Analytical applications, chemically modified electrode sensors

In this chapter, we discuss voltammetric methods and associated electrochemical sensors, including chemically modified electrodes. Voltammetric techniques use a microelectrode for microelectrolysis. Here, the potential is scanned and a dilute solution of the analyte produces, at a given potential, a limiting current (microampere range or less), which is proportional to the analyte concentration. Am-perometry is the application of voltammetry at a fixed potential to follow, via the current, changes in concentration of a given species, for example, during a titration. Amperometric measurements also form the bases of electrochemical sensors. 

Since the pioneering work of Updike and Hicks in developing enzyme electrodes and Lane and Hubbard in direct chemical modification of electrode surfaces, a great deal of attention has been paid to developing chemically modified electrodes. The result is that virtually every substance that is electroactive, or for which its chemical reaction can be coupled to the electrode-modifying matrix and/or electron transfer mediator, can now be detected electrocheniically. Principles, techniques, and scope of application of CMEs as sensors in analytical... 

While the variety of NPs used in catalytic and sensor applications is extensive, this chapter will primarily focus on metallic and semiconductor NPs. The term functional nanoparticle will refer to a nanoparticle that interacts with a complementary molecule and facilitate an electrochemical process, integrating supramolecular and redox function. The chapter will first concentrate on the role of exo-active surfaces and core-based materials within sensor applications. Exo-active surfaces will be evaluated based upon their types of molecular receptors, ability to incorporate multiple chemical functionalities, selectivity toward distinct analytes, versatility as nanoscale receptors, and ability to modify electrodes via nanocomposite assemblies. Core-based materials will focus on electrochemical labeling and tagging methods for biosensor applications, as well as biological processes that generate an electrochemical response at their core. Finally, this chapter will shift its focus toward the catalytic nature of NPs, discussing electrochemical reactions and enhancement in electron transfer. 

Clay modified electrodes are used in fundamental studies of electron transfer, and in the construction of sensor devices. The ionic aqueous environment in the interlayer region is highly amenable to electrochemical processes. Furthermore, this enviromnent is controllable and fairly innocuous, allowing many different species to retain their activity. Molecular recognition, chemical catalysis, electrocatalysis, and preconcentration of analyte molecules are all applications of this class of modified electrodes. A condensed list of clay modified electrodes is given in Table 8.4. Reference (96), Table 6.1 on pages 280-281 contains a more comprehensive list. 

Conducting polymer-modified electrodes have potential usage in the fabrication of chemical detector for analytical applications [8]. As many of such research activities have focused extensively on the application of polypyrrole and its derivatives in the fabrication of biosensor especially glucose sensors [9], we have worked on the development of analytical probes using polythiophenes and polybithiophenes. An amperometric glucose sensors fabricated using enzyme/polybithiophene modified electrode has recently been reported [10]. 

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