Mechanism And Method Of Rapid Sensing Content Of Tea Polyphenolsby Quenching With Carbon Quantum Dots Fluorescence Probe

Carbon quantum dots?CQDs?are new types of carbon nanomaterials with fluorescence properties of less than 10 nm in size,quasi-spherical shape and rich in organic functional groups.In addition to the good optical properties of traditional“quantum dot”,it has the advantages of low toxicity,environmental protection,low-cost,easy preparation,good biocompatibility and chemical stability,etc,and with a wide range of applications in thenanomedicine,detection,catalysis,biological imaging,sensing and other aspects.Tea is a popular flavor and functional health drink in the world,which greatly attributed to its important functional component—tea polyphenols?TP?.It not only determines the quality of tea,but also has anti-oxidation,anti-lipids,anti-tumor,anti-atherosclerosis and other effects.At present,conventional methods,which widely used to detect TP,include ferrous tartrate colorimetry,fulinphenol colorimetry,potassium permanganate titration,HPLC method,etc.Most of these traditional methods are tedious steps,time-consuming and labor-intensive,high-cost,environmentally unfriendly,and complicatedpre-processing,etc.Therefore,it is significantly important to develop a rapid?accurate and safe method for TP content detection.In this paper,we study about the rapid and efficient preparation of fluorescent carbon quantum dots and their properties and develop a new analysis method for TP detection,and then implement to rapidly detect TP content in the tea samples.The main contents and results are as follows:1.Rapid synthesis of fluorescent carbon quantum dots.The rapid preparation of high quantum yield fluorescent carbon quantum dots by high-throughput microwave digestion/extraction workstation.Carbon source structure,molar ratio of carbon and nitrogen source,temperature,power and time are mainly studied in this synthesis reaction.The emission spectrum of the synthesized carbon quantum dots under the different conditions described above was characterized by using a fluorescence spectrophotometer with an excitation light of350 nm.Experimental results indicate that the optimal conditions for the preparation of fluorescent carbon quantum dots were that the molar ratio of citric acid monohydrate?carbon source?and ethylenediamine?nitrogen source?was 1:1.5,temperature was 200?,power was500 W and time was 5 min.The relative fluorescence quantum of carbon quantum dots yield was about 73.66%,while prepared by the traditional hydrothermal method?the molar ratio of citric acid monohydrate and ethylenediamine was 1:1.5,temperature was 200?,time was 4h?was 66.08%.2.Properties of fluorescent carbon quantum dots.The carbon quantum dots prepared under the optimal conditions were characterized by UV-Vis absorption,fluorescence,transmission electron microscopy?TEM?and fourier transform infrared spectroscopy?FTIR?.The results indicate that the prepared carbon quantum dots have excitation wavelength-dependent properties.Its optimal excitation wavelength and emission wavelength are about 350 nm and445 nm,respectively.The CQDs are nearly spherical with some irregularities,well dispersed in water,the average particle diameter of about 6 nm and the surface with many functional groups including C-OH,CH,C=O,NH,etc.It exhibits blue with ultraviolet light irradiation and yellow in natural light.In addition,the stability of CQDs was also investigated.In the range of pH 4-10,the fluorescence intensity of CQDs did not change significantly,while it decreased significantly under the environment of pH 2?3?11 and other strong acids and bases.Therefore,it explains that the pH of the research system solution should be from 4 to 10.Fluorescence intensity of undiluted CQDs solution did not change significantly under different storage conditions,while the diluted CQDs solution were different under different storage conditions.The decrease of diluted CQDs fluorescence intensity was more obvious at room temperature without light storage,which was dropped by 74%after 30 days.3.Study on the rapid detection of EGCG and its promotion of oxidation.The effects of reaction time,reaction temperature and concentration of epigallocatechin gallate?EGCG?on the quenching of carbon quantum dot fluorescence probe in the presence of Cu²⁺were investigated,as well as promotion of oxidation.The results indicate that carbon quantum dots fluorescence appear well quenching under the Cu²⁺and EGCG concentrations of 2:1,40?water bath for 30 min.The detection limit of EGCG is 0.45?M in the concentration range of 0-50?M,R²=0.9986.Other substances similar to EGCG structure such as epicatechin gallate?ECG?,epigallocatechin?EGC?,epicatechin?EC?,tea polyphenols?TP?for carbon quantum dot fluorescent probes quenching has similar effects.In the presence of Cu²⁺,EGCG indicates pro-oxidative activity,which is producing.OH reactive oxygen radicals,and leading to fluorescence quenching of CQDs.4.Rapid detection of tea polyphenols in the tea sample.Based on the optimal conditions obtained in the above experiments,the effect of tea polyphenols?TP?concentration on the fluorescence probes of carbon quantum dots in the system was investigated.The results indicate that the linear range of TP detection for carbon quantum dot fluorescence probe was 0-30?g/m L,the limit of detection was 0.31?g/mL,and R²=0.9865.In addition,this method has no obvious effect on the other components of the tea water such as theophylline,L-theanine,L-alanine,Na⁺,K⁺,etc.This method can be used to detect TP tea sample,which is consistent with the national standard method,recovery rate from101.11%to 106.97%.