Chemically modified electrode surfaces, surface analysis

D. H. Karweik, C. W. Miller, M. D. Porter, T. Knwana, Prospects in the Analysis of Chemically Modified Electrodes , Surface Analysis, 1982, 89-119, American Chemical Society Conference Proceedings. 

This review gives a brief summary of the "types of chemically modified electrodes, their fabrication, and some examples of their uses. One especially promising area of application is that of selective chemical analysis. In general, the approach used is to attach to the electrode surface electrochemically reactive molecules which have electrocatalytic activity toward specific substrates or analytes. In addition, the incorporation of biochemical systems should greatly extend the usefulness of these devices for analytical purposes. 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

ADP AFP ab as ALAT AP ASAT ATP BQ BSA CEH CK CME COD con A CV d D E E EC ECME EDTA EIA /e FAD FET FIA G GOD G6P-DH HBg HCG adenosine diphosphate a-fetoprotein antibody antigen alanine aminotranferase alkaline phosphatase aspartate aminotransferase adenosine triphosphate benzoquinone bovine serum albumin cholesterol ester hydrolase creatine kinase chemically modified electrode cholesterol oxidase concanavalin A coefficient of variation (relative standard deviation) layer thickness diffusion coefficient enzyme potential Enzyme Classification enzyme-chemically modified electrode ethylene diamine tetraacetic acid enzyme immunoassay enzyme loading factor flavin adenine dinucleotide field effect transistor flow injection analysis amplification factor glucose oxidase glucose-6-phosphate dehydrogenase hepatitis B surface antigen human chorionic gonadotropin... 

To characterize the properties of molecules and polymer films attached to an electrode surface, a wide variety of methods have been used to measure the electroactivity, chemical reactivity, and surface structure of the electrode-immobilized materials [9]. These methods have been primarily electrochemical and spectral as indicated in Table I. Suffice it to say that a multidisciplinary approach is needed to adequately characterize chemically modified electrodes combining electrochemical methods with surface analysis techniques and a variety of other chemical and physical approaches. 

This paper will survey the current status of surface analysis in the examination of chemically modified electrode surfaces. In doing so, we shall take selected examples from our laboratory and the literature to illustrate some of the methods that have been employed to answer questions about surface topography, atomic and molecular speciation, and molecular orientation and bonding. 

Gorton and coworkers have been particularly active in this field and produced an excellent review of the methods and approaches used for the successful chemical modification of electrodes for NADH oxidation [33]. They concentrated mainly on the adsorption onto electrode surfaces of mediators which are known to oxidise NADH in solution. The resulting systems were based on phenazines [34], phenoxazines [35, 36] and pheno-thiazines [32]. To date, this approach has produced some of the most successful electrodes for NADH oxidation. However, attempts to use similar mediators attached to poly(siloxane) films at electrode surfaces have proved less successful. Kinetic analysis of the results indicates that this is because of the slow charge transfer between the redox centres within the film so that the catalytic oxidation of NADH is restricted to a thin layer nearest the electrode surface [37, 38]. This illustrates the importance of a charge transfer between mediator groups in polymer modified electrodes. 

Procedures suitable for the incorporation of chemical or biochemical substances or ion-exchange sites onto an electrode surface have been developed in various laboratories. " Such modified surfaces can then be used to perform a variety of functions. They have, for instance, been employed to preconcentrate analytes prior to voltammetric analysis and hence impove sensitivity. For example, dimethylglyoxime has been used to preconcentrate nickel and EDTA to preconcentrate silver." If this preconcentration can be achieved in an environment conducive to rapid electron transfer, then sensitivity will be enhanched even further. Alternatively electrocatalysts have been attached to electrode surfaces, and by speeding up what would otherwise be a sluggish electron transfer process, increased sensitivity has been attained. However, this approach yields no added selectivity and a large background must usually be tolerated. 

The adhesion power of metal electrode to perfluorosulphonic polymer is used to modify the surface of a single electrode for the analysis of electrochemical reactions of special interest. Nafion -coated electrodes have been developed by Rubinstein Bard" . Various electron-conducting materials (glassy carbon, gold, platinum) are used as support for the Teflon layer. With these coated electrodes, the mechanisms of mass and charge transfer in the perfluorosulphonic material have been investigated and also the catalytic and photochemical properties of polymer doped with various chemical species . ... 

The PVC is also applied in chemically modified forms with added hydroxyl, amine and carboxyl groups [24-27]. These types of polymers are often used in biosensors for clinical analysis, because they reduce the asymmetry potential of the membrane surface, due to deposition of proteins. Such polymers also improve the adhesion of the membrane to the electrode surface. 

Because of their stability against chemical dissolution and their effective promotion of oxidations of biochemical compounds, catalysts comprising inorganic polymers and other maCTomolecules that contain platinum-group metal centers will be the focus of the present report. The applications primarily will be as amperometric detectors in HPLC and in flow-injection analysis, in which case both surface-modified electrodes and doped CCEs are suitable. Specifically addressed will be the use of sol-gel processing as a means of immobilizing catalysts in composites. 

Arguably, carbon paste is the most useful material for laboratory preparation of various electrometric sensors. In the carbon paste electrode configurations, a selective agent (modifier) is commonly incorporated into the surface via the mixtures with graphite powder and a pasting liquid, when one obtains chemically modified carbon paste electrodes, CMCPEs. For many years, most of applications of CMCPEs in electroanalysis fall mainly amongst voltammetric determinations often, in combination with electrochemical stripping analysis (ESA). 

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