Self-assembled monolayers electrochemical behavior

Fe2(III,III)2+ states. By considering that organometallic dendrimers based on conjugated ferrocene units are of special importance since mixed-valence states have interesting electrical, redox, and magnetic properties, recently three generations of polypropylene amine) dendrimers, decorated at their periphery with 4, 8, and 16 (compound 6, Fig. 6.6a) BFc units, respectively, have been synthesized and the electrochemical behavior of the dendrimers complexed with (3-cyclodextrins ([3-CD) and adsorbed at self-assembled monolayers (SAMs) of heptathioether-functionalized [3-CD on gold (molecular printboard) has been studied.40... 

Several examples of catenanes and rotaxanes have been constructed and investigated on solid surfaces.1 la,d f 12 13 26 If the interlocked molecular components contain electroactive units and the surface is that of an electrode, electrochemical techniques represent a powerful tool to study the behavior of the surface-immobilized ensemble. Catenanes and rotaxanes are usually deposited on solid surfaces by employing the Langmuir-Blodgett technique27 or the self-assembled monolayer (SAM) approach.28 The molecular components can either be already interlocked prior to attachment to the surface or become so in consequence of surface immobilization in the latter setting, the solid surface plays the dual role of a stopper and an interface (electrode). In most instances, the investigated compounds are deposited on macroscopic surfaces, such as those of metal or semiconductor electrodes 26 less common is the case of systems anchored on nanocrystals.29... 

By their very nature, heterogeneous assemblies are difficult to characterize. Problems include the exact nature of the substrate surface and the structure of the modifying layer. In this chapter, typical examples are given of how surface assemblies can be prepared in a well-defined manner. This discussion includes the descriptions of various substrate treatment methods which lead to clean, reproducible surfaces. Typical methods for the preparation of thin films of self-assembled monolayers and of polymer films are considered. Methods available for the investigation of the three-dimensional structures of polymer films are also discussed. Finally, it will be shown that by a careful control of the synthetic procedures, polymer film structures can be obtained which have a significant amount of order. It will be illustrated that these structural parameters strongly influence the electrochemical and conducting behavior of such interfacial assemblies and that this behavior can be manipulated by control of the measurement conditions. 

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... 

In this section, selected studies are presented in which self-assembled monolayers have been used to address topics such as transition-state structures and sequential electron transfer. These studies were selected because they address fundamental mechanistic processes. SAMs have also been used to investigate such basic electrochemical phenomena as the potential profile near an electrode [134, 135], interfacial capacitance [136], the influence of redox [134] or polarizable [137] moieties on double-layer structure and the behavior of ultramicroelectrodes approaching molecular size [138]. These important topics are beyond the scope of this chapter, and the interested reader is directed to the literature for more information. 

In 1997, Hayes et al. [28] reported the physical and electrochemical properties of two-component self-assembled monolayers (SAMs) composed of both electroactive (4-aminothiophenol, 4-ATP) and electroinactive (w-octadecanethiol, ODT) species. LFM images obtained at three points near the critical region of the isotherm indicated that these two-component SAMs display complex phase behavior at relatively low 4-ATP coverage, the surface consists of small islands of 4-ATP embedded in an ordered film of ODT at higher coverage of 4-ATP, however, evidence was found for both separation into distinct phases and mixing of the two components. 

Schlereth (1999) used the SPR technique coupled with cyclic voltammetry to characterize monolayers of cytochrome-c and cytochrome-c-oxidase adsorbed on gold surfaces modified with different alkanethiol self-assembled monolayers [60]. Different behaviors for enzyme adsorption processes in the modified gold surface were observed. For modified mercapto propioni acid electrodes, the response observed for the cytochrome-c adsorbed may be explained as arising from a potential-dependent adsorption and for cytochrome-c-oxidase appears a conformational change between the two states of the adsorbed oxidase, which gives rise to two species with different electrochemical behaviour. 

S.-F. Wang, D. Du and Q.-C. Zou, Electrochemical behavior of epinephrine at L-cysteine self-assembled monolayers modified gold electrode, Talanta., 2002, 56, 687-692. 

W.A Hayes, H. Kim, X. Yue, S. Perry, and C. Shannon, Nanometer-scale patterning of surfaces using self-assembly chemistry. 2. Preparation, characterization, and electrochemical behavior of two-component organothiol monolayers on gold surfaces, Langmuir, 13(9), 2511—2518 (1997). 

In typical Au MFC ET experiments with SAMs, the clusters are linked to a metal electrode surface through a self-assembled alkyl chain monolayer with specific linking sites. The electron is presumed to be transferred to/from the tethered MFC through the chain linker to/from the electrode surface. Techniques used to measure ET rates in Au MFCs attached to SAMs are potential step voltammetry and electrochemical AC impedance. Rate constants determined through impedance measurements have the virtue of less distortion due to uncompensated resistance. Also, rate constants determined using cyclic voltammetry are explicitly based on the kinetic behavior of the subpopulation of MFCs that produces the most prominent current peaks. Fotential step voltammetry resolves more peaks and thus gives a better indication of the diversity of rate constants of the redox species in the monolayer. ... 

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