Preparation And Performance Study Of Chitosan-based Carbon Quantum Dots

Carbon quantum dots(CQDs)are a new class of zero-dimensional carbon nanomaterials with unique photoluminescence stability,biocompatibility and excellent charge transfer ability,which have been widely used in the fields of fluorescence sensing,photocatalysis and information security.In view of the challenges faced by the current research on carbon quantum dots,such as high cost of raw materials,unfriendly environment,complex preparation process,single fluorescence emission,and lack of effective optical performance regulation methods.In this paper,chitosan was used as carbon precursor,and the structure and composition of the product were modified and regulated by adjusting the reaction system to prepare chitosan based carbon quantum dots with adjustable optical properties.To explore the structure-activity relationship between its structure and fluorescence performance,and further expand its application scope,the research contents are as follows:(1)Chitosan-based carbon quantum dots NACs-Q with blue luminescence properties were prepared by solvothermal method with chitosan as raw material and glycine hydrochloride ionic liquid as solvent and dopant.The average particle size is3.8 nm,and the fluorescence excitation and emission wavelengths are 340 nm and432 nm.It has excitation-dependent property and p H sensitivity.The ionic liquid can effectively improve the intermolecular and intramolecular hydrogen bonding force of chitosan,and promote the reaction to form a stable aromatic conjugated system,which is conducive to the?-?*transition emission.A large number of electron-rich groups such as-NH₂and C=O on the surface of carbon dots make them useful as fluorescent probes to detect metal ions and small molecular compounds,and an"on-off-on"ion sensor was successfully constructed to detect the content of Fe³⁺and vitamin C.NACs-Q and polyvinyl alcohol were combined to prepare fluorescent film,which could detect acetone vapor with high sensitivity.(2)Based on the influence mechanism of the structural composition and surface properties of carbon dots on the fluorescence properties and applications,sodium hydroxide was introduced as modifier in the reaction process,and green luminescence carbon dots GQDs₂-6 with room temperature phosphorescence(RTP)were prepared by microwave hydrothermal method.The characteristic excitation and emission wavelengths were 460 nm and 502 nm,respectively.OH^-promoted the formation of stable sp~2aromatic domains and a large number of surface defects in carbon dots,which favored?-?*and n-?*transitions,making the emission wavelength red-shifted,and Na⁺contributed to the suppression of fluorescence quenching.The n-?*transitions generated by the surface defects of GQDs₂-6 is beneficial to promote the efficient filling of triplet excitons by inefficient intersystem crossing(ISC)to form a stable RTP phenomenon.When the ISC is combined with polyvinyl alcohol and boric acid,the phosphorescent lifetimes of the composite are 262 ms and 623 ms.The vacancy of the p orbital of B in GQDs₂-6/B₂O₃can attract?electrons to form a p-?conjugated system,which can effectively reduce the energy level of free orbital and promote the residual emission for 12 s.It can be successfully used for fingerprint encryption as an anti-counterfeiting material.(3)Compared with carbon dots with single fluorescence emission,the fluorescence response of multicolor luminescent carbon dots can be extended to the visible light region,which can broaden its application in the field of photocatalysis.Phosphoric acid was introduced into the reaction to regulate the structure and surface chemical state of carbon dots.By changing the concentration of phosphoric acid,multi-color luminescent carbon quantum dots(N,P_R-CQDs)with adjustable colors from blue,green to orange were prepared.Phosphoric acid as solvent promotes the formation of stable N-(C)₃functional groups on the surface of carbon dots,providing more p-delocalized electrons for carbon dots,which is conducive to the red shift of fluorescence emission.As a dopant,the incorporation of P can generate a large number of surface states favoring n-?*transitions assist in the capture of photoexcited electrons,enabling the carbon dots to exhibit double emission(503 nm and 620 nm)and excitation independence.The photocatalysts were prepared by compounding N,P_R-CQDs with carbon nitride.The new energy levels induced by related groups such as N and P can enhance the electron trapping behavior,promote the effective separation of electron-hole pairs,and broaden the spectral response of the composite catalyst to the visible region.In the visible region,the band gap is reduced from 2.75 e V to 2.40 e V.It was used for the visible light catalytic degradation of Rh B,and the degradation rate reached 98.39%within 60min,which was 40%higher than that of pure g-C₃N₄.