Electrochemical And Photoelectrochemical Sensors Based On Nano Semiconductor For The Detection Of Environmental Pollutants

Due to good biocompatibility and fairly photoelectrochemical activity, nano semiconductors have been intensively studied for fabrication of electrochemical and photoelectrochemical sensors. However, most nano semiconductors usually exhibit dissatisfactory photoelectrochemical activity, according to the high recombination rate of photo-generated electrons and holes. In order to improve their photoelectrochemical performance, the hybridization of graphene (GR), a new2D carbon nanomaterial with super conductivity, has been proposed as a suitable approach since GR coupling with nano semiconductors can effectively enhance the charge-separation rate. In this thesis, using liquid phase deposition, layer by layer and molecularly imprinting techniques, high performance electrochemical/photoelectrochemical sensors based on nano semiconductors such as TiO2and quantum dots were successfully fabricated and applied for the detection of some water pollutants. GR was utilized to improve the performance of developed photoelectrochemical sensors. The main contents of the thesis were described as follows:(1) A modified electrode preserving the bioelectrochemical and photoelectrochemical activities based on hemoglobin (Hb) and TiO2was prepared by liquid phase deposition (LPD) method. On this electrode, Hb was immobilized in TiO2film for studying the direct electron transfer of Hb. Using the LPD process, a hybrid film composed of Hb, TiO2and sodium dodecylsulfonate (SDS) was successfully prepared on the electrode surface. The cyclic voltammetric measurement indicated that the LPD hybrid film facilitated the electron transfer of Hb, which yielded a pair of redox peaks prior to the characteristic voltammetric peaks of TiO2. Due to the electrocatalytic activity of Hb towards H2O2, the LPD Hb/SDS/TiO2hybrid film could be utilized as a H2O2sensor, which showed a sensitive voltammetric response proportional to the addition of H2O2in the concentration range of5.0×10-7-4.0×10-5mol/L, with a detection limit (3S/N) of8.7×10-8mol L-1. At the same time, the LPD Hb/SDS/TiO2hybrid film preserved the photoelectrochemical activity of TiO2. The photovoltaic effect on the electrochemical behavior of LPD Hb/SDS/TiO2film was examined under long-time exposure to UV irradiation. It was found that, the UV irradiation treatment of proposed sensor caused significant improvement of calibration sensitivity of H2O2.(2) Water soluble CdS quantum dots (CdS QDs) were synthesized at low temperature in aqueous medium. The as-prepared CdS QDs were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD), and utilized to fabricate CdS QDs and DNA hybrid film (CdS QDs/DNA) on the graphite electrode surface via layer by layer (LBL) self-assembly technique. Under visible-light excitation, the CdS QDs/DNA electrode showed enhanced photoelectrochemical response toward methylene blue (MB), due to the interaction between MB and DNA. The photoelectrochemical current was linearly proportional to the concentration of MB in the range of3.0×10-8-3.0×10mol L-1. The detection limit (3S/N) was estimated to be1.4×10-8mol L-1. The proposed sensor had high sensitivity, high reproducibility, high stability and good selectivity for dyes.(3) A novel visible-light photoelectrochemical sensor was prepared based on FTO electrode modified with CdS quantum dots (CdS QDs), graphene (GR) and molecularly imprinted polypyrrole (MIP). The electrochemical impedance spectra (EIS) indicated that GR facilitated the electron transfer of CdS modified electrode. Small amount of GR doped in CdS film could dramatically improve the photoelectrochemical response of modified electrode under visible-light illumination. While a MIP layer of4-aminophenol was deposited on CdS-GR nanocomposites modified electrode, the photocurrent response towards4-aminophenol on the modified electrode was obviously promoted due to the specific binding of MIP with4-aminophenol. The photoelectrochemical response was linearly proportional to the concentration of4-aminophenol in the range of5.0×10-8mol L-1to3.5×10-6mol L-1, with a detection limit (3S/N) of2.3×10-8mol L-1. Moreover, the MIP/CdS-GR modified FTO electrode showed excellent selectivity. The proposed visible-light photoelectrochemical sensor based on MIP/CdS-GR/FTO provided a feasible tool for rapid, sensitive and selective determination of4-aminophenol.(4) A visible-light photoelectrochemical sensor for the detection of catechol was prepared based on CdS quantum dots (CdS QDs), pristine graphene (PGR) and DNA modified FTO electrode. The as-prepared CdS QDs-DNA-PGR film possessed photoelectrochemical activity of CdS QDs and bioactivity of DNA molecule. It was found that the presence of DNA on FTO electrode surface enhanced the photoelectrochemical response towards catechol, due to the interaction between DNA and catechol. The electrochemical impedance spectra (EIS) indicated that PGR dramatically facilitated the electron transfer of CdS QDs-DN A modified electrode. The PGR doped in CdS QDs-DNA film improved the photoelectrochemical response of modified electrode by more than50times in comparison to CdS QDs-DNA-FTO. The photoelectrochemical response of catechol was linear to its concentration from1.0×10-8mol L-1to1.0×10-6mol L-1. The detection limit (3S/N) was4.9×10-9mol L-1. The CdS QDs-DNA-PGR modified FTO electrode was demonstrated to photoelectrochemical sensing of trace catechol in water samples.