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Letters to Analytical Chemistry Development of a Plasma-Assisted Cataluminescence System for Benzene, Toluene, Ethylbenzene, and Xylenes Analysis Mohammad Reza Almasian, Na Na, Fang Wen, Sichun Zhang, and Xinrong Zhang* Department of Chemistry, Key Laboratory for Atomic and Molecular Nanosciences of the Education Ministry, Tsinghua University, Beijing 100084, P. R. China A novel method has been proposed to enhance efficiency of cataluminescence reaction by applying a plasma-assisted cataluminescence (PA-CTL) system. The obtained results clearly indicated that the PA-CTL system exhibited substantially higher sensitivity for the detection of benzene, toluene, ethylbenzene, and xylenes (BTEX) on the surface of nanosized ZrO2. There are two distinctive advantages in the PA-CTL system; on one hand, the plasma activates the BTEX molecules for the detection, and on the other hand, the working temperature range of the catalytic reaction is lowered with the plasma assistance. A detection limit (LOD ) 3σ) of 20 ng mL-1 was achieved for benzene in air samples. Using a graphite electrode in the designed plasma provides an additional opportunity for solid-phase microextraction (SPME) sampling of volatile organic compounds (VOCs) on its surface followed by PA-CTL detection. This ability has been investigated for detection of m-xylene in air samples. In recent years, increasing attention has been paid to the development of gas sensors for environmental applications. The study of cataluminescence (CTL) has lately provided new opportunities to develop optical gas sensors based on nanomaterials. CTL refers to the kind of chemiluminescence that is emitted during the catalytic oxidation of organic vapors on the surface of solid catalysts in an atmosphere containing oxygen. It was first observed by Breysse et al. during the catalytic oxidation of carbon monoxide on a thoria surface1 and further developed by Nakagawa’s group2 and Zhang’s group.3-8 Although CTL gas sensors * To whom correspondence should be addressed. E-mail: xrzhang@ chem.tsinghua.edu.cn. Tel: +86-10-6278-7678. Fax: +86-10-6277-0327. (1) Breysse, M.; Claudel, B.; Faure, L.; Guenin, M.; Williams, R. J. J.; Wolkenstein, T. J. Catal. 1976, 45, 137–144. (2) Nakagawa, M.; Yamashita, N. Springer Ser. Chem. Sens. Biosens. 2005, 3, 93–132. (3) Zhang, Z. Y.; Xu, K.; Xing, Z.; Zhang, X. R. Talanta 2005, 65, 913–917. (4) Liu, G. H.; Zhu, Y. F.; Zhang, X. R.; Xu, B. Q. Anal. Chem. 2002, 74, 6279– 6284. (5) Zhang, Z. Y.; Zhang, C.; Zhang, X. R. Analyst 2002, 127, 792–796. (6) Zhu, Y. F.; Shi, J. J.; Zhang, Z. Y.; Zhang, C.; Zhang, X. R. Anal. Chem. 2002, 74, 120–124. 10.1021/ac1006975 2010 American Chemical Society Published on Web 04/15/2010
have shown many promising features such as reversible response and durability, a high working temperature could result in backgroud emission from high incandescent radiation of the substrate during detection. It is even worse in the detection of airborne benzene, toluene, ethylbenzene, and xylenes (BTEX) due to their high stability against the oxidation reactions.9 Plasma assisted catalysis (PAC) has been studied for a decade which could be a very useful technique for decomposition of volatile organic compounds (VOCs). Decomposition of toluene and benzene via plasma assisted of a variety of catalysts has already been studied. For instance, Magureanu et al. investigated a PAC method for the oxidation of toluene in air.10 Also, benzene decomposition has been studied by Zhu et al. through a plasma driven catalyst system using a nano titania catalyst.11 Moreover, several review papers have been published lately regarding the application of plasma catalysis on VOC removal.12,13 Using a low temperature plasma (LTP) for production of detectable species from undetectable precursor molecules may improve a CTL-based reaction and generate stronger CTL emission so as to enhance the sensitivity of a CTL-based