Studies On Preparation And Sustained Release Of Thermosensitive ?-Lipoic Acid Molecularly Imprinted Polymers

The density functional theory(DFT),B3 LYP level and Gaussian 09 were applied to simulate and analyze the self-assembly system of the molecularly imprinted polymers with ?-lipoic acid(ALA)as template molecule and NIPAM as functional monomer based on quantum chemistry,and ALA-MIPs were prepared with ethylene glycol dimethacrylate(EGDMA)as cross-linkers,the azodiisobutyronitrile(AIBN)as the initiator and acetonitrile as solvent by non-covalent imprinting bulk polymerization using a heating initiation method,the sensitivity,microstructure,infrared spectroscopy and ALA content of ALA-MIPs was characterized,and the adsorption and release characteristics of ALA-MIPs were explored,which provided the basic theoretical support for the research of active packaging materials based on thermo-sensitive molecular imprinting materials.The results show that the active sites of ALA are mainly O atom of carboxyl group and H atom of hydroxyl group,while the active sites of NIPAM are mainly O atom of carbonyl group and H atom connected with N atom.In the prepolymerization of ALA-MIPs,ALA and NIPAM bind mainly via hydrogen bonding interaction.The number of hydrogen bonds did not increase to maximum until the molar ratio of the template and functional monomer reached to 1: 2,with strong hydrogen bonds,and the maximum binding energy of 345.86068 kJ mol-1.The results indicate that the template and functional monomer are expected to form a more stable complex at the above molar ratio.The triggering variation has a great influence on the thermo-sensitivity of ALA-MIPs.The thermo-induced polymerization of ALA-MIPs is thermo-sensitive due to the formation of poly N-isopropylacrylamide(PNIPAM),and the LCST of the prepared ALA-MIPs was 22.4 °C.However,the synthesized ALA-MIPs triggered by UV do not show any thermo-sensitive properties.The yield of ALA-MIPs and ALA content thereof are affected by the imprinting ratios.When the molar ratio of ALA and NIPAM reached to 1:2,the ALA content in ALA-MIPs was 14.443 mg/g with the yield of 57.34%.From the SEM images of NIPs and ALA-MIPs,it can be seen that the micro-morphology of ALA-MIPs has more three-dimensional holes,which has larger specific surface area and pore volume than NIPs.The infrared spectra results demonstrate that the carboxyl groups in ALA bind with the secondary amide groups in NIPAM in the forms of hydrogen bonds,indicating the formation of binding sites between ALA and NIPAM.The adsorption capacity of NIPs was significantly lower than that of ALA-MIPs.Moreover,Scatchard analysis of the static adsorption processes demonstrated that there was only one type of binding sites for ALA binding in NIPs compared with two different types in ALA-MIPs,with the equilibrium dissociation constants of 6.9697 × 10-5 mol/L and 1.5585 × 10-5 mol/L,respectively.Additionally,data indicate that the maximum adsorption capacities were 12.145 mg/g and 104.063 mg/g,respectively.The saturated adsorption capacity of ALA-MIPs to ALA was significantly higher than that of ALA-MIPs on DHLA and PBA.The imprinting factors of ALA-MIPs against ALA,DHLA and PBA indicated that ALA-MIPs have good adsorption selectivity to ALA and ALA-MIPs have better adsorption selectivity to ALA compared with NIPs.The release of ALA from ALA-MIPs at different temperature belongs to bi-directional restricted release.The cumulative release rate of ALA at 10~22 ? against time can be fitted by the first order equation(correlation coefficient is 0.9653,0.9775,0.9595),thereby indicating that the ALA release performance conforms to the first order kinetic release behavior,that is,the release behavior of ALA from ALA-MIPs is sustained release.