Synthesis,Properties And Biological Applications Of Red Fluorescent Carbon Dots

Carbon dots(CDs)are a new family of carbon nanoparticles with an average size usually less than 10 nm that present excellent fluorescent properties.CDs have been extensively used in various fields such as biomedicine,optoelectronic devices and catalysis etc.due to their adjustable fluorescence,good biocompatibility and high photostability.However,because of the limitation of the aggregation-caused quenching effect,the fluorescence of CDs in the aggregate/solid state is often quenched.Besides,the difficulty of achieving the long-wavelength region(from red to near-infrared)and the insufficient luminous efficiency limit the further practical application of CDs.Heteroatom-doping is one of the most effective methods for regulating the optical properties of CDs.Nevertheless,the precise regulation method and regulation mechanism are still controversial.This thesis focuses on the synthesis,properties and biological applications of red fluorescent CDs.Two kinds of CDs with red fluorescence emission were synthesized by solvothermal method with different reaction materials,and a series of studies on the morphology,structure,and optical properties biological application of the synthesized CDs were carried out.It consists of two parts:(i)The hydrophobic CDs(F,N,S-CDs)with solid red fluorescence are synthesized through fluorine doping.It is proved that F-doping can effectively increase the fluorescence quantum yield(QY)and cause the redshift of the emission wavelength.At the same time,the effects of F modulation on the morphology and structure of CDs,as well as the temperature-sensitive and solvation effects of F,N,S-CDs,are deeply studied.(ii)A new type of water-soluble red CDs(T-CDs)was synthesized from tea polyphenols and citric acid,and a variety of cells were used as models to evaluate the cytotoxicity and blood toxicity of T-CDs.Further study of its in vivo/in vitro fluorescence imaging effect were also taken out.Finally,the ability of T-CDs to generate reactive oxygen species after LED irradiation was tested,and it was successfully applied in photodynamic therapy(PDT)of cancer cells and solid tumor models.The main research contents and results of this thesis are as follows:(1)A new type of F,N and S co-doped CDs(F,N,S-CDs)was synthesized by solvothermal method in a reaction system with citric acid as the carbon source,3-fluoroaniline as the nitrogen and fluorine sources,DMSO as the solvent.Compared with the F non-doped CDs(N,S-CDs),F,N,S-CDs can resist the effect of aggregation-caused quenching,and exhibit bright red solid-state fluorescence(?_em=676 nm)in the solid state.Moreover,in the solution state,the yellow fluorescence of F,N,S-CDs has a redshift of 50 nm compared with the green fluorescence of N,S-CDs.At the same time,compared with N,S-CDs,the QY of F,N,S-CDs increased from 6.57%to 14.57%.The effect of F modulation on the structure and morphology of CDs was further studied.Transmission electron microscope(TEM)shows that F,N,S-CDs exhibit a spherical structure with a relatively uniform size distribution.The average size of F,N,S-CDs is 5.72 nm,which is significantly larger than that of N,S-CDs.More notably,in the Raman spectra,the intensity ratio of D band and G band of F,N,S-CDs increased significantly,indicating that F modulation induces more defects in F,N,S-CDs.The results of nuclear magnetic resonance fluorine spectroscopy,Fourier infrared spectroscopy(FTIR),the water contact angle,electron paramagnetic resonance spectroscopy(EPR)and X-ray photoelectron spectroscopy(XPS)all confirmed the existence of a large number of C-F bonds in F,N,S-CDs.They are mainly located on the periphery/edge or surface of the graphitized carbon core,and the content of F is as high as 8.39%.In addition,the study found that F,N,S-CDs exhibited a solvation effect.In the H₂O/DMSO mixed solvent,as the proportion of H₂O increases,the fluorescence emission changes from yellow to red(the emission peak is red-shifted from 556 nm to 663 nm).This is due to the hydrophobic C-F bond on the surface of F,N,S-CDs,which causes that F,N,S-CDs gradually aggregate with the addition of H₂O,showing an aggregation-induced discoloration behavior.Finally,the temperature-sensitive behavior of the solid-state fluorescence of F,N,S-CDs was studied.The solid-state fluorescence of F,N,S-CDs shows the strongest emission at 120 K,and the fluorescence intensity has a good linear relationship with temperature(in the range of 120-420 K).The research results show that F-doping is an effective strategy to modulate the optical properties of CDs.(2)Using citric acid and tea polyphenols as raw materials,T-CDs with blue and red emission were synthesized by solvothermal in formamide.After a series of purification,T-CDs with an average size of 2.45 nm were obtained,and the carbon core showed obvious lattice stripes.GPC result calculated that the average molecular weight of T-CDs was 60739Da.The study of the optical properties of T-CDs found that the aqueous solution of T-CDs exhibit a wide absorption spectrum in the ultraviolet-visible range,and show dual emissions in the blue and red regions.In addition,under the excitation of 680 nm laser,T-CDs also have the property of up-conversion fluorescence emission.Further,the exploration and research of T-CDs in the field of biological applications have been carried out.The cytotoxicity experiment proved that T-CDs showed no obvious cytotoxicity.Based on the wide absorption of T-CDs and the property of red emission,we study the in vitro cell imaging effect of T-CDs.The results of confocal laser scanning microscope experiment shows that T-CDs can completely enter the cell within 2 hours,pass through the nuclear membrane and concentrate in the nucleolar region,showing brighter red fluorescence.Furthermore,tumor-bearing mice cultivated with mouse breast cancer cells(4T1)cells were used for in vivo imaging experiments of T-CDs,The results show that T-CDs can penetrate the epidermal tissue of mice and exhibit excellent fluorescence imaging effects in tumors.T-CDs is an excellent in vitro/in vivo red fluorescent probe.More importantly,EPR spectroscopy proved that T-CDs can effectively produce hydroxyl free radicals after LED irradiation.Using 4T1 cells to construct the solid tumor model on the back of the mouse for PDT experiments,the results proved that T-CDs can effectively inhibit the growth of cancer cells and tumor tissues in both cells and solid tumors,showing excellent PDT effect.Further pathological analysis of the heart,liver,spleen,lung,and kidney of the mice after treatment showed that T-CDs would not cause lesions in the main organs of mice.In summary,T-CDs with good biocompatibility and low biotoxicity,is a promising red fluorescent imaging probe and a reliable new photosensitizer.