Detection Of Heavy Metal Ions By Multicolor Ratio Fluorescence Sensor Based On Carbon Quantum Dots And Gold Nanoclusters

Carbon quantum dots（CQDs）are a kind of green fluorescent nanomaterials.They have the characteristics of simple preparation,low toxicity,good biocompatibility and strong water solubility stable fluorescence properties which can be widely used in chemical sensing,biological imaging and photocatalysis.Fluorescent metal nanoclusters have attracted extensive attention in biomedical applications due to their unique optical properties.Gold nanoclusters（Au NCs）are usually composed of several to hundreds of Au atoms smaller than 3 nm.So it is equivalent to the Fermi wavelength of electrons.Au NCs has the advantages of simple preparation,good biocompatibility,low cytotoxicity,good light stability and easy to be functionalized with other biomolecules.The design of ratio fluorescent probe and sensor are a hot topic in recent years.Compared with the single fluorescent probe and sensor,the ratio type sensor greatly weakens the requirement of probe concentration,temperature and excitation intensity by establishing internal standard.Thus more accurate result a wider response range can be achieved.The research status of carbon quantum dots and gold nanoclusters,including synthetic methods,properties and applications of the two materials are firstly summarized in this paper.On this basis,the following research work has been done:Ⅰ.A blue fluorescent carbon dots（BCDs）,a nitrogen doped blue carbon dots（NCDs）and a red fluorescent gold nanoclusters（BSA-Au NCs）were synthesized separately by hydrothermal method.Transmission electron microscopy,Fourier transform infrared spectroscopy,ultraviolet visible absorption spectroscopy,fluorescence spectroscopy,X-ray photoelectron spectroscopy and X-ray diffraction were used to study the surface morphology,size,spectral properties,surface structure and elemental composition of the fluorescent materials.Ⅱ.A ratio fluorescent probe was designed and prepared by using blue carbon dots（BCDs）and red gold nanoclusters（BSA-Au NCs）with fluorescence intensity ratio of2:1.Under the single wavelength excitation of 330 nm,the probe has two emission peaks at 452 nm and 654 nm.The fluorescence of red gold nanoclusters at 654 nm can be gradually quenched by copper ions,while the fluorescence of blue carbon dots remains unchanged due to their photostability and chemical inertia.The color change from red to pink to purple to blue can be observed by naked eyes under the ultraviolet lamp.A smartphone can be used to record the RGB values of the solution for quantitative analysis for copper.Under the optimal condition,there was a good linear relationship between I₄₅₂/I₆₅₄ and Cu²⁺concentration(I₄₅₂/I₆₅₄=1.609C(Cu²⁺)+2.013,R²=0.9987)in the range of 0.05-1.85μM.The detection limit（LOD）was 16 n M.The ratio fluorescence probe has been successfully applied to the determination of copper ion in water samples with the recovery range from 90.8%to 109%.Compared with that displayed by other single probe methods,the current nanoprobe provides a high-quality and sensitively visible light platform for monitoring copper ions.Ⅲ.A ratio fluorescent probe was synthesized by using nitrogen doped blue carbon dots（NCDs）and red gold nanoclusters（BSA-Au NC_S）with fluorescence intensity ratio of 1:1.Under the excitation of a single wavelength of 400 nm,the probe has two emission peaks at 464 nm and 662 nm.The fluorescence of red gold nanoclusters is gradually quenched at 662 nm in the presence of different concentration of mercury ion,while the fluorescence of NCDs remains unchanged.The color change can be seen in the naked eye under the ultraviolet lamp.There was a good linear relationship between the concentration of mercury ion and the fluorescence intensity ratio I₄₆₄/I₆₆₂ in the range of 0.05-2.5μM(I₄₆₄/I₆₆₂=1.164 C(Hg²⁺)+0.952,R²=0.9935).The detection limit（LOD）was 2.7 n M.The method has been successfully applied to the determination of mercury in several water samples with recoveries of 90.5%～107.6%.By constructing a fluorescent paper-based sensor,a smartphone colorimetric determination for mercury ion can be achieved.