Anchoring Monolayers to Substrate

A large number of different molecules are used for the preparation of monolayers on a substrate. The most widely studied species belong to one of the following classes [1-8, 15]  

Organosulphur compounds represent the most common class of molecules exploited for formation of monolayers [1-8]. Many different kinds of similar molecules, e.g., aUcanethiols, dialkyl disulphides, dialkyl sulphides, thiophenes (Ths), thiophenols, and cysteines, have been reported to form monolayers on different substrates mainly consisting of (1) metals (e.g., Au, Ag, Cu, Pt, or Hg) or (2) semiconductors, such as indium tin oxide (ITO) and GaAs. Among the possible systems, the most extensively studied are those based on thiols adsorbed on Au. Although fundamental studies have also been carried out on the analogous Se and Te molecules, these have led to a limited number of applications in electroanalysis. 

The preparation is not straightforward, due to the strong influence of the ubiquitous presence of water most of the starting molecules are, in fact, sensitive to moisture, which leads to polymerization of the Si-based groups consequently, they are difficult to handle. The reproducibility and degree of order of the mono-layers are strongly affected by this variable, which constitutes a possible heavy drawback. 

Finally, monolayers based on phosphorus can be divided into two quite different classes, namely those based on phosphines and those on phosphonic acids [1-8]. The species belonging to the former class stably adsorb on metal surfaces, e.g., on Au, thanks to the presence of a lone electron pair interacting with the metal orbitals those of the latter class prefer oxide surfaces. 

As to grafting with marked covalent character, the most widely investigated systems are based on diazonium salts that can be anchored at a number of quite different electrode surfaces, e.g., C or Pd (Fig. 5.3) [23-26]. The grafting occurs through chemical or electrochemical reduction of the diazonium moiety, leading to the release of nitrogen from the precursor molecules and concurrent formation of C-C or C-metal bonds. It is worth noticing that the reaction is not self-limited, so that formation of multilayers is also possible. 

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