Preparation Of Excellent Adsorbing And Surface Molecular Imprinted Material Toward Flavonoids And Research On Their Adsorption Property

Flavonoids are a broad class of natural substance that exist extensively in plant tissuesand have various biological activities. Modern pharmacological studies indicate that theyhave multiple functions of free-radical scavenging, antioxidation, antitumor, sterilization,and so on. So they play an important role in natural medicine chemistry. Quercetin, rutin,genistein are three typical flavonoid compounds which have high bioactivity andpharmacological activities. Therefore, effective extraction and separation of them fromplants have attracted much attention in the chemistry and biomedical field. In this study, inorder to promote the development of extraction and separation of natural medicinecompounds, adsorbing materials which have strong adsorption properties to flavonoids areprepared，and flavonoids surface molecular imprinted polymers （MIPs） are also preparedby using a new surface molecular imprinting technique “grafting polymerizationsynchronized with imprinting” by means of molecular designing. So this research has greatscientific significance in biopharmaceutics and chemical sciences fields.First, micron-sized silica gel particles are surface-modified with coupling agentγ-Aminopropyltrimethoxysilane （AMPS）, obtaining the modified particles AMPS-SiO₂, onwhich primary amino groups are chemically attached. Distilled water and DMSO are usedas mixed solvents, and a redox initiation system is constituted with the amino groups onthe surfaces of AMPS-SiO₂particles and ammonium persulphate in a solution. Thispractice realizes the surface-initiated graft-polymerization of vinyl acetate （VAc）, andprepares the grafted particles PVAc/SiO₂with a high grafting degree of PVAc. PVAc canbe transformed into PVA by the alcoholysis reaction, which results in functional graftedparticles PVA/SiO₂with high grafting degree. The product particles are characterized byFTIR, SEM and TGA. The effects of main factors on the graft-polymerization of vinyl acetate are investigated emphatically. The experimental results show that the mixedsolvents composed of water and DMSO along with a redox initiation system constitutedwith the amino groups on the surfaces of AMPS-SiO₂particles and ammonium persulphatein a solution realize highly effective surface-initiated graft-polymerization of vinyl acetatewhich is a oil-soluble monomer. Then the functional material PVA/SiO₂is successfullyobtained through the alcoholysis reaction of PVAc.Subsequently, the adsorption investigation of three kinds of flavonoids, quercetin,rutin and genistein was conducted by using polyvinyl alcohol （PVA）-grafted silica gelparticles PVA/SiO₂as solid adsorbent. The effects of the main factors on the adsorptionproperties are investigated, the adsorption mechanism is explored in depth, and theadsorption thermodynamic is researched. The experimental results show that theconventional hydrogen bond is formed between the hydroxyl groups with high density onthe surfaces of PVA/SiO₂and the phenolic hydroxyl groups in flavonoids, while π-typehydrogen bond is formed between the hydroxyl groups of PVA/SiO₂and the conjugatedaromatic rings. It is the two types of hydrogen bond that make the functional compositeparticles PVA/SiO₂produce very strong physical adsorption toward quercetin, rutin andgenistein. The competitive adsorption of the solvent can have severe negative impact onthe adsorption capacity of flavonoids. In1,2-dichloroethane （DCE） as a weak polaritysolvent, the competing adsorption of the solvent barely exists. Therefore, when DCE isused as solvent, quercetin, rutin and genistein have very high adsorption capacities. Whilein ethanol as a protic solvent, the adsorption capacities of the three flavonoids declinegreatly as a result of the strong competing adsorption of the solvent. Increasingtemperature will weaken the hydrogen bond interaction between PVA/SiO₂particles andflavonoids, and even make the hydrogen bond rupture, leading to the decrease of theadsorption capacity. The existence of electrolytes in the protic solvent will affect theadsorption negatively. The adsorption process of PVA/SiO₂particles toward flavonoids isexothermic and driven by enthalpy. The adsorption isotherm data matches the Langmuirmodel. Lastly, by using the newly established surface molecular imprinting technique“grafting polymerization synchronized with imprinting”, genistein molecularly imprintedpolymer MIP-PMAA/SiO₂is successfully prepared. The main procedures are as follows.（1）In DMF solvent, monomer MAA gathers around template genistein with two types ofhydrogen-bond interaction.（2） A redox surface-initiated system is constituted betweenprimary amine group and the ammonium persulfate.（3） Monomer MAA and crosslinkerMBA carry out grafting/crosslinking copolymerization.（4） The template genistein isremoved, obtaining genistein molecular imprinted polymers MIP-PMAA/SiO₂. Theadsorption and recognition characteristics of MIP-PMAA/SiO₂for genistein are studied indepth by using static method, dynamic method and competitive adsorption experiment. Theexperimental results show that MIP-PMAA/SiO₂possesses very strong adsorption affinityfor genistein, and the saturated adsorption capacity could reach to0.36mmol·g^-1. Moreimportantly, MIP-PMAA/SiO₂possesses excellent specific recognition to genistein. Theselectivity coefficients relative to quercetin and rutin are5.4and11.8, respectively. Besides,the imprinting condition has a great influence on selective recognition properties ofgenistein MIPs. When the ratio of monomer MAA and genistein is4:1, monomer MAAand crosslinker MBA is5:1, MIP-PMAA/SiO₂has greatest selective recognitioncharacteristics.A similar method can be adopted to prepare quercetin molecularly imprinted polymerMIP-PMAA/SiO₂, and study its adsorption and recognition characteristics. Theexperimental results show that MIP-PMAA/SiO₂possesses very strong adsorption affinityand specific recognition for quercetin. And the saturated adsorption capacity could reach to0.33mmol·g^-1. The selectivity coefficients relative to genistein and rutin are4.4and7.69,respectively. Besides, elution experiment is performed by using sodium hydroxide solutionwith a concentration of0.001mol·L^-1（pH value of10） as eluting agent, within19bedvolumes, quercetin is eluted from MIP-PMAA/SiO₂column with a desorption ratio of98.1%. That way, the new adsorbent MIP-PMAA/SiO₂has outstanding elution propertyand it exhibits excellent reusability.