| Carbon dots(CDs)have excellent optical properties,good water solubility,stability,resistance to photobleaching,low toxicity,easy availability of raw materials,and biocompatibility.Therefore,the CDs are also widely applied in food and environmental monitoring,chemical analysis,catalyst preparation,energy development and other fields with good application prospects.In the field of food and environmental monitoring,the current analytical instruments for toxic and hazardous substances mainly include high-performance liquid chromatography-mass spectrometry,electrochemical instruments,fluorescence spectrometer or ultraviolet absorbance photometer,etc.Among them,the fluorescence method is widely applied in food and environmental analysis due to its advantages of simple operation,fast response,low cost,and high sensitivity.The CDs are fluorescent probes with high luminous efficiency.Due to the different raw materials,the difference of functional groups on the surface of the synthesized CDs and the influence of particle structure has specific fluorescence response to some harmful substances.According to the following features,we have built a fluorescent sensing platform to realize real-time monitoring of heavy metals,antibiotics,pH,and temperature.In this paper,based on the scientific research and a large number of related works of literature,small organic molecules containing organics with various functional groups as carbon source,and the luminescence effect can be tuned by doping N elements or changing the pH of the solution to synthesize fluorescent CDs with unique properties,and those CDs can detect toxic and hazardous substances in food and environment.The specific content is as follows:(1)Preparation of green fluorescent CDs and their application to the naked eye for identifying and detecting three tetracyclinesGreen fluorescent carbon dots(G-CDs)have been successfully synthesized through the one-step hydrothermal treatment of o-phthalic acid and ethylenediamine.The G-CDs possessed higher fluorescence quantum yield,up to 39.22%,exhibiting distinguished optical property,water solubility and stability.The G-CDs that emitting the strong blue-green fluorescence can visually identify and determine tetracycline(TC),oxytetracycline(OTC),chlortetracycline(CTC).TC quenched the fluorescence of G-CDs at 500 nm owing to the inner filter effect,OTC behaved similarly but the emission wavelength of G-CDs was red-shifted to 515 nm.Inversely,once CTC was introduced to G-CDs solution,the fluorescence increased and the emission peak was blue-shifted to 450 nm.Bandgap transition and electrostatic interaction were proposed to be the mechanisms for the detection of OTC and CTC by G-CDs.Wide linear relationships were established for TC,OTC and CTC with the limits of detection to be50 nM,36 nM and 373 nM,respectively.Furthermore,the nano-scale probe constructed by this system has been applied to detect tetracyclines(TCs)in complex samples with satisfying recoveries(93.2-114%)and was designed as portable test strips sensor for visually on-site TCs of honey sample screening.The preparation process of the nano fluorescent probe is simple,environmentally friendly,and G-CDs has the specific recognition ability for tetracyclines.The synthesized G-CDs in this work provide a new orientation for fast,effective,and visual real-time detection of tetracycline in actual samples.(2)Preparation of blue-emitting CDs and their application in the sensing of Cr(Ⅵ)and p-nitrophenolIn this work,blue-emitting carbon dots(B-CDs)as a luminescence sensing platform for the quantitative detection of Cr(Ⅵ)and p-nitrophenol(p-NP).Employing m-phthalic acid and ethylenediamine as precursors,B-CDs with strong blue fluorescence were synthesized by the hydrothermal method.Because the excitation spectrum of B-CDs had good overlap with the absorption spectrum of Cr(Ⅵ)and p-NP,the fluorescence of B-CDs was decreased because of the inner filter effect(IFE).Due to amino groups distributed in the surface of B-CDs,as H~+acceptor,once mixed with p-NP,the p-nitrophenol easily generated p-nitrophenol ions due to the charge interaction and the solution turned from colorless to yellow.Therefore,p-nitrophenol can also be detected by colorimetry.The B-CDs not only had a good linear response with Cr(Ⅵ)in the concentration range of 0.2-80μM with the detection limit of 89 nM.Moreover,it had a good fluorescence response with p-nitrophenol in the wide linear range of 0.2-140μM with a detection limit of 74 nM.Besides,the synthesized B-CDs have been used for the quantitative monitoring of Cr(Ⅵ)and p-nitrophenol in tap water,lake water and milk samples and the recoveries(93.2%-102%)were satisfactory.The analytical method constructed by this system provides a new direction for realizing low-cost,fast and convenient determination of hazardous substances in industrial wastewater.(3)Colorimetric and fluorescent detection of pH based on the red-emitting carbon dotsThe one-step hydrothermal method was used to synthesize red-emitting carbon dots(R-CDs).The fluorescent probe was prepared by hydrothermal pyrolysis of o-phenylenediamine and hydrochloric acid.The R-CDs that prepared after optimizing the conditions of temperature,time,and hydrochloric acid concentration emitted red fluorescence,the ultraviolet absorption wavelength was from 400 nm to 600 nm and had a high fluorescence quantum yield(25.4%).The R-CDs have the advantages of photobleaching resistance,unique salt resistance and excellent stability.The synthesized R-CDs showed high fluorescence response to pH,and the linear range of response to pH in fluorescence mode from 3 to 10;and the linear range of response to pH in colorimetric mode from 2 to11.The experimental data proved that the specific response of the R-CDs to pH may be derived from the state or the degree of surface protonation between the aggregation and dispersion of R-CDs.The R-CDs have been successfully applied to monitor pH in various water sources.Therefore,the prepared R-CDs have great potential for the application of biomedical imaging. |
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