Study On The Quantitative Determination Of Cr(?) Based On Carbon Quantum Dots As Fluorescence Probes

Carbon quantum dots are a kind of novel nano fluorescent probes,which are widely used in metal ion recognition and detection due to their high water solubility,easy functionalization and good photoluminescence properties.Hence,carbon quantum dots have been reported for the detection of Cu²⁺,Hg²⁺,Ag+ and other metal ions,but not including Cr???ions.The chemical character of Cr???ions is similar to that of V???ions,which leads to the interference of Cr???in the vanadium extraction during the processes of vanadium hydrometallurgy.In addition,the environmental harm of aqueous solutions of vanadium hydrometallurgical processes has been aggravated by the existence of Cr???ions due to their toxicity.Therefore,the monitoring of Cr???ions in the aqueous solutions of vanadium hydrometallurgy is an urgent problem to be solved.In this thesis,the microstructure,chemical composition,fluorescent properties of carbon quantum dots and the influence of Cr???and V???on their fluorescent properties were systematically investigated.Based on results above,it was realized to selectively recognize and quantitatively determine the Cr???ions in the mixed solution of Cr???and V???ions.The application potential of carbon quantum dot as fluorescent probes has thus been explored in monitoring the aqueous solutions of vanadium hydrometallurgy.By using citric acid as the carbon source and ethylenediamine as the surface modifier,the carbon quantum dots with relative fluorescence quantum yield up to 49.5% were prepared through the one-step pyrolysis method.Results show that the carbon quantum dots are amorphous carbon with an average particle size of about 3.2 nm.They consist of four elements,C,N,O and H,and are rich in hydrophilic functional groups such as amide bonds,leading to their good dispersibility in aqueous solution.The carbon quantum dots have excellent photoluminescence properties,with the maximum absorption wavelength of 340 nm and the maxiumum emission wavelength of 440 nm.Results show that V???ions did not influnece the fluorescence of carbon dots.But Cr???ions bonded coordinatively with the amide groups of carbon quantum dots and forms a p-? conjugated system,resulting in the fluorescence quenching of carbon quantum dots.There was a good linear relationship between the quenching ratio and Cr???concentration.When the Cr???concentration was in the range of 0.25²5 ?mol/L,the linear equation was y=1.080+0.015x?R2=0.997?,with the limit of detection?LOD?of 0.13 ?mol/L.When the Cr???concentration was in the range of 19.2⁹600 ?mol/L?1⁵00 ?g/m L?,the linear equation was y=2.03²⁺0.071x?R2 = 0.998?.The carbon quantum dots were applied to determine the concentration of Cr???ions in the aqueous solutions of vanadium hydrometallurgy,including the leaching solution of sodium roasted vanadium slag and industrial wastewater after ammonium vanadate precipitation.The relative deviation of detection was less than 0.3% while the recoveries were in the range of 99%¹03.8% with the relative standard deviation of 1.5% or less.Results also show that V???,Ca²⁺,Al³⁺ ions had no influence on the carbon dots' fluorescence quenching caused by Cr???ions in the aqueous solution of vanadium hydrometallurgy.it was found that only Fe³⁺ ions could interfere with the fluorescence intensity and they could be masked by the masking agent of Na F to achieve the selective detection of Cr???.