| The freshness of aquatic products is a crucial factor in determining the quality of aquatic products and one of the essential factors affecting consumer choice.It is usually necessary to determine the freshness of aquatic products by measuring indicators related to freshness.However,these traditional detection methods require equipment and complex operational processes.How to detect the freshness of aquatic products quickly and in realtime is an urgent problem to be solved.The colorimetric indicator card can monitor the freshness of food in real-time through the color change of substrate to convert the freshness information into color information and realize the rapid detection of freshness.Colorimetric indicator cards generally contain two parts:color-changing chromogenic substrates and matrix materials.Among them,nanozymes have unique surface electron transfer and oxygen storage capacities,which can rapidly change color through redox reactions and become an excellent choice for chromogenic substrates.Therefore,colorimetric indicator cards based on nanozymes have recently become a research hotspot.Based on this,this paper proposes a cerium oxide nanozyme(CeNPs)indicator card,which uses cerium oxide’s redox discoloration ability to monitor the hypoxanthine(HX)content produced during shrimp spoilage.The change of HX content theoretically represents the degradation of ATP in shrimp muscle and then realizes the rapid detection of shrimp freshness.CeNPs were mainly used as redox amplifiers and chromogenic elements in the colorimetric indicator card,and cellulose filter paper and gelatin film were used as carriers to construct nanozyme colorimetric indicator cards.The main results are as follows:1.Process optimization and performance study of cerium oxide nano-enzyme paper-based indicator cards.Firstly,the sol-gel method constructed a paper-based indicator card with good mechanical properties by fixing cerium oxide on cellulose filter paper.The color development principle and feasibility of HX detection by CeNPs were further verified and analyzed.Then,the preparation process of the sensor was optimized with the concentration of the soaking solution,ultrasonic time,soaking time,and detection time as indicators,and the most suitable preparation process parameters were obtained.The concentration of CeNPs soaking solution was 8%,the ultrasonic time was 6 h,the soaking time was 10 min,and the detection time was 10 min.The structure of the paper indicator card was analyzed by scanning electron microscopy(SEM)and Fourier transform infrared spectroscopy(FTIR).The results showed that cerium oxide was uniformly deposited on cellulose filter paper.The paper-based indicator card produced a color change from pale yellow to orange for different concentrations of HX,and the sensitivity was 0.025 mmol/L.However,due to the limited content of paper-based loaded cerium oxide,the color change of the sensor needs to be more apparent,and the sensitivity needs to be higher.Therefore,we further introduced CeNPs into the gelatin film matrix in the subsequent research to improve the detection performance of the sensor.2.Process optimization and performance study of cerium oxide nano-enzyme film-based indicator card.In the previous part of the study,it was found that the color change of the paper-based indicator card needed to be more noticeable,and the sensitivity needed to be higher.Therefore,this paper constructed a stable three-dimensional network structure by crosslinking gelatin and glutaraldehyde to increase the amount of cerium oxide in the sensor,thereby improving the coloration and sensitivity of HX.Among them,the effects of different CeNPs additions on the brilliant film’s structure,performance,and sensitivity were studied to determine the most suitable CeNPs addition.The results showed that hydrogen bond interaction was formed between nanoparticles and gelatin after adding CeNPs to the gelatin matrix.With the increase of CeNPs content,the dense structure of gelatin/glycerol/glutaraldehyde(G/Glu/Gly)film was destroyed.The results of the morphological analysis showed that CeNPs were well deposited on the film after adding CeNPs.The roughness of the intelligent film increased,but the surface was still smooth and had a larger contact area.The intelligent film’s tensile strength(TS)did not change much,and the elongation at break(EB)tended to decrease.The water vapor permeability(WVP)increases first,then decreases,and then increases,but the barrier performance is still excellent.The color of the innovative film gradually deepened,and the transmittance decreased.In conclusion,the optimum addition amount of CeNPs is 25%(w/w,gelatin).The three-dimensional network structure of gelatin increased the amount of CeNPs immobilized on the gelatin matrix,improved the sensitivity of the film-based indicator card(0.0062 mmol/L),and the film-based indicator card had high specificity and correlation for HX y=63.605x+20.170(R2=0.9823).3.Application of film-based indicator card in shrimp freshness detection.In order to accurately monitor the freshness of shrimp,the pH value,total volatile base nitrogen(TVB-N),and total bacterial count,(TVC)of shrimp during storage were measured,and the change process of shrimp freshness from fresh,sub-fresh to spoilage was constructed.Then the above film-based indicator card was applied to the detection of shrimp freshness,and the correlation between the freshness index of shrimp and the color value of the film was constructed.The results showed that the color change of the film had the same trend as the freshness indexes such as pH,TVB-N,and TVC value of shrimp,which showed a significant upward trend.During the detection process,the film showed a visible color change from milky white to deep orange,which could clearly distinguish the three states of fresh,sub-fresh,and completely deteriorated shrimp.At the same time,the accurate HX content in shrimp samples was further determined by the liquid phase.The results showed that the detection results of the film-based indicator card were the same as the actual content trend of HX,which verified the reliability and feasibility of the film-based indicator card in detecting shrimp freshness. |
