment leads the field in analytical instrumental sales, particularly in the United States of America. ‘For the 1990s shipments of Chromatographic instruments- the leading product category-are projected to increase in dollar terms from $725 million in 1990 to $990 million in 1994. Included are analytical gas, liquid, ion and supercritical fluid chromatographs as well as detectors employed in these instru- ments.Columns, supplies and accessories or preparatory chromatographic systems are not included’ (TrAC, 1991, 10, V). However, with the introduction of new speciality columns for chiral and environmental compounds and columns for transi- tion metals, carbohydrates, biochromatography and capillary microbore systems, and their application to veterinary medi- cine, bioseparations and toxicology studies, this is a rapidly expanding market. The figures quoted above for chromatographic sales are supported by figures for sales of instrument data systems, of which dedicated chromatography integrators make up the largest segment Therefore, there is a vast amount of chromatography being carried out, both in industry and academia. For these reasons, and also to increase our profile in the US by supporting our US Associate Editor, Professor Julian Tyson, the Analytical Editorial Board has decided to recruit two new US members to the Advisory Board of The Analyst. They are Dr. Joe P. Foley (Villanova) and Professor John G. Dorsey (Cincinnati), both of whom are well known chromatographers; this issue contains brief biographies and also papers they have contri- buted to this landmark issue. Harp Minhas, Editor

ISSN:0003-2654    

DOI:10.1039/AN9911601203

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, DECEMBER 1991, VOL. 116 1205 Faraday the Man, Faraday the Genius* Sir John Meurig Thomas Fullerian Professor of Chemistry, The Royal Institution of Great Britain, 21 Albemarle Street, London WIX 4BS, UK That Michael Faraday combined singular gifts of intellectual power, technical virtuosity, intuition and moral perfection is indisputable. These qualities won the hearts and minds of his contemporaries and have secured for him a permanent place in the pantheon of science. So much has already been written about Faraday that one hesitates to add yet another version of the essence of his supreme humanity and his genius. However, this is the bicentenary of his birth. This is also a time when I myself, on the eve of my departure as Director of the Royal Institution, wish to muse upon my debt to Faraday.Having lived and worked for 5 years in Faraday’s home and laboratory, my initial interest in, and curiosity about, the great scientist has developed with a passionate admiration for all that he stood for and achieved. His scientific and spiritual presence at the Royal Institution confers a unique aura that pervades the whole place. One cannot escape it. Each time I enter the Main Theatre I am conscious that it is redolent of Faraday and seems haunted by his spirit. Whenever I stand at the lecturer’s desk there, Wordsworth’s reference to ‘the spiritual presence of absent friends’ comes to mind. I W CHEMISTRY AND PHYSICS. MICHAEL FAR.ID.lY, D.C.L., F.R.S., L O S D O N : BlCE4RD TAYLOR A.VD WILLIAM FRASCIS, PNKIERS AND P C R ~ ~ S ~ ~ K R S TO Tns L S I Y P R S ~ ~ I or LON DO.^.RED LIOY COURT, FLEET STREET. 1859. Fig. 1 Title page of the copy of Experimental Researches in Chemistry and Physics presented to the Royal Institution by Michael Faraday * Based in part on the Michael Devlin Lecture given at St Patrick’s College, Maynooth, Ireland, in March, 1991, and on the Krishnan Memorial Lecture given at The National Physical Laboratory, New Delhi, January, 1991. An analysis of Faraday’s life reveals a number of interesting paradoxes. Though a public figure, known to all the famous people and the majority of the general public of the Victorian era, he remained throughout his life a private person uninterested in social ambition and in the pursuit or attrac- tions of wealth. Festooned with honorary degrees from numerous universities and elected a member of all the premier academies of the world,l and proud to acknowledge such honours after his name in the title pages of the books he wrote (see Fig.l), he nevertheless shunned publicity and instructed that his gravestone should read simply, Michael Faraday. Though he frequented the occasional soiree, held in the salon of the wealthy philanthropist Baroness Burdett Coutts,2 with whom he had considerable rapport , or in Great Marlborough Street, where he mingled with artists and musicians (including the painter J . W. M. Turner), he was happier in the privacy of his own home, and in the company of his wife, niece and brother. Serene in the security of his religious conviction, he was untroubled by the apparent conflict between science and religious beliefs.He could excoriate the spiritualists for their naivety of faith while at the same time accept, as did his fellow Sandemanians, the literal truth of the Bible. Resolute in his pursuit of excellence as a lecturer and dedicated to the attainment of the highest standards in the Royal Institution at Albemarle Street, he accepted with equanimity the primitive theological pronouncements of his fellow worshippers in the Paul’s Alley Meeting House,3 off Aldersgate Street. His successor, John Tyndall, reveals much about Faraday the man.4 ‘We have heard much of Faraday’s gentleness and sweetness and tenderness. It is all true, but it is very incomplete. You cannot resolve a powerful nature into these elements, and Faraday’s character would have been less admirable than it was had it not embraced forces and tendencies to which the silky adjectives “gentle” and “tender” would by no means apply.Underneath his sweetness and gentleness was the heat of a volcano. He was a man of excitable and fiery nature; but through high self-discipline he had converted the fire into a central glow and motive power of life, instead of permitting it to waste itself in useless passion. “He that is slow to anger”, saith the sage, “is greater than the mighty, and he that ruleth his own spirit than he that taketh a city”. Faraday was not slow to anger, but he completely ruled his own spirit, and thus, though he took no cities, he captivated all hearts’. Tyndall poses the question: ‘Is he not all the more admirable, through his ability to tone down and subdue that fire and that excitability, so as to render himself able to write thus as a little child? I once took the liberty of censuring the closure of a letter of his to the Dean of St Paul’s.He subscribed himself, “humbly yours”, and I objected to the adverb. “Well, but, Tyndall”, he said, “I am humble; and still it would be a great mistake to think that I am not also proud”. This duality ran through his character. A democrat in his defiance of all authority which unfairly limited his freedom of thought, and still ready to stoop in reverence to all that was really worthy of reverence, in the customs of the world or the characters of men’. That he was earnest and studious is abundantly apparent from the wealth of information disclosed in his notebooks, correspondence, published papers and evidence given at1206 ANALYST, DECEMBER 1991, VOL.116 public enquiries and select committees. But he was also capable of enjoying himself in a variety of rather simple ways. His rich imagination pertained to things other than the scientific. He once described himself when a young apprentice as ‘a very lively imaginative person’ who ‘could believe in the Arabian Nights as easily as in the Encyclopaedia; but facts were important to me. I could trust a fact’. The beauty of nature, especially the hills of Devon, the vales of South Wales, all the Alpine landscapes and the seascapes of Brighton or the Isle of Wight, could move him to lyrical ecstasy. And in contemplating waterfalls, the rainbow or lightning, his responses were often Wordsworthian, though never expressed in verse.Music and the theatre appealed to him; and so did the craftsmanship required in attending to his hobbies as a collector and to the custodial side of his duties as the Superintendent of the House of the Royal Institution. On quiet evenings he probably attended to attaching his name- plates (Fig. 2) on his furniture5 or to sticking interesting letters and cuttings into large albums (Fig. 3) with lithographs and engravings for illustrations. The Faradays were childless, but they had a blissfully happy married life. Sarah Barnard never shared the scientific interests that drove Faraday with such febrile intensity: she said she was happy to be ‘the pillow of his mind’.The company of young people, especially his nieces, Jane and Constance Reid, who lived for a few years with the Faradays at the Royal Institution, meant a great deal to him. Occasionally the party went up the river Thames in a rowing boat to picnic. Dafydd Tomos’ Faraday in Wales tells of a delightful incident in the Vale of Neath when Faraday engaged himself in conversation with a 10 year old girl. He was struck by the fact that he could understand her, and appreciate all her winning charm, even though she spoke to him in a foreign tongue. His niece has recalled how, when she visited him in the laboratory, he would drop a piece of potassium into a trough of water for her to watch it dart across the surface. She has also told of how, at children’s parties, Faraday rode a velocipede in the corridor round the back of the lecture theatre! In a letter that I received recently from Dr.J. D. C. Innes, whose great grandmother was Faraday’s sister, we read: ‘He would occasionally take some physical exercise6 by cycling on his velocipede as far as Hampstead’. The company of children added an extra resonance to his life, not surprising perhaps in someone whose childlike innocence and per- cipience contributed to his genius. Faraday often declared publicly the importance of returning to the innocence of childhood. At the commencement of one of his series of Christmas Lectures he asserts: ‘I will return to second childhood and become, as it were, young again among the young’. And in his famous series on The Chemical History of a Candle presented in 1848-9 and 1860-61, he says in his opening lecture ‘.. . I mean to pass away from those who are seniors amongst us. I claim the privilege of speaking to juveniles as a juvenile myself‘. Faraday paid great attention to his appearance and deport- ment. Recognizing his quintessential humility and his reclu- sive tendencies we are a little surprised by the stance he takes in some of the more famous photographs of him (see Figs. 4 and 5 ) , but this may be the result of the prevailing public attitudes to photography as a relatively new science and art form. After all, film speeds were slow in those days and exposure times correspondingly long: a somewhat rigid appearance was therefore obligatory on the part of the person being photographed. There is a good deal of contemporary evidence that Faraday was charming and congenial. This was certainly the view of the wife of Richard Owen (the redoubtable antagonist of Darwin- ism and first Director of the Natural History Museum). She also commented on Faraday’s tact when he rebuked some of the male members of the audience who had invaded the ladies’ gallery.As Geoffrey Cantor’ tells us, another female member of Faraday’s audiences was impressed by his ‘great talent, great goodness, and the wonderful simplicity of his nature. Despite (if not because of) his humble background and his membership of a Christian sect that set him apart from polite Victorian society, he appeared as a polished gentleman before his audience’. Faraday’s public persona was partially shaped by his religion which emphasized how a convinced Christian should deport himself or herself in public.The Sandemanians’ concern with love and fellowship is a counterpart to Faraday’s Fig. 3 A typical example of the content of successive pages in Faraday’s folio of portraits and correspondence. The young Prince Edward (inset) was present, sitting to the left of HRH Prince Albert, at Faraday’s Christmas Lecture series given in 1855-56 Fig. 2 Faraday took great delight in attaching brass nameplates to his furniture, many pieces of which are still in use at the Royal InstitutionANALYST, DECEMBER 1991, VOL. 116 1207 interpersonal skills which helped him relate directly to his audience. Much has already been written about the role of religion in Faraday’s life.8 Some insight into the specific religious precepts which he held to be important may be gained from the extensive markings and marginalia in, as well as a few notes and emendations made on, some of Faraday’s much-thumbed Bibles.He heavily marked Timothy, VI, 10: ‘The love of money is the root of all evil’, and the cry of Job: ‘If I justify myself, my own mouth shall condemn me: if I say I am perfect, it shall also prove me perverse’. But many of his moral principles, to which he adhered with such incorruptible tenacity throughout his life, were of the kind that would appeal to people of goodwill, religious or otherwise, in all ages. Who would dispute the wisdom of the following declaration? ‘A philosopher should be a man willing to listen to every suggestion but determined to judge for himself.He should not be biased by appearances, have no favourite hypothesis, be of no school and in doctrine have no master. He should not be a respecter of persons, but of things. Truth should be his primary object. If to these qualities be added industry, he may indeed hope to walk within the veil of the temple of nature’. The profound belief, a powerful propellant and guiding star throughout all his scientific quests, in the interconnection and unity of natural forces and phenomena was a conviction that he possessed from an early age. Most religious people in the early 19th century believed in underlying unity. ‘God had fashioned the world; it is a whole, so everything should surely be connected’, was how the argument went. This unified view of the world was also espoused by many non-religious ancient and more modern philosophers. In his 1821 historical survey Fig.4 A photograph Of Faraday that he had prepared as a young man One Of the glass ’pecimens (p. 21), Faraday drkw speiific attention to Brsted’s constancy in pursuing inquiries ‘respecting the identity of chemical, electrical and magnetic forces’. Even earlier, Davy, in one of the lectures taken down by Faraday in 1812, alluded to a future, simpler science that ‘will connect mechanical and chemical sciences’. Faraday was not alone in London’s scientific circles in the 1840s in seeking correlations between various natural forces. W. R. Grove, the man credited with the enunciation of the First Law of Thermodynamics, pursued this subject with conspicuous success in an extensive course given at the London Institution in 1842.Grove’s Bakerian lecture to the Royal Society in 1847, as well as his other publications, dwell on this theme. W. R. Grove’s book Correlation of Physical Forces was much consulted from the 1850s onwards. In the preface to the last edition of this book (1884), Grove writes: ‘Every one is but a poor judge where he is himself interested, and I therefore write with diffidence; but it would be affecting an indifference which I do not feel if I did not state that I believe myself to have been the first who introduced this subject as a generalized system of philo- sophy, and continued to enforce it in my lectures and writings for many years, during which it met with the opposition usual and proper to novel ideas’. Faraday’s inner convictions led him to devote most of his energies to two principal activities: pursuing the eternal verities in his role as natural philosopher; and serving his small circle of friends in the Sandemanian sect and the even smaller circle of family members.In the former, his task was not only to prove but also to explain: in the latter to offer selfless devotion and compassionate care, which spilled over into work of a charitable kind. He lived a contented, if exhausting, life. And the nature, if not the secret, of that contentedness is revealed in correspondence with his life-long friends-see especially the letter to Auguste De La Rive, of Geneva, a long standing friend.9 An elaboration of that contentedness as well as his general advice on dealings with other scientists, emerges Fig.5 A photograph of Faraday in his study at the Royal Institution in 1863 at the age of 721208 ANALYST, DECEMBER 1991, VOL. 116 from his letter to John Tyndall, in connection with an acrimonious discussion at a meeting of the British Associ- ation, October 6th, 1855: ‘My dear Tyndall, These great meetings, of which I think very well altogether, advance science chiefly by bringing scientific men together and making them to know, and be friends with each other, and I am sorry when that is not the effect in every part of their course. I know nothing except from what you tell me, for I have not yet looked at the reports of the proceedings; but let me, as an old man who ought by this time to have profited by experience, say that when I was younger I found I often misinterpreted the intentions of people and found that they did not mean what at the time I supposed they meant, and further that as a general rule, it was better to be a little dull of apprehension where phrases seemed to imply pique, and quick in perception when, on the contrary they seemed to imply kindly feeling.The real truth never fails ultimately to appear, and opposing parties, if wrong, are sooner convinced when replied to forebear- ingly than when overwhelmed. All I mean to say is that it is better to be blind to the results of partisanship and quick to see good will. One has more happiness in oneself in endeavouring to follow the things that make for peace. You can hardly imagine how often I have been heated in private when opposed-as I have thought unjustly, and super- ciliously, and yet I have striven, and succeeded I hope, in keeping down replies of a like kind.And I know I have never lost by it. I would not say all this to you did I not esteem you as a true philosopher and friend’. Faraday had little compunction about declining high honours, in turning down invitations to socialize, or in avoiding interruptions. The delegation that implored him to become President of the Royal Society were left in no doubt about his refusal of that honour. Shortly thereafter he remarked: ‘Tyndall, I must remain plain Michael Faraday to the last; and let me now tell you, that if I accepted the honour which the Royal Society desires to confer upon me, I would not answer for the integrity of my intellect for a single year’.He had earlier declined the Presidency of the Chemical Society, which was formed in his fiftieth year. He hardly ever attended its meetings in nearby Burlington House, not becayse he was antipathetic to the aspirations of the Society, but because he felt disinclined to risk losing yet more time from his beloved laboratory and family. It is not surprising, given these tendencies, that Wheatstone, in a letter to his friend, W. F. Cooke, dated October 4th, 1838, should have said: ‘I called on Faraday this morning and was told that this was one of the days on which he denies himself to everybody for the purpose of pursuing uninterruptedly his own researches. He will be visible tomorrow’. His laboratory hours were gruelling; nine in the morning until eleven at night was not untypical.The only person who ever actively assisted him in his experimental work, and who shared his laboratory, was an ex-Sergeant of the Royal Artillery named Charles Anderson. He had been recruited to maintain the special furnace installed in the Royal Institution for the glass project started in 1827. Hours would pass without a word spoken between them, and when the silence was broken, it was only for a few minutes. Faraday’s letters to the Press, an imperfect measure of one’s involvement in the affairs of the day, were very infrequent. Once he wrote to draw attention to the polluted state of the river Thames, an action which prompted Punch to publish the cartoon shown in Fig. 6.Faraday drifted down the Thames inserting strips of cardboard into the water until he could not see the bottom edge and then marking the depth of insertion Fig. 6 The cartoon published in Punch, July 21st, 1855, after Faraday had written to the editor of The Times describing how he tested the polluted state of the Thames (see text) and the place where the test took place. The letter, published by The Times, July 7th, 1855, reads as follows: ‘Sir ,- I traversed this day by steamboat the space between London and Hungerford Bridges, between half-past one and two o’clock. It was low water, and I think the tide must have been near the turn. The appearance and smell of the water forced themselves at once on my attention. The whole of the river was an opaque pale brown fluid.In order to test the degree of opacity, I tore up some white cards into pieces, and then moistened them, so as to make them sink easily below the surface, and then dropped some of these pieces into the water at every pier the boat came to. Before they had sunk an inch below the surface they were undistinguish- able, though the sun shone brightly at the time, and when the pieces fell edgeways the lower part was hidden from sight before the upper part was under water. This happened at St Paul’s Wharf, Blackfriars Bridge, Temple Wharf, Southwark Bridge, and Hungerford, and I have no doubt would have occurred further up and down the river. Near the bridges the feculence rolled up in clouds so dense that they were visible at the surface even in water of this kind.The smell was very bad, and common to the whole of the water. It was the same as that which now comes up from the gully holes in the streets. The whole river was for the time a real sewer. Having just returned from the country air, I was perhaps more affected by it than others; but I do not think that I could have gone on to Lambeth or Chelsea, and I was glad to enter the street for an atmosphere which, except near the sink-holes, I found much sweeter than on the river. I have thought it a duty to record these facts, that they may be brought to the attention of those who exercise power, or have responsibility in relation to the condition of our river. There is nothing figurative in the words I have employed, or any approach to exaggeration. They are the simple truth.If there be sufficient authority to remove a putrescent pond from the neighbourhood of a few simple dwellings, surely the river which flows for so many miles through London ought not to be allowed to become a fermentingANALYST, DECEMBER 1991, VOL. 116 1209 sewer. The condition in which I saw the Thames may perhaps be considered as exceptional, but it ought to be an impossible state; instead of which, I fear it is rapidly becoming the general condition. If we neglect this subject, we cannot expect to do so with impunity; nor ought we to be surprised if, ere many years are over, a season give us sad proof of the folly of our carelessness’. Another occasion on which he wrote to the Press was in 1853 when he was bombarded with requests and enquiries pertaining to the supposed phenomenon of table-turning.This allegedly arose because of the consequences of the spiritualist movement, which began in 1848 in Hydesville, New York, with the rappings and knockings of the Fox girls.10 It spread rapidly to other parts of the United States, Britain and continental Europe. The spiritual phenomena encompassed the tilting and levitation of tables and chairs, and the movement of objects in the dark became the subjects of several books published in England and France in 1853. The rather rapid and uncritical acceptance of spiritualistic forces greatly disturbed Faraday because of what they revealed about the general level of intelligence. His exasperation may be gauged from the following excerpt of a letter that he wrote to Professor C.F. Schonbein, the German-Swiss scientist who discovered ozone. On July 25th, 1853, he wrote: ‘I have not been at work except in turning the tables upon the table-turners, now should I have done that, but that I thought it better to stop the inpouring flood by letting all know at once what my views and thoughts were. What a weak, credulous, incredulous, unbelieving, superstitious, bold, frightened, what a ridiculous world ours is, as far as concerns the mind of man. How full of inconsistencies, contradictions, and absurdities it is . . .’. Faraday’s letter to The Times ‘On Table-turning’ ended with the sentence: ‘I think the system of education that could leave the mental condition of the public body in the state in which this subject has found it, must have been greatly deficient in some very important principle’.His concerns about public credulity and general state of awareness were raised in a famous lecture on ‘Mental Education’ that he gave at the Royal Institution in May, 1854, in the presence of The Prince Consort. Several scientific anecdotes have centred on Faraday’s life and attitudes. Two are very widely quoted, although their authenticity is not unquestioned.11 Irrespective of whether these anecdotes are true or not, they carry an interesting message. They appear in various forms in the works of 19th and 20th century writers. When Faraday was endeavouring to explain to Prime Minister Robert Peel or to the Chancellor of the Exchequer, W. E. Gladstone, an important new discovery in science, the politician allegedly comments ‘But, after all, what use is it?’ Whereupon Faraday replied ‘Why sire, there is the probability that you will soon be able to tax it’.It would be surprising if Faraday did retort in these terms: he seemed to have been singularly uninterested in patenting his own inventions or in the mechanics of wealth creation and taxation. The other retort, for which Faraday is often given authorship, again involves the Prime Minister or any other earnest en- quirer after hearing of Faraday’s scientific discovery. The question this time is ‘What good is it?’ The sagacious reply: ‘Of what good is a new-born baby?’ is thought to have been first used by Benjamin Franklin in Paris in 1783. An Analysis of Faraday’s Genius In Owen Meredith’s Last Words of a Sensitive Second-Rate Poet we read that: ‘Genius does what it must, and talent does what it can’.This certainly applies to Faraday just as do other definitions of genius. For example: ‘A supreme capacity of taking trouble’ (Samuel Butler); or, ‘A greater aptitude for patience’ (Comte De Buffon). But none of these goes sufficiently deep. Perhaps Emerson went somewhat deeper if we accept that his words apply more to the realm of natural rather than to moral philosophy: ‘To believe your own thoughts, to believe that what is true for you in your private heart is true for all men-that is genius’. But this is still inadequate, so far as Faraday is concerned. Every great human being of first rank is unique, and Faraday’s genius is the consequence of a unique combination of a uniquely large sub-set of major qualities: an infinite capacity to take pains; restless intellectual energy; and inexpugnable intellectual honesty, coupled with a measure of technical virtuosity that encompassed the manipulative dex- terity and constructive imagination to produce new instru- ments and new techniques of unsurpassed power and sensitiv- ity.(His torsion balances and coulometers were more sensi- tive, his electromagnets stronger, his glass specimens heavier and of superior optical quality, than those of his predecessors or contemporaries.) Always convinced that to the problems he pursued there were solutions and that to the questions he raised there were intelligible answers, he had the supreme gift of selecting those that were really important and also of knowing precisely what to do next.Both his strategy and his tactics were impeccable. Add to all this his prodigious physical stamina, endless curiosity, penetrating intuition, complete mastery of detail and an exceptional facility for arguing from the particular to the general-on his own, and with his own brand of self-criticism and self-discipline-and one sees why even those who themselves are regarded as princes among experimenters elevate Faraday to the status of paragon. Rutherford spoke for all scientists when, in 1931, he said: ‘The more we study the work of Faraday with the perspective of time, the more we are impressed by his unrivalled genius as an experimenter and a natural philo- sopher. When we consider the magnitude and extent of his discoveries and their influence on the progress of science and of industry, there is no honour too great to pay to the memory of Michael Faraday-one of the greatest scientific discoverers of all time’.Much earlier, John Tyndall in his book Faraday as Discoverer analysed the qualities that made Faraday such a successful scientist: ‘He united vast strength with perfect flexibility. . . . The intentness of his vision in any direction did not apparently diminish his power of perception in other directions, and when he attacked a subject expecting results, he had the faculty of keeping his mind alert, so that results different from those he expected should not escape through preoccu- pation’. To all this analysis must be added four other factors. Firstly, Faraday wrote and spoke about his work in memorable ways.Secondly, he wrote down everything that he observed experimentally at the time of the observation. (His diaries reveal that he invariably recorded the key points of each experiment; he also had the habit of writing up his work promptly for publications.) Thirdly, almost all the successful experiments that he carried out he proceeded to refine, with a view to demonstrating them publicly at Discourses in the Royal Institution. They were intended to leave an indelible impression and in this he succeeded triumphantly. Lastly, he had the extra good fortune to have as one of his interpreters arguably one of the greatest physicists since Newton, J. Clerk Maxwell. Maxwell, who was born the year that Faraday made his most momentous discovery (of electromagnetic induction) in 1831, selected ‘Faraday’s Lines of Force’ as the title of his extraordinary paper delivered to the Cambridge Philosophical Society in December, 1855, and February, 1856, when he was a 24 year old Fellow at Trinity College, Cambridge.With that1210 ANALYST, DECEMBER 1991, VOL. 116 monumental work12 mathematical precision and quantitative prediction were added to Faraday’s qualitative views on field theory in general and to electromagnetism in particular. With this event a new era dawned. References Thomas, J. M., Michael Faraday and The Royal institution, Adam Hilger, Bristol and Philadelphia, 1991, Appendix 11, p. 215. This is a complete listing of the 71 academies and learned societies to which Faraday was elected.Angela Burdett-Coutts, a member of the banking family, was persuaded by Faraday to join the Royal Institution. They were both patrons at the Orphan Asylum in the 1840s. On May 29th, 1856, Faraday joined guests on the roof of the Baroness’s home to celebrate the end of the Crimean War. It is reported (see Edna Healey’s Lady Unknown (Sigwick Jackson, London, 1978, p. 62) that, on watching the fireworks, Faraday ‘hallooed out with wonderful vivacity, “there goes magnesium, there’s potassium”’. An editorial published in The Referee, June 21st, 1891, discloses that ‘Michael Faraday was one of the elders of our chapel; another was a butcher, another a gas fitter, and a fourth, if I remember rightly, a linen draper. I heard Faraday read the Bible & expound often during my childhood, and I remember I liked him best of all the elders because he didn’t waggle his head and whine & tremble like some of the others’.Tyndall, J., Proc. R. Inst. G.B., 1868, V, 214. Some of it is still in use in the Director’s second-floor flat and in offices and public sectors of the Royal Institution. 6 7 8 9 10 11 12 His physical strength never deserted him. As adviser to Trinity House, he was called upon to make visits to various lighthouses. At the age of 70 he braved snow and storm, crossed fields and hedges, and put up with other discomforts in his tour of duty. See Cantor, G., ‘Educating the Judgement: Faraday as a Lecturer’ (lecture given at the American Chemical Society Annual Meeting, Atlanta, Georgia, USA, April, 1991), Bull. Hist. Chern., in the press. See, for example, Marryat, H., The Times, September 21st, 1931; Cantor, G. A.. Br. J . Hist. Sci., 1989, 22. 433; Pratt, H. 7’. , American Chemical Society Meeting, Atlanta, Georgia, USA, April, 1991 (abstract). Part of Faraday’s letter, written when he was 61 years of age (in October, 1852) reads as follows: ’. . . It may seem very trite to say that content appears to me to be the great compensation for these various cases of natural change; and yet it is forced upon me, as a piece of knowledge that I have ever to call afresh to mind, both by my own spontaneous and unconsidered desires and by what I see in others. No remaining gifts though of the highest kind; no grateful remembrance of those which we have had, suffice to make us well and be content under the sense of the removal of the heart of these which we have been conscious of‘. Numerous other examples of Faraday’s contentedness emerge from his other letters: see The Correspondence of Michael Faraday Vol. 1, 1811-1831, ed. James, F. J. L., Institution of Electrical Engineers, London. 1991. See Sci. Mon., 1956 (September), 145. See Webb, C. J., and Cohen, I. B . , Nature (London), 1946,157. 196; Gregory, R. A., Nature (London), 1946, 157, 305. Proc. Camb. Phil. SOC., 1856, X, Part I.

