Research And Application Of Semiconductor Gas Sensor Based On WO₃ Nano Materials For Rapid Detection Of Listeria Monocytogenes In Food

With the increasing demand of amounts and quality of food products,the food industry is developing rapidly.However,there are many food safety issues happened every years.Among them,the issues caused by food-borne pathogens are in large number and post great threat to public health.Listeria monocytogenes is a food-borne pathogen that can survive in broad temperature and p H conditions.Listeria monocytogenes can attack people by releasing many kinds of toxic factors such as Listerolytic toxin,with a fatality rate of up to 20%-30%.The detection to Listeria monocytogenes has been a main research point in the field of food safety control.However,the traditional detection methods such as plate colony counting can reach high accuracy but usually suffer from long-time and laborious process.Hence,it is of great importance to develop a rapid and accurate in site Listeria monocytogenes detection method.During the metabolism,Listeria mnocytogenes can produce specific volatile organic compounds 3-hydoxy-2-butanone,which is considered as the biomarker.Therefor,the detection of Listeria monocytogenes can be achieved by indirectly determining 3-hydoxy-2-butanone.In this regard,semiconductor gas sensors own superiority owing to its advantages of small size,easy operation and high sensitivity to target gas.Among the semiconductor metal oxides,WO₃is widely used in gas detection for its stable properties,low-cost and environmental friendliness.But WO₃is commonly limited by long response and recovery time,and low selectivity.Usually,the morphology control and noble metal decoration strategies can be used to improve the sensing performance of the semiconductor nanomaterials.Herein,a variety of WO₃nanomaterials with different morphology and flower-like WO₃nanomaterial decorated with bimetal have been synthesized,the gas sensing performance of the WO₃nanomaterial have been investigated,and the sensitivity mechanism as well as the reason for the improvement of gas sensing performance have been illustrated in detail.At the same time,the detection to the Listeria monocytogenes in milk and salmon have been achieved by the prepared gas sensors,aiming to propose a rapid and accurate method to detect Listeria monocytogenes.The main research contents are as listed below:1.The effective and rapid detection of 3-hydoxy-2-butanone by the gas sensors based on WO₃nanomaterials with different morphology.WO₃nanobulks,WO₃nanoclusters and WO₃nanoflowers were synthesized by controlling the reaction solvent system and reaction temperature and time,using Na₂WO₃·2H₂O and WCl₆as tungsten sources.Their morphologies were studied,and the WO₃nanoparticles were purchased for the evaluation of the sensing performance of the as-synthesized nanomaterials.The WO₃nanoflowers based sensors exhibited the best sensing performance.Under the optimal working temperature（290°C）,its response to 20 ppm 3-hydoxy-2-butanone was 81,and response and recovery times were 4 s/9 s.This can be attributed to the hierarchical nanostructure of WO₃nanoflowers with large specific area（40.815 m²/g）which can provide many pathways for the transport of gas molecules and lots of active sites for chemical reactions on the material surface.2.The rapid and selective detection of 3-hydoxy-2-butanone by the gas sensors based on Au Pd-WO₃nanoflowers.The Au Pd nanoparticles were synthesized by ascorbic acid reduction method and were decorated to the surface of WO₃nanoflowers by impregnation method.The Au Pd-WO₃nanoflowers modified with different amounts were successfully prepared,and WO₃nanoflowers modified with single metal Au and Pd were used as control group.The 3-hydoxy-2-butanone detection results showed that the gas sensing properties of WO₃nanoflowers decorated by Au Pd bimetallic namoparticles were significantly enhanced,which mainly benefited from the chemical sensitization effect of Au,the electronic sensitization effect of Pd and the synergistic effect of Au and Pd.Among all the WO₃nanoflowers decorated by Au Pd bimetallic namoparticles,the 1.0wt%Au Pd-WO₃nanoflowers showed best gas sensing performance.Its optimal working temperature decreased to 250°C and the detection limit decreased to 100 ppb.3.The rapid and selective detection of Listeria monocytogenes by the gas sensors based on 1.0 wt%Au Pd-WO₃nanoflowers.The 1.0 wt%Au Pd-WO₃based sensors were used to detect the Listeria monocytogenes cultured in BHI broth.The response of the sensors increased with the increase of the concentration and culture time of the Listeria monocytogenes.And the sensors showed excellent specificity to Listeria monocytogenes.Moreover,the 1.0 wt%Au Pd-WO₃based sensors have been successfully used to determine the milk and salmon contaminated by Listeria monocytogenes.The detection limit of the sensors were 10²CFU/m L,which illustrated the potential of the Au Pd-WO₃based sensors to the effective detection of Listeria monocytogenes.