Molecularly Imprinted Electrochemical Sensor Prepared By Electro Polymerization Method And Its Analytical Applications

As one of the most important researches of electrochemical biosensors, molecularimprinting sensors obtained great achievements over the past10years. Several procedures areused to prepare molecularly imprinted polymer （MIP） films on transducer surfaces, includingdrop-coating or spin-coating of a solution of a pre-prepared polymer, in situ chemicalpolymerization and electropolymerization. In comparison with other procedures of MIP filmpreparation, the MIP films prepared by electropolymerization have superior properties withrespect to adherence to the transducer surface, simplicity and speed of preparation, easycontrol of the film thickness and high reproducibility. Five types of MIP sensors wereprepared respectively based on covalent or noncovalent interaction by electropolymerizationin this study. For the preparation of MIP sensors based on the noncovalent interaction,o-phenylenediamine with two functional groups is selected as the monomer. Triclosan,diethylstilbestrol and salbutamol, with the similar molecular weight but the different hydroxylnumber, were selected as the target molecules. The effect of hydroxyl number in templates onthe properties of MIP sensors was studied in detail. In order to increase the sensitivity ofsensors, carbon nano-material was modified on the surface of electrodes and the enhancedsensitivity was realized. In the preparation of MIP sensor based on the covalent interaction,3-aminophenylboronic acid was used as the monomer and dopamine was used as the targetmolecules, and a sensor with special recognition capacity towards dopamine was constructed.The main innovative works in this paper are described as follows:1. An evaluation of literatures on the recent achivements aiming at the MIP sensors baseon electropolymerization is presented. The main focus is on the choice of functionalmonomers and the removal of templates in the electrosynthesized preparation of MIP films.The application of the electrosynthesized molecularly imprinted polymers （MIPs） asrecognition elements of chemical sensors is summarized. The future development and thelimitations in this research field are discussed.2. Base on the noncovalent interaction, a MIP amperometric sensor was prepared byelectropolymerization using o-phenylenediamine （o-PD） as a monomer in the presence oftemplate triclosan which contains a hydroxyl group. Thickness of thepoly（o-phenylenediamine）（POPD） film with the nanometer was determined by QuartzCrystal Microbalance （QCM） technique. Factors affecting the properties of sensor, such as pHvalue of supporting electrolyte, polymerized potential, scan number and the ways of removingthe template were investigated in detail. The noncovalent interaction between the templateand the monomer is no strong because triclosan contains only one hydroxyl group. Thetemplate can be quickly removed by immersing the electrode in NaOH solution for10min.The sensor was applied for the detection of triclosan by an indirect method taking potassiumferricyanide, an electrochemical probe, as the mediator between the imprinted electrodes andsubstrate solutions. The sensor responses sensitively to triclosan over a linear range of2.0×107to3.0×106mol/L and the detection limit as low as8.0×108mol/L is obtained. Thissensor provides an efficient way for eliminating interferences from compounds with similar structures to that of triclosan compared to bare electrode.3. In order to verify the flexibility of o-PD as a functional mononer used forelectropolymerized preparation of MIP, diethylstilbestrol, a molecular containing twohydroxyl groups, was selected as a template. The prepared MIP sensor had the best propertieswhen the electropolymerization of o-PD was conducted by cyclic voltammetry （20scans） inthe range of0⁰.8V （scan rate50mV/s） in a PBS buffer （pH7.2）. The interaction forcebetween the monomer and the template is still the noncovalent interaction, butdiethylstilbestrol with two hydroxyl groups can form more hydrogen bonds with the polymermatrix, which makes it is more difficult to remove the template from the polymer thantriclosan imprinted sensor.50%ethanol-water solution was used to get rid of diethylstilbestrolembedded in the polymer matrix and10min is needed in order to reach complete remove.The sensor was applied for the detection of diethylstilbestrol by an indirect method takingpotassium ferricyanide as the mediator between the imprinted electrodes and substratesolutions. The sensor responses sensitively to diethylstilbestrol over a linear range of1.0×10^-75.1×10^-6mol/L and a detection limit as low as3.0×10^-8mol/L is obtained. Thissensor provides an efficient way for eliminating interferences from compounds with similarstructures to that of diethylstilbestrol compared to bare electrode.4. Carbon nano-materials have the excellent properties, such as the large surface area,high electrical conductivity, and are often used for the construction of sensors with highsensitivity. In order to improve the sensitivity of the MIP sensor, bare electrode was modifiedby carbon nano-materials before the formation of the MIP film. Triclosan imprinted sensorwas first prepared using o-PD as a monomer and a single-walled carbon nanotubes （SWNTs）modified GC electrode as a work electrode. Comparative trial between the bare electrode andthe SWNTs modified electrode verified the contribution of SWNTs to the enhanced sensitivity.Salbutamol imprinted SWNTs modified sensor was then prepared at the similarelectropolymerized conditions. Salbutamol contains three hydroxyl groups and can form morehydrogen bonds with the polymer matrix than both triclosan and diethylstilbestrol, whichmakes it embedded deeply in polymer matrix and very difficult to be removed. Onlyelectrochemical methods can remove the templates. On the other hand, with the increase ofhydrogen bonds between the monomer and the template, number of templates in the polymermatrix increases, the imprinted cavities increase and the linear range is widened accordingly.The sensor responses sensitively to salbutamol over the linear range of2.0×10^-75.0×10^-5mol/L and the detection limit as low as7.0×10^-8mol/L at the optimum conditions using linearstripping voltammetry. The results showed that SWNTs can increase the sensitivity of MIPsensors dramatically. The MIP sensor modified by SWNTs has the good recognition abilitytowards diethylstilbestrol.5. In order to increase the selectivity of MIP sensors, a MIP sensor based on thereversible covalent interaction between3-aminophenylboronic acid （APB） and dopamine wasdeveloped. The boronic acid group is known to rapidly and reversibly form complexes withdiols in aqueous solution. Cyclic voltammetry and Electrochemical Impedance Spectroscopy（EIS） was performed to characterize the dopamine imprinted sensor. Factors affecting theproperties of sensor, such as pH value of supporting electrolyte, polymerized potential, scan number and the ways of removing the template were investigated in detail. The prepared MIPsensor had good stability and selectivity when the electropolymerization of APB wasconducted by cyclic voltammetry （20scans） in the range of0¹.2V （scan rate50mV/s） in aPBS buffer （pH7.0）. The templates can be removed by potential cycling between0¹.5V （10scans） in0.50mol/L sulfuric acid. The response of the imprinted sensor to dopamine waslinearly proportional to its concentration over the range0⁶.0×10^-5mol/L, with a lowdetection limit of5.0×10^-8mol/L. The imprinted sensor showed high recognition ability andaffinity for dopamine and could eliminate the interference of the same concentration ofascorbic acid.