Study On Electrocatalysis And Biosensing Of Nanomaterials

Nanomaterials have special structure, which results in series of interesting physical and chemical properties. The application of nanomaterials has involved in nanoelectronics, nanochemistry, catalysis, biosensing, environmental controlling, medicine, nanomachine, biotechnology and so on. This thesis was concentrated on the investigation of nanomaterials (such as noble-metal nanoparticles and carbon nanotubes grown directly on substrate) in electrocatalysis and biosensing. The micrographs, structure and electrocatalytic properties of the resulting nanomaterials for the electrochemical reaction of oxygen, methanol and glutathione have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray energy dipersive spectrum (EDS), cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy. Also the fabrication of enzyme, DNA and tumor marker biosensors based on noble-metal nanoparticles and carbon nanotubes and its electrochemical characteristics have been evaluated. The main points of this thesis are summarized as follows: (1) Highly dispersed platinum nanoparticles were electrodeposited on graphitic carbon nanofibers (GCNFs) by cyclic voltammetry (CV). GCNFs used in this paper are grown directly on graphite disk. The micrographs and element composition of Pt/GCNFs/graphite electrode were characterized by SEM and EDS. The electrocatalytic properties of Pt/GCNFs/graphite electrode for methanol oxidation have been investigated by CV. The prepared Pt nanoparticles dispersed uniformly on GCNFs and the particles size is in the range of 40 to 50 nm. Excellent electrocatalytic activity for methanol oxidation can be observed at Pt/GCNFs/graphite electrode. Compared with Pt/graphite electrode, the electrocatalytic activity of Pt/GCNFs/graphite is two times as large as that of Pt/graphite electrode. The methanol oxidation is controlled by diffusion of methanol and the Pt/GCNFs/graphite electrode shows good long-term stability. (2) The well-aligned carbon nanotube (CNT) arrays that grown on titanium (Ti) substrate were used as supporting material for platinum catalysts for the first time. The platinum nanocatalysts/well-aligned CNT/Ti electrode was prepared by a potential-step method. The structure and element composition of the resulting Pt/aligned-CNT electrode were characterized by SEM, TEM and EDS. The electrocatalytic properties of the Pt/aligned-CNT electrode for oxygen reduction and methanol oxidation reactions have been investigated by cyclic voltammetry and linear sweep voltammetry. SEM and TEM results demonstrate that the prepared Pt nanocatalysts are dispersed uniformly on the surface of aligned-CNTs. The Pt/aligned-CNT electrode shows good electrocatalytic activity for oxygen reduction and methanol oxidation reactions. The kinetics results suggest that oxygen reduction and methanol oxidation reactions at Pt/aligned-CNT electrode are controlled by the diffusion process of oxygen and methanol; Compared with Pt/tangled-CNT and Pt/graphite electrodes, the Pt/aligned-CNT electrode is more favorable for oxygen reduction and methanol oxidation. (3) A platinum overlayer was prepared on Au nanoparticles that were electrodeposited on a graphite electrode by replacement of the under-potential deposition (UPD) Cu monolayer. The micrographs and element composition of Au nanoparticles and UPD-Cu/AuNP/graphite electrodes were characterized by SEM and EDS. The electrocatalytic properties of Pt/AuNP/graphite electrode for methanol oxidation have been investigated by cyclic voltammetry. The results demonstrate that the Pt/AuNP/graphite electrode has high electrocatalytic activity and good long-term stability for methanol oxidation. The core (Au nanoparticle)-shell (Pt overlayer) nanocatalyst obtained by the above method may have good potential applications in designing noble metal catalysts with super low loading at the atomic level. It also provides interesting method for preparation of nano-electrocatalysts. (4) Based on the good electrocatalytic property of carbon nanotubes for active biomolecules, the well-aligned carbon nanotube arrays that grown on graphitic substrate were used for direct electrochemical oxidation of glutathione (GSH) for the first time. The micrographs and electrochemical performance of the aligned-CNT electrode were characterized by SEM and cyclic voltametry. The electrochemical behavior of GSH at aligned-CNT