Preparation Of Carbon Quantum Dots And Their Study In Fluorescent Quenching And Fluorescent Plla Films

Carbon quantum dots（CQDs）are quasi-spherical carbon nanoparticles with ultra-small size（<10 nm）,which have a number of merits,such as simple preparation process,low cost,excellent luminescence properties and good biocompatibility.CQDs have now become one of the research hotspots in luminescent materials.It can be used in broad platforms as fluorescent sensor by using the fluorescence quenching effect.On the other hand,Poly（L-lactic acid）（PLLA）is a non-toxic biodegradable material,but suffers from poor crystallinity,slow crystallization rate and brittle fracture,etc.Adding small amounts of CQDs into PLLA can compensate for the shortcomings.In this dissertation,nitrogen-doped CQDs（N-CQDs）were first synthesized,and their fluorescence quenching mechanism was discussed.Then they were used as fluorescent probes for the detection of the hypochlorite ions（ClO^-）in water.Lastly,the N-CQDs were added into PLLA matrix to form PLLA/N-CQDs nanocomposite films,and the optical properties,the structure polymorphs and the performance of the nanocomposite films were systematically investigated.The main works of this dissertation are as follows.1.Spherical N-CQDs were synthesized by hydrothermal method using citric acid（CA）and ethylenediamine（EDA）as raw materials.their particle sizes are distributed in the range of 1-5 nm with an average size of 2.8±0.7 nm.the N-CQDs consists of the conjugated aromatic carbon nuclei doped with N atoms and the hydrophilic functional groups on the nuclei’s surface.The optimal excitation and emission wavelengths of the N-CQDs were 342 nm and 440 nm,respectively,and the fluorescence quantum yield reaches up to 44%with a fluorescence lifetime of 12.66 ns.The N-CQDs have an excitation-independent emission behavior and excellent fluorescence stability.2.The quenching effects of different cations and anions on the fluorescence of N-CQDs were investigated.It is found that the oxidants with an electrode potential of 0.72 V can quench the fluorescence of the N-CQDs.This quenching is caused by the electron transfer between the oxidants and the abundant electron-donating groups on the surface of N-CQDs.Therefore,a redox-driven fluorescence quenching mechanism of N-CQDs was proposed.For KMnO₄,its quenching under neutral conditions is the combined effect of the internal filtration effect and the electron transfer effect,in which the internal filtration effect accounts for no more than 22.2%of the total fluorescence quenching.3.A fluorescence analysis method was developed for the sensitive and selective detection of ClO^-in water by using the reducibility of N-CQDs.The Stern-Volmer values of N-CQDs show a linear response to the ClO^-concentration in the range of 1.0-10.0μM,and the limits of detection（LOD）and the limits of quantification（LOQ）are 0.43μM and 1.04μM,respectively.The developed method was successfully applied to the quantitative determination of ClO^-concentrations in swimming pool water and in local tap water.Besides,this methodology is also successfully applied in anti-counterfeiting and paper encryption.4.PLLA/N-CQDs nanocomposite films were prepared by solvent casting method.Due to the interaction between PLLA molecular chains and N-CQDs nanoparticles,the added N-CQDs induced the crystallization of the amorphous PLLA which improved the UV shielding,the water vapor barrier and the fracture strength of the films.FTIR spectroscopy and imaging verified that the N-CQDs nanoparticles were uniformly dispersed in the PLLA matrix,and the higher the adding amount,the higher the crystallinity and theα-crystalline content of PLLA.However,when the amount of N-CQDs was larger than 2 phr,the crystallinity increased slowly and the crystalline conformation（i.e.,α-/α’-ratio）also tended to be stable.The as-prepared nanocomposite films showed excellent comprehensive performance.In addition,only 0.5 phr of N-CQDs can make the PLLA nanocomposite films emit strong fluorescence.