Preparation And Applications Of Graphene-based Nanocomposites

Graphene is the name given to a two-dimensional sheet of sp2-hybridized carbon, which can be seen as the basic building block of carbon allotropes. The 2D structure and long-rangeπconjugation of graphene bring unique thermal, mechanical and electrical properties. Therefore graphene exhibits significant potentials in both theoretical studies and practical applications. In this work, for solving the crucial problems existing in biosensors, graphene and Au nanoparticles were utilized to promote the electrons transportation and keep the activity of the enzyme to construct novel biosensors with high performance. Besides, graphene-based nanocomposites were successfully synthesized through solvothermal reaction by using graphene oxide and metal salt precursors as starting materials. The applications of the nanocomposites were also studied.Water soluble graphene was prepared chemically in chapter 2. First, we synthesized graphene oxide using a Hummers method and followed sonication in water. Before reduction with hydrazine, graphene oxide layers were sulfonated to modify the surface with sulfonate groups. The obtained graphene layers could keep separate in water through the electrostatic repulsion due to the presence of negatively charged -SO3- groups.The water soluble graphene was utilized to construct novel hydrogen peroxide biosensors and glucose biosensors respectively in chapter 3. (1) Graphene and horseradish peroxidase (HRP) were co-immobilized into biocompatible polymer chitosan. Then a glassy carbon electrode was modified by the biocomposite, followed by electrodeposition of Au nanoparticles on the surface of the modified electrode to fabricate a hydrogen peroxide biosensor. Cyclic voltammetry demonstrated that the direct electrochemistry of HRP was realized. This biosensor exhibited an excellent performance in terms of electrocatalytic reduction towards H2O2, with a response time of 3 s. The linear range to H2O2 was from 5×10-6 M to 5.13×10-3 M, and the detection limit was 1.7×10-6 M. (2) Graphene, Au nanoparticles and glucose oxidase were co-dispersed into a Nafion solution. Then the dispersion was cast on the surface of a glassy carbon electrode to construct a glucose biosensor. This biosensor responded rapidly upon the addition of glucose and reached the steady state current within 5 s, with the presence of hydroquinone. The linear range is from 15×10-6 M to 5.8×10-3 M, and the detection limit is approximate 5×10-6 M.In chapter 4, we studied the preparation of graphene-metal oxide composites through solvothermal reaction in one step by using graphene oxide and metal salts as starting materials. (1) Graphene-Fe3O4 composites (G-Fe3O4) were synthesized with solvothermal reaction by using graphene oxide and FeCl3 as raw materials. The oxygen groups were decomposed under the high temperature and high pressure inside the sealed vessel and graphene oxide was reduced into graphene, while the Fe3+ ions adsorbed on the surface of graphene were reduced and grown to Fe3O4 in situ. The size and density of the Fe3O4 microspheres distributed on the graphene can be easily controlled by altering the starting Fe3+ concentration. The saturation magnetization of the composites at room temperature was about 45.5 emu g-1. Considering the high specific surface area and good biocompatibility of graphene, the obtained G-Fe3O4 composites were very suitable for the immobilization and delivery of drugs. The saturated loading capacity of the composites to doxorubicin could achieve 65%. We also immobilized the G-Fe3O4 on the surface of an electrode to construct a H2O2 biosensor, which also displayed excellent performance. (2) Graphene-TiO2 composites (G-TiO2) were synthesized through solvothermal reaction by using graphene oxide and tetrabutyl titanate as raw materials. TiO2 particles of anatase phase with a narrow size distribution were dispersed on the surface of graphene uniformly. In comparison with pure TiO2, the G-TiO2 showed an obvious red shift of absorption edge in the UV-vis absorption spectra. The structure of the G-TiO2 composites depended on the time of solvothermal reaction. Prolonging the reaction time moderately could promote the chemical bonding of TiO2 on graphene, which was beneficial to obtain more homogeneous products. The fluorescence quenching of the G-TiO2 indicated that graphene could hinder the recombination of electron-hole pairs of TiO2. The photocatalytic activity of the G-TiO2 was also affected by the amount of graphene in the composites. Under the optimal conditions, more than 75% of methylene blue would be degraded in 3 hours by the photocatalysis of the G-TiO2 with the irradiation of a simulated sunlight.The investigation of the graphene analogues was also carried out in this work. Ultrathin MoS2 layers with few or even single layer were prepared with a liquid-phase exfoliation. The photoluminescence of the exfoliated MoS2 was detected, which was absent in bulk MoS2. And the intensity of the fluorescence could be further enhanced by using Ag@SiO2 composite as an enhancing substrate based on the metal-enhanced fluorescence mechanism.