Monolayers voltammetric properties

General Voltammetric Properties of Redox-Active Monolayers... 

Kakiuchi, T., Usui, H., Hobara, D., and Yamamoto, M. (2002) Voltammetric properties of the reductive desorption of alkanethiol selfassembled monolayers from a metal surface. Langmuir, 18, 5231-5238. 

The thiolated j -CD forms monolayers with substantial defect densities due to the lack of favorable lateral interactions between the CD molecules. In order to patch the holes in these monolayer assemblies without clogging the CD cavities, we devised procedures for the preparation of mixed monolayers containing both the thiolated P-CD receptors and pentanethiol molecules. These monolayers were characterized by reductive desorption and electrode capacitance measurements. The monolayer blocking properties were also investigated, i.e., the voltammetric response of solution redox probes such as Fe(CN) and Ru(NH3)6 at the monolayer-covered electrodes were recorded. Our data supported the monolayer structure depicted in the Scheme 3 [26]. 

At higher potentials, positive of the Hupd OHad exchange, the C Vs of the Pt island-modified Ru(OOOl) surface closely resemble those of the ft-fiee Ru(OOOl) electrode, except for the lower currents/charges in the characteristic features. This simply reflects the fact that at these potentials, the surface reactivity is dominated by the electrochemical properties of the remaining exposed Ru surface. As already mentioned, the Pt monolayer islands themselves contribute only little to the voltammetric behavior, which is due to the weak bonding and hence low adsorbate coverages on these islands. 

Hexacyanoferrates were immobilized on Au covered with SAM of 3,3 -thiodipropionic acid [86]. It has been found from voltammetric studies that the surface coverage of hexacyanoferrate is close to one monolayer and such an electrode exhibits very good surface redox behavior. Cheng et al. [87] have described the formation of an extremely thin multilayer film of polybasic lanthanide heteropolytungstate-molybdate complex and cationic polymer of quaternary poly(4-vinylpyridine), partially complexed with osmium bis(2,2 -bipyridine) on a gold electrode precoated with a cysteamine SAM. Consequently, adsorption of inorganic species might also be related to the properties of SAMs. This problem will be discussed in detail in a separate section later. 

Electrochemical properties of Ni(lll) prepared and characterized in UHV were examined in 0.1 H KOH by cyclic voltammetry using an UHV-electrochemistry transfer system. A saturation coverage (0.5) of CO was used to protect the Ni(lll) from possible contamination during the transfer. The results indicated that the CO layer remained intact up to the moment of contact with the electrolyte and could be further electro-oxidized. Only one monolayer of Ni metal was involved in the electrochemically formed film in the first voltammetric cycle and less than two monolayers upon repeated cycling. Surfaces which were not protected by CO displayed essentially the same behavior. 

Recently, much effort has been made on the facilitation of direct electron transfer of the SODs by self-assembled monolayers (SAMs) confined onto Au electrodes. For instance, Ohsaka et al. have formed various kinds of SAMs of alkanethiols onto an Au electrode and studied the electron transfer properties of the SODs [98]. Here, we will use the SAM of cysteine as an example to demonstrate the electron transfer of the SODs promoted by the SAMs of alkanethiols. Figure 6.1 depicts cyclic voltanunograms (CVs) obtained at a cysteine-modified Au electrode (curves a and b) in 25 mM phosphate buffer containing 0.56 mM Cu, Zn-SOD (the concentration used represents that of the Cu or Zn site of Cu, Zn-SOD). For comparison, the CV obtained at a bare Au electrode (curve c) under the same conditions was also given. As shown, the cysteine-modified electrode exhibits one pair of well-defined voltammetric peaks in the SOD-containing phosphate buffer (curve a). These redox peaks were not obtained at the bare Au electrode (curve c). This observation suggests that the direct electron transfer between Cu, Zn-SOD and Au electrode does not occur actually at the bare electrode, but it can be significantly promoted at Au electrode modified with the SAM of cysteine. 

A very pronounced double-layer effect is also observed in the voltammetry of adsorbed outer-sphere reactants." For instance, the voltammetry of self-assembled monolayers (SAMs) of alkane-thiols containing a terminal redox group (e.g., ferrocene) is strongly influenced by the electric field across the SAM, and this is manifested in peak broadening and a shift in the half-wave potential. Because redox-active SAMs are frequently geometrically very well defined, and their dielectric properties can be measured, the electric field across these layers can be readily computed from electrostatics to obtain the electric potential at the redox center. This in turn can be used to compute the influence of the electric potential distribution on voltammetric response, which can then be quantitatively compared to experiment. 

There are a few indirect methods for characterizing molecule-size physical recognition sites, such as examining the extent of monolayer penetration by probe molecules as a fiinction of their van der Waals radii and other chemical and physical properties (Scheme HI, Frame 4). We have used an electrochemical version of this approach, which assumes that the defect sites define an array of ultramicroelectrodes, to analyze our composite SAMs (Scheme IV). In these experiments, the shape of the cyclic voltammetric wave is correlated to the size and number density of sites through which the probe molecules can penetrate, as shown on the right side of Scheme IV (6). 

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