Synthesis And Controlled-release Performance Study Of Thermosensitive ?-lipoic Acid Molecularly Imprinted Polymer

This study aims at preparing molecularly imprinted polymers(MIPs) which can adsorb the “universal antioxidant”—?-lipoic acid(ALA) selectively, with ?-lipoic acid as template, chitosan(CS) as functional monomer, epichlorohydrin and glutaraldehyde as two types of cross-linkers. The adsorption capacity plays a critical role in optimization of the assembled mole ratio of ?-lipoic acid and chitosan. The morphology and chemical structure were characterized by scanning electron microscope(SEM) and fourier transform infrared spectroscopy(FTIR). Furthermore, the adsorption kinetics and static adsorption experiments were conducted to study the selectivity of MIPs to ALA, so as to discuss the recognition mechanism and evaluate the recognition performance. After the thermosensitive compounds including PVA and NIPAM being entangled with CS or grafted onto CS, two novel thermosensitive MIPs were prepared at the optimized molar ratio of ALA and CS. The morphology and chemical structure were characterized by scanning electron microscope(SEM) and fourier transform infrared spectroscopy(FTIR). Finally, ALA release kinetics was researched at different temperatures. Both thermosensitive MIPs play an important role for the development of innovative advanced food packaging materials that combine the advocated advantages of MIPs with antioxidability of ALA.Two novel MIPs were synthesized with ?-lipoic acid as template, CS as functional monomer, epichlorohydrin and glutaraldehyde as two types of cross-linkers. Glutaraldehyde being as cross-linker, the first MIP was synthesized with different mole ratios of CS(—NH2) and ALA and then freeze dried. Then the template was removed by elution with methanol/acetic acid(9:1, v/v). Static adsorption results showed that MIPs made by CS(—NH2):ALA=5:1 has maximal adsorption capacity(19.72 mg/g). The adsorption process reached equilibrium after 5 hours. Comparatively, the maximal adsorption capacity of NIPs was only 8.85 mg/g and adsorption equilibrium reached in 3 hours. Scatchard analysis demonstrated that both MIPs and NIPs have single binding site to the template, their linear regression equations were Q/Qs(MIPs)=646.32-21.56 Q and Q/Qs(NIPs)=225.71-17.49 Q, respectively. FTIR analysis illustrated hydrogen bonds formed between ALA and —OH of CS, and —C=N— formed for cross-link between glutaraldehyde and —NH2 of CS. Another MIP was prepared via emulsion polymerization with different mole ratios of CS(—NH2) and ALA. After the template being eluted by methanol/acetic acid(9:1, v/v), adsorption studies demonstrated that MIPs made by CS(—NH2):ALA=5:1 has maximal adsorption capacity(13.60 mg/g). Both adsorption processes of MIPs and NIPs reached equilibrium after 5 hours. However, adsorption capacity of NIPs to ALA was just 5.02 mg/g. Scatchard analysis demonstrated that both MIPs and NIPs have single binding site to the template with linear regression equations of Q/Qs(MIPs)=332.08-18.44 Q and Q/Qs(NIPs)=81.61-8.37 Q, respectively. FTIR analysis illustrated hydrogen bonds formed between ALA and —OH or —NH2 of CS.The first thermosensitive molecularly imprinted hydrogel was synthesized with PVA as thermosensitive monomer. The release kinetics of ALA from this hydrogel was studied at two different temperatures. Specifically, after one type of thermosensitive monomer(PVA) being entangled with CS, the first thermosensitive MIP was synthesized with ALA and glutaraldehyde as template and cross-linker respectively and freeze dried. Swelling experiments showed that the swelling equilibrium reached in 30 min and this MIP had highest swelling rate in solutions of pH=3.00. Furthermore, the stronger acidity of the solutions, the higher swelling rate of MIPs. The ALA release kinetics of this MIP in phosphoric buffer solutions of pH=3.00, 4.00 and 5.00 demonstrated that ALA released most quickly in 1st day but more slowly from 2nd day to maximum. Moreover, the final release capacity in pH=5.00 was more than those in pH=3.00 and 4.00. FTIR results showed that hydrogen bonds formed between CS and PVA and PVA was just entangled around the CS molecule chain. The ALA release kinetics of this MIP at 20? and 30? demonstrated that the released amount at 20? was more than that at 30?.The second thermosensitive MIP was prepared with CS-g-NIPAM(NIPAM grafted onto CS) as thermosensitive monomer. Then the release kinetics of ALA from this MIP was studied at two different temperatures. Specifically, After NIPAM being grafted onto CS to be CS-g-NIPAM by the initiator ceric ammonium nitrate, the second thermosensitive MIP was prepared with ALA as functional monomer, ECH as crosslinker at the optimized molar ratio of CS and ALA. That NIPAM being grafted on —NH2 of CS was evidenced by FTIR. Furthermore, DSC analysis demonstrated that lower critical solution temperature(LCST) of this thermosensitive MIP was between 21? and 22?. The ALA release kinetics of this MIP showed that the released amount at 15? was much more that at 35?.Based on the aforementioned research, two novel thermosensitive MIPs were prepared. The ALA release kinetics depended on temperatures, namely temperature variation could lead to variation of ALA release amount. This study could promote the development of thermosensitive antioxidant active packaging materials.