Redox Active Chemically Modified Electrodes

A way to circumvent the first problem is to ensure that all of the active material is present at the electrode surface. That is, employ a chemically modified electrode where a precursor to the active electrocatalyst is incorporated. The field of chemically modified electrodes Q) is approaching a more mature state and there are now numerous methodologies for the incorporation of materials that exhibit electrocatalytic activity. Furthermore, some of these synthetic procedures allow for the precise control of the coverage so that electrodes modified with a few monolayers of redox active material can be reproducibly prepared. Q)... 

Some of the materials highlighted in this review offer novel redox-active cavities, which are candidates for studies on chemistry within cavities, especially processes which involve molecular recognition by donor-acceptor ii-Jt interactions, or by electron transfer mechanisms, e.g. coordination of a lone pair to a metal center, or formation of radical cation/radical anion pairs by charge transfer. The attachment of redox-active dendrimers to electrode surfaces (by chemical bonding, physical deposition, or screen printing) to form modified electrodes should provide interesting novel electron relay systems. 

The interest in chemically modified electrodes that developed during the 1980s resulted in the synthesis of many redox-active polymers and surface-confined redox couples, including ferrocenes. These were subsequently adapted to electrochemical biosensors, and both surface-confined and polymeric ferrocenes have been widely used. Typically, polymeric ferrocenes that have been exploited in this way include poly(vinyl) ferrocenes, polysiloxanes, polyethylene oxide with covalently attached ferrocenes, poly(allylamine) ferrocene, and polacrylamide ferrocene cross-linked hydrogels." ... 

At chemically modified electrodes, NADH can be oxidized to enzymatically active NAD at much lower potentials than at bare electrode surfaces. Specified functionalities have been introduced to the electrode surface by a number of different immobilization techniques (see Section 14.4). It is presumed that a redox compound able to oxidize NADH in solution may also act as a suitable mediator when it is fixed to an electrode surface. In cyclic voltammetric experiments, the current of the oxidation wave of the mediator must be significantly increased in the presence of NADH, while the corresponding reduction current is decreased (Figure 14-9). 

Electrochemistry on Thiol-based SAMs Modified at Electrodes Electrode reactions consist of the combination of elementary processes, that is, electron-transfer reaction processes between redox-active species and electrodes coupled to the preceding and/or succeeding chemical processes [479]. If these chemical processes are fast relative to the electron-transfer reactions, the chemical processes are thermodynamically reflected in the overall electrode reaction. If these chemical processes are slow, the overall electrode reactions are kinetically characterized. Therefore, the electrode reaction can express the specificity depending on the characteristics of the chemical processes spatially and temporally coupled to the electron-transfer reaction. We can take advantage of this fundamental principle of electrochemical reactions to provide a wide variety of specificity into the nanoscale electrochemistry on the exposed... 

Fig. 14.9 Schematic representation of the dominant processes assumed to influence the voltammetric response when a chemically modified electrode consisting of a redox-active microparticle adhered to an electrode and coated with a layer of ionic liquid is placed in contact with aqueous electrolyte, dj and 2 are the thicknesses of solid-state phase and ionic liquid phase respectively. Adapted with permission from Zhang et al.. Anal. Chem. 2003, 75, 6938-6948 [23]. Copyright 2013, American Chemical Society...

Because gold displays very weak chemisorbing properties, the activated chemisorption model of electrocatalysis is assumed to be inapplicable in the case of this metal in aqueous media. The alternative, which is well established in the chemically modified electrode [48] and redox sensor [49] area, is the interfacial cyclic redox mediator model which, in the case of gold in aqueous media, is sometimes referred to as the incipient hydrous oxide/adatom mediator (IHOAM) [18,33] model. In the case of the Group 11 metals the mediator systems are unusual in that their redox transitions involve couples with nonequilibrium (or metastable) reduced and oxidized states. 

Several approaches have been undertaken to construct redox active polymermodified electrodes containing such rhodium complexes as mediators. Beley [70] and Cosnier [71] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD+ applied a poly-Rh(terpy-py)2 + (terpy = terpyridine py = pyrrole) modified reticulated vitreous carbon electrode [70]. In the presence of liver alcohol dehydrogenase as production enzyme, cyclohexanone was transformed to cyclohexanol with a turnover number of 113 in 31 h. However, the current efficiency was rather small. The films which are obtained by electropolymerization of the pyrrole-linked rhodium complexes do not swell. Therefore, the reaction between the substrate, for example NAD+, and the reduced redox catalyst mostly takes place at the film/solution interface. To obtain a water-swellable film, which allows the easy penetration of the substrate into the film and thus renders the reaction layer larger, we used a different approach. Water-soluble copolymers of substituted vinylbipyridine rhodium complexes with N-vinylpyrrolidone, like 11 and 12, were synthesized chemically and then fixed to the surface of a graphite electrode by /-irradiation. The polymer films obtained swell very well in aqueous... 

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. 

In the synthesis of vitamin C, the oxidation of diacetone L-sorbose to diacetone 2-keto-L-gulonic acid proceeds at an Ni-anode in the presence of hydroxide. Under these conditions, the nickel hydroxide surface is anodically transformed to NiOOH, the nickel peroxide, which acts as chemical oxidant via hydrogen atom abstraction. Thus, a chemically modified redox-active electrode acts as a heterogeneous redox catalyst [13] ... 

The most interesting reaction scheme is the electrocatalytic one. Electrocatalysis at modified electrodes is accomplished by an immobilized redox mediator, which is activated electrochemically by applying an electrical perturbation (potential or current) to the supporting electrode. As a result, the chemical or electrochemical conversion of other species located in the solution adjacent to the electrode surface (which does not occur, or occurs very slowly in the absence of the immobilized catalyst) takes place [1, 92-94]. The main advantage of this kind of electrocatalyzed reactions lies in the large number of synthetic procedures for... 

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. 

The immobilization of a photoisomerizable material that can be switched by light between redox-active and redox-inactive or conductive and insulating states offers an encouraging route toward integrated molecular memory devices. Figure 7.2 shows a photoisomer state A in which the molecular unit is redox-inactive and no electronic signal is transduced. Photoisomerization of the chemical component to state B generates a redox-active assembly, and the electron transfer between the electrode and the chemical modifier yields an amperometric (electrochemical) indicator of the state of the system. 

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