摘要:

AbstractVarious approaches to the production of miniaturized chemical sensors for in vivo applications are reviewed with special regard to the measurement of chemical transients in the mammalian brain. The basic tenants of biosensor principles are discussed in terms of the temporal response of enzyme‐modified electrodes and selection of redox enzymes. The effects of electron transfer mediators and differential measurements to gain selectivity are also examined. A broad range of immobilization methods including membrane entrapment, physical and chemical adsorption, cross‐linking agents, polymeric entrainment, electropolymerized films and covalent derivatization are examined. The use of polymer layers and redox polymers to gain selectivity are discussed. The discussion is limited to microelectrode surfaces (<100 μm diameter) and various electrode types are examined, including bare and platinized carbon fibers, platinum microelectrodes, conducting organic salts and carbon paste electr

ISSN:1040-0397    

DOI:10.1002/elan.1140070502

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY

摘要:

AbstractAn isomerizable monolayer‐modified electrode is used to control interfacial electron transfer by means of external signals. A spiropyran (SP) monolayer undergoes thermal isomerization to the positively charged merocyanine monolayer, MRH+, while the latter isomer monolayer is photoisomerized, λ>475 nm, to the SP‐monolayer state. Pyrroloquinolino quinone (PQQ) is used as redox component for probing the interfacial electron transfer. The MRH+‐monolayer electrode attracts the negatively charged PQQ and effective electron transfer is attained. Upon photochemical isomerization to the SP‐monolayer electrode, the interfacial electron transfer is inhibited. Reversible activation and deactivation of the interfacial electron transfer is accomplished by cyclic isomerization of the monolayer through the MRH+and SP‐states, respectively, using thermal and photochemic

ISSN:1040-0397    

DOI:10.1002/elan.1140070503

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY

摘要:

AbstractDifferent crown ethers were electrochemically polymerized or adsorbed onto a platinum electrode for the analysis of catechols by either static potentiometric, potentiometric FIA or amperometric FIA. The static potentiometric responses of all the crown ether modified electrodes were linear over about three decades of catechol concentration (10−5−10−8M). The potentiometric FIA response of all the crown ether modified electrodes exhibited linear responses over four decades of catechol concentration (10−2−10−6M). The amperometric FIA responses were linear over a 10−2−10−5M range. Comparing the different techniques; static potentiometric, potentiometric FIA, and amperometric FIA, all had markedly different response slopes for the different crown ethers. Overall, the crown ether modified electrodes, except for an open‐crown ether, displayed excellent response stability for successive injections in both FIA modes. The lowest detection limits were about 0.5 × 10−6M for the potentiometric FIA with minimal interference from ascorbic acid, uric acid and acetaminophen. For amperometric FIA the detection limits were about 0.5 × 10−5M, but for ascorbic acid, uric acid, and acetaminophen the interference was significant. Experimental parameters which influence the mechanism of the potentiometr

ISSN:1040-0397    

DOI:10.1002/elan.1140070504

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY

摘要:

AbstractFacilitated ion transfer reactions of the ammonium, potassium, and sodium ions with the ionophores dibenzo‐18‐crown‐6, nonactin, and valinomycin were investigated at the water/ 1, 2‐dichloroethane interface. Association constants for the various reactions studied were calculated to determine the selectivity imparted by the ionophores on the transferring ions. Of the ionophores studied, nonactin was found to be the most selective towards the ammonium ion, with a calculated association constant

ISSN:1040-0397    

DOI:10.1002/elan.1140070505

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY

摘要:

AbstractThe electrochemical treatment of an electropolymerized (NiIIL)nthin film (where L represents 5,7,12,14‐tetramethyl‐dibenzo[b,i]‐1,4,8,11‐tetraaza [14]annulene) gives rise to a nickel‐based catalytic deposit on conducting substrates [glassy carbon (GC), platinum, or gold electrodes], which shows strong similarities to the nickel hydroxide electrode. The effect of potential cycling on surface composition, chemical status, and morphology of this nickel‐based chemically modified electrode (Ni‐CME) has been investigated by cyclic voltammetry (CV) [1], X‐ray photoelectron spectroscopy (XPS), and scanning electron microscopy. XPS analysis was very useful for characterizing the chemical changes in the nickel coordination caused by electrochemical treatments in alkaline solution. The featureless surface of the as‐prepared GC/(NiII)Ln, film compared with the fractal nature of the GC/Ni‐CME was revealed by electron micrographs. The Ni‐ CME exhibits high electrocatalytic activity towards the oxidation of carbohydrates in alkaline solution. The electrooxidation capability of the Ni‐CME is highlighted by the amperometric detection of sugars after anion‐exchange liquid chromatographic separations wi

ISSN:1040-0397    

DOI:10.1002/elan.1140070506

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY

摘要:

