摘要:

AbstractThe double‐layer capacitance of self‐assembled unsubstituted and ω‐substituted n‐alkanethiol layers on gold electrodes was studied using a potential step technique. A four‐element equivalent circuit model was used for the evaluation of the current response. The double‐layer capacitance and the conductivity were both found to decrease with increasing chain length for alkanethiol homologues. Substitution with COOH, OH, and NH2in the ω‐position caused the double‐layer capacitance to rise dramatically, whereas the general pattern of decreasing capacitance with increasing chain length was confirmed for a series of homologue ω‐mercaptoalcohols. The effect of polar substituents is explained by the introduction of permanent dipoles into the self‐assembled layers. The capacitance was independent of the applied potential for dodecanethiol but increased with voltage for butanethiol when tested in the range of −200 to 500 mV versus Ag/AgCl. Buffer strength had little influence on the double‐layer capacitance above 5 mM (phosphate buffer, pH 7.0). The self‐assembly of mercaptoethanol was completed within 3 minutes from an aqueous solution and that of alkanethiols within 10 minutes using acetone solutions. The electrodes were

ISSN:1040-0397    

DOI:10.1002/elan.1140041002

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY

摘要:

AbstractDenatured calf thymus deoxyribonucleic acid (DNA) and synthetic poly(dG)poly(dC) have been covalently immobilized onto oxidized glassy carbon electrode surfaces using the water‐soluble carbodiimide, 1‐(3‐dimethylaminopropyl)‐3‐ethylcarbodiimide. Immobilized DNA has been detected using the complex Co(bpy) 33+(bpy = 2,2′ ‐bipyridine). This complex has a reversible one‐electron oxidation (Eo′ = 0.33 V versus SHE), binds electrostatically in the minor groove of the DNA double helix, and preconcentrates at the surface of the DNA‐modified electrode prior to a voltammetric scan. Our cyclic voltammetric data indicate that larger Co(bpy) 33+/2+signals are observed at low ionic strength; this is consistent with the larger association constants obtained for the binding of Co(bpy) 33+with calf thymus DNA at low ionic strength. While immobilizations were attempted for four species, poly(dA)poly(dT), native calf thymus DNA, denatured calf thymus DNA, and poly(dG)poly(dC), only the last two species could be successfully immobilized with the carbodiimide procedure; these results show that guanosine and/or cytosine bases will bind covalently to the o‐acylisourea o

ISSN:1040-0397    

DOI:10.1002/elan.1140041003

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY

摘要:

AbstractA rotating ring‐disk electrode, (RRDE) of which the platinum disk was coated with a polypyrrole‐glucose oxidase (PPy‐GOD) film by electrochemical polymerization and the ring was left bare, has been used to characterize the PPy‐GOD electrode. Polypyrrole‐GOD films of various thicknesses grown in the presence of 100 and 500 U/ml of GOD in the deposition solution were used. The amperometric current generated at 0.7 V versus saturated calomel electrode (SCE) by the electrochemical oxidation of hydrogen peroxide, generated by the enzymatic reaction, has been evaluated at both the coated disk and uncoated ring at various rotation speeds. The collection efficiency of the RRDE was 22% and was used to evaluate the total current of the PPy‐GOD electrode. The total current of a polypyrrole electrode was defined as the sum of the disk current and the total ring current taken as the observed ring current divided by the collection efficiency of the RRDE. From this study, it clearly appears that the biotransformation of glucose takes place mainly at the polypyrrole film/solution interface independently of the rotation speed and that the catalysis is not limited by oxygen. The Eadie‐Hofstee type plots obtained for disk current with 10–225 mM of glucose show a strong positive deviation for glucose concentrations higher than 100 mM. On the other hand, linear plots are obtained when the total current of the enzyme electrode is considered. No definitive explanation can be given for these behaviors, except that at a higher glucose concentration, the flux of glucose is high enough to permit the diffusion of glucose deeper in the film, where it can be oxidized by the enzyme. Thus, the catalytic reaction is taking place in a thicker la

ISSN:1040-0397    

DOI:10.1002/elan.1140041004

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY

摘要:

AbstractA voltammetric study has been made of copper electrodes in the presence and absence of amino acids and proteins. The results are discussed in terms of oxide formation on the copper surface and the complexation of the biomolecules with the copper. It is shown that in the presence of strongly complexing ligands, the copper surface is effectively self‐cleaning, and reproducible potentiometric results can be obtained without any rigorous electrode precleaning procedures. In the presence of weakly complexing ligands, on the other hand, it is shown why it is necessary to reduce the electrode surface beforehand in order to carry out reproducible potentiometr

