Rapid Fabrication Of Carbon Dots And Carbon Dots-Based Composites And Their Applications In Fluorescent Analysis

Carbon dots（CDs）are newly emerging fluorescent carbon-based nanomaterials with sizes often below 10 nm.CDs not only inherit low-toxicity and excellent biocompatibility from carbon-based materials,but also possess many unique features,for instance,highly tunable fluorescent emissions,good water solubility,non-blinking,easy preparation and convenient surface functionalization,which show broad prospects of applications,including bio-imaging,drug delivery,photoelectric device and fluorescent analysis.Up to now,considerable synthetic strategies for CDs have been explored.However,most of them suffer from various drawbacks,including expensive starting materials or rigorous experimental conditions,the use of strong acid or oxidizing chemicals,complicated preparation and purification process,which greatly impede the large-scale synthesis of CDs.Meanwhile,the application of CDs as fluorescent probes has aroused scientific attentions in recent years,but it is still challenging to develop novel analytical model and explore the detection mechanism in depth.In this thesis,we are committed to develop facile,efficient and green preparative routes for CDs with high fluorescent quantum yield,and explore the formation and fluorescence emission mechanism.Combining the unique optical properties of CDs and the absorption ability of other materials,we hope to design multi-functional composites and establish novel fluorescent probes for the sensitive and selective detection of metal ions or organic pollutants in food and environmental samples.In preface part,we firstly give a brief introduction to the preparation and properties of carbon dots,as well as their recent progress in fluorescent analysis.Then,we conduct our research from the following five aspects:1.Fluorescent CDs were prepared by a one-pot microwave-assisted method in the presence of phosphates.Many small-molecular organic compounds,such as citric acid and ammonium citrate,could serve as the carbon precursors.The CDs obtained from ammonium citrate showed the highest fluorescent quantum yield（¹4.6%）.The morphology,structure and optical properties were characterized by transmission electron microscopy,X-ray diffraction spectrometer,Fourier transform infrared spectrophotometer,UV-vis absorption and fluorescence spectrophotometer.The as-prepared CDs showed good water solubility,chemical stability and non-blinking emissions.Hg²⁺ could efficiently quench the fluorescence of CDs via electron or energy transfer process.Subsequently,the fluorescence was recovered with the addition of histidine（His）,cysteine（Cys）and glutathione（GSH）due to their relatively strong affinity with Hg²⁺.On this basis,the CDs were applied to the rapid fluorescent detection of His,Cys and GSH in human serum samples with satisfactory results.2.Water-soluble CDs were prepared by microwave pyrolysis acetic acid in an immiscible system.This method was facile,efficient and free of any complicated purification process.The CDs exhibited strong emissions at 445 nm with a high quantum yield of ⁴0%.Meanwhile,we firstly reported the possible delayed fluorescence emissions from the CDs,and the corresponding emission mechanism has been speculated.The CDs demonstrated obvious p H-dependent emissions due to the protonation-deprotonation of surface carboxyl groups.Based on this phenomenon,we established a sensing platform for the detection of p H.Common anions,cations or compounds showed little effects on the fluorescence of the CDs.Therefore,the as-prepared CDs could serve as sensitive p H sensors in environmental water and urine samples.3.Many synthetic strategies have been explored for the rapid fabrication of CDs,but most reported CDs only show strong emissions in blue-light region,which restricts their practical applications.Meanwhile,the fundamental investigation of the preparation mechanism is always neglected.In this part,we developed a solvent-free preparative route for N-doped CDs（N-CDs）.The as-prepared N-CDs showed a uniform size distribution,bright yellowish-green emission and a high fluorescent quantum yield of ⁷3.2%.The products obtained at different formation stages were characterized systematically,and a possible formation mechanism has been proposed including intermolecular dehydration,polymerization and carbonization.Afterwards,the N-CDs were applied to the Fe3+ and Fdetermination based on the fluorescence “on-off-on”.The detection limits of Fe3+and Fwere 50 nmol L-1 and 75 nmol L-1,respectively.4.Fluorescent CDs were encapsulated in Zn（OH）₂ nanosheets through a template-free microwave-assisted route.The CDs/Zn（OH）₂ composites demonstrated uniform size distribution,excellent fluorescence property and fast adsorption capability for metal ions.Hereon base,CDs/Zn（OH）₂ composites were employed as an ultrasensitive fluorescence sensor for the rapid removal and detection of Cu²⁺ and Hg²⁺ in water samples.Aided by suitable chelating agents,exclusive and reproduced detection could be expected.This assay showed remarkable advantages,including operational simplicity,low cost,high sensitivity,low detection limits and acceptable reproducibility.It is envisioned that such sensing platform is promising in the enrichment,removal and detection of heavy metals in both online and off-line modes.5.A simple microwave-assisted route was utilized to graft molecularly imprinted polymers onto the surface of CDs via a typical sol-gel polymerization.Tetracycline（Tc）served as template.The obtained composites not only demonstrated good fluorescence stability and imprinted recognition sites for Tc,but also could selectively accumulate template molecules.A sensitive and selective fluorescence assay has thus been established.Under optimal conditions,the detection linear range for Tc was 0.02¹4 μM with a detection limit of 5.48 n M.The practicality of this sensing platform for Tc determination in milk samples was also validated,revealing satisfactory sensitivity,selectivity and reproducibility.In the spiked experiments,the recoveries of Tc were in the range of 97.3¹05.3%.