Development of sensitive carbon disulfide sensor by using its cataluminescence on nanosized-CeO2

Sensors and Actuators B 136 (2009) 218–223

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Development of sensitive carbon disulfide sensor by using its cataluminescence on nanosized-CeO2 Yuelan Xuan a , Jing Hu a , Kailai Xu a , Xiandeng Hou a,b , Yi Lv a,∗ a b

Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China

a r t i c l e

i n f o

Article history: Received 22 July 2008 Received in revised form 6 October 2008 Accepted 21 October 2008 Available online 30 October 2008 Keywords: CeO2 Cataluminescence Carbon disulfide Gas sensor

a b s t r a c t When carbon disulfide (CS2 ) was passing through the surface of nanosized-CeO2 (nano-CeO2 ), strong chemiluminescence (CL) emission was observed. Based on this phenomenon, a novel sensitive gas sensor was demonstrated for the determination of CS2 . The luminescence characteristics and optimal parameters, such as the morphology of the CeO2 , wavelength, temperature and the flow rate of carrier gas, were investigated in detail. Under the optimal conditions, the CL intensity versus the concentration of CS2 was linear in the range of 0.9–12.6 ␮g mL−1 , with a linear correlation coefficient (R) of 0.999 and a limit of detection (S/N = 3) of 3.7 ng mL−1 for CS2 . The relative standard deviation (R.S.D.) for the determination of 3.6 ␮g mL−1 CS2 was 1.2% (n = 5). There is no or weak response to foreign substances, such as alcohol (methanol, ethanol, and n-butanol), aldehyde (formaldehyde and acetaldehyde), acetone, ethyl acetate, chloroform, carbon tetrachloride, toluene, chlorobenzene, hexane and hydrogen sulfide. The stability and durability of the sensor were examined by sampling 7.2 ␮g mL−1 CS2 vapor into the sensor everyday for 3 h over two weeks, with a long-term R.S.D. of less than 6.0%. Finally, the proposed sensor was applied to the determination of CS2 in artificial air samples. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Interest in determining volatile organic compounds (VOCs) in air has increased in the last several decades. Carbon disulfide, one of VOCs, is not only extensively used in industry and the laboratory as a reactive reagent or solvent, but also frequently used for dry cleaning and as an insecticide for the conservation of grains. Longtime exposure to even low concentrations of CS2 can also cause various health risks such as deficiency of vitamin B6 , depletion of the level of essential trace metals and so on [1]; and it has been considered to be one of the most toxic substances causing accelerated atherosclerosis, coronary artery disease and nervous system diseases [2]. Therefore, the American Conference of Government Industrial Hygienists (ACGIH) has strictly controlled the level of CS2 in air with its threshold limit value (TLV) of 31 mg/m3 . On the other hand, with the characteristics of flammability and explosive nature (explosion in air in the concentration range of 1.25–50%), CS2 poses a great risk to human’s safety. It is, therefore, of current concern to develop a simple, rapid and reliable method for the sensitive determination of CS2 in many cases. Great efforts have been made to establish various methodologies to determine low con-

∗ Corresponding author. Tel.: +86 28 85412798; fax: +86 28 85412798. E-mail address: [email protected] (Y. Lv). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.10.014

centrations of CS2 [1,3–11], such as flame photometric detection (FPD), pulse flame photometric detection (PFPD), photo ionization detection (PID), electron capture detection (ECD), spectrophotometric methods, mass spectrometry (MS) and atomic emission detection (AED). Most of these techniques are sensitive for the determination of trace CS2 , but generally need to be connected to gas chromatography (GC). To furthermore improve the sensitivity, a preconcentration procedure is usually necessary with more organic reagents involved, which is time-consuming and may result in secondary environmental pollution. Consequently, they are possibly complicated and not suitable for in-situ analysis. Therefore, there is still a demand for a simple, rapid, sensitive and potentially portable method for the determination of trace CS2 . Cataluminescence (CTL), chemiluminescence emission generated on the surface of catalytic solid materials, is an interesting phenomenon which was first observed by Bresse et al. [12] in 1976 in catalytic oxidation of carbon monoxide on the surface of thoria. Owing to its advantages of fast response, high sensitivity and simple instrumentation, CTL has attracted much attention of analytical chemists [13]. Nakagawa and co-workers [14–17] have observed CL phenomena of organic vapor, such as alcohol, acetone and hydrocarbon, by passing it through the surface of bulk materials of ␥-Al2 O3 or ␥-Al2 O3 doped with Dy3+ . They also found that different gases could be discriminated by measuring the temperature dependence. In recent years, attributing to their specific features, such as

