The design, fabrication, characterization and application of photo-responsive moleculary imprinted materials containing azobenzene chromophores

Abstract

Receptor sites that are capable of recognizing specific molecular species can be conveniently imprinted into polymer matrices via a template-directed polymerization technique known as molecular imprinting. With a suitable choice of functional monomers for the fabrication of molecularly imprinted polymers (MIPs), regulation of substrate affinity of imprinted receptor sites by externally applied stimuli should be possible. Light irradiation is one of the most frequently adopted external stimuli for stimuli-responsive polymeric materials (SRPM) as it is convenient to apply and easy to control. Amongst the numerous well-established photo-responsive molecular systems, the UV-visible photoinduced trans-cis isomerization of azobenzene and its derivatives is probably the most extensively studied. The trans-cis photoisomerization of azobenzene brings about large changes in molecular geometry and dipole moment to the chromophore. When incorporated into imprinted receptor sites, the photoinduced configurational changes should bring about significant alteration of receptor geometry and, hence, affect host-guest binding. In this Ph.D. thesis, the feasibility of such a molecular imprinting approach to induce photo-regulatable responses in functional polymer materials is explored. A series of functional monomers with azobenzene chromophores are studied for their photoisomerization properties and their ability in generating photo-responsive molecularly imprinted materials. Chapter 1 introduces the theory of molecular imprinting, from its concept, the imprinting process, optimization of polymerization parameters to the evaluation and characterization of MIPs. Literature examples of applications of MIPs are also given. Chapter 2 is a brief review of stimuli responsive polymeric materials, especially on those photo-responsive materials that contain azobenzene chromophores. Chapter 3 concerns the fabrication, characterization and photo-regulated chemical release and uptake properties of a photo-responsive molecularly imprinted polymer (MIP) material based on a specially designed azobenzene-based functional monomer, 4-[(4-methacryloyloxy)phenylazo]benzonic acid (MPABA). The trans-cis photoisomerization properties of MPABA were found to be retained after incorporation into the rigid three-dimensional cross-linked polymer matrix of the MIPs. Caffeine was adopted as the molecular template for imprinting. Substrate affinity of the MIP receptor sites was found to be photo-switchable. This is attributable to the photoisomerization of azobenzene chromophores within the MIP receptors that alters their geometry and the spatial arrangement of their binding functionalities. The favorable binding constant of the MIP receptors for caffeine was 5.48 × 104 M-1. Density of the caffeine-specific receptor sites in the MIP material was 0.95 μmol/g-MIP. Upon irradiation at 365 nm, 58.3 % of receptor-bound caffeine was released from the MIP material. Subsequent irradiation at 440 nm caused 96.4 % of the released caffeine to be rebound by the MIP material. This nearly quantitative uptake of the released caffeine indicates the reversibility of the receptor site configuration and substrate affinity in the course of photo-switching of their azobenzene chromophores. Although the photo-regulated substrate release and uptake processes were generally repeatable, gradual reduction in substrate release and rebinding was observed. This may be attributed to the slow deformation of MIP receptors during the course of repetitive photo-switching. L-Tryptophan methyl ester (L-Trp-OMe) possesses stable fluorescence properties and can be excited at wavelengths different from those used for the photo-switching of azobenzene chromophores. Thus, it is considered a convenient analyte for the monitoring of the photo-regulated chemical release and uptake process of photoresponsive MIPs. In this context, Chapter 4 reports the fabrication and photoswitching studies of another azobenzene-based photo-responsive MIP material based on MPABA for L-Trp-OMe. Density of receptor sites in the MIP material was 20.24 μmol/g-MIP. The favourable binding strength of the imprinted receptor sites in the MIP for L-Trp-OMe was found to be 9.56× 