Monolayers electroactive

Other common techniques have been applied to the assembly of layers or films of TTF-derived molecular conductors. Compound 29 is an example of an amphiphilic TTF derivative. It forms conducting Langmuir-Blodgett charge transfer films with the acceptor TCNQF4 (30).98 Self-assembly of compound 31 on gold by electrochemical methods yielded an electroactive monolayer which was remarkably stable to electrochemical cycling.99... 

Numerous bisthiols have been observed to form spontaneously multilayers on gold and silver on the basis of the oxidative formation of disulfides.15-27 Nonetheless, most of these compounds lack electroactive character, with few notable exceptions.23,27 In principle, the introduction of redox centers at the core of these molecules and their subsequent assembly into multilayers can be exploited to generate electroactive films. The concentration of redox centers within the resulting electrode coatings, as well as their thickness, can be significantly larger than those possible with electroactive thiols such as 1-4 (Fig. 7.1).11 14 In addition, the transition from electroactive monolayers to electroactive multilayers can translate into a significant enhancement in stability and a much more effective protection of the electrode surface. 

Recently, a numerical solution has been obtained for the LSV response to a homogeneous catalytic reaction at an electroactive-monolayer-film-covered rotating disc electrode [279]. 

Mediation of electron transfer between a dissolved redox probe and the electrode by surface-confined redox groups has also been used to assess the integrity of electroactive monolayer assemblies. As an example, the reduction of Ru (NH3)e, normally occurring at about 0.1 V vs. Ag/AgCl, is fully blocked by an assembly containing tethered A-ethylviologen redox probes. However, at the observed reduction potential of the viologen probes (ca. —0.4 V), these probes mediate the reduction of Ru (NH3)g [87]. Similarly, the delayed oxidation of Fe (CN)6 at... 

Chapter 13 was largely concerned with adsorbed species that are not electroactive. In this chapter we consider electroactive monolayers and thicker films on conductive substrates these are frequently called chemically modified electrodes. This area of electrochemistry has been a very active one in recent years, and a number of reviews discussing the preparation, characterization, and electrochemical behavior of chemically modified electrodes are available (1-14). These electrodes are often prepared by the modification of a conductive substrate to produce an electrode suited to a particular function, whose properties are different from those of the unmodified substrate. Modified electrodes can be prepared in several different ways, as discussed in vSection 14.2, including irreversible adsorption, covalent attachment of a monolayer, and coating the electrode with films of polymers or other materials. 

Impedance methods have been more useful in studying electron-transfer kinetics in electroactive monolayers in the absence of an electroactive solution species (71-73), such as alkylthiol layers with tethered electroactive groups (Section 14.5.2). The equivalent circuit adopted is shown in Figure 14.3.18, where the adsorbed layer is represented by Cads = (F AT)/4RT and the electron-transfer kinetics by = (2RT)/F ATkf, so that... 

Figure 14.3.18 Equivalent circuit for an electroactive monolayer. Rfi = solution resistance, = double-layer capacitance, = charge-transfer resistance, and Cads capacitance of the adsorbed layer.

Studies of tethered electroactive species are less sensitive to pinholes than experiments with solution reactants and blocking layers, although heterogeneity and roughness of the substrate and film defects can still play a role. The rate constant, k, in this case has units of a first-order reaction (s ). Rate constants can be determined by a voltammetric method as described earlier for electroactive monolayers (Section 14.3.3). In addition potential-step chronoamperometry can be employed, in which case the current follows a simple exponential decay (88, 90, 91) ... 

We now use an equivalent circuit representing an electrode/solution interface where the electrode surface is covered by an electroactive monolayer. The simplest circuit is shown in Fig. 2.18. We assume that the molecules in a Langmuir monolayer undergo an n-electron transfer reaction in response to ac and that the ER signal is exclusively due to this faradaic process [69]. The faradaic process of the surface-confined species at the formal potential is represented by a series connection of a constant capacitance associated with the redox reaction of the adsorbed species Q and a charge transfer resistance Ret. where Q is written for a Nernstian process as... 

Electroactive monolayers of thiol-derivatized hydroquinones on the surface of platinum electrodes were prepared and studied by Hubbard and coworkers even before Nuzzo and Allara reported formation of the first gold-thiol monolayers . The redox potentials of quinone- or hydroquinone-containing monolayers vary with pH with a slope of 60 mV pH indicating a 2e -I- 2H+ reaction . This observation was used in an elegant study by Hickman and coworkers. They codeposited hydroquinone and ferrocene-containing thiols on the surface of a gold electrode. The half-wave potential of the latter does not depend on the pH value of the solution, and therefore can be used as a reference. The whole assembly acts thus as a pH-sensitive sensor which does not require any separate reference electrodes . 

The BV model has been very successful in the parameterisation and classification of the kinetics of systems where the electroactive species diffuse in the electrolytic solution instead of being attached to the electrode surface. On the other hand, for the case of electroactive monolayers, significant deviations have been reported with respect to BV predictions [11]. Moreover, the BV model is very limited with respect to understanding the connection... 

Figure 6.4. The treatment of electrochemical systems with adsorption is significantly more complicated given that we must select a suitable model to describe the adsorption process which will introduce new variables, uncertainties and approximations. Moreover, as will be discussed below, in general the models will lead to non-linear terms in the mathematical problem. For all the above reasons, it is common practice to try to minimise the incidence of adsorption by means of the experimental conditions (mainly the electrode material and solvent). However, in some situations adsorption cannot be avoided (being even intrinsic to the process under study) or it can be desirable as in the modification of electrodes with electroactive monolayers for electroanalysis or electrocatalysis.

As can be seen in Figure 6.5, the signal of an electroactive monolayer is quite different from that of an electroactive species diffusing in solution. Thus, considering a reversible process, the voltammograms are symmetric with respect to and the potential axis such that both the reduction... 

Yeo WS, Hodneland CD, Mrksich M (2001) Electroactive monolayer substrates that selectively release adherent cells. Chembiochem 2(7-8) 590-593... 

Ohtani M (1999) Quasi-reversible voltammetric response of electrodes coated with electroactive monolayer films. Electrochem Commun 1 488-492... 

The incorporation of electroactive hydroquinone moieties into SAMs has also been utihzed to generate electroactive monolayers. Hydroquinone moieties can be reduced by electrochemical oxidation to generate benzoquinone and this has been used to release Hgands attached to a substrate surface [12]. For example, Yeo et al. detached Arginine-Glycine-Aspartic acid (RG D) peptides, which function as a cell adhesive, from the substrate surface this triggered the release of cells adhered to the peptides [13]. Furthermore, benzoquinone units resulting from this reduction... 

A derivation for electron-transfer kinetics in electroactive monolayers with an amount of adsorbed species F (mol/cm ) resulted in expressions for the adsorption capacitance (C s) and adsorption resistance (R pj) as ... 

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