Selective detection of trace p-xylene by polymer-coated QCM sensors

Abstract

Two types of polymer-coated QCM sensors were developed, which exhibited high sensitivity, stability and selectivity for the detection of trace p-xylene vapor in air over toluene, butyl acetate and 1-butanol, which are usually produced in the coating process of films on the back of solar cell array. The polymers were poly(4-vinylbenzyl chloride) (PVBC) and poly(4-vinylbenzyl chloride-co-methyl methacrylate) (P(VBC-co-MMA)), respectively. Both PVBC and P(VBC-co-MMA)-coated QCM sensors were reusable and can be reactivated by releasing the adsorbed vapor in vacuum. The QCM data also indicated that the incorporation of MMA can enhance the sensitivity of the polymer-coated QCM sensor toward p-xylene vapor. The lowest detection limit can reach 54 ppm for the QCM sensor coated with 119 nm thick P(VBC-co-MMA) thin film, leading to the frequency shift of 9.6 Hz (−ΔF₃). These two types of polymer-coated QCM sensors may find potential application in the monitoring and controlling the concentration of p-xylene vapor in the coating process of thin films on the back of solar cell array, ensuring the health of the related personnel in the factory of solar cell array.

Introduction

Nowadays, natural environmental pollution, which is resulted from the rapid industrial growth, becomes more and more serious. One kind of pollutants is volatile organic compound (VOC), which will lead to the air pollution and cause harm to human health [1]. Several governments in the world have laid down some rules in order to reduce the emission of VOCs. So how to effectively detect VOCs in air with a quick and simple way becomes one of the most important tasks all over the world [2], [3], [4], [5], [6], [7]. In the past decade, solar energy is regarded as one of the resources of green energies and a growing number of the applications of solar energy appear via the use of solar cell. However, the production of solar cell results in environmental pollution because VOC pollutants are produced in the coating process of films on the back of solar cell array.

Currently, most monitoring methods in the detection of pollutants, such as Fourier transform infrared spectroscopy (FTIR), fluorescence spectroscopy and gas chromatography–mass spectrometry (GC–MS), need off-site analysis [8]. Although these methods have high sensitivity and selectivity, they are costly and time-consuming because of sample collection. Real-time sensors, which are placed in situ, as a reformation, are paid more and more attentions. These alternatives can save the time spent in collecting sample, and reduce the cost of analysis.

In order to detect VOCs selectively and sensitively, some polymer films that have a strong affinity for certain VOCs [9], [10], [11], [12], have been developed by various groups. Additionally, the applications of the polymer films are expanded by combining them to several appropriate sensors (like the quartz crystal microbalance or the surface acoustic wave device). One of the widely used devices is quartz crystal microbalance (QCM) [13], which is a resonant sensor. The sensing film was coated onto the electrode surface of quartz crystal and can absorb the target molecules, resulting in a shift of the resonance frequency of the coated quartz crystal. It can detect the trace mass change in the nanogram range onto the electrode surface of quartz crystal in air or liquid. So the sensitivity in monitoring of VOCs has been greatly enhanced by QCM. In the recent decade, the polymeric materials used to modify the surface of quartz crystal have attracted more and more attention. Many polymers are widely chosen for detection of target molecules by QCM, such as heavy metal ions [14], [15], [16], protein [17], [18], organic gas [2], [19], [20], [21] and organic compounds [22], [23]. However, QCM sensors in the application of detecting VOCs have met many challenges (e.g. low selectivity, low response, reproducibility), which promoted the researches on the material sensors in environmental monitoring.

During the coating process of thin films on the back of solar cell array, four typical kinds of VOCs, i.e. toluene, p-xylene, butyl acetate and 1-butanol, are usually produced. Therefore, in order to protect the health of personnel, it is necessary and important to develop the sensing systems, which can be used to monitor or detect the concentration of these kinds of VOCs simultaneously or respectively. In our previous report [24], a novel copolymer, poly(2-hydroxyethyl methacrylate-co-methyl acrylate) (P(HEMA-co-MA)), with molar ratio of HEMA:MA = 1:1 (mol:mol) was successfully designed and synthesized, which can be used to selectively detect the trace amount of 1-butanol with the lowest detection limit of 72 ppm by combining with the QCM technique. We continue our efforts in seeking suitable or specific polymers for the selective detection of trace toluene, p-xylene, butyl acetate, or 1-butanol vapors, which are produced in the coating process of solar cell array.

In the present work, we report on the selective detection of trace p-xylene with polymer-coated QCM sensors. One of the most general volatile organic compounds is xylene, which is irritating and anesthetic. There were several reports concerning the detection of xylene by using QCM sensors. Wang et al. [11] have compared QCM with electrochemical sensor in measuring the response to xylene isomers. They chose calix[4]arenes as receptors, because of the shape-persistent hydrophobic binding pockets of these materials in their cone conformation. They found that the QCM measurements are less selective than the chemiresistor results for the different xylene isomers, because a thicker film was needed to produce a sufficient signal-to-noise ratio. Finklea et al. [20] reported a QCM sensor for organic vapor detection in dry air based on molecularly imprinted polymers. They chose hydroquinone and phenol as templates, acrylic acid and methacrylic acid as monomers, ethylene glycol dimethacrylate as cross-linker, and synthesized several molecularly imprinted polymers. The templates were removed and shape-selective cavities were generated in polymer matrix. The imprinted polymers exhibited high sensitivity and selectivity toward planar analytes, especially toward toluene. However, the cross-linked polymers were insoluble and needed to be bound on the electrode surface of quartz crystal by adhesive. Syu et al. [21] have developed a poly(β-cyclodextrin-co-maleic anhydride)-coated QCM sensor. They synthesized a crosslink polymer by using β-cyclodextrin as monomer and maleic anhydride as cross-linker. The polymer can be dissolved in water, and polymer films were prepared by spin-coating. They have studied the detection of benzene, toluene, and p-xylene in low concentrations. Linear calibrations were achieved for the three gases with linear ranges of 400, 300, 150 ppm for benzene, toluene and p-xylene, respectively. Additional, p-xylene exhibited the highest sensitivity among them, which was 1.236 ± 1.994 Hz/ppm in average.

Here, we reported another two simple polymers, poly(4-vinylbenzyl chloride) (PVBC) and poly(4-vinylbenzyl chloride-co-methyl methacrylate) (P(VBC-co-MMA)), which exhibited highly sensitive and selective detection of p-xylene by combining with QCM technique. 4-Vinylbenzyl chloride was chosen as main monomer because it contains aromatic skeleton and may form π–π–electron–donor–acceptor complexes with aromatic compounds. Methyl methacrylate was used as co-monomer for adjusting the affinity of the resultant copolymer to the p-xylene. The chemical structures of the two monomers are shown in Scheme 1. These two polymers, PVBC and P(VBC-co-MMA), can be simply obtained via typical free radical polymerization of the corresponding monomers. The polymer-coated QCM sensors were fabricated by spin-coating of the polymer solution onto the gold electrode surface of quartz crystal. The selectivity and sensitivity of these polymer-coated QCM sensors were systematically studied in response to the trace vapors of toluene, p-xylene, butyl acetate and 1-butanol with various concentrations.