Fluorescence Adjustment Mechanism And Application Of Ion Detection Of Nitrogen Doped Carbon Quantum Dots

Nitrogen doped carbon quantum dots(N-doped C-dots)have attracted growing attention due to theirhigh luminous efficiency and adjustability of internal properties.At present,because of many defects about the existing synthetic methods andmaterials,the performance of the as-prepared C-dots isnot satisfactory.For example,the short wavelength range of the fluorescent emission,the lower fluorescence quantum yield andnon directional controllable structure and propertiesofthe as-prepared C-dots limiteditsapplication.Particularly,though many mechanisms for the fluorescent emission of carbon dots have been proposed,there is no any universal interpretation in the literature.The properties of C-dots can not be regulated freely due tolack of deep understanding about fluorescence emission origin,which seriously hindered the further development of C-dots.In order to solve these problems,this paper focuses on the researchesabout thefluorescence mechanism and the application development of N-doped C-dots.The maincontentsof the study are as follows:Firstly,the ammonium citrate derived C-dots were synthesized via hydrothermal approach at different temperatures.The structures and optical properties of the as-prepared C-dots were studied.The formation mechanism of C-dots was systematically studied.Corresponding to the different stages of the hydrothermal treatment,the distinct C-dots with different crystal structure andemission behaviorwere obtained.Carbon core can not be formed when the synthesis temperature is lower than the nucleation temperature.After reaching the nucleation temperature,it is found thatthe as-prepared C-dots with smaller carbon nuclei and more surface fluorophores are formed under the lower synthesistemperatures,and exhibit excitation-independent emission behavior.The fluorescence emission is provided by the surface fluorescence group.As the increasein hydrothermal temperatures,thecarbon nuclei ofas-prepared C-dotsincrease;whilethe surface functional groupsof as-prepared C-dotsdecrease.The crystalline nature of the as-prepared C-dots becomes stronger and the fluorescence emission model haschanged gradually from excitation-independent to excitation-dependent.The fluorescence emission is provided by both the carbon core and the surface fluorescence group.When the temperature is increased to ahigher temperature,the size of the carbon core is further increased,and the surface fluorescence group is further reduced.The luminescence behavior is dominated by the carbon core.Soy flour derived carbon dots were prepared via a facile one-step hydrothermal approach with assistant of NaOH for the first time.The physical and chemical structures and optical properties of the as-prepared C-dots were studied via variouscharacterizations.The relationship of fluorescence enhancement of C-dots and their structuredefects was deeply studied through changing the contents of N defects and O defects of C-dots.Thechange degree of structure defects was controlled by varying the synthesis temperatures and further passivating the as-prepared C-dots with ethylenediamine.It is found that soy flour derived carbon dots show good water-solubility,strong fluorescence and high photostability.It is proved concretely that N defects,such as C-N defects and N-H defects,are finely tuned by substituting for partial O defects(e.g.,C-O,C=O),reducing non-radiative recombination and enhancing fluorescence intensity.The ammonium citrate derived C-dots were synthesized via pyrolysisapproach at different temperature.The physical and chemical structures and optical properties of the as-prepared C-dots were investigated.Meanwhile,themechanismof different luminescence behavior was studiedbycomparingthe influences of carbon source and synthesis methods on fluorescence properties of C-dots.The as-prepared C-dots also showgood water-solubility,strong photoluminescence and high photostability.lt is found thatthe C-dots with excitation independent fluorescence should be synthesized at nucleation temperatureor just over nucleation temperature,and carbon source is the key factor influencing the luminescence behavior of C-dots.It is due to small molecular compounds(e.g.,ammonium citrate)as carbon sourcesare readilyto form C-dots with excitation independent fluorescence,while high molecular polymers(e.g.,soybean flour)as carbon sourcesnormally form carbon dots with excitation dependent fluorescence.Due to itsfluorescence quenching by Fe3+,soy flour derived C-dots were successfully used for Fe3+ detection.The ammonium citrate derived C-dots weresuccessfully used for Cr(?)and sulfitesdetection based on its fluorescence quenching and fluorescence recovery.Severalfactorsaffecting the detectionwere investigated to determine the proper detecting conditions.Furthermore,the mechanism of fluorescence quenching of Cr(?)and Fe3+ was studied.Thesoy flour derived C-dots fluorescenceprobe is highly selective and sensitive for Fe3+ detectionand the detection limit is 0.021?mol/L.Theammonium citrate derived C-dots fluorescence probe is highly selective and sensitive for Cr(?)detection,and the detection limit is 0.01 ?mol/L.Theammonium citrate derived C-dots fluorescenceprobe can be reusableand used for tap water samples for Cr(?)detection.Using the system of ammonium citratederived C-dots and Cr(?)as a fluorescent probe for detecting sulfite,the lowest detection limit was 0.3 5 mol/L.Theoretically,the detection mechanism ofCr(VI)via the ammonium citrate derivedC-dots probe is basically originated fromelectronexchange.The mechanismof fluorescence quenching of Fe3+ should be attributed to the combined action ofphotoinduced electron transfer andelectronexchange.