Research On The Detection Of Fish Freshness Based On Room Temperature Semiconductor Gas Sensor Array

Fish is susceptible to spoilage during processing,storage or transportation,which will seriously affect the flavor,quality and human health of fish.Freshness is a key indicator of the quality and safety of fish products including fish,therefore,detection and assessment of freshness is a key aspect of fish quality and safety control,which is related to the vital interests of consumers.Traditional freshness testing methods,such as sensory analysis,physical and chemical analysis and existing nondestructive testing equipment have limitations such as sample breakage,subjectivity,cumbersome steps,time-consuming,requiring large instruments and equipment and professional personnel to operate,resulting in the existing technology cannot meet the requirements of objective real-time and rapid testing.With the development of agricultural economy and the progress of science and technology,intelligent detection has become the future development trend of edible agricultural products industry.Therefore,there is a need to develop fast and effective alternative technologies to detect and evaluate the freshness of fish in different periods or conditions.Gas sensor technology is becoming increasingly important in the qualitative and quantitative assessment of high-protein foods,including fish.Among them,metal oxide semiconductor resistive(MOSR)sensors are gradually showing promising practical applications due to their low cost,small size,easy integration,and high sensitivity.However,the application of this technology in the field of fish freshness detection still suffers from limitations such as high operating temperature,insufficient detection limits,performance drift and low accuracy.To address the above key issues,this thesis analyzes the possible volatile gases generated during fish storage and selects three gases as fish freshness biomarkers;designs and develops three new room-temperature semiconductor based gas sensors,elucidates their working mechanism and verifies their practicality;builds a room-temperature array system based on fewer semiconductor gas sensors,and uses common sea fish(salmon,cod and tuna)and freshwater fish(pangasius)in the market as the target.The study was conducted with sea fish and freshwater fish in the market,and the change pattern between fish freshness and resistance signal was obtained by combining sensory analysis and pH characteristics of fish body,and the method of fish freshness identification based on principal component analysis and linear discriminant analysis was established to verify the effectiveness of the room temperature semiconductor gas sensor array system.This thesis provides an important proof of concept and scientific basis for the gas sensor detection technology of fish freshness,and has guiding significance and application value for the development of real-time online detection equipment and the promotion of the development of the fish industry.The specific research contents and results are as follows:(1)Spoilage mechanism and characteristic odor information extraction of fish at different freshness stages during storage.The main causes of fish spoilage are briefly described in terms of fish structure and environmental factors.The types of gases and volatile organic compounds detectable in fish from freshness to spoilage and their biochemical production mechanisms were summarized,and hydrogen sulfide(H2S),ammonia(NH3)and trimethylamine(TMA)were identified as the key indicators of fish freshness.In addition,the change pattern of freshness during fish spoilage was investigated by traditional sensory analysis and physicochemical indexes(fish pH).It provides a research basis for the design of a room temperature semiconductor gas sensor array for subsequent freshness detection.(2)Design and fabrication of a new room temperature semiconductor H2S gas sensor.Gas sensors based on pure Bi2WO6 nanosheets and WO3-Bi2WO6 microflowers with different WO3 mass ratios were fabricated by a facile hydrothermal synthesis and coating technique for the detection of ppb-level H2S.The results showed that 20 WO3-Bi2WO6 microflowers showed the best room temperature sensing performance:the response value to 50 ppb H2S was as high as 4.4,the response/recovery time was 52/119 s,the detection limit reached 2 ppb,and it showed high sensitivity to low concentration of H2S,fine selectivity,good resistance to humidity interference and excellent long-term operational stability.The enhanced gas sensing performance is explained by the synergistic strategy consisting of the catalytic effect of WO3 nanoparticles,the modified hierarchical structure with large specific surface area,and the n-n heterojunction structure.In addition,the research on the application of 20 WO3-Bi2WO6 microflower gas sensor to fish freshness detection showed that the response value of the sensor increased linearly(R2=0.9922)with the storage time of pangasius at 20℃ and showed a strong effect on fish volatilization.Good transient response characteristics of markers.This trend further confirmed the effective detection of different H2S concentrations generated by the decomposition of sulfhydryl-containing amino acids during the spoilage of pangasius by the sensor.(3)Design and fabrication of a new room temperature semiconductor NH3 gas sensor.A series of Ce-doped TiO2 nanocrystal gas sensors were developed and fabricated by microwaveassisted solvothermal and coating techniques for detecting low-concentration NH3 at room temperature.The results show that 0.43 at%Ce-TiO2 nanocrystals exhibit excellent NH3 sensing performance at room temperature:the response value to 1-20 ppm NH3 reaches 1.223.99,and the response/recovery time to 10 ppm NH3 is 55/192 s,detection limit as low as 140 ppb,excellent NH3 selectivity,repeatability and operating stability.The enhanced sensing mechanism can be explained by the synergistic strategy of high-energy facet growth,oxygen vacancies,and large specific surface area,which enhances the surface activity and charge transfer efficiency,resulting in excellent gas-sensing performance.In addition,the gas sensor was used to detect NH3 released from 25 g pangasius during normal temperature(23℃)storage to evaluate its practical application potential in the field of rapid and nondestructive detection of fish freshness.With longer storage time,the response value increased from 1.66 to 7.98.Th results further confirm the great practical application potential of the 0.43 at%CeTiO2 gas sensor for the low-concentration NH3 detection at room temperature.(4)Design and fabrication of a new room temperature semiconductor TMA gas sensor.A series of g-C3N4/Bi2MoO6 heterogeneous composite gas-sensing materials were developed by a facile solvothermal synthesis and coating process for the detection of TMA,a major volatile biomarker of fish freshness,at 22℃.The results showed that 1 wt%E-g-C3N4/Bi2MoO6 achieved excellent TMA sensing performance at 22℃,including the best response(Ra/Rg=10.6@20 ppm)and faster response/recovery speed(42/209 s),high selectivity,longterm working stability and low detection limit(1.3 ppm),meeting the theoretical criteria for freshness testing(fresh:<10 ppm TMA).The practical application research results showed that during the storage of cod samples at room temperature,the response value of the 1 wt%E-g-C3N4/Bi2MoO6 gas sensor increased from 1.25 to 11.71 with the storage time.In addition,the response value of the gas sensor has a linear proportional relationship with the storage time(R2=0.975).The results further confirm the great potential of the 1 wt%E-g-C3N4/Bi2MoO6 gas sensor for fish freshness detection.(5)Construction and practical application of a new room temperature semiconductor gas sensor array.A sensor array based on self-developed room temperature semiconductor gas sensors was designed and constructed to study and validate its practicality and reproducibility for freshness detection of different fish species,including its response characteristics to volatile marker gases,performance stability and reliability for continuous detection.Based on the comprehensive evaluation results of fish color and tissue status,sensing characteristics,principal component analysis(PCA),linear discriminant analysis(LDA)and pH tests,an effective two-dimensional correlation model of physical electrical signal-fish freshness quality visualized by the gas sensor array was established,in which the total contribution rate in PCA was higher than 97%and in LDA was higher than 98.8%for all types of fish samples.The feasibility and reliability of the application of the novel MOSR room temperature gas sensor array for nondestructive testing of fish freshness was elucidated.