ISSN:0003-2654    

DOI:10.1039/AN9911601205

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, DECEMBER 1991, VOL. 116 1211 lon-selective Electrode Studies on Novel Organic Molecule Sensors* J. D. R. Thomas School of Chemistry and Applied Chemistry, University of Wales College of Cardiff, P.O. Box 912, Cardiff CFI 3TB Researches on prospects for novel ion-selective electrodes, based on organic molecule sensors, are described. The organic molecules are large crown ethers extending from bis( metaphenylene)-26-crown-8 to bis(metaphenylene)-38-crown-l2, small crown ethers, bis-crown ethers, and acyclic polyethers consisting of diphenyl ethers of tetraethylene glycol and receptor molecules of planar and tetrahedral tripodal types. Keywords: Acyclic polyether sensor; crown ether ion sensor; ion-selective electrode; neutral carrier electrode The various organic molecules based on structures containing ethoxylate (-CH2CH20-) units owe their function as poten- tiometric ion sensors to the ability of the ethoxylate structure to either crown, or coil around, or clasp analates.Such complexation is facilitated by the organic molecules contain- ing the ethoxylate units carrying a sequence of localized charges of sufficient energy to form ion-dipole bonds with appropriate cations. These charges are frequently com- plemented by features of other substituents within the molecule, such as phenolic, carboxylic and aromatic ring substituents. Overall, the conformation of the molecule permits a solvation type shell around the cation effectively to replace the ion hydration shell. The charged cationic complex thus formed is electrically balanced by anions.The impetus for studies on structure with polyethoxylate units as ion sensors is attributable to the observation of Moore and Pressman’ that valinomycin is capable of actively trans- porting potassium across rat mitochondria membranes. This led to the very successful valinomycin sensor for potassium.2-3 Also relevant is the description by Pedersen4 of the function of crown ethers in promoting the dissolution of salts in which they arc otherwise insoluble. However, it is to be noted that cation adducts of polyethylene glycols had already been known for several years,5-7 and it was this category of ethoxylates that was first exploited in ion-selective electrodes (TSEs). The special properties of polyethylene glycols in this application are attributable to the ethoxylate units (EOUs) in the polyethoxylate complex assuming a tight helical conforma- tion of appropriate ring radius, for holding the ion in a ‘cage’ of oxygen atoms of the EOUs by ion-dipole interaction.8 For the particular instance of barium ions complexing with a nonylphenoxypolyethoxylate (NP) [Antarox (or Igepal) CO- 880 with 30 EOUs], 12 EOUs are involved in holding the Ba*+ ions in a tight helical arrangement with a ring radius of about 1.3 A, the cage for the barium ions being formed by the 12 oxygens in two loops of 6 EOUs each.8 For the crown ethers, some of the earliest ISE studies were by Rechnitz and Eya19 on dicyclohexyl- and dibenzo-18- crown-6 ligands.Shortly afterwards, Petranek and Ryba’O.1’ assessed various types of crown-6 and larger crown com- pounds for sensing potassium.They favoured dimethyldiben- zo-30-crown-10 with dipentyl phthalate in poly(viny1 chloride) (PVC) as the best of the series. Although these and later studies on crown ether sensors showed that the selectivity for potassium does not exceed that by valinomycin, researches on polyethers as potentiometric sensors have yielded many interesting features and new directions in sensing. This paper outlines studies in Cardiff on attempts to assess and exploit additional molecular systems based on EOUs as potentio- * Presented at thc Royal Socicty of Chemistry 150th Anniversary Congress, Imperial College, London, 8th-11th April, 1991. metric ion sensors in PVC matrix membranes with appropriate plasticizing solvent mediators.The types of structures dis- cussed consist of: (i) large crown ethers for sensing diquat (DQT), paraquat (PQT) and guanidinium; (ii) small crown ethers for lithium; (iii) bis-crown ethers for alkali metal cations; and ( i v ) acyclic structures, consisting of ‘clasp’ type diphenyl ethers of tetraethylene glycol and ‘scorpion’ or ‘grasp’ types based on planar and tetrahedral tripodal receptor molecules for alkali and alkaline earth cations, and guani- dinium. Large Crown Ethers as Sensors Diquat and Paraquat The way in which a selection of crown ether type neutral carrier molecules for ion-sensing can be helped by structural studies on interactions between the carriers and ions has been well demonstrated by Stoddart and co-workers in Sheffield (now at Birmingham), and Williams in London. Their work showed interesting features on the interactions between macrocyclic pol yether molecules and cations according to three types of bonding (for relevant references, see ref.12): (i) coordination via [N-H . . . 01 hydrogen bonds as in complexation between polyether and primary alkylammo- nium ions; (ii) coordination through [C-H . . . 01 linkages to the DQT dication, alkylphosphonium cations, etc. ; and (iii) metal cation coordination to oxygen atoms of the crown ether compound. The structural information obtained by Stoddart and co-workers has been directed in Cardiff12 to the development of ISE sensors for DQT and PQT dications based on dibenzo-30-crown-10 (DB30C10). This is an important appli- cation in view of the use of DQT and PQT as contact herbicides.The maximum stability’3-16 for dibenzo-3n-crown-n (DB3nCn) complexes with DQT occurs when n = 10 (Table 1) while the X-ray structure shows the plane of the DQT molecule to be enclosed in a U-shaped cavity14 formed by DB30C10. The complexation involves three types of Table 1 Stability constants ( K , ) and free energies of formation (AC,) of DQT.DB3nCn complexes in acetone* Crown cther K,ldm3 mol-1 AGfIkJ mol-1 DB27C9 4.1 x 102 15.0 DB30C10 1.8 x 104 24.3 DB33C11 1.1 x 104 23.1 DB36C12 2.0 x 103 18.9 DB30ClO(-OCH2)2Ph - 31.0 * Data from refs. 14, 15 and 17.1212 ANALYST, DECEMBER 1991, VOL. 116 H CH3 H CH3 lo -0-0-0-0 wowowo~o a: DB3nCn DB-crown 0 1 BMP-crown BPP-crown Fig. 1 Chemical structures (top) of (a) 4,4'-dipyridinium, (b) paraquat and (c) diquat, showing some dimensions, and structure (bottom) of dibenzo-type 3n-crown-n crown ethers, showing the positions of the benzene 0,O-disubstitution in dibenzo (DB), bis(metapheny1ene) (BMP) and bis(parapheny1ene) (BPP) crown ether derivatives (from ref.17) DB30C10 (host)-DQT (guest) interactions: (i) DB30C10 catechol-oxygen electrostatic interaction with the positively charged nitrogen atoms in DQT. For this, the crown ether catecholo-0 separation (2.6 A) and N-N separation in DQT (2.8 A) are similar (Fig. 1) so that the catechol oxygens are neatly directed above and below the DQT nitrogens in the [DQT.DB30C10]2+ complex; (ii) DB30C10 benzene ring x-electron charge transfer to the electron deficient DQF+ ; and (iii) hydrogen bonding between H6 and H6' (Fig.1) with oxygen atoms in the DB30C10 framework, and as mentioned above. A wide ranging study was then undertaker1,~3>*~ during which various crown ethers, namely, DB30C10, bis-meta- phenylene-32-crown-10 (BMP32C10), bis-metaphenylene-38- crown-12 (BMP38C12), bis-paraphenylene-34-crown-10 (BPP34C10), bis-paraphenylene-37-crown-l l (BPP37Cll) and dinaphthalene-36-crown-10 (DN36C10) were tested for their efficacy as sensor systems for DQT and PQT and their conformational behaviour in their complexation with the two dications and with the related 4,4'-dipyridinium (4,4'-DPy). Also studied18 was the use of ion-pairing reagents in ISEs for the dications, these being related also to searches for the best type of anion excluder for use in PVC ISE type membranes with the crown ether neutral ligand.The best electrodes for DQ-T are basedl3J7 on DB30C10 plus DQT-2TPB (TPB = tetraphenylborate) with either 2- nitrophenyl phenyl ether (NPPE) or 2-nitrophenyl octyl ether (NPOE) as solvent mediator [Fig. 2(a)] but good electrodes were also obtained with just DQT-2TPB and the solvent mediators [Fig. 2(3)]. The other crown ethers studied gavel7 good ISE properties for 4,4'-DPy of calibration slope between 33 and 41 mV decade-', extending down to 1.7-6 pmol dm-3. However, the expectation in this study18 was the discovery of a PQT ISE based on BPP34C10 as a complex exists19 (of stability constant 730 dm3 mol-l) through the 0-0 separation (5.5 A) in this host being not too far removed from the N-N separation of 7.0 8, in PQT [Fig.1(6)]. The expectation was 8 6 4 2 0 -2 -4 I l I I I I 1 1 1 I I 1 m -,g 5 a0 Y m 4 3 -J 2 1 0 -1 -2 -3 DOT POT Mg Ca Ba Li Na K NH4 Gu PhNH, Et2NH2 B Fig. 2 (a) Effect of plasticizing solvent mediator on the selectivity of diquat ISEs based on DB30C10 plus DQT.2TPB. (b) Effect of plasticizing solvent mediator on the selectivity of diquat ISEs based on DQT-2TPB without crown ether. 0, NPOE; El, NPPE; A, DOPP; A, dinonyl phthalate (DNP); and M, DBP. Lines joining points connect the various solvent mediator types (from ref. 17) not realized,17 probably because the [PQT.BPP34C10]2+ complex is considerably less stable than [DQT-DB30C10]2+ (stability constant 18 000 dm3 mol-1) and, therefore, much too weak to exhibit a potentiometric function. A wide range of anion systems, namely, phosphorus hexafluoride (PF6) , anthraquinone-2-sulphonate (AS), octyl sulphate (OS), picrate (PIC) , dipicrylaminate (DPA), Diam- ine Green B (DGB), tetraphenylborate (TPB) and tetrakis-4- chlorophenylborate (T4ClPB) have been studied as possible ion-pairing agents for use in ISE membranes for PQT, DQT and 4,4'-DPy, but only PF6, TPB and T4ClPB gave good electrodes.18 The PF6 electrodes were not selective, thus leaving just the TPB and T4CIPB systems as suitable for exploitation.In this respect, electrodes based entirely on DQT-2TPB or PQT.2TPB had already been found12 to give excellent ISEs with appropriate solvent mediators. The calibration response of a DQT.2T4CIPB based ISE for DQT during an appraisal study20 was found to be stable for 55 d with a near-Nernstian slope and detection limit in the pmol dm-3 range.The electrode was of fast response (3 s at 1 mmol dm-3 and 25 s at 1 pmol dm-3) and was useable at pH 2-12 over a sample temperature range of 2-50 "C. Samples could be analysed by the standard additions method with about a -5% error and a precision of 7-8%. For comparison, DQT was also determined by titration with sodium tetra- phenylborate using DQT.2TPB and tetrabutylammonium tetraphenylborate (TBAeTPB) ISEs as sensors. Here, the errors were 12-20% for DQT analysis in de-ionized water, sodium chloride solution or simulated serum.** Guanidinium The importance of guanidinium in the biological and medical fields has led to interest in ISE development. For this, the guanidinium cation, [(H2N)3C]+, can complex with crownANALYST, DECEMBER 1991, VOL.116 1213 ethers of between 18 and 33 ring atoms.21-23 The 27-mem- bered ring is the most selective towards the guanidinium cation (Gu+), the stability constant trend being K > Gu > alkylammonium > other metal cations.21-23 X-ray and nuclear magnetic resonance (NMR) spectroscopy data21-24 support the [NH . . . 01 arrangement of hydrogen bonds to yield stable complexes of 1 mol host to 1 mol guest in most instances and, occasionally, 2 : 3, depending on the solvent and guanidinium salt . In initial studies25326 on crown ether based ISEs. for guanidinium, the most suitable system was based on dibenzo- 27-crown-9 (DB27C9) using dibutyl phthalate (DBP) in PVC. Dioctyl adipate (DOA) and dioctyl sebacate (DOS) plasticiz- ing solvent mediators with the same sensor yielded the next best electrodes,26 while NPOE, NPPE and dioctylphenyl- phosphonate (DOPP) proved to be unacceptable as solvent mediators.It is to be noted that functional Gu+ ISEs may be obtained with just guanidinium tetraphenylborate as sensor with either DBP or DOPP as solvent mediator, each with good all-round selectivity.26 The pH interference-free range (4-10) was narrower for the electrodes without crown ether26 than those with crown ether, when the pH range was 3-12. In a consideration27 of the relative merits of DB27C9, DB30C10 and bis( metaphenylene)-26-crown-8 (BMP26C8) as sensor, the 26-membered ring system of BMP26C8 was deemed to have the optimum crown ether ring size for sensing guanidinium. This worked well with either DBP or DOA as solvent mediator, and functioned both with and without either guanidinium tetraphenylborate or guanidinium tetrakis(4- chloropheny1)borate as the anion excluder.Hence, the metaphenylene component of the crown ether led to an optimum crown to improve on the previous best 27-membered system. The electrodes were functional over several weeks (>16), while the main interferents were pyridine, tetraethy- lammonium and PQT.27 Small Crown Ethers as Sensors Lithium is an effective therapeutic agent for the treatment of manic depression. Hence, a great deal of attention has been given to the development of lithium ion sensors. The analytical constraint is the clinical requirement of maintaining the lithium level between about 0.5 and 1.0 mmol dm-3; this being in a background of approximately 0.15 mol dm-3 sodium means that the sodium level is about 200 times the planned lithium level and 1400 times the lowest lithium level.Therefore, a successful lithium ion sensor should have sufficient selectivity for lithium to discriminate against large sodium backgrounds. In the development of lithium ISEs, the most encouraging results