Use in electroanalysis

S.2.2 Carbon Electrodes Solid electrodes based on carbon are currently in widespread use in electroanalysis, primarily because of their broad potential window, low background current, rich surface chemistry, low cost, chemical inertness, and suitability for various sensing and detection applications. In contrast, electron-transfer rates observed at carbon surfaces are often slower than those observed at metal electrodes. The electron-transfer reactivity is strongly affected by the origin... 

As a final note, the reader is reminded that the intent of this chapter is descriptive rather than comprehensive. Although glassy carbon and carbon paste are commonly used in electroanalysis, there are a variety of alternative carbon... 

Previously, Nafion was used in electroanalysis as electrode covering membrane to immobilize cationic species (7,8) and as a barrier to anionic species (9,10). In biosensor field, the main limitation in the use of Nafion for enzyme entrapment is related to the solubility of the Nafion in lower aliphatic alcohols which are not compatible with the enzyme activity (11). Until now, only glucose oxidase was successfully immobilized in Nafion. 

Liquid junctions are found in almost all electrochemical cells used in electroanalysis. In general, there is a potential drop across the liquid junction and it is important to be able to evaluate it. Because of the different electrolyte compositions and concentrations involved, the liquid junction is associated with an irreversible mass transfer process. In this section, methods of estimating the potential drop due to the liquid junction are outlined. 

A partially blocked electrode is a macroscale electrode that is partially covered in electrochemicaUy inert microscale particles which block the dif-fusional path of electroactive solution-phase species to the electrode surface [14]. The inverse situation is an inert surface modified with a distribution of (usually hemispherical or spherical) electroactive nanoparticles such systems are currently finding widespread use in electroanalysis [15]. 

Principal ICPs Derivatives Used in Electroanalysis 2.3.1 Polypyrrole... 

Fig. 2.8 Main thiophene (Th) derivatives used in electroanalysis 2,2 -bithiophene (BTh), 3-methylthiophene (MTh), and 3,4-ethylendioxythiophene (EDOT)...

Metallopolymers in which Ni(II) [141-143], Pd(II) [142, 144], and Au(III) [142, 144] are complexed by sulphur-containing ligands are reported in the literature, under the form of dithiols or dithioethers. Side Th groups are suitable to lead to the corresponding metallopolymers. These have been synthesized and characterized, but are not used in electroanalysis. On the other hand, they are often poorly conductive, and p-doping is only obtained at relatively high potentials. 

This chapter deals with a number of non-conducting polymers possessing net positive or negative charge. They impart the electrode a specific reactivity toward charged species in solution, exploitable in many electroanalytical contexts, as detailed in the following. Table 4.1 reports the most common anionic and cationic polymers used in electroanalysis they are, for the most part, commercially available. 

Novel nanosized materials have been proposed at an increasing rate over the past two decades, and have influenced many different scientific and technological fields as soon as the community revealed and developed their particular characteristics [1-5]. Their use in electroanalysis has become so widespread that electrode modifications based on these materials are often the preferred solution to the realization of effective amperometric sensors. 

Fig. 6.2 Carbon-based materials used in electroanalysis schemes for (a) single- and (b) multiwall carbon nanotubes, (c) graphene, (d) C o fullerene (e) transmission electron microscopy images of carbon black particles (Reproduced from Refs. [45,54,55], and [56] with the permission of Springer, American Chemical Society and InTech, respectively)...

The most important organic nanostructures of use in electroanalysis are based on intrinsically conductive polymers (ICPs). A large number of polymeric nanostructures, both supported on the electrode surface and dispersed in solution, have been prepared through the use of a template, taking advantage of the strategies described in Sect. 6.4. A few examples are reported in Table 6.4 and Fig. 6.10. 

No other material shows as much versatility as an electrode as does electrically conducting, CVD diamond. The material can be used in electroanalysis to provide low detection limits for analytes with superb precision and stability for high current density electrolysis (1-10 A/cm ) in aggressive solution environments without any morphological or micro-structural degradation and as an optically transparent electrode (OTE) for spectroelectrochemical measurements in the ultraviolet-visible (UV-Vis) and infrared (IR) regions of the electromagnetic spectrum. 

One of the first demonstrations of diamond s usefulness in electroanalysis was the oxidative detection of azide anion in aqueous media [29,30,37]. Sodium azide has been widely used commercially and, in the past, as an inflator for automotive airbags. Azide anion is highly toxic and presents a... 

Electrically conducting diamond is a new type of carlxm electrode material that is beginning to find widespread use in electroanalysis (86-88). The material possesses properties superior to other forms of carbon that include (i) low and stable background current over a wide potential range, (ii) wide working potential window in aqueous media, (iii) relatively rapid electron-transfer kinetics for several redox systems without conventional pretreatment, (iv) weak molecular adsorption, (v) dimensional stability and corrosion resistance, and (vi) optical transparency. The material is now available from several commercial sources and is not overly expensive, as commonly perceived (89). 

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