Sensors and Actuators B 169 (2012) 261–266
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A cataluminescence gas sensor for triethylamine based on nanosized LaF3 –CeO2 Liu Xu, Hongjie Song, Jing Hu, Yi Lv, Kailai Xu ∗ College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China
a r t i c l e
i n f o
Article history: Received 14 November 2011 Received in revised form 7 April 2012 Accepted 29 April 2012 Available online 4 May 2012 Keywords: Gas sensor Cataluminescence (CTL) Triethylamine LaF3 –CeO2
a b s t r a c t A novel and highly sensitive gas sensor for triethylamine (TEA) was proposed based on its cataluminescence (CTL) from the catalytic oxidation on the surface of nano-sized LaF3 –CeO2 . The LaF3 –CeO2 nanoparticles were successfully prepared by using a calcination method. The luminescence characteristics and the optimal conditions were investigated in detail. Under the optimal conditions, the present gas sensor exhibited a linear range of 0.9–54 ppm, with a correlation coefﬁcient (R) of 0.9987 and a limit of detection (S/N = 3) of 0.2 ppm. The relative standard deviation (R.S.D.) for 36.5 ppm triethylamine was 4.3% (n = 6). There was no signiﬁcant change in the catalytic activity of the sensor after 6 days, with a R.S.D. less than 5%. The proposed triethylamine sensor showed the advantages of good sensitivity and high selectivity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As one of the inﬂammable, explosive and excitant gases, triethylamine is widely used as organic solvents, catalyst, preservatives or synthetic dyes [1–3]. It exhibits a great damage on health, especially on respiratory due to its strong pungency, causing pulmonary edema and even death. Many evidences indicated that it may also endanger our atmosphere, and its steam mixing with air can form explosive mixture and cause combustion when touching the ﬂame. Various methods have been established for the determination of triethylamine, such as gas detection tube  and gas chromatography . Although these traditional techniques are powerful and sensitive for trace triethylamine, they also exhibit some disadvantages, for example, for real time monitoring in the actual site. Therefore, there is a strong demand for a simple portable sensor for the determination of triethylamine at very low concentration levels. Recently, cataluminescence (CTL) gas sensors based on various nanomaterials have been reported for the determination of volatile organic compounds (VOCs) (e.g. ethanol [6,7], formaldehyde , acetaldehyde , acetone, [10,11], hexane , and chlorinated volatile organic compounds ) due to their unique advantages of high sensitivity, rapidity, high selectivity and simplicity. Therefore, it is possible to develop a CTL gas sensor for triethylamine based on nanomaterials. Recently, lanthanide compounds have attracted a lot of interests due to their catalytic performances and special optical applications, such as optical ampliﬁers , labels for biomolecules ,
∗ Corresponding author. Tel.: +86 28 8541 5695; fax: +86 28 8541 5695. E-mail address: [email protected]
(K. Xu). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.04.079
luminescence devices  and electric ﬁeld  probes. They have also been successfully used as phosphors in numerous applications (e.g., organic light-emitting diodes (OLEDs)) ﬂuorescent lamps, lasers, and luminescent probes in biology ). Rare earth doped insulator nanoparticles such as Y2 O3 :Eu3+ , LaF3 :Eu3+ and LaPO4 :Tb3+ represent a new type of luminescent materials with high-efﬁciency [19,20]. Zhou et al.  studied the transfer cataluminescence on nanosized YVO4 :Eu3+ surface. Zhang et al. reported that the luminescence of Eu(TTA)3 phen revealed a great enhancement of the 5 D0 -7 F2 hypersensitive transition, leading to a considerably higher luminescent efﬁciency of the Eu3+ ions. Cerium (IV) oxide (CeO2 , also known as ceria) nanoparticles have good characteristics of high stability at high temperature, unique UV absorption ability and high hardness, well adsorption and reactivity [22,23]. Thus, CeO2 and CeO2 -based materials have been applied in areas of catalysis, luminescence materials , gas sensors, etc. . In comparison with the conventional oxide-based luminescent materials, ﬂuorides show advantages of lower phonon energies and higher luminescence quantum yields . Hence, various kinds of metal ﬂuorides in nano/microscale have been intensively researched in recent years [27–29]. Xu group  investigated the effects on the guest and host on the optical properties of rareearth complexes. Cross et al.  studied the luminescence of 5% LaF3 :Eu3+ , which showed a strong emission. Recently, Lunstroot et al.  have studied the luminescence of LaF3 :Ln3+ , and Yao et al.  have reported the emission phenomenon of LaF3 –Ce3+ , which were synthesized in dimethyl sulfoxide. In this work, nanosized LaF3 –CeO2 was synthesized using a simple calcination method. Strong CTL emission could be observed when triethylamine vapor passed through the surface of nanosized LaF3 –CeO2 , while pure LaF3 or pure CeO2 shows no CTL
L. Xu et al. / Sensors and Actuators B 169 (2012) 261–266
Fig. 1. Schematic representation of the CTL (cataluminescence) sensor based on nanosized LaF3 –CeO2 .
when exposed to TEA. Based on this phenomenon, a gas sensor for the determination of triethylamine was developed and the results showed that the sensor was highly sensitive and selective for triethylamine.