have been obtained for sensor membranes consisting of neutral carrier molecules, admixed with compatible solvent mediators in PVC matrices. The essential guideline is that the neutral carrier must preferably have 4-5 coordinating sites, and that the ligand should be sufficiently flexible to allow a fast exchange, despite the rigidity in arrangements of coordination sites.These points are important in relation to reversibility for the electrode. Of course, lithium being small with a high charge density tends to be highly solvated; hence, a suitable neutral carrier should be capable of stripping the water of hydration off the lithium ion (the enthalpy of hydration of lithium is high, namely, 510 kJ mol-1). In relation to the above, research on the development of crown ethers for lithium ISEs has centred on 12-crown-4 through to 16-crown-4 type molecules.28 The lithium ion (0.68 A diameter) must fit snugly into the polyether cavity, and the crown ethers that fit such a specification range from 12-crown- 4 to 14-crown-4. Others, such as 15-crown-4 and 16-crown-4 have larger cavity diameters and are selective for larger cations.The 12-crown-4 ether (Fig. 3) with its 'ideal' cavity size for lithium gave an electrode29 of near-Nernstian response at R W- n (4) Fig. 3 Some crown-4 molecules studied as lithium ion s e n ~ o r s ~ , ~ * 1 x 10-5-1 x 10-4 mol dm-3, but was of poor selectivity, particularly with respect to sodium. A later investigation30 modified the 12-crown-4 by having a substituent R (-CH20C18H37 or -CH20C( O)CI7H35) designed to hinder the formation of sandwich complexes with the larger alkali metal cations. This proved to be unsuccessful in that the kR,'Na values deteriorated from 0.12 for the simple 12-crown-4 material to 81 and 280 for the respective derivatives. Of a comprehensive range of crown-4 derivatives studied by Kitazawa et aZ.30 covering 12-, 13-, 14-, 15- and 16-membered rings, it was found that lithium ion selectivity was dramatically enhanced for the 14- and 15-crown-4 derivatives with keT,tNa being in the range 0.032-0.062, but deteriorated for larger ring sizes.Of course, it is important to realize that selectivity ultimately depends on the solvent mediator, and several have to be investigated before dismissing the prospects of any single sensor. For the crown ethers mentioned above, the mediator was NPOE.30Jl A benzo-14-crown-4 derivative (Fig. 3) proved to be disappointing32 as did the corresponding material without the right-hand phenoxy group (unpublished work). The ISEs from both gave similar lithium ion responses, but the selectivity was inadequate for overcoming the sodium back- ground of blood serum.32 The sodium uptake by the mem- brane containing this derivative during permeation experi- ment@ with radiotracers was deemed to be mainly due to PVC-solvent only contributions.Sensing With Bis-crown Ethers A reason for the poor selectivity of the 12-crown-4 ether (Fig. 3) towards lithium is the possibility of sandwich compound formation between two 12-crown-4 ether ligands and one sodium i0n.29 This leads to prospects for bis-crown ethers as ion sensors. There has been enquiry in this direction,34-3* for example, Shono and co-workers35~36 have demonstrated systems with good potassium ion selectivity for some bis(l5- crown-5) derivatives, and one containing a dodecyl link exhibits good lipophilicity and longer lifetime for the resulting ISEs.36 Further studies in Cardiff37 on bis( 1,4,7,1O-tetraoxa- dodecan-2-ylmethyl) 2-dodecyl-2-methylmalonate ( 5 ) and bis-( 1,4,7,10 , 13-benzopentaoxacyclopentadecin- 15-ylme th yl) heptanedioate (6) (Fig.4) show the former to be selective to sodium and the latter to potassium, both being best used with NPOE plasticizing solvent mediator and potassium tetrakis-4- chlorophenylborate anion excluder in PVC matrices.1214 0 *C cY C- /ClZHZ5 C //O ANALYST, DECEMBER 1991, VOL. 116 (7) R=H (8) R=CHZPh Fig. 4 Bis( l74,7,10-tetraoxadodecan-2-ylrnethyl) 2-dodecyl-2-methylmalonate (5) and bis( 1,4,7,10,13-benzopentaoxacyclopentadecin-15- ylmethyl) heptanedioate (6). Also shown is 1,ll-bis(2-hydroxy-5-formylphenoxy)-3,6,9-trioxaundecane (7) and its benzyl ether (8) Although the above electrodes offer promising alternatives to glass electrodes for sodium and to valinomycin electrodes for potassium, the data for measurements of the ions in blood serum demand further research in order to improve the correlations with flame photometric measurements.37 The molecules shown in Fig.4 have also been assessed as possible uranyl ion sensors.38 The pimelate failed to respond, while the malonate gave a steady uranyl response over the 18 d study period with a near-Nernstian slope. The solvent mediator in each instance was NPOE. The fact that the malonate-based ISE failed to respond after storage for 5 months was attributed mainly to minor radiation damage.38 Sensors Based on Ethoxylate-type Acyclic Structures The most exciting acyclic polyether structures studied at Cardiff for their scope in potentiometric sensing are clearly the various polyalkoxylate types, and especially those based on nonylphenoxypolyethoxylate (Antarox types) with different numbers of alkoxylate units, and referred to earlier in this paper.These were initially developed for their barium ion sensing qualities, but a bonus was the ability to sense alkoxylates themselves. Further, the inflection in the calibra- tion provides a facility for measuring the critical micellization concentration (CMC) of alkoxylate-type non-ionic surfac- tan t s .39-41 With regard to CMC, the inflections observed in the e.m.f. versus log[alkoxylate] graphs are definite and the breaks40 compare favourably with literature CMC data42-44 and agree with a derived equation.42 log (106 x CMC) = An + B (1) where A and B are constants for a particular hydrophobic group and n is the average number of EOUs in the molecule. Diphenyl Ethers of Tetraethylene Glycol Two derivatives of these have been studied27,32,38,45 namely, 1,l l-bis(2-hydroxy-5-formylphenoxy)-3,6,9-trioxaundecane (7) and its benzyl ether (8) (Fig.4). These can be looked upon as ‘clasping’ cations in ion sensing, and they were initially assessed as possible alkali and alkaline earth metal ion sensors in association with seven different solvent mediators.45 The response characteristics of compound (7) are dependent on the solvent mediator. Also, the systems with tris(2-ethylhexyl) phosphate and DOPP, each without a sensor, exhibited a significant response towards lithium ions.However, the over-all characteristics were slower, noisier, less stable and showed greater drift than the corresponding systems with a sensor present. Further evaluation32 as a sensor for lithium showed compound (7) to be much inferior to a lipophilic diamide. Continuing studies27 with compounds (7) and (8) yielded greater success with guanidinium ion sensing. Hence, each when used with either DBP or DOA solvent mediator is better than the previously recommended26 DB27C9 but, as already suggested above, they were not as good as the now recommen- ded27 BMP26C8. Sensor (8), when used with DBP or DOPP as solvent mediator, turned out to be a potential uranyl ISE sensor. This was largely on the premise of the marked reduction of iron(m) interference with sodium tetraphenylborate as anion excluder.Planar and Tetrahedral Tripodal Receptors Nine acyclic pol yethers, representing examples of planar and tripodal ‘scorpion-like’ molecules (Fig. 5), each with oligo- ether tails and a pair of anionic type ‘pincers’ together designed to ‘grasp’ ions, have recently been evaluated6 as possible sensors for barium ISEs. The chosen solvent media- tor was NPPE. The general performance of the resulting series of electrodes46 was inferior to the traditional ISE based on the tetraphenylborate of the barium complex with Antarox CO-880 (a nonylphenoxypolyethoxylate with 30 EOUs) . However, a general barium ion response seems to be favoured by a tetrahedral tripodal structured sensor (where X is benzyl in Fig. 5) with its design promoting good ion-dipole interac- tions, as seen in another on association constants of these acyclic polyethers with barium ions.ANALYST, DECEMBER 1991.VOL. 116 1215 C? p-x Fig. 5 studied as possible sensors for barium I S E S ~ ~ Planar and tetrahedral tripodal receptor type molecules Conclusion The studies presented demonstrate the underlying benefits in sensor development of structural-backed syntheses related to experience of sensor behaviour. The resulting comparative work has yielded promising sensors for a number of cations, including DQT, PQT, Gu+ and uranyl, and promising leads for lithium, sodium, potassium and barium. The author thanks his many co-workers for their dedication, and also Professor J. F. Stoddart (University of Birmingham) and Dr.D. J. Williams (Imperial College, London) and their co-workers for structural information and the provision of a range of materials. The various sponsors to whom acknow- ledgement has already been made in the cited references are also thanked for generous financial support, and steered by grants from the Science and Engineering Research Council under the sensors initiative for the main thrust in the research programme. 1 2 3 4 5 6 7 8 9 10 11 12 13 References Moore, C., and Pressman, B. C., Biochem. Biophys. Res. Commun., 1954, 15, 62. Stefanac, Z., and Simon, W., Chimia, 1966,20,436. Stefanac, Z . , and Simon, W., Microchem. J., 1967, 12, 125. Pedersen, C. J., J . Am. Chem. SOC., 1967, 89, 7017. Neu, R., Arzneim. Forsch., 1959, 9, 585. Uno, T., and Miyajima, K., Chem.Pharm. Bull., 1963,11,75. Levins, R. J., and Ikeda, R. M., Anal. Chem., 1965, 37, 370. Levins. R. J.. Anal. Chem., 1971,43, 1045. Rechnitz, G. A., and Eyal, E., Anal. Chem., 1972,44, 370. Petranek, J., and Ryba, O., paper presented at the IUPAC International Symposium on Selective Ion-Sensitive Electrodes, UWIST, Cardiff, UK, 9th-12th April, 1973, paper No. 13. Ryba, 0.. and Petranek, J., J. Electroanal. Chem., 1973, 44, 425. Moody, G. J.. Owusu, R. K., and Thomas, J. D. R., Analyst, 1987, 112, 121. Colquhoun, H. M., Goodings, E. P., Maud, J. M., Stoddart, J. F., Williams, D. J., and Wostenholme, J. B., J. Chem. SOC., Perkin Trans. 1, 1985, 2, 607. 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 Kohnke, F.H., Stoddart, J. F., Alwood, B. L.. and Williams, D. J., Tetrahedron Lett., 1984, 26, 1681. Kohnke, F. H., and Stoddart, J. F., Tetrahedron Lett., 1984,26, 1685. Allwood, B. L., Kohnke, F. H., Slawin, A. M. Z., Stoddart, J. F., and Williams, D. J., J. Chem. SOC., Chem. Commun., 1985, 311. Moody, G. J., Owusu, R. K., and Thomas, J. D. R., Analyst, 1988, 113, 65. Moody, G. J., Owusu, R. K., and Thomas, J. D. R., Analyst, 1987, 112, 1347. Colquhoun, H., Stoddart, J. F., and Williams, D. J., New Sci., 1986, May, 44. Moody, G. J., Owusu, R. K., and Thomas, J. D. R., Anal. Lett., 1988, 21, 1653. Kyba, E. P., Helgeson, M. K., Madan, K., Gokel, G. W., Tanowski, T. L., Moore, S. S., and Cram, D. J., J . Am. Chem. SOC., 1977,99,2564. Lehn, J.-M., Vierling, P., and Hayward, R.C., J. Chem. SOC., Chem. Commun., 1979,296. Stolwijk, T. B., Grootenhuis, P. D. J., van der Wal, P. D., Sudholter, J. W. H. M., and Kruise, L. J., J . Org. Chem., 1986, 51, 4891. Colquhoun, H. M., Goddings, E. P., Maud, J. B., Stoddart, J. F., Williams, D. J., and Wostenholme, J. B., J. Chem. SOC., Chem. Commun., 1983, 1140. Bochenska, M., and Biernat, J. F., Anal. Chim. Acta, 1984, 162, 369. Assubaie, F. N., Moody, G. J., and Thomas, J. D. R., Analyst, 1988, 113, 61. Assubaie, F. N., Moody, G. J., and Thomas, J. D. R., Analyst, 1989, 114, 1545. Gadzekpo, V. P. Y., Moody, G. J., Thomas, J. D. R.. and Christian, G. D., Ion-Sel. Electrode Rev., 1986, 8, 173. Gadzekpo, V. P. Y., and Christian, G. D.. Anal. Lett., 1983,16, 1371. Kitazawa, S., Kimura, K., Yano, H., and Shono, T., J . Am. Chem. SOC., 1984, 106,6978. Kitazawa, S., Kimura, K., Yano, H., and Shono, T., Analyst, 1985, 110, 295. Beswick, C. W., Moody, G. J., and Thomas, J. D. R., Anal. Proc., 1989, 26, 2. Harris, N. K., Moody, G. J., and Thomas, J. D. R., Analysf, 1989, 114, 1555. Lindner, E., T6th, K., Orvath, M., Pungor, E., Agai, B., Bitter, I., Toke, L., and Hell, Z., Fresenius 2. Anal. Chem., 1985,322, 157. Kimura, K., Mazeda, T., Tamura, H., and Shono, T., J. Electroanal. Chem., 1979, 95, 91. Kimura. K., Tamura, H., and Shono, T., J . Chem. SOC., Chem. Commun . , 1983,492. Moody, G. J., Saad, B. B., and Thomas, J. D. R., Analyst, 1989, 114, 15. Johnson, S.. Moody, G. J., Thomas, J. D. R., Kohnke, F. H., and Stoddart, J. F., Analyst, 1989, 114, 1025. Moody, G. J., and Thomas, J.D. R., in Non-ionic Surfactants: Chemical Analysis, ed. Cross, J.. Marcel Dekker, New York, 1986, p. 117. Jones, D. L., Moody, G. J., and Thomas, J. D. R., Analyst, 1981, 106, 439. Jones, D. L., Moody, G. J., Thomas, J. D. R., and Birch, B. J., Analyst, 1981, 106, 439. Hsiao, L., Dunning. H. N., and Lorenz, P. B., J. Phys. Chem., 1956, 60,657. Schick, M. J., Atlas, S. M., and Elrich, F. R., J. Phys. Chem., 1962, 66, 1326. Alexander, P. H. V., Moody, G. J., Thomas, J. D. R., and Birch, B.. Analyst, 1987, 112, 849. Moody, G. J., Saad, B. B., Thomas, J. D. R., Kohnke, F. H., and Stoddart, J. F., Analyst, 1988, 113, 1295. Feng, Y. P., Goodlet, G., Harris, N. K., Islam, M. M., and Thomas, J. D. R., Analyst, 1991, 116,469. Bartlett, J. S., Costello, J. F., Mehanic, S., Ramdas, S., Slawin, A. M. Z., Stoddart, J. F., and Williams, D. J., Angew. Chem. Znr. Ed. Engl., 1990, 29, 1404. Paper 1/01 721 G Received April 15th, 1991 Accepted May 5th, 1991

