Sensitive and selective acetone sensor based on its cataluminescence from nano-La2O3 surface

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Sensors and Actuators B 132 (2008) 243–249

Sensitive and selective acetone sensor based on its cataluminescence from nano-La2O3 surface Li Tang a , Yaming Li a , Kailai Xu a , Xiandeng Hou a,b , Yi Lv a,∗ a

Key Laboratory of Green Chemistry & Technology of MOE, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China b Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China Received 4 November 2007; received in revised form 13 January 2008; accepted 16 January 2008 Available online 31 January 2008

Abstract In the present paper, a highly sensitive and selective sensor for acetone was demonstrated, which was based on intensive cataluminescence (CTL) emission on the surface of nano-sized La2 O3 . The CTL characteristics and the optimum conditions for the sensor, including La2 O3 morphology, wavelength, working temperature, and airflow rate, were investigated in detail. Under the optimized conditions, the CTL intensity versus concentration of acetone was linear in the range of 0.19–140 ␮g mL−1 , with a correlation coefficient (r) of 0.9981 and a detection limit (signal-to-noise ratio is 3; and noise is the width of baseline) of 0.08 ␮g mL−1 . The relative standard deviation (R.S.D.) for 9.3 ␮g mL−1 acetone (n = 5) was 1.4%. There was no or weak response to common foreign substances, such as ammonia, toluene, benzene, formaldehyde, ethanol, methanol, ethyl acetate, and acetaldehyde. The gas sensor also exhibited good stability and durability for continuously introducing 19 ␮g mL−1 acetone for 80 h over 8 days, with a long-term R.S.D. less than 5%. © 2008 Elsevier B.V. All rights reserved. Keywords: La2 O3 ; Cataluminescence; Acetone; Gas sensor

1. Introduction Volatile organic compounds (VOCs) are ubiquitous in the air we breathe and include a multitude of components which can cause short- or long-term adverse health effects. A number of VOCs sources exist, like solvents, dry cleaning compounds, degreasers, paints, chemical intermediates, and assorted industrial products [1]. As one of highly volatile organic compounds, acetone is widely used as an organic solvent (plastic, fibre and spray-paint) or chemicals intermediate (dyestuff, rubber, and lubricating oil). Although it is popularly regarded as relatively low toxic, some investigations have indicated that chronical exposure may damage to the liver and kidney or nerve, and cause inflammation [2]. Accordingly, persistent efforts have been directed so far to develop the detection of acetone by many analytical methods, such as electrochemistry [3], gas chromatography coupled to flame ionization detection [4] or mass spectrometry [5,6], etc. These techniques, in spite of their effec-

∗

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.2008.01.031

tiveness for trace acetone measurement with merits of high performance and sensitivity, are known to have inherent disadvantages for online monitoring in the actual locale of the contamination. Therefore, there is a strong demand for stable, simple and portable sensors for the detection of acetone at very low concentration level. In recent years, there has been a growing interest in CTL that usually results from the interaction between gases and solid surfaces [7]. In 1976, Breysse et al. [8] reported that the catalytic oxidation of carbon monoxide on the surface of thoria could produce a weak chemiluminescence (CL) emission, and established a concept of “cataluminescence” for the first time. The phenomenon of CL emission based on the catalytic oxidation of organic vapors showed a promising usefulness for the design of gas sensors and specific detectors for liquid chromatography or capillary electrophoresis [9–11]. On the other hand, nanomaterials have attracted widespread attention since the 1990s due to their specific features that differ from bulk materials. Especially, the application of nanomaterials to the design of chemical sensors is nowadays one of the most active research fields, mainly owing to their high activity, good selectivity, tremendous specific surface area and small size [7]. Accordingly, the CTL

