Silica imprinted polymer layers

Mechanical Stability. For application in HPLC and as catalysts the mechanical stability should be high. Especially polymers with ethylene glycol dimechacrylate as a crosslinking agent show good mechanical stability. Further improvement can be achieved by preparing these polymers on a silica support (see "Imprinted Polymer Layers on Silica"). 

Imprinted Polymer Layers on Silica. Wide pore silicas (mean pore diameter 500 and 1000 k) were surface-modified by 3-(trimethoxysilyl)propyl methacrylate by formation of siloxane bon s. To such a silica, layers of approximately 50 - 100 A thickness of the usual monomeric mixture containing 1 were applied (56,57). The monomeric mixture was radically polymerized giving silicas with pores coated... 

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... 

Burow and Minoura performed a similar kind of investigation to prepare protein imprinted polymers [48]. They used methacrylate modified silica particles as the carrier matrix on which imprinted sites were created. Using acrylic acid as the functional monomer and A,TV -1,2-diethylene bisacrylamide as the cross-linker, template polymerisation was carried out in the presence of glucose oxidase. This approach led to formation of a thin layer of cross-linked polymer film on the silica surface. After removing the template protein, substrate selectivity of the polymer was tested. Preferential affinity of the polymer for its template suggests the formation of substrate-selective binding sites in the polymer matrix. 

Optical isomer separations that are carried out on a chiral layer produced from C-18 modified silica gel impregnated with a Cu(II) salt and an optically active enantiomerically pure hydroxyproline derivative, on a silica layer impregnated with a chiral selector such as brucine,on molecularly imprinted polymers of alpha-agonists,or on cellulose with mobile phases having added chiral selectors such as cyclodextrins have been reported mostly for amino acids and their derivatives. Mixtures of sorbents have been used to prepare layers with special selectivity properties. 

Independent of our work, Mosbach et al. (78) prepared cavities in chin polymer layers on the surface of silica. They used certain dyes for imprinting. The polymer-modified silicas were used also for HPLC investigations. 

Chromatography with Imprinted Polymers. Chromatographic separations of the two enantiomers of phenyl-a-mannoside on a stationary silica phase with a polymer layer imprinted with 1 were performed 5T). In spite of the thin layer of polymer Che peaks were still rather broad. This cannot, therefore, be attributed to diffusion resistance in the polymer matrix, since these very thin layers should not show substantial diffusion hindrance. 

Preformed polymers can also be employed to prepare imprinted core-shell particles [143]. The group of Chang recently prepared a poly(amic acid) bearing oestrone as a template molecule covalently bound to the polymer through a urethane linker (see Fig. 2). A layer of this polymer was subsequently deposited on silica particles (10 pm diameter) prefunctionalised with amino groups at their surface. Thermal imidisation of the polymer yielded finally a polyimide shell (thickness about 100 nm) on the silica particles. Subsequent template removal yielded the imprinted cavities, which exhibited selective rebinding of oestrone in HPLC experiments. 

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). 

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