Theoretical Design,Preparation And Adsorption Properties Of Graphene Based Surface Molecularly Imprinted Polymers For Flavonoids

Flavonoids are a class of naturally occurring polyphenolic compounds among the most prevalent classes of compounds in the plant kingdom.Owing to the complicated structures and diversified types,it is quite challenging to the separattion and purification of the flavonoids from natural products.At present,the surface molecularly imprinted polymers of flavonoids exhibit low adsorption efficiency,low adsorption capacity and low selectivity.In this paper,graphene and graphene oxide(GO)based molecularly imprinted polymers(MIPs/Graphene and MIPs/GO)for the selective recognition of flavonoids were prepared by surface molecular imprinting technique on the basis of the quantum chemical calculations by designing the imprinted preassembly system of quercetin(Qu).The morphology,structure,composition and adsorption properties of the prepared materials were investigated,and the adsorption mechanism was discussed.The main results are as follows:(1)The optimization of functional monomer and its ratio to Qu in the imprinted preassembly system.By optimizing the structure and conformation of Qu by quantum chemical calculations,the size of Mulliken charge on hydrogen atom of the hydroxyl group can be obtained and its size is in the order of the position 3′(0.290),4′(0.273),3(0.266),7(0.264)and 5(0.258),suggesting that the hydrogen atom in the position of 3′ is most likely to form the hydrogen bond.On this basis,the theoretical models of the hydrogen-bond complexes were constructed by using 4-vinylpyridine(4-VP),methacrylic acid(MAA)and acrylamide(AM)towards Qu.The structure parameters of hydrogen-bond and the interaction energy were calculated.It was shown that the hydrogen bond formed by 4-VP and Qu displays the shortest bond length,the largest bond angle and the biggest interaction energy(﹣36.877 k J mol-1).Therefore,molecularly imprinted polymers(MIPs)were prepared by using 4-VP as functional monomer.Moreover,through the simulation of the interaction energy of 4-VP and Qu at different ratios,the most suitable ratio between 4-VP and Qu was 4∶1 and the intramolecular hydrogen bond is formed by the hydrogen atom on the hydroxyl group of positon 5 and the oxygen atom on the carbonyl group of position 4 in Qu.In brief,by the quantum chemical calculations towards the imprinted preassembly system,the 4-VP monomer and the ratio of 4∶1 between 4-VP and Qu were used to prepare the MIPs.(2)The preparation and adsorption property of MIPs/Graphene.MIPs/Graphene were prepared by free radical polymerization using graphene as support on the basis of the functional modification of C=C double bond in graphene.The results of selective adsorption tests showed that MIPs/Graphene have no selectivity to Qu.It was found that the main factor is the low oxygencontaining levels and defects on graphene,which lead to the incomplete imprinted layers on graphene and the poor selectivity of MIPs/Graphene to Qu.In addition,through the theoretical calculation,it was found that when graphene coexists with the complex formed by 4-VP monomer and Qu molecule,its binding ability to Qu is greater than that of 4-VP monomer to Qu molecule,indicating that graphene has a strong adsorption ability for Qu.The results of this study paved the way for the preparation of MIPs/GO.(3)The preparation and adsorption properties of MIPs/GO.MIPs/GO were fabricated by free radical polymerization using GO as support on the basis of the functional modification of oxygen-containing groups in GO.The obtained MIPs/GO were characterized by the atomic force microscopy and the results showed that the thickness of the MIPs/GO increases to 31.46 nm compared with that of GO,indicating that the MIPs layer is grafted onto the GO surface.The adsorption properties of MIPs/GO were tested,the results showed that the adsorption capacity of MIPs/GO at the initial concentration of 100 mg L-1 Qu solution was 30.61 mg g-1 and the adsorption equilibrium time was about 30 min at 298 K.The adsorption kinetic and adsorption isotherm can be well described by the pseudo-second-order kinetic model and Langmuir isotherm adsorption model,respectively.Finally,the selective adsorption experiments showed that the values of selectivity coefficient of MIPs towards Qu over Rutin and Kaempferol are 4.765 and 2.017,respectively,indicating that MIPs/GO exhibit good selectivity for Qu in the mixed solution of flavonoids.