Synthesis And Their Application Of Carbon Nanomaterials

Recently, carbon-based nanomaterials such as graphene and carbon quantum dots (CDs) emerge as new stars in carbon family and attract extensive attentions due to their extraordinary physicochemical properties. It is known graphene or reduced graphene nanosheets (RGN) are highly conductive but not luminescent because they are nearly zero-bandgap materials. However, fluorescent CDs with excellent optical properties can be obtained when size of carbon materials decrease to below10nm, which have potential applications in optoelectronics and bioimaging devices. There are a variety of methods to prepare CDs, which result in their different structures and thus optical properties. In this paper, a new carbon source, kojac flour (KF) was used to synthesize nitrogen-doped CDs (N-CDs) by one-step pyrolysis process. The morphology, structure and photoluminescent (PL) property of the obtained N-CDs have been studied. And their applications in live cell imaging and photocatalysis have also been investigated. In addition, different reducing agents such as tartaric acid, malic acid and oxalic acid were employed to treat graphene oxide nanosheets (GO), which created diverse surface characteristics that would in turn affect their electrochemical performances. The main works are included as follows:1. A facile and green method to synthesize N-CDs with high quantum yield (QY).The N-CDs were prepared by pyrolysis of KF under mild conditions followed with a simple extraction by ethanol and water repeatly. The QY of the N-CDs in distilled water and ethanol is13%and22%, respectively. Moreover, the PL properties of the N-CDs have been studied, and the results can be summarized as follows:(1) The N-CDs has an excitation-dependent behavior;(2) The N-CDs exhibit an excellent pH-switched luminescence, and show strong emission intensity in the alkaline medium (pH-9), which is two-fold higher than that in acidic solution (pH-5), but the luminescence does not quench under acidic condition;(3) The N-CDs show a concentration-dependent behavior, the optimum excitation and emission wavelength of the CDs will red-shift with the increasing of concentration of N-CDs, and the PL intensity of N-CDs reaches the highest value at concentration of1.0mg mL-1;(4) the PL intensity of N-CDs can be enlarged by non amine-terminated PEG passivation with increasing molecular weights;(5) the N-CDs show a fluorescence "off-on" behavior by mixing with Fe3+and NaOH, L-Arg, or L-lysine, that is to say, Fe3+can quench the fluorescence of N-CDs, upon the addition of NaOH, L-Arg, or L-lysine into CDs/Fe3+solutions, the fluorescence of N-CDs can be recovered. More interestingly, the addition of L-Lys will lead to an obvious red-shift (Δλ,=20nm) of PL emission while the color of the mixture of CDs/Fe3+and L-Lys aqueous solution turns from blue into green. Moreover, the quenched N-CDs/Fe3+complex will show fluorescence again in cells after they are internalized by HeLa cells.2. The cytotoxicity of N-CDs and their application in live cell imaging in vitro. The HeLa cells were selected as a model cell line to study the cytotoxicity of the CDs by a double-standard MTT assay. The results indicate that the cytotoxicity of the N-CDs show a concentration-dependent behavior, and the HeLa cells still retained viability more than75%even treated with the N-CDs at a concentration of200mg mL-1for24hrs. Further, after2h incubation with the N-CDs of200mg mL-1, the N-CDs were taken up by cells and localized in both cell membrane and cytoplasm under observation of a confocal laser scanning microscopy in z-stacked scanning mode, in the meanwhile, the cells with N-CDs showed multi-colors under different excitation wavelengths.3. The photocatalytic activity of N-CDs. The naked N-CDs were used to degrade methyl orange and Rhodamine B solutions at room temperature and the visible light obtained from a300W Xe arc lamp equipped with an ultraviolet cutoff filter (λ>420nm). The commercial TiO (P25) nanoparticles were also used as a control. The results show that the degradation rate of50mL10mg L"1methyl orange and Rhodamine B solutions can reach95.4%and69%after adding2mg N-CDs, respectively, which is as good as that of P25.4. Different surface characteristics of GO after reduction by three different acids. The GO was synthesized by modified Hummers, and the RGN1, RGN2and RGN3were prepared by treating GO with tartaric acid, malic axid and oxalic acid, respectively. The results reveal that oxygen-containing groups such as-OH,-COOH and-C=O on the GO surface have been removed and thus RGN are obtained with residual C-O-C groups. Interestingly, the number of sp2domains of RGN increases as treated by tartaric acid<malic acid<oxalic acid along with the steric hindrance decreasing and the altering ionization constants for the three acids (malic acid<tartaric acid<oxalic acid). Furthermore, the conductivity of GO has been improved and the specific capacitances of GO have been greatly promoted from2.4F g’1to100.8,112.4, and147F g-1after treated with the three acids respectively. The difference in both steric hindrances as well as ionization constants of the three acids is the main reason for different surface characteristics and reduction degree of GO, and the steric hindrance of acids might play a more important role than ionization constants of acids for the reduction of GO in this work.