Electroanalytical applications

One important electrochemical technology where diamond electrodes have made a significant impact is in the area of electroanalysis. CVD diamond offers advantages over other electrodes, especially sp carbon (e.g., glassy carbon), in terms of linear dynamic range, limit of detection, response time, response precision, and response stability. Some of the reported applications of diamond in electroanalysis are highlighted below. Unless stated otherwise, all the diamond electrodes mentioned below are boron-doped, microcrystalline thin films deposited on a conducting substrate (e.g.. Si). 

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Particularly attractive for numerous bioanalytical applications are colloidal metal (e.g., gold) and semiconductor quantum dot nanoparticles. The conductivity and catalytic properties of such systems have been employed for developing electrochemical gas sensors, electrochemical sensors based on molecular- or polymer-functionalized nanoparticle sensing interfaces, and for the construction of different biosensors including enzyme-based electrodes, immunosensors, and DNA sensors. Advances in the application of molecular and biomolecular functionalized metal, semiconductor, and magnetic particles for electroanalytical and bio-electroanalytical applications have been reviewed by Katz et al. [142]. 

It should be noted here that the ultra thin-layer cells (UTLC) which result from the close approach of an STM tip to a conducting substrate may have important electroanalytical applications in studies other than STM imaging (64). This is because extremely large current densities should be attainable in such cells, and also because of the fast transit times (e.g., 50 nsec for d - 10 nm) for reactants across the cell. Thus, such UTLC s might facilitate the determination of fast heterogeneous rate constants or the study of reactive electrochemical intermediates (64). 

The high surface areas of these electrodes make them ideal for electroanalytical applications where the high surface area is exploited to improve detection limits and/or detection range. Evans et al. [61] have demonstrated the production of platinum mesoporous electrodes and their application into the detection of H2O2. 

Given the tremendous development of SAMs over the past two decades it is dear that this chapter is able to cover only a fraction of the spectrum of topics related to the combination of SAMs and electrochemistry. For a comprehensive picture the reader is referred to a number of additional review articles, one of which is the excellent and extensive account of organized monolayers on electrodes by Finklea [23]. Besides this one, which comprehensively covers the literature up to the mid-1990s, other more focused reviews are available that address various developments over the past decade in areas of sensor development and electroanalytical applications [22, 24—28] and electrochemical metal deposition on SAM-modified electrodes [29, 30]. 

Electroanalytical application of hemispherical [35,36], cylindrical [37,38] and ring microelectrodes [39] has been described. A hemispherical iridium-based mercury ultramicroelectrode was formed by coulometric deposition at -0.2 V vs. SSCE in solution containing 8 x 10 M Hg(II) and 0.1M HCIO4 [35]. The radius of the iridium wire was 6.5 pm. The electrode was used for anodic stripping SWV determination of cadmium, lead and copper in unmodified drinking water, without any added electrolyte, deoxygenation, or forced convection. The effects of finite volume and sphericity of mercury drop elecPode in square-wave voltammetiy have been also studied [36]. 

In practical applications, where the maximum yield of a product or electricity in electrochemical energy conversion systems at the lowest energy cost is desirable, the rate of mass transport should be fast enough in order not to limit the overall rate of the process. For electroanalytical applications, such as polarography or gas sensors, on the other hand, the reaction must be limited by the transport of the reactant since the bulk concentration which is of interest is evaluated from the limiting con-vective-diffusional current. 

In the previous edition of this book, Dryhurst and McAllister described carbon electrodes in common use at the time, with particular emphasis on fabrication and potential limits [1]. There have been two extensive reviews since the previous edition, one emphasizing electrode kinetics at carbon [2] and one on more general physical and electrochemical properties [3]. In addition to greater popularity of carbon as an electrode, the major developments since 1984 have been an improved understanding of surface properties and structure, and extensive efforts on chemical modification. In the context of electroanalytical applications, the current chapter stresses the relationship between surface structure and reproducibility, plus the variety of carbon materials and pretreatments. Since the intent of the chapter is to guide the reader in using commonly available materials and procedures, many interesting but less common approaches from the literature are not addressed. A particularly active area that is not discussed is the wide variety of carbon electrodes with chemically modified surfaces. 

Section I identified the performance criteria that determine the suitability of a given electrode for an electroanalytical application. We now turn to the question of what aspects of the carbon determine its performance and electrochemical behavior. Since the structure of sp2 carbon materials is more complex than that of pure metals like Pt, there are more structural variables that affect behavior. As a consequence, sp2 carbon can vary widely in conductivity, stability, hardness, porosity, etc., and care must be taken to choose and prepare the carbon material for an electrochemical application. Before discussing particular carbon electrode materials, we first consider which structural variables affect the electrochemical observables discussed in Section II. 

PG and HOPG materials play an important role in understanding carbon electrode structure and reactivity due to their anisotropic nature [2]. In some special cases, basal plane HOPG and EPG electrodes are of particular value in electroanalytical applications for both fundamental and pragmatic reasons. Examples are shown in Figures 10.7 to 10.9. Figure 10.7 shows the voltammetry... 

The main features of electrolysis cells as used in electroanalytical applications are described in Chapter 9. For application in the laboratory and in industry, some other aspects are important. These aspects are discussed here. 

The oxidised form of indigo is only sparingly soluble in aqueous solution, which has serious consequences for electroanalytical purposes. Indeed, oxidising the reduced and water-soluble form of indigo to its insoluble oxidised form results in the formation of a deposited layer of the oxidation product. This causes blocking of the electrode surface and prevents it from further use in electroanalytical applications. However, this problem can be cir-... 

Finally, it has to be indicated that the actual trend for miniaturising analytical systems implies the use of ERDs or NEs in electroanalytical applications. With this aim, highly packed ERD or NE arrays have demonstrated to be suitable candidates from the analytical and economical criterion. 

Jacques Chevallier, Hydrogen Diffusion and Acceptor Passivation in Diamond Jurgen Ristein, Structural and Electronic Properties of Diamond Surfaces John C. Angus, Yuri V. Pleskov and Sally C. Eaton, Electrochemistry of Diamond Greg M. Swain, Electroanalytical Applications of Diamond Electrodes... 

It has been reported that ACS provides an effective way for dispersion of CNTs in aqueous solution in electroanalytical applications [75, 76]. The FESEM images of MWCNTs-ACS-coated ITO, and Ag-MWCNTs-ACS-coated ITO are shown in Fig. 6.5a, b, respectively. It can be seen in Fig. 6.5a that the MWCNTs are well dispersed in the MWCNTs-ACS nanocomposite. The formation of Ag in MWCNTs-ACS after electrochemical deposition can be seen in Fig. 6.5b. Ag particles were with diameters about 100 nm in the MWCNTs-ACS nanocomposite and exhibited some spherical nanostructures. The Ag nanoparticles are in electrical contact with the ITO substrates throughout the MWCNTs-ACS nanocomposite. The Ag particle size prepared by this method is suitable and provided a high number of good hot spots for SERS. 

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