Desorption thiol

At the same time that reduction of the nitro group occurs in NBT SAMs, cross-linking also occurs between adjacent biphenyl groups. This may be exploited to yield micro- and nanostructures. Eck et al. showed that freestanding sheets of cross-linked biphenyls may be formed by electron-induced cross-hnking and released by exposure of the samples to iodine vapor (which oxidizes the S-Au bond). Alternately, NBT SAMs may be cross-linked and the unexposed adsorbates removed by thermal desorption (thiols readily desorb from gold surfaces because the activation barrier for conversion of two thiolates to a disulfide is small). ... 

Example Peptides often contain sulfur from cysteine. Provided there are at least two cysteines in the peptide molecule, the sulfur can be incorporated as thiol group (SH, reduced) or sulfur bridge (S-S, oxidized). Often, both forms are contained in the same sample. At ultrahigh-resolution, the contributions of these compositions to the same nominal m/z can be distinguished. The ultrahigh-resolution matrix-assisted laser desorption/ionization (MALDI) FT-ICR mass spectrum of native and reduced [D-Pen jenkephalin gives an example of such a separation (Fig. 3.25). [39] The left expanded view shows fully resolved peaks due to and C2 isotopomers of the native and the all- C peak of the reduced compound at m/z 648. The right expansion reveals the peak of the native plus the... 

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.

Figure 5.10 Current transients of the desorption of octadecane thiol from Au(l 1 1) recorded in 0.1 m KOH. The times indicate the time of formation at a potential of —0.2 V vs. SCE. To record the curves the potential was stepped to —1.31 V. Printed with permission from Ref [158].

In contrast to the detailed work on the Au(l 11) surface, desorption studies from the other low-index surfaces are scarce with, for example, MC9 and MC4/8 on Au(l 10) [45, 46] and MC4 [47] on Au(l 00). Compared to Au(l 11) thiols are more stable on Au(l 10) as reflected by a negative shift of the desorption peak by 200-300 mV, which was explained by the difference in the pzc for both surfaces [46]. N o obvious differences in the shape of the desorption peaks were found for Au( 1 0 0) compared to Au(l 11). Interestingly, for MC4 a higher thiol coverage compared to both MC4 on the Au(l 11) and MC2 Au(l 0 0) was concluded from the desorption studies. For polycrystalline surfaces the desorption signal is more complicated with additional features, possibly due to the presence of different crystallographic domains [94, 163, 164]. 

Figure 5.11 Desorption of a SAM of dodecanethiolate from a rotating Cu-disk electrode, (a) Current measured at the Cu electrode, (b) Current measured at a Au-ring electrode indicating oxidative adsorption of a thiol. Electrolyte 0.1 NaOH + H2O (5%) in methanol. Reproduced with permission from Ref [165].

Figure 5.15 Stability change of a SAM of propane thiol on Au/ mica by UPD of a series of metals. The linear sweep voltammograms were recorded in 0.5 M KOH at a scan rate ofO.l V/s. The electrode area was 0.36 cmf In the case of Cu UPD no desorption is discernible since the stability is increased to such an extent that the desorption peak shifts negative beyond the range shown into the region of hydrogen evolution. Reproduced with permission from Ref [202].

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. 

There are at least three possibile ways in which the inhibitor can bind to the active site (1) formation of a sulfide bond to a cysteine residue, with elimination of hydrogen bromide [Eq. (10)], (2) formation of a thiol ester bond with a cysteine residue at the active site [Eq. (11)], and (3) formation of a salt between the carboxyl group of the inhibitor and some basic side chain of the enzyme [Eq. (12)]. To distinguish between these three possibilities, the mass numbers of the enzyme and enzyme-inhibitor complex were measured with matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI). The mass number of the native AMDase was observed as 24766, which is in good agreement with the calculated value, 24734. An aqueous solution of a-bromo-phenylacetic acid was added to the enzyme solution, and the mass spectrum of the complex was measured after 10 minutes. The peak is observed at mass number 24967. If the inhibitor and the enzyme bind to form a sulfide with elimination of HBr, the mass number should be 24868, which is smaller by about one... 

The preparation and application of SAM systems patterned by STM and their use in catalysis was demonstrated by Wittstock and Schuhmann [123]. The patterning (local desorption) of SAMs from alkane thiols on gold was performed by scanning electrochemical microscopy (SECM), followed by the assembly of an amino-deriva-tized disulfide and coupling of glucose oxidase to form a catalytically active pattern of the enzyme. The enzymatic activity could be monitored/imaged by SECM. 

The lower the thiol concentration, the longer the deposition time that is required to achieve a given surface coverage. In order to investigate the influence of these and other parameters on the quality of a monolayer, as well as the completeness of the surface coverage, the process of reductive desorption has been employed. 

The measurement of the charge involved in the reductive desorption of layers prepared under various conditions provides very important information about the surface coverage and the nature of the oxidative adsorption. The influence of time and thiol concentration on both the potential and the charge of the reductive desorption has been investigated for decanethiol adsorbed on Au(lll) [125] (see Fig. 8). 

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. 

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... 

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