Preparation Of Fluorescent Carbon Quantum Dots And Its Application In Ions And Amino Acids Detection

Carbon quantum dots（CQDs）,commonly known as"artificial atoms"in nanomaterials,have a very small size,usually less than 10 nm,so they have quantum size effects and surface effects,making CQDs have special photoelectromagnetic properties,and are therefore used in fluorescent probe detection,chemical sensing and light-emitting diodes,etc.,with good application prospects.The original CQDs have a large number of defects on the surface,as well as weak luminous intensity and low quantum yield（QY）,and CQDs can be modified by surface functionalization and heteroatom doping.At present,most of the modified CQDs have the advantages of stable fluorescence performance and specific detection of specified substances,but most of them are concentrated in short-wavelength emission.This project takes the improvement of optical properties of CQDs as the starting point,and adopts a bottom-up approach to appropriately select precursors containing aromatic rings and heteroatoms to prepare CQDs with long wavelength emission and high QY.Finally,blue,green and yellow fluorescent CQDs with high QY were prepared and applied to the analysis and detection of metal ions and amino acids.The main studies of the paper are as follows:1.Blue fluorescent CQDs were prepared by the bottom-up method using tryptophan（Trp）as a precursor for the detection of Fe³⁺and Cys.The CQDs were characterized and their morphology and optical properties were studied,showing blue fluorescence with bright fluorescence color and stable fluorescence intensity,with an excitation peak at 370nm and an emission peak at 445 nm,and its QY reached 52.64%.Adding ions to detect the substance for specific detection,it was found that only Fe³⁺could make CQDs blue fluorescence extinguished,and on the basis of quenching,amino acids were added to the substance to be detected,and it was found that only cysteine（Cys）could restore the fluorescence quenched by CQDs.Subsequently,the type and concentration gradient tests of the detected substances showed that CQDs had high sensitivity and selection specificity in detecting Fe³⁺and Cys,and the LODs detected by the electrochemical method were basically consistent with those of the fluorescence method.Subsequent CQDs were successfully applied to the detection of real samples of water samples and human urine.2.The conjugated structure of the precursor Trp in part 1 is not large and has no major advantage in long wavelength emission,so this part uses the organic semiconductor perylene-3,4,9,10-tetracarboxylic acid dianhydride（PTCDA）,which has a large conjugated structure and excellent performance,as the substrate raw material,and adds a certain volume of Na OH to prepare green fluorescent quantum dots G-CQDs by hydrothermal method,which can be used for the detection of p H and aspartic acid（Asp）.G-CQDs have a fluorescence quantum yield of 76.98%,show bright green fluorescence,and the fluorescence intensity is very stable,and there is no large change in fluorescence intensity for at least 30 days.The morphology,elements,functional groups and properties of G-CQDs were characterized by TEM,FT-IR,XPS and UV-vis.G-CQDs can visually detect p H and compare it with a p H test strip and a p H meter.By detecting 12 amino acids,only Asp could greatly quench the fluorescence of G-CQDs,analyze their fluorescence lifetime and UV-vis spectrum,and show that the mechanism was dynamically quenched,and verified by theory.Subsequently,G-CQDs were applied to the detection of actual samples,which showed feasibility and reliability.3.On the basis of part 2,heteroatom doping is considered to adjust the surface state to achieve longer wavelength emission.In this part,yellow fluorescent quantum dots YN-CQDs with 20.73%QY were prepared by adding urea on top of the prepared G-CQDs using the bottom-up method,which can be used for the specific detection of Fe³⁺.At the same time,G-CQDs-TH with thiourea and G-CQDs-CA with citric acid were prepared to study the effect of adding different substances on the wavelength of fluorescence emission.The introduction of nitrogen in urea can generate new surface state energy levels,which makes the degree of graphitic nitrogen increase and facilitates the products to move to longer wavelengths and exhibit yellow fluorescence.A series of characterizations of YN-CQDs were carried out to obtain information about the morphology,elemental composition and fluorescence of YN-CQDs.At the excitation wavelength of 470 nm,the influence of different metal ions on the fluorescence intensity of YN-CQDs was studied,and it was found that only Fe³⁺could effectively quench the fluorescence intensity of YN-CQDs,which was specific,and proved that the quenching mechanism was static quenching.YN-CQDs perform well in performance testing and can be applied to actual samples.