Preparation And Characterization Of Mass-sensitive Gas Sensors For Detection Of Dimethyl Methyl Phosphonate

The frequently chemical terrorism cases in 20^th Century arose great attentions in the world, and these demonstrate that there is a critical need for detectors and sensors that are able to warn about imminent chemical warfare agents (CWA) danger, to enable people to safely leave a contaminated zone or to protect themselves. The dissertation focused on the mass-sensitive piezoelectric sensors, which have been widely used as detections of CWA. Especially, a great deal investigations have been done on the polymers as sensitive materials and sensitivity characteristics towards CWA. Furthermore, the application of kinds of polymers in a sensor array to improve the selective performance of single sensor was also described. The main research results were as follows:1. Quartz crystal microbalance (QCM) sensors coated with poly (vinylidene fluoride) (PVDF) were fabricated for the first time, and the sensitive characteristics were investigated in detail. It was found that the frequency shifts were linear to the concentrations of dimethyl methyl phosphonate (DMMP), a simulant of nerve agents, in the range of 5-60 ppm with a correlation coefficient of above 0.997, and the absolute frequency responses of the sensor all exhibited negative Arrhenius temperature dependencies at different concentrations. The results also showed that the sensitivity to DMMP vapor was almost identical in various humidity circumstances, and the sensitivity to DMMP was absolutely greater than to the interferences. Lanmuir-Blodgett (LB) method was introduced to prepare the PVDF film, and besides successfully preparation of the QCM sensor with PVDF LB film, the sensing properties to DMMP were also studied.2. Different kinds of polymers were choosed due to various analyte compounds, and thus good sensitivities were reached. (1) Poly (methyl-3, 3, 3- trifluoropropyl-siloxane) (PMTFPS) was oxygen plasma treated and then grafted with sulfosalicylic acid (SSA). After that, the sensitivity of this material to DMMP was found to be enhanced from 9.91 Hz/ppm to 11.96 Hz/ppm, and the response time was reduced from 141s to 112s. (2) Poly (vinyl pyrrolidone) (PVP) with hydrogen-bond basicity was used as humidity sensitive coating. PVP complexed with SSA depressed the hydrogen-bond ability between PVP and water, and so that both the reversibility and response time of this sensor were improved.3. A series of polysiloxane materials synthesized in our group were analysed. The frequency shifts behaved linear to the concentrations of analyte in various film thicknesses, whether the poly methyl [3-(2-hydroxyl)phenyl] propyl siloxane (PMPS) film fabricated on QCM or surface acoustic wave (SAW) sensors, and they showed rapid response, good repeatability, and favorable selectivity. The capability of PMPS-SAW was corresponded with BSP3-SAW, although the stability was a little worse. Compared three polysiloxane materials with different amount of fluorin atom in QCM and SAW sensors, which were PMPS, poly methyl [3-(2-hydroxyl-3-fluoro) phenyl] propyl siloxane (PMFPS) and poly methyl [3-(2-hydroxyl-3, 4-difluoro) phenyl] propyl siloxane (PMDFPS), the results both indicated that PMPS, which only had single fluorin atom, had the least sensitivity to DMMP vapor, and the selection of PMPS and PMDFPS needed compromising between response time and magnitude.4. Pattern recognition techniques, both principal component analysis (PCA) and hierarchical cluster, were applied to frequency shift data obtained from 15 piezoelectric sensors, to obtain the least number of sensors composed an array for CWA detection. Combined with hierarchical cluster method, the results of PCA indicated that the four-sensor array providing the greatest degree of discrimination would consist of sensors coated with PVP, PVDF, poly(isobutylene) (PIB) and poly (2-methoxy-5-octyloxy)-1, 4-phenylene vinylene (PMOCOPV). DMMP, N, N- dimethylacetamide (DMA), 1, 5-dichloropentane (DCP) and dichloroethane (DCE), used as the simulants of organophosphorus nerve agents and vesicant agents, were tested by the above sensor array. It can be observed that DMMP, DMA, DCP and DCE can be clearly distinguished, and 95.46% information can be preserved with only two principal components, which indicated the successful distinguishability of the four vapors with this sensor array.5. The gas-sensing mechanism of polymers to DMMP was investigated. The response of each QCM sensor to different DMMP concentration was modeled with BET adsorption isotherms, and results showed that the affinities of different sensors to vapor can be described well with the BET parameters. The ordinal magnitudes of the adsorbent specific surface areaes of the three polysiloxane materials were calculated out as PMPS>PMDFPS>PMFPS, which were consistent with the experimental results. We treated the chemical sensor as a linear time-invariant (LTI) system, and equations were established to simulate the response curves caused by an impulse signal of analyte concentration. The model was validated by comparing its predictions to experimental data. Both the kinetic process and the linear salvation energy relationship (LSER) were regarded to explain that the responses between analytes and sensing materials were integrated all the ingredients of interaction.