Syntheses, Properties And Recognition Mechanism Of Molecularly Imprinted Micropheres And Nanofibers For Bisphenol A

A large number of chemical substances are discharged into our environment as a result of human production and living activities. These chemical substances in the environment do a great harm on the health of humans and wildlifes, and then affect the propagation and living of wild animal species. Among them, the most remarkable is environmental hormone compounds, which can cause decline of reproductive function of animals, reproductive organ tumour, decline of im-munity, and a variety of physiological abnormalities. Following the ozone layer depletion and the greenhouse effect, the environmental hormone pollution has become another global environmental problem. Research on environmental hormone has become one of research forefronts and hottop-ics of international environmental science. Bisphenol A (BPA), which is known as endocrine dis-ruptor, have an adverse effect on organisms in extra-diluted concentration. Therefore, for the ben-efit of environmental protection, materials with high selectivity and recognition to BPA should be prepared to determine trace BPA accurately, conveniently and quickly, and to romove BPA from micro-polluted source water. In this respect, nanomaterials with molecular recognition show good capacity of separation and recognition. In this paper, molecular imprinting technology and elec-trospinning technology were utilized to prepare molecularly imprinted microspheres, molecularly imprinted nanofibers and composite molecularly imprinted nanofiber membranes for recognition of BPA, and the recognition properties and recognition mechanism of these materials were studied. These novel molecular recognition materials can be used for solid phase extraction and advanced treatment of wastewater, to determinate trace BPA in water samples, and to remove BPA in wastewater. The main research and findings were as follows:1. Four types of novel materials with recognition for BPA were prepared.(1) BPA molecularly imprinted polymer particles were prepared by modified bulk polymeri-zation method in toluene and liquid paraffin mixture. This method can be used to prepare directly BPA-MIPs particles with a small amount of solvent, sparing follow-up treatment of grind and sieve, by which MIPs particles were obtained in bulk polymerization.(2) BPA molecularly imprinted polymer microspheres were prepared by precipitation poly-merization method in acetic acid solvent, avoiding use of highly toxic acetonitrile and environ-mental pollution. Prepared in protonic solvent, the BPA-MIPs microspheres were suitable for de-termining in water phase.(3) Composite molecularly imprinted nanofiber membranes (CMIMs) were prepared by elec-trospinning polymer solution mixed with molecularly imprinted microspheres. Compared with conventional separation membranes, CMIMs had high flux values and permselectivities. (4) BPA molecularly imprinted nanofiber membranes (MINMs) were prepared by electros-pinning polyethersulfone, which was used as structural and functional polymer in nanofibers. MINMs showed several advantages over the traditional MIPs including low cost, easy preparation and fast mass transfer. Various MINMs can be prepared by selecting various functional polymers and templates.2. Effects of preparation conditions on the morphology and molecular recognition of resultant materials were investigated.Effects of preparation conditions on the morphology and diameter of MIPs particles and na-nofibers were studied by field emission scanning electron microscope. The interaction between the functional monomer and template molecules was investigated by Fourier transform infrared spec-tra. The surface areas were determined using dynamic nitrogen adsorption specific surface area and pore size distribution analyzer. Effects of viscosity, surface tension and electrical conductivity of polymer solution on the morphology and diameter of electrospun nanofiber were researched. Effects of preparation conditions on the binding characteristics of MIPs particles and nanofibers were studied through batch binding experiments. The results showed that:(1) Under the condition that molar ratio of monomer to crosslinker was 1:5, volume ratio of toluene and liquid paraffin mixture to crosslinking agent was 2.25:1, and volume ratio of toluene to liquid paraffin was 2:3-1:2, MIPs particles prepared by modified bulk polymerization showed high imprinted effect and selectivity. For the MIPs particles prepared in 2:3 volume ratio of tolu-ene to liquid paraffin, the binding equilibrium constants of the specific binding sites was 1.82x 105 L/mol, appearent maximum numbers of the specific binding sites was 0.46 mmol/g, partition fac-tor (KD) for BPA was 24.0 L/g in 0.2 mmol/L BPA solution, and imprinting factor was 27.1. With increase of the amount of mixed solvent and the proportion of liquid paraffin, the resultant MIPs showed regular morphology, larger size, and good specific selectivity for BPA. It was also shown that the hydrolysis rate and the binding amounts of imprinted polymers noticeably increased after MIPs particles with high hydrophobicity, prepared in toluene/liquid paraffin nonpolar solvent, were wetted by ethanol or acetone.(2) BPA-MIPs microspheres were prepared by precipitation polymerization in acetic acid solvent using BPA dimethacrylate (BADM) as complex of functional monomer and template mo-lecule, divinyl benzene (DVB) as crosslinker, azobisisobutyronitrile (AIBN) as initiator. The mi-crospheres showed high surface area (551.9 m2/g), high binding numbers (0.45 mmol/g). The binding constant of specific binding sites for BPA was 3.64x 105 L/mol, and partition factor KD for BPA was 47.6 L/g in 0.2 mmol/L BPA, and the selectivity was higher than that of BPA-MIPs mi-crospheres prepared in acetonitrile.