Preparation And Application Of Biomass Derived Carbon Quantum Dots

Graphene quantum dots?GQDs?are a type of carbon quantum dots?CQDs?with higher crystallinity.GQDs and CQDs are novel nanocarbon materials with excellent optical stability and water dispersibility,as well as adjustable fluorescence emission/excitation properties.They are ideal materials to replace inorganic quantum dots,which show unique application prospects in the fields of metal ion detection,biological imaging,disease diagnosis and treatment,and solar cells.Biomass is a green and renewable resource for preparation CQDs because of their rich C,N,S,P atoms.Most biomass CQDs are synthesized by hydrothermal method which had the limitation of lower quantum efficiency and optical stability.In this paper,chitosan,gelatin,and soybean protein were dissolved in the citric acid-urea system,and then synthesized by one-step hydrothermal method to obtain N-doped carbon quantum dots?NCQDs?,N-doped graphene quantum dots?NGQDs?and N,S double-doped graphene quantum dots?N,S-GQDs?respectively.The CQDs powder was obtained by a rapid and simple separation method using ethanol precipitation.The structures,optical properties and detection of metal ions of NCQDs,NGQDs and N,S-GQDs were studied,respectively.Their luminescence mechanism was discussed,which was contribute to the application of CQDs.The main innovations of this thesis as follows:1)Nitrogen-doped carbon quantum dots?NCQDs?and nitrogen-doped graphene quantum dots?NGQDs?were prepared using hydrothermal method and ethanol precipitation from chitosan or gelatin,respectively.2)Nitrogen and sulfur double-doped graphene quantum dots?N,S-GQDs? were obtained from soy protein by hydrothermal method.3)The NGQDs synthesized by gelatin-citric acid-urea system has the characteristics of up-conversion luminescence spectrum,excitation wavelength independence and high fluorescence quantum yield?28.50%?.4)NCQDs,N,S-GQDs and NGQDs have excellent water dispersibility and sensitive to Fe³⁺and Pb²⁺,respectively.The main contents and conclusions of this thesis as follows:1.Nitrogen-doped carbon quantum dots?NCQDs?was prepared from chitosan through hydrothermal method.When ethanol precipitation was used as the purification method,a high product yield of 85.3%was obtained.Strong blue fluorescence emission with a high quantum yield?QY?of 6.60%was observed from the NCQDs aqueous dispersion.Physical and chemical characteristics of the NCQDs were carefully investigated by transmission electron microscope?TEM?,X-ray diffraction?XRD?,Fourier transform infrared spectrum?FTIR?,Raman spectrum,X-ray photoelectron spectroscopy?XPS?,and transient fluorescence spectrum.Experimental results showed that diameters of the NCQDs were in the range of 2-10nm.The carbon quantum dots possess good water dispersibility and precipitation by ethanol.When used for metal ion detection,detection limit of the NCQDs for Fe³⁺was as low as 1.57?M.This work proposed a facile method to synthesize NCQDs from chitosan with high yield and demonstrated that carbon quantum dots derived from chitosan were promising for ion detection.2.N-doped graphene quantum dots?NGQDs?were synthesized by hydrothermal method from gelatin-citric acid-urea systems.The effects of gelatin concentration on the quantum yield of NGQDs were studied.The structures and properties of NGQDs were characterized by FTIR,UV-vis,Raman,XPS,fluorescence spectrum,TEM and zeta potential.The results showed that NGQDs had the high fluorescence quantum yield of 28.50%when the mass ratio of gelatin to citric acid-urea was 7.7 wt.%.NGQDs were a typical graphene quantum dot with a lattice fringe spacing of 0.24 nm and an FFT diffraction ring.NGQDs owned independent of excitation wavelength properties with the emission wavelength at 470 nm.The emission spectrum of NGQDs has up-conversion and down-conversion characteristics.NGQDs have good water dispersibility,pH sensitivity,sodium chloride stability and good sensitive for lead ions.The detection limit of the NGQDs for Pb²⁺was as low as 0.48?M.This work proposed a facile method to synthesize NGQDs with up-conversion,down-conversion and excitation wavelength independence from gelatin with high quantum yield.This work might show unique application in biological imaging.3.Nitrogen and sulfur double-doped graphene quantum dots?N,S-GQDs?were obtained from soy protein by hydrothermal method,in which the effects of the quantum efficiency of N,S-GQDs was studied.Structures and properties of N,S-GQDs were characterized by FTIR,UV-vis,XPS,fluorescence spectrum,TEM and zeta potential.The results show that N,S-GQDs possess good water dispersibility and precipitation by ethanol.N,S-GQDs water dispersibility showed good bright blue fluorescence.When the mass ratio of soybean protein to citric acid-urea was 5 wt.%,the fluorescence quantum yield of N,S-GQDs was as high as 9.23%.The detection limit of the N,S-GQDs for Fe³⁺was as low as 0.95?M.This work provided a facile method to synthesize N,S-GQDs from soy protein with high quantum yield.