Synthesis Of Fluorescent Probes Based On Carbon Dots And Their Applications In The Detection Of Heavy Metal Ions And 2019-ncov Antibodies

Carbon dots(CDs),with characteristic sizes less than 10 nm,have become a rising star in carbon nanomaterials.Carbon dots have many unique physicochemical properties,such as excellent optical properties,less biotoxicity,better biocompatibility,and higher chemical stability.They are widely used in imaging,sensing,and catalysis.In recent years,carbon dots have attracted more attention in the field of fluorescence sensing.This demonstrates its efficient,fast,highly sensitive,and highly selective sensing performance for pollutants in the environment.By optimizing the synthesis parameters of the carbon dots and modifying the surface of the carbon dots,more excellent fluorescence properties were obtained.In this way,its sensing efficiency is improved.In this paper,nitrogen doped carbon point fluorescence sensor(N-CDs-CTAC)induced by cetyltrimethylammonium chloride(CTAC)aggregation,and nitrogen and sulfur co-doped carbon point fluorescence sensor modified by polyethylene imine(PEI)surface were designed based on the design principle of carbon point fluorescence probe.They were respectively applied to the detection of Cr(Ⅵ)and Pb2+ in the environment,and were successfully applied to the detection of actual water bodies.In addition,a graphene quantum dots(GQDs)-Ag@Au nanoparticles(Ag@AuNPs)complex fluorescent sensor was established to achieve sensitive detection of novel coronavirus antibodies(2019-nCoV mAb).And the fluorescence sensor was successfully applied in artificial serum.The main research contents are as follows:(1)The study on the detection of Cr(Ⅵ)by CTAC aggregation-induced carbondot fluorescent probes:Nitrogen-doped green fluorescent carbon dots(N-CDs)were successfully synthesized by a solid-state reaction method using ammonium citrate and urea as precursors.The carbon dots obtained the best fluorescence emission spectrum at 535 nm under the excitation wavelength of 405 nm.After the introduction of other metal ions including Cr(Ⅵ),the fluorescence of N-CDs did not change significantly.The addition of the cationic surfactant cetyltrimethylammonium chloride(CTAC)to the N-CDs resulted in the aggregation of the N-CDs due to the electrostatic interaction between them,resulting in a significant enhancement of the fluorescence of the N-CDs.Because of the sensitization effect of surfactants on N-CDs,N-CDs can be used for selective detection of Cr(Ⅵ).By comparing the emission spectrum of N-CDs-CTAC with the absorption spectrum of Cr(Ⅵ),as well as the measurement of the fluorescence lifetime of N-CDs-CTAC without Cr(Ⅵ)and containing Cr(Ⅵ),it is proved that the fluorescence quenching of N-CDs-CTAC by Cr(Ⅵ)is a dynamic quenching process.The degree of fluorescence quenching(F0-F1)and the logarithm of the concentration of Cr(Ⅵ)showed a good linear relationship in the two ranges of 0.5-100 μM(R2=0.9960)and 100-1000 μM(R2=0.9970).The detection limit of Cr(Ⅵ)was calculated to be 39.98nM.This method is simple,fast,low-cost and environmentally friendly.It has been successfully applied to the detection of Cr(Ⅵ)in actual water samples.(2)The study on the recognition of Pb2+by PEI surface-modified carbon-dot fluorescent probes:Nitrogen-sulfur co-doped carbon dots(CDs)with green fluorescence were synthesized by a simple one-pot method using L-cysteine and hydrogen peroxide as precursors.The surface modification of CDs by the highly branched polymer capping agent PEI,PEI-CDs with better fluorescence properties were prepared.Under excitation at 410 nm,the carbon dots obtained the best emission spectrum at 515 nm.The fluorescence of CDs without PEI modification can be quenched by various metal ions(Pb2+,Hg2+,Co2+).However,the fluorescence quenching phenomenon of CDs modified by PEI occurs only after adding Pb2+.The selectivity of the fluorescent probe was improved.This is because highly branched polymer end sealers provide high surface coverage for carbon points and effectively protect the CDs surface.This allows other metal ions to bind at the end of the branched PEI,preventing them from binding to the CDs surface.However,Pb2+is only loosely bound to PEI due to its larger ionic radius and closed-shell electron configuration.Therefore,it can penetrate the capping layer and coordinate with the carboxyl groups on the surface of CDs,which in turn leads to the quenching of the fluorescence of CDs.The ratio of fluorescence quenching degree and PEI-CDs fluorescence intensity(F0-F1)and Pb2+ concentration in the range of 0.5200 μM(R2=0.9980)and 200-400 μM(R2=0.9986)have a satisfactory linear relationship.The lowest detection limit was 0.270 μM.The fluorescence sensor was successfully applied to tap water and lake water samples,and it is an effective and rapid detection method.(3)Fluorescence immunoassay rapid detection of 2019-nCoV antibody based on the fluorescence resonance energy transfer between graphene quantum dots and Ag@Au nanoparticle:Severe acute respiratory syndrome coronavirus 2(SARS-CoV-2),which has dramatically changed the world,is a highly contagious virus.The timely and accurate diagnosis of SARS-CoV-2 infections is vital for disease control and prevention.Here in this work,a fluorescence immunoassay was developed to detect 2019 Novel Coronavirus antibodies(2019-nCoV mAb).Fluorescent graphene quantum dots(GQDs)and Ag@Au nanoparticles(Ag@AuNPs)were successfully synthesized and characterized.Fluorescence resonance energy transfer(FRET)enables effective quenching of GQDs fluorescence by Ag@AuNPs.With the presence of 2019-nCoV mAb,a steric hindrance was observed between the Ag@AuNPs-NCP(2019-nCoV antigen)complex and GQDs,which reduced the FRET efficiency and restored the fluorescence of GQDs.The fluorescence enhancement efficiency has a satisfactory linear relationship with the logarithm of the 2019-nCoV mAb in a concentration range of 0.1 pg mL-1-10 ng mL-1,and the limit of detection was 50 fg mL-1.The method has good selectivity.When the serum sample was spiked with 2019-nCoV mAb,the recovery rate was between 90.8%and 103.3%.The fluorescence immunosensor demonstrates the potential to complement the existing serological assays for COVID-19 diagnosis.