| The detection of trace gases has a wide range of applications in many national economic aspects,such as detection of NOx,SOx CH4,formaldehyde and toluene for air pollution and green house gas control,acetone and ammonica for diagnostics,monitor of hazardous and toxic vapors for various industries and detection of ethylene and secaline for food quality evaluation.Detection of trace gases requires sensors with high sensitivity,while current detection methods always requires expensive and equisite instruments but still can't fulfill the demand for rapid response to instant cases.The surface acoustic wave?SAW?gas sensor is a high-sensitivity sensor that can work at room temperature.It has high selectivity,good sensitivity and fast response.It is a promise candidate to trace-level gas detectecting and monitoring.Sensing film is a key factor to SAW sensor since it decides the selectivity and halfly decide the sensitivity.Hydrogen bond acidic polymers?HBA?are specific sensitive material,which are easy to form hydrogen bond with hydrogen-bond basic compounds,achieving a relative strong adsorption and a reversible desorption process.Therefore,the main research in this paper is the preparation of HBA-polymer-based SAW sensor and its detection of three kinds of hazardous trace-level gases,including chemical warfare agents,nitroaromatic explosives and malordorous organic sulfur vapors.The detailed content mainly includes several aspects as follows:1.Hydrogen bond acidic polymers Linear polysiloxane fluoroalcohol?LSFA?and Linear functionalized polymer?PLF?were synthesized by utilizing polymethyl hydrosiloxane?PHMS?as main chain reacted with 2-vinylhexafluoroisopropanol and2-allyl hexafluoroisopropanol through one-step hydrosilylation reaction.Similarly,the3,5-bistrifluoromethylphenol functional group was obtained by Williamson synthesis andClaisenrearrangementandthenaddedtoPHMStoproduce poly{methyl[3-?2-hydroxyl,4,6-bistrifluoromethyl?phenyl]propylsiloxane}?DKAP?.After synthesis,the molecular structures of the synthesized polymers were then characterized by Fourier transform infrared spectroscopy?FTIR?and nuclear magnetic resonance spectroscopy?NMR?to ensure the desired products.Subsequently,through a high-pressure spraying technique,these HBA polymers were evenly sprayed into the SAW substrates.Then integrated with a customized signal generation and reading circuit designed by our group,the HBA polymer-based SAW sensors were prepared.2.The real-time detection of four maloldorous organosulfur compounds,including methyl mercaptan?MM?,dimethyl sulfide?DMS?,dimethyl disulfide?DMDS?,and carbon disulfide?CS2?,by HBA polymer-based SAW were investigated for the first time.The test results show that when the VOSCs concentration is 10 mg/m3,the response of LSFA to MM,DMS,DMDS,CS2 are 13.58,7.19,3.05 and 5.16 kHz respectively,and the extrapolated limit of detection?LOD?are 0.044,0.103,0.039 and 0.034mg/m3,respectively;the response of PLF to them are:18.5,17.3,8.6 and 9.49kHz,and the LOD are 0.026,0.032,0.081 and 0.059mg/m3,respectively;the response of DKAP are:12.9,12.8,6.4 and 8.2kHz,and the LOD are 0.034,0.036,0.074 and 0.058mg/m3,respectively.Additionally,the responses of organosulfur are much higher than interference gas.Furthermore,the formation of hydrogen bonding between the hydrogen atom in the hydroxyl group of HBA polymers and the sulfur atom in VOSCs was verified by Fourier-transform infrared?FTIR?spectroscopy.3.To further verify the formation of hydrogen bonding,the adsorption energies between three HBA polymers and four VOSCs were studied respectively by the first-principle theoretical calculation.Firstly,the configurations of polymers and small gas molecules were calculated and the bonding angle,bonding length as well as simulated infrared spectrum were analyzed in detail.Secondly,the adsorption energies between polymer and small molecule were calculated as negative and is euqual to median hydrogen bond strength,which indicated the spontaneous reaction processes and matches the experimental results perfectly.Further analysis of molecular electrostatic potential demonstrated that the most negative potential was concentratedly distributed over sulfur atoms of the organosulfur vapors,while the most positive sites were near the hydrogen atoms in hydroxyl groups of the HBA polymer.This potential distribution undoubtedly favored the hydrogen bonding formation.At last,combined with frontier molecular orbital theory,the highest occupied molecular orbitals?HOMOs?for these four molecules vapor donors were well below the lowest unoccupied molecular orbital?LUMO?of the sensing polymer while they were all above the HOMO of the polymer.Therefore,the hydrogen bonds were favorably formed between the organosulfur vapors and the sensing polymer via HOMO-LUMO interaction.In summary,the interaction of the polymer with the organic sulfur gas due to the formation of a medium-strength hydrogen bond was further evidenced.4.Two strong hydrogen-bond acidic?HBA?polymers DKAP and PLF,were used as sensitive films for rapid detection of an explosive compound,2,4-dinitrotoluene(2,4-DNT in the concentration range of 200 ppb to 1 ppm?1.626 to 8.13mg/m3?.The DKAP sensor exhibits a high response of 4.6 kHz to 200 ppb 2,4-DNT,with an extrapolated LOD of 0.1 ppb,which is comparable to most sensitive polymer-coated sensors previously reported.Furthermore,Sensor responses to possible common interferences were found to be 11–35 folds lower than to 2,4-DNT and both adsorption and recovery time are much shorter than other related sensors.The results also showed that the DKAP sensor has a 4.6 times higher sensitivity to 2,4-DNT than PLF at the concentration of 1 ppm,while a further comparison indicated that their sensitivities to dimethyl methylphosphonate?DMMP?are essentially identical.The reason is that DMMP only has strong hydrogen bond basicity,while nitrobenzene explosives not only have hydrogen bond basicity,but also have strong dipole polarity;therefore,the performance of DKAP with both hydrogen bond acidity and even polarity is better.5.A linear hydrogen-bond acidic?HBA?polymer PLF,was chosen as sensitive film for detection of nerve agent sarin?GB?and related simulant DMMP,blister agents mustard gas?HD?and related simulants chloroethyl ethyl sulphide?2-CEES?,and1,5-dichloropentane?DCP?.The results show that the sensor is highly sensitive to GB and DMMP,and has low sensitivity to HD and DCP.However,the sensor possesses an unexpected high sensitivity toward 2-CEES.We believe the lone pair electrons around the sulphur atom of 2-CEES provide an electron-rich site,which facilitates the formation of hydrogen bonding between PLF and 2-CEES.On the contrary,the electron cloud on the sulphur atom of the HD molecule is offset or depleted by its two neighbouring strong electron-withdrawing groups,hence,hydrogen bonding can hardly be formed.Thus the detection results between real agents and simulants may be totally different. |
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