sensor for the BTEX detection. Herein, a plasma-assisted cataluminescence (PA-CTL) system has been proposed for the analysis of BTEX in environmental samples. A laboratory-built LTP device, based on a coaxial cylindrical dielectric barrier discharge, was constructed and coupled to a CTL reaction cell. The gas sensor was made of ZrO2 nanoparticles, which were directly deposited on a homemade heating filament to form a layer with a thickness of 0.1 mm. The sample vapors were delivered by the argon carrier gas and, after passing through the LTP tube and mixing with air (7) Na, N.; Zhang, S. C.; Wang, S. A.; Zhang, X. R. J. Am. Chem. Soc. 2006, 128, 14420–14421. (8) Zhang, Z. Y.; Jiang, H. J.; Xing, Z.; Zhang, X. R. Sens. Actuators, B: Chem. 2004, 102, 155–161. (9) Lu, J.; Cao, X.; Pan, C.; Yang, L.; Lai, G.; Chen, J.; Wu, C. Sensors 2006, 6, 1827–1836. (10) Magureanu, M.; Mandache, N. B.; Eloy, P.; Gaigneaux, E. M.; Parvulescu, V. I. Appl. Catal. B: Environ. 2005, 61, 12–20. (11) Zhu, T.; Li, J.; Jin, Y. Q.; Liang, Y. H.; Ma, G. D. Int. J. Environ. Sci. Tech. 2009, 6 (1), 141–148. (12) Chen, H. L.; Lee, H. M.; Chen, S. H.; Chang, M. B.; Yu, S. J.; Li, S. N. Environ. Sci. Technol. 2009, 43, 2216–2227. (13) Durme, J. V.; Dewulf, J.; Leys, C.; Langenhove, H. V. Appl. Catal. B: Environ. 2008, 78, 324–333.
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Figure 1. Schematic diagram of the plasma assisted cataluminescence (PA-CTL) system.
Figure 3. Comparison of the working temperature range of CTL emitted from m-xylene on the surface of nanosized ZrO2 with and without plasma (with the flow rate of 80 mL min-1 for the air and the discharge gases).
Figure 2. (A) Comparison of the intensity of CTL signals from BTEX vapors with and without plasma. (B) Comparison of the plasma assistance factors for the selected VOCs (on the surface of nanosized ZrO2 at 170 °C and with the flow rate of 80 mL min-1 for the Ar and air gases).
through a Y-shape connector, reached the surface of the sensor. A power supply (6 × 4 × 3 cm, Beili Guoke Co. LTD, Beijing, China) with an alternating voltage of 3.5-4.5 kV at frequencies of 18.0-25.0 kHz was used for plasma production, and the argon, helium, nitrogen, and air were tested as discharge gases. The CTL intensity was measured with a chemiluminescence analyzer. A schematic diagram of the detection system is shown in Figure 1. In our studies, with the same catalyst, CTL was not detectable (assumed to be equal with the lowest detection limit in Figure 2) for BTEX at a temperature below 300 °C. However, the CTL intensities have been increased dramatically with the plasma assistance. A comparison of the intensity of CTL signals acquired for the BTEX vapors at 170 °C with and without plasma is shown in Figure 2A which indicates the enhancement of signals due to the plasma effect. We found that there is a distinguishable difference between PA-CTL responses for BTEX and the other examined VOCs. For instance, the VOCs, such as methanol, ethanol, acetone, 1-butanone, isopropyl alcohol, benzaldehyde, acetic acid, n-butanoic acid, ethyl acetate, butyl acetate, cyclohexane, and pyridine showed no significant increase in their CTL intensities with plasma assistance. We can define a plasma assistance factor for VOCs in the PA-CTL method as follows; Plasma assistance factor for each VOC ) Ion/Ioff, in which Ion refers to PA-CTL intensity with plasma (plasma on), and Ioff refers to PA-CTL intensity without plasma (plasma off). Figure 2B shows that the different plasma assistance factors were obtained for various VOCs. A major reason for this difference could be the different activation energy needed for the oxidation of each compound on the surface of catalyst. It has been studied by several authors that the BTEX have low reactivities (much 3458
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Figure 4. Linear dynamic range, the calibration curve, and reproducibility for detection of m-xylene by the PA-CTL method.
