SAMs reductive desorption

It is obvious that the change in stability upon UPD of Cu and Ag (Figure 5.15) can be harnessed for manipulation of SAMs, as demonstrated by the scheme of Figure 5.20a where nanopores were created in a sequence of steps involving UPD of both Ag and Cu as well as reductive desorption and adsorption of thiols [219,220]. 

In a scheme complementary to the one just presented where thiols are removed by reductive desorption of thiols, molecules can also be removed during stripping of a UPD layer. This was demonstrated by Shimazu et al. [221] where an alkane thiol SAM was deposited onto a Au(l 11) that had been modified with Pb. Oxidative stripping of the lead also caused thiols to be removed. The empty sites were then subsequently filled with mercaptopropionic acid (MPA). A remarkable result is that the binary SAMs exhibit only one desorption peak. From this it was concluded that a well-mixed layer forms that is very different from the mixed SAM obtained by adsorption from solution containing both types of thiols. In this case the layer exhibits singlecomponent domains that are refiected by two desorption peaks. 

In the voltammogram of Au(lll) modified with binary SAM of 1-undecanethiol and 11-mercaptoundecanoic acid, only one reductive desorption peak was formed for any value of mixing ratio of both thiols [139]. Such a response suggests that both thiols are well mixed in the SAM. 

Also, reductive desorption of SAMs of such asymmetrical disulfides as butyl hexadecyl disulfide and decyl-2(perifluoro-hexyl)ethyl disulfide has been studied on Au(lll), using CV [168]. Peak potentials corresponding to electrochemical desorption waves of the adsorbed species were different from those obtained for monolayers... 

Reconstruction of Au(lll) is observed in STM images as double rows separated from each other by 6.3 nm [335]. Some model calculations have been performed [362] to show that the energy difference of the reconstructed and unreconstructed Au(lll) is small. The effect of Triton X-100 on the reconstruction process of Au(lll) surface has been studied in chloride media [363] applying CV and double potential-step chronocoulometry. It has been found that adsorption of Triton X-100 stabilizes the reconstructed face of Au(lll). Hobara etal. [364] have used in situ STM to study reconstruction of Au(lll), following reductive desorption of 2-mercaptoethanesulfonic acid SAMs. 

Widrig et al. [196] have studied voltam-metrically, the SAMs of several -alkanethiols formed on pc-Ag electrodes. Analysis of data showed that during adsorption, the hydrogen of thiol group is lost and the sulfur is oxidized by one electron. Based on the charge required for the reductive desorption of the mono-layer, the surface coverage was found to be 7.0 X 10-10 j -2... 

Multifarious patterns of differently functionalized alkanethiol SAMs have been mapped to single-molecule and sub-molecular resolution by in situ STM in aqueous electrolyte, strongly supported by electrochemical studies of reductive desorption in particular. In situ STM is, however, rooted in electronic conductivity and quantum mechanical tunneling. Theoretical support is therefore needed in detailed image interpretation of all the many facets of alkanethiol-based SAM packing and in situ STM contrasts ]163]. 

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