| Carbon Dots(CDs),as emerging star in carbonaceous nanomaterials since their discovery by purifying single-walled carbon nanotubes in 2004,have attracted great attention due to their excellent fluorescent properties,low environmental hazard,great biocompatibility and robust chemical inertness.The rising potential in applications of CDs in bioimaging,cancer therapy,sensor,optoelectronics and catalysis require for a kind of low-cost and large-scale production of this nanomaterial.Although variously synthetic techniques have been developed for preparations of CDs,they usually suffer from expensive precursors,special equipment,harsh synthetic conditions and complex process,which lead to low production yield and bottleneck of large-scale synthesis.How to synthesize CDs from the carbon source is another important issue.However,little attention has been paid to the formation process of CDs due to the difficulty of monitoring their complicated reaction path,which leads to the unclear formation mechanism.A deeper insight into the formation mechanism of CDs is of great importance not only for understanding the construction chemistry of carbon nanomaterials but also for guiding the large-scale synthesis.Electrochemical exfoliation of carbon source is an ideal large-scale production for synthesis of CDs due to not only facile method,easy operation and common precursors but also its ability of controlling the sizes of CDs by adjusting conditions of reaction.On the other hand,if CDs with special properties are obtained,it is necessary to further explore their applications.Hence,it is of great importance for theory and applications that the investigation of exfoliated CDs by electrochemical method in detail is carried out under different voltages and in various electrolyte.This dissertation includes the following five parties:1 The mixture ofionic liquid and distilled water as electrolyte electrochemical exfoliation of carbon dotsAn electrochemical approach have been introduced for synthesis of carbon dots(CDs)by exfoliating graphite rod at a voltage of 15 V in an mixed electrolyte consisting of water and ionic liquids.It can be easily found that these CDs were monodispersed and quasi-spheres in shape from transmission electron microscopy(TEM).High-resolution TEM(HRTEM)images show that CDs possessed distinct crystal lattice with lattice spacing around 0.21 nm,agreeing well with the(100)lattice plane of graphite,which is an sufficient evidence that the CDs were carbon genic materials.Fourier transform infrared(FTIR)and X-ray photoelectron spectroscopy(XPS)show the CDs to have oxygen functional groups on their surface which strongly improve solubility.Size-controlled CQDs were achieved by changing the voltages applied to the electrodes and adjusting the content of ionic liquid in electrolyte.The monodisperse and spherical CQDs with the size of 2.9,4.4 and 6.6 nm were obtained at voltages of 9,15 and 30 V,respectively.lt is found that the size of the CDs can be tuned by varying the fraction of water in the mixed electrolyte;CDs in sizes of 4.9,4.1 and 3.1 nm are obtained if the electrolyte contains water in fractions of 24,38 and 56%,respectively.Comparing to these two methods,it is easily found that the adjustment of voltage would obtained the CDs with larger sizes distribution.Although these CDs possess different sizes,they have similar behaviors in fluorescent emission and surface groups,which is attributed to similar reaction environment.2 Inorganic solutions as electrolyte electrochemical exfoliation of carbon dotsAlthough organic ionic liquids as electrolyte have a better effect,its high cost actually limit large-scale application.Hence,a further explore on low-costInorganic solutions as electrolyte has been made to instead of expensive ionic liquid.Some of inorganic salts such as sodiumchloride,potassiumchloride,sodiumnitrate,potassiumnitrate,sodiumbicarbonate and sodium hydrogenphosphate were used as electrolytes,and their influence on behaviors or properties of the obtained CDs was investigated.TEM images show that CDs exfoliated in the same anion solution possessed similar shape such as sodiumchloride,potassiumchloride,which indicated that the property of anion would affect the shape of CDs.Under the same conditions,CDs exfoliated in solution of nitrate ion were clearer than ones in other solutions from TEM images.This phenomenon may be result of the appearance of nitric acid during reaction,which is recognized to contribute to exfoliation.The spectra of FTIR and XPS show CDs have oxygen functional groups on their surface which strongly improve solubility,which are similar to each other.Applied voltage and concentration of solution have no effect on the sizes of obtained CDs.3 Distilled water as electrolyte electrochemical exfoliation of carbon dotsAfter finishing the investigation of organic and inorganic solution as electrolyte,distilled water was used as the only electrolyte for CDs exfoliated from carbon gods using electrochemistry approach.Due to conductibility of water very lower than inorganic solution,increasing applied voltage was used to accelerate the speed of reaction.The as-exfoliated CDs show weaker fluorescent intensity,but fluorescence spectra are the narrowest and only 10 nm of full width at half maximum(FWHM).Only very simple functional groups existed on the surface of CDs from X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy results.The narrowest FWHM of fluorescence spectra indicated the as-grown CDs possessed excellent monochromaticity and pure emission color,which may be attributed to simple surface groups.13C NMR spectrum of the CDs shoVed two main signals in the range of 120 to 200 ppm corresponding to sp2 carbons.The signals in the range of 120 to 150 ppm arose from aromatic carbon,while the signals in the range of 170 to 200 ppm were attributed to carboxylic carbon.The signals of 134 and 170 ppm represented C=C and C=O in the CDs,respectively,which were well agreement with and were also evidenced by the XPS and FTIR results.4 Bioimaging of bacteria using the exfoliated CDs as biomarkerAs the most important application,CDs have been widely investigated and utilized in the bioimaging or biological labeling.However,to the best of our knowledge,most of bioimaging experiments were carried out using various cancer cells,and there are few studies using bacteria as labeled specimen.After successfully synthesized CDs,we took leading in bioimaging bacteria using CDs.The bacteria of ShewanellaoneidensisMR-1 were cultured in 10 mL fresh LB containing aqueous solution of CDs at room temperature for a week.Then,exciting at three different wavelengths,the bacteria incubated with CDs showed three different colors,which was an evidence of eliminating autofluorescence.There was obvious and bright difference between bacteria and background in the higher resolution images with single bacteria.These evidenced that the CDs have been internalized by the bacteria... |
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