Tetramethoxysilane molecules

A second, equally powerful means to prepare such materials relies on traditional inorganic polymerization tools, most notably sol-gel polymerization.24 25 A number of excellent reviews have appeared on this subject as well.5,12,17 In sol-gel processing, the functional monomer [i.e., an organoalkoxysilane such as 3-aminopropyltrimethox-ysilane (APTMS)] is combined with the cross-linking agent [i.e., a tetrafunctional alkoxysilane such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS)], a catalyst (such as hydrochloric acid or ammonia), and the template molecule. The resultant sol can be left to gel to form a monolith, which can then be dried, sieved, and extensively washed to remove the template. Alternatively, the sol can be spin coated, dip coated, or electrodeposited on a surface to yield a thin film, which can be subsequently washed with a solvent to remove the template and yield the imprinted cavities. 

Spin-casting techniques have also been used to prepare imprinted thin films. Makote and Collinson recently prepared metal oxide thin films imprinted with recognition sites for dopamine [18], The dopamine template was loaded at 4 mole % in a sol with a 10 1 ratio of tetramethoxysilane and phenyltrimethoxysilane. The prepared film had a thickness of ca. 450 nm. CV analysis found that 90% of the templates could be removed by washing the films with pH 7 phosphate buffer. The opened receptor sites offered selective binding for related molecules containing catechol amines, such as dopamine, epinephrine and norepinephrine, as determined by CV. The use of phenyltrimethoxysilane turns out to be an essential ingredient for the gel matrix and is believed to provide some complementary affinity for the catechol amines via hydrophobic and/or 7i-stacking interactions. 

The difference between I(EtOSi) and I(MeOSi) is strikingly small, which is mainly caused by the too high retention index for tetra-methoxysilane. Accordingly, the positive charge on the methyl carbon atoms in tetramethoxysilane renders the molecule more soluble in the stationary phase than expected for normal behaviour. Judging from the 6I(CH2) values, the influence of the silicon (and oxygen) atom does not reach appreciably beyond the 6 CH2 group in this instance either. 

The hydrolysis rate is high under an acidic environment relative to that of condensation, and acid catalysts promote the development of more linear or polymer-like molecules in the initial stages. In addition to the pH of the reaction, the concentration of reagents and the size of the alkoxy group can also influence the hydrolysis and condensation reactions through a steric or leaving-group stability effect. As a result, species such as tetramethoxysilane (TMOS) tends to be more reactive than tetraethoxysilane... 

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