Gold-thiol monolayers

Many examples of surface-surfactant interactions which promote self-assembly are known. Apart from gold-thiol monolayers which are formed because of the creation of the strong S—Au bond, other commonly studied monolayers include alkyltrichlorosilane layers on hydroxylated surfaces (such as SiC>2)6, fatty acids on metal oxide surfaces7 8 and alkyl phosphonate salts on zirconium9. 

The above advantages of the gold-thiol system have resulted in the extensive use of gold-thiol monolayers by the whole scientific community. A brief research of the literature shows that more than 2000 papers have been published since 1983 studying self-assembled monolayers of thiols on gold. We have therefore made no attempts to make the present review comprehensive however, we have endeavoured to include all recent seminal work in the area. The subject is highly interdisciplinary, and we put perhaps more emphasis on the chemical aspects of the monolayer science than the authors of some other reviews. 

It will be going beyond the scope of the present review to give a detailed description of the techniques available, or to provide a comprehensive list of all methods used in surface analysis. Such information could be found in a number of monographs and review papers414,15. Analytical Chemistry publishes reviews every two years on the latest developments in surface characterization methods16. We will attempt to discuss briefly the most powerful and most readily available methods applicable to routine gold-thiol monolayer analysis with particular emphasis on the kind of information which can be obtained from these systems. 

In view of the limited stability of gold-thiol monolayers to laser light and X-ray, electron and metastable atom beams reported recently (see Section VI), one has to exercise caution to avoid damage of the monolayer in such experiments as SERS, XPS, GIXD, LEED etc. 

IV. PREPARATION OF GOLD-THIOL MONOLAYERS A. Experimental Procedures... 

The ability of the acidic and basic groups to exchange protons at the solid-liquid interface is different from the corresponding process in bulk media, and a significant number of studies was performed to establish the acidity and basicity of the functional groups on the surface of gold-thiol monolayers. 

Table 3 lists some ionization properties of functionalized gold-thiol monolayers and relevant alkylsiloxane monolayers together with the appropriate bulk values. Monolayers with carboxylate terminal groups show abnormal wetting behaviour, which makes it difficult to determine accurately their surface pKa values308. Apart from contact angle titration, other methods were also used to study proton transfer equilibria at the mono-layer surfaces, such as quartz crystal microbalance (Table 3, line 1), measurements of the adhesion force between the monolayer deposited at the surface of an AFM tip and the same monolayer deposited on the substrate (chemical force microscopy, Table 3, lines 3, 4, 15), FT-IR spectroscopy (Table 3, line 7), adsorption of polyelectrolytes (Table 3, line 5) and differential capacitance measurements (Table 3, lines 12, 13). 

It is known, however, that isothiocyanates on their own can form stable self-assembled monolayers on gold (Section V.E), and amine-terminated monolayers were reported to show unusually low reactivity towards electrophilic reagents329,330. It is therefore possible that the observed immobilization of eosin is due not to reaction with amino groups, but to either insertion of the eosin isothiocyanate into a disordered layer of cysteamine, or even replacement of cysteamine monolayer with the molecules of S=C=N—eosin. Because the experimental data on reactions in gold-thiol monolayers are scarce, chemical transformations described in this section should be interpreted with some caution. 

Polymerized diacetylenes show remarkably improved stability as compared to monomeric gold-thiol monolayers (see Section VI). 

Electron transfer to the perfect blocked electrode is also possible due to electron tunneling, and gold-thiol monolayers provide a unique opportunity to study this phenomenon. Miller and coworkers showed that the level of defects in the monolayers of m-hydroxyalkanethiols of medium chain length (6 < n < 18) has only a minor contribution to the electron transfer, and they performed detailed studies of the electron tunneling in this system367-369. 

FIGURE 23. Incorporation of an electroactive group in the mixed gold-thiol monolayers... 

Other electroactive groups incorporated in gold-thiol monolayers include tetrathiaful-valene365 and fullerene393. Miller and coworkers prepared thiol-derivatized oligoimides with naphthalene diimide electroactive groups. Electron transfer to each electroactive function was observed394,395. 

No such acceleration effects have so far been reported in gold-thiol monolayers. The existing studies are mainly concerned with the diminished reactivity of a functional group buried under the monolayer surface, due to partially blocked access of an external reagent to this group. 

Exposure of octadecanethiol monolayer to the small phospholipid vesicles leads to their unrolling and adsorption of a single lipid layer on top of the octadecanethiol film. Unrolling of vesicles on the surface of the hydrophilic monolayer, however, leads to the formation of the lipid bilayer on top of the original film. This principle was used by Evans and coworkers to create lipid bilayers supported by cholesterol moieties present in a mixed gold-thiol monolayer (Figure 28)430. 

A study of non-specific binding of proteins to the surface of functionalized gold-thiol monolayers showed that mixed monolayers containing hydrophobic and hydrophilic... 

FIGURE 28. Formation of a lipid bilayer on the surface of a mixed gold-thiol monolayer. Reprinted with permission from Reference 430. Copyright (1997) American Chemical Society... 

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