## 4 Photo-Responsive Molecularly Imprinted Polymers
The results showed that the photo-isomerization was greatly related to the spacer length of the both ends of the cross-linkers, that is the rate of photo-isomerization gradually increased with spacer length. However, the template binding capacity of the imprinted receptors dropped with the increasing flexibility of the hydrogels. In an optimal condition, the MIP hydrogels showed excellent specific binding capacity and repeatable photo-regulating release/uptake properties in biocompatible aqueous media. In addition, the relative low cross-linking degree and good optical transparency in aqueous media awarded the MIPs hydrogels faster photo-responsive rate, moreover, the use of water-soluble functional monomer and cross-linkers demonstrated their wider applications in the biological field. [PERSON] _et al._ also used a water-soluble azobenzene derivative containing two carboxylic acid groups (-COOH), 5-((4-(methacryloyloxy)phenyl) diazenyl)isophthal acid (MAPDIA) as the functional monomer, and developed a photo-responsive MIP for guanine using a new water-soluble triethanolaminetrimethacrylate (TEAMA) as the cross-linker [79]. The MIP can release and take up guanine with irradiation at 365 and 440 nm, respectively. In addition, the MIP was successfully applied to the photo-controlled solid-phase extraction of guanine from complex real samples (beer) with good recovery. Subsequent photo-controlled release of guanine in aqueous media allowed for convenient quantitative analysis with high efficiency. Recently, [PERSON] also demonstrated a new photo-responsive molecular recognition system for 2,3,4,5,7,8,9,10-octafluorophenazine (PAF) uptake through fluorine-fluorine interaction [80]. The new kind of photo-responsive functional monomer containing fluorine-substituted azobenzene chromophore, named (4-methacryloyloxy)nonafluoroazobenzene (MANFAB), was designed and synthesized for the MIPs. The reversible release and uptake of PAF from toluene can be photo-regulated by alternate irradiation at 315 and 440 nm, indicating a possibility for photo-responsive molecular recognition directed by fluorine-fluorine interaction. In addition, [PERSON] _et al._ synthesized a novel photo-responsive functional monomer bearing diaminopyridine and azobenzene moieties, which was applied to the preparation of photo-regulated MIPs for recognizing porphyrin derivatives carboxylic acids through hydrogen bonding [26]. [PERSON] _et al._ designed a photo-responsive cross-linker with bisurea binding functionalities serving as well as a functional monomer towards dicarboxylate moieties (Figure 8) [68]. Despite of the rigid polymer, the photo-isomerization properties of the azobenzene chromophore are retained. This new approach to form imprinted sites can be used for non-covalent MIPs preparation with potential improved performance, so that more functionality can be introduced without suffering affinity performance losses due to reduced cross-linking. For photo-responsive materials, the efficient area of irradiation on the materials is one of the most crucial factors for rapid and high-efficiency photo-isomerization. Therefore, besides the materials with a great transparency [70] or prepared as a membrane with large exposed area, micro/nano-particle materials are also appropriate candidates for photo-responsive MIPs because of their large specific surface area. However, photo-responsive bulk MIPs always need a time-consuming grinding and sieving procedure to particles, as well leading to great loss of binding sites [113]. Precipitation polymerization has proven very versatile for preparing MIPs micro-/sub-microspheres for its easy operation and lack of need for any surfactant or stabilizer. Recently, [PERSON] _et al._ described a successful preparation of azobenzene (azo)-containing MIP microspheres with photo-responsive template binding properties via precipitation polymerization [74].