ISSN:1040-0397    

DOI:10.1002/elan.1140041005

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThree different techniques of immobilization of the enzyme butyrylcholinesterase (E.C. 3.1.1.8.) and their use in association with pH glass electrodes for the determination of certain insecticides (Carbofuran, Carbaryl, and Paraoxon) by inhibition of the enzyme, have been studied and the results obtained are compared. The immobilizations are done by cross‐linking a mixture of the enzyme and human serum albumin with glutaraldehyde onto nylon nets, by fixing the enzyme onto commercially available polyamide pre‐activated PALL™ membranes and by physically retaining the enzyme in a synthetic prepolymer of acrylamidemethacrylamide hydrazides which is cross‐linked with glyoxal. There are no significant differences with regard to the sensitivities or the detection limits among the sensors using the different membranes. The detection limits of the insecticides tested varied from 0.2 to 10 ppm. In terms of signal reproducibility, stability, and cost, the nylon net membranes appear most promising. Membranes prepared in lots show better sensor signal reproducibility compared to those prepared individually. For the development of “disposable” biosensors, both nylon net and PALL™ membranes are very apt, since these membranes are easily mass produced at low cost and maintain their original enzyme activities even after 3 years wh

ISSN:1040-0397    

DOI:10.1002/elan.1140041006

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY

摘要:

AbstractSome compounds with the general formula(I) have an antiparasitic effect on animals and are used against heart disease induced by trypanosomes. A rapid, precise and selective analysis of species (I) and RNHC6H4AsO3H2(II), RNHC6H4As  O (III), and R'SH (IV), appearing as intermediates, is necessary in order to regulate the synthesis processes if (I) and (II), to identify and measure the products before using them in the reactor, and to control their purity before scaling up. Tast polarography and differential pulse polarography convert quickly the concentration of these compounds into current intensity suitable for control of the process. The first method makes the assay in oxidation of (IV) possible, but the resolution is insufficient to obtain the species (III) and (I). The assay, in reduction of (II) is possible, but with interferences of (III). The second method, on the other hand, using oxidation of (I) in a basic medium provides sufficient resolution to analyze (I) and (III) with a precision on the order

ISSN:1040-0397    

DOI:10.1002/elan.1140041007

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY

摘要:

AbstractA procedure for the logarithmic analysis of polarographic waves with nonparallel initial and limiting currents is developed and its application illustrated with two experimental systems: phosphine‐metriphonate reduction and methylene blue (MB) reduction and reoxidation. The procedure is based on the presumed identity of the wave function and the change of a double‐layer capacitance along the w

ISSN:1040-0397    

DOI:10.1002/elan.1140041008

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThe AgI‐coated ion‐selective electrode surface is more stable than either the AgCl or AgBr surfaces if exposed to solutions containing ions such as Cl−, Br−, I−, Ag+, and NH3for a week. A mixed AgClsI1−or AgBrsI1−sphase is formed upon a week's treatment of the AgI surface with either Cl−or Br−, withSmuch smaller than 1. After a week's treatment with high concentrations of the ions, no new surface layer consisting of only AgCl or AgBr is formed, as was observed in the case of AgCl and AgBr electrodes. Although AgI is much less soluble than either AgCl or AgBr, the absorption of chloride and bromide ions into the AgI surface nevertheless in

ISSN:1040-0397    

DOI:10.1002/elan.1140041009

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY

摘要:

AbstractA highly sensitive, catalytic adsorptive stripping voltammetric (AV) method for the determination of ultratrace cobalt concentrations in the supporting electrolyte consisting of ammonia buffer, dimethylglyoxime (DMG), and NaNO2has been evaluated and compared with the well‐known AV method with DMG and other dioximes. Conditions for maximal sensitivity and the simultaneous determination of Co and Ni are described. The sensitivity of the cobalt‐DMG complex method is improved about 20‐fold by exploitation of the catalytic effect observed in the presence of nitrite. The detection limit of the method is 4 × 10−11M. As a result of the selective nature of the catalytic response for cobalt, the method is more sensitive and selective against Ni and Zn than the commonly used classical DMG method (without nitrite). The method, which uses inexpensive and widely available reagents (DMG, NaNO2), is suitable for the determination of Co in the 10−10M–10−9M concentration range even when the Ni concentration is 10 to 50 times higher, as is the case in many practical situations. The method is also advantageous when the background current from sources other than Ni makes the use of the less sensitive method with DMG impossible. However, the selectivity of the method over the Ni and Zn responses is inferior than obtained with previously published but more expensive catalytic methods based on nioxime‐NaNO2or α‐benzil d

ISSN:1040-0397    

DOI:10.1002/elan.1140041010

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY

摘要:

AbstractTrace levels of niobium can be determined by adsorptive stripping voltammetry, with controlled interfacial accumulation of the niobium/cupferron complex onto the hanging mercury drop electrode. Optimal conditions (5 times; 10−6M cupferron in acetate buffer (pH 4.0), a preconcentration potential of −0.2 V, and differential pulse stripping) allow convenient quantitation of μg/L (ppb) concentrations. The detection limit is 83 ppt (10 minute preconcentration); the response is linear over the 0–20 ppb range: and the relative standard deviation (at the 5 ppb level) is 2.0%. Most cations do not interfere in the determination of niobium. Complexation with cupferron also allows analogous trace measurements of va

ISSN:1040-0397    

DOI:10.1002/elan.1140041011

出版商:VCH Verlagsgesellschaft mbH    

年代:1992

数据来源: WILEY