Preparation And Fluorescence Detection Of Dual-emissive Heteroatom-doped Carbon Dots

In recent years,multifunctional materials that can respond to various environmental factors have attracted significant attention.Carbon dots（CDs）,as a new type of luminescent nano-material,have shown enormous application potential in fields such as biosensing,chemical analysis,and environmental monitoring due to their simple preparation,good biocompatibility,superior optical properties,and low toxicity.However,most carbon dots reported in the literature currently only exhibit single emission band,and the according fluorescence probes only exhibit changes in monochromatic fluorescence intensity during the sensing process,which makes their detection results susceptible to various factors and limits the detection application in real life.Therefore,this article starts with dual-emitted carbon dots,and aims to explore various aspects of fluorescence detection.Two types of heteroatom-doped fluorescent carbon dots with dual emission were synthesized,focusing on the ratiometric fluorescent detection in the fields of temperature,pH,organic solvent,moisture in solvent,and metal ions.The specific research content is as follows:（1）The preparation conditions,morphology,structure,and optical properties of dual emission nitrogen-doped carbon dots（N-CDs）were studied.Based on the strategy of single heteroatom doping,we successfully synthesized dual-emitted N-CDs using citric acid as the carbon source,urea as the nitrogen source,and N,N-dimethylformamide（DMF）as the solvent using a one-step solvothermal method.N-CDs emit 440 nm（blue）and 580 nm（orange）emissions under 330 nm excitation.It is important to adjust the intensity ratio of the two emissions by changing the mass ratio of citric acid and urea,and N-CDs with the chromatographic coordinates in the International Commission on Illumination（CIE）of pure white light（0.33,0.33）are prepared.In addition,the morphology,element composition,crystal structure,optical properties and the stability of N-CDs were investigated.（2）The fluorescence detection function of N-CDs in temperature,pH,solvents,and metal ions was studied.Firstly,the relationship between N-CDs and temperature was studied.The experimental results showed that the blue fluorescence of N-CDs at440 nm had a good linear relationship with temperature in the range of 15～70℃（R²=0.998）,which can be used to sensitively detect temperature.Secondly,based on the color changes of N-CDs in solutions with different pH values and the good linear relationship between the orange fluorescence intensity and pH values in the range of4.0 to 7.0（R²=0.997）,a dual mode colorimetric and fluorescence detection of pH was achieved.At the same time,N-CDs also exhibit special solvatochromism,generating a new green fluorescence band when dispersed in different organic solvents.Based on the unique phenomenon,the detection of water in N-methylpyrrolidone（NMP）solvent was achieved with a detection limit of 0.075%.Finally,under the pH condition of 6.8,N-CDs can also be used to selectively detect Fe³⁺,Cu²⁺,and Ag⁺simultaneously,with detection limits of 5.7,0.18,and 0.46μM,respectively。（3）The preparation method,morphology,structure,and optical properties of dual emission boron and nitrogen co-doped carbon dots（BN-CDs）were studied.Based on the strategy of dual heteroatom doping,we successfully synthesized dual-emitted fluorescent BN-CDs using boric acid as the boron source,urea and ammonia as the nitrogen source,citric acid as the carbon source,and DMF as the solvent using a one-step solvothermal method.At the excitation wavelength of 310 nm,BN-CDs exhibit dual emissions at 438 nm（blue）and 570 nm（yellow）,and the CIE chromatographic coordinates are white light（0.31,0.33）.In addition,the morphology,structure,elemental composition,surface functional groups,and optical properties of BN-CDs were studied through various characterization methods,and the stability of BN-CDs was investigated.（4）The fluorescence detection function of BN-CDs in temperature,pH,solvents,and metal ions was studied.The experimental results indicate that BN-CDs exhibit significant temperature dependence（15～70℃）,and based on the good linear relationship（R²=0.997）and reversibility exhibited by blue fluorescence at 438 nm,BN-CDs can be used as fluorescent probes for temperature detection.BN-CDs also exhibit sensitive pH response behavior,displaying different colors in different pH solutions,and their yellow emission intensity exhibits a good linear relationship with pH values in the range of 6.0 to 12.0（R²=0.992）,achieving dual mode detection of pH by colorimetry and fluorescence.In addition,BN-CDs exhibit special solvent induced discoloration,and based on the sensitivity to water in solvents,BN-CDs can serve as ratio fluorescence probes for detecting water in DMF solvents.Finally,BN-CDs achieved efficient and sensitive detection of Fe³⁺in a buffer solution with a pH value of 5.0.