Construction Of Carbon Dot Ratiometric Fluorescent Probes And Their Application In Food Analysis

Food analysis is essential in monitoring the quality of food for public health risk assessment.Conventional analytical methods can meet the requirements of routine laboratory analysis of food products.However,the critical food safety situation urgently requires rapid,time-saving and low-cost analytical methods,or even means to achieve on-site,portable portable and home testing.Fluorescence analysis has significant advantages such as high sensitivity,ease of operation,low cost and fast result output,and has great potential for application in the field of food analysis.Carbon dots（CDs）are new carbon nanomaterials that have emerged in recent years and are expected to replace traditional fluorescent materials.Compared with traditional fluorescent materials such as organic fluorescent dyes,metal nanoparticles/nanoclusters and quantum dots（QDs）,CDs have the advantages of simple preparation,low cost,excellent optical properties,low cytotoxicity,good biocompatibility and resistance to photobleaching.Therefore,CDs-based fluorescence sensing provides a good analytical platform for food composition analysis,detection and monitoring of trace hazardous substances.Furthermore,to overcome the challenge of low sensitivity and selectivity of sensors disturbed by various components in complex food matrices,self-calibrated ratiometric fluorescence sensing based on CDs is considered as an effective strategy for specific identification of target analytes in food matrices.In this paper,CDs were prepared using organic small molecules containing different functional groups as carbon sources.CDs-based ratiometric fluorescence sensing platform have been constructed for applications in food composition analysis and food safety testing.The details of the study are as follows:1.We synthesized blue fluorescent CDs by the solvothermal strategy using 2,5-dihydroxyterephthalic acid as the precursor.The as-prepared CDs surfaces contain abundant carboxyl and hydroxyl groups,while show excitation-independent emission as well as excellent chemical stability.Interestingly,with the increment of water contents in organic solvents,the CDs underwent a visual fluorescence color transition from blue to green,which could be attributed to the excited-state intramolecular proton transfer（ESIPT）effect via the intermolecular hydrogen bonding between CDs and water.Based on this,we designed an effective ratiometric fluorescence water sensor in organic solvents,which featured self-calibration,visualization,rapid response（<20 s）,and high sensitivity.The linearity range is 0.2%-6%（v/v,ethanol）and the limit of detection（LOD）is 0.052%.The smartphone-based quantitative assay was also realized using CDs-loaded paper strips through a color processing application“Color Analyzer”and successfully applied to monitor the water content in spirit samples.Humidity sensing was further evaluated through the as-prepared CDs/polymer films.Remarkably,the CDs-based sensors were well recyclable both with paper strips and polymer films.The as-prepared CDs also hold great potential for anti-counterfeiting.2.Metal ions-mediated reaction strategies usually offer powerful approaches for detection of small molecule biological analytes.The properties of metal ions,such as the unpaired electrons,empty orbitals and heavy atom effect,contribute to cause the emission turn-off of fluorophores by energy or electrons transfer.Inspired by the above-mentioned dual-emission property,a ratiometric fluorescence sensor for the detection of hydrogen sulfide（H₂S）based on Cu²⁺-mediated fluorescent CDs was further developed,which has a high sensitivity and fast response for the detection of H₂S and can be clearly identified by the fluorescence color change.The linear range is 10-500μM and LOD is 5.48μM.The ratiometric sensor was successfully used for the detection of H₂S in beer samples with recoveries of 89.1%-97.4%due to the superb binding ability of S^2-to Cu²⁺.In addition,we successfully applied CDs-Cu²⁺loaded glass films for monitoring gaseous H₂S.Thus,the sensing platform we have developed has a good application prospect for H₂S detection in complex environments.3.Though dual emissive probes are free from labeling and facilitate ratiometric approaches,their application is limited to finding suitable fluorophores that has ground-state and excited-state transformations which can provide band-shifting and generation of new bands,with fluorescence color changes typically occurring in adjacent bands that may affect the reliability of visual detection.The selection of a suitable reference fluorophore can overcome this problem.To this end,we have developed a histamine-responsive CDs-based sensing system for visual monitoring of food spoilage.In this system,the aldehyde group on the surface of CDs serves as the recognition sites to combine with the amino group on histamine to form a Schiff base that effectively enhances the fluorescence.On the other hand,1-aminoanthraquinone（ANQ）was introduced as a reference signal to endow the fluorescent probe with self-calibrating capacity.As the histamine concentration increases,linear ratiometric response was obtained accompanied with a distinguishable fluorescence color change from red to blue.The linear range is 0.2-20μM with an LOD of 0.036μM.Besides,test strips-based smartphone visual assays were successfully used to evaluate and monitor the histamine levels in fish samples.Our proposed sensing strategy has been demonstrated to be efficient and robust along with great potential for real-time visual monitoring of food spoilage.