DAS et al. used benzoyl peroxide as an initiator to undergo free radical polymerization reaction on the QCM electrode to form a polymer film, which was used to detect volatile organic compound VOCs gas. They systematically studied the response of the sensor to different concentrations of VOCs (toluene, p-creluenol, o-xylene, benzene), and Figure 1 shows the sensor's response to VOCs at a concentration of 5 25 ppm, with the sensor having the highest sensitivity for p-cresol gas (041Hz ppm), the lower detection limit reaches 5ppm; The return of the response frequency to the base value after nitrogen introduction also indicates good repeatability of the material. The disadvantage is that the sensor has a similar response curve to each VOC in the experiment, which also indicates that there are still shortcomings in the selection of adsorption with this material. Si et al. coated thiophene and thiophene derivatives conductive polymers onto QCM electrodes to prepare a new type of QCM gas sensor, and constructed a sensor array of eight QCM sensors coated with different conductive polymer thin layers, because the sensitive materials have different sensitivities and selectivity to VOCs components with different polarities. The results show that the sensor array has the qualitative ability to separate and detect polar VOCs gases in non-polar (weakly polar) and polar VOCs mixtures. At the same time, the experiment of detecting the concentration of a single component in the mixed gas was carried out, and the concentration of a single component was successfully detected in the mixed gas.
Fig. 1 Sensor response curves of polymerized flaxseed oil to VOCs at concentrations of 5 25 ppm: (a) p-cresol, (b) o-xylene, (c) benzene, (d) toluene.
Fan et al. prepared a QCM sensor for the detection of n-butanol by spin coating the copolymer poly(methyl 2-hydroxyethyl acrylate methyl acrylate) [P(Hema-Co MA)] as a sensitive material. They investigated the sensitivity, stability, and repeatability of the sensor for different VOCs gases (toluene, paraxylene, butyl acetate, and n-butanol). According to the experimental results, the QCM sensor has high sensitivity to butyl acetate and n-butanol, but has better sensitivity and better stability to n-butanol, and the sensor can complete the desorption of n-butanol in vacuum for 24 hours, indicating that it is repeatable. They also used a QCM sensor built from a copolymer to detect paraxylene. The sensitive film P (VBC Co MMA) is composed of poly-chloromethyl styrene (PVBC) and polymethyl methacrylate (PMMA) copolymerized, and a single polymer PVBC is used as the sensitive material to construct a QCM sensor for comparison. From the experimental results, it can be seen that P (VBC CO MMA) QCM and PVBC QCM have higher sensitivity for both paraxylene and toluene, and have higher sensitivity for paraxylene, and PVBC with PMMA enhances the adsorption of paraxylene. The QCM sensor with a thickness of 119 nm P (VBC Co MMA) was used to achieve a lower detection limit of 54 ppm for paraxylene and a frequency shift of 96 Hz (as shown in Figure 2).
Fig. 2 Frequency variation of P(VBC Co MMA)-modified QCM sensors for different concentrations of toluene and paraxylene. P (vbc co MMA) thin layer thickness is 119 nm
VOCs gas sensors are mainly used to monitor places that are prone to VOCs gases, but the ambient air humidity will reduce the sensitivity and selectivity of the sensor to the target gas, so there is an urgent need for sensors that can be used under a certain air humidity. Andreeva et al. successfully modified the superhydrophobic thin layer onto the QCM electrode by microwave plasma-enhanced chemical vapor deposition (MPECVD) to prepare a VOC gas sensor, which has a large specific surface area, which can adsorb more target gas molecules and increase the sensitivity of the sensor. From the experimental results, it can be found that the superhydrophobic film can effectively inhibit the adsorption of water molecules, and more selects the adsorption of formaldehyde and toluene molecules. When N2 was introduced for desorption experiments, it was found that the response frequency quickly returned to the initial frequency value, indicating good reproducibility. However, this study work selects fewer analytes of interest, and more VOCs need to be tested to account for their selectivity. In addition, superhydrophobic membrane micropores can be modified to increase their specific surface area, making it possible to use them as excellent sensitive materials for the detection of VOCs gases.
Bachar et al. demonstrated a QCM sensor array constructed with polycyclic aromatic hydrocarbon (PAH) derivatives as sensitive materials, and the sensor-sensitive film is composed of a double-layer membrane, single-walled carbon nanotubes as a cushion, and a thin layer of PAH derivatives with different side chain structures as the outer structure. Polar and non-polar VOCs were selected from alcohols, alkanes, ethers and aromatic hydrocarbons as detection objects, and experiments were carried out in different humidity ranges (5% 80% RH), and the experimental results showed that the non-polar side-chain PAH derivatives had lower detection limits than the polar side-chain PAH derivatives. When combined into a sensor array from a single sensor, non-polar side-chain PAH derivatives have excellent classification capabilities for VOC gas polarity, chemical classification, and mixed gas separation. In terms of ambient humidity, the sensitive thin layer with hydrophobic ends does not have a significant effect on the frequency change in a wide range of humidity. Therefore, the QCM sensor array modified by PAH derivatives is expected to be used as a universal sensing platform for detecting VOCs gases. In order to be able to apply it more broadly in practice, it is necessary to have a deep understanding of the sensing mechanism, and it is necessary to conduct research on related aspects, such as comprehensive research on the length of the side chain of the PAH molecule and the functionalization of the end groups.
February** Dynamic Incentive Program