Preparation And Application Of The Electrochemical Sensor Based On Magnetic Molecularly Imprinted Nanoparticles

With the increase of environmental and food safety issues, some large-scale instrumentscannot be satisfied with the request of sensitive, rapid and on-site detection. Electrochemicaldetermination have been attracted widely attention because of its low cost, high sensitivity,easy operation and online testing. This dissertation was committed to solving theelectrochemical detection of a class of substances with low electrochemical response andpassivation to the electrode surface. Admicelles formed on the electrode surface by surfactantwere used to enhance the sensitivity and pollution resistance of the sensor. Magneticmolecularly imprinted nanoparticles (MMIPs NPs) as a sensitive element was fixed on thesurfactant modified magnetic electrode by magnetic attraction, improving stability andreproducibility of the magnetic molecularly imprinted sensor. The sensor based on MMIPsNPs and magnetic sensor modified with admicelles could determine target molecules rapidly,sensitively and selectively. The main contents of the dissertation include the followingaspects:Three-step miniemulsion polymerization was firstly used to prepare MMIPs NPs. Theeffect of polymerization method, organic solvent, cross-linker, co-stabilizer andminiemulsification approach on the size and morphology of MMIPs NPs were investigated bytransmission electron microscopy (TEM), scanning electron microscopy (SEM), thermalgravimetric analysis (TGA) and vibrating sample magnetometer (VSM). Some importantexperimental phenomena were observed. Firstly, mixed organic solvents with differentpolarity used in miniemulsion polymerization caused polydispersity of particles. Secondly,appropriate crosslinking degree was necessary to provide adequate adsorption capacity, aswell as high selectivity. Thirdly, co-stabilizer played a crucial role to ensure goodmonodispersity. Finally, the third step of miniemulsifying was indispensable and important toobtain good regularity and monodispersity of MMIPs NPs. Because in the thirdminiemulsified step, stability of either magnetic nanoparticles or the oil phase stabilized bysurfactant was destroyed, only hybrid particles composed of magnetic nanoparticles and oilphase could be obtained. Salicylic acid (SA) MMIPs NPs were prepared under the optimumconditions, the imprinted particles were98nm with RSD of6.6%(n=100). It is importantthat MMIPs NPs possessed regular morphology, monodisperse size, uniform core-shellstructure and high saturation magnetic intensity. So MMIPs NPs had adsrorption capacity toSA was1.37mg/g and achieve the adsorption equilibrium in100min. MMIPs couldspecifically recognize SA molecules in solutions which indicated by selective adsorptionexperiment, the separation factors for SA to benzoic acid (BA) and p-hydroxybenzoic (PHBA) were ?SA/BA=2.5, ?SA/PHBA=2.1, respectively.Bisphenol A (BPA) as environmental estrogen was selected as the target molecule, BPAMMIPs were prepared with threep-step miniemulsion polymerization. MMIPs with uniformcore-shell structure were characterized by SEM and TEM, and the average diameter was123nm with RSD of4.9%(n=50). The saturation magnetization intensity of MMIPs was28.005emu/g which characterized by VSM. Thermodynamic and kinetic adsorption experimentsshowed that the saturation adsorption capacity of MMIPs for BPA was up to121.3mg/g, andthe adsorption equilibrium could reached in120min. The separation factors for BPA to?-estradiol (E2), estriol (E3) and diethylstilbestrol (DES) was ?BPA/E2=23.6, ?BPA/E3=8.8,?BPA/DES=3.7, respectively. Based on a selective enrichment of BPA by the magneticmolecularly imprinted solid phase extraction, the contents of BPA in lake water and plasticbottle were detected with UV spectrophotometer.The effect of surfactants on the sensitivity, reproducibility and anti-passivation of thecarbon paste electrode (CPE) were researched. In the dissertation, two anionic surfactants(SDS and SDBS), three quaternary ammonium cationic surfactants (CTAB, TTAB andDTAB), and two quaternary ammonium gemini surfactants (12-3-12and12-4-12) were