Rapid High-yield Preparation Of Carbon Dots For Food Analysis Applications

Food safety is an important global concern,and the development of novel rapid detection technology is of great importance for the evaluation and control of food quality.Nanomaterial-based fluorescent sensors have great potential for food safety analysis.Carbon Dots（CDs）,as a new class of carbon nanomaterials,have the advantages of excellent luminescent property,chemical stability,photostability,high fluorescence tunability,and low toxicity,and have received increasing attention in the fluorescence sensing for food samples.In this paper,we explored a rapid and high-yield synthesis techniques for CDs based on self-exothermic reaction,burning reaction and sunlight photocatalytic reaction,respectively,and developed a series of fluorescence sensing procedures forβ-Glu activity,acid value of vegetable oil,acetic acid content in vinegar and ethanol content in wine based on the excellent luminescent properties of the prepared CDs.The main contents are as follows:（1）G-CDs were rapidly prepared within 5 min using ganpu tea as the carbon source and the self-exothermic reaction between hydrogen peroxide and diethylenetriamine（DETA）as the heat source.The product yield was about 2.5 g.The maximum excitation and emission wavelengths for G-CDs were 320 nm and 385 nm,respectively,with a fluorescence quantum yield of 14.9%.It was found that the absorption peak（400 nm）of p-nitrophenol（p-NP）had a large spectral overlap with the fluorescence emission peak（385 nm）of G-CDs,which bring a significant fluorescence internal filter effect（IFE）on G-CDs.An analytical method for p-NP was established based on IFE with a detection limit of 14.6 n M,and the method was successfully applied to the determination of p-NP in human urine and environmental water samples.Furthermore,p-NP is one of the hydrolysis products of p-nitrophenyl-β-D-glucuronide（p NPG）in the presence ofβ-glucosidase（β-Glu）,and a fluorescence method for indirect determination ofβ-Glu activity was established based on fluorescence response to the product of enzyme reaction.The developed method has been successfully applied to the determination ofβ-Glu activity in fruits with a detection limit of 0.074 U L^-1.（2）Gram-scale synthesis of B-CDs through a burning reaction within 20 min using o-phenanthroline as the carbon source and ethanol as the solvent and fuel.The product yield was about 4.0 g.The prepared B-CDs exhibited obvious solvent-dependent optical properties.In acetonitrile solvent,the optimal excitation and emission wavelengths of B-CDs were 335 nm and 362 nm,respectively;while in ethanol solvent,the optimal excitation and emission wavelengths of B-CDs were 355 nm and 386 nm,respectively,with a fluorescence quantum yield of 8.2%.Based on the response of B-CDs to solvent polarity,a fluorescence method for the detection of water content in organic solvents was established,and wide linear ranges（0.025-90%for water content in acetonitrile and 10-90%for water content in ethanol）and low detection limits（0.016%for water content in acetonitrile and 1.3%for water content in ethanol）were achieved.The developed method was successfully applied to the determination of water content in acetonitrile solvent and the indirect determination of ethanol content in wine.In addition,the B-CDs solution exhibited acid-base response characteristics.It was found that the fluorescence intensity of ethanol solution of N-CDs showed a good linear response with respect to NaOH concentration.Based on this,a fluorescence method for the indirect determination of acid value of edible acids and acetic acid content in vinegar.The analytical results by this method were in good agreement with those by the national standard method（GB 5009.229-2016）.The developed method has the features of less sample consumption,simple analysis,rapid and sensitive compared with the national standard method.（3）N,S-CDs were successfully prepared in DMSO solvent within 5 hours under sunlight irradiation using catechol and diethylenetriamine as carbon source and modifier,respectively,and biomass CDs as catalyst.The single yield was about 1.5 g,and the fluorescence quantum yield was 14.1%.The prepared N,S-CDs have bright green fluorescence（the optimal excitation and emission wavelengths are 410 nm and 510 nm,respectively）.It was found that Pd²⁺could form a complex with the functional groups on the surface of N,S-CDs,which caused the static quenching of N,S-CDs.Based on this,a method for the determination of Pd²⁺was established with a detection limit（3σ）of 3.7 ng m L^-1.The method was successfully applied to the determination of Pd²⁺in environmental water and mineral water samples.