Detection Of 3, 4-Dihydroxybenzoic Acids With Molecularly Imprinted Electrode

Molecularly imprinted polymers (MIPs) are becoming an important class of synthetic materials mimicking molecular recognition by natural receptors. MIPs provide synthetic receptors with high selectivity and stability, while electrochemical sensors could offer good sensitivity, at low cost, with possibility of easy design, manufacture and miniaturization. Though the main applications continue to be in the separation field, on the basis of the studies undertaken, it is no doubt that a new generation of MIPs-based electrochemical sensors will be established in the future.Limitations exist in functional monomers, cross-linkers and polymerization methods used in traditional imprinting technique. The MIPs are usually thick and highly cross-linked, which introduce difficulties for electrochemical sensing application, such as incomplete template removal, broad guest affinities and selectivities, slow mass and charge transfer, high detection limit, bad reversibility and reproducibility, et al. Studies are encouraged to find new imprinting matrices and methods to satisfy the requirements of electrochemical sensing elements. By the way, at present, still little is known about the relative operation mechanism of binding sites, the configuration of the MIPs and the mass transfer mechanism, further investigations are needed, as how to understand the imprinting and recognition process from molecular level. In addition, natural molecular recognition process is always carried out in aqueous solution, however, the imprinting and recognition process in the preceding studies are done mostly in non-polar organic solution. It is difficult how to use special interactions between molecules and imprint recognize in aqueous.3,4-dihydroxybenzoic acid (3,4-DHBA) has great importance to everyday life. Up tu date, many methods such as Flow injection analysis, High-performance liquid chromato- graphy,Gas chromatography / Mass spectrometry, Fluorescence chromatography, Capillary electrophoresis, etc, have been uesd for the detection of the 3,4-DHBA. But some have low sensitivity, and also expensive instruments are needed. It is reported that electrochemical method is fewer than others.Hereby in this dissertation the applications of two kinds of new matrices in molecular imprinting are studied, some of the difficulties associated with traditional matrices and detection are thus overcomed, and some new electrochemical sensors are constructed succesfuly by coupling these imprinted matrices with electric transducers for the detection of 3,4-DHBA. The main content is listed as follows:Molecular imprinted over-oxidized polypyrrole (OPPy) modified glassy carbon electrode (3,4-DHBA-OPPy/GC) was prepared by electrochemical polymerization of pyrrole with cyclic voltammetry in the presence of template molecule 3,4-DHBA, and was followed by over-oxidized at 1.3V in phosphate buffer solution (pH=9.05). The modified electrode could successfully avoid polymerization of oxidized 3,4-DHBA and the interference of 2,4-dihydroxybenzoic acid (2,4-DHBA). A linear response curve was obtain from 1.0×10-5 to 1.6×10-3mol/L, with the detection limit of 5.0×10-6mol/L. The main driving force for recognition are hydrogen bonds and comnlementarv cavity effect.Thin films of a molecularly imprinted sol-gel polymer were studied. Sol-gel derived silicate molecularly imprinted films were electrochemically deposited on the surfacesof glassy carbon electrode from a sol consisting of tetramethoxysilane and phenyltri- methoxysilane for the first time. Compared to the dip-coating and spin-coating, short preparing time and thin films are the advantages of this method. The problem of crack in preparation can be controlled effectivly. The electrochemical results indicate that the thin films show high selectivity and affinity toward the template molecule. The influence of 2,4-DHBA and polymerization of oxidized 3,4-DHBA can be avoided effectively. A linear response curve was obtain from 1.0×10-5 to 8.0×10-4 mol/L, with the detection limit of 5.0×10-6mol/L. The main driving force for recognition are hydrophobic interaction, hydrogen bonds and complementary cavity effect.