Graphene-based Nanomaterials For Fabricating The Electrochemical Sensing Platform And Its Application

The rapid development of nanoscience and nanotechnology provides new opportunities for the sustainable progress of analytical science. The introduction of nanomaterials into analytical chemistry implants their novel functions into analytical sensing systems, resulting in many novel analytical techniques being tested and some highly sensitive and selective systems fabricated. This rapidly extending interdisciplinary area has attracted great research efforts from chemists, physicists, biologists and materials scientists. Recently, graphene-based analytical systems are a very active area due to their unique electronic, optical, and mechanical properties associated with their planar structure.Nanoelectroanalytical chemistry is a rising interdisciplinary field, which combines characteristics of electrochemistry (e.g., simplicity, speed, high selectivity and high sensitivity) with unique properties of nanomaterials (e.g., electronic, optical, magnetic and catalytic) to become one of the most exciting topics. Graphene not only could act as an advanced support with very large surface area for immobilizing different targets, but also effectively promote the electron transfer between electrode and analytes.Aptamers, generated from "systematic evolution of ligands by exponential enrichment" (SELEX) technique, are short single stranded oligonucleotides (DNA or RNA) that bind to specific targets ranging from small organic molecules, metal ions, proteins, biological cells, to tissues. It is facile to label the DNA or RNA with different kinds of functional groups, such as electroactive and fluorescence groups. In addition, aptamers also exhibit other unique features over antibodies, such as in vitro selection, chemicalstability, low immunogenicity, automated synthesis and numerous choices of nucleotide or phosphate functional groups. These exceptional properties make aptamers capable of serving as favorable molecular recognition elements in the design of high-performance biosensing devices. The marriage of two-dimensional nanomaterials and aptamers has emerged many ingenious aptasensing strategies for applications in the fields of chemistry, physical, materials, biology and interdisciplinary science.This dissertation was focused on the synthesis and fabrication of functionalized graphene nanomaterials based on graphene oxide (GO) nanosheets, The as obtained functionalized graphene nanomaterials were characterized by UV-vis spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, atomic force microscopy, transmission electron microscopy and cyclic voltammetry, which confirmed that functionalized graphene nanomaterials had been successfully synthesized. The constitute and development of biosensor and electrochemical sensor based on as obtained functionalized graphene nanosheet have been studied. The application of these electrochemical sensors in biomolecules and environmental pollutants were investigated.1. In the text, the characteristics of nanomaterials and the synthesis and the functionalization of graphene have been expounded, we also outlined the research progress on biosensors and aptasensors so far, especially the progress of electrochemical aptasensor and the applications of functionalized graphene in electrochemical aptasensor introduced in detail.2. A facile strategy for the preparation of meso-terakis (4-methoxyl-3--sulfonatophenyl) (4-methoxyl-3-sulfonatophenyl) porphyrin(T(4-Mop)PS4)-graphene hybrid nanosheets (TGHNs) was demonstrated for the first time, which combines the features of both graphene (high conductivity and surface area) and porphyrin (prospective photochemical electron-transfer ability and excellent electrochemical activity). The as obtained TGHNs were characterized by UV-vis spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, atomic force microscopy, transmission electron microscopy and cyclic voltammetry, which confirmed that porphyrin had been effectively functionalized on the surface of graphene. Combining with the the high affinity and specificity of aptamer, a simple, rapid, sensitive and label-free electrochemical aptasensor was successfully fabricated for adenosine triphosphate (ATP) detection. The proposed TGHNs based aptasensor achieved the direct electron transfer of porphyrin and showed good affinity and specificity towards ATP, which avoided the interference such as the introducing of labeled-substance and the [Fe(CN)6]3-/4- probe. The preferable linear range for ATP was from 2.2 nM to 1.3 μM (R=0.9982) with the detection limit of 0.7 nM. Integrating the advantages of graphene nanomaterials with optoelectronically active porphyrin molecules and high selectivity of aptamers, the fabricated biosensor shows an expanded linear range, excellent sensitivity and selectivity against other nucleoside family.3. We report on an electrochemical aptasensor for the ultrasensitive determination of thrombin. A glassy carbon electrode modified with a graphene-porphyrin nanocomposite exhibits excellent electrochemical activity and can be used as a redox probe in differential pulse voltammetry of the porphyrin on its surface. The thrombin aptamer is then immobilized via p stacking interactions between aptamer and graphene and π-π stacking with porphyrin simultaneously. The resulting electrochemical aptasensor displays a linear response to thrombin in the 5-1,500 nM concentration range and with a limit of detection of 0.2 nM (at an S/N of 3). The sensor benefits from the synergetic effects of graphene (with its high conductivity and high surface area), of the porphyrin (possessing excellent electrochemical activity), and of the aptamer (with its high affinity and specificity). This kind of aptasensor conceivably represents a promising tool for bioanalytical applications.4. In this part, atomic layer deposition is applied to coat graphene nanosheets with TiO2(TiOGNs). The produced TiOGNs composites are characterized by X-ray photoelectron spectroscopy, fourier transform infrared spectroscopy, and transmission electron microscopy. It is revealed that TiO2 are effectively deposited on the surface of graphene. The coatings have a highly controlled thickness.5. The electrochemical properties of the obtained TiOGNs composites are investigated for detection of lead (Pb2+) and cadmium (Cd2+) ions by differential pulse anodic stripping voltammetry (DPASV). It is found that the TiOGNs composite exhibits improved sensitivity for detection of these metal ions. The linear dynamic ranges are from 1.0×10-8 M to 3.2×10-5 M for Pb2+ and 6.0×10-7 M to 3.2×10-5 M for Cd2+, respectively. The detection limits (S/N=3) are estimated to be 1.0×10-10 M for Pb2+and 2.0×10-8 M for Cd2+, respectively.