Study On The New Colorimetric Method For Detecting Hg²⁺ In Food Based On G-C₃N₄@AuNPs Mimic Enzyme

Natural enzymes are widely found in organisms,can efficiently and specifically catalyze various physiological and biochemical reactions.It is related to food detection,food analysis,food composition and so on.For instance,enzyme sensor can be used for the detection of heavy metal ions,additives and pesticide residues and other food contaminants,these methods have the advantages of high sensitivity,short time consumption and good specificity.However,the characteristics of natural enzyme such as variability,low content,high price restrict its wide application in the food field.Nanoenzyme has the advantages of low cost,easy storage,good stability,simple preparation,easy surface modifications,good catalytic activity and so on,which can make up the deficiency of natural enzyme.These advantages make the nanoenzyme have a good prospect in food detection.Gold nanoparticles(AuNPs)is one of the most used nanomaterials in metal nanoparticles and it exhibit size-dependent unique properties that differ from those of bulk gold due to the quantum confinement effect and exceedingly large surface-to-volume ratio.The application of AuNPs in the fields of catalysis,biology and detection have received great attention.However,gold is a rare precious metal and AuNPs usually suffer severe aggregation,which limits the use of AuNPs in the field of food detection.The aim of this paper is to synthesize novel g-C3N4@AuNPs mimetic enzyme by dispersing and stabilizing gold nanoparticles with ultrathin graphite phase carbon nitride(g-C3N4)nanosheet as carrier.It has good stability,dispersibility and specific peroxidase catalytic activity.Based on the enhancing effect of Hg2+ on the peroxidase activity of g-C3N4@AuNPs,a new method for colorimetric detection of Hg2+has been constructed,which effectively broadened the application of nanomaterials in the field of food detection.Its main contents are as follows:Firstly,g-C3N4@AuNPs mimic enzyme was successfully synthesized by ultrasound-assisted reduction.UV-Vis spectrophotometer,fluorescence spectrophotometer(FL),X-ray electron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FT-IR)were used to characterize the synthesized g-C3N4@AuNPs.At the same time,the morphology and size of g-C3N4@AuNPs before and after mercury addition were characterized by transmission electron microscopy(TEM).The average particle size of AuNPs on the g-C3N4 surface was 2.55±0.06 nm,and the average particle size of the composite nanoparticles was 4.84±0.30 nm.Secondly,by studying the mimic enzyme properties of g-C3N4@AuNPs,it can be concluded that g-C3N4@AuNPs possess peroxidase mimic enzyme activity.In the presence of hydrogen peroxide(H2O2),the substrate 3,3’,5,5’-tetramethylbenzidine(TMB)can be catalytically oxidized and the product of the reaction has a characteristic absorption peak at 652 nm.Through a series of optimization,the optimum conditions for the colorimetric detection of Hg2+were as follows:the pH value was 7.5,the volume of g-C3N4@AuNPs reaction is 4μL(diluted 3 times),the reaction time was 2 min,the TMB concentration was 0.8 mM,the concentration of H2O2 was 0.5 M,the reaction temperature was at 25℃.The results showed that the catalytic kinetics of g-C3N4@AuNPs conforms to the Michaelis-Menten equation,and exhibited similar affinity with HRP to the substrate TMB,With the increase of Hg2+ concentration,the Km values of substrate TMB and H2O2 gradually decrease,and the value of Vm gradually increases,indicating that Hg2+ can enhance the affinity of g-C3N4@AuNPs mimetic enzyme to the substrate.Thirdly,studies have shown that Hg2+ can increase the activity of g-C3N4@AuNPs peroxidase.Based on this,a new Hg2+ colorimetric detection method based on g-C3N4@AuNPs mimetic enzyme was constructed.The linear range of Hg2+detection for was 5.0-500 nM and the detection limit was 3.0 nM.At the same time,the test results showed that this nanocomposite has high specificity and can be stored stably for two months in the dark at room temperature.The method was applied to the detection of Hg2+ in water samples and fruit juices.The recovery was 92.6-109.3%,and the RSD was 0.7-7.7%.It shows that this method can be used in the detection of real samples.The method had the advantages of simple operation,low cost,short time consuming,signal visualization and high specificity,and can be used as a new method for rapid detection of Hg2+ in food.