Fabrication And Kinetic Characterization Of Au@Ag Core-shell Nanorods Time Temperature Indicator (TTI)a Nd Its Application In Pasteurized Milk

Temperature is one of the most important environmental factors affecting the quality and safety of perishable foods such as milk,meat,fish,vegetables and fruits.The health,nutrition,sensory quality and deterioration of perishable foods are highly dependent on time and ambient temperature.Time temperature indicators?TTIs?are food quality recording tools with high cost performance.TTIs indicate food quality changes and remaining shelf-life information by monitoring and recording the temperature accumulation effects of the product,which can be used to prevent temperature abuse in the cold chain process.In the paper,our Au@Ag core-shell nanorods TTI?Au@Ag NRs TTI?constructed based on the extremely sensitive dependence of the plasmon resonance on the shape and composition of Au@Ag NRs bimetallic nanocrystals,its dynamic characteristics and temperature dependence are characterized,and it is applied to real-time quality indication of pasteurized milk.The main research contents and results of this paper are as follows:?1?The preparation,optimization and characterization of Au NRs and Au@Ag NRs in Au@Ag NRs TTI.Au NRs were prepared by the seed growth method with binary mixed surfactant of hexadecyltrimethylammonium bromide and sodium oleate.By adjusting the addition amounts of silver nitrate,ascorbic acid and hydrochloric acid,Au NRs of different sizes and different surface plasmon resonance effects were obtained,and their colloidal solutions showed different colors.The morphology and spectral characteristics were analyzed by transmission electron microscope and UV-visible spectrophotometer,and the colloid solution color changes were recorded.The results indicated that increasing the amount of silver nitrate and hydrochloric acid can make the wavelength of longitudinal plasmon resonance peak of Au NRs toward the long wavelength,while increasing the amount of ascorbic acid causes the wavelength of the longitudinal plasmon resonance peak of Au NRs to shift toward the short wavelength.Finally,Au@Ag NRs TTI was prepared by using Au NRs with transverse and longitudinal plasmon resonance absorption peaks at 510 nm and 864 nm,respectively,with an aspect ratio of 4.6.Au@Ag NRs was synthesized by adding silver nitrate and ascorbic acid to the prepared Au NRs,and characterized by transmission electron microscope and UV-visible spectrophotometer.?2?Study on the kinetic characteristics and temperature dependence of Au@Ag NRs TTI.The blue shift rate of the longitudinal plasmon resonance absorption peak wavelength of Au@Ag NRs is adopted as the kinetic response parameter of the Au@Ag NRs TTI.The longitudinal plasmon resonance absorption peak wavelength of Au@Ag NRs When the longitudinal plasmon resonance absorption peak wavelength of Au@Ag NRs moved to 690 nm,Au@Ag NRs TTI reaction reached the end point,at which time the color of Au@Ag NRs colloidal solution turned green.The results showed that the blue shift rates of the longitudinal plasma absorption peak wavelength of Au@Ag NRs are 17.25 h^-1,10.387 h^-1 and 2.5833 h^-1respectively,at the experimental temperatures of 35?,25?and 15?.According to Arrhneius equation,the activation energy of Au@Ag NRs TTI reaction system is 70.36 k J/mol.The results indicated that the Au@Ag NRs TTI prepared in this experiment can indicate the activation energy of the products in the range of 45.36?95.36 k J/mol.Simultaneously recording a color change Au@Ag NRs colloidal solution.At the three experimental temperatures,he color of the solution changed from initial red to orange,yellow and greenish yellow,and then to green at the specified reaction end point.TEM image analysis showed that the silver shell deposited on Au NRs surface successfully generated Au@Ag NRs.?3?Pasteurized milk was selected as the Au@Ag NRs TTI-indicated product and the kinetic characteristics of its quality parameters were studied.The pasteurized milk was selected as the research object,and the changes in acidity,p H value,total bacterial count and E.coli were measured under the storage of 4?,15?,25?and 35?.Appropriate growth kinetic model were used to fit the kinetic growth characteristics of these parameters,and then the effect of temperature on the growth kinetics of these parameters was characterized according to Arrhenius equation.The results showed that the activation energies calculated by taking acidity,p H value and total bacterial count of pasteurized milk as spoilage indicators were 65.67 k J/mol,127.87 k J/mol and 6.28 k J/mol,respectively.The difference between the activation energy of acidity and the activation energy of Au@Ag NRs TTI is 4.69 k J/mol less than 25 k J/mol.In addition,the matching of pasteurized milk and Au@Ag NRs TTI under a certain time-temperature history condition was studied through dynamic temperature condition test.The results showed that the difference of effective temperature T_eff is 0.1?and the shelf time error is 5.4%.Therefore,the Au@Ag NRs TTI prepared in this experiment can be applied to the quality monitoring of pasteurized milk.