Design And Preparation Of Multifunctional Fluorescent Carbon Dots And Application In Heavy Metal Detection

Carbon quantum dots have the advantages of simple synthesis,low cost,good chemical stability and photostability,no scintillation fluorescence,low cytotoxicity,good biocompatibility,and adjustable excitation and emission spectra,especially in biosensors,chemical analysis,environmental monitoring and other practical applications have a good application prospects.The purpose of this paper is going to synthesize fluorescent carbon quantum dots by one step hydrothermal method（folic acid,ascorbic acid,glutathione as carbon source）.The surface of the obtained carbon quantum dots contains hydroxyl groups,amino groups and other groups which are easy to combine with heavy metal ions,resulting in the change or quenching of the fluorescence color.In addition,we also explore the applications of carbon quantum dots in paper sensors,cell imaging and fluorescent ink.A new blue emission carbon dots（FA-CQDs）are synthesized by one-step hydrothermal method.The fluorescence spectrum of FA-CQDs shows the maximum emission wavelength of 445 nm and the excitation wavelength of 360 nm.The Hg²⁺quenches the fluorescence of FA-CQDs with the limit as low as 1.29 nM.With adding saturated EDTA solution to the FA-CQDs+Hg²⁺complex solution,the fluorescence color of the initial FA-CQDs solution was quickly restored.The reusability of the“on-off-on”fluorescent switch has been proved to be a good performance of the recycling experiment with alternating addition of Hg²⁺.Moreover,paper sensors are manufactured and used for the visual detections of Hg²⁺with the limits of 0.1μM.Besides,the biological studies shown that the FA-CQDs have the characteristics of high fluorescent contrast and low cytotoxicity in cells.Thus,the obtained FA-CQDs can be treated as fluorescent ink and application in cell imaging.The highly luminescent blue fluorescent carbon quantum dots（CQDs）has been demonstrated via the one-step hydrothermal method using L-Ascorbic acid（LAA）as a carbon source.The strongest fluorescence emission was obtained at410 nm under excitation at 330 nm.The carbon quantum dots（LAA-CQDs）demonstrated high sensitivity and selectivity for detecting Cu²⁺and Hg²⁺based on the efficient fluorescence quenching effect with the lowest limits were 7.59nM and 2.29 nM,respectively.Moreover,paper sensors were fabricated and showed a clear fluorescence color cancellation after exposed to certain amount of the Cu²⁺and Hg²⁺with the visual detection limits of 5μM and 3μM,respectively.The cyclic utilization of the paper sensors was realized by restoring the fluorescence of paper sensors after soaking them into saturated ethylenediamine tetraacetic acid,which greatly reduced resource consumption.The LAA-CQDs has been successfully applied to detection in real environmental water samples with good results.In addition,we have verified the low toxicity and good biocompatibility of LAA-CQDs by cytotoxicity experiments and cell imaging.A dual emissive fluorescent carbon dots（GSH-CQDs）were synthesized by green thermal treatment method and used as a colorimetric fluorescent probe for multi-detection of Zn²⁺,Mn²⁺and Cu²⁺individually or simultaneously.The GSH-CQDs exhibit dual fluorescence emission at 460 nm and 683 nm respectively,which resulted in cyan fluorescence emission under UV light.The dual emissive GSH-CQDs can be used as a multifunctional colorimetric fluorescent nanoprobe for precise and quantitative detection of Zn²⁺,Mn²⁺and Cu²⁺by dual color fluorescence conversion.With the action of Zn²⁺,the fluorescent color of GSH-CQDs was transformed from cyan to pink.When exposed to Mn²⁺or Cu²⁺,a distinguishable fluorescence color transformed from cyan to blue.When the three ions existed simultaneously,the cyan fluorescence emission changed to nattier blue.The minimum limits of detection（LOD）for Zn²⁺,Mn²⁺and Cu²⁺were calculated to be 9.64 nM,3.24 nM and 1.7 nM,respectively.As far as we know,this work was the first time to report the dual-emissive carbon dots for multi-detection of heavy metal ions.Moreover,the multi-detection for Zn²⁺,Mn²⁺and Cu²⁺in real environmental water by GSH-CQDs was also investigated.We reported a dual emissive multicolorful fluorescent-nanoprobe which was composed of gold nanocluster（BSA-Au NCs）and carbon dots（CQDs）with a certain proportion,i.e.Au NCs-CQDs.The dual emissive Au NCs-CQDs showed pink fluorescence under UV light,and the dual emission peaks at 625 nm and 444 nm were corresponded to the existence of Hg²⁺and Cr（VI）respectively.The fluorescence color transformed from pink to blue when in presence of Hg²⁺.The blue fluorescence emission of CQDs selectively quenched by Cr（VI）,whereas the red fluorescence of BSA-Au NCs was stable against Cr（VI）,which can be treated as internal reference.The difference response of the two emissions to Hg²⁺and Cr（VI）resulted in a distinguished fluorescent color change,which can be noticed by the naked eyes clearly.When the Hg²⁺and Cr（VI）were exposed to the Au NCs-CQDs simultaneously,the dual-emission of the Au NCs-CQDs were all reduced and exhibited the complete quenching of the pink emission.The detection limits for Hg²⁺and Cr（VI）were 1.85 nM and 5.34 nM,respectively.Significantly,it was successfully applied to determine the existence of Hg²⁺and Cr（VI）in real samples,which offered the advantages of simplicity,environmentally friendly and cost saving efficiency.