ISSN:0003-2654    

DOI:10.1039/AN9911601211

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, DECEMBER 1991, VOL. 116 1217 Disposable Electrochemical Biosensors* Paul 1. Hilditch and Monika J. Green MediSense Inc., Units 3 & 4, 14/15 Eyston Way, Abingdon, Oxon OX14 1TR, UK The market for decentralized clinical testing is undergoing expansion. Electrochemical biosensors represent one approach to the different demands of this market. A range of sensing systems are described which use electrochemical techniques for the measurement of various analytes and which have been demonstrated to be applicable to the manufacturing methods required for single-use disposable tests. Keywords: Amperometry; enzyme; mediation; blood; assay Without doubt the most important market for biochemical sensors is that for clinical diagnostics. The volume of testing has increased dramatically over the last 50 years as chemical analyses have become more important in the diagnosis and management of clinical conditions.Another wave of change is taking place in the pattern of clinical testing. Traditionally, samples from patients in hospital wards and clinics were delivered to central laboratories for analysis, a system that arose in part from the inflexible nature of the clinical organizations and in part from the specialized and expensive equipment then needed. Centralized testing is being super- seded by ‘distributed’ or ‘decentralized’ testing,’ the main feature of which is the use of diagnostic tests that can be performed at the patient’s bedside in a hospital ward, or in a physician’s surgery as the patient waits. The advantages for improved patient care and better use of overburdened medical resources are clear.A responsibility lies with the device manufacturer to ensure that the decentralized test performs comparably to the centralized analyser, while being suf- ficiently uncomplicated for staff with minimal training in its use, and inexpensive enough to allow the purchase of several instruments by a hospital or clinic. The biosensor has been widely heralded as the ideal solution to many OF the problems of the clinical chemist. Sadly, few of the exciting ideas for biosensors advanced over the last few years have found their way from the proof-of-principle stage to commercial realization. This paper examines a number of biosensor systems that are, however, in or near to commercial production.The scope of this review has been limited to electrochemical systems; in the first place these are the systems on which we are best qualified to comment, lying as they do within our experience, but we also believe that electrochemical systems offer real advantages over some of the optical alternatives, particularly in the distributed-testing or home-testing situations where convenience and robustness are of paramount importance, The sensor systems described have been arranged according to the species or moiety that the electrochemical reaction measures-the ‘true analyte’ of the sensor. The species of clinical interest that the system as a whole is designed to measure, the ‘effective analyte’, is coupled with the detection of the true analyte, usually by means of one or more enzymes. Sensors for Determining a Product of an Enzyme React ion Sensors in which the true analyte is the product of an enzyme reaction have been devised in several formats.In the first instance, the effective analyte is a substrate for an enzyme, which converts it into a species (the true analyte), which can be detected. In the second instance, the effective analyte is the enzyme itself, and a substrate is provided, which the enzyme * Presented at the Royal Society of Chemistry 150th Anniversary Congress. Imperial College, London, 8th-11th April, 1991. converts into a quantifiable product. This principle has also been extended to the class of sensors in which an enzyme is used as a label or other reporter of analyte presence. The classical example of this would be in enzyme-labelled immu- noassay, but other applications, such as nucleic acid hybridiza- tion assays and enzyme inhibition assays, are possible.Analyte is a Substrate for the Enzyme Few analytes of interest are immediately detectable electro- chemically; the use of an enzyme to convert the analyte into an electrochemically detectable form also provides specificity. Two examples will be presented here of this type of sensor, for the two widely used analgesics, acetyl salicylate and acet- aminophen. Acetylsalicylic acid, known as aspirin , is rapidly converted by hepatic esterases into salicylic acid, which is the analyte of clinical interest. Monitoring of salicylate levels in the blood- stream is of importance in cases of acute poisoning and in chronic therapy.Therapeutic concentrations of salicylate in plasma are in the range 1.1-2.2 mmol dm-3; levels higher than this are considered toxic. Salicylate is currently determined by a variety of methods. One of the most frequently used is a spectrophotometric procedure relying on the formation of a coloured complex between salicylate and iron(m) ions. Although convenient, this test suffers from a lack of specific- ity, a problem which can be overcome by the use of the enzyme salicylate hydroxylase (E.C. 1.14.13.1).2 This enzyme, of bacterial origin, converts salicylic acid into catechol in the presence of oxygen; the cofactor nicotinamide adenine dinucleotide (reduced form) (NADH) or nicotin- amide adenine dinucleotide phosphate (reduced form) (NADPH) is required: moH + 0 2 + NADH + 2H+ - The progress of this reaction may be followed spectropho- tometrically by measuring the decrease in absorbance due to reduced cofactor.Electrochemical approaches have included potentiometric and amperometric sensors, which measure the uptake of oxygen and release of carbon dioxide.3.4 An alternative approach to a disposable single-use sensor repor- ted by Frew et al. 5 involves the amperometric determination of catechol at a carbon electrode on a screen-printed disposable sensor strip. Biological components necessary for the reac- tion, including enzyme and NADH cofactor, were provided in a labile matrix printed onto the carbon electrode; addition of blood caused this layer to reconstitute, allowing the reaction to occur. Determination of the catechol generated was1218 ANALYST, DECEMBER 1991, VOL.116 performed chronocoulometrically , at a measurement poten- tial of +300 mV (versus Ag-AgCI), by using a printed reference electrode adjacent to the carbon working electrode. The kinetics of the enzyme were not ideally suited to the concentrations of salicylate of interest, and gave a sharply non-linear response over the clinical range. A more linear response was obtained by the inclusion in the reagent layer of sodium benzoate. An analogue of salicylate, this compound is able to uncouple the hydroxylase activity of the enzyme from its cofactor oxidation activity, modifying its apparent kinetic parameters towards salicylate. The dry-strip sensor for salicylate described is an instance where the use of an enzyme reaction combined with electro- chemical detection produces a rapid, convenient assay with good performance. Another example of this type of sensor is that for acetaminophen (N-acetyl-p-aminophenol; parace- tamol).In this instance the reaction is that catalysed by the enzyme aryl acylamidase (E.C. 3.5.1.13), which deacylates the acetaminophen molecule to yield p-aminophenol: yHCOCH3 y42 8 + H20- 0 +CH,COOH / / b H OH p-Aminophenol is detectable electrochemically at much lower potentials than would be required for the oxidation of acetaminophen itself, which reduces the background currents and the danger of interference by other compounds.6 A sensor based on this principle was described by Jones et a1.;7 again based on disposable screen-printed strip technology, the system displayed excellent precision and accuracy when compared with a reference assay.Analyte is the Enzyme Itself Levels of a number of enzymes are significant in the diagnosis and monitoring of some clinical conditions. The example that will be described here is the enzyme a-amylase (E.C. 3.2.1.1), important in the diagnosis of acute and chronic pancreatitis, in which levels of a-amylase can be elevated 20-fold above their normal values. &-Amylase catalyses the random cleavage of glycosidic linkages in a-( 1-4) linked glucose polymers (starch, glycogen, etc.). Spectrophotometric methods for the determination of a-amylase activity (e.g., reference 8) involve the use of p-nitrophenol-derivatized oligosaccharides as substrates for the a-amylase.The enzyme a-glucosidase is added to the reaction mixture, which will hydrolyse short (n < 4) oligosac- charide products of the amylase reaction to liberate p-nitro- phenol, which is detected spectrophotometrically. An adapta- tion of this assay format for an electrochemical sensor is described by Batchelor et aZ.,9 in which a p-aminophenyl oligosaccharide is used as substrate, with a-glucosidase employed as coupling enzyme. The electrochemical reaction is again the oxidation of p-aminophenol (Scheme 1). Enzyme is a Label or Other Reporter of Analyte Presence Given the ability to measure the activity of therapeutically significant enzymes, it is a small conceptual step to the assay of enzymes used as labels in, for example, immunoassays. Spectrophotometric enzyme immunoassays typically involve the use of one of four enzymes: horseradish peroxidase, g-galactosidase, alkaline phosphatase or glucose oxidase.Electrochemical techniques exist for the assay of all four of these enzymes. Alkaline phosphatase in particular is a popular I Scheme 1 choice for immunoassays because of its high catalytic activity and broad substrate specificity. The electrochemical assay of alkaline phosphatase activity is possible by use of p-amino- phenyl phosphate as substrate for the enzyme; hydrolysis releases p-aminophenol, which is electrochemically distin- guishable from the phosphate: NH2 I I OP032- Frew et a1.10 applied this to immunoassays, in which the anti-asthmatic drug theophylline was determined by a compet- itive assay and the anticonvulsant drug phenytoin (dilantin) by a displacement assay. An alternative assay for theophylline was described by Foulds et al.11 This exploits the property that, at micromolar concentrations, theophylline is a particularly potent inhibitor of several isoenzymes of alkaline phosphatase, particularly that from the liver.An electrochemical detection system for alkaline phosphatase such as that already described is, therefore, ideally suited for the determination of theophylline in a sample. Disposable sensor strips consisting of a carbon working electrode and an Ag-AgCI reference electrode were fabricated; the p-aminophenyl phosphate used as substrate for the enzyme was provided in a layer above the electrode, which was designed to reconstitute on addition of sample.p-Amino- phenol generated by the enzyme reaction was determined by chronoamperometry at a potential of +SO mV. Sensors Measuring the Rate of an Enzyme Reaction In this section assays will be considered in which the effective analyte undergoes a reaction catalysed by an enzyme, and the sensor measures the rate of progress of this reaction by mediating with redox centres of the enzyme or with soluble redox cofactors. The true analyte in these examples is, therefore, the mediating compound. By Mediation With Enzyme or Prosthetic Group The classic example of this class of reaction is the electrochem- ical determination of glucose via interaction of glucose oxidase (E.C. 1.1.3.4; GOD) with ferrocene [bis(cyclopentadienyl)- iron] derivatives.12713 Glucose oxidase catalyses the oxidationANALYST, DECEMBER 1991, VOL. 116 1219 of glucose to form gluconolactone, with concomitant con- By Mediation With a Cofactor sumption of oxygen a n d production of hydrogen peroxide. Oxidation of the glucose occurs with reduction of the flavin prosthetic group within the enzyme; the reduced flavin can then reduce O2 to form hydrogen peroxide. The oxidized forms of many ferrocene derivatives (ferricinium ions) are capable of competing with O2 for reduced flavin. It is, therefore, possible to set up a mediation cycle in which ferricinium is reduced by flavin and re-oxidized at an electrode, producing a current dependent on the glucose concentration (Scheme 2). Scheme 2 A commercial glucose sensor based on this chemistry is available.14315 The sensor strips are constructed, using screen- printing technology to incorporate a carbon working electrode and an Ag-AgC1 reference electrode; a reagent 1ayer.contain- ing GOD and a ferrocene derivative is printed above the carbon electrode, which reconstitutes on addition of a drop of blood. A second-generation product 16 provides further improve- ments including temperature compensation, ease of sample application and almost complete freedom from interference by other compounds. The last of these is achieved by the provision of a third electrode on the sensor strip; the reagent layer for this electrode contains all the components of the normal working electrode except for the enzyme. Any redox-active interferents present in the sample will affect the response of this ‘dummy’ electrode in the same way as the working electrode, allowing a correction to be performed by the meter to provide the correct glucose value. Another enzyme-sensing system that employs mediation to redox sites in an enzyme is that developed for measuring the activity of the haem enzyme horseradish peroxidase (E.C.1.11.1.7; HRP). Although HRP is not a flavoprotein like GOD, ferrocenes are capable of interacting with the enzyme, regenerating it from the oxidized form, after reaction with hydrogen peroxide, to its reduced state, with production of ferricinium. This lends itself to a cyclic mediation scheme (Scheme 3). Scheme 3 Apart from the potential application of this system to enzyme-labelled immunoassays, the measurement of hydrogen peroxide levels is of significance in many coupled assay schemes.For example, cholesterol can be determined by its reaction with cholesterol oxidase (E.C. 1.1.3.6), forming cholesten-3-one and hydrogen peroxide. A cholesterol esterase (E.C. 3.1.1.13) is necessary to release the cholesterol from the lipoprotein complexes in which it is predominantly found. A cholesterol sensor, using this chemistry on dispos- able sensor strips, is now commercially available. Most enzymes used in clinical assays are not amenable to direct mediation with ferrocenes or other compounds, but many utilize soluble cofactors such as NAD(H) or NADP(H). It is possible to measure the levels of NADH by its electrochemical oxidation; however, this requires a high overpotential and alternatives have been sought involving use of lower potential redox compounds.Several types of com- pound have been employed to this end. Batchelor et al. 18 used 4-methyl-o-quinone (4MQ) as a redox mediator in a screen- printed system for the determination of 0-hydroxybutyrate, which is of clinical significance in the management of diabetes. The enzyme used was D-3-hydroxybutyrate dehydrogenase, which was incorporated into a reagent layer on a sensor strip containing 4MQ and NAD. The NADH generated by the enzyme reaction is oxidized by the 4MQ, which is converted into 4-methylcatechol, which is then oxidized at the electrode at +350 mV versus Ag-AgC1. Sensors for Determining a Product of a Metabolic Process The sensors described thus far determined individual chemical analytes of therapeutic or diagnostic interest.At least as important a field for sensor application is that of microbiolog- ical testing. Tests are performed for the identification of specific organisms and for the measurement of antibiotic susceptibilities, but the most frequently used test is that for the measurement of total bacterial biomass, typically by agar plate culture and colony counting. This method suffers from disadvantages of a long response time (>24 h) and limited accuracy. Many efforts have been made to design a more rapid and convenient biosensor for total bacterial measurement; that described here19 employs the electrochemical interroga- tion of respiratory electron transport. The respiratory electron transport system of a bacterium is located in its cell membrane; electrons liberated during the oxidative degradation of metabolic substrates are passed between redox carriers within and peripheral to the mem- brane.Certain compounds are known to be capable of intercepting these respiratory electrons; for example, sodium hexacyanoferrate(Ii1) will become reduced to hexacyanofer- rate(I1) in the presence of respiring bacteria. A more efficient mediator is 1,6benzoquinone, which is used in the assay described. Interception of respiratory electrons causes this compound to be reduced to its hydroquinone; this can be determined electrochemically by oxidation at a potential of +400 mV versus Ag-AgCl. Straightforward incubation of benzoquinone with a bac- terial suspension will cause the hydroquinone to be generated at a rate reflecting the rate of respiration and hence the cell numbers.Although this approach has been used in bacterial sensors, there are two problems with the simplistic approach. Firstly, the sensitivity of the approach is limited by the rate of electron transport expected for bacterial cells in the light of the likely measurement threshold of appropriate electronic systems. Secondly, any redox active components present in the sample could interact with the benzoquinone or the hydroquinone product causing a false result. These problems are addressed in the assay described by collecting bacterial cells in the sample on a filter contained in a disposable element, which incorporates the electrodes needed for the electrochemical determination of the hydroquinone generated.Not only does this extend the sensitivity of the system by concentration of cells in the sample, but also soluble redox interferents are eliminated by washing the loaded filter with buffer solution before filling the assembly with benzo- quinone solution and incubating at a controlled temperature. The hydroquinone generated is determined by chronocoul- ometry after a 10 min incubation. The performance of the system has been validated with a number of species of bacteria1220 ANALYST, DECEMBER 1991, VOL. 116 in pure and mixed culture and in food and environmental samples, and appears to have a sensitivity of about 104 cells ml-1 with an assay time of about 15 min. Sensors Measuring Changes in Potential Potentiometric techniques have been used for many years in chemical sensors; typical examples are the familiar glass electrodes and the membrane-based gas-sensing electrodes. These systems suffer from the disadvantages associated with re-usable sensors and tend also to be less than robust.The design of a truly disposable, single-use potentiometric sensor presents a particular challenge. The system described here20 measures concentrations of potassium ions in blood, which is of interest in cardiovascular management during major surgery and in the diagnosis of cardiac disease. The sensor, the operation of which is based on an ion-selective electrode technique, is constructed on a dispos- able strip, which has an integral calibration mechanism. A gelatin hydrogel containing potassium chloride is in contact with an Ag-AgC1 reference electrode and is covered by a poly(viny1 chloride) membrane containing the potassium ionophore valinomycin.Potassium in a sample applied to the other face of the membrane changes its electrical properties; in this instance the circuit is completed by a second gelatin hydrogel that is in contact with another Ag-AgC1 electrode, but which has no ion-selective membrane. Calibration of the system is achieved by means of a further gel, containing a known concentration of potassium, which is positioned above and in contact with both electrode systems. After measure- ment of the potential by the meter, the calibration gel is removed by the user, and the sample is applied. The difference in potential between the calibration gel and the sample is logarithmically related to the potassium content of the sample.The system has been validated in clinical trials21 to be substantially equivalent to currently available products, and will offer considerable advantages for convenience and speed of use. Conclusions The examples described above illustrate some areas of bioelectrochemistry , which have been applied to disposable sensor devices. The challenges and limitations in applying further examples will lie not in conceiving the chemistry, but in designing the devices, which must be in a form that can be manufactured in large numbers at low cost. The devices should not require the addition of extra reagents at the time of use, and must be stable for at least 6 months. It is this design and development phase that is the most critical for the survival of a product.References 1 Hilton, S., Br. J. Gen. Pract., 1990, 40, 32. 2 White-Stevens, R. H., and Kamin, H., J. Biol. Chem., 1972, 247, 2358. 3 You, K., Clin. Chim. Acta, 1985. 149, 281. 4 Fonong, T.. and Rechnitz, G. A., Anal. Chim. Acta, 1984,158, 357. 5 Frew, J. E., Bayliff, S. W., Gibbs, P. N. B., and Green, M. J., Anal. Chim. Acta. 1989, 224, 39. 6 Bramwell, H., Cass, A. E. G., Gibbs, P. N. B., and Green, M. J., Analyst, 1990, 115, 185. 7 Jones, A. F., McAleer, J. F., Braithwaite, R. A., Scott, L. D., Brown, S. S., and Vale, J. A., Lancet, 1990,335 (8692), 793. 8 Rauscher, E., Neumann, U., Schaich, E., von Bulow, S., and Wahlefeld, A.-W., Clin. Chem. (Winston-Salem, N.C.), 1985, 31, 14. 9 Batchelor, M.J., Williams, S. C., and Green, M. J., J. Electroanal. Chem., 1988,246, 307. 10 Frew, J. E., Foulds, N. C., Wilshere, J. M., Forrow, N. J., and Green, M. J., J. Electroanal. Chem., 1989, 266, 309. 11 Foulds, N. C., Wilshere, J. M., and Green, M. J., Anal. Chim. Acta, 1990, 229,57. 12 Cass, A. E. G., Davis, G., Francis, G. D., Hill, H. A. O., Aston, W. J., Higgins, I. J., Plotkin, E. V., Scott, L. D. L., and Turner, A. P. F., Anal. Chem., 1984, 56, 667. 13 Green, M. J., and Hill, H. A. O., J. Chem. SOC., Faraday Trans., 1986, 82, 1237. 14 Matthews, D. R., Bown, E., Watson, A., Holman, R. R., Steemson, J., Hughes, S . , and Scott, D., Lancet, 1987,1(8536), 778. 15 Matthews, D. R., Burton, S. F., Bown, E., Chusney, G., Dornan, T., Gale, E. A. M., McKinnon, G., and Steemson, J., Diabetic Med., in the press. 16 Matthews, D. R., Burton, S. F., and Smith, E., in Proceedings, Artificial Insulin Delivery Systems, Pancreas and Islet Trans- plants (AIDSPIT), 1991. 17 Ball, M. R., Frew, J. E., Green, M. J., and Hill, H. A. O., Proc. Electrochem. Soc., 1986, 86-14, 7. 18 Batchelor, M. J., Green, M. J., and Sketch, C. L., Anal. Chim. Acta, 1989, 221, 289. 19 Hilditch, P. I., Carter, N. F., Barrett, C. B., Sullivan, D. J., Charman, K. M., Green, M. J., and Williams, S. C., in Advances in Bioreactor Monitoring, ed. Wang, N. S . , Academic Press, New York, in the press. 20 Franklin, C. J., McAleer, J. F., Orman, H. J., Hirst, J. A., Andrews, M. P., and Ward, R. N., in Methodology and Clinical Applications of Blood Gas, pH, Electrolytes and Sensor Tech- nology, eds. Moran, R. F., and VanKessel, A. L., MVI Publishing, Copenhagen, 1990, vol. 12, p. 165. 21 Smith, M., McAleer, J. F., Orman, H. J., Hirst, J., Andrews, M. P., and Franklin, C. J., Br. Med. J., submitted for publication. Paper 1103020E Received June 19th, 1991 Accepted July 18th, 1991

