Silanization Reactions

A second general method of attaching organic molecules to silicon is through silanization reactions on hydroxyl-terminated silicon surfaces. This approach uses the wide array of methods and organosilane reagents that have been developed for surface modification of glass. A good review and introduction to the formation of silane monolayers can be found within the comprehensive review of SAMs by Ulman [105]. Ruckenstein and Li [106] have reviewed silane SAM formation in the context of subsequent graft polymerization. As illustrated schematically in Figure 4.5, the silanizing agent (an organosilane) typically has one organic group and three alkoxy or halogen (usually chlorine) groups attached to it. The alkoxy or chlorine groups react with surface hydroxyls to form Si-O-Si linkages. They then condense with each other to form a cross-linked siloxane layer on the surface. In some cases, the silanizing compound has a single-reactive (alkoxysilane or chlorine) group, with methyl groups in place of the other two, in which case a noncross-linked layer can be formed. The chlorosilane reagents can form denser, higher quality monolayers, but are very reactive with water. Trace water is required to prepare a cross-linked layer from them, but in the presence of more than trace amounts of water, they will polymerize in solution. This makes it relatively difficult to achieve reproducible results with them. The alkoxysilanes are much less reactive. They generally will not form dense, high-quality SAMs, but they are much easier to handle and may lead to more reproducible results. Here, we briefly review some examples of this silanization chemistry applied to attach biologically relevant molecules to flat and porous silicon surfaces as well as examples of the application of this approach to silicon nanoparticles.

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