Silica molecularly imprinted

The sorbents that are most frequently used in environmental analysis are Cig-silica based sorbents, polymeric sorbents (usually styrenedivinilbenzene) and graphitized carbon. In order to increase the selectivity of these sorbents, immunosorbents (35, 36) have been developed and used with good results, while recently, molecularly imprinted polymers have started be to used (35, 36). 

A. Katz and M. E. Davis, Molecular imprinting of bulk, micro-porous silica, Nature, 2000, 403, 286. 

Molecular imprinting is not limited to organic polymer matrices, but can also be applied to silica-based materials and even proteins. Proteins freeze-dried in the presence of a transition state analogue as template have been used successfully as catalysts, e.g., for the dehydrofluorination of a fluorobutanone. For instance, lyophilized 3-lactoglobulin imprinted in this manner with N-isopropyl-N-ni-trobenzyl-amine could accelerate the dehydrofluorination by a factor of 3.27 compared to the non-imprinted protein see Table 5 [62]. In a similar procedure, BSA was imprinted with N-methyl-N-(4-nitrobenzyl)-S-aminovaleric acid and showed an enhancement of the catalytic effect by a factor of 3.3 compared to the control protein for the same reaction see Table 5 [113]. 

Silica particles surface-imprinted with a TSA of a-chymotrypsin were applied for the enantio-selective hydrolyzation of amides. Surprisingly, the particles showed reverse enantio-selectivity, i. e., the sol-gel imprinted with the L-isomer of the enzyme s TSA showed a higher selectivity for the D-isomer of the substrate [125]. Also Ti02 gels have been imprinted, e.g., with 4-(4-propyloxypheny-lazo)benzoic acid. QCM coated with ultrathin films of this gel were prepared by an immersion process and showed selective binding of the template [ 126]. These examples demonstrate once more the broad applicability of the concept of molecular imprinting. 

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

Figure 3. Synthetic scheme to produce silica containing site-isolated aminopropyl groups through molecular imprinting approach [29].

Chang DK, Chul O, Oh S-G, Chang JY. The use of a thermally reversible bond for molecular imprinting of silica spheres. J Am Chem Soc 2002 124 14838-14839. 

Katz A, Davis ME. Molecular imprinting of bulk, microporous silica. Nature 2000 403 286-289. 

Metal nanoparticles have also been included into MIPs. Such particles can be used, for example, as nanoantennae for the enhancement of electromagnetic waves (plasmonic enhancement). It has been shown by He et al. [122] that a thin layer (20-120 nm) of testosterone-imprinted silica could be synthesized around 350 nm silver particles in a controlled way. The composite material showed specific binding of the testosterone target. Matsui et al. [123] reported a molecularly imprinted polymer with immobilized Au nanoparticles as a sensing material for spectrometry. The sensing mechanism is based on the variable proximity of the Au nanoparticles... 

Another nice example of nanostructuring an MIP layer is the work published by Wu et al. [138, 139] who developed a label-free optical sensor based on molecularly imprinted photonic polymers. Photonic crystals were prepared by self-assembly of silica nanospheres. The space between the spheres was then filled with MIP precursor solution. After polymerization, the silica was dissolved, leaving an MIP in the form of a 3D-ordered interconnected macroporous inverse polymer opal (Fig. 15). The authors were able to detect traces of the herbicide atrazine at low concentrations in aqueous solution [139]. Analyte adsorption into the binding sites resulted in a change in Bragg diffraction of the polymer characterized by a color modification (Fig. 15). 

Toth B, Laszlo K, Horvai G (2005) Chromatographic behavior of silica-polymer composite molecularly imprinted materials. J Chromatogr A 1100(l) 60-67... 

Abstract Molecular imprinting has grown considerably over the last decade with more and more applications being developed. The use of this approach for the generation of enzyme-mimics is here reviewed with a particular focus on the most recent achievements using different polymer formats such as microgels and nanogels, beads, membranes and also silica nanoparticles. 

In a reported example the precursor 29 reacted with an excess of TMOS (TMOS/precursor = 150) to achieve molecular imprinting in silica. A chemical... 

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