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high surface areas, good adsorption characteristics and high activity [18,19], nanomaterials have attracted considerable interests. Zhang’s research group has taken great efforts to explore a series of gas sensors or sensor arrays based on CTL on the surface of various nanomaterials, for measuring ethyl acetate [20], formaldehyde [21], acetaldehyde [22], hydrogen sulfide [23], ethanol [24–26], acetone [26] and other compounds [27,28]. Recently, Lu and co-workers have successfully established a CTL sensor by using zeolite as a catalyst for n-hexane [29] and Al2 O3 for acetone [30]. In addition, by using ZnO or La2 O3 nanoparticles, our research group [31,32] has explored new CTL sensors for ethanol and acetone, respectively. Ceria, a metal oxide semiconductor, has been proposed as a sensing material for the detection of oxygen [33,34], ethanol [35], or acetone and NO [36]. In the present work, strong CTL emission was observed when CS2 vapors passed through the surface of nanoCeO2 . Based on this phenomenon, a new gas sensor for CS2 was proposed. Optimal parameters and the luminescence characteristics of CS2 on the surface of nano-CeO2 were discussed in detail. Finally, the proposed sensor was successfully used for the determination of CS2 in artificial samples. The proposed sensor is a simple, rapid, sensitive and selective one. 2. Experimental 2.1. Reagents and apparatus Standard carbon disulfide (purity ≥99.5%) was purchased from Kelong Reagent Factory (Chengdu, China). Hydrogen sulfide was purchased from National Institute of Measurement and Testing Technology (Chengdu, China). The reagents used for the selectivity study of the sensor were analytical grade from Kelong Reagent Factory (Chengdu, China). Analytical grade Ce(NO3 )3 ·6H2 O, NaOH, urea and H2 O2 (30 wt%) from Kelong Reagent Factory (Chengdu, China) were used to prepare the nano-CeO2 . A simplified schematic diagram of the CS2 sensor is shown in Fig. 1. It consists of an air pump to supply continuous stream of air flow, a sample valve in which the sample was injected, vaporized and then driven by the air flow to pass through the CL reaction chamber, where the CTL emits, a temperature controller to adjust the surface temperature of nano-CeO2 , and a commercial BPCL ultra weak chemiluminescence analyzer (Biophysics Institute of Chinese Academy of Science, China) for the detection of the CTL. In the CL reaction chamber, a cylindrical ceramic tube (4 mm i.d.), on which a layer of 0.5 mm of the CeO2 nanomaterial was sintered, was inserted in to a quartz tube (8 mm i.d.). 2.2. Synthesis and characters of catalysts A hydrothermal method was used to synthesize the sensing nanomaterial ceria as follows: (1) 0.868 g of Ce(NO3 )3 ·6H2 O and

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Fig. 2. X-ray diffraction pattern of CeO2 .

2.4 g of NaOH were dissolved in 5 mL and 35 mL deionized water, respectively; (2) a mixture of these two solutions was kept stirring for 30 min as soon as they were transferred into a Teflon bottle; (3) the Teflon bottle with this mixture was held in a stainless steel vessel autoclave, which was subsequently sealed tightly and then was subjected to hydrothermal treatment at 100 ◦ C for 12 h; and (4) the resultant precipitate was washed with deionized water and ethanol several times, followed by drying at 60 ◦ C in air for 10 h. In order to compare the capability of ceria with different morphology, the established procedures in the literature [37,38] were employed for the synthesis of ceria of different morphology. X-ray diffraction (XRD) was carried out with a Philips X’pert Pro MPD diffractometer (Philips Analytical, Netherlands) equipped with a plumbaginous-monochromated Co K␣ radiation source working at 40 kV and 40 mA. The samples were scanned from 20◦ to 110◦ (2) with a step size of 0.03◦ . From Fig. 2, it can be seen that these peaks are assigned to the pure cubic crystal according to the XRD standard spectrum of CeO2 crystal. The morphologies of the synthesized CeO2 were characterized with a transmission electron microscope (JEM-100CX) at an accelerating voltage of 80 kV. These CeO2 using different preparation procedures are shown in an overview TEM image in Fig. 3, with Fig. 3a–c corresponding to the products resulted from the present method and the established method in the literatures [37,38], respectively. It can be seen that the morphology characteristic of the ceria using the present method was the nanorod. As described in the literatures [37,38], Fig. 3b and c were ceria nanocubes and ceria microspheres, respectively. 3. Results and discussion 3.1. CL spectra of CS2 on nano-CeO2 with different morphology

Fig. 1. The schematic diagram of the CS2 sensor.

