Tuning The Fluorescence Performance Of Carbon Dots And Studying Its Fluorescence Quenching

Since Xu firstly discovered carbon dots (CDs) in 2004, it has became one of the most popular nanomaterials among the nanocarbon family, including fullerene, carbon nanotube and graphene, has attracted intensive interest. CDs possess the excellent optical characteristics of high fluorescence intensity, against photo-bleaching and blinking and adjustable emission wavelength.At the same time CDs possess the advantages of good biocompatibility, low toxicity and so on.Based on these advantages, CDs is expected to replace quantum dots in biolabeling and bioimaging. Besides, CDs can as a drug carrier with the observability and traceability, also as a probe to detect metal ions, anions, organic small molecules and biological molecules. Due to its excellent physical and chemical properties (such as photoinduced electron transfer and oxidation reducing), CDs has a good application foreground in the field of photocatalyst and luminescent material, such as laser, LED, fluorescent ink, etc.Although CDs has many advantages and applications, the fluorescence of single grain CDs is weaker than that of single grain of quantum dots. In addition, the emission wavelength of CDs is mainly concentrated in the blue and green areas. Moreover, CDs as excitation-dependent emission fluorescence materials, which mean its emission state changes along with the excited state, their fluorescence are very weak as the excitation wavelength deviates from the optimal excited states seriously. These drawbacks become the bottleneck to restrict the application of CDs. Therefore, it is very necessary and urgent to tune the fluorescence performance of CDs. In this paper, the main research on tuning and studying the fluorescence properties of CDs as follows:(1) We chosed NaBH4 and LiAlH4, respectively, to reduce ordinary carbon dots, and got two kinds of carbon dots with different reduction degrees,labeled as r-CDsl and r-CDs2, tuning the excitation and emission wavelength of CDs carbon dots and fluorescence quantum yield improvement, successfully. Moreover, some original CDs with poor performance could be further grown into better ones by reduction treatment. We confirmed that the change of fluorescent properties of CDs has nothing to do with particle size. In fact, the different fluorescence characteristics of these three kinds of CDs were due to their different surface groups. At the same time we explained the mechanism of tuning the fluorescence performance of CDs through calculating the energy gap and the first excitation energy. Our study opens an avenue for preparing and tuning CDs with excellent fluorescence performance, as well as developing CD-based fluorescent probes.(2) We found that the gold nanoparticles (AuNPs) could effectively quench the fluorescence of CDs. The effects of CDs concentration, pH, temperature and incubating time effects on fluorescence quenching were investigated. Under the optimal conditions a quenching constant of 9.1×10 mol L-1 was achieved. It was worth noting that with increasing incubating temperature, the quenching constant decreased. On the other hand, the fluorescence lifetime of CDs hardly changed in the prescence of AuNPs. Therefore, we speculated that the fluorescence quenching of CDs by AuNPs was static quenching.