Preparation Of Carbon Dots Based On Cyclodextrin And The Fluorescence Detection For Metal Ions

With the continuous development of industry,more and more metal ions are discharged into the environment and accumulated into the human body through bioaccumulation.The presence of Hg²⁺and the abnormal concentration of Fe²⁺,Fe³⁺and Al³⁺can cause harm to the human body,so the detdction of metal ions is necessary.There are many detection methods of metal ions,among which fluorescent carbon dots（CDs）,as the new nanofluorescent metal,is considered to be the most potential fluorescent sensor for detecting metal ions because of its advantages of good water solubility,rapid detection,biocompatibility,easy functionalization,high selectivity and sensitivity.In this paper,three fluorescent carbon dots（Na-CDs,N/S-CDs,La/N-CDs）were prepared by doping sodium acetate,reactive black 5（RB5）and lanthanum nitrate withβ-cyclodectrin（β-CD）as the carbon source.The three carbon dots were characterized by Zeta potential,particle size and FT-IR.The optical properties of three carbon dots were analyzed by UV-vis absorption spectroscopy and fluorescence spectroscopy and the fluorescene response of Hg²⁺,Fe²⁺,Fe³⁺and Al³⁺to carbon dots was studied as flollow:（1）The carbon dots（Na-CDs）which doped sodium acetate were synthesized by using a one-step hydrothermal method withβ-CD as the carbon source.Na-CDs maintained good stability at different p H,temperature and xenon lamp durations and could be used in the fluorescence detection studies of Hg²⁺.Na-CDs had high selectivity and good immunity to interference for the detection of Hg²⁺.The linear correlation coefficient between the fluorescence intensity of Na-CDs and the concentration of Hg²⁺was 0.9916 and the detection limit of Na-CDs was 3.83μmol/L at the concentration of 4-20μmol/L.After particle size and Zeta potential changed,it was speculated that the fluorescence annihilation was based on the principle of aggregation-caused quenching（ACQ）.（2）Withβ-CD as the carbon source and RB5 as the nitrogen source and sulfur source,the carbon dots（N/S-CDs）were prepared through the hydrothermal method.FT-IR spectral analysis could prove that nitrogen and sulfur was successfully doped into the carbon dot.N/S-CDs selectively identified Fe²⁺and Al³⁺over a wide range of p H and the equilibrium time was 55 min and 3 min,respectively.Other metal ions basically did not interfere with N/S-CDs detected Fe²⁺and Al³⁺.The fluorescence intensity of N/S-CDs was linearly related to the concentrations of Fe²⁺and Al³⁺in their respective detection range.The detection limit of Fe²⁺and Al³⁺was 0.68 nmol/L and0.48 nmol/L by calculating.It was assumed that Fe²⁺enhanced fluorescence by chelation with the carbocyl on the N/S-CDs surface.Al³⁺was combined N/S-CDs to increase the particle size,result in aggregation and enhance fluorescence based on the principle of aggregation-induced emission enhance（AIEE）.（3）The carbon dots（La/N-CDs）were synthesized by one-step hydrothermal method with usingβ-CD and lanthanum nitrate.p H and ion strength tests showed that La/N-CDs had good stability.La/N-CDs could detect four metal ions(Hg²⁺,Fe²⁺,Fe³⁺,Al³⁺)through visual recognition and fluorescence analysis.La/N-CDs could detect Hg²⁺,Fe²⁺,Fe³⁺and Al³⁺over a wide p H range and the minimum equilibrium time of 20 min,20 min,15 min and 3 min,respectively.La/N-CDs basically had good anti-interference,well sensitivity and low detection limit when detected four metal ions.By inference,it was proved that Hg²⁺quenched fluorescence of La/N-CDs by the principle of aggregation-caused quenching（ACQ）;Fe²⁺made the fluorescene quenching by non-radiating galloping;Fe³⁺suppressed excitation fluorescence by facilitating charge transfer and restraining the radiative recombination of excitons and Al³⁺enhance the fluorescene by increasing the rigidity of the aromatic system with chelating with the hard alkali（-OH）on the surface of La/N-CDs.