Richard P. Baldwin

COMPARISON OF METALLIC ELECTRODES FOR CONSTANT-POTENTIAL AMPEROMETRIC DETECTION OF CARBOHYDRATES, AMINO ACIDS AND RELATED COMPOUNDS IN FLOW SYSTEMS

Abstract Constant-potential amperometric detection of carbohydrates, amino acids, and other aliphatic organic compounds is possible by means of their oxidation in alkaline solution at a variety of metal/metal oxide electrodes including Pt, Au, Cu, Ni, Ag and Co. The experimental conditions required for optimum detection and the analytical performance obtainable vary widely for different electrode materials and analytes. In this work, the cyclic voltammetric behavior exhibited by selected analytes (glucose, glycine, lactic acid, ethylamine and ethanol) at each of these electrodes was used to determine the optimum potentials suitable for flow detection so that the capabilities of the different metal electrodes could be evaluated and systematically compared. In general, the Cu electrode was found to provide superior detection capabilities in terms of its range of response, detection limits and especially stability. Despite the fact that Pt and Au are typically used only with a pulsed applied potential, both can provide long-lived constant-potential detection of carbohydrates and other analytes at low concentrations if the potentials ere carefully chosen and the electrodes are allowed to undergo an initial stabilization period.

Characterization of carbohydrate oxidation at copper electrodes

The electro-oxidation of carbohydrate compounds at copper electrodes in strongly alkaline solution was investigated by cyclic voltammetry, hydrodynamic voltammetry and coulometry experiments. Product analysis was carried out by 1H and 13C nuclear magnetic resonance spectroscopy. Model carbohydrate compounds examined included, in addition to simple monosaccharides, alditols and aldonic, aldaric and uronic acids. All these oxidations were found to consist of many-electron processes. For glucose itself, 12 electrons were involved in complete oxidation at +0.60 V vs. Ag|AgCl|3 M NaCl, and the sole product appeared to be the formate ion, with no partial oxidation species seen as either intermediate or final products. For other carbohydrates, carbonate was assumed (but not observed) to be a possible oxidation product as well. In view of the fact that various alditol compounds were oxidized even more readily than glucose and other monosaccharides, it appeared that the aldehyde or ketone functionality is not required for oxidation at the Cu electrode. Rather, voltammetry results obtained for a series of polyhydroxyl compounds revealed that the minimum structural requirement for facile oxidation is the presence of at least two, and preferably more, nearby hydroxyl groups in the compound to be oxidized.

Detection of pesticides using an amperometric biosensor based on ferophthalocyanine chemically modified carbon paste electrode and immobilized bienzymatic system

A new highly sensitive amperometric method for the detection of organophosphorus compounds has been developed. The method is based on a ferophthalocyanine chemically modified carbon paste electrode coupled with acetylcholinesterase and choline oxidase co-immobilized onto the surface of a dialysis membrane. The activity of cholinesterase is non-competitively inhibited in the presence of pesticides. The highest sensitivity to inhibitors was found for a membrane containing low enzyme loading and this was subsequently used for the construction of an amperometric biosensor for pesticides. Analyses were done using acetylcholine as substrate; choline produced by hydrolysis in the enzymatic layer was oxidized by choline-oxidase and subsequently H(2)O(2) produced was electrochemically detected at +0.35 V vs. Ag/AgCl. The decrease of substrate steady-state current caused by the addition of pesticide was used for evaluation. With this approach, up to 10(-10) M of paraoxon and carbofuran can be detected.

Chemically modified electrodes in liquid chromatography detection: A review

Recent trends in the use of chemically modified electrodes (CMEs) in electrochemical detection systems used in flow-injection analysis or high-performance liquid chromatography are reviewed, with the objective of indicating the most promising approaches for practical CME applications. Four specific application areas of CMEs are identified: (1) those using perm-selective coatings to enhance selectivity and inhibit surface fouling, (2) those using immobilized electron-transfer mediators to catalyse slow electrode reactions, (3) those using enzymes as modifiers to provide biological activity on the electrode surface and (4) those using ion-exchange coatings for the electrochemical detection of non-electroactive anions and cations. By virtue of these approaches, the judicious use of CMEs has already had a significant impact on the performance and scope of electrochemical detection in flow systems. The most important improvements have been in CME selectivity, which produces simpler assay procedures with less need for sample treatment, and increased usefulness for complex samples, and in CME reactivity, which expands the range of analytes amenable to electrochemical monitoring.

Determination of carbohydrates, sugar acids and alditols by capillary electrophoresis and electrochemical detection at a copper electrode

Abstract Capillary electrophoresis combined with electrochemical detection at copper electrodes has been shown to provide a simple and sensitive method for the direct analysis of samples containing a wide range of carbohydrate compounds including simple sugars, sugar acids and alditols. In this approach, both the separation and the detection required the use of a strongly alkaline medium whose hydroxide content could be varied to optimize conveniently the migration times and resolution obtained. Detection consisted of a direct oxidation that required no derivatization and yielded detection limits at or below the fmol level for most of the carbohydrate species.

