## 2 Thermo-Responsive Molecularly Imprinted Polymers
Besides the co-monomers mentioned above, the polymeric network was added with a cleavable cross-linker, such as \(N\),_N_-cystaminebis(acrylamide) (BAC), whose disulfur bridges in the pendant cystamine groups can be cleaved and oxidized to form a pair of sulfonic functions, or 2,3-dihydroxy-_N_,_N_',_N_'-tetramethyl _N_'',_N_''-bis(3-((2-methylacryloyl)aminopropyl)-1,4-butanediammonium) dibromide, whose C-C with vicinal diol can be cleaved and leading to a pair of adjacent tertiary amines. Since the pair of sulfonic functions or adjacent tertiary amines located face to face with each other, the hydrogels prepared in this case were capable of specifically recognizing divalent cations or molecules containing two anionic functions, respectively. Moreover, they showed significant volume phase transition and reversible recognition properties on varying the temperature. [PERSON] _et al_. proposed thermo-responsive imprinted polymer gels capable of recognizing l-pyroglutamic acid (PGA) and 4-aminopyridine (APY) molecules [43] through multiple-pointelectrostatic interactions. The gels were synthesized starting from _N_-isopropylacrylamide and methacrylic acid as temperature-responsive and functional monomers, respectively. In their subsequent investigations, reversible recognition and release of target molecule are both realized by a change in temperature. Since the imprinted polymer hydrogels have good thermal responsivity, selectivity and reusability, they are potentially applicable in the separation field and in drug controlled release systems. Recently, \"on/off\"-switchable catalysis by a smart enzyme-like thermo-responsive imprinted polymer was reported by [PERSON] _et al_. [54]. This unique imprinted polymer was also based on PNIPAm while being imprinted with _p_-nitrophenyl phosphate, a transition state analogue for the hydrolysis of the substrate _p_-nitrophenyl acetate. At a relatively low temperature (such as 20 \({}^{\circ}\)C), the polymer networks were hydrophilic, enabling the substrate to access the catalytic sites resulting in enhanced catalytic efficiency in the hydrolysis of _p_-nitrophenyl acetate. On the contrary, at higher temperatures (such as 40 \({}^{\circ}\)C), this polymer demonstrated a dramatically increased hydrophobicity, rendering the imprinted sites less accessible and hence resulting in poor catalysis. Thus, the polarity switch inherent in the use of a thermo-responsive PNIPAm endowed the polymer with a switchable catalytic activity. In consideration with the application of MIPs in the biological field, some biomacromolecules, e.g., proteins have also been used as a template to design thermo-responsive MIPs [24,30,38,48]. A thermo-sensitive macroporous PNIPAm hydrogel showing selective recognition for lysozyme was developed by an imprinting procedure based on metal coordinate interaction [35]. In contrast to a traditional functional monomer, a metal chelate monomer (_N_-(4-vinyl)-benzyl iminodiacetic acid)forming coordination complex with the template protein in the presence of Cu\({}^{2+}\) was used. Finally, this imprinted hydrogel was used to purify the protein template from an artificial mixture of proteins and areal sample, in both cases resulting in a high selectivity. Another example of lysozyme imprinting based on a PNIPAm nanogel was proposed by [PERSON] _et al_. [24]. AA and DMAPMA, containing a negatively charged carboxyl group and a positively charged tertiary amine group, respectively, were used as co-monomers and MBAAm as a crosslinker. Protein-imprinted spherical nanogel particles were readily prepared via aqueous precipitation polymerization. Compared to the non-imprinted counterparts, the lysozyme-imprinted nanogels possessed higher rebinding capacity, more rapid rebinding kinetics, and a much higher specificity toward lysozyme. Importantly, both the rebinding and release characteristics of lysozyme-imprinted nanogels showed dramatic temperature-dependence, with clear on-off transition around 33 \({}^{\circ}\)C. [PERSON] _et al_.