AbstractPyrolytic graphite (PG) electrodes are coated with films of dodecylamine (DDA), dimethyldioctadecylammonium bromide (DODAB) or composite layers based on a mixture of phosphatidylcholine (PC) and cholesterol (CH). Cyclic voltammetry is used to investigate the incorporation of two negatively charged species, Fe(CN) 63−and spinach ferredoxin, into the DDA or DODAB layers, as a function of time and composition of the cast films. No signal corresponding to the spinach ferredoxin redox reaction is detected at the PG electrode simply coated with a mixed (PC‐CH) layer. Well‐shaped responses denoting redox reaction of ferredoxin are observed when the (PC‐CH) layer is doped with DDA (or DODAB). The results support the electrostatic nature of the interactions between negatively charged species and the positive charges in the lipid layer.Such coated electrodes seem well‐suited for studying negatively charged species. They provide a good model for mimicking physiological redox protein‐membrane

ISSN:1040-0397    

DOI:10.1002/elan.1140070507

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY

摘要:

AbstractIn this work riboflavin was determined by square wave adsorptive stripping voltammetry (SWASV): riboflavin was initially adsorbed on a mercury film electrode (MFE) at a potential of 0.0 V (vs. Ag/AgCl) and at pH 12; the subsequent reductive stripping step was carried out by applying a square wave potential‐time waveform on the electrode. Both a batch method (based on the rotating disk electrode configuration) and a flow approach [based on the wall‐jet flow‐through electrode and flow injection analysis (FIA)] were employed. Both instrumental and chemical parameters were investigated and optimized for analytical use of the technique. The limit of detection was 0.5 nmol L−1(for 10 min of preconcentration), the precision was 8% for the batch system (at the 10 nmol L−1level) and 6% for the flow system (at the 100 nmol L−1level); the linear range depended on the preconcentration time and the mass transfer conditions during the accumulation step. Both methods were automated and applied to the determination of riboflavin in commercial pharmaceutical products with recovery values

ISSN:1040-0397    

DOI:10.1002/elan.1140070508

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY

摘要:

AbstractThe electroreduction mechanism on mercury of the aldoses glucose and galactose, the ketoses fructose and tagatose, and the disaccharides lactose and lactulose was investigated in neutral and weakly alkaline solutions. Measurements at different temperatures ranging from 0 to 45°C indicate that the chemical reaction of the hemiacetal ring opening, which controls the polarographic limiting current, proceeds mainly in the adsorbed state below room temperature and in the nonadsorbed state above room temperature. Along the rising portion of the polarographic wave the electrode process is under the mixed control of the chemical reaction of ring opening and of a subsequent electrochemical step involving the uptake of the first transferring electron, with no protonation step in between

ISSN:1040-0397    

DOI:10.1002/elan.1140070509

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY

摘要:

AbstractFlow‐injection selectivity coefficients for a solid‐state iodide‐selective electrode were determined for three moderately interfering species—bromide, thiocyanate and thiosulfate–by injecting mixed iodide–interferent solutions into selected reagent streams. The use of neutral and acidified iodate and bromate oxidant streams was found to increase the selectivity of the iodide electrode towards the three interferents. The acidified streams also visibly modified the surface of the electrode membrane, although the resultant coating, ascribed to the formation of elemental sulfur and iodine, was easily removed by manual polishing with slurried alumina and a lint‐free cloth. X‐ray photoelectron spectroscopy (XPS) performed on pressed AgI/Ag2S pellets following immersion in the acidified iodate and bromate solutions revealed substantial surface modification, with the formation of AgIO3and AgBr, respectively. Use of ascorbic acid or metabisulfite reducing streams resulted in higher selectivity towards iodide than that obtained using either of the neutral oxidant streams, but less than that obtained in the presence of a simple nitrate reagent stream. In contrast, the use of a thiosulfate stream irreparably damaged the surface of the electrode to the point where subsequent flow‐injection experiments showed excessive peak broadening and very poor baseline recovery. XPS studies performed on the damaged membrane revealed a surface dominated by hydroxides and almost totally devoid of e

ISSN:1040-0397    

DOI:10.1002/elan.1140070510

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY

摘要:

AbstractThe electrochemical reduction of O2was studied at Ni‐Cu alloy electrodes in aqueous alkaline solution (0.1–6.0 M KOH) and in unbuffered neutral solution (0.1–2.0 M KCl) using the rotating disk electrode technique. Voltammetric measurements showed that the overall O2reduction process at the electrodes containing 70–90% Cu is transport‐limited at cathodic potentials, and that they can be used as indicator electrodes for galvanic‐type oxygen sensors. An industrially produced Ni‐Cu alloy “melhior”, which contains ∼ 30% Ni, was also studied, and this alloy proved to be an excellent O2reduction catalyst. Melhior electrodes were utilized as indicator electrodes for oxygen sensors, and showed high output current stability and a long lifetime (

ISSN:1040-0397    

DOI:10.1002/elan.1140070511

出版商:VCH Verlagsgesellschaft mbH    

年代:1995

数据来源: WILEY