Preparation Of Photoelectrochemical Aptasensors For Heavy Metal Detection Based On Semiconductor Nanomaterials

With the rapid development of industry and the continuous improvement of urbanization level,solid waste and effluent containing heavy metals have been discharged to nature,and the environmental pollution is more and more serious.Due to the strong toxicity and undegradability,heavy metals can accumulate in the organism by the food chain and cause deformity,desease and even death,which are affecting the ecological environment and human health seriously.Therefore,it is significant to investigate analysis methods for heavy metal ions with advantages of high efficiency,sensitivity and accuracy.Photoelectrochemical?PEC?analysis is a convenient method with the merits of simple device,convenient operation and high sensitivity which is developed on the basis of electrochemical analysis introducing illuminant as excitation deveic.Combined with the specific recognition property of aptamer,PEC aptasensors with high selectivity and sensitivity have been designed,which take different photoactive materials as transducer and aptamers as target recognition element.The exploiting of photoactive materials with high conversion efficiency and designing of signal amplification sensing route are effective methods to the development of high performance PEC aptasensors.In this thesis,PEC aptasensors for heavy metal ions detection with high sensitivity and selectivity have been prepared by using aptamer as biological recognition element and designing a variety of signal amplification approaches,which are based on the preparation of photoactive materials with excellent performance.The main research contents are summarized as follows:?1?A PEC aptasensor was constructed by TiO₂ nanorods and Au nanostars modified on ITO conductive glass as photosensitive substrate and self-assembled mercapto modified Hg???aptamer as probe?S/Au/TiO₂/ITO?,which was applied to detection of Hg???.The morphology of the modified material,the modification process and the sensing performance of the aptasensor were characterized by scanning electron microscopy?SEM?,electrochemical impedance spectroscopy?EIS?and current-time?i-t?curve techniques.The results showed that the photocurrent of the aptasensor after Hg???recognition had a good linear relationship with Hg???concentration in the range of 1.0×10^-9 to 5.0×10^-7 mol/L with a detection limit as3.1×10^-10 mol/L.Also the aptasensor showed good repeatability and stability.?2?The photoelectrode was prepared via electrodeposition of ZnO and further modified by Au nanochains and aptamers?complementary sequences S1 and S2?successively.Thus a double-stranded DNA modified PEC aptasensor?S2/S1/Au/ZnO/ITO?was constructed for Pb???specific recognition.The morphology and structure of ZnO on the electrode were characterized by X-ray diffraction?XRD?,Raman and SEM.The property of the modified aptasensor was check by i-t curve method.ZnO nanorods on the ITO surface can exhibit good photocurrent response with large specific surface area for the further loading of Au nanoparticles and aptamer.The electrode interface modified by double-stranded DNA had higher negative charge density,and the photocurrent changed more significantly before and after Pb???recognition with improved sensitivity.A linear detection range from 1.0×10^-10 to5.0×10^-8 mol/L was obtained by plotting the photocurrents against Pb???concentrations with a detection limit of 3.5×10^-11 mol/L.The method can be used for the determination of Pb???in lake water sample.?3?ZnO-TiO₂ nanocomposites were synthesized by hydrothermal method,and characterized by transmission electron microscopy?TEM?,X-ray photoelectron spectroscopy?XPS?,SME,XRD and Raman.Then Au nanochains and 3'terminal amino modified Cd???aptamer were decorated on ZnO-TiO₂ modified electrode with the carboxylated g-C₃N₄covalently bound to the free end of aptamer through coupling agent to obtain a signal amplified Cd???aptasensor?g-C₃N₄/S/Au/ZnO-TiO₂/ITO?.The spatial distance between g-C₃N₄ and the electrode surface changed before and after Cd???recognition,resulting in significant photocurrent changes.The electrode modification process and sensing performance were checked by i-t curve,and a linear detection range of 3.0×10^-11?4.0×10^-8mol/L was obtained by plotting the photocurrents against Cd???concentrations with a detection limit of 1.1×10^-11 mol/L.The method had been successfully utilized for the analysis of Cd???concentration in lake water sample.?4?CdS sensitized TiO₂ nanocomposites were prepared through one-step hydrothermal method,which were characterized by SEM,XRD,XPS and Raman.CdS was loaded on the surface of TiO₂ uniformly and cooperated with it to obtain strong photocurrent response.A signal enhanced Pb???aptasensor?QC-Cu/S2/S1/Au/CdS-TiO₂/ITO?was constructed by complementary double-stranded DNA self-assembled on the surface of photoelectrode with the quercetin-copper complex?QC-Cu?embedding into the double-stranded DNA as photosensitizer.Owing to the specific recognition of Pb???by the sensing interface,the Pb???-aptamer complex was formed with the double-stranded DNA unwound and accompanied by the detachment of QC-Cu,which led to the sensitive change of photocurrent response.The analysis method for Pb???detection was established according to the linear relationship between photocurrent response and Pb???concentration in the range of5.0×10^-12?1.0×10^-8 mol/L with a lower detection limit as 1.6×10^-12 mol/L.This aptasensor was applied to the determination of various real samples with satisfactory results.?5?Au/ZnO-reduced graphene oxide?rGO?composites designed as photoactive materials were fabricated by electrodeposition and investigated by SEM,XRD,XPS and Raman in detail.The rGO with good conductivity can promote the photogenerated electron transfer and obtain good photoelectric conversion property of ZnO.After double-stranded DNA self-assembled on the photoelectrode,the Cd???aptasensor?MB/S2/S1/Au/ZnO-rGO/ITO?was prepared by combining the photosensitizer methylene blue?MB?into the double-stranded DNA through electrostatic force or intercalation.Meanwhile,a double-working-electrode detection system was introduced to realize the redox cycle of electron donor dopamine?DA?on the surface of the first?modified electrode?and the second working electrode?glassy carbon electrode,GCE?,forming a stable and continuous photocurrent signal.The photocurrent changed linearly as the Cd???concentration from 5.0×10^-12 to 2.0×10^-88 mol/L with a detection limit of 1.8×10^-12 mol/L.The aptasensor showed good sensitivity and stability,which was used for the detection of practical samples.