The Fabrication Of Four Nanocomposites Based QCM Gas Sensors And Their Applications In Detecting The VOCs Of Grass Carp Fillets

In the production,processing,transportation and storage of agricultural products,a wide variety of volatile organic compounds(VOCs)with low content are produced.It is of great significance to develop high-performance gas sensors to detect VOCs in agricultural products.The Quartz Crystal Microbalance(QCM)gas sensor can detect the mass change of nanogram level,which is very suitable for detecting VOCs in agricultural products with ppm level.In this study,graphene oxide and nano cuprous oxide(GO/Cu₂O),hydrophobic silica nanoparticles and amino-functionalized graphene oxide(AGO),hydrophobic silica nanoparticles and cysteine sensitized nano cuprous oxide(Cu(I)-Cys),hydrophobic silica nanoparticles and molecularly imprinted polymers(MIPs)based quartz crystal microbalance(QCM)gas sensors were developed,respectively.Field Emission Scanning electron microscopy(FE-SEM),Fourier infrared spectroscopy(FT-IR),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),Energy dispersive spectrometer(EDS)and other characterization methods were used to characterize the morphologies,compositions and structural characteristics of the as-prepared nanocomposites.The sensing performances of the sensors such as sensitivity,selectivity,repeatability and stability towards target VOCs(trimethylamine;hexanal,octanal and nonanal;hexanal and 1-octene-3-ol;hexanal)were tested,respectively.The sensing mechanisms of the sensors towards target VOCs were discussed by means of chemical model and Gaussian calculation,and the sensors were applied to the detection of target VOCs in grass carp fillets.The main research contents and conclusions are as follows:(1)GO/Cu₂O nanocomposite based QCM gas sensor via layer-by-layer self-assembly(LBLSA)method was developed for trimethylamine(TMA)gas sensing under concentrations of5 ppm.Under the condition of room temperature and 30%relative humidity(RH),the sensor response increased linearly with the concentration of TMA,which was consistent with the Saurebrey equation.It also had good sensitivity,reversibility,selectivity and stability in 60 days.The detection limit of 230 ppb was achieved which was the lowest detection levels among recently reported QCM gas sensor towards the detection of TMA under room temperature.The sensing mechanism of the GO/Cu₂O nanocomposite based QCM gas sensor towards TMA was proposed based on the chemical model as follows:an adsorption-desorption process via hydrogen bonding between the GO surface carboxyl groups and TMA molecules;enhanced physical adsorption of TMA gas molecules via LBLSA method due to enlarged surface area and p-n junction of GO/Cu₂O nanocomposite.Solid phase micro extraction and gas chromatography-mass spectrometry(SPME-GC-MS)results demonsrated that there was no TMA in the grass carp fillets during 0-3 refrigerated(4?)days,while there was TMA in the grass carp fillets during 4-7 refrigerated days.The response amplitudes of the sensor towards the headspace air of grass carp fillets during 0-3 refrigerated days were below 10 Hz,while the response amplitudes of the sensor towards the headspace air of grass carp fillets during 4-7refrigerated days were beyond 20 Hz,which indicated the as-prepaerd QCM gas sensor still had some selectivity towards TMA in complex gases.(2)A QCM gas sensor modified with the hydrophobic AGO nanocomposite for aldehydes detection was reported.The sensor was exposed towards air streams containing aldehydes(hexanal,octanal,and nonanal in single,binary,and tertiary mixture)vapors with different concentrations under 80%RH,and the sensor responded well towards aldehydes within 1-45ppm.The sensor characteristics were validated by hydrophobicity,sensitivity,reversibility,selectivity and stability analyses.The sensing mechanism was proposed as resistance to water vapor by hydrophobic silica nanoparticles;hydrogen bonding interactions between the primary amine groups,secondary amine groups and of AGO and aldehydes based on Gaussian calculation results.This sensor was successfully applied to the detection of aldehydes in grass carp fish fillets during 0-4 refrigerated days.SPME-GC-MS was used to detect aldehydes in the same fillets samples,and the results of the sensor and SPME-GC-MS were high related with a correlation coefficient of 0.98.(3)A QCM gas sensor was fabricated for hexanal and 1-octen-3-ol detection at room temperature with a hydrophobic Cu(I)-Cys nanocomposite as the sensing layer.The QCM gas sensor exhibited good reproducibility,hydrophobicity,reversibility,sensitivity,selectivity and stability towards 1-50 ppm hexanal and 1-45 ppm 1-octen-3-ol under 80%RH.The sensing mechanism of the sensor towards hexanal and 1-octen-3-ol was proposed as resistance to water vapor by hydrophobic silica nanoparticles;formation of sulfhydryl compound between cysteine and Cu₂O nano cubes;hydrogen bonding interaction between the carboxyl group of Cu(I)-Cys and hexanal(?H=-45.2 k J/mol)as well as the amino group of Cu(I)-Cys and 1-octen-3-ol(?H=-60.3 k J/mol)based on Gaussian calculation results.This hydrophobic Cu(I)-Cys nanocomposite based QCM gas sensor was applied to discriminate refrigerated grass carp fillets during 0-4 refrigerated days.Validation of the proposed sensor was carried out by correlating the frequency shifts with the contents of hexanal and 1-octen-3-ol obtained from SPME-GC-MS.The sensor responses showed a good congruence with the SPME-GC-MS analysis with a correlation coefficient of 0.96.(4)A QCM gas sensor modified with a hydrophobic MIPs nanocomposite for hexanal detection was reported.Hexanal could selectively bind with the MIPs through hydrogen bonding while the hydrophobic silica nanoparticles could give the electrode resistant to water vapor.The QCM gas sensor showed the highest response towards hexanal among the other tested vapors,and had little responses towards TMA,NH₃,ethanol,acetone,acetic acid and diethyl ether.The QCM gas sensor was reliable for hexanal detection in the range of 1-80 ppm with about 80%RH and able to detect hexanal amount in grass carp fillets during 0-7 refrigerated days.The results displayed positive correlation with SPME-GC-MS analysis of the same grass carp fillets samples,and the correlation coefficient was 0.98.These achievements may offer some new ideas for gas sensors development with high performances in areas of other characteristic VOCs detection in agricultural products and so on.