Preparation Of Nanomaterials And Their Applications In The Construction Of Electrochemical Biosensors

As a new kind of analytical tool, biosensors which have the characteristics of good selectivity, high sensitivity, fast response, low cost and continuous on-line detection in complex system, have been widely applied in the clinical diagnosis, environmental monitoring, and food control etc. Because of the good physical, chemical, electrocatalytic and biocompatibility properties of nano-materials, applying nano-materials for the fabrication of biosensors will greatly improve the performance of the resulting biosensors. In this thesis, many nano-materials with different morphologies were synthesized using different methods and were applied on the surface of electrodes as modified materials or enzyme immobilizing matrix for the preparation of biosensing interface. The resulting biosensors exhibited high sensitivity, low detection limit and fast response to the corresponding substance, validating the enhancing effect of nano-materials on biosensors'performance. The main results are as follows:Carbon nanotubes (CNTs) represent a new kind of promising carbon material for the modification of electrode owing to the unique electrical properties such as high surface-to-volume ratio, excellent electric ability and high chemical stability. Multi-walled carbon nanotubes (MWCNTs) were firstly treated with a mixture of concentrated sulfuric and nitric acid, which could shorten the chain lengths of CNTs, thereby enabling them to be immobilized easily and stably on an electrode. In addition, carboxylic acid groups could be introduced in the ends and side walls of CNTs during the acid treatment, which can interact with positively charged polyelectrolytes or biomolecules.Amperometric choline biosensors based on MWCNTs and layer-by-layer (LBL) assembled multilayer films composed of poly(diallyldimethylammonium chloride)(PDDA) and choline oxidase (ChOx) were constructed, also a polymer film of (PAA/PVS)3 to be used as a permselective layer. MWCNTs was characterized by transmission electron microscopy (TEM), and properties of the resulting choline biosensors were measured by electrochemical measurements. Among the resulting biosensors, MWCNTs/(PAA/PVS)3/(PDDA/ChOx)8 based choline biosensor was the best. At the applied potential of+0.6 V vs. Ag/AgCl, the biosensor showed a linear range of 0.75μM～0.15 mM, with a sensitivity of 10.89μA/mM, and a detection limit of 0.3μM (S/N=3). Moreover, it exhibited good suppression of interference as well as long-term stability, and MWCNTs played the roles of electron transfer promoting and response speeding, thereby improved the properties of the sensor.MWCNTs-gold nanoparticles (GNp) nanohybride was synthesized and used as an enzyme matrix to immobilize ChOx. PDDA was employed to disperse the mixture of MWCNTs-GNp-ChOx and also used a binder material to electrostatically adsorbe the enzyme. A novel choline biosensor based on the nanocomposite film composed of ChOx, MWCNTs, GNp and PDDA was developed. The microscopic structure and composition of the MWCNTs-GNp hybride were characterized by TEM, energy dispersive X-ray spectroscopy (EDX) and UV-visible asorbance spectroscopy (UV-Vis), and properties of the resulting choline biosensors were monitored by electrochemical measurements. At the applied potential of+0.35 V vs. Ag/AgCl, it showed a linear range of 0.001～0.5 mM, with the sensitivity of 12.97μA/mM, and a detection limit of 0.3μM (S/N=3). Moreover, the biosensor with the nanohybride of MWCNTs-GNp exhibited significant improvement of sensitivity, which was 3 times and 2 times of biosensors with GNp only and MWCNTs only, respectively.Shaped silver nanoparticles (Ag NPs) with like-spheres and well-defined dendrites were fabricated on glassy carbon (GC) electrodes by the direct electrodeposition process from an aqueous solution of AgNO3 in the presence of no template or catalyst, and the shape of Ag NPs was AgNO3 concentration-dependent. Scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and TEM were employed to characterize Ag NPs. The electrochemical behaviors of the silver dendrites and its reduction catalytic activity towards hydrogen peroxide were investigated by electrochemical measurements. At the applied potential of-0.2 V vs. Ag/AgCl, it showed a linear range of 0.005～12 mM, with a sensitivity of 7.39μA/mM, and a detection limit of 0.5μM (S/N=3), meanwhile, eliminating the interference from solved oxygen and other electric active species.Silver nanowires (Ag NWs) with diameter of 200 nm were synthesized by an L-cysteine-assisted poly (vinyl pyrrolidone) (PVP)-mediated polyol route. A novel strategy for fabricating a hydrogen peroxide sensor was developed by direct drop-casting Ag NWs on Pt electrode. UV-Vis, XRD, SEM and TEM were employed to investigate the prepared Ag NWs. The electrochemical properties of Ag NWs and the sensor were characterized by electrochemical measurements. It was found that the sensitivity of the modified electrode would be greatly enhanced after cyclic scan in phosphate buffer solution (PBS), which might be due to the enlarged surface areas and the increased active sites of Ag NWs during the scan. After scanned for 50 cycles, at the applied potential of-0.2 V vs. Ag/AgCl, the resulting sensor showed a linear range of 0.5μM～30 mM, with the sensitivity of 9.45μA/mM, the detection limit of 0.2μM (S/N=3). Furthermore, the sensor exhibited good anti-interference, reproducibility and long-term stability.Based on optimization of Ag NWs synthesis method, by casting the prepared Ag NWs on Pt electrode, a new kind of sensing interface was fabricated for determination of chloride, bromide and iodide in solution. Because of the well-separated redox potential, chloride, bromide and iodide could be simultaneously monitored in a mixture. The electrochemical properties of Ag NWs towards the three kinds of ions were investigated by cyclic voltammetry. By measuring the oxidation peak currents of the respective silver halides, the calibration graph was linear from 200μM～20 mM for chloride,50μM～20 mM for bromide and 50μM～20 mM for iodide, which was narrower than the linear range in single determination. And the sensitivity was 0.059μA/mM,0.042μA/mM and 0.032μA/mM for chloride, bromide and iodide, respectively.It is a significant way to make platinum as nanomaterials to improve its catalytic efficiency. In this thesis, NaHB4 as the reductant, polyvinyl alcohol (PVA) as the stabilizer, the PVA-protected spherical Pt nanoparticles (Pt NPs) were prepared by a reduction method in aqueous solution. The as-prepared Pt NPs were characterized by XRD and TEM, and a new kind of sensing interface was developed by drop-casting the Pt NPs on GC electrode. Cyclic voltammetry was employed to investigate the electrocatalytic activity of Pt NPs towards methanol and hydrogen peroxide. The results demonstrated that the modified electrode exhibited sensitive response for the electro-oxidation of methanol and good catalytic activity to the reduction of hydrogen peroxide, which could be monitored at 0 V vs. Ag/AgCl with good sensitivity.