usedto modified the CPE. Different kinds of surfactants adsorbed on the CPE were studied withatomic force microscopy, impedance spectroscopy, contact angle and the electrochemicalbehavior of BPA. The results suggested that as the concentration was greater than the CMC,the quaternary ammonium cationic surfactant, such as CTAB, TTAB,12-3-12and12-4-12formed the cylindrical admicelles on the CPE, while the saturated monolayer adsorption forDTAB and SDS. The electrochemical catalytic mechanism of the admicelle for BPA wasresearched, the results showed that the electrochemical sensitization effect to BPA was mainlydue to the BPA solubilized in the admicelle. Furthermore, cation-? effect between cationicsurfactants and BPA was the key for BPA solubilization. Therefore, CTAB admicelle modifiedCPE was used to detect BPA sensitively and repeatedly.Magnetic nanoparticles, surface imprinting technique, combined with admicellesmodified magnetic electrode were firstly applied to prepare a novel magnetic imprinted sensorfor BPA. A direct electrochemical detection of BPA depended on close-packed admicelle onthe electrode surface formed by cationic surfactant molecules, and MMIPs particles as thesensing element to selectively recognize BPA. Moreover, MMIPs could tightly cling to thesurface of the magnetic sensor because of the strong magnetic attraction, which improving thestability and reproducibility of the sensor. The experiments showed that the magneticimprinted sensor could selectively recognize BPA, the separation factors for BPA to E2, E3and DES was ?BPA/E2=16.5, ?BPA/E3=17.3, ?BPA/DES=6.6, respectively. The RSD of oxidationcurrent Ipa recorded on the same sensor was1.4%(n=7), and RSD of Ipa detected with threedifferent imprinted sensors prepared under the same conditions was4.3%. In addition, the Ipa decreased only by8.6%after the sensor was kept in air at room temperature for30days.These results indicated that the imprinted sensor has good reproducibility and stability. Underoptimized conditions,0.19?mol/L BPA was detected in lake water and not found in drinkingbottle sample. The recoveries at two spiked levels (2?mol/L and20?mol/L) were between95%and112%with the RSD in the range of2.7%to4.8%.Based on the selective solvent method, magnetic composite nanoparticles were obtainedby co-assembling of magnetic nanoparticle and amphiphilic polymers. P(MMA-co-MAA),P(St-co-MAA) and P(SMA-co-MAA) with different monomer ratio were synthesized.Amphiphilic random copolymer and OA-Fe3O4were dispersed in solvent, and then thisorganic phase was dropwise added to water phase to induce self-assembly of the polymer.Compared with the traditional selective solvent process, this operation was simple, and astable co-assembly could be formed without stir for long time and dialysis operations. It couldbe understood that the magnetic attraction and strong hydrophobic effect between OA-Fe3O4and hydrophobic segments of the polymer promoted hydrophobic segment of polymer to fastshrink in self-assembly process, so a stable composite particles could be formed in a shorttime. Moreover, the magnetic attraction improved the stability of the composite particles, andthe bulk of the solvent could be removed when composite particles were separated bymagnetic separation. In the process of co-assembling of P(MMA-co-MAA) and OA-Fe3O4,TEM and zeta potential and particle size analyzer were used to study the effect of the ratio ofhydrophobic to hydrophilic monomer, water-oil ratio, the amount of magnetic nanoparticlesand polymer dosage on the morphology and size of magnetic composite particles. Underoptimized conditions, BPA as a template molecule, MMIPs NPs were prepared by theco-assembly method using P(MMA-co-MAA) with monomer ratio of9:1and OA-Fe3O4.Compared with MMIPs NPs prepared with three-step miniemulsion polymerization,thermodynamic and kinetic adsorption experiments showed that MMIPs NPs obtained higheradsorption capacity (201.5mg/g), but required a longer time to reach adsorption equilibrium(210min), and had a poor selectivity. The separation factors for BPA to E2, E3and DES was?BPA/E2=3.1, ?BPA/E3=2.9, ?BPA/DES=3.7, respectively.