ISSN:0003-2654    

DOI:10.1039/AN9911601217

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, DECEMBER 1991, VOL. 116 1221 Ion Permeability Through Bilayer Lipid Membranes for Biosensor Development: Control by Chemical Modification of InterFacial Regions Between Phase Domains* Dimitrios P. Nikolelis,t John D. Brennan, R. Stephen Brown, Graham McGibbon and Ulrich J. KrullS Chemical Sensors Group, Department of Chemistry, Erindale Campus, University of Toronto, 3359 Mississauga Road North, Mississauga, Ontario, Canada 151 I C6 Based on the results of studies on cystic fibrosis, which implicated hydroxystearic acid (HSA) as a contributing factor in altered biomembrane function, solvent-free bilayer lipid membranes (BLMs) and monolayer films were prepared from a lipid mixture containing (by mass) 34% phosphatidylcholine, 19% dipalmitoylphos- phatidyl serine, 47% cholesterol and variable amounts of 10- and 12-HSA (0-50%).Ion currents, resulting from K+ permeation through BLMs that were supported in 0.1 mol dm-3 KCI solutions buffered to pH 7.4, were monitored with use of a d.c. circuit. The structures of monolayer films at the air-water interface of a Langmuir-Blodgett trough were studied by pressure-area correlations and by further correlation with microscopic phase separation as revealed by fluorescence microscopy. In order to elucidate the role of the hydroxyl moieties in ion permeability, the transmembrane ion current was corrected for the effect of the negative surface charge of the carboxylic acid by replacement of the HSA component with stearic acid. The ion current was found to increase with the molar proportion of the HSAs.Two models for ion conduction through BLMs were considered: ‘hopping‘ via hydrophilic sites within the hydrophobic zone of the BLMs, introduced by the hydroxyl moiety of 10- or 12-HSA; and transport through interfacial regions between phase domains that represent areas of low steric density and low structural order within monolayers. Although the two mechanisms are not distinct, the ion permeability results indicate a change in the response of ion current to HSA concentration at 35 mol-%, suggesting a change in the relative proportion of the mechanisms. The results implied that the magnitude and physical location of ion permeation through BLMs could be controlled by using specific concentrations of HSA such that permeation occurs predominantly through interfacial zones at the edges between domains, and that artificial ‘ion channels’ had been generated by virtue of the edge activity of HSA.Keywords: Bila yer lipid membrane; biosensor; ion permeability; domains; phase structure Perturbation of the structure of artificial lipid membranes can be monitored by electrochemical methods, and this offers opportunities for development of chemically selective sen- sors.1 A large number of biochemical reactions based on enzyme-substrate, antibody-antigen, lectin-saccharide and hormone-receptor interactions have been monitored by observation of the transmembrane ion current. An important advantage of such an electrochemical sensing system is the improved sensitivity that is derived from an intrinsic amplifica- tion process.A single selective binding event between a receptor and a target molecule can result in an increase of the transmembrane conduction that involves thousands of ions, provided that a suitable conductive pathway forms through the lipid membrane. An interesting extension of artificial lipid membranes for electrochemical sensing was the construction of ‘biomimetic ion-channel sensors’.2 These devices were based on Langmuir -Blodgett deposition of multilayers of acidic lipid membranes on glassy carbon electrodes. More than three layers of lipid were necessary to block the permeation of Fe(CN)64- , which was used as a marker ion. In the presence of the stimulant Ca2+, a change in the structure of the lipid occurred on the glassy carbon surface arising from electrostatic complexation of Ca2+ with acidic phosphate headgroups of the lipids, resulting in an increase of the marker ion penetration to the electrode surface.When ethylenediaminetetraacetic acid was added to the solution in a slight excess of that required to complex the Ca2+, the electrode signal for Ca2+ was elimi- nated. This indicates that conductive zones can be opened ~ ~~~~ * Presented at the Royal Society of Chemistry 150th Anniversary t University of Athens, Laboratory of Analytical Chemistry, $ To whom correspondence should be addressed. Congress, Imperial College, London, 8th-11th April, 1991. Panepistimiopolis, Kouponia, GR 157 71, Athens, Greece. reversibly by interaction of an analyte at a membrane in the absence of ion-channel proteins. However, the nature of such a conductive zone is poorly understood.A number of models of the physical mechanism for ion transport in bilayer lipid membranes (BLMs) have been proposed.3 Although the dynamics of even pure phospholipid BLM systems are complex, experimental results have identi- fied membrane fluidity/packing, dipolar potential, relative permittivity, surface charge and thickness as the physical parameters that contribute most significantly to ion per- meability.4 Empirical studies on phospholipid-cholesterol membranes have correlated the transmembrane ion current with average molecular packing and surface potential by using an Arrhenius rate equation.s.6 Theoretical and experimental reports have indicated that disordered states of lipid structure, as encountered during phase transitions, are highly permeable to ions.7 This is relevant to the process of ion permeation through lipid membranes as bilayer and monolayer lipid membranes can be composed of mixed phases in co-existence.The interfacial boundary between different phases is a zone of low structural order, and this could provide a highly conduc- tive pathway for ion permeation, conductivity being modu- lated by aspects of molecular density, mobility and electro- static charge. It has been suggested that high intramembrane concentra- tions of 10-hydroxystearic acid (10-HSA) are responsible for changes in the permeability of gases and ions through membranes of alveolar cells of cystic fibrosis patients.8.9 Characterization of the mechanism and control of the magni- tude of ion permeability through membranes by 10-HSA could provide an insight into criteria for the design of conductive artificial ‘ion channels’ in chemically selective membranes (or analogues of natural tissues’*).In order to determine the effect of 10-HSA in alterations of lipid membrane structure , the electrochemical K+ permeabil-1222 ANALYST, DECEMBER 1991, VOL. 116 ity of solventless BLMs was studied, and structural features of monolayer films at an air-water interface were investigated by fluorescence microscopy.11 The introduction of 10-HSA into BLMs was expected to lead to alteration of the ion conduction properties via the introduction of hydrophilic sites into the hydrophobic environment in the interior of the membrane. A similar change in the ion current would also be expected when the 12-analogue of the 10-HSA was used for membrane formation.Increased ion permeability could be rationalized in terms of a 'hopping mechanism' during which a migrating ion would be energetically stabilized by polar interactions with the hydroxyl moiety. If ion conduction was inhibited by steric packing of the acyl chains in the bilayer systems, it could be inferred that transport would occur through regions of lowest chain density or 'defect' structures in the membrane. Hence, the alteration of interfacial regions between phase domains would provide a mechanism for controlling ion conduction. Experimental Reagents The phospholipids used were lyophilized egg phosphatidyl- choline (PC; Avanti Biochemicals, Birmingham, AL, USA) and dipalmitoylphosphatidyl serine (DPPS; Sigma, St. Louis, MO, USA).The steroid, which was cholest-5-en-3-01, and the 12-HSA were also from Sigma. The fluorescent probe used in the monolayer experiments was N(7-nitrobenz- 2,1,3-oxadiazol-4-yl)dipalmitoyl phosphatidylethanolamine (NBD-PE) from Avanti Biochemicals. The 10-HSA was provided by Dr. Ian Campbell of the Department of Zoology, University of Toronto, and was extracted from bacterial culture. Water was purified by passage through a cartridge filtering system (Milli-Q system, Millipore, El Paso, TX, USA) with a minimum specific resistivity of 18 MS2 cm. All other chemicals were of analytical-reagent grade. Apparatus The solventless bilayer membranes were prepared under 0.1 mol dm-3 KCI electrolyte solution in a circular aperture of 0.32 mm diameter located in a 10 pm thick Saran Wrap sheet (Dow Chemical Co., Sarnia, Canada).The aperture in the film was punched with a perforation tool, similar to that described previously,'2 and was always circular; the diameter was measured by using a microscope with a micrometer grating in the ocular. The support housing for BLMs consisted of two identical Perspex half-cells, each with a volume of about 10 ml and an air-water interface area of about 3 cm*. In order to isolate the solventless BLMs from mechanical vibrations, the membrane support housing was placed on a cement tile that was supported on a partially inflated bicycle tyre. An external potential of 25 mV d.c. was applied across the BLM between a pair of Ag-AgC1 wire electrodes.Currents were measured with a digital electrometer (Model 616B, Keithley Instruments, Cleveland, OH, USA). The electrochemical cell and electrometer were shielded from external electromagnetic interference in an earthed copper screen Faraday cage. The electrical capacitance of the membranes was measured by applying rectangular voltage pulses of 25 mV through the Ag-AgC1 electrodes to the membrane. The resulting capaci- tive current was displayed and stored with a Model DS102 10 MHz digital storage oscilloscope converter (Polar Instru- ments, Guernsey, UK) and a 20 MHz Model HM203-5 oscilloscope (Hameg, Frankfurt, Germany). The Keithley digital electrometer was inserted between the electrodes and the digital storage oscilloscope converter, which served as a current-to-voltage converter.The capacitance was also measured directly with a Model SR-530 lock-in amplifier (Stanford Research Systems, Sunnyvale, CA, USA) by applying a sinusoidal waveform (set to a frequency of 1 kHz) to the Ag-AgC1 electrodes and observing the out-of-phase component of charge flow. The monolayer work was carried out at the air-water interface of a Langmuir-Blodgett thin-film balance (Lauda Model 1974, Sybron-Brinkman Instruments, Toronto, Ontario, Canada) and involved the use of a fluorescence microscope, as described previously.11 Procedures The mixtures used for the preparation of solventless BLMs consisted of 0.125 mg of PC, 0.075 mg of DPPS, 0.175 mg of cholesterol and various amounts of 10- or 12-HSA in a solvent system containing 0.8 ml of hexane with 0.2 ml of absolute ethanol.The membranes were formed by using a variation of the 'monolayer folding' technique described previously. Is15 A 5-10 p1 amount of lipid solution was added dropwise to the water surface in the one-cell compartment. The drops were applied near the partition so that the lipid layer was swept away from the partition and removed any surface impurities. Over a period of 5 s the electrolyte level was brought below the aperture level and then raised again by means of a disposable syringe located with the tip inside the compartment and perpendicular to the plane of the monolayer. The formation of the membrane was checked by monitoring the ion current through the aperture. The formation of a membrane took only 5 s, although each membrane required about 10 min to stabilize before quantitative measurements were begun.This modified arrangement for membrane formation was found to yield stable membranes that lasted for more than 4 h, and in most instances the formation of BLMs was immediately successful. No pre-treatment of the aperture with petroleum jelly or squalene was found to be necessary to facilitate the formation of the film.15 All experiments were performed at 21 k 1 "C, and the ion current values shown in the figures are the mean values sampled from five separate experiments. The lipid solutions used for monolayer formation had a composition identical with those used for the preparation of solventless BLMs. The NBD-PE was added to obtain a final concentration of 1 mol-% .These lipid solutions were stored at a temperature below 0 "C and were sonicated at room temperature for 30 min before use. Monolayers were formed by adding the lipid solution dropwise to the air-water interface in 1 p1 increments over the entire surface of the trough, followed by a delay of 15 min for solvent evaporation before compression. Compression and expansion rates were 15 cm* min-1. Results Pertinent considerations for the successful reconstitution of solventless BLMs are given elsewhere. l6 Bilayers that were free of solvent were used in this study because the retention of solvent decreases experimental precision between experi- ments, and the properties of solvent-free membranes more closely resemble those of natural membranes. Saran Wrap was used as a partition,l7 as it was found that when a thicker partition was used the membranes were unstable. A similar reduction of physical stability was apparent when the diameter of the aperture was larger than 0.32 mm.Closing one of the solution compartments has been reported to increase the mechanical stability of membranes,lg but this was considered unnecessary as the BLMs formed here were stable for over 4 h. Electrical capacitance was measured to test whether the BLMs that were formed via the modified 'monolayer folding' method were free of solvent. The time constant, t, of the current versus time profile was measured when rectangular voltage pulses were applied through the Ag-AgC1 electrodes across the membrane. The capacitance of a membrane was calculated from the relationship: C, = t JdVANALYST, DECEMBER 1991, VOL.116 1223 where Jo is the current extrapolated to zero time and V is the applied voltage. The mean value of the specific capacitance of the BLMs was found to be 0.735 k 0.074 pF cm-2 (n = 10). This value of specific capacitance was in agreement with that obtained from the out-of-phase component observed during application of a sinusoidal waveform (C, = 0.715 k 0.049 pF cm-2; n = 3) and within the range given in the literature for solventless lipid bilayers.13-15 Using the above values of the specific capacitance, the hydrocarbon thickness, d, of the membrane can be calculated from the equation: where e0 represents the permittivity of free space and E is the relative permittivity of the hydrocarbon zone of the mem- brane and is assumed to be equal to 2.1.The membrane can be represented as a large capacitor (polar layers) in series with a smaller capacitor (the hydrophobic core of the membrane), and d is, therefore, almost equal to the thickness of the hydrocarbon layer of the membrane (experimentally deter- mined to be 2.6 & 0.2 nm in this work). The bimolecular thickness of the membranes was also confirmed by conduc- tance changes that were induced when the channel-forming agent gramicidin was added to the supporting electrolyte solution. 