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on the surface of nanomaterials has been successfully introduced into the exploration of gas sensors by several research groups. For example, Nakagawa et al. [12–15] and Zhang et al. [16–21] have made persistent efforts for exploration of CTL phenomena on nanoparticles or nanoporous materials, such as ␥-Al2 O3 , V2 Ti4 O13 , BaCO3 , Fe2 O3 , TiO2 , SrCO3 , and ZnO, and established many sensitive and selective sensors for the determination of VOCs, including formaldehyde [16], acetaldehyde [17], hydrogen sulfide [18], chlorinated volatile organic compounds [19] and alcohols [20–22]. It is worth mentioning here that Lu et al. also developed CTL sensors based on zeolite [23] for n-hexane and Al2 O3 [24] for acetone. Coupled with a headspace solid-phase microextraction (HS-SPME) pretreatment, the latter provided a simple, rapid, solvent-free, and sensitive methodology for the determination of acetone in blood. In addition, CTL also provides a novel sensing strategy for optical chemo-sensor array [25] and screening array [26], owing to the ability of discriminating CTL on different nanomaterials and the potential of miniaturization. Rare earth oxides are well known to display interesting catalytic properties [27] in the petrochemical engineering, environment protection, organic synthesis, and synthetic ammonia. Lanthanum oxide is the most representative compound among these substances. Consequently, rare earth oxides and other rare earth compounds have been developed as sensors of electrochemistry to detect ethanol [28], carbon monoxide [29], carbon dioxide [30], etc. In this work, we proposed a novel acetone sensor based on CTL on lanthanum oxide. The intensive CTL emission was observed when acetone passed through the surface of nano-sized La2 O3 . The results showed that this CTL sensor was not only sensitive but also highly selective to acetone with advantages of short response time and good stability. The proposed gas sensor was preliminarily used for the determination of acetone in artificial samples. 2. Experimental 2.1. Synthesis and characterization of catalysts All reagents used were of analytical grade and purchased from Chengdu Kelong Chemical Co. Ltd. (Chengdu, China). 0.1 mol L−1 La(NO3 )3 and 1% (v/v) NH3 ·H2 O solutions were prepared with sub-boiled distilled water. PEG-4000 and PEG20000 were used as surfactants. The La2 O3 catalyst was prepared using a coprecipitation procedure as follow: (1) 90 mL 1% NH3 ·H2 O solution was slowly added into a mixed solution containing 100 mL 0.1 mol L−1 La (NO3 )3 , 2.5 g PEG-4000 and 2.5 g PEG-20000 with vigorous magnetic stirring; (2) the resultant product, a gelatinized precipitate La(OH)3 , was washed with distilled water and ethanol three times respectively to remove residual NO3 − ; and (3) the precipitate was dried in a vacuum chamber for 2 h at 100 ◦ C, and then calcined in a muffle furnace for 2 h at temperatures of 650, 750, and 850 ◦ C to obtain pure La2 O3 particles with different sizes. X-Ray power diffraction (XRD) was carried out with a Philips X’pert Pro MPD diffractometer (Philips Analytical, Nether-

Fig. 1. X-ray power diffraction pattern of La2 O3 calcined at 650 ◦ C, 750 ◦ C, and 850 ◦ C.

lands) using Cu K␣ radiation working at 40 kV and 40 mA. The samples were scanned from 20◦ to 80◦ (2θ) with a step size of 0.02◦ . From Fig. 1, it can be seen that these peaks are attributed to pure and highly crystalline hexagonal La2 O3 . The morphology of the synthesized La2 O3 was characterized with a transmission electron microscope (JEM-100CX) at an accelerating voltage of 80 kV. From Fig. 2, it can be found that the particle size approximately increases with an increase of temperature in the range of 650–850 ◦ C. Furthermore, their shapes are not uniform. As described later, it was indicated that the effect of particle size of La2 O3 on the CTL intensity was very obvious, and eventually the material calcined at 750 ◦ C was selected for use. 2.2. Apparatus Fig. 3 shows the schematic diagram of the apparatus for sensing measurement. It can be seen that a cylindrical ceramic heater with a diameter of 4 mm was put in a quartz tube of 6 mm inner diameter. This ceramic heater was operated at required temperature by using an electric wire inserted into the ceramic tube. A 0.5 mm-thick layer of La2 O3 powder was sintered on the outside surface of the ceramic tube. The main reaction between the sensor and sample molecules occurred on the surface of nano-sized La2 O3 powder. The CTL emission was detected and recorded by a computerized BPCL ultra-weak luminescence analyzer (BP-II, Institute of Biophysics, Academia Sinica, Beijing, China). An air pump (SGK-5LB, Beijing Dongfang Jinghuayuan Technology Co. Ltd., Beijing, China) was used for air supply. 3. Results and discussion 3.1. La2 O3 morphology and possible mechanism To explore the effect of the La2 O3 morphology on the CTL, three kinds of La2 O3 particles with different size were employed as a sensing material. At a flow rate of 160 mL min−1 , the CTL emission on the La2 O3 particles surface was investigated

