Synthesis Of Graphene Quantum Dots And Their Applications In Photoluminescent Sensing

Graphene and fuctionalized-graphene were much excitement in recent years in material science. Covalent or noncovalent functionalization of graphene with photochemically or electrochemically active groups has been the focus of investigations. Graphene also serves as an acceptor as a fluorescent quencher in fluorescence resonance energy transfer biosensor, with applications in DNA, protein, ATP analysis etc. Graphene rarely serves as a donor of fluorescent emission, due to their weak photoluminescent properties. Large graphene sheets can be tailored through manipulating the size and shape by reduction chemistry. Graphene sheets smaller than100nm with oxygenic groups, generally termed graphene quantum dots. The smaller graphene has the superior photoluminescence characteristics. However, to date, nearly all experimental work on graphene quantum dots has focused on their synthesis. And the investigation of their applications is still in its initial stages. The main results of this research are given as follows:(1) Synthesis of Graphene Quantum Dots from Graphene Sheets by the Hydrothermal ApproachThe obtained graphene quantum dots, blue light emitting material, were characterized by the fourier transform infrared spectrum, shows there are carboxylate groups on the surface of the graphene quantum dots. Then it was analyzed by atomic force microscopy, transmission electron microscope and photoluminescence spectrum. The results show that graphene quantum dots are single or bilayered and their diameters are mainly distributed in the range of10-20nm. The topographic heights are mostly between0.5and1.0nm. The graphene quantum dots also exhibit excitation-dependent photoluminescence behavior. When the excitation wavelength changes from310to380nm, the photoluminescent peak shifts from450to510nm correspondingly and its intensity decreases rapidly. The salt resistance of graphene quantum dots is high; the photoluminescence intensity was no clear change in the presence of NaCl. The fluorescence process of graphene quantum dots is pH-dependent. Under alkaline conditions, the graphene quantum dots emit strong fluorescence, whereas, under acidic conditions, the fluorescence intensity is weak. Graphene quantum dots have good solubility in water, biological compatibility, low toxicity and resistance to photobleaching properties.(2) Graphene Quantum Dots Combined with Europium Ions as Photoluminescent Probes for Phosphate SensingEu3+ions, lanthanide series, act as a bridge when it is coordinated with the carboxylate groups on the surface of the graphene quantum dots, lead to the aggregation of graphene quantum dots. As a consequence, the photoluminescence of graphene quantum dots is quenched through energy-transfer or electron-transfer processes. Then, the graphene quantum dots aggregates may be dissociated after the introduction of phosphate because Eu3+ions display a higher affinity for the oxygen-donor atoms in phosphate than for the carboxylate groups on the graphene quantum dots surface. In this case, the subsequent redispersion of graphene quantum dots results in the restoration of photoluminescence. That could be used for the quantitative detection of phosphate based on the fluorescence recovery. The method showed a linear response for phosphate in the range of0.5μmol/L to190μmol/L, and the detection limit is as low as0.1μmol/L. In order to prove the feasibility of this method, our method might be directly applied to detecting Pi in real samples from artificial wetlands. The recovery percentage is high enough. We have demonstrated a new type of rapid, sensitive, and specific photoluminescence assay for the detection of phosphate.(3) Application of Graphene Quantum Dots in the Detection of Ferric Ion and Hydrogen PeroxideIron is one of the main component of hemoglobin, plays a very important role in the human body, thus ferric ion detection is necessary. The carboxylate groups on the surface of the graphene quantum dots would coordinate with ferric ion. That led to the fluorescence quenching through energy-transfer or electron-transfer processes. The particles of the coagulation of graphene quantum dots are bigger, caused the light scattering intensity increasing. Hydrogen peroxide can oxidize ferrous ion to ferric ion due to its high oxidation-reduction potential. Grapheme quantum dots can be used for the detection of hydrogen peroxide.