The CTL emission from the catalytic oxidation of CS2 on the surfaces of nano-CeO2 of three types of morphology was investigated. Considering substantially high CL intensity was observed when CS2 vapor of 7.2 ␮g mL−1 passed through the surface of nano-CeO2 , the optimization of the detection wavelength was firstly carried out at 275 ◦ C and a carrier flow rate at 300 mL min−1 . From Fig. 4, it can be seen that the profile of each curve in the figure is similar and each curve has the same peak at 460 nm, and this approximately demonstrates that the same luminescence specie is produced on the three types of materials of nano-CeO2 . However, Fig. 4 also

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Fig. 4. The CL spectra on (a) ceria nanorods; (b) ceria nanocubes; and (c) ceria microspheres. Temperature, 275 ◦ C; carrier flow rate, 300 mL min−1 . Error bars stand for ±S.D. (standard deviation).

CL intensity of 3.6 ␮g mL−1 CS2 on the nano-CeO2 was studied, with the result shown in Fig. 5. It can be seen that both the signal and the S/N (signal-to-noise ratio) increase gradually from 200 ◦ C to 282 ◦ C, most probably due to the higher catalytic activity of nano-CeO2 at higher temperature. However, the signal and the S/N sharply decrease above 282 ◦ C, so 282 ◦ C was chosen for use in this work. 3.3. Optimization of air flow rate The flow rate of carrier gas has an important influence on the diffusion rate and the residence time of the sample on the nano-CeO2 , and this may affect the reaction rate of the catalytic oxidation reaction. The air flow rate dependence of the CTL intensity was, therefore, carefully studied in the range of 200–1000 mL min−1 . Fig. 6 demonstrated that the CTL intensity of 3.6 ␮g mL−1 CS2 increased gradually and reached the maximum at 600 mL min−1 , then slowed down with the increase of the flow rate over 600 mL min−1 . The flow rate of 600 mL min−1 , therefore, was selected for further use.

Fig. 3. TEM photo of nano-CeO2 with different morphology. (a) Ceria nanorods; (b) ceria nanocubes; and (c) ceria microspheres.

shows that the CTL intensity from the surface of ceria nanocubes is clearly higher than those from the other two nano-CeO2 materials. Thus, the detection wavelength of 460 nm and the ceria nanocubes were selected for the subsequent investigation in this work. 3.2. Optimization of working temperature Temperature plays a key role in the catalytic oxidation reaction. It is well known that the activity of a catalyst generally increases with the temperature. Therefore, the effect of temperature on the

Fig. 5. Effect of temperature on the CTL intensity and the S/N. Air flow rate, 300 mL min−1 ; wavelength, 460 nm; and surface material, ceria nanocubes. Error bars stand for ±S.D.

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Fig. 6. Effect of air flow rate on the CTL intensity. Heating voltage, 135 V (corresponding to 282 ◦ C at the air flow rate of 500 mL min−1 ); and wavelength, 460 nm. Error bars stand for ±S.D.

3.4. Temporal CL profiles of carbon disulfide on ceria nanocube The CTL response profile was investigated by injecting CS2 vapor with different concentrations. The corresponding CTL response profiles of CS2 vapor were shown in Fig. 7. Curve 1, 2 and 3 denote the results for different concentration of 1.8, 3.6 and 7.2 ␮g mL−1 , respectively. It can be seen that the CTL is proportional with the CS2 concentration. Due to the high toxicity of CS2 , it is of great importance for a CS2 sensor to respond rapidly. The results showed that the signal sharply increased and reached the maximum immediately after CS2 vapor was introduced. The fast response and recovery time were less than 3 s and 25 s, respectively, because of the fast catalytic oxidation reaction and the small dead volume. 3.5. Selectivity of the CS2 sensor A series of foreign substances, such as alcohol (methanol, ethanol and n-butanol), aldehyde (formaldehyde and acetaldehyde), acetone, ethyl acetate, chloroform, carbon tetrachloride, toluene, chlorobenzene, hexane and hydrogen sulfide, were used to investigate whether these compounds interfere with the determination of CS2 under the optimal experimental conditions. Their

Fig. 7. Typical temporal CTL emission profiles. Carbon disulfide vapor concentration is (1) 1.8 ␮g mL−1 ; (2) 3.6 ␮g mL−1 ; and (3) 7.2 ␮g mL−1 , respectively. Air flow rate, 600 mL min−1 ; temperature, 282 ◦ C; and wavelength, 460 nm.

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Fig. 8. Selectivity study for the CS2 sensor. Temperature, 282 ◦ C; and air flow rate, 600 mL min−1 .

