Construction Of The Fluorescent Probes Based On The Novel Carbon Quantum Dots And Its Applications To Detection Of Heavy Metals Ions

Carbon quantum dots（CQDs）are a new type of carbon nanomaterials whose size are smaller than 10nm,which belong to zero-dimensional（0D）carbon nanoparticles.CQDs have a broad application prospect in sensor,biomedicine,catalysis and other fields due to its advantages of simple synthesis method,small size,good photostability,biocompatibility,tunable photoluminescence（PL）and good chemical stability.In particular,CQDs,as electron donor and acceptor,have excellent optical properties and can produce chemiluminescence and electrochemiluminescence,so it is widely used to construct fluorescent probes to detect metal ions and organic small molecules.The chemical structure of carbon cores and surfaces of CQDs can be adjusted to realize the response to different analytes through doping and surface functionalization.As a result,three types of fluorescent probes based on different carbon quantum dots were constructed in this paper,which were applied to the detection of cadmium ions(Cd²⁺),mercury ions(Hg²⁺)and lead ions(Pb²⁺).The main research contents can be summarized into the following three aspects:（1）2-Amino-3-hydroxypyridine and sodium borohydride were introduced as nitrogen source and boron source to fabricate nitrogen and boron co-doped carbon quantum dots（N,B-CQDs）with high fluorescence quantum yield（21.2%）,which were synthesized through one-pot hydrothermal method.Fourier transform infrared spectroscopy（FT-IR）and X-ray photoelectron spectroscopy（XPS）demonstrated the successful doping of N and B elements.The experimental results showed that N,B-CQDs and Cd²⁺form chelates,resulting in the phenomenon of chelation-enhanced fluorescence（CHEF）.Accordingly,a highly sensitive and selective fluorescence method for the detection of Cd²⁺was established with a linear range of 2.5-22.5μM and the limit of detection（LOD）of 0.45μM.At the same time,the fluorescence of N,B-CQDs recovered obviously after adding L-cysteine（L-Cys）because the binding force of L-Cys to Cd²⁺was greater than that of N,B-CQDs to Cd²⁺.Therefore,N,B-CQDs can also be used to detect L-Cys with a linear range of 2.5～17.5μM and a LOD of 0.28μM.The probe can be used for the detection of Cd²⁺and L-Cys in actual samples（tap water,lake water and human urine）,indicating that the fluorescent probe has good system applicability.（2）Sulfhydryl functionalized carbon quantum dots（HS-CQDs） were synthesized from dimercaptosuccinic acid as raw material.Based on the experimental results,Hg²⁺was shown to be capable of quenching HS-CQDs fluorescence by the static quenching effect.The non-fluorescent ground state complexes were formed between HS-CQDs and Hg²⁺and electron transfer occurred,resulting in the use of the HS-CQDs as a fluorescent probe.The fluorescent probe had a good linear relationship in the range of concentration of Hg²⁺from 0.45 to 2.1 m M,and the LOD was12 n M.Furthermore,the HS-CQDs were applied to the detection of Hg²⁺in lake water and domestic wastewater with recovery rates ranging from97.79%to 105.7%,which illustrated that the fluorescent probe has good performance and can meet the requirements of detecting Hg²⁺in the actual environment.（3）A kind of water-soluble carbon quantum dots（CB-CQDs） was prepared by one-step hydrothermal method using green,cheap and readily available coffee beans as precursors.The CB-CQDs can be used as an“on/off”fluorescent probe for the detection of Pb²⁺and pyrophosphate（PPi）.The fluorescence probe was quenched in the presence of Pb²⁺based on the internal filtration effect（IFE）.Meanwhile,due to the strong coordination between PPi and Pb²⁺,Pb²⁺was dissociated from the surface of CB-CQDs,which effectively restoring the fluorescence of CB-CQDs and realizing the detection of PPi.The experimental results showed that the linear ranges of CB-CQDs for the detection of Pb²⁺and PPi were 12～260μM and 37.5～125μM,respectively,and the LODs were 3.85μM and 0.18μM,respectively.The fluorescent probe can be used to identify Pb²⁺and PPi in lake water and human urine,and the recoveries ranged from97.0～105.5%,indicating that the fluorescent probe is reliable.In addition,the hemolysis rate experiments showed that the probe has good biocompatibility,which provides the possibility for applications such as bioimaging.