Theoretical Design And Preparation For Melamine Molecular Imprinting Polymers

Melamine, an organic chemical material, is mainly used in the manufacture of plastics, adhesives, glues, and laminates. Owing to the high nitrogen content(66.7%), the addition of MAM in food will cause a false increase of protein level, which is a serious threat to human health. In recent years, the MAM detection has increasingly become a hot spot because of the event that milk powder polluted by MAM making baby kidney stone. Therefore, it is of great significance to improve the method of detecting MAM from food samples with high sensitivity and accuracy that we can separate, purify and enrich the MAM target molecules rapidly and accurately at a low cost.Molecular imprinting technique is a procedure to prepare molecularly imprinted polymers(MIPs) with specific selectivity for the target molecules which is taken as template or imprinted molecules. Due to its high mechanical and chemical stability, predetermination, and specific recognition, MIPs exhibit great application prospect in the fields of solid phase extraction, sensors, chromatographic separation, membranes and catalysts. The performance of MIPs is influenced by the reaction conditions such as functional monomers, imprinting mole ratio, solvent and cross-linking. In order to improve the affinity, selectivity and recognition properties of MIPs to template molecules, computer-aided molecular simulation was carried out to investigate the system of MIPs.Gaussian 09 software was used to optimize the structure of MAM with the B3 LYP, PBE1 PBE, LC-wPBE, and WB97 XD methods as well as the 6-31G(d, p) and 6-31+G(d, p) basis sets. The results show that the WB97XD/6-31G(d, p) method is the best one to simulate the system of MAM-MIPs. Then, we used the WB97XD/6-31G(d, p) method to calculate the geometry optimization of the action sites, the bonding situation between MAM and five functional monomers, and optimize the cross-linking agents and solvents according to the binding energy(ΔE). In this process, MAM was used as the template molecule, acrylamide(AM), methacrylic acid(MAA), N, N’-methylenebisacrylamide(MBA), itaconic acid(IA) and trifluoromethacrylic acid(TFMAA) were used as the functional monomer, ethylene glycol dimethacrylate(EGDMA), divinylbenzene(DVB), pentaerythritol triacrylate(PETA), and trimethylolpropane trimethylacrylate(TRIM) were used as the cross-linking agent, respectively. The nature of the imprinting effect was also studied by the atoms in molecules(AIM) theory and Infrared spectrum. The results of theoretical calculations showed that MAM interacts with the functional monomers via hydrogen bonds, and the MAM-MIPs with molar ratio of 1:6 have the largest number of hydrogen bonds and the most stable structure. DVB is the best cross-linking agent for the MAM-MIPs comparing with other crossing-linking agents. The MAM-MIPs are expected to have higher recognition and selection ability in methanol solvent.Based on the stable configuration of MAM-MIPs, all atomic coordinates were fixed after removal of the template MAM, then the optimized structural analogues(cyromazine(CYR), cyanuric acid(CYA), and trithiocyanuric(TRI)) were added in the action sites as possible as to form a new complex configuration, respectively. The rebinding energies of the new optimized complexes were used to predict the selectivity of MIPs. The results showed that MAM-MIPs had a good selective adsorption property to MAM.A series of MAM-MIPs with different functional monomer, imprinting molar ratio, cross-linking agent and solvent were synthesized by precipitation polymerization based on the results of theoretical calculation. The experiment results showed that MAM-MIPs had the lowest bond energy and the most stable structure when IA was used as fictional monomer and the molar ratio of MAM and IA was 1:6; DVB was the most suitable cross-linking agent and methane was the most suitable solvent, which is consistent with theoretical calculation results. The Scatchard plot revealed that the maximum adsorption capacity for MIPs towards MAM was 20.79 mg/g and the dissociation equilibrium constant was 58.82 mg/L. The adsorption quantity of MAM-MIPs to melamine is obviously higher than that of cyromazine, cyanuric acid, and trithiocyanuric MIPs, respectively.Finally, MAM was added to the crude extract of pure milk samples, and the MAM-MIPs were used to enrich and separate MAM from the milk products. The results show that the average recovery rate was 99.41%. This study provides a basis for the selective separation, enrichment and detection of MAM in dairy products.