QCM sensors modified with sensitive materials of organic polymers have been extensively studied in the detection of relative humidity, volatile organic compounds (VOCs), and difficult volatile compounds. Organic polymers have high sensitivity and good reversibility, but the adsorption of polymer molecules to target gas molecules mainly depends on physical adsorption, such as hydrogen bonding, van der Waals force, etc., and has a wide selectivity.
Research in humidity detection
Yoo et al. prepared QCM humidity sensors using polymethyl methacrylate (PMMA) as a sensitive film. In order to prepare large specific surface area adsorbent materials, they first added polylactic acid (PDLL) to PMMA, and then the NaOH solution dissolved the PDLL in PMMA to prepare a sensitive film with a porous structure and large specific surface area, and the water vapor entered the inside of the thin layer of PMMA through the pores, thereby increasing the water vapor selection adsorption site. The experimental results are shown in Figure 1, when PMMA and PDLL are mixed 1:1, the sensitivity of the PMMA PDLL-sensitive membrane treated with NaOH solution to water vapor increases by 37 times, so a better water vapor detection effect can be obtained by increasing the specific surface area of PMMA. However, the problem is that polymers, as moisture-sensitive films, are less stable at higher humidity and have average repeatability.
Fig.1 Relationship between water vapor adsorption capacity and PMMA content in copolymers.
Research in the detection of chemical warfare agents
At present, the sensitive materials used to detect the chemical warfare agent sarin, mainly hydrogen-bonded acidic polymers, adsorb sarin molecules through the force of hydrogen bonding. Zuo and other synthetic fluorosiloxane polymers were used as QCM electrode coating materials for the detection of sarin reagent, and the frequency shift of the sensor showed good linearity with the sarin gas concentration in the experiment, and the lower detection limit could reach 015ppm；In a repeat experiment, the concentration of 0 was detected 7 times with the same sensor42ppm sarin reagent, the response value relative standard deviation is less than 3%, showing good reproducibility; At the same time, the selectivity of the sensor was studied, and the response value of the sensor to mustard gas was found to be weak, indicating that the sensitive material had good selectivity for sarin molecules. The accuracy of the sensor is still relatively high after 100 repeated detections, indicating that the fluorosiloxane-modified QCM sensor has high stability. Combined with the above advantages, the sensor has the potential to be used for the actual detection of sarin molecules.
Fig.2 Real-time response curves of polymer-modified QCM sensors to three chemical warfare agent simulants: (A) 2,2'-dichlorodiethyl ether, (B) dimethylphosphonate, and diethyl chlorophosate.
Zhang et al. synthesized the copolymer polymethacrylic acid, dimethylaminoethyl ester polyvinyl alcohol (PDMAEMA PVA) by emulsion synthesis method, and modified it as a sensitive film onto the QCM electrode to assemble a gas sensor (PC-QCM) to evaluate its sensing and detection effect on chemical warfare agent simulants, and the experiment focused on the effects of PDMAEMA PVA copolymer with different mass ratios on 2,2-dichlorodiethyl ether (DCE) and dimethyl methylphosphonate (DMMP) and the degradation and adsorption of diethyl chlorophosphate (DCP). The results show that the higher the content of PDmaema in the polymer, the better the degradation effect of the corresponding DCP in the QCM sensor coated with a thin layer of PDMAEMA PVA. However, considering that the protective material not only needs good adsorption and degradation ability, but also needs good moisture permeability, so based on the above conditions, PDMAEMA has good adsorption and degradation ability and moisture permeability at 80WT%, which is the most suitable as a protective material. As can be seen from Figure 2(a), the 80wt% content of PDMAEMA has good sensitivity and good reproducibility for DCE sensing detection. Figure 2(b) shows that the adsorption performance of the sensitive film is better than that of DCE (Figure 2(a)) and DMMP at the same concentration. The thin copolymer layer not only directly degrades sarin reagent mimics (DCP and DMMP), but also adsorbs mustard gas mimics (DCE). Therefore, the thin layer of copolymer (PDMAEMA PVA) has the potential to play an important role in military and public safety as a better chemical warfare protection material.
DU et al. used polysiloxane (PMTFMPS) as a sensitive material to construct a QCM sensor for the detection of DMMP, and its sensitivity reached 27Hz ppm in the range of 10 50 ppm DMMP vapor concentration, which had efficient adsorption capacity, but the response time was long, and the air humidity would also adversely affect the detection of DMMP, which was not conducive to real-time accurate monitoring.
Although hydrogen-bonded acid polymers have high sensitivity to these agents, QCM sensors modified with such sensitive materials often have long response and recovery times, and for sensors that play an early warning role in actual use, too long response time often loses alarm significance, which is also the disadvantage of QCM sensors modified with sensitive films.
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