Visual Detection Of Heavy Metal Ions In Aqueous Phase

Water is the most important natural resource.With the further development of modern industry,heavy metal ion pollution in the water environment has great harm to the environment and human health,especially mercury ions(Hg²⁺).Although the traditional detection method using large-scale precision instruments as the detection method has high precision,the instrument is expensive and cannot achieve in-situ detection.Therefore,it is urgent to establish a simple,fast response,and highly sensitive detection method.Colorimetric and fluorescence-based detection methods have the advantages of simple operation,low cost and high degree of visualization,and have been widely used in various analyte detection fields.In this study,three fluorescence sensors were designed and synthesized mainly based on small organic molecules and fluorescent carbon quantum dots（hereinafter referred to as carbon dots,CDs）as fluorescence signal reporters,and their applications in the visual detection of heavy metal ions were deeply studied.The main research contents and results of this paper are listed as follows:The chemical sensor S has been synthesized by condensation of3-formyl-4-hydroxy-1,8-naphthalomide with p-aminobenzoic acid.Spectroscopy studies show that S can emit a bright yellow-green fluorescence in aqueous solution.The Cu²⁺-based ensemble(S-Cu²⁺)was constructed through the complexation between S and Cu²⁺,which presented fluorescence quenching.Fluorescence recovery sensing toward CN^-was achieved based on the dissociation of Cu²⁺-based ensemble and the regeneration of sensor S.Therefore,it can be applied in visual（fluorescence“on-off-on”）successive detection of Cu²⁺and CN^-with the detection limits（LOD）of 0.12 and 0.27μmol/L.The visual detection of Cu²⁺and CN^-could be achieved based on the test strip in tab water.N,S co-doped bule-emitting carbon dots（NS-CDs）was prepared by solvothermal（methanol）method using citric acid as a carbon source and thiourea as a nitrogen and sulfur source,respectively.Owing to relatively poor sensitivity of NS-CDs for heavy metal ions in aqueous solution（LOD=0.47μmol/L）.We framed thymine-functional fluorescence carbon dots（T-NS-CDs）by post surface modification strategy.Spectroscopic studies show that T-NS-CDs with the high sensitivity towards Hg²⁺,and LOD was reduced from 0.47μmol/L to1.56 nmol/L.Interestingly,after addition of Hg²⁺chelation agent,T-Hg²⁺-T specific base pairs were dissociated and NS-CDs would be released,rendering the fluorescence recovery.T-NS-CDs has been successful applied to detection of Hg²⁺in tap water with satisfactory recoveries（96.97%～101.38%）.R-CDs was prepared by hydrothermal method under acidic environment with o-phenylenediamine and thiosemicarbazide as carbon and sulfur sources,respectively.R-CDs emitted a bright red fluorescence at 600 nm and 640 nm under the irradiation of 365 nm ultraviolet lamp.The characterization results showed the surface of R-CDs rich in nitrogen,sulfur and oxygen related groups.It displayed significant fluorescent response toward the three heavy metal ions of Hg²⁺,Cu²⁺and Fe³⁺.The LOD for them were 19.02 nmol/L,0.19nmol/L and 0.95 nmol/L,respectively.In the standard addition recovery experiment in tap water samples,the recoveries were found to be99.36%～100.91%,97.47%～103.25%and 99.92%～104.60%.This indicates that the detection method is highly accurate.