103 M-1. This substrate affinity can be photo-regulated by illumination at 365 nm and 440 nm. There was no obvious reduction in substrate binding strength after 9 recycles of the photo-regulated substrate release and uptake process. Compounds having structures analogous to that of L-Trp-OMe were also tested. The relative extent of release and uptake was clearly smaller than that in L-Trp-OMe. Chapter 5 reports another photo-responsive imprinted polymer fabricated from an azobenzene-containing functional monomer 4-[(4-methacryloyloxy)phenylazo] pyridine (MPAP) that uses a basic pyridine functionality for interaction with acidic substrates. In this case, we have used 2,4-dichlorophenoxyacetic acid (2,4-D) as substrate. Trans – cis and the reverse cis – trans isomerization of the chromophore were achieved by irradiation at 330 and 440 nm respectively. Scatchard analysis revealed that the favourable substrate binding constant was 2.49 × 103 M-1, and the density of imprinted binding sites was 7.9 μmol/g-MIP. Like the other photoresponsive MIPs in the previous chapters, substrate affinity for the imprinted binding sites for 2,4-D can be photo-regulated. This demonstrates the versatility of this class of azobenzene chromophore--containing MIP materials for the photo-controlled transfer of chemical substrates. Chapter 6 reports a water-soluble azobenzene-containing functional monomer 4-[(4- methacryloyloxy)phenylazo]benzenesulfonic acid (MAPASA) for the fabrication of a photo-responsive molecularly imprinted hydrogel material that can function in the bio-compatible aqueous media. Paracetamol, N-(4-hydroxyphenyl)acetamide, was adopted as the molecular template for the imprinting. Photoisomerization of the hydrated sulfonated azobenzene chromophores within hydrogel was found to be sterically demanding highly dependant upon the nature of cross-linkers. A series of bisacrylamide and bismethacrylamide cross-linkers were used to crosslink MAPASA to produce hydrogels that are optically transparent in aqueous media. When incorporated into the relatively confined and rigid hydrogel environment generated by the most commonly adopted polyacrylamide cross-linker, N,N’- methylenebisacrylamide, these chromophores were not able to undergo photoisomerization. Lengthening the spacers separating the polymerizable acrylamide/methacrylamide functionalities at both ends of the cross-linkers from ethylene to octylene enhanced the flexibility of the resultant hydrogel matrices and resumed the photoisomerization properties of the chromophores. The rate of photoisomerization gradually increased with spacer length. On the other hand, substrate binding strength of the imprinted receptors dropped with the increasing flexibility of the hydrogels. To balance these factors, the cross-linker N,N’- hexylenebismethacrylamide was subsequently selected as the optimal cross-linker for the fabrication of the photo-responsive imprinted hydrogel. Scatchard analysis revealed that the specific and non-specific binding strength of the resultant imprinted hydrogel was 1.96 × 105 and 747.0 M-1, respectively. The density of the imprinted receptors in the hydrogel was 0.47 μmol g-1. Affinity of the hydrogel for paracetamol can be photo-regulated. Upon irradiation at 353 nm, 83.6 % of receptor-bound paracetamol was released from the imprinted hydrogel. Subsequent irradiation at 440 nm caused 94.1 % of the released paracetamol to be rebound by the hydrogel again. Such a photo-regulated release and uptake process is repeatable. Chapter 7 reports the photo-responsive covalently imprinted material for ibuprofen via sol-gel fabrication. After imprinting, the sol-gel organic-inorganic hybrid material was hydrolysis in acid to remove ibuprofen. The new molecularly imprinted sol-gel hybrid material was found to behaviour similarly to the previous organic polymer based photo-responsive MIPs. The favourable binding strength of the imprinted receptor sites in the MIP for ibuprofen was found to be 2.28 x 103 M-1. Density of receptor sites in the MIP material was 4.0 μmol/g-MIP. The release and uptake of ibuprofen from THF solution by the material can be photo-regulated by irradiation at 365 and 440 nm, respectively. The concluding chapter (Chapter 8) summarizes what we have achieved in our work on the application of molecular imprinting to fabricate smart responsive materials.