Silica direct imprinting

Direct Imprinting in Silica. As already pointed out at the beginning Dickey (2A) used the imprinting procedure to produce selectively adsorbing silicas. Other research groups applied this technique to various substrates (106, 107). and it was also possible to prepare silicas imprinted by optically active substances and achieve some enantiomeric resolution (27-29 108). 

Markowitz et al. developed a different approach, again in an attempt to overcome some of the inherent difficulties that arise when imprinted bulk materials are used as catalysts [82], Here, the authors used a template-directed method to imprint an a-chymotrypsin TSA at the surface of silica nanoparticles, prepared with a number of organically modified silanes as functional monomers. Silica particle formation was performed in a microemulsion, where a mixture of a non-ionic surfactant and... 

After a steady flow of publications over a period of 15 years, the interest in imprinted silica experienced a temporary decline (it has been pursued more recently by several groups). We have not been able to find any direct explanations from studying the literature, other than those put forward in the excellent review by Bartels and Prijs [31], which principally relate to the limitations in stability and reprodueibility of the imprinted silica materials. 

Among the many desirable properties that are obtained with sol-gel formed metal oxide materials, it is perhaps the hydrophilic/hydrophobic duality of the matrix that allows the successful imprinting of molecular species. In the formation of the gel, templates will direct the placement of siloxane and silanol groups to complementarily interact with various hydrophobic and hydrophilic sites on the template. Figure 8.4 shows a possible form of an imprinted site for propyl orange in a silica matrix. Various aspects of molecular imprinting, such as these concepts, will be explored in the following sections. 

Plunkett and Arnold [3] reported imprinted supports based on metal-chelate interactions made in the pores of 10 pm Lichrosphere beads. These were highly stable, even after prolonged use for several hundred hours, and achieved reasonable separations of related Z w-imidazole model compounds. Unfortunately no direct comparison with the performance of bulk-imprinted material was presented. A photograph of part of a silica bead containing imprinted polymer is shown in Fig. 12.1. 

Markowitz MA, Kust PR, Deng G, Schoen PE, Dordick JS, Clark DS, Gaber BP (2000) Catalytic silica particles via template-directed molecular imprinting. Langmuir 16 1759... 

Chitosan has been used as a molecular template to promote formation of nanosized PPy particles on silica.171 Interestingly, another biomolecule (Heparin) has been shown to have a similar templating effect. In the case of Heparin, it acts as both the molecular dopant and a (nano) structure-directing template.172 Interestingly, the fibrils continue to grow in this form even after they leave the constraints of the membrane,173 indicating that once a pattern of growth is imprinted, it continues. 

Imprinting approaches using inert clusters to direct attachment of ligands at appropriate points on the silica matrix, followed by removal and replacement of the inert metal ions with more reactive metals present a possible alternative to direct immobilization of labile complexes. Such approaches are largely unexplored in sol-gel silica materials, generally because there are significant difficulties associated with synthesis of suitably modified (i.e., silylated)... 

However, an alternative to using surfactant systems is to use nanopartide-based PSPs directly. They are more compatible with mass spectrometric (MS) detection and do not hamper electrospray ionization (ESI) [49]. In that respect, nanopartides from silica [50], gold [51] and polymers [52] and even molecularly imprinted nanoparticles [53] have been used. Imprinting is based on a technique for tailor-making network polymers, where templates for a specific solute give a high separation affinity. 

Surface functional monomer-directing strategy for the highly dense imprinting of 2,4,6-trinitrotoluene (TNT) molecules at the surface of silica nanoparticles... 

Gao, D. Zhang, Z. Wu, M. Xie, C. Guan, G. Wang, D. (2007). A surface functional monomer-directing strategy for highly dense imprinting of TNT at surface of silica nanoparticles. Journal of the American Chemical Society, 129, 7859-7866. 

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