Synthesis And Application In Lung Cancer Biomarkers Detection Of Zinc Oxide Based Gas Sensors

Lung cancer is one of the most dangerous diseases worldwide.Traditional lung cancer screening methods such as CT and blood tests are difficult to achieve large-scale screening of the target population,and a large number of lung cancer patients miss the best time for treatment because they cannot be diagnosed early.Early diagnosis through breath analysis is a simple and effective method.Until now,the most used breath analysis method is to analyze the concentration changes of lung cancer biomarkers in exhaled breath by gas chromatography combined with mass spectrometry(GC-MS),but GC-MS equipment is expensive,not portable and requires trained technicians to use.In the past ten years,electronic nose has developed rapidly,and lung cancer diagnosis by using electronic nose has become a hot topic.To realize the detection of complicated and low-concentration VOCs,the sensors in the e-nose sensor array need to have a high response and different kinds of signals to different VOCs.Metal oxide gas sensors are good candidates for this kind of e-nose system due to their low price,easy-to-fabricate and high response.In this thesis,to realize the diagnosis of lung cancer by e-nose,we chose isopropanol,acetone,formaldehyde and ammonia as target gases and prepared five different zinc oxide(ZnO)based metal oxide semiconductor gas sensors for the detection of low concentrations of lung cancer biomarkers in the exhaled breath.The effect of synthesis parameters and modification methods on the gas sensing performance were studied as well as the gas sensing mechanism.The detailed results are introduced below.(1)ZnO nanorod powders with a high specific surface area were synthesized by hydrothermal method and then fabricated the sensing layers.The effects of synthesis parameters on the structure and properties were investigated.The XRD results showed that ZnO was well crystallized when the reaction time was longer than 2 h.The concentration of CTAB in the precursor can affect the oriented growth of ZnO.The amount of non-polar facets increases with the increase of CTAB concentration.When the percentage of non-polar facets is low,the response is high,which can reach 23 for 5 ppm isopropanol,but the response-recovery rate is slow,with a recovery time of over 20 min.The selectivity varies with the more exposed facets.The gas-sensitive mechanism of the ZnO sensor was investigated by both experiments and theoretical calculations.DFT calculation was used to calculate the adsorption energy of the target gases on the non-polar facet of ZnO and the adsorption energy of HCHO at the center of ZnO layer is much lower than that of NH3 on the ZnO surface.(2)5 at%Na was doped into the ZnO nanorods with the assist of citric acid which worked as the complexing agent,the lattice constant of Na-doped ZnO increased,indicating that Na+substituted Zn2+in the ZnO lattice.the SEM images showed that Na had no significant influence on the morphology of ZnO.the XPS and ICP-OES results showed that citric acid played an important role as a complexing agent in Na doping.The sensor response and selectivity improved significantly with Na doping,its response to 5 ppm HCHO was about 6 times higher than that of pure ZnO,and the best target gas was converted from isopropanol to HCHO.The working temperature had a significant influence on the selectivity of Na-ZnO due to the introduction of additional acceptor levels by Na doping which changed the bandgap structure of ZnO.(3)The ZnO nanorods were doped with Fe and the effect of Fe doping on the sensor structure and performance was investigated.It was found that Fe doping effectively reduced the size and increased the specific surface area of ZnO nanorods.Fe doping also significantly improved the gas sensing performance of ZnO nanorods.The optimal working temperature of 5 at%Fe-ZnO was 275℃ for isopropanol detection,it showed a high response to 250 ppb of isopropanol,and the response can reach 4.7.Also,the sensor showed good stability to humidity and time.The gas sensing process was simulated by Langmuir adsorption theory and compared with the experiment for the explanation of the gas adsorption and gas sensing mechanism of FeZnO.(4)ZnO nanoflowers were prepared by adding Co to the precursors by chemical precipitation.Depending on the ratio of Co and Zn in the precursors,the form of Co in ZnO differs.When the content is lower than 30 at%,The main form of Co is Co2+ and it substituted Zn2+in the ZnO lattice;when the Co content is higher than 30 at%,Co3O4 appears.the Co content determines the optimal working temperature and response of the sensor.10 at%Co-ZnO sensor showed the highest response to alcohols at the optimal operating temperature of 225℃,its response to 5 ppm isopropanol could reach 24,while the 30 at%Co-ZnO showed the best response to acetone at 250℃ and the response to 5 ppm acetone was 15.The improved sensing performance was attributed to the doping of Co and the formation of p-n heterojunctions,while the different forms of Co affected the operating temperature and selectivity of the sensors.To verify the feasibility of Co-ZnO in real exhaled breath analysis,the response to 500 ppb isopropanol was tested in synthetic air with 5%CO2 and artificial breath.The Co-ZnO sensor still showed a significant response to ppb-level isopropanol.(5)MXene derived TiO2-ZnO sensor was prepared in this thesis.Ti3C2Tx MXene was obtained by etching Ti3AlC2 with hydrofluoric acid followed by high-speed centrifugation and agitation.Then ZnO nanoparticles were prepared on the MXene surface via the hydrothermal method.After heat treatment under high temperatures,the derived TiO2 successfully maintained the 2D structure of MXene,and ZnO nanoparticles successfully attached to the MXene surface as well.1:1 TiO2-ZnO sensor showed a significant change in selectivity and optimal working temperature compared with both pure TiO2 and ZnO.The operating temperature rose to 350℃,which was much higher than the two pure metal oxides and the response varied slightly with temperature.The response of the sensor to VOCs was not high in absolute value,but the signal-to-noise ratio was good,and the resistance changed significantly when in 250 ppb isopropanol.The change in its gas sensing performance can be explained by the nn heterojunction created at the interface.