Preparation Of Fluorescent Carbon Quantum Dots By Microwave Method And Their Application

Carbon quantum dots (CDs) have many advantages such as high fluorescence intensity, good stability, low toxicity to biological cells, a relatively broad excitation wavelength and thus they have widely potentials in chemical analysis, bio-analysis and optoelectronic devices as a new research hot-point. In this work, microwave synthesis method was applied to prepare the fluorescent CDs in water solution. They were further transferred into different organic solvents and the photoluminance in various solvents were investigated systematically. The interactions between gold nanoparticles, various metal ions and anions with CDs were also explored by quenching effect as a fluorescent probing method. The main contents and results are as follows:1. Using the L-cystine as carbon source, fluorescence CDs were prepared by microwave method. Technologies of transition electron microscope (TEM), UV-visible spectroscopy, FT-IR, X-ray photoelectron spectroscopy (XPS) and fluorescence spectroscopy were applied to characterize the structure, morphology, elemental composition and fluorescent properties of CDs. Experimental results showed that the prepared CDs can be well dispersed in the water with the uniform average diameter of 1.7-3.5 nm. Effects of microwave reaction time and temperature on the fluorescence of CDs were studied.2. The fluorescent intensity of the CDs product changed with the pH value of the reaction solution. The maximum excitation wavelength and the maximum emission for the aqueous CDs were 424 nm and 502 nm, respectively. As the pH value of the reaction solution increased from 2 to 13, the fluorescence intensity of the CDs solution increased. XPS analysis showed that the CDs dispersed in water phase contained much more amount of N and S elements, indicating that the CDs surface was covered by the unreacted L-cystine groups.3. CDs were transferred into different organic solvents and the fluorescence properties of the organic phases were studied. Experimental results indicated that CDs could be successfully transferred into hexane, ether, toluene, chloroform, dichloromethane, ethyl acetate organic solvents with increased fluorescence intensity. The maximum excitation wavelength and the maximum emission for the organic CDs were 365 nm and 425 nm, respectively. The fluorescence intensity varied in different organic solvents. XPS results showed that surface carbon content of CDs dispersed in the organic phase was 88%, and the surface was covered with little amount of sulfur element.4. Interactions between CDs and gold nanoparticles, metal cations and common anions were studied by fluorescence quenching experiments. The possible fluorescence quenching mechanism was proposed. It was found that the gold nanoparticles could quench the fluorescence of CDs water phase; Hg2+, Cu2+, Fe2+ and Fe3+ were of typical fluorescence quenching property to CDs. The selected anions in experiments did not show obvious quenching effect as they were interacted with CDs.These results provided a scientific basis of further understand the nature of CDs and extend its range of applications.