13 Typical values for the specific resistance of the solventless BLMs were about 108 Q cm-2. These results were calculated from the inverse of the slope of current-voltage recordings obtained when applying a linear ramp of voltage versus time in the region 0 to about 300 mV at a rate of 1.4 mV s-1, and from the steady-state values of current when applying voltage increments of 20 mV over the same voltage range.The dielectric breakdown voltage of these membranes was about 350 mV. Fig. 1 indicates how the ion current through solventless BLMs changes in relation to the mass percentage of 10- or 12-HSA. The general trend for all concentrations of these acids was to increase the ion current above the level that a phospholipid-cholesterol mixture exhibited in the absence of these compounds. Incorporation of the carboxyl group anion of stearic acid (at pH 7.4) in the membrane structure would be expected to increase the negative surface charge density and surface potential of the bilayer membrane at each membrane/elec- trolyte interface;*g-21 therefore, as the concentration of stearic acid increases, more cations will preconcentrate at each interface, while anions will be repelled.As a result, the cation permeability and ion current through the BLM will increase. In order to measure the ion current increase due only to the hydroxyl moiety of the 10- or 12-HSA, the effect of incorporation of stearic acid on the ion current was studied. Fig. 1 shows that an increase of stearic acid concentration in the membrane results in an increase of the transmembrane ion current in a linear fashion. This increase of the ion current is c, = €odd (2) 1.2 i"/ ./ 1 .o a 0.8 r I 0 0.6 g 2 0.4 0.2 0 10 20 30 40 50 Residual ion current as a function of fatty acid content: A, Fatty acid ("/.m/m) Fig. 1 stearic acid; B, 10-HSA; and C, 12-HSA less than that obtained when the 10- or 12-hydroxy fatty acid is incorporated in the membrane. Fig. 2 shows the results of the difference between the ion currents obtained when the stearic acid is replaced by HSA for membrane formation. The graph is plotted on a semilogarithmic scale and shows that there is an approximately exponential increase of the ion current as the molar content of the 10- or 12-HSA in the membrane increases. An inflection point near the 35 mol-% level is present for both 10- and 12-HSA mixtures. Monolayer pressure-area (n-A) curves for membranes containing various amounts of 10-HSA are shown in Fig. 3. The compression curve for PC-DPPS+holesterol exhibits a transition at about 1 .OO nm2 per molecule, which corresponds to the separation of fluid cholesterol and fluid lipid phases.Concurrent fluorescence images such as those of Fig. 4(a)-(c) revealed the formation of bright circular domains, consisting of relatively more fluid lipid phases, surrounded by darker, more condensed areas likely to be rich in cholesterol. The bright domains reduced in diameter as the molecular area decreased to 0.50 nm2 per molecule, and then became irregular structures as the molecular area decreased to 0.40 nm* per molecule. The introduction of a small amount (12 mol-%) of 10-HSA caused a substantial condensation of the original phospho- lipid+holesterol monolayer. An inflection point was observed as shown in curve B of Fig. 3. No significant change in microscopic images was observed.An increase of 10-HSA to a composition of 35 mol-% caused further condensation of the membrane and reduced the pressure at which the inflection point was observed. The surface pressure associated with the inflection point became constant beyond the 35 mol-% composition of 10-HSA in the monolayers. The microscopic images changed dramatically at the higher concentrations of 10-HSA. Initial structures -8.5 -9.0 = -9.5 a v, = -10.0 I - a -10.5. ? v, -11.0 0 2 -11.5 - 12.0 I I I I 1 1 I 0 10 20 30 40 50 60 70 n-HSA (mol-%) Fig. 2 Transmembrane ion current corrected for the contribution of the carboxylic acid headgroup (determined from the effect of stearic acid) as a function of the composition of hydroxy fatty acid in a semilogarithmic mode: A, 10-HSA; and B, 1ZHSA I I I I 1 0 0.20 0.40 0.60 0.80 1 .oo Area per molecule/nrn2 Fig.3 amounts of 10-HSA: A, 0; B, 12; C, 27; D. 35; and E, 48 mol-% Pressure-area isotherms of monolayers containing various1224 ANALYST, DECEMBER 1991, VOL. 116 changed from simple circles to ‘crescent’ shapes, and a greater variety of structures was apparent as shown in Fig. 4(d)-(f). Compression caused a decrease in the proportion of the bright or fluid region, with a breakdown of the structure, shown in Fig. 4(d), occurring at molecular areas less than that for the inflection point in the JC-A curves of Fig. 3. The amount of NBD-PE that was added to the solutions for the monolayer studies was deliberately small so as not to alter substantively the structure of the membranes.The effect of NBD-PE on the ion permeability of BLMs was also tested. Solventless BLMs were formed from the solutions used for the monolayer studies, and the ion current was found to be the same (within the relative standard deviation) as for the original membranes without fluorophore. Discussion The complicated chemical composition of the membranes was chosen to provide a lipid-cholesterol system with features similar to those of natural membranes22.23 in consideration of the proposed effect of 10-HSA, but this made it difficult to provide a detailed interpretation of the structure of the membranes. In a simplified view, two models for the transport process in BLMs composed of lipid, cholesterol and 10- or 12-HSA are possible. The first model considers the hydroxyl moiety at the 10- or 12-position of the acyl chain as a ‘polar defect’ distributed homogeneously throughout the hydro- phobic interior of the membrane.The interaction of hydrated K+ with polar hydroxyl groups would be energetically Fig. 4 Fluorescence microscopy images (full scale of image 250 pm) showing domain structure of monolayers at different pressures and with different compositions of 10-HSA. Images (a)-(c) represent monolayers containing 12 mol-% 10-HSA at average molecular areas of 0.85,0.50 and 0.40 nm2, respectively. Images (d)-(f) represent monolayers containing 35 mol-% 10-HSA at average molecular areas of 0.85, 0.52 and 0.38 nm2, respectivelyANALYST, DECEMBER 1991, VOL. 116 1225 f’ A n II K li Fig. 5 Schematic representation of the two models associated with ion permeation through lipid membranes containing some HSA (smaller headgroup and one hydroxylated acyl chain).The lipids are represented by larger headgroups and two acyl chains, and are shown to form two zones rich in lipid which differ in density (phase domains). Conductivity through zone ‘A’ is by electrostatic stabilization of ions by hydroxyl moieties in domains that are rich in HSA. Conductivity in zone ‘B’ also involves stabilization of charge by hydroxyl moieties, but also shows the steric disorder that can exist between domains favourable in the non-polar interior of a BLM. Therefore, the ions could be transported across the membrane via ‘polar defects’ in a ‘hopping’ mechanism. The second model consid- ers that the same energetics associated with the first model are applicable, but that the 10- or 12-HSA is heterogeneously distributed in zones of high relative HSA concentration.These models are illustrated schematically in Fig. 5. It is plausible that, at high local concentrations, associative effects between hydroxyl groups would further reduce electrostatic and steric barriers to ion permeation. The 10- and 12-HSA have only one acyl chain, whereas the lipids (PC and DPPS) each have two acyl chains per molecule. The lipids also have a glycerol backbone that is not present in the 10- or 12-HSA. Therefore, the average molecular area decreased as the molar proportion of 10- or 12-HSA was increased. The LO- or 12-HSA has a carboxylic acid headgroup that is deprotonated at pH 7.4, indicating that the anionic form was present in the membranes used for these investiga- tions.The negative charge present on the 10- or 12-HSA implies that electrostatic repulsions exist between 10- or 12-HSA and DPPS molecules. Hence, on structural and electrostatic bases, the 10- or 12-HSA represents an impurity in domains otherwise rich in either lipid or cholesterol components. The lowest energy state of the system could be expected to arise if the 10- or 12-HSA initially partitioned into interfacial regions between the lipid and cholesterol phases. Interfacial energetics are known to dominate cholesterol-lipid interactions at the molecular level24 and it is, therefore, expected that the introduction of 10- or 12-HSA would alter the phase structures in the mixed cholesterol-lipid systems. Fluorescence microscopy indicated that sufficient concen- trations of 10-HSA could significantly alter the shape and distribution of domains, resulting in a substantial increase in the total interfacial area of the domains.The formation of domains with ‘crescent’ shapes has been previously linked to interfacial processes? The four-component mixture used in these experiments complicates predictions of phase structure and the extent of domain co-existence. The trends observed from the x-A curves show similarities to previous work in which two-component mixtures of 12-HSA with long-chain fatty acids were studied.25 The evolution of an inflection point in the x-A curves for a low concentration of 10-HSA could be due to the formation of a miscible eutectic mixture.The large condensation effect caused by relatively small concentrations of 10-HSA further suggests that the material is not homogeneously distributed as a bulk constituent of the monolayer, but that it is located in interfacial regions which then control phase distribution. The inflection point in the n-A isotherms occurs at a constant pressure once the concentration of 10-HSA is above 35 mol-% , suggesting that an immiscible mixture is formed,*S resulting in a distinct phase that is rich in 10-HSA. The appearance of a distinct phase in a BLM should alter transmembrane ion permeability in comparison with results obtained for lipid compositions containing less than 35 mol-% 10- or 12-HSA. Fig. 2 indicates that there is an inflection in the conductivity near the 35 mol-% HSA composition.The slope of ion permeability versus 10- or 12-HSA content is much greater for HSA compositions above 35 mol-% , indicating that a new pathway with a lower energy barrier is available for ion permeation. This pathway is likely to be associated with a phase that is rich in 10- or 12-HSA, and could involve dipolar interactions of hydroxyl residues. This cooperativity is consis- tent with the logarithmic relationship of HSA composition and ion permeability. A further significant feature of Fig. 2 is that extrapolation of the plot to an HSA composition of zero yields a current greater than that actually observed for BLMs that did not contain HSA (greater by 1.1 x 10-12 and 5.4 x 10-13A for LO- and 12-HSA, respectively).This again suggests that the HSA is heterogeneously distributed in the BLMs, in a location that affects ion permeability in a non-linear fashion. The interfaces between lipid and cholesterol phases rep- resent the most likely pathways for ion conduction. On a molecular level the cholesterol phase is a condensed fluid. There exists a higher degree of order in this region than in the surrounding lipid regions as determined by fluorescence images? However, the lipid phase also has a high degree of order, even in the fluid state, because of the packing of the acyl chains. The interfacial zone where lipid and cholesterol interact is, by comparison, the region of lowest order. As permeation of an ion through a BLM is largely hindered by a steric barrier that can be associated with the acyl chain region,6 decreased order corresponds to a lower steric barrier, which would be expected to lead to increased ion permeation.The presence of the hydroxyl moieties of 10- or 12-HSA in this region would increase the steric disorder and increase electrostatic stabilization by dipole forces. Conclusions The effects of preferential spatial partitioning of HSA, and, therefore, the selective introduction of polar residues at defined zones and defined depths within a membrane, indicates how ion conduction through lipid membranes can be localized to conductive zones or artificial ‘ion channels’. This work has shown how the leakage current through a membrane1226 ANALYST, DECEMBER 1991, VOL. 116 can be fine-tuned. Further applications to sensor development would link the concept of the artificial ‘ion channel’ with a selective chemical reaction.The reaction would be used to activate ‘channel’ conductivity by processes such as induction of phase separation for ‘channel’ formation, or alteration of electrostatic/structural barriers to ion entry to ‘channel’ zones (analogous to gating in a natural ion channel), or production of permions at the ‘channel’ site. Work is currently being carried out to exploit the localized ion transport mechanism in order to prepare chemically induced artificial channel activity for development of a chemically activated switch. We are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support of this work, and for the provision of a fellowship to J.D. B. References Krull, U. J., and Thompson, M., TrAC, Trends Anal. Chem. (Pers. Ed.), 1985, 4, 90. Sugawara. M., Kojima, K., Sazawa, H., and Umezawa, Y., Anal. Chem., 1987, 59, 2842. McLaughlin, S., in Current Topics in Membranes and Transport, eds. Bonner, F., and Kleinzeller, A., Academic Press, New York, 1977, vol. 9, p. 71. Buck, R. P.. in CRC Critical Review in Analytical Chemistry, ed. Campbell, B., CRC Press, Cleveland, OH, 1976, vol. 5 , p. 323. Krull, U. J., Thompson, M., Vandenberg, E. T., and Wong, H. E., Anal. Chim. Acta, 1985, 174. 83. Krull, U. J., J. Electrochem. SOC., Electrochem. Sci. Technol., 1987, 134, 1910. Cadenhead, D. A., in Structure and Properties of Cell Mem- branes, ed. Benga, G., CRC Press, Boca Raton, FL, 1985. vol. 111, ch. 2, p. 21. Campbell, I. M., Crozier, D. N., and Caton, R. B., Pediatrics, 1976,57, 480. Campbell, 1. M., Crozier, D. N., and Pawagi, A. B., Eur. J . Clin. Microbiol., 1986, 5 . 622. 10 11 12 13 14 15 16 17 38 19 20 21 22 23 24 25 Krull, U. J., Anal. Chim. Acta, 1987, 197, 203. Krull, U. J . , Brennan, J. D., Brown, R. S . , Hosein, S., Hougham. B. D., and Vandenberg, E. T., Analyst, 1990, 115, 147. Tancrede, P., Paquin, P., Houle, A., and Leblanc, R. M.. J. Biochem. Biophys. Methods, 1983, 7,299. Montal, M., and Mueller, P., Proc. Natl. Acad. Sci. USA, 1972. 69, 3561. Montal, M., in Methods in Enzymology, eds. Colovick, S. P., and Kaplan, N. O., Academic Press, New York, 1974, vol. 32, Benz. R., Frohlich, O., Lauger, P., and Montal, M., Biochim. Biophys. Acta, 1975,394, 323. White, S. H., in Ion Channel Reconstruction, ed. Miller. C.. Plenum Press. New York, 1986, p. 3. Labarca, P., Coronado, R., and Miller. C., J. Gen. Physiol.. 1980, 76,397. Vodyanoy, V., and Murphy, R. B., Biochim. Biophys. Acta, 1982, 687, 189. Ohki, S., Physiol. Chem. Phys., 1981, 13, 195. Clementz, T., Christiansson, A., and Wieslander, A., in Advances in Membrane Fluidity, eds. Aloia, R. C., Curtain, C. C., and Gordon, L. M., Alan R. Liss, New York, 1988, vol. 3, ch. 2. p. 41. Blank, M., in Progress in Surface and Membrane Science, eds. Cadenhead, D. A.. and Danielli, J . F., Academic Press, New York, 1979, vol. 13, p. 125. Robertson, B.. Rev. Clin. Chem., 1983, 3, 97. Demel, R. A., Paltauf, F., and Hauser, H.. Biochemistry, 1987, 26,8659. Weiss, R. M., and McConnell, H. M . , J. Phys. Chern., 1985,89, 4453. Matuo, H., Mitsui, T., Motomura. K., and Matuura, R., Chem. Phys. Lipids, 1981. 29, 55. p. 545. Paper 1102281 D Received May 15th, 1991 Accepted June 6th, 1991