(3) The morphology and diameter of MIPs particles, which were prepared by precipitation polymerization in acetonitrile solvent, varied with the functional monomer (methacrylic acid (MAA) or 4-vinyl pyridine (4-VP)), the crosslinker (DVB, ethylene glycol dimethacrylate (EGDMA) or trimethylolpropane trimethacrylate (TRIM)). When DVB was used as the crosslink-er, the diameter of P(MAA-co-DVB) imprinted polymer microspheres was 669 nm, while diame-ter of P(4-VP-co-DVB) imprinted polymer microsphere was 3.5 times larger than the former. The diameter of non-imprinted microspheres is larger than that of imprinted microspheres. When 4-VP was used as functional monomer, the size of MIPs and NIPs particle prepared using EGDMA or/and TRIM as crosslinking agent were small (<300 nm). With the increase of the amount of template molecule, the size of the imprinted microspheres decreased. The size of MIPs micro- spheres prepared in acetonitrile/toluene mixture was smaller than that prepared in acetonitrile and a secondary nucleation was observed, but the former had a high specific surface area (444.9 m2/g) and high static allocation factor (39.7 L/g) (at 0.2 mmol/L BPA concentration). It was shown that irregular MIP particles were obtained by precipitation polymerization in toluene solvent. When N, N-dimethyl formamide was used as solvent in precipitation polymerization, coagulum was ob-tained instead of particles.(4) MIPs particles were successfully encapsulated in the nanofiber by electrspinning. The composite molecularly imprinted nanofiber membranes (CMIMs) showed molecular recognition. The solution concentration had a significant effect on the morphology and diameter of electrospun fiber; the diameter of composite fiber increased with the increase of particle concentration. Elec-trospun by using high concentration and small size of monodisperse microspheres, the composite nanafibers showed uniform particle distribution and specific selectivity. CMIMs showed high permselectivity. In the presence of bis-(4-hydroxyphenyl) methane (BPF) and 4-tert-butylphenol (TBP)), CMIMs showed high selectivity for BPA, and the selectivity coefficient (ai) of BPA, BPF and TBP were 1,0.23 and 0.55, respectively.(5) Electrospinning appropriate polymers solution blended with template was a feasible me-thod for preparing molecularly imprinted nanofiber membranes (MINMs) that could selectively rebind the target molecule. Template amount, electrospinning voltage and solvent had an affect on the formation of recognition sites. After removal of the template by methanol, the imprinted nano-fibers maintained a clear shape and physical stability. The MINMs, which were prepared by elec-trospining 25 wt%PES (DMF/acetone (3.5:1, W/W) as solvent) concluding 10%BPA (BPA:PES, W/W) and were eluted with methanol to remove template, had high imprinted effect and selectivity. The surface area of the prepared MINMs was 127 m2/g, and the diameter of nanofiber was 850±150 nm, and the binding constant of specific binding sites was 2.23×105 L/mol, and the binding numbers of specific binding sites were 0.0437 mmol/g.3. The binding properties and recognition mechanism of MIPs particles and MIPs nanofibers for BPA was investigated.The imprinting efficiency and selectivity of MIPs particles and MIPs nanofibers for BPA were investigated by batch binding experiments and competition binding experiments. The bind-ing properties of MIPs particles and MIPs nanofibers for template were studied through data fit-ting by using Scatchard equation and two-site binding model. Binding thermodynamics and kinet-ics of the MIPs particles was researched. Dynamic adsorption experiment was carried out to un-derstand the penetration selectivity of adsorption column.The results showed that:(1) MIPs particles and MIPs nanofiber had two types of binding sites, and the binding prop-erties could be described by two-site Langmuir equation.(2) MIPs particles and MIPs nanofibers showed good imprinting effect and specific selectiv-ity for BPA, and MIPs particles had better specificity than MIPs nanofibers.(3) BPA binding process of MIPs particles was exothermic reaction, and system entropy de-creased in binding process, which was controlled by enthalpy.(4) At lower concentration, two-site Lagergren first-order kinetic equation could be employed to describe binding velocity of MIPs particles and MIPs nanofibers.(5) Composite MIPs nanofiber and MIPs nanofiber had smaller column resistance and higher water flux.4. MIPs particles and MIPs nanofibers could be used for solid phase extraction with satisfac- tory results.In this paper, BPA-MIP particles and nanofibers with molecular recognition were prepared by simple methods. The resultant BPA-MIP particles and nanofibers, which were employed for solid phase extraction, had high selectivity, separation efficiency and low matrix interference, and overcomed unfavorable factors of complex environmental and biological samples and cumber-some pretreatment procedures. BPA-MIP particles and nanofibers, which were used for removal of trace amounts of BPA in micro-polluted source water, can remove selectively BPA and its struc-tural analogues without removal of useful substances for the human body, and the operating process was simple and convenient. Preparation of the MIPs nanofibers by electrospinning was a new idea for preparing MIPs, which provided a new way for preparation of separation membranes with high permselectivity and flux.