Detection Method Of Heavy Metal Ions Based On Fluorescent Carbon Dots Sensing

As one of the global environmental problems,heavy metal pollution has received a great deal of attention in its related testing research.So far,traditional methods that can realize the sensitive detection of heavy metal ions include atomic absorption spectroscopy,mass spectrometry,fluorescent probe method,etc.However,some detection methods are limited by the tedious detection process,difficult to carry instruments or strong professional operability and other factors,which can’t meet the requirements of on-site rapid monitoring and real-time analysis.With the rapid development of optical sensor research in recent years,some nanoparticles,conjugated polymers,organometallic frameworks,quantum dots and organic dyes have been widely used as optical sensors in various analysis and detection.As a new type of carbon nanomaterials,carbon dots（CDs）have unique properties due to the excellent photoluminescence properties,abundant precursor sources,easy surface functionalization for different purposes,good biocompatibility and low toxicity.It has aroused great interest of researchers and has been widely used in environmental field.Its fluorescence response mainly has four modes:fluorescence quenching,fluorescence enhancement,fluorescence response energy transfer and ratiometer response.Based on the design principle of fluorescence sensor,this paper constructs sulfur,nitrogen doped carbon dots fluorescence sensor,CTAB aggregation-induced carbon point sensor and gold nano-cluster hybrid carbon point fluorescence sensor,respectively.A fluorescence method for selective determination of chromium Cr（VI）,mercury Hg（Ⅱ）,lead Pb（Ⅱ）and ascorbic acid AA was established and successfully applied to environmental water.The main research contents are as follows:（1）S,N co-doped carbon dots and its fluorescent film sensor research on the identification method of Cr（VI）and ascorbic acid:A simple,economical and green method was proposed to prepare sulfur and nitrogen co-doped carbon dots with strong fluorescent properties（S,N-CDs）,the quantum yield was 17%.The sensing principle is due to the construction of a new internal filter effect（IFE）system,which causes the Cr（Ⅵ）to quench the fluorescence of S,N-CDs.The presence of AA could reduce Cr（Ⅵ）to Cr（Ⅲ）and eliminate the IFE effect,thereby restoring the fluorescence of S,N-CDs.The sensor showed good selectivity and anti-interference ability to more than 30 common interfering substances in water.The linear range and detection limit of Cr（VI）were 0.03-50μmol/L and 21.14 nmol/L,respectively.The linear range and detection limit of AA were 1-1000μmol/L and 0.28μmol/L,respectively.In addition,a portable thin film sensor was obtained by encapsulating S,N-CDs in a polyvinyl alcohol（PVA）gel matrix,which could detect Cr（VI）and AA intuitively and sensitively.The linear range/detection limit for Cr（VI）and AA detection were 0.1-50μmol/L and 10-500μmol/L（R²=0.998）,and the detection limits were 92.48 nmol/L and 6.99μmol/L,respectively.In order to prove the practicability of the fluorescent substrate,it was further applied to the detection of Cr（VI）and AA in lake water and commercial orange juice.（2）Research on Recognition of Hg（Ⅱ） Based on CTAB Aggregation Induced Fluorescent Carbon Dots:This work reported an economical and sensitive fluorescence sensor for Mercury（II）Ions based on the cetyltrimethylammonium bromide-mediated nitrogen-doped carbon dots.Nitrogen-doped carbon dots（NCDs）were synthesized by a one-step hydrothermal technique using citric acid,urea and N,N-dimethylformamide.At this time,the quantum yield of NCDs was low（6.2%）,and it could be quenched with a variety of metal ions(Cu²⁺,Cr⁶⁺,Hg²⁺,Co²⁺,Fe³⁺).After the addition of cetyltrimethylammonium bromide（CTAB）,the electrostatic interaction between NCDs and CTAB resulted in the aggregation of NCDs,the quantum yield of NCDs was increased to 31.7%.The spatial structure formed by CTAB/NCDs could selectively capture Hg²⁺and made it interact with the surface groups of NCDs to cause fluorescence quenching.The fluorescence intensity and Hg²⁺concentration showed a terrific linear response in the range of 0.15-1 mmol/L and 0.16-10.24μmol/L（R²=0.998）,and the detection limit was 85.71 nmol/L.The fluorescent probe was used to determine Hg²⁺in environmental waters,and satisfactory results were obtained.（3）Quantitative Detection of Pb²⁺by Fluorescent Probe Based on Nitrogen doped Carbon Dots-Gold Nanocluster Ratio:The preparation of a double emission carbon point gold nanocluster（NCDs-Au NCs）composite was introduced.Using citric acid,ethylenediamine and water as raw materials,NCDs was prepared by hydrothermal synthesis.Based on this,a composite sensor（Au NCs-NCDs）of gold nanoclusters and carbon points was prepared with glutathione as reductant.The prepared sensor showed double emission peaks with high fluorescence resonance energy transfer at 460 nm and620 nm at a single excitation wavelength of 360 nm.The addition of Pb²⁺significantly enhanced the red fluorescence of Au NCs,while the blue fluorescence of NCDs was quenched.Quantitative detection of Pb²⁺was carried out based on the changes in the specific gravity of their fluorescence.There was a good linear relationship between the fluorescence efficiency and the concentration of Pb²⁺in the range of 0.86-20μmol/L,and the detection limit was calculated to be 0.68μmol/L.The fluorescence sensor has been successfully used to detect Pb²⁺in real water samples and made into a paper-based sensor,which can realize visual semi-quantitative determination with naked eyes.