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report [18], the occurrence of CL emission requires not only the catalytic oxidation of gases on the catalyst but also the formation of CL intermediates. Although many gases would be adsorbed and then catalytically oxidized on the catalyst, resulting in the change of electric conductivity of the catalyst, a few of them could produce CL intermediates. So, a possible mechanism of CTL with acetone on lanthanum oxide can be deduced as follows: first, when acetone passes over the surface of the catalyst of lanthanum oxide, it is catalytically oxidized by oxygen in the air, and then electronically excited oxidative molecules are produced during the reaction to generate CL when they return to their ground state. The more exact mechanism awaits further exploration. 3.2. Optimization of working temperature The temperature plays an important role in the reaction rate of the catalytic oxidation reaction. To investigate the effect of temperature on the CL intensity, a 19 ␮g mL−1 acetone sample was determined at varied temperatures. Fig. 5 presents the signal intensity and the signal to noise ratio (S/N) versus the catalysis temperature at a carrier gas flow rate of 160 mL min−1 . The results showed that the cataluminescence intensity increased with increasing the temperature from 266 to 361 ◦ C. According to the Yang’s report [33], this may be attributed to the higher collision efficiency between acetone and O2 and the higher catalytic activity of nano-sized La2 O3 at higher temperature. However, the experiment also indicated that the background signal, which mainly arose from the heat radiation, also increased with increasing the temperature even faster than the CL intensity at higher temperatures. Furthermore, at 361 ◦ C the signal and the S/N reached a maximum on the La2 O3 particles surface. Under or above this temperature, the S/N decreased. Therefore, 361 ◦ C was chosen for the following experiments. 3.3. Optimization of air flow rate

Fig. 2. TEM photo of La2 O3 calcined at (a) 650 ◦ C, (b) 750 ◦ C, and (c) 850 ◦ C.

by introducing acetone gas of a concentration of 19 ␮g mL−1 , through a series of interference (band-pass) filters in the region of 400–555 nm for the selection of proper detection wavelength for the maximum CTL signal. The results are shown in Fig. 4. It is obvious that the three kinds of La2 O3 particles all gave the maximum CTL emission at about 490 nm for acetone, while the La2 O3 particles calcined at 750 ◦ C was the best in terms of signal intensity. A widely accepted CL mechanism is the formation of an intermediate of a highly reactive endoperoxide [31,32]. Light is emitted during the transition of these excited intermediates to the electronic ground state. Additionally, according to the previous

The flow rate dependence of the CTL intensity was studied in the range of 50–600 mL min−1 at 361 ◦ C with a band pass filter of 490 nm. As shown in Fig. 6, it can be seen that the CTL intensity of acetone increased gradually with increasing the air flow rate in the range of 50–200 mL min−1 . However, at higher flow rates (>200 mL min−1 ), the reaction time between acetone and La2 O3 would possibly not be sufficient, so that the sensitivity of the CTL detection system became lower. Furthermore, the S/N tended to be maximal at 200 mL min−1 . Therefore, a flow rate of 200 mL min−1 was finally used for the determination of acetone. 3.4. Temporal CTL profiles of acetone on La2 O3 Temporal CTL profiles of acetone vapor on the surface of La2 O3 were investigated by injecting the acetone vapor of different concentrations into a carrier gas at 200 mL min−1 and 361 ◦ C. The results are shown in Fig. 7. Curves a–c denote the results for different concentrations of 0.93, 4.65, and 9.30 ␮g mL−1 , respectively. It can be seen that the temporal pro-

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Fig. 3. Schematic diagram of the apparatus for sensing measurement.

files of cataluminescence emission were essentially similar to each other, and the signal rapidly increased from the baseline to the maximum value within less than 5 s. The recovery time of the CL intensity for each curve was about 25 s. This indicates that the proposed sensor has an advantage of high throughput.

3.5. Analytical characteristics

Fig. 4. Typical recordings of CTL emission on La2 O3 calcined at (a) 650 ◦ C, (b) 750 ◦ C, and (c) 850 ◦ C. Air flow rate: 160 mL min−1 ; and temperature: 390 ◦ C. Error bars in the graph stand for ±S.D. (standard deviation).

Fig. 6. Effect of flow rate on the CTL intensity and S/N ratio. Temperature: 361 ◦ C; and wavelength: 490 nm. Error bars stand for ±S.D.

Fig. 5. Effect of temperature on the CTL intensity and S/N. Air flow rate: 160 mL min−1 ; and wavelength: 490 nm. Error bars stand for ±S.D.

Under the optimized conditions, the calibration curve of CTL intensity versus acetone vapor concentration was linear in the range of 0.19–140 ␮g mL−1 with a detection limit (S/N = 3) of 0.08 ␮g mL−1 , as shown in Fig. 8. The line regression equa-

Fig. 7. Typical temporal profiles of CTL emission. Acetone vapor concentration: (a) 0.93 ␮g mL−1 , (b) 4.65 ␮g mL−1 , and (c) 9.30 ␮g mL−1 . Wavelength: 490 nm; and temperature: 361 ◦ C; air flow rate: 200 mL min−1 .

