## 2 Thermo-Responsive Molecularly Imprinted Polymers
Correspondingly, the phase transition from swollen state to collapsed state occurs in water when a moderate cross-linked PNIPAm network is used around the LCST [88,92]. This feature endows the MIPs containing PNIPAm blocks the fascinating property of regulatory binding capacity accompanied by the size changes of binding sites derived from temperature variation. To date, PNIPAm has been the most employed synthetic polymer to prepare thermo-responsive MIPs, due to its LCST which is close to the temperature of the human body [93]. One of the earliest thermo-responsive MIPs was reported by [PERSON] _et al._[87]. The MIPs based on PNIPAm hydrogels were prepared via redox polymerization of NIPAm, AAc, MBA, and template such as dl-norepinephrine hydrochloride or dl-adrenaline hydrochloride in 1,4-dioxane. The imprinted and non-imprinted gels showed a volume change in aqueous solution as a function of temperature but still retained the molecular recognition ability in the shrunken states. When the guest molecule was present in a saturated solution, the polymers exhibited another phase (\"molecular recognition phase\"), whose volume was responsive to the concentration of the guest molecule. Further on, [PERSON]'s group reported a similar imprinted polymer gel based on PNIPAm that swelled and shrank reversibly in response to temperature changes. The addition of a small amount of functional monomer such as methacrylamidopropyltrimethylammonium chloride (MAPTAC) resulted in gels able to capture target molecules such as pyranine-3 or pyranine-4 via a multiple-point electrostatic interaction [94]. From the relationship between swelling ratio and affinity of the gel as a function of temperature (Figure 2), it is seen that the recognition of the resulting polymeric gels towards the target molecule can be changed reversibly by more than one order of magnitude, accompanied by reversible swelling and shrinking of polymeric network in response to temperature. In their shrunken state, the imprinted sites of the gels exhibit higher affinity presumably due to the ability of the templates to ion-pair with three (pyranine-3, charge: 3\(-\)) or four (pyranine-4, charge: 4\(-\)) MAPTAC monomers in the gel. While in the swollen state, the affinity of the imprinted sites decreases dramatically because MAPTAC monomers are well separated from one another, which means the target molecules can only be captured by MAPTAC with a single contact. This reversible molecular adsorption by using multiple-point interaction based on PNIPAm gels definitely provides a pioneering idea to prepare thermo-responsive MIPs, and the MIPs with reversible  
Figure 2: (**a**) The chemical structures of pyranine-3 and pyranine-4; (**b**) The affinity of the gels to pyranine-3 and pyranine-4 as a function of temperature. Reprinted with permission from Reference 94. Copyright 1999, Science. recognition towards various target molecules such as ions, small organic molecules, and even proteins were realized by using the same approach. [PERSON] _et al_. have reported a PNIPAm based temperature-sensitive polymeric gel to adsorb and release divalent ions reversibly by two-point interaction [50]. The imprinted gels prepared with calcium methacrylate (CaMAA2) or lead methacrylate (PbMAA2) showed higher affinity for target molecules as compared to randomly polymerized gels containing methacrylic acid (MAA) or lithium methacrylate (LiMAA) as functional monomers. Due to the reversible volume phase transition caused by temperature, the affinity decreased in the swollen state but recovered upon shrinking. A slightly different procedure was also employed to prepare temperature-responsive imprinted polymers for capturing divalent cations or anions without using a template [95].