concentrations were about 250 ␮g mL−1 with the exception of hydrogen sulfide of 500 ppm. As shown in Fig. 8, there were no detectable CTL signals expect for acetone and ethyl acetate, which had relatively weak CTL signals in comparison with that of 7.2 ␮g mL−1 CS2 . Therefore, the CTL sensor possesses high selectivity for CS2 . According to a previous report [23], the high selectivity of the present sensor may principally result from the fact that few of these gases can be catalytically oxidized and produces CTL intermediates on the nano-CeO2 . 3.6. Analytical characteristics Under the optimal conditions, the CTL intensity is proportional to the concentration of CS2 in the range of 0.9 ␮g mL−1 to 12.6 ␮g mL−1 in clean air. The linear regression equation is described as Y = 28538 + 54485X (R = 0.999), where Y is the relative CTL intensity, X is the concentration of CS2 and R is the linear regression coefficient, as shown in Fig. 9. The limit of detection based on 3N/S (N refers to the noise and S refers to the slope of the calibration curve) is 3.7 ng mL−1 , which is much lower than the threshold limit value (TLV) – time weighted average (TWA) of CS2 in air of 31 mg/m3 recommended by ACGIH. Theoretically speaking, the TLV can be determined by the present sensor. In fact, it is difficult to quantify the lower concentration of CS2 exactly due to the current conditions in our laboratory. Excitedly, it has the potential application in

Fig. 9. The calibration curve for carbon disulfide. Temperature, 282 ◦ C; air flow rate, 600 mL min−1 ; and wavelength, 460 nm.

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Table 1 Determination of carbon disulfide in the artificial samples by the proposed method. Mixture

Spiked values (␮g mL−1 )

Measured valuesa (␮g mL−1 )

Recovery (%)

1

Carbon disulfide Carbon tetrachloride

3.60 11.4

3.46 ± 0.11

96

2

Carbon disulfide Chlorobenzene

3.60 1.60

3.54 ± 0.16

98

3

Carbon disulfide Carbon tetrachloride Chlorobenzene

3.60 11.4 1.60

3.53 ± 0.04

98

Sample no.

a

Average ± S.D. (n = 3).

high sensitivity, cost-effectiveness, long life-time, simplicity and compactness. Thus, it has potential application for in-situ determination of CS2 in many cases, especially in industrial work place and CS2 warehouse. Acknowledgements We gratefully acknowledge the Ministry of Education of China (NCET-07-0579) and the National Natural Science Foundation of China (No. 20605013 and 20875066) for financial support for this project. References

Fig. 10. Typical results obtained from six replicate injections of carbon disulfide. Temperature, 282 ◦ C; air flow rate, 600 mL min−1 ; and wavelength, 460 nm.

CS2 warehouse, that is the sensor could warn when the concentration of CS2 is near to its explosive limit. Relative standard deviation (R.S.D., n = 5) for 3.6 ␮g mL−1 CS2 is 1.2%, as shown in Fig. 10. The lifetime of the sensor was also examined. The CTL intensity was detected everyday for 3 h by continuously introducing 7.2 ␮g mL−1 CS2 in the air carrier through the surface of the CeO2 nanocubes for two weeks. The day-to-day R.S.D. (n = 11) was less than 6.0%. 3.7. Sample analysis Carbon tetrachloride and chlorobenzene are two commonly used organic solvents. They are likely to be coexistent with carbon disulfide in polluted air. Three artificial air samples were prepared and analyzed by the proposed method. As shown in Table 1, Sample 1 was a mixture of carbon tetrachloride and carbon disulfide, Sample 2 was a mixture of chlorobenzene and carbon disulfide, and Sample 3 was a mixture of carbon tetrachloride, chorobenzene and carbon disulfide. After completely vaporized in an airtight bottle, the sample was flushed onto the surface of CeO2 nanocubes for the detection the CTL with the sensor. As shown in Table 1, carbon disulfide in the three samples is detected with satisfied recoveries of 96–98%. 4. Conclusion A new carbon disulfide sensor was developed based on the strong CL emission from the oxidation of carbon disulfide by air on nano-CeO2 . Due to its characteristics of high selectivity, satisfactory stability and good linearity, the sensor was successfully applied to the determination of trace carbon disulfide in artificial air samples. In addition, this sensor has the advantages of fast response,

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Biographies Yuelan Xuan received her B.S. degree in 2006 from Henan University, China. Now she is an M.S. candidate of analytical chemistry at Sichuan University. Her research interest is focused mainly on luminescence-based sensors. Jing Hu received her B.S. degree in 2006 from Sichuan University, China. Now she is a Ph.D. candidate of analytical chemistry at Sichuan University. Her research interests are focused mainly on luminescence-based sensors. Kailai Xu received her Ph.D. in 2004 from Sichuan University, China. Now she is an associate professor of College of Chemistry at Sichuan University, China. Her major research interests focus on optical methods and computational chemistry. Xiandeng Hou received his Ph.D. from University of Connecticut in 1999. He is currently a professor of analytical chemistry and the director of the Analytical & Testing Center at Sichuan University, Chengdu, China. His main research interest is analytical spectroscopy. Yi Lv received his Ph.D. from Southwest China Normal University (the current Southwest University) in 2003. He is currently a professor of analytical chemistry at Sichuan University, Chengdu, China. His research interests are mainly in the areas of luminescence-based sensors and nanomaterials for analytical chemistry.