Preparation And Analytical Applications Of Fluorescent Carbon Dots

Carbon dots（CDs）as a newly emerging carbon nanomaterial at the beginning of this century have shown promising application prospects in some fields such as detection,in vivo/vitro imaging and disease diagnosis,due to the luxuriant carbon sources,easy preparation,facile modification,water-solubility,excellent photostability,low cytotoxicity and altitudinal biocompatibility.Based on the preparation and application of carbon dots,this paper realized the macro-preparation of green and yellow fluorescent carbon dots from suitable organic carbon sources via simple and quick methods.The sensing platforms based on as-prepared carbon dots and their conjugates were also established for detection of drug molecule and imaging identification of cancer cell.The specific contents are as follows.Chapter 1:Carbon dots were summarized from aspects of composition,structure,optical characters and biological security,and further review was written on preparation methods and analytical applications of carbon dots.Chapter 2:The green fluorescent carbon dots with high yield and good properties were rapidly synthesized by microwave heating method using active dry yeast as the bio-carbon source.The average synthetic yields of50.59%±0.98%for repeated experiment and 50.76%±1.29%for scale-up preparation,manifesting the glorious repeatability and reproducibility,allowed the CDs macro-preparation.Characterization results indicated that synthesized CDs were monodisperse and spherical particles with an average particle diameter of about 3.4 nm and possessed water-soluble groups like–OH,–COOH and–NH₂on their surface.The CDs exhibited a UV absorption peak at 280 nm,a maximum emission at 522 nm and a quantum yield of about 13%.The sensitive detection method for pantothenic acid was established based on fluorescence quenching due to the hydrogen-bond interaction between CDs and PA.The detection linear range was 5.0×10^-86.0×10^-66 mol·L^-11 and detection limit was 2.5×10^-99 mol·L^-1.The established analytical method was unaffected by common ions,carbohydrates,amino acids and some other water-soluble vitamins so exhibited satisfactory selectivity.Besides,the proposed method had been successfully used to detect PA in urine samples and the recovery rates were 95.20⁹8.62%.Chapter 3:In view of the strong affinity between folic acid（FA）and folate receptor（FR）on the surface of cancer cells,the FA was grafted to the surface of CDs prepared in the previous chapter by EDC/NHS cross-linking reaction to obtain folic acid-carbon dots（FA-CDs）conjugates with targeting ability.The successful coupling of FA and CDs was confirmed by comparing characterizations of FA,CDs and FA-CDs conjugates.And the immobilized amount of FA on the surface of CDs was approximately 309μg·mg^-1.Imaging studies indicated that the CDs showed nearly the same fluorescence results in HepG2 and PC12 cells imaging,having no recognition function,while FA-CDs conjugates showed obvious fluorescent differences because they retained both the green fluorescence of CDs and the specific recognition ability of FA.FA-CDs conjugates could effectively recognize HepG2 cells with highly-expressed FR from the mixed model of HepG2 and PC12 cells by FR-mediated endocytosis.The FA-CDs probes presented desired targeting imaging quality and had a promising application prospect in sensitive and specific early cancer diagnosis.Chapter 4:The yellow fluorescent carbon dots with high synthesis yield and high fluorescence quantum yield were prepared by the low-temperature heating method using saccharides with different relative molecular masses as carbon sources.It was found that the smaller the relative molecular mass of saccharide,the shorter the required reaction time.Then glucosamine was individually chosen as the carbon source to synthesize CDs and the orthogonal design was used for selecting the optimal synthesis conditions.Under optimal conditions,the synthesis yield of repeated experiment was18.25%±0.34%,which demonstrated the feasibility of macro-preparation.Characterizations results indicated that the CDs was spherical,granular and well-dispersed and possessed an average size of around 4.5 nm,a UV absorption at 283 nm,a maximum emission at 538 nm and a high fluorescent quantum yield of 23.56%as well as–OH,–COOH and–NH₂ on their surface.FA and CDs were bonded to form FA-CDs conjugates and the immobilized amount of FA on CDs was about 162μg·mg^-1.Imaging results studying PC12,SMMC7721 and MCF7 cells demonstrated that CDs were unable to distinguish these cells whereas the probe based on FA-CDs conjugates could clearly recognize cancer cells according to the expression level of FR.And the yellow fluorescence imaging could improve the sensitivity of identification effectively,which may provide new tools for diagnosis of early cancer and play an invaluable role in fluorescence-assisted surgical resection of tumor.