Preparation And Properties Of Carbon Dots Based Polymer Composite Fluorescent Films

Carbon dots were first discovered in 2004.Because of their good photoluminescence,biocompatibility,simple preparation process,low toxicity or non toxicity,they were widely used in ion detection,catalysis,information anti-counterfeiting,biological cell labeling and other fields.The synthesis methods of carbon dots included hydrothermal method,arc discharge and electrochemical oxidation.Heteroatom doping was an effective method to modify carbon dots and improve their properties.It can adjust the fluorescence characteristics.It was considered to be the most promising process to produce strong fluorescent carbon dots.In recent years,there had been reports on the combination and application of carbon dots and polymer materials.There were many papers about the preparation and application of hydrogels and films.Many functional groups such as-OH,-NH₂,and-COOH on the surface of carbon dots can be easily formed by hydrogen bonding with polymer materials,and the doping of heteroatoms will give new properties to the films.In this paper,carbon dots were synthesized by heteroatom doping,and then composite films were prepared with polymer materials by tape casting,which gave new properties to polymer films.A variety of characterization methods were used to systematically explore the structural composition and optical properties of carbon dots,thermal and mechanical properties,flame retardant,light conversion and adsorption properties of films.The specific contents include the following parts:（1）Nitrogen and phosphorus doped carbon dots（N,P-CDs）were synthesized by one pot hydrothermal method with m-phenylenediamine and phosphoric acid as raw materials.PVA/N,P-CDs composite films were synthesized by tape casting after carbon dots diluent were mixed with polyvinyl alcohol（PVA）solution.The microstructure and composition of carbon dots were observed by transmission electron microscope,infrared spectrum and X-ray photoelectron spectroscopy.The fluorescence spectrum showed that N,P-CDs emitted green fluorescence under ultraviolet lamp irradiation（emission wavelength was 515 nm）.The PVA/N,P-CDs films were characterized by IR,transmittance,atomic force microscope,polarizing microscope,mechanical properties and thermal properties.The results showed that N,P-CDs were successfully mixed into the films and had little effect on the transmittance of the films.The tensile strength,elongation at break and thermal stability of the films were also improved.The flame retardant experiment showed that the addition of a small amount of N,P-CDs can make the PVA flame retardant film reach VTM-V0 level and higher limiting oxygen index（33.2%）.The tests of microcalorimetry,in-situ infrared,scanning electron microscope,energy spectrum and Raman spectrum showed that the mixing of N,P-CDs can effectively improved the thermal stability of the film.The film showed excellent flame retardant properties.（2）Firstly,anhydrous citric acid（CA）was grafted onto chitosan（CS）by EDC/NHS activator,and then chitosan grafted nitrogen doped carbon dots（P（CS-g-CA）CDs）were synthesized by hydrothermal method with N-（2 hydroxyethyl）ethylenediamine.Then the carbon dots were blended with corn starch（CST）to prepare CST/P（CS-g-CA）CDs composite films.P（CS-g-CA）CDs were characterized by XPS,FT-IR,UV and fluorescence spectra.The results showed that there were-COOH,-OH and-NH₂groups on the surface of P（CS-g-CA）CDs.The fluorescence spectra of carbon dots showed that the carbon dots were light yellow transparent solution under sunlight and blue fluorescence under UV lamp irradiation.P（CS-g-CA）CDs were characterized by FT-IR,TGA,tensile and fluorescence spectra.The results of film infrared spectroscopy showed that P（CS-g-CA）CDs interacted with corn starch through hydrogen bond.The addition of P（CS-g-CA）CDs can not only effectively improve the mechanical strength,thermal stability and mechanical properties of the film,but also effectively slow down the decomposition of CST main chain and increase the amount of carbon residue.In addition,the introduction of CST/P（CS-g-CA）CDs gives the film good light conversion characteristics,that was 381 nm ultraviolet light can be converted into 474 nm blue light beneficial to plants.This study provides a feasible and effective strategy for the preparation of light conversion,high mechanical properties and degradable films（3）PVA/P（CS-g-CA）CDs composite films were prepared by tape casting and applied to dye adsorption.The structure of the films was studied by FTIR,TGA,XPS and tensile tests.The results show that the addition of carbon dots can effectively improve the thermal and mechanical properties of the films.In addition,the film was applied to the adsorption of methyl blue dye.The effects of film dosage,different pH value,different adsorption temperature and different adsorption time on the adsorption performance of methyl blue were studied,and the adsorption mechanism was explored.The results showed that the film showed good adsorption for methyl blue dye,showed stable adsorption effect at pH 4-9,and reached the maximum adsorption capacity at pH=5;PVA/P（CS-g-CA）CDs composite film reached saturation when the concentration of methyl blue dye was 500 mg/L,so the maximum adsorption capacity of the film was about 304.5 mg/g.In addition,the kinetic and thermodynamic processes of methyl blue adsorption on the film were explored.The results showed that the adsorption kinetics accorded with the pseudo second-order kinetic model and the thermodynamics accorded with the Langmuir model.Finally,through the cyclic adsorption test,it showed that PVA/P（CS-g-CA）CDs film still had good adsorption performance after 7 times of desorption and adsorption,and the dye removal rate reached more than 70%,The adsorption mechanism may be the electrostatic interaction between the-COO^-Group on the carbon point surface and the-NH⁺structure of the dye.