Green Preparation Of Lignin-based Fluorescent Carbon Dots And Their Application In Metal Ion Detection

Carbon quantum dots,an emerging class of carbon nanomaterials with good photoluminescent properties,have a promising application prospect in fields of fluorescent probe,biological imaging,etc.Lignin is one of the three major components of biomass.Black liquor,the byproduct of pulping industry,contains a large amount of lignin which is a potential material for preparing carbon quantum dots.In this paper,carbon quantum dots were prepared using dealkaline lignin and black liquid as the material with a green method,and their optical properties and physicochemical properties were explored.Based on the principle of fluorescence quenching of carbon quantum dots,a metal ion detection method was proposed.Carbon quantum dots with good fluorescence properties were synthesized from dealkaline lignin in one step in solvents such as ethanol,dioxane,isopropanol and n-dodecane under conventional heating.L-CDs prepared from dealkaline lignin in ethanol were characterized by TEM,FTIR,XPS,Raman spectroscopy,UV–vis spectra and fluorescence spectra.The results show that L-CDs were carbon nanoparticles with approximate spherical and uniform particle size between 20 and 50 nm,possessing carbon structure of graphene,and abundant carbonyl,carboxyl,and hydroxyl groups.The fluorescence spectra of L-CDs exhibited the characteristics of multiple emission centers and the common dependence of excitation waves of biomass-based carbon quantum dots.The fluorescence quantum yield of L-CDs reached 18.9%.Carbon material M-L-CDs were prepared from dealkaline lignin under microwave heating which took only 30 min.The results show that M-L-CDs had similar surface functional groups and core structure to L-CDs.The fluorescence quantum yield of M-L-CDs was 19.6%.Carbon quantum dots M-B-CDs were prepared from black liquor under microwave heating.The results show that M-B-CDs were carbon nanoparticles with spherical shape and particle size distribution between 50 and 100 nm.M-B-CDs had good solubility and photoluminescence properties,and the fluorescence quantum yield was 9.7%.The prepared carbon quantum dots were applied in the detection of metal ions.The results showed that L-CDs had a quantitative detection ability for Fe³⁺in the range of 0～0.5mmol,presenting a linear relationship as y=0.9811-1.0187x with the correlation coefficient of 0.9939.The detection limit of L-CDs for Fe³⁺was 0.083 mmol.Besides,L-CDs also showed high sensitivity to Cu²⁺and K⁺.M-B-CDs had a quantitative detection ability for Fe³⁺in the range of 0～0.2 mmol,presenting a linear relationship a y=0.9926-2.1104x with the correlation coefficient of 0.99008,and the detection limit was 0.018 mmol.M-B-CDs also showed high sensitivity to Cu²⁺.In summary,a green and efficient preparation method of carbon quantum dots was proposed in this paper using dealkaline lignin and black liquid as the materials without strong acid or base additives,and the carbon quantum dots performed well in the detection of metal ions.This study promotes the large-scale preparation and utilization of carbon quantum dots from lignin and black liquor.