Studies On The Synthesis Of N-Doped Fluorescent Carbon Dots And Their Applications In Cellular Imaging Analysis

Carbon quantum dots also named as carbon dots (CDs), have been widely applied in many fields, due to their excellent biocompatibility and electro-optic properties. As one of the fluoresent nanomaterials family, CDs are mainly used in bioimaging and bioanalysis. However, most of them show low quantum yields and short emssion wavelengths in blue light region with high backgroud noises, which limits their applications in bioimaging. Hence, it’s highly desirable to fabricate CDs with high photoluminescence and long emssion wavelength. To obtain ideal CDs, a lot of research have been exploited, including extending new raw materials, improving synthesis methods, doping and chemical modification, et al. All these efforts above have greatly improved the fluorescent properties of CDs. Here, we used different raw materials to synthesize nitrogen doped carbon dots with different characteristics and applied them in cell imaging, pH sensing and constructing the long wavelength emission system. The detail research contents as followings:(1) One-pot hydrothermal synthesis of highly luminescent nitrogen-doped amphoteric carbon dots for bioimaging from bombyx mori silk.In order to sythesize CDs with high quantum yield, we used bombyx mori silk as raw material to hydrothermally synthesize the nitrogen doped CDs under alkaline condition. The TEM results showed CDs had uniform size was about 5 nm. The FTIR, XPS and Raman results revealed that CDs contained carbon, nitrogen, oxygen elements and had rich hydroxyl, carboxyl, amino groups, which made them possess good solubility in water. The UV-vis and fluorescence spectra showed CDs held good absorption and emission property, high quantum yield. Furthermore, CDs presented excellent photostability and the fluorescent intensity scarcely showed any change even if they were dispersed in acid and alkali, high ionic strength solution, most organic solvent and even in the complex biologic samples such as human serum. The cytotoxicity investigation showed the CDs were well biocompatibile. Meanwhile, we have investigated the effect of CDs as bioimaging reagents and the result of the intracellular uptake experiment showed CDs could well light the cells. Therefore, they could be served as a potential alternative for organic dyes which are easy to photobleach or semiconductor QDs which have certain biotoxicity in bioimaging.(2) A general quantitative pH sensor developed with dicyandiamide N- doped and high quantum yield graphene quantum dots.To prepare high photoluminescent CDs and broden their application in biochemical analysis, herein, we reported a facile hydrothermal route to synthesize N-doped graphene quantum dots (GQDs) by using citric acid as carbon source and dicyandiamide as N source, respectively. The morphology of the GQDs was characterized by using transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results indicated the average size of the GQDs was about 2.3 nm with a relatively narrow size distribution and the height of GQDs approximately was 0.4-0.7 nm about 1-2 graphene layers with a lattice spacing of 0.34 nm. XRD, Raman, FTIR, XPS were performed to show the GQDs were amorphous crystal with sp2 and sp3 carbon. Meanwhile, they contained carbon, nitrogen, oxygen elements and had rich functional groups such as hydroxyl, carboxyl, amino. The fluorescence spectrum and the characterization of stability displayed GQDs had a strong emission with quantum yield of 36.5% and excellent photostability.The fluorescent intensity kept invariant in salt of high concentration and complex system. The cytotoxicity investigation showed that GQDs scarcely imposed toxicity to the cells, which has important significance for their application in biochemical analysis. In addition, GQDs were pH-sensitive and the fluorescence intensity was significantly enhanced with increasing pH from 2 to 9. What’s more, the fluorescence intensity of GQDs had an excellent linear relationship with pH in this region. The pH detection result of water samples from GQDs almost agreed with that from 510 pH meter. We further applied GQDs in pH detection of Hep-2 cells and the detection results were reliable and satisfied. Therefore, the GQDs could be unitized as pH sensor for the pH detection in real water body and cells.(3) The construction of the long wavelength emission system based on fluorescence resonance energy transfer of carbon dots and investigations on their influence factors.For the long wavelength emmssion of CDs, based on the fluorescence resonance energy transfer (FRET) between carbon dots and meso-tetra-(p-sulfonatophenyl) porphyrin (TPPS4), we established a FRET system with long wavelength emission and investigated how the concentration of carbon dots and TPPS4, pH, salinity, and viscosity influenced the efficiency of the energy transfer between carbon dots and TPPS4. The results of the investigations were following:the concentration of carbon dots little effect the efficiency of the energy transfer, however, the efficiency of the energy transfer would be induced when the concentration of TPPS4 was too high or too low. The influence of pH was complex because it influenced not only the absorption spectrum but the fluorescence spectrum of TPPS4, which all related to the efficiency of the energy transfer. The influence of salinity was accordance with viscosity, that is, the efficiency of energy transfer reduced with the increase of the salinity and viscosity.The investigations above showed that bombyx mori silk is a natural protein with high content of nitrogen, CDs from which with excellent photoluminescent properties and biocompatibility have been successfully applied in bioimaging, broadening raw materials for preparation of CDs. GQDs from citric acid and dicyandiamide were pH sensitive with a good response range and linear relationship, which could be applied in the detection pH of the real water samples and cells, which broadened the scope of the applications of CDs. Furthermore, the preparation route for fabricating two kinds of CDs above was simple, green and could be used for mass production. But CDs above had a short emission wavelength. To solve the problem, the long wavelength emission system based on fluorescence resonance energy transfer between carbon dots and TPPS4 was constructed and the influence factors were ivestigated, which was significant for the research of the energy transfer system with CDs as energy donor.