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Fig. 8. The calibration curve for acetone. Conditions: wavelength, 490 nm; temperature, 361 ◦ C; and air flow rate, 200 mL min−1 .

tion is described as Y = 246.7 + 444.6X (correlation coefficient r = 0.9980, ‘Y’ is the CL intensity, ‘X’ is the concentration of acetone gas, and three replicates for each data point). Relative standard deviation (R.S.D., n = 5) was 1.4% for a 9.3 ␮g mL−1 acetone sample, as shown in Fig. 9. 3.6. Gas selectivity of the La2 O3 sensor Under the optimum working conditions, gas selectivity of the La2 O3 sensor was studied. Common foreign substances, which may interfere with the measurement of acetone in the industry grounds, such as methanol, ethanol, formaldehyde, acetaldehyde, ethyl acetate, benzene, toluene, and ammonia, were investigated with a same concentration of 18.6 ␮g mL−1 for the influence on the acetone determination. The results are shown in Fig. 10. It can be seen easily that acetone has the highest emission intensity, compared to all the foreign species. Ethanol, methanol, ethyl acetate, and acetaldehyde exhibited insignificant interference (less than 5%) and there was almost no response from other foreign substances tested. Consequently, for the commonly used organic solvents, the CTL detector exhibits high selectivity for the determination of acetone.

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Fig. 10. Selectivity of the La2 O3 sensor. Conditions: wavelength, 490 nm; temperature, 361 ◦ C; and air flow rate, 200 mL min−1 .

3.7. Long-term stability of the sensor The experiments about stability and durability of the sensor were carried out by continually injecting a 19 ␮g mL−1 acetone sample into the sensor chamber under the optimum conditions. There were no significant changes of the catalytic activity of La2 O3 for 80 h reaction over 8 days, with an R.S.D. less than 5% by collecting the CTL intensity at every hour. 3.8. Sample analysis Three artificial air samples were analyzed to evaluate the analytical application. Because acetone, ethanol, benzene and toluene are the common organic compounds in the manufacturing, a series of mixed samples of known concentrations were prepared for the measurements. The artificial gas samples were prepared by the following procedures: a certain volume of the mixed sample was injected into a 10 mL airtight bottle which was placed in a thermostatic oil bath (120 ◦ C). After an incubation for 5 min for the mixed sample evaporation, the vapor sample was carried into the CTL chamber for measurement. Sample 1 was a mixture of acetone and ethanol, sample 2 was a mixture of acetone and toluene, and sample 3 was a mixture of acetone, benzene, and toluene. The results are shown in Table 1. Table 1 Determination of acetone in the artificial samples by the proposed method

Fig. 9. Typical results obtained from five replicate injections of acetone. Conditions: wavelength, 490 nm; temperature, 361 ◦ C; and air flow rate, 200 mL min−1 .

Sample no.

Mixture

1

Acetone Ethanol

2

3 *

Standard values (␮g mL−1 )

Measured values* (␮g mL−1 )

Recovery (%)

18.6 93.0

18.0 ± 1.1

96.8

Acetone Toluene

9.3 140.0

9.1 ± 0.5

97.8

Acetone Toluene Benzene

9.3 140.0 140.0

8.9 ± 0.3

95.7

Average ± S.D. (n = 3).

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4. Conclusion In summary, a new acetone sensor was developed based on CTL emission on the surface of La2 O3 . The sensor was successfully applied to the determination of acetone in artificial air samples. This gas sensor has the advantages of fast response, high sensitivity, satisfactory stability, good selectivity and low cost. This work may be useful for developing miniaturized equipment for the determination of acetone in the industry grounds.

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[18]

Acknowledgements The authors gratefully acknowledge the financial support for this project from the Ministry of Education of China (NCET-07-0579, 20070610020) and the National Natural Science Foundation of China (No. 20605013).

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Biographies Li Tang received her BS degree in 2006 from Sichuan University, China. Now she is an MS candidate of Sichuan University and majors in analytical chemistry. Her research interests is focused mainly on luminescence-based sensors. Yaming Li received his BS degree in 2003 from Chengdu University of technology, China. Now he is an MS candidate of Sichuan University and majors in analytical chemistry. His research interests is focused mainly on chemiluminescence analysis. Kailai Xu received her PhD degree in 2004 from Sichuan University, China. Now she is an associate professor of College of Chemistry at Sichuan University,

L. Tang et al. / Sensors and Actuators B 132 (2008) 243–249 China. Her major research interests focus on optical detectors and computational chemistry. Xiandeng Hou received his PhD 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 and its applications.

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Yi Lv received his PhD degree in Department of Chemistry at Southwest China Normal University (the current name is Southwest University) in 2003. He is currently professor of College of Chemistry at Sichuan University, Chengdu, China. His research interests are mainly in the areas of luminescence-based sensors and nanomaterials for analytical chemistry.