Self-assembled monolayers electrochemical studies

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... 

In this part we will describe recent achievements in the development of biosensors based on DNA/RNA aptamers. These biosensors are usually prepared by immobilization of aptamer onto a solid support by various methods using chemisorption (aptamer is modified by thiol group) or by avidin-biotin technology (aptamer is modified by biotin) or by covalent attachment of amino group-labeled aptamer to a surface of self-assembly monolayer of 11-mercaptoundecanoic acid (11-MUA). Apart from the method of aptamer immobilization, the biosensors differ in the signal generation. To date, most extensively studied were the biosensors based on optical methods (fluorescence, SPR) and acoustic sensors based mostly on thickness shear mode (TSM) method. However, recently several investigators reported electrochemical sensors based on enzyme-labeled aptamers, electrochemical indicators and impedance spectroscopy methods of detection. 

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... 

Bryce and co-workers reported that the crown-annulated TTF derivatives 98 and 99 were used for UV-Vis spectroscopic and electrochemical studies of metal complexation <1996J(P2)1587>. Solution electrochemical studies showed that metal complexation to the crown unit leads to a small anodic shift in the first oxidation potential of the TTF system. Langmuir-Blodgett films of amphiphilic 99 have been assembled on solid substrates by Y-type deposition. Compounds 100-104 were used to prepare self-assembled monolayers on gold and platinum surfaces <1998AM395, 2000JOC8269>. The self-assembled monolayers of 104 were the most stable in this series of TTF-crowns. Electrochemical data for the self-assembled monolayers of 100-104 in MeCN showed two reversible one-electron waves, typical of the TTF system. The self-assembled monolayers of 102-104 exhibited an electrochemical response in aqueous electrolyes, which was observed between 50 and 100 cycles. 

Organothiols chemisorbed on gold surfaces are especially usefiil for electrochemical and chemical studies. For example, carboxylic acid terminated monolayers are often used. An example is the attachment of cytochrome C with its side chain lysine groups to acid terminated self assembled monolayer gold electrodes. Carbodiimide mediated esterification of ferrocene-l,l -dicarboxylic acid with phenols is used to construct thermotropic liquid crystals. ... 

This section describes the evolution of the systems and techniques that have made possible detailed studies in which electron transfer has been investigated using self-assembled monolayers. After a brief presentation of the important electrochemical methods for assessing the quality of the electroactive SAMs and determining ket across monolayer spacers, studies of electron transfer across SAMs are presented. 

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. 

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. 

Fluoride is trapped as NaF and under certain conditions the yield of per-florotetralin can be maximized. Lewis acidic sites in crystal surface defects are thought to assist in the C-F activation process and at 470°C the system is active for demineralization of chlorofluorocarbons to afford NaF, NaCl and carbon [76]. Detailed electrochemical studies of the reduction of C-F bonds in aryltrif-luoromethanes and fluoroalkoxyarenes have been reported [77]. Free radicals cleave C-F bonds of fluorinated self-assembled monolayer surfaces [78]. 

Recently, Brzozowska et al. used NR and ex situ electrochemical techniques to characterize an innovative type of monolayer system intended to serve as a support for a bUayer lipid membrane on a gold electrode surface [51]. Zr ions were used to noncovalendy couple a phosphate-terminated self-assembled monolayer (SAM) formed on a gold surface to the carboxylate groups of negatively charged phos-phatidylserrne (PS). This tethered surface was then used for the formation of a PS hpid bilayer structure formed by vesicle fusion and spreading. NR studies revealed the presence of an aqueous environment associated with the tether layer which arises from nonstoichiometric water associated with the zirconium phosphate moieties [52]. 

A few groups examined larger systems by in situ STM. Examples include the deposition of polymers, polypyrrole, polyaniline,and polymethylthiophene. Hagenstrom et al. reported studies on imaging of self-assembled monolayers (SAM). "" They showed the possibihty of imaging detailed stmetures, and order-disorder transitions in dependence on electrochemical parameters. 

The ability of compounds of sulphur in different oxidation states, such as sulphinates and sulphonates, to form self-assembled monolayers has been studied. The relative adsorp-tivities of aryl sulphur species were found to be ArSC>3 2 < ArS-. While sulphonates can only adsorb electrochemically at positive potentials, sulphinates form self-assembled monolayers at normal conditions263. Sodium benzenesulphinate adsorbs from aqueous solution, but does not form monolayers from benzene. These monolayers of sulphinates are readily replaced by thiols264. 

Wang, R., lyoda, T., Tiyk, D. A, Hashitnoto, K., and Fujishima, A. Electrochemical modulation of molecular conversion in an azobenzene-terminated self-assembled monolayer film An in situ UV-visible and infrared study. Langittuir 1997,13, 4644-4651. 

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