Synthesis And Sensing Application Of Red-Emitting Carbon Dots

In recent years,as a rising star in the field of photoluminescence（PL）materials,fluorescent carbon quantum dots（CQDs）have attracted great attention.With its many advantages,such as excellent optical properties（adjustable emission and high light stability）,excellent biocompatibility,good water dispersion,simple preparation method using cheap raw materials and surface modification,CQDs has been proved to be a promising luminescent material in various applications,especially in the biomedical field.However,most of the carbon dots reported so far are excited by short wavelengths,and the emitted light is mostly blue or green,which easily causes autofluorescence interference.However,red-emitting（>600 nm）CQDs have received more and more attention in the past few years due to their low damage to biological substrates,large tissue penetration depth and minimal autofluorescence background in biological samples.This thesis uses cheap and easily available materials to synthesize red fluorescent CQDs through a simple hydrothermal method.The specific research content mainly includes the following three parts:1.Using citric acid as the carbon source and p-phenylenediamine as the nitrogen source,red fluorescent carbon dots CQDs were prepared by a one-step hydrothermal synthesis method.The optimal excitation and emission wavelengths of the carbon dots were 490 and 618 nm,respectively.Ce⁴⁺can catalyze the oxidation of the colorless substrate 3,3′,5,5′-tetramethylbenzidine（TMB）to obtain the blue oxidation substrate ox TMB,which has a strong UV absorption peak at 652 nm.There is a large overlap with the emission peak of CQDs,so due to the inner filter effect,the presence of ox TMB will reduce the fluorescence intensity of CQDs.However,α-glucosidase can specifically hydrolyze L-ascorbic acid-2-O-α-D-glucopyranosyl（AAG）to produce ascorbic acid.Ascorbic acid reduces ox TMB to TMB,which leads to the disappearance of the inner filter effect and the recovery of fluorescence.This method can detectα-glucosidase activity as low as 0.02 U m L^-1and provides a dynamic linear range of 0.5-5.5 U m L^-1.In the presence of interfering substances,this strategy has good selectivity for the detection ofα-glucosidase.This method has been successfully used to determine theα-glucosidase activity in spiked human serum samples with satisfactory results.2.A method for the detection of trypsin activity was constructed.Using citric acid as the carbon source and p-phenylenediamine as the nitrogen source,red fluorescent carbon dot CQDs were obtained.β-Cyclodextrin（β-CD）is a cyclic oligosaccharide with a special molecular structure composed of a hydrophobic inner cavity and a hydrophilic outer surface.The cavity structure ofβ-CD enables it to form inclusions with other molecules.It can effectively enhance the fluorescence intensity of the system during the process of the inclusion of the phosphor.Experiments have found thatβ-CD can interact with the groups on the surface of CQDs to increase the fluorescence intensity of CQDs to form aβ-CD/CQDs system.In addition,it was found that Cu²⁺can quench the fluorescence ofβ-CD/CQDs system.Bovine serum albumin（BSA）is hydrolyzed by trypsin to produce amino acids or polypeptide fragments,which chelate Cu²⁺in the system to restore the fluorescence of theβ-CD/CQDs system.The concentration of trypsin is in the range of 5-30μg m L^-1,and the fluorescence recovery degree of the system is proportional to the concentration of trypsin.Based on this,a simple and efficient method for detecting trypsin has been established,with a linear range of 5-30μg m L^-1,and successfully used for the detection of trypsin concentration in actual samples.3.A ratiometric fluorescent probe was designed to detect Cu²⁺and S^2-.First,use chlorophyll extracted from fresh green leaves of heather as raw material,red fluorescent biobased quantum dots（BQDs）were obtained by solvent extraction and solvothermal method.The optimal excitation emission peaks of BQDs were 410 nm and 678 nm,respectively.When Cu²⁺was added into the composite system of BQDs and OPD,2,3-diaminophenazine（DAP）was synthesized in situ due to the oxidation of OPD by Cu²⁺.DAP showed a fluorescence emission peak at 570 nm under 410 nm excitation.At the same time,due to Cu²⁺will chelate with the pheophytin produced by chlorophyll,resulting in the quenching of the fluorescence of BQDs at 678 nm.Therefore,the ratio of the fluorescence intensity at 570 nm and 678 nm(I₅₇₀/I₆₇₈)can be used as the signal output of the ratiometric fluorescent probe to detect the concentration of Cu²⁺sensitively.In addition,due to the strong binding capacity between Cu²⁺and S^2-,this ratio fluorescent probe can also be used to quantitatively detect S^2-.Subsequent experiments found that the linear range of this probe to Cu²⁺and S^2-was 0.5-10μM and 1-12μM,respectively.This probe was also successfully used for the detection of S^2-in tap water samples.