SAMs electrochemistry

Detailed studies about metal deposition from the gas phase onto SAMs have been published [108-110], The central question for the system substrate/SAM/deposit there (as well as in electrochemistry) is the exact location of the deposited metal On top of the SAM or underneath Three clearly different situations are easily foreseen (Fig. 31). (1) Metal on top of the SAM. Depending on a strong or weak chemical interaction between metal and SAM (e.g., functional end group of the SAM), the deposit will spread out on top of the SAM or it will cluster on the SAM. (2) Metal penetrating the SAM (e.g., at defects in the SAM) and connecting to the metal substrate underneath the SAM. This configuration is often pictured as a mushroom, with a thin connective neck and a large, bulky head. (3) Deposited metal is inserted be-... 

SAMs controlling electrochemistry, whereas the reverse holds for the other one, both topics are inseparably intertwined as exemplified by the underpotential deposition of metal on SAM-modified electrodes where patterned SAMs allow localization of metal UPD, which in turn affects the monolayer. 

Given the tremendous development of SAMs over the past two decades it is dear that this chapter is able to cover only a fraction of the spectrum of topics related to the combination of SAMs and electrochemistry. For a comprehensive picture the reader is referred to a number of additional review articles, one of which is the excellent and extensive account of organized monolayers on electrodes by Finklea [23]. Besides this one, which comprehensively covers the literature up to the mid-1990s, other more focused reviews are available that address various developments over the past decade in areas of sensor development and electroanalytical applications [22, 24—28] and electrochemical metal deposition on SAM-modified electrodes [29, 30]. 

Similar to other fundamental studies on SAMs, alkane thiols on Au(l 11) have also been prevailing in electrochemistry with, however, aromatic thiols receiving increased attention. A major topic has been formation and stability of SAMs and... 

Carbon nanotubes are increasingly recognized as a promising tool for surface functionalization. M.J. Esplandiu presents a state-of-the-art overview of their applications in electrochemistry. As with SAM s of organic molecules the great potential of carbon nanotubes lies, among others, in biochemical applications and in molecular electronics. 

Reversible attachment of nanostructures at molecular printboards was exemplified by the adsorption and desorption of CD-functionalized nanoparticles onto and from stimuli-responsive pre-adsorbed ferrocenyl-dendrimers at a CD SAM (Fig. 13.7).65 Electrochemical oxidation of the ferrocenyl endgroups was employed to induce desorption of the nanostructure from the CD SAM. An in situ adsorption and desorption of ferrocenyl dendrimers and CD-functionalized Au nanoparticles (d 3 nm) onto and from the molecular printboard was observed by a combination of surface plasmon resonance spectroscopy (SPR) and electrochemistry. Similar behavior was observed when larger CD-functionalized silica nanoparticles (d 60 nm) were desorbed from the surface with the aid of ultrasonication. 

A number of important conclusions were drawn from this study, as follows. Electrochemical reversibility in electroactive self-assembled monolayers depends upon concentration and polarity of a covalently attached redox probe. Reversible surface electrochemistry is observed for the well-diluted ferrocenyl ester. However, reversibility decreases with steric congestion of redox probe because higher redox probe concentrations lead to disorder due to cross-sectional mismatch of the redox probe and the alkyl chain. Reversibility also decreases with a nonpolar redox probe the alkylferrocene (System 4) yields broad peaks with long tails positive of E°, consistent with kinetic dispersion of the redox probes and their differential solvation in the SAM. 

Faulkner et al. performed surface-confined electrochemistry at high pressures to probe the structure of the transition state during the oxidation of a tethered ferrocene probe (analogous to System 4) [139]. In these studies, the ferrocene-containing SAMs on gold were subjected to pressures between 1 and 6000 atm. The pressure dependence of the anodic peak potential reveals a positive volume of activation for oxidation, which is consistent with a solvent reorganization in the transition state, which allows ion complexation. This study demonstrates the importance of structural and environmental effects on surface-confined electron-transfer processes. 

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