Applications Of Several Transition Metal Oxides And Their Nanocomposites As Electrochemical Sensors

In this thesis, the preparation and electrocatalytic ability of several transition metal oxide and their nanocomposites were studied and they were further applied for the rapid detection of glucose, trehalose, insulin, nitrite and hydrogen peroxide. This thesis contains 7 chapters, which can be classified into 3 parts: 1) design of electrochemical biosensor interface based on manganese oxides?MnOx? and carbon nanofibers?CNFs?/MnOx composites, 2) preparation of CNFs supported nickel oxide and its application in electrochemical insulin detection, and 3) application of copper oxide and CNFs-based nanocomposites as a nonenzymatic electrochemical sensor.In Chapter 1, an overview on the principle and classification of electrochemical sensors, electrocatalytic ability, structure characteristics, as well as corresponding application of the transition metal oxides and CNFs in electrochemical sensors were presented. Meanwhile, the research purpose and approaches of this thesis were also introduced.In Chapter 2, a glassy carbon electrode was modified with ?-manganese dioxide nanowires??-MnO₂ NWs? dispersed in Nafion solution and exhibited high electrocatalytic ability and improved sensitivity toward hydrogen peroxide?H₂O₂?.After glucose oxidase?GOx? was immobilized in the surface and ?-MnO₂ NWs acts as a mediator, the electrochemical biosensor enables amperometric detection of glucose with a high sensitivity and a fast response time. This study also demonstrates the feasibility of realizing inexpensive, reliable, and high-performance biosensors using MnO₂ nanowires.In Chapter 3, on the basis of the work of Chapter 2, by combining the advantages of manganese dioxide nanoparticles?MnO₂ NPs? and carbon nanofibers?CNFs?, a biosensing electrode surface as a high-performance enzyme biosensor was designed.The MnO₂ NPs and CNFs nanocomposite?MnO₂-CNFs? was prepared by using a simple hydrothermal method and then characterized by scanning electron microscopy,powder X-ray diffraction, fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy and electrochemistry. The results showed that the nanocompositehad a stable structure and MnO₂ NPs were uniformly attached to the surface of CNFs.Meanwhile, the MnO₂-CNFs nanocomposite as a supporting matrix can provide an efficient and advantageous platform for electrochemical sensing applications. On the basis of the improved sensitivity of MnO₂-CNFs modified electrode toward H₂O₂ at low over potential, a MnO₂-CNFs-based glucose biosensor was fabricated by monitoring H₂O₂ produced by an enzymatic reaction between GOx and glucose. The constructed biosensor exhibited excellent characteristics, such as high sensitivity and selectivity, short response time, and the relatively low apparent Michaelis-Menten constant.In Chapter 4, the Mn₃O₄-CNFs nanocomposite was successfully prepared by using an easy solution-phase reflux method. A potential application of the Mn₃O₄-CNFs nanocomposite modified electrode as a biosensor to monitor trehalose has been investigated. A bienzymatic electrode incorporating trehalase?Tre? and glucose oxidase?GOx? bound to the surface of Mn₃O₄-CNFs modified electrode was designed and assembled and its catalytic properties were examined by electrochemical methods in phosphate buffer solutions?PBS, pH 7.0?. Relying on the binding of Tre and GOx to the surface of the modified electrode, acting in series, the built biosensor can accomplish the conversion of trehalose into gluconolactone. At the same time, the enzyme induced H₂O₂ was produced and the electrochemical signal was generated on the electrode surface. Plots of the steady-state currents versus the concentration of trehalose over the range examined, i.e., 2-25 mM, were found to be linear. The construction of the bienzyme sensing platform also provided us with the methodology and research content to understand and realize the transformation of biological energy in the future.In Chapter 5, the uniform nickel oxide?NiO? nanoparticle decorated on CNFs hybrid with the assistance of ethylenediamine?EDA? following a simple on-spot pyrolysis route was fabricated and used for insulin electrocatalytic oxidation. The fabricated hybrid displayed superior catalytic performance due to the synergetic effect between NiO nanoparticles and CNFs. Excellent analytical features, including high sensitivity?1.55 ?A/?M?, short response time?< 3s?, low detection limit?12.1 nM?and satisfactory linear dynamic range?20-1020nM? were achieved. Moreover, this EDA-CNFs-NiO hybrid showed good stability and antifouling property in 0.1 M NaOH electrolyte toward insulin after successive potential cycling, which is highly required to a promising insulin electrocatalyst.In Chapter 6, flower like copper oxide?CuO? composed of many nanoflake was synthesized by a simple hydrothermal reaction and characterized using field-emission scanning electron microscopy?SEM? and X-ray diffraction?XRD?. CuO modified glass carbon electrode?CuO/GCE? was fabricated and characterized electrochemically.A highly sensitive method for the rapid amperometric detection of hydrogen peroxide?H₂O₂? and nitrite?NO2-? was reported.In Chapter 7, a new electrocatalyst Cu₂O/EDA-CNFs nanocomposite was prepared through a mild liquid phase reduction reaction with EDA modified CNFs as support. The Cu₂O/EDA-CNFs-based electrochemical sensor can be used for the highly sensitive detection of H₂O₂ with a fast response time. The detection limit for H₂O₂ detection was estimated to be 0.12 ?M. Because of the remarkable analytical advantages, the as-made sensor can be applied to determine H₂O₂ released from human cervical cancer cells. The high levels of H₂O₂ closely pertain to high oxidative stress and are associated with cancer and neurodegenerative disease, thus this electrochemical sensor has the potential applications in clinical diagnostics for assessing oxidative stress of living cells.