Redox-active monolayers, electrochemical

Ferrocenylated ODNs were first immobilized in a self-assembled redox-active monolayer on Au electrodes by Letsinger and co-workers.Upon hybridization of a complementary strand, the electrochemical potential of the ferrocene changes. In addition to applications as electrochemical DNA sensors, such self-assembled DNA monolayers with electro-active groups may provide information on the mechanism of electron transfer through DNA, and indirectly also on molecular mobility within short stretches of DNA. " We have recently extended this idea by the use of immobilized metallocene-labeled PNA on Au electrodes. Because PNA is an uncharged molecule, a surface with improved properties forms, and electrochemical detection, also of single mismatches, is facilitated. 

The electrochemical behavior of molecules attached to conducting surfaces (i.e., electrode surfaces) forming electro-active monolayers is discussed in the following sections. This situation has frequently been called modified electrodes in the literature [39]. The electro-active character of these monolayers arises from the presence of redox molecules on them which are susceptible to transfer to or receive charge from the supporting electrode as well as from species in solution (in this last situation, the attached molecules act as redox mediators between the electrode and the solution and are responsible for the appearance of electrocatalytic processes [39, 40]). Multipulse and Sweep Electrochemical techniques like SCV and CV have proven to be very necessary tools for understanding the behavior of these interfaces and the processes taking place at them. 

As outlined above, the electrochemical properties of this redox species are strongly pH-dependent and this behavior can be used to illustrate the supramolecular nature of the interaction between the polymer backbone and the pendent redox center. The cyclic voltammetry data shown in Figure 4.17 are obtained at pH = 0, where the polymer has an open structure and the free pyridine units are protonated (pKa(PVP) = 3.3). The cyclic voltammograms obtained for the same experiment carried out at pH 5.7 are shown in Figure 4.18. At this pH, the polymer backbone is not protonated and upon aquation of the metal center the layer becomes redox-inactive, since protons are involved in this redox process. This interaction between the redox center and the polymer backbone is typical for these types of materials. Such an interaction is of fundamental importance for the electrochemical behavior of these layers and highlights the supramolecular principles which control the chemistry of thin films of these redox-active polymers. Finally, it is important to note that the photophysical properties of polymer films are very similar to those observed in solution. Since the layer thickness is much more than that of a monolayer, deactivation by the solid substrate is not observed. 

As discussed in Chapter 4, a wide variety of functionalized alkane thiols, HS(CH2) -2, where 5 < n < 16, form highly ordered self-assembled monolayers. As illustrated in Figure 5.1, redox-active species can be covalently bound to these bridges. The seminal work of Chidsey [2], Acevedo and Abruna [3] and Finklea and Hanshew [4] has demonstrated that electroactive adsorbed monolayers can exhibit close to ideal reversible electrochemical behavior under a wide variety of experimental conditions of time-scale, temperature, solvent and electrolyte. These studies have elucidated the effects of electron transfer distance, tunneling... 

Cysteamine was used to couple redox-active carboxylalkyl-4,4 -bipyridinium salts to the gold surface . The nonordered monolayer assembly was then transformed into a densely packed monolayer with 1-hexadecanethiol and cyclic voltammetry of the surface bound viologen was performed. The electron transfer rate constants to the bipyridinium sites depended on the alkyl chain length Abridging the redox site to the electrode. Electron transfer rate constants followed the Marcus theory. Cysteic-acid-active ester monolayers chemisorbed on gold were used to electrode-immobilize the protein glutathione reductase, then a bipyridinium carboxylic acid was condensed onto the enzyme in the presence of urea to wire the protein towards electrochemical reduction (Figure 6.26). 

A further system providing photoswitchable redox-activated properties with amplification features via a secondary electrocatalytic vectorial electron transfer reaction has been exemplified by diarylethene (45) molecules incorporated into a long-chain thiol monolayer adsorbed on a Au electrode due to hydrophobic interactions [85]. In the closed isomeric state (45a), the monolayer demonstrates well-defined reversible cyclic voltammetry, whereas the open (45b)-state is completely redox-inactive. The electrochemically active 45a-state provides electrocatalytic reduction of Fe(CN)g-, thus enabling a vectorial electron cascade that amplifies the photonic input. 

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