Synthesis Of Highly Luminescent Carbon Quantum Dots And Their Applications

Carbon quantum dots(CDs) are a carbon nanomaterials emerging in recent years. To fulfill the demands for practical application, proper ready-made or self-made carbon sources were chosen to prepare CDs taking the previous fabrication approaches into consideration. The effects of carbon sources and process conditions on the yields and performances of CDs were studied. The microstructure, surface state, thermal stability, the crystalline state and photoluminescence properties were analyzed by HRTEM, SEM, FTIR, XPS, TG, XRD, PL and other means. The relations between the physical and chemical characteristics of CDs and their fluorescent properties were explored. Then the CDs were tried to mark the cells. While CDs had shown great potentials in the photocatalytic area, the composites of CDs and titanium dioxide(Ti O2) were systhesized and the roles that CDs played in the enhancement of photocatalytic properties were stuied. The concrete work completed in this paper was summarized as follows:(1) Water-soluble fluorescent(CDs) were synthesized on a large scale by a facile, one-step and mild pyrolytic route using ammonium citrate as carbon source in air. The average diameter of CDs was 6 nm around and the higest quantum yield reached 26.1%. CDs were of amorphous graphite nature with abundant hydrophilic groups on the surface. With the rise of temperature, the yield and relative fluorescence quantum yield of CDs tended to decrease. However, compared with the CDs prepared in air, the fluorescence quantum yield of CDs obtained under nitrogen atmosphere decreased relatively slowly. And the surface functional groups of CDs prepared under N2 were richer. The CDs were of a typical wavelength-dependent PL feature. With the increase of the excitation wavelength, the maximum emission peak position shifted to longer wavelength gradually, accompanied with remarkable decrease of PL intensity. Compared with the CDs prepared in air, emission spectra of CDs obtained in N2 had a blue shift phenomenon under the same excitation wavelength. Furthermore, the red shift rate of CDs obtained in N2 was relatively uniform. With good water solubility, light stability and low cytotoxicity, CDs had been initially applied in fluorescence labeling of Hep G2 cells. CDs inside the Hep G2 cells could emit fluorescence with different colors under different excitation wavelengths. Other salts of citric acid, such as dihydrogen ammonium citrate and hydrogen diamine citrate, were also used to prepare CDs under the same conditions. And the fluorescent properties of obtained CDs were quite close to that CDs using ammonium citrate as carbon source.(2) CDs were prepared using citric acid as carbon source. The effects of surface passivation and metal doping on the composition, microstructure and fluorescence properties were studied. Mg-EDA-CDs were well distributed among a narrow range between 0.8 – 2.8 nm without apparent agglomeration. The CDs without any doping exhibited almost analogous particle size distribution and morphology compared with Mg-EDA-CDs. It could be inferred that no differences could be found on the final morphology of CDs with different carbon sources when the other process conditions were the same. Ana the resulting CDs were mainly amorphous carbon structure without complete and continuous crystal structure. Because of extremely high PL, blue fluorescent emission from aqueous solution of Mg-EDA-CDs could be easily observed under a UV lamp or even under a natural light. And the quantum yield of Mg-EDA-CDs was further enhanced to as high as 83.0%, which is the highest value for CDs reported so far and almost equal to that of organic dyes or semiconductor quantum dots. The non-doped CDs showed a weaker peak at 459 nm compared with that of Mg-CDs at 437 nm. Likewise, EDA-CDs exhibited an emission peak at 449 nm in contrast with a 437 nm peak of Mg-EDA-CDs. The results revealed that an unknowable blue-shift phenomenon occurred to the CDs once doped with magnesium. Besides N-mediated surface passivation by ethylenediamine, the Mg-citric acid chelate played the roles of introducing Mg and preserving the carboxyl group, both greatly contributing to the photoluminescence enhancement of the final CDs. With the increase of the excitation wavelength, the maximum emission peak position of Mg-EDA-CDs shifted to longer wavelength gradually but in a non-uniform mode, simultaneously accompanied with remarkable decrease of PL intensity. The PL intensity of Mg-EDA-CDs was very stable over the p H range of 3 – 11, which was favorable for CDs applications in the physiological and pathological environments with a normal p H range of 4.5 – 9.5. The Mg-EDA-CDs exhibited low cytotoxicity with more than 90% cells retaining viability when incubated in the medium containing 250 μg/m L or lesser CDs. Mg-EDA-CDs labeled L929 cells emitted blue, green and red light excited at 405 nm, 488 nm and 543 nm, respectively, whereas the control cells(without CDs labeling) showed nearly no visible fluorescence detected under the same conditions. And the isopropyl acrylamide gels would show multicolor when CDs were introduced into the polymerization process.(3) We designed a one-step solvothermal process to fabricate nanohybrid composites of CDs and Ti O2 in situ. The mixture of citric acid and tetrabutyl titanate dissolved in ethanol acted as carbon source and Ti O2 precursor separately. The H2 O generated from the dehydration and impartial carbonization of citric acid was immediately utilized to initiate the hydrolysis of tetrabutyl titanate, which was turned into the earliest sol-gel Ti O2 and final Ti O2 nanocrystal. In this way it is expected to realize a maximum contact between the as-prepared CDs and Ti O2 in the final composites for the benefit of an enhanced photocatalytic performance. In the panoramic view of SEM, the samples were all microspheres with diameter no more than 10 μm. The basic unit of the microspheres was the observed nanoparticles, which further joined together to form the final microspheres. The basic nanoparticles were probably composed of CDs, Ti O2 nanocrystals and a very few residues of partially reacted precursor molecules. It is easy to find the existence of Ti O2 as an imperfect crystal of anatase in the CDs-Ti O2 composites. But with prolonged reaction time, the diffraction peaks attributing to anatase Ti O2 became visible and acute gradually from sample C-6 to C-12. But after a further calcination merely at 400°, rutile Ti O2 was found, together with the predominant anatase Ti O2. The proportion of rutile Ti O2 increased quickly with enhanced annealing temperature, and the ratio of anatase/rutile in sample C-450 was very close to that of commercial Degussa P25. Almost all the characteristic peaks of the CDs-Ti O2 composites could be found in the lineshape of CDs. While the further calcination realized the finely doping of inert carbon and Ti O2, as most of surface functional groups were sacrificed in the calcination. The further calcination mainly consumed CDs and previous unreacted precursors in sample C-9, which probably converted the CDs-Ti O2 composites into C-Ti O2 composites. Sample C-9 was a nanohybrid of CDs and Ti O2, and most CDs were converted into inert and insoluble carbon materials in sample C-450 after the calcination. When a 10 min irradiation of UV light was carried out on the 10 ppm MB solution containing sample C-9, decontamination of MB was almost complete(ca. 95%). In fact, all the CDs-Ti O2 composites were efficient on photo-degradation of MB under UV light, and sample C-9 was the best one. A complete photo-degradation of MB(ca. 96%) for C-450 required 40 min of UV light irradiation. The CDs-Ti O2 composites were good at the photo-degradation of MB. As to MO and RB, the C-Ti O2 composites exhibited more advantages. The CDs-Ti O2 composites were efficient on photo-degradation of MB, besides a special absorption capability for MB. However, the photocatalytic activity would be weakened if too many MB were absorbed. The superior adsorption of MB for the CDs-Ti O2 composites did not only stem from their big specific surface areas, but also probably from the microporous structure and aboundant surface functional groups introduced by CDs. However, it seemed that the superior adsorption of CDs-Ti O2 composites functioned on MB only. As to MO and RB, the big specific surface areas and proper proportion of anatase/rutile content of the C-Ti O2 composites were more advantageous.