Recent advances in electrochemical detection in capillary electrophoresis

Recent advances in the design and application of electrochemical (EC) detection systems in capillary electrophoresis (CE) are reviewed, with the objective of providing the nonelectrochemist with a state‐of‐the‐art picture of CEEC instrumentation and an overview of the principal analytes for which CEEC is best suited. The detection schemes considered here include those based both on amperometry and on potentiometry as both kinds of EC systems are being actively developed in CE and have the potential for broad application in analysis. Over the three‐year period covered by this review, an important direction that CEEC has taken is the construction of more complex electrode systems beginning with the use of multiple EC electrodes and culminating with the adaptation of EC detection to microfabricated „lab‐on‐a‐chip”︁ analysis devices. In addition, CEEC applications have now grown to include a broad variety of inorganic, organic, and biochemical analytes and samples.

Selective Oxidation of Thiols to Disulfides at Polymeric Cobalt Phthalocyanine Chemically Modified Electrodes

Cysteine, glutathione, and several other aliphatic and aromatic thiols were exhaustively oxidized at chemically modified reticulated vitreous carbon containing a polymer coating of cobalt phthalocyanine (CoPC). Coulometric measurements indicated that, for all thiols examined, the electro-oxidation was a one-electron process. Subsequent end product analysis by high performance liquid chromatography, polarimetry, and 13 C nuclear magnetic resonance spectrometry confirmed that, for cysteine and glutathione, the corresponding disulfide species were formed very cleanly with no evidence of further oxidation to the sulfinic or sulfonic acid. The polymeric CoPC electrodes were compatible with a wide range of solvent and pH conditions. Unlike most conventional electrodes, which are rapidly passivated upon attempted oxidation of organosulfur compounds, the CoPC electrodes were extraordinarily durable, exhibiting turnover numbers of greater than 10 4 for each Co 2+ center with little or no loss of activity

Integrated Capillary Electrophoresis/Electrochemical Detection with Metal Film Electrodes Directly Deposited onto the Capillary Tip

The practical application of electrochemical detection in capillary electrophoresis has been hampered by irreproducibility and inconvenience related to capillary/electrode alignment. In order to eliminate these problems, a simple, flexible method by which the capillary and the working electrode were integrated into a single operational unit was devised and evaluated. The electrodes were formed by sputtering a thin conductive layer of Au or Pt onto the exit tip of the capillary. Depending on the size of the capillary used (i.e., both inner and outer diameters), Au on-capillary electrodes (OCEs) gave detection limits at the micromolar level and slightly below for the test analytes dopamine and catechol. More important, operation of the OCEs required no alignment procedures beyond immersion in the CE buffer reservoir/detector cell. OCEs used in this manner exhibited relative standard deviations of 2-4% for repeated injections even if removed from solution between runs. Finally, the Au and Pt OCEs could themselves be modified further by conventional electrochemical procedures. Here, Cu OCEs, formed by electrodeposition onto Au, were used to detect carbohydrate compounds; and an enzyme OCE, formed by adsorption of glucose oxidase onto Pt, was used to detect glucose.

Electrochemical determination of carbohydrates: Enzyme electrodes and amperometric detection in liquid chromatography and capillary electrophoresis

In recent years, electrochemical detection (EC) methods have become increasingly important for the determination of carbohydrate compounds in a variety of biological and pharmaceutical samples. In this work, recent advances in the design and application of EC approaches are reviewed, with the goal of providing the non-electrochemist with a basic understanding of the most important EC approaches to carbohydrate detection and an overview of their current applications. Two specific EC detection strategies are considered in detail: enzyme electrodes and electrodes used for HPLC or capillary electrophoresis detection.

Electrochemistry and chromatographic detection of monosaccharides, disaccharides, and related compounds at an electrocatalytic chemically modified electrode

Abstract Many carbohydrate compounds undergo electro-oxidation at modest positive potentials at chemically modified electrodes (CMEs) made by adding cobalt phthalocyanine (CoPC) electrocatalyst to a conventional carbon paste mixture. The active carbohydrates which were oxidized at the CME at +0.4 to 0.5 V vs. Ag/AgCl under the very basic pH conditions required, included not only monosaccharides but also oligosaccharides, deoxy sugars, and even simple polyalcohols such as glycerol. When used for carbohydrate detection after liquid chromatography, the CoPC CME yielded detection limits in the 10–50 pmol range. Electrode stability and selectivity was such that no sample preparation other than dilution and particulate filtration was required for carbohydrate determinations in sample matrices including food products and physiological fluids.