| In recent years,food contamination has posed a great threat to human health,and foodborne pathogens are one of the direct causes of foodborne disease.Foodborne pathogens produce specific volatile biomarkers,which can be effectively monitored by metal oxide semiconductor(MOS)-based gas sensors in real time,thus providing timely warning and reducing the risk of human infection with pathogenic bacteria.In this paper,the biomarker dimethyl trisulfide(C2H6S3)produced by Listeria monocytogenes is selected as the detected gas,while the microstructure(exposed facet and defects)of WO3 is regulated and the structure-function relationship between the microstructure and gas-sensitive performance is established.In addition,the gas-sensitive mechanism is revealed by combining density functional theory(DFT)calculation with in situ spectral characterization.The main research work is as follows:Firstly,WO3 nanorod bundles with high crystallinity with exposed(002)facet are prepared by in situ hydrothermal growth method,which shows high responsiveness and selectivity to C2H6S3.The DFT calculation shows that there exists a strong adsorption between the S atom of the detected gas and the W site(Eadv =-1.310 e V),and the in-situ Raman further confirms the existence of the W···S bond,revealing that the catalytic action of the W site enhances the gas-sensitive response.Secondly,Prussian blue is used as template to prepare WO3 nanosheets(with a thickness of about 65 nm)with exposed(020)facet by chemical etching method.Compared with WO3 nanorod bundles with exposed(002)facet,the response value of WO3 nanosheets has a nearly 2.5-fold increase to 500 ppb C2H6S3,and the detection limit is reduced from 100 ppb to 5 ppb.Meanwhile,the sensor based on WO3 nanosheets exhibits excellent selectivity,good stability,and insensitivity to humidity.DFT calculation shows that the detected gas molecules exhibit stronger adsorption on the(020)facet(Eadv=-1.552 e V).In order to avoid drawbacks such as low yield and complex process of etching method,pothole-rich WO3 nanosheets(with a thickness of about 11 nm)with exposed(020)facet is successfully synthesized by hydrothermal method combined with annealing process,and its response value to 500 ppb C2H6S3 was 9.2,which was higher than that of thick WO3 nanosheets.The DFT calculation results show that the improvement of performance is attributed to the edge effect: the abound dangling bonds on the edge of the pores provide more active sites for the detected gas molecule,which enhances the adsorption energy of the gas molecules with WO3(edge Eadv =-1.715 e V).In addition,XPS characterization results also confirm the formation of W-S bond and the existence of the strong chemisorption between C2H6S3 and WO3.Pothole-rich WO3 nanosheets with oxygen vacancy are prepared by hydrogenation process,and the concentration of oxygen vacancy of WO3 increases after hydrogenation process.Compared with WO3 nanosheets without oxygen vacancy,the response of WO3 nanosheets with oxygen vacancy to 100 ppb C2H6S3 increases by 63%.The adsorption characteristics between dimethyl trisulfide molecules and oxygen vacancy-containing WO3 is simulated by DFT.It is found that the target gas molecules exhibit higher adsorption energy(Eadv =-2.030 e V)and there exists more charge transfer between C2H6S3 and WO3.The strong chemical adsorption makes it difficult for the gas molecules to desorption,leading to a certain increase in the recovery time.Finally,in order to solve the problem of long recovery time of WO3 nanosheet under the action of thermal field,photothermal excitation is induced to shorten the recovery time of the sensor based on hydrogenated WO3 nanosheet(from 527 s to 155 s),and the mechanism of photothermal excitation to improve the recovery performance is discussed.In conclusion,a variety of technical means are used to achieve the effective control of WO3 microstructure.The structure-function relationship between the microstructure and properties of WO3 is established.By combining with spectroscopy method and DFT calculation,the gas sensing mechanism is successfully revealed,and the real-time detection of C2H6S3 with high sensitivity,low detection limit,and rapid response is realized,which lays a solid technical foundation and provides theoretical guidance for the MOS-based sensors to be applied in the field of microbial detection. |
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