electrode was investigated by square wave voltammetry and cyclic voltametry. The results suggest that aligned-CNT electrode exhibits excellent electrocatalytic activity and good anti-fouling property for direct electrochemical oxidation of GSH. The electrochemical oxidation of GSH ataligned-CNT electrode is controlled by diffusion process and the exchange current density for GSH oxidation is 4.78×10-8 A cm-2. In view of fast response, high sensitivity and low detection limit, the aligned-CNT electrode holds potential promise as electrochemical detector for the amperometric determination of GSH or other thiol compounds. (5) Combining the advantages of carbon nanotubes with Pt nanoparticles, a glucose amperometric biosensor, based on adsorption of glucose oxidase (GOD) at platinum nanoparticle/carbon nanotube electrode, is prepared. CNTs were grown directly on the graphite substrate. The resulted GOD/Pt/CNT electrode was covered by a thin layer of Nafion to avoid the loss of GOD in determination and improve the anti-interferent ability. The morphology of the resulted electrode was characterized by SEM. The electrochemical performances and the parameters of the resulted enzyme electrode have been investigated in detail. The results suggest that the prepared enzyme electrode has good current response to glucose. The linear determination range is 0.1-13.5 mM with a regression coefficient of 0.9993; the response time is within 5 s. The apparent Michaelis-Menten constant appK m is 10.11 mM, and the maximum current of the enzyme electrode is 1.176 mA cm-2. The sensitivity of the biosensor is 91 mA M-1 cm-2. The use of Nafion film gives the enzyme electrode good anti-interferent ability. The enzyme electrode has good stability, reproducibility and applicability to whole blood analysis. (6) The surface of directly-grown carbon nanotubes was functionalized by electrochemical oxidation of ethylene diamine. Calf thymus dsDNA was immobilized on ethylene diamine modified CNT (EDM-CNT) electrode by potential-controlling method. The micrographs of the resulting EDM-CNT and dsDNA/EDM-CNT electrodes were characterized by SEM and the electrooxidation of the immobilized dsDNA was investigated by square wave voltammetry. Additionally, the interaction between promethazine hydrochloride and immobilized dsDNA was investigated by electrochemical impedance spectroscopy and square wave voltammetry. The possibility of using functional CNT electrode immobilized with dsDNA to sense promethazine hydrochloride was also explored. The results suggest that the EDM-CNT electrode has better electrochemical performance and is favorable for enrichment and electrochemical oxidation of trace dsDNA (10 ng mL-1).The intercalative binding of promethazine hydrochloride with dsDNA results in an increase of electrochemical impedance during dsDNA oxidation and a decrease of dsDNA oxidation peak current. According to the change of dsDNA oxidation peak current before and after interaction with promethazine hydrochloride, the dsDNA/EDM-CNT electrode can be used for determination of promethazine hydrochloride with a linear detection range of 0.025 to 2.1 μM. (7) Combining the gold nanoparticles (AuNPs) with nonconducting film (poly σ-aminophenol), a label-free electrochemical immunoelectrode for the detection of carcinoembryonic antigen (CEA) was designed. The glutathione (GSH) monolayer-modified AuNPs that were prepared by one-step direct chemical reduction were used to covalently immobilize the CEA antibody. And the antibody-AuNP bioconjugates were electro-copolymerized with α-aminophenol on polycrystalline gold electrode to fabricate the immunoelectrode (POAP/CEAAb-AuNP/Au). The formation of CEA antibody-antigen complexes on the electrode was probed by [Fe(CN)6]3-/4-redox pair and monitored by cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical performance of the immunoelectrode was investigated in detail. The results suggest that formation of CEA antibody-antigen complexes increases the electron transfer resistance of [Fe(CN)6]3-/4-redox probe and decreases the redox current of the probe at the POAP/CEAAb-AuNP/Au electrode. The use of CEA antibody-AuNP bioconjugates and POAP film can enhance the sensitivity and anti-nonspecific binding property of the resulting immunoelectrode. The immunosensor can detect the CEA with a detection limit of 0.1 ng mL-1 and the linear determination range was from 0.5 to 20 ng mL-1.