Surface-imprinted silica

Fig. 6.4. Chromatographic separation of d,l a-methylphenethylamine on a column packed with surface imprinted silica particles prepared by using L-phenylalanine as the template.

Other results involving liquid chromatography separations with imprinted metal oxides have been published in recent years. Norrlow et al. prepared surface imprinted silica gels functionalised with boronic acids that could form covalent linkages with riboses via boronate ester formation [43]. The gels were imprinted with templates containing two (nicotinamide adenine dinucleotide (NAD)) or four ribose units (bis-NAD 1, bis-NAD II). Columns were packed with the imprinted... 

A report by Lulka and Chambers [27] describes the properties of surface-imprinted silicas using A-acetyltryptophanamide (NATA) or fluorescein (Fig. 20.9) as templates. Silica gel was mixed with bis(2-hydroxyethyl)-aminopropyltriethoxysi-lane (as functional monomer), tetraethoxysilane and the template and the slurry... 

Markowitz, M. A. Kust, P.R. Klaehn, J. Deng, G. Gaber, B.P. Surface-imprinted silica particles the effects of added organosilanes on catalytic activity. Anal. Chim. Acta. 2001,... 

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. 

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... 

Yang and coworkers also used a surface imprinting approach to prepare nanotube membranes that exhibit selectivity for the molecule estrone.77 In this study, the silica nanotubes with pore diameters of 100 nm were synthesized within the cylindrical pores of nanopore alumina membranes. A covalent assembly approach was used to prepare the imprint.77 Zhang and coworkers have also recently shown how silica nanotubes can be used as an imprinting scaffold.78 In this example, trinitrotoluene (TNT)... 

The imprinting of metal oxides is a relatively new method for the immobilization of metal complexes. The case of Rh2(CO)4Cl2 on silica is illustrative. The dimer is first adsorbed onto the surface of silica, most likely through a surface hydroxyl group, and then reacted with trimethylphosphite, P(OMe)3. Amorphous silica is then... 

In addition it was found that molecules which only differ in one methyl group can be distinguished [48] and that there seemed to be a lower limit for a suitable template size in these systems since, e.g., dimethylaniline and diethylani-line exhibited the same adsorption power [37]. The most recent trend in imprinted silicas uses aluminium ions as dopants to introduce more specific interactions with the template. The silica gel is only surface-modified in the presence of e.g. phenanthrene by treatment with diazomethane. After removal of the template, the silica shows a stronger retention for all polycyclic aromatic compounds, but no particular affinity towards the template [52]. 

Waksmundzki et al. extensively examined the surface areas and microporosities of imprinted silica surfaces [44]. It was found that although the template itself had little effect on the total surface area, the sizes of the micropores were positively correlated to the size of the template. Subsequent studies on the sorption of template to silicas imprinted with pyridine [45-50], quinoline and acridine [45-47], and 2-picoline, 2,4-lutidine and 2,4,6-collidine [50], combined with thermodynamic studies on the heat of wetting of template or methanol/water sorption [47,51-53], led to the conclusion that these templates were adsorbed as multilayers to the silica. This observation supported the association mechanism hypothesis. The possibility of a footprint mechanism and an association mechanism coexisting in a concentration dependent fashion does not appear to have been considered. 

Fig. 8.7. Surface imprinting with a covalently coupled diimine template has been used to position two amines onto a flat silica surface in an effort to isolate the effect of functional group placement in molecular imprinting. The rectangular spacer between the imines represents either a phenyl or diphenylmethane. Adapted from [37].

Surface imprinting has recently been used in our laboratory to prepare phosphate and phosphonate receptors in silica xerogels [38]. In an effort to obtain faster on/off binding kinetics and gain thermodynamic control of the host-guest interaction, we positioned the receptor sites as close as possible to the gel surface. This resulted in observed equilibration binding times of less than a minute. 

Bass JD, Katz A (2006) Bifunctional surface imprinting of silica Thermolytic synthesis and characterization of discrete thiol-amine fnnctional gronp pairs. Chem Mater 18 1611... 

N-acetylated conotoxins Silica gel bis(2-hydroxyethyl)-aminopropyltriethoxysilane Surface imprinting on silica [123]... 

Phenylphosphonic acid Tetraethoxysilane-based gel EtOH 1-trimethoxysilyl-propyl-3-guanidium chloride or 3-trimethoxysilylpropyl-1-trimethylammonium chloride Surface imprinting on silica xerogels guanidium or ammonium surface-functional zation solid-state 31P NMR characterization [125]... 

Example 8.2 Imprinting of CyD on the surface of silica gel support (Fig. 8.2.) [4] Although a conventional crosslinking agent such as MBAA does not provide the polymer with enough stiffness as a stationary phase of HPLC, a silica gel support can reinforce this soft polymer. By introducing vinyl group on the silica-gel surface, the imprinted polymer can easily be immobilized on its surface by the conventional polymerization. 

Matrices used for molecular imprinting are not necessarily restricted to organic materials, and inorganic materials are also available [11, 12]. Modification of the surface of silica gel for the stationary phase of HPLC is known [13]. Recently, unique molecular imprinting by the use of inorganic materials showing various characteristic properties has been proposed. 

Fig. 8-9 Schematic illustration of the preparation of catalytic silica by surface imprinting in the presence of TSA as a template by use of the water-in-oil emulsion technique...

Morihara s research group worked very intensively on preparing catalysts on the surface of silica gel (see, e.g., [113, 148]). In a process described as footprint catalysis , commercial silica gel is treated with Al ions and imprinted with a transition-state analogue. Similar accelerations to those of the foregoing examples were obtained. 

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