ISSN:0003-2654    

DOI:10.1039/AN9911601221

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, DECEMBER 1991, VOL. 116 1227 Universal Chemiluminescence Detection Using the Luminol Reaction and the Displacement Ion Effect* Bolei Yan and Paul J. Worsfoldt Department of Environmental Sciences, Polytechnic South West, Drake Circus, Plymouth, Devon PL48AA, UK Kevin Robards School of Science and Technology, Charles Sturt University-Riverina, P.O. Box 588, Wagga Wagga 2650, Australia A universal detection scheme for ions in solution, based on the stoichiometric displacement of copper(l1) ions from a strongly acidic ion-exchange column, is described. The eluted copper(l1) ions are detected by their catalytic effect on the chemiluminescent luminol reaction. In conjunction with flow injection, this method of detection is simple, rapid and reproducible, and pg I-' detection limits can be achieved for mono-, di- and trivalent cations, counter anions and weak acids.Keywords: Chemiluminescence; flow injection; displacement ion effect; luminol; ion exchange Solid-phase ion exchangers are widely used in flow injection (F1)'-9 for the preconcentration of analytes, matrix removal and indirect detection, based on the displacement of, or conversion into, chromogenic or luminescent species. Indirect detection methods include the determination of total anions or cations in aqueous solution by the displacement of hydroxide ions from a strong anion-exchange resin and detection by a pH e1ectrode;l.s determination of anions by displacement of thiocyanate from an anion-exchange resin by sample anions, followed by reaction with Methylene Blue6 or i r o n ( ~ i i ) , ~ determination of sulphide by precipitation with cadmium ions and detection by atomic absorption spectrometry,8 and determination of sample anions by displacement of an anionic luminol derivative with subsequent chemiluminescence (CL) detection .g The temperature variability and limited detection capability of conductimetry have encouraged the use of other detection methods in ion chromatography.1(&12 One possible alternative is the use of replacement ion chromatography (RIC), first reported by Downey and Hieftje12 in 1983. Following the analytical and suppressor columns, a third ion-exchange column containing replacement ions is used, and separated analyte ions or their counter ions are quantitatively exchanged for the replacement ions, which are then monitored down- stream with a suitable detector.Applications reported to date include the replacement of lithium ions from a cation- exchange column followed by flame photometric detec- tion,'2.13 replacement of nitrate and iodate ions from an anion-exchange column with ultraviolet detection,l4 and replacement of potassium ions's from a cation-exchange column and of bromide ions16 from an anion-exchange column followed by potentiometric detection. Chemiluminescence, the phenomenon of light emission from chemical reactions, has been exploited in a number of analytical applications, particularly in conjunction with FI and liquid chromatography. 17-20 The absence of a source elimi- nates light scatter and source noise, making CL detection inherently more sensitive than other spectrophotometric methods.The hydrogen peroxide induced oxidation of lumi- nol, one of the most well-known CL reactions, has, for example, been used for the determination of metal ions, such as those of cobalt(ii), copper(ii) and chromium(iii), by their catalytic effect on thc reaction." * Presented at the Royal Society of Chemistry 150th Anniversary t To whom correspondence should be addressed. Congress, Imperial College, London, 8th-1 l t h April. 1991. This paper describes an ion-displacement scheme for FI with luminol CL detection. Copper(i1) ions are used as the displacement source from an ion-exchange resin and are subsequently detected by their catalytic effect on the luminol reaction, Experimental Reagents High-quality de-ionized water from a Milli-Q system (Milli- pore, Milford, MA, USA) was used throughout, and all chemicals were of AnalaR grade (BDH, Poole, Dorset, UK) unless otherwise indicated. Working solutions of luminol and hydrogen peroxide in 0.1 mol dm-3 carbonate buffer were prepared daily from stock solutions of 1 x 10-3 rnol dm-3 luminol (Sigma, St.Louis, MO, USA) and 0.1 rnol dm-3 hydrogen peroxide (100 vol., approximately 9 rnol dm-3) (BDH), and the pH of the working solutions was adjusted to 11.7 with 0.2 rnol dm-3 sodium hydroxide. Working solutions of various cations and strong and weak acids were prepared daily by serial dilution of 1000 pg ml-1 stock solutions. Displacement Ion Column The displacement ion column (3.5 cm glass tube; 2 mm i.d.) was slurry packed with approximately 0.1 ml of Dowex 50W (H+ form, 4% cross-link, dry mesh 100-200, wet exchange capacity 1.7 mequiv ml-1) (Sigma), a strong cation-exchange resin, which was thoroughly washed with de-ionized water before use.A small plug of glass wool was inserted at each end of the column, and silicone rubber tubing (0.8 mm i.d.) was used to connect the column to the FI tubing. An aqueous solution (1 rnol dm-3) of either cobalt(ii) chloride or copper(i1) chloride was pumped through the column for 30 min at 1.5 ml min-1 to convert the resin into the corresponding salt form. The displacement column was then washed with de-ionized water for 30 min to obtain a stable baseline and flushed with de-ionized water before use. FI Manifold Optimum conditions are given throughout.A schematic diagram of the flow system used for ion displacement with CL detection is shown in Fig. 1. The carrier stream of de-ionized water was pumped at 1.5 ml min-' via a peristaltic pump (Ismatec Mini S820, Zurich, Switzerland). Standards (50 PI) were introduced into the stream by a rotary poly-1228 ANALYST, DECEMBER 1991, VOL. 116 50 pl ml min-I I - R1 1.5 R2 1.5 - Waste Fig. 1 FI manifold for copper(n) ion displacement with CL detection via the luminol reaction: C = deionized water; R1 = 5 x 10-5 rnol dm-3 luminol at pH = 11.7; and R2 = 5 X 10-3 rnol dm-3 hydrogen peroxide at pH 11.7 (tetrafluoroethylene) (PTFE) vaive (Rheodyne 5020, Cotati, CA, USA). The injected sample zone was passed through the displacement ion column (omitted for direct measurement) in which sample cations were stoichiometrically displaced by cobalt(rr) or copper(ir), and merged 15 cm downstream with the reagent stream.The luminol and hydrogen peroxide streams were pumped at 1.5 ml min-1 via a second peristaltic pump (Gilson Minipuls 2, Worthington, OH, USA). The three merged streams were passed directly into a coiled glass flow cell (1.1 mm i.d., 125 pl volume). Poly(tetra- fluoroethylene) tubing (0.8 mm i.d.) was used throughout. The flow cell was sandwiched between a silvered mirror and the end window of a photomultiplier tube (Thorn EM1 9789 QB, Ruislip, Middlesex, UK) and the complete assembly was contained in a light-tight , radiofrequency-shielded housing. 19 The output from the photomultiplier tube was fed to a strip-chart recorder (Kipp & Zonen BD 8, Bohemia, NY, USA), and the peak heights were measured manually.Results and Discussion Direct CL Detection of Cobalt(I1) and Copper(1r) Reagent concentrations and flow rates for maximum CL emission from the luminol reaction, with cobalt(i1) or cop- per(ii) as the catalyst, and with an FI manifold, were simplex optimized. Optimum reagent concentrations of 5 x 10-5 rnol dm-3 luminol and 5 x 10-3 rnol dm-3 hydrogen peroxide in 0.1 rnol dm-3 carbonate buffer at pH 11.7, and optimum flow rates of 1.5 ml min-1 per channel, were used in all subsequent experiments.20 The manifold was as shown in Fig. 1, except that the displacement ion column was removed. A typical CL emission intensity versus cobalt(ii) concentra- tion profile is shown as a log-log plot in Fig.2, with a linear range from 1 x 10-10 to 1 x 10-7 rnol dm-3. A similar profile for copper(i1) is shown in Fig. 3, with a linear range from 1 x 10-8 to 5 X 10-5 rnol dm-3. The detection limits (defined as twice the peak-to-peak noise) for cobalt(I1) and copper(1i) were 1 x 10-11 and 1 x 10-9 rnol dm-3, respectively. The relative standard deviations (RSDs; n = 4) for a 1 X 10-8 rnol dm-3 cobalt(i1) standard and a 1 X 10-6 rnol dm-3 copper(i1) standard were 2.0 and 1.4%, respectively. Clearly, cobalt(ri), with a lower limit of detection, is the more effective catalyst. Ion Displacement With CL Detection In order to assess the effectiveness of a displacement ion column in an FI manifold with CL detection, a cation- exchange column was incorporated, as shown in Fig.1. The displacement ion effect is also known as RIC when used in conjunction with ion chromatography. Replacement ion chromatography involves a stoichiometric exchange of the eluted sample ion with an ion that can be detected more 4 5 . 3 E . c x In Q) -I 0 J .- .- E 2 0, 0 0 0 0 I I 1 I I I I -11 -10 -9 -8 -7 -6 -5 0 Log( [cobalt]/mol dm -3) Fig. 2 CL emission intensity versus cobalt(rr) concentration o o 0 0 - 0 0 - - 0 0 0 0 0 4.. I I I I I I .- -9 -8 -7 -6 -5 -4 -3 Log([mpper]/mol dm-3) Fig. 3 CL emission intensity versus copper(1r) concentration sensitively than the sample ion itself. Most cations can be determined by RIC, regardless of their relative affinities for exchange sites on the resin; even cations with a lower affinity than the replacement ion are stoichiometrically replaced.'* The same behaviour was observed in this work when cobalt(ii) and copper(I1) were used as the displacement ions, because of their catalytic effect on the oxidation of luminol by hydrogen peroxide.As stated above, under the optimum conditions, the detection limit was two orders of magnitude lower for cobalt(i1) than for copper(ii), and the sensitivity for cobalt(ii) (expressed as mV ng-1 of metal ion at a metal ion concentra- tion of 1 X rnol dm-3) was greater by a factor of 141. This increased sensitivity was also seen in the ion displacement work. However, the limiting factor, when using ion displace- ment, is the background signal produced by displacement ion bleed from the column resulting from ionic impurities and dissolved carbon dioxide (which produces hydrogen ions) in the water carrier stream.This is a major source of background noise and hence degrades precision and detection limits. The greater background signal and increased signal noise, when using cobalt(i1) as the displacement ion, favoured the use of copper(1i) as the displacement ion in this study. No deteriora- tion in column performance was observed after the passage of 150 ml of 1 x 10-4 rnol dm-3 magnesium chloride through the column (equivalent to 3000 injections). The effect of column length (2.0-6.5 cm; 2 mm i.d.) on the maximum CL emission intensity was investigated by injecting 5 x 10-6 rnol dm-3 magnesium chloride. The results presented in Table 1 show that a 3.5 cm column gave the largest response, owing to the complete exchange with copper(i1) ions and to minimum dispersion of the sample zone.The fact that there was no breakthrough of analyte ions when using a 3.5 cm column was shown by injecting 1 x 10-5 rnol dm-3 magnesium chloride and cobalt(I1) chloride sequen-ANALYST, DECEMBER 1991, VOL. 116 1229 tially into the manifold. Any breakthrough would have produced a larger response for cobalt(n) because of its catalytic effect on the luminol reaction, but the response for magnesium(r1) and cobalt(I1) was the same. Quantitative Analysis The response for a number of monovalent (sodium and potassium) , divalent (magnesium, calcium , cadmium and cobalt) and trivalent (iron and aluminium) metal ions, strong acids (hydrochloric and nitric) and weak acids (tartaric and succinic) was investigated by using the manifold shown in Fig.1. The effect of the counter ion (chloride, nitrate or sulphate) was also studied. Calibration data and detection limits (twice the peak-to-peak noise) are listed in Table 2. The background signal of 640 mV, due to bleed of copper(n) ion from the column, was used as the baseline. A de-ionized water blank gave a negligible response and RSDs ( n = 4) were less than 4.0% in all instances. The response was the same for all cations of the same charge and was independent of the nature of the counter ion. The higher the charge, the greater the displacement of copper(r1) and the larger the response. As with other RIC techniques, anions can also be quantitatively determined by displacement of the counter cation.The detection limit is governed by the peak-to-peak noise on the background signal, which is primarily due to cyclic variation in the flow rate through the detector caused by the peristaltic pumps. A practical and easy to measure limit of detection was defined as twice the peak-to-peak noise. For all of the analytes investigated this was in the range 1.2 X 10-7-3.5 X 10-7 mol dm-3. By comparison, conductivity detection (Waters 431, Milford, MA, USA), with use of a single-channel de-ionized water carrier stream pumped at 1.5 ml min-1, a 50 PI injection volume and a displacement ion column, gave a limit of detection for potassium chloride of 5 x 10-7 mol dm-3 (RSD = 10%; n = 4). More detailed calibration graphs for potassium, magnesium and iron(m) chlorides over the range 0-3 x 10-5 mol dm-3 are Table 1 Effect of column length on maximum CL emission intensity for 5 x 10-6 mol dm--3 magnesium chloride Column lengthkm CL intensity/mV 0 2.0 3.5 5.0 6.5 0 1585 1820 1740 1600 shown in Fig.4. The relative responses over the linear ranges [which are governed by total copper(n) concentration] are of the order of 1 : 2 : 3 for the mono-, di- and trivalent cations, respectively . The signals obtained from partially dissociated weak acids (tartaric and succinic) are similar to those for monovalent cations and higher than would be expected on the basis of their acid dissociation constants. This is because displacement of copper(r1) ions by hydrogen ions within the column shifts the equilibrium completely towards the dissociated form of the acid.13 The low detection limits achieved for weak acids by ion displacement with CL detection suggest that this procedure could be used in conjunction with ion chromatography for the selective detection of such species, provided that the ionic strength of the eluent is sufficiently low.Weak acids can be separated by ion-exclusion chromatography with use of only de-ionized water as the mobile phase, which has a negligible effect on copper(I1) ion bleed from the displacement column. Detection limits of the order of 1 X 10-6 mol dm-3 have been reported for formic, acetic, propionic, butyric and valeric acids by using this approach.20 Current work involves an investigation of the tolerance of the detection system to eluents of higher ionic strength.Conclusions The results presented here demonstrate the feasibility of using copper(i1) ion displacement from a strong cation-exchange resin in conjunction with CL detection via the luminol reaction for the rapid, sensitive and universal detection of mono-, di- 0 5 10 15 20 25 30 Concentration/pmol dm-3 Calibration graphs for A, K+; B, Mg2+; and C, Fe3+ using Fig. 4 copper(i1) ion displacement with CL detection Table 2 Response data for a range of compounds, using FI with a displacement ion column [copper(ii) form] and CL detection via the luminol reaction Analyte concentratiodmol dm-3 5 x 10-7 1 x 10-6 5 x 10-6 1 x 10-jm CL emission intensity/mV Analyte NaCl NaN03 KCI HCI HN03 MgC12 MgS04 CdC12 COC12 AlC13 A W W 3 CaC& FeC13 Tartaric acid Succinic acid 140 150 165 115 130 275 250 250 260 250 305 285 325 115 110 245 275 260 225 245 320 340 320 340 345 500 520 530 270 250 850 880 890 825 875 1800 1775 1725 2025 1875 2825 2900 2950 1290 995 1875 1900 1925 2000 1950 3875 3825 3825 3900 3900 4 100 4100 4075 3600 2850 LOD* 10-7mol dm-3 3.2 3.0 3.2 3.5 3.0 2.0 3.0 2.5 1.2 1.5 1.2 1.5 1.2 3.0 3.5 pgI--' 7.4 6.9 12.5 0.4 0.3 4.9 7.3 10.0 13.5 8.8 3.2 4.0 6.7 45 41 * Limit of detection of cation (twice the peak-to-peak noise).1230 ANALYST, DECEMBER 1991, VOL.116 and trivalent cations, their counter anions and strong and weak acids. The detection technique is compatible with liquid chromatography, provided that a mobile phase of low ionic strength is used. One of us (B. Y,) thanks the Sino-British Scholarship Scheme for financial support.References Ramsing, A., RGiieka, J., and Hansen, E. H., Anal. Chim. Acta, 1980, 114, 165. Sakamoto-Arnold, C. M., and Johnson, K. S., Anal. Chem., 1987, 59, 1789. Fang, Z., Xu, S., and Zhang, S., Anal. Chim. Arra, 1984, 164, 41. Liu, Y., and Ingle, J. D., Jr., Anal. Chem., 1989, 61, 525. Olsen, S., Pessenda, L. C. R., RGiiEka, J., and Hansen, E. H., Analyst, 1983, 108, 905. Ducret, L., and Ratouis, M., Anal. Chim. Acta, 1959, 21, 91. Faizullah, A. T., and Townshend, A., Anal. Chim. Acta, 1986. 179, 233. Petersson, B. A., Fang, Z., RQiiCka, J., and Hansen, E. H., Anal. Chim. Acta, 1986, 184, 165. Copper, M. M., and Spurlin, S. R., Anal. Lett., 1986,19,2221. 10 11 12 13 14 15 16 17 18 19 20 21 Fritz, J. S., Gjerde, D. T., and Pohlandt, C., in Zon Chromaro- graphy, eds. Bertsch, W., Jennings, W. G., and Kaiser, R. E., Huthig, Heidelberg, 1982, pp. 185-199. Sherman, J. H., and Danielson, N. D., Anal. Chem., 1987,59, 490. Downey, S. W., and Hieftje, G. M., Anal. Chim. Acta, 1983, 153. 1. Galante, L. J., and Hieftje, G. M., Anal. Chem., 1987,59,2293. Galante. L. J., and Hieftje, G. M., Anal. Chem., 1988,60,995. Trojanowicz, M . , and Meyerhoff, M. E., Anal. Chim. Acta, 1989, 222, 95. Trojanowicz, M., Pobozy, E., and Meyerhoff, M. E., Anal. Chim. Acta, 1989, 222, 109. Abbott, R. W., Townshend, A., and Gill, R., Analyst, 1986, 111, 635. Kawasaki, T., Meada, M., and Tsuji, A., J. Chromatogr., 1985, 328, 121. Yan, B.. and Worsfold, P. J., Anal. Chim. Acta, 1990,236,287. Yan, B.. Lewis, S. W.. Worsfold. P. J.. Lancaster, J. S., and Gachanja, A., Anal. Chim. Acta, 1991, 250, 145. Fernandez-Gutierrez, A., and Munoz de la Pena, A.. in Molecular Luminescence Spectroscopy. Methods and Applica- tions: Part I , ed. Schulman, S. G., Wiley, New York, 1985, pp. 468-475. Paper 1102090K Received May 2nd, 1991 Accepted July 9th, 1991

