| Graphene quantum dots (GQDs) have attracted extensive attentions because of their chemical inertness, biocompatibility and low toxicity. They can be used for bioimaging, disease detection, drug delivery and photovoltaic devices. In this thesis, a series of studies have been directed to the design and preparation of graphene quantum dots, in addition to their applications in bioimaging and sensing. First, GQDs with a large number of carboxylic groups were prepared by microwave-assisted hydrothermal synthesis. It proves that the number of carboxylic groups can bring a J-type self-assembled aggregation induced unusual emission transformation. Furthermore, graphene quantum dots containing pyrrole ring were prepared by microwave irradiation in the presence of ammonia. The surface-functionalized graphene quantum dots with both maximum excitation and emission wavelengths fall into the visible-light region can be used for labelling cells and for the detection of mercury ions. Finally, in-situ growth of silver nanoparticles on the graphene quantum dots has been conducted based on the high peroxidase activity of graphene quantum dots. This material can be used for hydrogen peroxide and glucose sensing. The major achievements in the present thesis include the following:1. Graphene quantum dots have been prepared by a one-step hydro-thermal procedure in microwave with graphene oxide (GO) as raw material. The as-prepared GQDs were characterized by HRTEM, AFM and XPS. Their size is3-5nm, with single-layer and contains a lot of carboxylic groups. It exhibits an unusual emission transformation in strong acidic medium and at high concentration levels, induced by self-assembled J-type aggregation under restrained π-π interactions. More importantly, we have proposed that it is possible to develop new type of sensors based on monitoring the transformable emission maximum with GQDs as label. 2. The GQDs preparation procedure is improved and effectively shortened the preparation time. We report for the first time pyrrole-ring surface-functionalized graphene quantum dots (p-GQDs) prepared by a two-step hydrothermal approach under microwave irradiation in an ammonia medium. As the most distinct feature of the functionalized GQDs, both of its excitation-and emission-wavelengths fall into the visible-light region. The p-GQDs are excited by visible light at λex490nm (2.53eV) to emit excitation-independent photoluminescence at a maximum wavelength of λem550nm. This is hitherto the longest wavelength near infrared region reported for GQDs. Stable photoluminescence is achieved at pH4-10with an ionic strength of1.2mol L-1KCl. The cytotoxicity of p-GQDs was investigated by using a standard3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium(MTT) assay. The viability remains high at about85%even at a very high p-GQD concentration of800μg mL-1. These features make the p-GQDs an excellent probe for bio-imaging and bio-labeling which has been well demonstrated by imaging of live HeLa cells. The stable photoluminescence of p-GQDs makes it an excellent probe for sensitive and selective detection of mercury ions.3. We report a facile green approach for in-situ growth of silver nanoparticles (AgNPs) on the surface of graphene quantum dots (GQDs). GODs serve as both reducing agent and stabilizer, and no additional reducing agent and stabilizer is necessary. The GQDs/AgNPs hybrid exhibits superior absorbance fading response toward the reduction of H2O2. A simple colorimetric procedure is thus proposed for ultra-sensitive detection of H2O2without additional chromogenic agent. It provides a record detection limit of33nM so far reported for the detection of H2O2by AgNPs-based sensing system. This colorimetric sensing system is further extended to the detection of glucose, by combining with the specific catalytic effect of glucose oxidase for the oxidation of glucose and formation of H2O2, giving rise to a detection limit of170nM. The favorable performances of the GQDs/AgNPs hybrid are due to the peroxidase-like activity of GQDs. |
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