## 4 Photo-Responsive Molecularly Imprinted Polymers  
Photo irradiation is one of the most frequently adopted external stimuli for stimuli-responsive polymeric materials. Those photo-sensitive materials commonly contain photosensitive chromophores, such as azobenzene [106], stilbene [107], spiropyran and spitooxazines [108] and fugides [109]. Figure 6: pH effect on the catalytic rate of the imprinted hydrogels in Tris-HCl buffer. Reprinted with permission from Reference 64. Copyright 2010, Wiley. Amongst the numerous photosensitive chromophores, the UV-visible photo-induced _trans-cis_ isomerization of azobenzene and its derivatives is probably the most extensively studied for its chemical stability, large change in dipole [110], and high quantum yield [111]. The azobenzene chromophore exhibits two isomeric states, the thermodynamically more stable _trans_-isomer and the meta-stable _cis_-isomer. The _trans_-form is converted into the _cis_-isomer by UV light irradiation or heat, and the _cis_-isomer can return to the _trans_-form photo-chemically under visible light irradiation or thermally in the dark (Figure 7). Since the _trans-cis_ photoisomerization of azobenzene brings about large changes in molecular geometry and dipole moment to the chromophore [112], when the azobenzene chromophore was incorporated into imprinted receptor sites, the photo-induced changes in orientation and space should result in a significant alteration of receptor geometry and, consequently, affect the binding capacity of the host-guest. The first successful precedent for designing photo-responsive MIP membrane was demonstrated by [PERSON] _et al._ using a derivative of azobenzene, \(p\)-phenylazoacrylanilide (PhAAAn). The mixtures of EGDMA and tetraethylene glycol diacrylate (TEGDA) were used as the cross-linkers due to the formation of mechanically stable, but flexible, polymer membranes. Since photo-isomerization of a chromophore had a great influence on the binding site, the resulted MIP membranes exhibited a reversible absorption/release of the template dansylamide (DA), and a better selectivity towards its analogue, dansyl-l-leucine (dl) and \(N\),\(N\)-dimethylnaphthylamine (DMN). However, the binding capacity and the selectivity of the MIP membranes were not high, which may be attributed to the weaker interactions between the azobenzene functional monomers and target molecules [82]. For the purpose of a more powerful interaction between template and functional monomer, [PERSON] _et al._ employed an novel azobenzene-based functional monomer, 4-((4-methacryloyloxy)phenylazo)benzoic acid (MPABA), which contained a carboxyl group on the para-position of benzene ring [67]. The authors made a detailed study on the influence of cross-linker/monomer ratio, and found that lower cross-linker/monomer ratios (e.g., 1:1 and 3:1) displayed no photo-isomerization properties, which was attributed to the higher chance for azobenzene monomers to aggregate and pack with each other, leading to the reduction of free volume necessary for the reorientation of azobenzene chromophores during their isomerization. The optimized bulk MIP material possessed the ability of reversible release and caffeine  
Figure 7: Schematic figure of the _trans-cis_ isomerization of azobenzene. uptake more than four times greater upon irradiation at 365 and 440 nm, respectively, which demonstrates its potential applications in smart separations and drug delivery systems. Various interactions have been employed in photo-responsive molecular recognition systems. By the same group, a water-soluble azobenzene-containing functional monomer 4-(((4-methacryloyloxy)phenylazo) benzenesulfonic acid (MAPASA) was developed for the fabrication of a photo-responsive molecularly imprinted hydrogel material through electrostatic interaction [70]. The photo-responsive MIP hydrogels were designed by cross-linking with various bisacrylamide and bismethacrylamide.