lower than other organic compounds), making it much harder to decompose. It has also been shown that nonthermal plasma technology has considerable potential for the destruction of BTEX.10–14 Furthermore, the plasma-assisted system effectively decreases the working temperature of the catalytic reactions. For BTEX, a working temperature range of 150 to 250 °C has been achieved in a PA-CTL system, using various discharge gases, despite that those were inactive at a temperature lower than 300 °C and a working temperature range above 400 °C has been reported for those catalytic reactions.9,14 However, Figure 3 shows that a higher CTL intensity at lower temperature can be obtained with the use of a plasma assisted cataluminescence with argon discharge gas. The analytical characteristics have been examined, under the optimal conditions. The calibration curves with linear regression coefficients (R) of >0.99 for BTEX were obtained. Figure 4 shows the linear dynamic range, the calibration curve, and reproducibility for the m-xylene sample as a representative compound. The linear ranges of the method were over 2 orders of magnitude. The instrumental limits of detection (LOD ) 3σ) were 20 and 100 ng mL-1 for benzene and the other substances, respectively. Table 1 summarized the analytical characteristics for BTEX. (14) Kim, H. H.; Oh, S. M.; Ogata, A.; Futamura, S. Appl. Catal. B: Environ. 2006, 56, 213–220.
Table 1. Linear Regression Parameters of BTEX Detection by the PA-CTL Method
benzene toluene ethylbenzene o-xylene p-xylene m-xylene
linear range (ppbv)
intercept
slope
linear regression coefficient
200-28000 200-20000 200-28000 200-40000 200-30000 200-25000
1325.4 199.7 15.3 108.8 1.0 674.4
0.592 0.695 0.584 0.481 0.371 0.299
0.995 0.999 0.993 0.995 0.992 0.998
In addition, the central electrode in the LTP device is made of graphite which is able to adsorb analytes on its surface as a solidphase microextraction (SPME) fiber.15,16 The capability of the graphite electrode for adsorption of BTEX on its surface and direct desorption of samples in a plasma device, followed by CTL detection, were investigated in three artificial air samples. The sampling conditions and the resulting PA-CTL for each sample are summarized in Table S1 (Supporting Information). It has been demonstrated that the PA-CTL responses produced from enriched samples on the graphite electrode surface were increased proportionally with the sampling time and the sample concentration. The expectation is that the proposed combination of SPME with PA-CTL potentially is applicable to the analysis of water samples as well as air samples, through a headspace sampling methodology. CONCLUSIONS We developed a new plasma assisted cataluminescence method with significantly higher sensitivity and lower working temperature (15) Almasian, M. R.; Yang, C.; Xing, Z.; Zhang, S.; Zhang, X. Rapid Commun. Mass Spectrom. 2010, 24, 742–748. (16) Djozan, Dj.; Assadi, Y. Chromatographia 2004, 60, 313–317.
lowest detection limit (ppbv) 20 100 100 100 100 100
S/N S/N S/N S/N S/N S/N
) ) ) ) ) )
3 3 3 3 3 3
range, especially for the detection of some VOCs such as BTEX which are difficult to detect by conventional CTL. Further development of the PA-CTL methodology by the use of a different catalyst may lead to a wider application area for the CTL-based methods. ACKNOWLEDGMENT This work is supported by grants from the MOST (Grant Numbers 2008IM040600 and 2009AA03Z321) and NSFC (Grant Number 20875053). SUPPORTING INFORMATION AVAILABLE Further details about the experimental conditions and other additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
Received for review March 17, 2010. Accepted April 12, 2010. AC1006975
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