Sensitive Properties Of The Surface Acoustic Wave Sensors Based On Hydrogen-bond Acidic Polymers

Chemical warfare agents, explosives and other harmful gases are threatening human health and life security, which promotes the development of the real-time chemical sensors. Surface acoustic wave(SAW) sensor is a promising candidate for the detection of these gases with the characteristics of small size, good portablility, high sensitivity and selectivity. A thin film of the sensitive material is needed to coat onto the surface of the sensor to adsorb the target gas. Hydrogen-bond acidic(HBA) polymers are a set of molecules fuctionalized with fluorinated alcohol or fluoriated phenol groups in the polymer main chains, which are mainly polysiloxane or polycarbosiloxane. These polymers have low glass to rubber transition temperature(Tg) and exhibit large segmental motion in the polymer chains, which promote the permeability of the target molecule into the polymer film. The hydroxyl group in the side chain of the HBA polymer is active, and can form hydrogen bond with hydrogen bond basic compounds.This work mainly focuses on the synthesis of three HBA siloxane polymers, whose functional groups were fluorinated alcohol or fluoriated phenol groups, seperately. These polymers were dissolved into chloroform, and subsequently coated onto the surfaces of the SAW devices by airbrush apparatus, separately. The three fabricated SAW sensors were tested to real nerve agents and its simulant dimethyl methylphosphonate(DMMP), 2,4-DNT, simulant of mustard gas 2-cloroethyl ethyl sulfide(2-CEES), styrene, respectively. The sensitivities of the sensors were investigated, and furthermore, the sensitive mechanisms of these sensors to each analyte were studied. To be specific, this thesis includes:1. The molecular structure and synthetic route of LSFA were designed based on the structural characteristics of the HBA polymers. The synthetic routes of the other two polymers, PLF and DKAP, whose molecular structures were reported before, were designed by our group. LSFA and PLF were synthesized via one-step hydrosilylation, which avoided using highly toxic hexafluoroacetone to introduce hexafluoroisopropanol group into the polymer backbone; DKAP was synthesized by Williamson synthesis, Claisen rearrangement and hydrosilylation reaction, successively. FT-IR and HNMR spectra were used to characterize their structures. Then the three polymers were spray-coated onto the surface of 434 MHz SAW devices separately to fabricate LSFA-SAW, PLF-SAW and DKAP-SAW sensors.2. The sensitivities of LSFA-SAW, PLF-SAW and DKAP-SAW sensors to real agents and DMMP were investigated via SAW test platforms. Results revealed that the three sensors exhibited responses of 8.7, 14.2 and 26.6 kHz respectively when tested to DMMP at a concentration of 5 mg/m3. Furthermore, the responses of these sensors were fast and reproducible. The sensitivities of PLF-SAW and DKAP-SAW sensors to real agents were investigated by exposing to various concentrations of sarin and soman vapors. The two sensors could reach responses of 11.72 and 27.93 kHz separately to sarin vapor of 10 mg/m3 when the test time was 3 min, and responses of 1.22 and 4.31 kHz were exhibited when test to soman vapor of 1 mg/m3. The sensitive property comparison of the sensors to real agents and DMMP was studied, and the discrepancy was attributed to the difference in the molecular structure of the analytes. The reason leading to the response of the sensor was that the oxygen atom connected directly to the phosphorus atom of the organophorus compound exhibited high electronegativity, and could interact with the active hydroxyl group of the HBA polymer by hydrogen bond.3. The sensitive properties of the PLF-SAW and DKAP-SAW sensors to 2,4-DNT were investigated by exposing to the analyte with 3 min of pure nitrogen, followed by 3 min of 2,4-DNT vapor. The 2,4-DNT vapor generator was constructed based on a permeation tube. The PLF-SAW sensor exhibited a response of 1.48 kHz to 2,4-DNT of 100 ppb at 3 min, and the response of the DKAP-SAW sensor could reach 4.2 kHz when tested to 2,4-DNT at a concentration of 200 ppb. The response rate of the DKAP-SAW sensor was faster than the PLF one. Since the nitroaromatic compounds consist of one or more nitro-groups attach directly to the aromatic ring. They are highly polarizable molecules and possess multiple basic lone electron pairs in oxygen atoms of the nitro-groups, which lead to strong hydrogen-bond basicity. The sensitive mechanism of the sensor was that the hydroxyl groups of the HBA polymers can form strong hydrogen bond with the nitro-groups of the nitroaromatic compounds.4. The sensitivities of the LFSA-SAW and PLF-SAW sensors to 2-CEES were firstly investigated by exposing them to the analyte at the concentrations ranging from 1 mg/m3 to 20 mg/m3. The two sensors exhibited both good adsorpion and desorption abilities. To 2-CEES vapor at a concentration of 10 mg/m3, the responses of the two sensors were 12.42 kHz and 19.87 kHz, respectively. Compared to another simulant of mustard gas 1,5-dichloropentane(DCP), the responses of these sensors to 2-CEES were 20 times larger. Furthermore, the two sensors were not sensitive to real agent mustard gas, either. The ethyl and the chlorethylidene groups are electron-donating and electron-withdrawing groups, respectively. For mustard gas, the electron-withdrawing effect of the two chlorethylidene groups decreases the density of electron cloud on the sulphur atom, which is believed to lead to the low sensitivity of the sensor. With regard to DCP, there is no sulphur atom in the molecular structure. As to 2-CEES, the reason leading to the high responses is speculated to be that the lone pair electrons on the sulphur atom can form weak hydrogen bonding with the hydroxyl in PLF.5. The sensitivities of the LFSA-SAW and PLF-SAW sensors to styrene were firstly investigated by exposing them to the analyte at the concentrations ranging from 1mg/m3 to 19 mg/m3. Results revealed that the two sensors exhibited fast responses, good repeatability and reproducibility. Responses of 6.49 kHz and 12.8 kHz were recorded when the sensors tested to styrene vapor at a concentration of 10 mg/m3. Tests to various interference gases were also investigated, and both the two sensors showed good selectivity. The sensitive mechanisms of the two sensors to styrene vapor were studied. Firstly, the hydrogen bonding interaction between the HBA polymers and styrene molecules were verified by FT-IR spectra; Then, test comparisons of the two sensors to toluene and styrene were investigated, and the sensors exhibited more than 20 times larger responses to styrene than to toluene; Finally, the sensitivity of the PLF-SAW sensor to 1-butene at the concentration of 19 mg/m3 was investigated, and the result showed the sensor showed a 5 times larger response to 1-butene than to toluene. Based on the above, a sensitive mechanism was proposed that the sensitivity of the sensors to styrene vapor was attributed to the OH-Ï€ hydrogen bonding interaction between the HBA polymers and styrene molecules.