Thiol SAMs

Herein the focus is on SAMs of trichlorosilanes and thiols. SAMs of carboxyUc acids are important as a connection between the LB and self-assembly techniques, but studies of their formation and stmcture have been relatively limited. SAMs of carboxyUc acids on AI2O2, AgO, and CuO have also been carried out (113—124). 

B. Ge and F. Lisdat, Superoxide sensor based on cytochrome c immobilized on mixed-thiol SAM with a new calibration method. Anal. Chim. Acta. 454, 53-64 (2002). 

For advanced electrochemical applications of SAMs in this area, their design is, therefore, a key issue. While SAMs are often perceived to form easily well-defined structures, a closer look into the literature reveals that thiol SAMs, in fact, very often lack the structural quality anticipated. Contrasting their ease of preparation, orga-nosulfur SAMs represent systems whose structure is determined by a complex interplay of interactions and if those are not properly taken into account, a SAM of limited structural quality and performance will result. To optimize SAMs for electrochemical applications and to exploit their properties for electrochemical nanotechnology it is, therefore, crucial to identify the factors controlling their structure. For this reason we start with an account of the structural aspects of SAMs. 

SAMs, in general, and thiol SAMs, in particular, are very often perceived as systems that easily form layers of high structural quality and this view is reflected in oversimplifying cartoons where a SAM is represented by a two-dimensional crystalline arrangement of molecules on a surface, similar to the one depicted in Figure 5.1b. For some systems one can get quite close to this ideal picture, as seen from Figure 5.2a, however, the more common case exemplified by Figure 5.2b is quite different. While... 

Figure 5.8 CV of a hexadecane thiol SAM on Au/mica recorded in 0.5 m KOH at a scan rate of 20 mV/s. KOH is used to shift the hydrogen-evolution reaction negativeofthethiol desorption peak.

Table 5.1 Compilation of thiol SAMs for which electrochemical metal deposition has been studied. 

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. 

Figure 5.22 Cu UPD on a Au/mica substrate modified by a hybrid nanostructure consisting of a hydrogen-bonded network and a thiol SAM. (a) Cartoon ofthe hybrid structure and molecular structures of the components. The hydrogen-bonded network is composed of 1,3,5-triazine-2,4,6-triamine (melamine, triangles) and perylene-3,4,9,10-tetracarboxylic di-imide (PTCDI, rectangles) that interact via a triple hydrogen bond (dotted lines). Pores ofthe...

M. Buck reviews in great depth the literature on self-assembled monolayers (SAMs) of thiols on gold, a classic means of surface modification. The wide variety of functional groups that is provided by synthetic chemists makes thiol-SAMs an exciting playground for applications where the gap between two worlds, the inorganic and the organic, needs to be closed. Examples are molecular electronics and biochemistry. 

Similar to thiol SAMs, nonspecific interactions can be reduced by mixing PEG-terminated silanes into the monolayer. Another simple method is rinsing of the silanized surface with Tween 80 after the preparation [29]. 

By choosing the appropriate head-group functionality of the thiol, SAMs of different chemical nature can be generated (carboxylic and amine are the most frequently used examples). Modified ONDs can be attached to these SAMs using common coupling methods [46], or thiolated ONDs can be directly attached to the gold surface. 

Fig. 5.1 Scheme for patterned deposition of PbS. (a) Self-assembled monolayer (SAM) of long-chain mercapto-carboxylic acid. On exposure to UV radiation through a mask, the exposed thiol group is oxidized to a sulphonate group (b) that is weakly bound to the An substrate and can be easily rinsed away (c). Subsequent formation of a long-chain alkyl thiol SAM occurs only on the exposed Au (d). CD of PbS occurs only on hydrophilic car-boxylate endgroups and not on hydrophobic methyl groups (e). (See Ref. 41). 

CaSki, HeLa, 3T3 fibroblasts Release of OEG-thiol SAM from gold electrodes 2008 [167]... 

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