The Fabrication Of QCM Gas Sensors Based On Composites And Their Applications In Detecting The Freshness Of Three Kinds Of Food

Food freshness is one of the important indexes to evaluate food quality,and it is of great significance to obtain food freshness information to ensure food safety.Characteristic volatile organic compounds（VOCs）of low concentration released by food are effective media for obtain food freshness information.However,the current detection methods for VOCs in food still have some shortcomings,such as long analysis time,unintuitive detection results and difficulty in quantitative analysis.QCM gas sensors can monitor the tiny mass changes on the electrodes,which is suitable for the detection of characteristic VOCs on ppm level of food freshness.Based on this,in this thesis,using QCM as the detection platform,gas sensitive materials were designed and synthesized,and QCM gas sensors were fabricated to detect the freshness of three kinds of food.Moreover,the adsorption mechanism of the gas sensitive materials and the target gas molecules was further explored,and the feasibility of using the prepared QCM gas sensors for food freshness detection was verified.The main research contents and results are as follows:（1）For TMA gas under low concentration,,Sn S_x（x=1,2）heterojunction modified QCM gas sensors with high sensitivity were fabricated.Firstly,a series of Sn S_x heterojunction materials were synthesized by a one-step hydrothermal method.The QCM gas sensors were prepared by drop-casting Sn S_x heterojunction materials on Au electrodes,respectively,and the response to TMA under 2～10 ppm was investigated（40%RH,25℃）.Among them,Sn S_x-6 modified QCM gas sensor with the highest content of Sn S/Sn S₂ heterostructure had highest sensitivity（-22.29Hz/ppm）,relatively long response time（466.1 s for 10 ppm TMA）,low LOD（0.77 ppm）,good selectivity and stability within 40 days.However,the response time of the prepared sensor to TMA gas increased significantly with the increase of the concentration,and the response time for10 ppm TMA gas was 466.1 s.Then,the adsorption mechanism of TMA gas molecules on Sn S_xheterostructure materials was further investigated based on density functional theory（DFT）,the Sn-site on Sn S structure was proved to be the dominant adsorption site,and Sn S/Sn S₂heterostructure could effectively enhance the sensing performance of the QCM gas sensor.Finally,the prepared Sn S_x-6 modified QCM gas sensor was employed to detect the freshness of shrimps.The frequency response value of the sensor increased with the increase of storage time during third to seventh day,which was strongly correlated with HS-SPME-GC-MS results.The results showed that the prepared QCM sensor was able to realize the freshness detection of shrimps.（2）In order to solve the problem that the above QCM gas sensor has a long response time,a highly sensitive QCM gas sensor modified with WS₂ and MWCNTs was fabricated.Firstly,MWCNTs was designed to be the growth skeleton of WS₂ nanosheets,which can reduce the agglomeration of WS₂.Subsequently,WS₂/MWCNTs nanocomposites were modified on the QCM electrodes to fabricate QCM sensors,and their responses to TMA gas under the concentration range of 0.25～2 ppm were investigated.Compared with single WS₂ and MWCNTs at 25℃and 50%RH,the QCM gas sensor modified with WS₂/MWCNTs nanocomposites had higher sensitivity（-111.74 Hz/ppm）,faster response time（244.5 s for 500 ppm TMA gas）and lower LOD（76 ppb）.Moreover,the adsorption amount of the QCM sensor for TMA molecules was more and the response value was higher compared with amine and other interfering gas under same concentration.Based on this,the adsorption mechanism of TMA molecules on WS₂/MWCNTs nanocomposites was further analyzed.TMA gas molecule preferred to adsorbed on the H-site of WS₂,and the S edge exposed by WS₂ could greatly promote the gas sensing performance,which proved that the high specific surface area and more effective adsorption sites of WS₂/MWCNTs composites can effectively enhance the adsorption of TMA molecules.Finally,the prepared WS₂/MWCNTs modified QCM gas sensor was employed to detect the freshness of frozen yellow beef slices.During the third to fifth day,the frequency response value of the sensor increased with the increase of storage time,which had a strong correlation with HS-SPME-GC-MS results.The results showed that the prepared QCM sensor was able to realize the freshness detection of frozen yellow beef slices.（3）In order to improve the selectivity of the QCM gas sensor in the same type of gas,an extremely sensitive QCM gas sensor modified with ZIF-67 and H-Si O₂ was designed and fabricated.Firstly,ZIF-67 grains crystals were synthesized on the surface of H-Si O₂ microspheres,and the existence of H-Si O₂ could inhibit the growth of ZIF-67 crystals,which make the morphology and structure of ZIF-67 crystals more stable.Then,the ZIF-67/Si O₂ composites were modified on the QCM electrodes by drop-casting method to fabricate QCM gas sensors,and the response of the QCM gas sensor to E,E-2,4-heptadienal vapor under 2～10 ppm was investigated.At 25℃and 50%RH,ZIF-67/Si O₂ composites based QCM gas sensor had extremely high sensitivity（-418.03 Hz/ppm）,fast response time（223.5 s to 10 ppm target vapor）,relatively low LOD（1.67 ppm）,good selectivity and stability.Based on this,the adsorption mechanism of E,E-2,4-heptadienal vapor on ZIF-67/Si O₂ composites was further analyzed.The target gas molecules preferred to adsorb on the imidazole group of ZIF-67 and form hydrogen bonds with H atom.Moreover,the free energy of E,E-2,4-heptadienal gas molecules was stronger than other aldehydes,which verified the good selectivity of the composites.Finally,the prepared QCM gas sensor modified with ZIF-67/Si O₂ composites was used to detect the freshness of first-grade rapeseed oil.During the 6th～20th day,the frequency response value of the QCM sensor increased with the increase of storage time,which was strongly correlated with HS-SPME-GC-MS results.The results showed that the prepared QCM sensor was able to realize the freshness detection of first-grade rapeseed oil.