Studies On Solid-phase Preparation And Luminescence Properties Of Carbon Dots

Carbon dots?CDs?,as a new class of luminescent nanomaterials,have far reaching application potential in the fields of light-emitting devices,drug delivery,chemical analysis and biosensing owing to their small particle size,wide range of raw materials,low toxicity,good water solubility,excellent biocompatibility,strong chemical stability,high photobleach resistance and easy functionalization.The liquid-phase synthesis method was used widely so far to preparate CDs,such as hydrothermal/solvothermal methods.However,they suffered from some limitations of wide temperature window,complicated reaction process and post-processing,and hard to large-scale preparation.Therefore,based on the simplicity of solid-phase synthesis,we developed this method to prepare CDs with excellent optical performance and diversified applications.First,we proposed a solid-phase synthesis method with simple process operation and easy large-scale preparation through direct calcination of the only carbon source 1,2,4-triaminobenzene for preparing red emissive carbon dots?r-CDs?.The important factors such as calcination temperature and time were optimized to obtain r-CDs with high quantum yield and excellent fluorescent performance.It was found that the r-CDs have a good response to the change of the pH of the solution in the colorimetric and fluorescent modes.As the pH of the buffer solution increases from 4.0 to 8.0,the color of the r-CDs solution changes from red to orange and then to yellow under daylight,and under ultraviolet irradiation,the fluorescence emission peak of the r-CDs solution moves from 650 nm?purple?to 610 nm?orange?,then blue-shifts to 585 nm?yellow?.The above phenomenon can be easily observed by the naked eyes.Through experimental characterizations such as Fourier transform infrared spectroscopy,nuclear magnetic resonance spectroscopy and density functional theory?DFT?,it can be preliminarily deduced that r-CDs have a structure similar to methyl orange,and the pH response results from the reversible transformation between the azo and quinone structures during protonation process.Owing to the feature of pH response,r-CDs can be used as probes to monitor pH fluctuations in Hela cells.Given the advantages of solid phase synthesis,we designed a molten salt?MS?method to prepare carbon dot?CDs@MS?composite materials with room temperature phosphorescence?RTP?.KNO₃ and NaCl were used as the inorganic salts,and 1,2,4-triaminobenzene was chose as the only carbon source.Then KNO₃?7.5 g?,NaCl?2.5 g?and 1,2,4-triaminobenzene?0.025g?were thoroughly mixed before adding into a crucible,and then the mixture was transferred to a muffle furnace and heated to 350°C at a rate of 10°C×min^-1 and kept for 3 h.After cooling to room temperature,the CDs@MS powder was obtained.Molten salt?MS?is salt which is solid under ambient conditions but enters the liquid phase at an elevated temperature.During the melting and recrystallization process,CDs were formed and incorporated into crystalline salt matrices,which could restrain non-radiative relaxation through locking the triplet exciton and inhibiting the intermolecular collision.Therefore,the RTP phenomenon was obtained.Furthermore,the melting temperature and different ions can be precisely controlled by the selection of a large amount of readily available salts.Herein,we chose different kinds of inorganic salts to adjust the reaction temperature.The as-prepared CD@MS exhibited long lifetime RTP?up to 886 ms?and excitation-dependent phosphorescence,that is,the emission can be facilely tuned from 510 nm to 573 nm?green to yellow?by changing the excitation wavelength.It was found that the high charge density of metal ions plays a critical role in reducing the energy gap for realizing effective intersystem crossing.CD-based RTP materials with yellow phosphorescent emission are achieved from a variety of carbon sources including aliphatic carbon chain,heterocycle and aromatic compounds?such as citric acid,ascorbic acid and m-phenylenediamine?.This method is simple,efficient and can be used as a gram-scale synthetic method and the excitation dependent RTP feature of CDs@MS nanocomposites could provide a novel dual security protection strategy in high-level information anticounterfeiting.Although CDs@MS exhibited good RTP behaviorin the solid phase,the RTP of these CDs disappeared with the collapse of salt matrices.Nevertheless,the achievement of RTP in aqueous solution is still a highly challenging task.Herein,KNO₃,MgCl₂ and KH₂PO₄ are selected as inorganic salt systems to construct rigid matrix around the CDs?CDs@MP?,which can protect RTP characteristics for CDs in aqueous solution.Meanwhile,the size of CDs@MP can be well controlled through adjusting the carbonization temperature,reaction time and the content of carbon source in the system,so that they have good dispersibility in water.The resulting CDs@MP exhibited bright yellow RTP,with a quantum yield of 26.4%,a lifetime of 1.28 s,and last for approximately 6 s to the naked eyes.This is the first time that the long-lived CDs@MP composites with RTP are achieved in aqueous solution owing to the synergistic effect of crystalline confinement and aggregation-induced phosphorescence.Further investigations reveal that three key processes may be responsible for the observed RTP of the composite materials:?1?The rigid crystalline salt shell can preserve the triplet states of CDs@MP in water and suppress the nonradiative deactivation;?2?The addition of high-charge-density metal ions Mg?II?and phosphorus element in the composites facilitates the singlet-to-triplet intersystem crossing process and enhances the RTP emission;?3?The aggregation of CDs@MP composites enables the matrix shell to self-assemble into a network,which further improves the rigidity of the shell and prevents the intermolecular motions,hence prolonging the RTP lifetime.The unique RTP feature and good water dispersibility allow the CDs-based composite materials to be applicable in detection of temperature and pH in the aqueous phase.Our approach for producing long-lived RTP CDs@MP is effective,simple,and low-cost,which opens a new route to develop RTP materials tha are applicable in aqueous solution.