ISSN:0003-2654    

DOI:10.1039/AN9911601227

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, DECEMBER 1991, VOL. 116 123 1 1941-1951 : The Golden Decade of Chromatography Leslie S. Ettre Department of Chemical Engineering, Yale University, P.O. Box 2159, New Haven, CT 06520, USA Introduction Today, chromatography is the most widely used laboratory technique: no chemical or biochemical laboratory can func- tion without it. We can find chromatographs everywhere in the world, and even in space, to analyse the products of industry, to check the purity of the food we eat, to help in the proper diagnosis of illnesses, to search for the variations in living organisms and to monitor the purity of our environ- ment. Chromatography was invented about ninety years ago by M. S. Tswett, a Russian scientist studying plant pigments. Initially, he had only a few followers; in fact, for some time his results were greeted with scepticism and mistrust. The method finally achieved recognition in the 1930s when, in a few years, it became an important tool in both organic chemistry and biochemistry laboratories. However, during this first renais- sance of chromatography, the technique still had many limitations and was, essentially, a manual technique requiring considerable skill, manipulation and time.The foundations for the next chapter in the history of chromatography were laid fifty years ago. This development essentially changed the principle on which separation was based from adsorption to partition and, by providing a theoretical treatment of the process, opened the way for further conscious improvements. In the subsequent decade two additional developments further enhanced the applicabil- ity of chromatography. Finally, at the end of the decade, it was extended into a new domain: while up to then separation took place in a liquid system, this was now changed to separation in a moving gas stream.This new development opened the way for the construction of more-or-less automated systems, instruments, which in turn provided laboratory-to-laboratory uniformity in carrying out the chromatographic separation. Because of the importance of these developments we may call the period between 1941 and 1951 The Golden Decade of Chromatography. These milestones in chromatography, the developments during the golden decade, are all associated with one person: Archer John Porter Martin. In 1941, together with R.L. M. Synge, he developed partition chromatography, as an exten- sion of which, in 1944, now together with R. Consden and A. H. Gordon (see page 1245 for a personal view from Dr. Gordon) , he developed paper chromatography. In 1950, together with G. A. Howard, he broadened the applicability of liquid-liquid partition chromatography by developing reversed-phase chromatography. Finally, in 1950-1952, now with A. T. James, he further extended partition chromato- graphy to systems in which the sample molecules are transported by a gas instead of a liquid. These pivotal developments changed, in many ways, how scientific investi- gations are carried out and represented such a fundamental achievement that it is not surprising that for its foundation, the invention of partition chromatography, in 1952 Martin and Synge received the Nobel Prize for Chemistry (Fig.1). On the occasion of the fiftieth anniversary of the beginning of this golden decade, the invention of partition chromato- graphy, let us briefly survey the contributions of Martin and co-workers during this period to the evolution of chromato- graphy * The Development of Partition Chromatography The story of the development of partition chromatography had been told many times (see, for example, refs. 1-3) hence it is outlined only briefly here. This work was carried out in the laboratories of the Wool Industries Research Association, in Leeds. As mentioned by R. L. M. Synge in a later recollec- tion,3b this laboratory was established and the research funded by the International Wool Secretariat, established by the wool growers of Australia, New Zealand and South Africa, mainly due to the foresight of a couple of scientific advisors who realized that fundamental studies on the nature of wool would ultimately help in improving the quality of the product.The aim of the work of Martin and Synge was to separate the monoamino monocarboxylic acids present in the wool. A continuous-flow, 40-plate, countercurrent extractor was con- structed for this purpose using chloroform and water as the two solvents; based on a detailed publication4 written towards the end of the operation life of this machine, it had to be a very complex system. It is a pity that no photograph of the whole apparatus was ever published! According to Martin,3a ‘it was a fiendish piece of apparatus’ and working up a mixture took one whole week.The obvious inadequacies of this system led Martin, one day, to the idea of selecting a different approach. In his own words: ‘I suddenly realized that it was not necessary to move both liquids; if I just moved one of them the required conditions were fulfilled’.3a They took silica gel, ground and sieved it, and then wetted it with water (the ‘stationary phase’). This material was packed into 30 cm X 10 mm i.d. columns. For separation, the amino acid solution was added by pipette to the top of the column and washed through it using chloroform containing a small amount (about 0.5%) of an alcohol as the mobile phase, The movement of the separated bands along the column was monitored by adding an indicator to the column packing (Fig.2). Their results were first presented at a meeting of the Biochemical Society on June 7th, 1941. Six papers were presented there and the brief report on the meetings gave the abstracts of four papers: for two including the one by Martin and Synge, only the title was printed. Synge, in his recollec- tions,3b indicates that during their lecture, they also ‘demon- strated . . . experiments in liquid-liquid chromatography’. Evidently, further refinements needed five more months until the final manuscript of their work was submitted on November 19th, 1941, fifty years ago, to Biochemical Journal.6 In addition to reporting on partition chromatography and demonstrating its application for the separation of amino acids, the paper by Martin and Synge presented the mathe- matical treatment of the theory of chromatography, introduc- ing the theoretical plate concept, and discussed the physical factors which affect the resolving power of a chromatographic column.In this, it indicated how further improvements would be possible, by using very small particles and a high-pressure difference along the column, advice which was followed only 25 years later. The paper also contains the famous prediction that partition chromatography could also be carried out by using a gas instead of a liquid as the mobile phase. This paper can certainly be considered as one of the key publications in the evolution of science. Thus, it is not surprising that in 1952, the authors were honoured by being awarded the Nobel Prize in Chemistry.Paper Chromatography The development of paper Chromatography was a direct continuation of the development of partition chromatography and its use to separate amino acids. Liquid-liquid chromato-1232 ANALYST, DECEMBER 1991, VOL. 116 Fig. 1 Sweden. (Courtesy of the Nobel Foundation, Stockholm, Sweden) A. J. P. Martin and R. L. M. Synge (in background) receiving the 1952 Nobel Prize in Chemistry, from King Gustav VI Adolphus ofANALYST, DECEMBER 1991, VOL. 116 1233 Fig. 2 Page of the notebook of A. J. P. Martin and R . L. M. Synge describing their experiment using silica gel with water packed in a column, and using chloroform as the mobile phase. [Courtesy of R. L. M. Synge. Originally published in ref. 3(a)] graphy in a column containing silica gel as the support for the stationary phase permitted only the separation of monoamino monocarboxylic acids: they could not make the system work for the dicarboxylic or basic amino acids.It was clear that another, less active support was needed. The use of paper was based on Martin's knowledge of the analysis of dyes on paper and also of the so called capillary analysis of Goppelsroeder first described in 18677 and further improved by a number of researchers, among them Liesegang.8.Y Meanwhile, Synge had left the Wool Research Laboratory in 1943 and now Martin had two new collaborators, R. Consden and A. H. Gordon. They impregnated a piece of filter-paper with water, spotted a drop of the amino acid solution at its edge and let the mobile phase (n-butanol) rise by capillary attraction and carry the amino acids at different speeds depending on their retardation by the stationary phase fixed on the paper.When the mobile phase reached the other edge of the paper it was dried and then sprayed with ninhydrin solution to reveal the spots corrsponding to the separated amino acids. They also realized that separation can be further improved by adding a second development, by turning the paper after the first separation through a right angle and using a different solvent as the mobile phase. Their first report10 described such two-dimen- sional paper chromatography (Fig. 3) and also discussed the theory of the technique, introducing the concept of the retardation factor, RF. The particular advantages of paper chromatography were the simplicity of the method, the fact that only very simple equipment was needed and that a number of samples could be analysed simultaneously.Thus, it is not surprising that, in a very short space of time, the technique became universally accepted. One can even say that until the advent of gas-liquid partition chromatography, it was paper chromatography which popularized partition chromatography.1234 ANALYST, DECEMBER 1991, VOL. 116 RF Values in collidine 0 0.2 0.4 0.6 0.8 1 .o I I I I 0 Cy (Decomp) As 0 0 Glu 0 La O C Y 0 % Gly 0 0 Th TY 1.0 ' Fig. 3 A two-dimensional paper chromatogram of amino acids, prepared with phenol-ammonia (0.3%) and collidine solvents. Al, alanine; Ar, arginine; As, aspartic acid; Cy, cystine; Glu, glutamic acid; Gly, glycine; H, histidine; HP, hydroxyproline; IL, isoleucine; La, lanthionine; L.leucine; Ly, lysine; M. methionine; NL. norleu- cine; NV, norvaline; Or, ornithine; @AI, phenylalanine; P, proline; Se, serine; Th, threonine; Tr, tryptophan; Ty, tyrosine; V, valine. [Courtesy of R. L. M. Synge. Originally published in Ref. 3(a)] 0.20 0.15 3 9 s 2 0 u 0 m F 0 0.10 .- - 5 E - E 0.05 n u 50 100 150 200 Eluate (ml) Fig. 4 The separation of a mixture of lauric, myristic, palmitic and stearic acids. Chromatogram marked X: mobile phase. 70 and 80% v/v aqueous methanol saturated with n-octane. Stationary phase: n-octane saturated with 70% v/v aqueous methanol. Mobile phase was changed from 70 to 80% at B. Chromatogram marked with Y: mobile phase: 55, 68 and 70% v/v aqueous acetone saturated with liquid paraffin.Stationary phase: liquid paraffin saturated with 55% v/v aqueous acetone. Mobile phase changed from 55 to 68% at A and to 70% at C. Ambient temperature: 35 "C. From ref. 11. (Courtesy of Biochem. J.) Reversed-phase Chromatography Even while developing paper chromatography, Martin con- tinued to be interested in liquid-liquid column chromato- graphy. The next major development in this field came in 1948-1950, as a result of his involvement in the analysis of long-chain fatty acids, at the Lister Institute of Preventive Medicine in London. It was soon clear that the original approach used in the work with Synge was not applicable hpranicp c i i r h camnlpc wniild f2vniir tnn miirh the lpcc nnlnr 4 NH3 I I I I I 0 10 20 30 40 Time (min) Fig.5 Chromatogram of ammonia and methylamines, obtained by gas-liquid partition chromatography. Column: 4 ft x 4 rnm i.d.; column packing: (CI1-OH + liquid paraffin 85 + 15) on Celite 545 ( 3 : 7 ) . Carrier gas: nitrogen, at 5 ml min-1. Column temperature: 78.6 "C. A, Experimental curve (titration); B, differential of experimental curve. From ref. 18. (Courtesy of Biochem. J.) mobile phase and thus there would be no partition between this and the more polar stationary phase. Therefore, it was Martin's idea to reverse the situation: have the less polar phase as the stationary phase; and the more polar as the mobile phase. A special problem arose with finding the proper support for the less polar phase; this was solved by employing kieselguhr treated with dichlorodimethylsilane vapour, which was unwettable for the strongly polar mobile phase.This new variant of liquid chromatography was named by Martin 'reversed-phase chromatography' (RPC) because it was considered as a reversal of the relative polarities of the two phases used until then. The detailed paper co-authored by G. A. Howard" also introduced another improvement in liquid column chromatography: changing the eluent strength during the chromatographic run (Fig. 4). This may be considered as the precursor of gradient-elution chromato- graphy described by Tiselius and coworkers two years later. 12 Today, RPC is the predominantly used variant of liquid column chromatography; thus, one may admire the foresight of the compilers of the 'Recommendations on Nomenclature for Chromatography, published by IUPAC in 1974,13.14 according to which RPC is 'a term of historical interest' only.. . Gas-Liquid Partition Chromatography As mentioned earlier, the 1941 paper of Martin and Synge predicted the possibility of using a gas as the mobile phase in partition chromatography. However, this prediction was not verified for almost ten years until, in 1950, Martin, now associated with the National Institute for Medical Research in London, initiated an investigation in order to determine the feasibility of this prediction. He was joined by A. T. James, a young scientist who had previously been associated with Synge. Their first project was unsuccessful and therefore Martin decided to go back to the old prediction to see whether gas-liquid partition chromatography (GLPC) was indeed feasible.A particular impetus inducing this decision was that they had a real problem to solve: a more refined separation of the lower molecular mass fatty acids was desired than could be obtained by paper chromatography. The development of the new technique was first hindered by dimerization of the short-chain fatty acids on the column. This problem was solved by the addition of a non-volatile acid (stearic acid) to the stationary phase.3c From this point on, the development proceeded rapidly and on October 20th, 1950, at a meeting of the Biochemical Society, they reported their preliminary results.15 At that time, manual titration of the column effluent was used for detection; an automated titrator was developed later by Martin and this was described in theirANALYST, DECEMBER 1991, VOL.116 final paper. At that time, they started to show their results to visitors: for example, we know about a visit on October 18th, 1950, by N. H. Ray of Imperial Chemical Industries Ltd,*6 with whom Martin and James discussed the possibility of using GLPC for the separation of hydrocarbons. Speaking about this visit Kay also mentioned3d that Martin suggested the use of a thermal conductivity detector because titration was not a feasible detection method for hydrocarbons. It took about eight more months until Martin and James were ready to publish a detailed description of the new technique: their manuscript was submitted on June 5th, 1951, to Biochemical Journal and published at the beginning of 1952.17 This paper further expanded the theory of partition chromatography by considering the compressibility of the gas used as the mobile phase.This fundamental paper was followed within a few months by two additional papers18.19 extending the application of the technique to the separation of basic compounds (Fig. 5 ) . The visit of Dr. Ray to the laboratory of Martin and James at the early stage of development indicated the interest of scientists in industrial laboratories in this new technique. This early interest, resulting from Martin’s contact with industrial laboratories, helped in the rapid acceptance of the new method. Another factor was that soon after the publication of their first reports, on September 4-9, 1952, one of the most important meetings took place in Oxford, England: the First International Congress on Analytical Chemistry at which senior representatives of the major chemical laboratories in Europe and America participated.There, Martin presented a major lecture describing GLPC, which was published in The Analyst.’” From then on, the technique spread rapidly, revolutionizing analytical chemistry. Conclusions The discussed developments in the decade between 1941 and 1951 changed the way in which chemical and biochemical investigations are carried out in laboratories. All these changes are due to the work of A. J . P. Martin, together with his co-workers R. L. M. Synge, R. Consden, A. H. Gordon, G. A. Howard and A. T. James. In 1969, at a Symposium sponsored by the Ciba Foundation, S.R. Lipsky of Yale University, himself a pioneer in both gas and modern liquid chromatography, finished his lecture21 with the following words. ‘In conclusion, I would like to pay tribute to the man whose genius was most responsible for it all, Professor A. J. P. Martin. He has twice made outstanding contributions to this field, in his discovery of partition chromatography and in his pioneering work on gas chromatography. He has thus 1235 altered for the better the lives of many of us. We, his scientific colleagues, thank him for allowing us to share with him this wonderful adventure.’ There is nothing one could add to these words. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 References Nobel Lectures: Chemistry 1942-1 962.Elsevier, Amsterdam, 1964: (a) Martin, A. J. P., pp. 359-371; (b) Synge, R. L. M., pp. 274-389. Martin, A. J. P., ‘Historical Background’, in Gas Chromato- graphy in Biology and Medicine. A Ciba Foundation Symposium (February 5-4, 1969), ed. Porter, R., published by J. & A. Churchill Ltd., London, 1969, pp. 2-10. 75 Years of Chromatography, A Historical Dialogue, ed. Ettre, L. S., and Zlatkis, A., Elsevier, Amsterdam. 1979: (a) Martin, A. J. P., pp. 285-296; (b) Synge, R. L. M., pp. 447-451; (c) James, A. T.. pp. 167-172; (d) Ray, N. H., pp. 345-350. Martin, A. J. P., and Synge, R. L. M., Biochem. J., 1941,35,91. ‘214th Meeting of the Biochemical Society, June 7th, 1941, National Institute for Medical Research, London’, Chem. Znd., 1941, 19, 487. Martin, A. J. P., and Synge, R. L. M., Biochem. J., 1941. 35, 1358. In 1910 Goppelsroeder published a collection of his most important publications between 1861 and 1909: Goppelsroeder, F., Capillaranalyse, beruhend auf Capillaritats- und Adsorp- tionserscheinigungen, Steinkopff, Dresden, 1910. Liesegang, R. E., and Schmidt, H . , Kolloidchemische Tech- nologie, Steinkopff, Berlin, 1927. Liesegang, R. E., Naturwiss., 1943,31, 348. Consden, R., Gordon, A. H., and Martin, A. J . P., Biochem. J., 1944, 38, 224. Howard, G. A., and Martin, A. J . P., Biochem. J., 1950, 46, 532. Alm, R. S., Williams, R. J. P., and Tiselius, A., Acta Chem. Scand., 1952, 6, 826. Pure Appl. Chem . , 1974. 37, 445. IUPAC, Analytical Chemistry Division, Compendium of Analytical Nomenclature, Pergamon Press, Oxford, 1978, pp. 74-87. ‘Abstracts of the 290th Meeting of the Biochemical Society, October 20th, 1950’, Biochem. J., 1951, 48 (l), vii. Ray, N. H., personal communication dated May 9th, 1978; see also ref. 3(d). Martin, A. J. P., and James. A. T., Biochem. J., 1952,50,679. James, A. T., Martin, A. J. P., and Smith, G. H., Biochem. J., 1952,52, 238. James, A. T., Biochem. J., 1952, 52, 242. James, A. T., and Martin, A. J. P., Analyst, 1952, 77, 915. Lipsky, S. R., ‘Gas Chromatography: A Scientific Revolution’, in Gas Chromatography in Biology and Medicine. A Ciba Foundation Symposium (February 5-6, 1969), ed. Porter, R., published by J . & A. Churchill Ltd., London, 1969, pp. 11-16.

ISSN:0003-2654    

DOI:10.1039/AN9911601231

出版商:RSC    

年代:1991

数据来源: RSC