3. Results and discussion
3.1. Characterization of catalysts
The nanostructures of catalysts were characterized by power X-ray diffraction (XRD, Philips Netherlands) with a plumbaginous˚ radiation. Fig. 2 shows monochromatized Cu K␣ ( = 1.5406 A) the powder patterns (b) and (c) for LaF3 and CeO2 , respectively, which are in good agreement with the structures of LaF3 and CeO2 described in JCPDS cards 32-0483 and 04-0593, respectively. Obviously, the pattern (a) represented the mixture of LaF3 and CeO2 . The morphology and particle size were investigated with a transmission electron microscope (TEM-100CX) at an accelerating voltage of 80 kV. As shown in Fig. 3, the particle size decreased after combined with the CeO2 (the size of LaF3 is about 30–50 nm, and the size of LaF3 –CeO2 is about 10–30 nm), and the density of particles increased, resulting in the poorer transmissivity. Strong CTL emission could be observed when triethylamine vapor passed through the surface of nanosized LaF3 –CeO2 , while pure CeO2 shows no CTL emission and pure LaF3 shows weak signal when exposed to TEA. It is probably that the coexistence of LaF3 and CeO2 could improve the possibility of energy transfers from excited intermediates to ground state, which may also enhance the luminescence efﬁciency. To explore the effect of CeO2 doping concentration on the CTL intensity, 36.5 ppm triethylamine was introduced through the surfaces of different particles with three doping concentration of CeO2 (1%, 5%, and 10%) under the air ﬂow of 360 mL min−1 and at 205 ◦ C with a ﬁlter of 490 nm. The results showed that the catalyst with 5% CeO2 doping concentration had the maximal CTL intensity. Therefore the nanosized LaF3 –CeO2 (5%) was chosen for the subsequent experiments.
All reagents used in this experiment were of analytical grade. Standard triethylamine (purity ≥99.5%) was purchased from Kelong Reagent Factory (Chengdu, China). La(NO3 )3 , NH4 F and CeO2 were purchased from Chengdu Kelong Co. Ltd. (Chengdu, China). 2.2. Synthesis of LaF3 –CeO2 nanoparticles In this work, the LaF3 –CeO2 was synthesized by a calcination method. 6.5 g of La(NO3 )3 ·6H2 O(AR) and 2.55 g of NH4 F(AR) was respectively dissolved in deionized water to prepared 0.3 mol/L La(NO3 )3 and 1.0 mol/L NH4 F aqueous solution. These two solutions were then mixed rapidly with stirring for 3 min. Then 0.325 g of CeO2 was added into the prepared mixture solution, and LaF3 –CeO2 (5%) was obtained and stirring for 5 min. The resulting suspension was ﬁltrated with Millipore ﬁlter (0.22 m). Precipitates were collected and calcined at 300 ◦ C for 3 h. 2.3. Apparatus The CTL sensor system was assembled according to our previous work , and the schematic diagram was shown in Fig. 1. It consists of an air pump to supply continuous stream of air ﬂow, a sample valve of 30 mL in which the sample of 1–5 L was injected, vaporized and then driven by the air ﬂow to pass through the CTL reaction chamber, a temperature controller to adjust the surface temperature of nano-LaF3 –CeO2 by adjusting the voltage of the heating cylindrical ceramic rod. Nanosized LaF3 –CeO2 0.03 g were sintered as a layer on a ceramic heating rod (about 0.5 mm). The rod was put into a quartz tube (i.d. = 8 mm), to build a CTL reaction chamber. The triethylamine vapor was sent into the chamber by the air ﬂow and oxidized on the surface of the catalyst by the oxygen in the air. The CTL intensity was directly measured with a commercial BPCL Ultra Weak Chemiluminescence analyzer (Biophysics Institute of Chinese Academy of Science, China). Then the resultant gas from sensor was collected in a 3 L sampling bag (Hede Biotechnology Company, Dalian, China) and introduced into the QP2010
GC/MS system (Shimadzu Technologies, Japan) for the analysis of the reaction products.
3.2. Optimization 3.2.1. Optimization of emission wavelength The optimal emission wavelength was a signiﬁcant factor for CTL intensity, which was investigated with a series of interference ﬁlter in the region of 400–620 nm (400, 425, 440, 460, 490, 535, 555, 575 and 620 nm) using 25 ppm triethylamine at the ﬂow of 300 mL min−1 and the working temperature at 200 ◦ C The results are shown in Fig. 4. The CTL intensity exhibited maximum peak at 490 nm, which was used for the subsequence study.
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Fig. 2. X-ray power diffraction patterns of (a) LaF3 –CeO2 , (b) LaF3 and (c) CeO2 .
3.2.2. Optimization of working temperature Temperature plays an important role in the cataluminescence of triethylamine on LaF3 –CeO2 . In order to obtain the optimal temperature, 25 ppm triethylamine was determined under different temperatures. Fig. 5 shows the CTL intensity increased with the temperature from 165 ◦ C to 225 ◦ C, and the S/N increased with the temperature from 165 ◦ C to 200 ◦ C. Then both of them sharply decreased due to the thermal radiation. Taking these two factors into consideration, 205 ◦ C was ﬁnally chosen as the optimal temperature and for the further study. 3.2.3. Optimization of air ﬂow rate The ﬂow rate of carrier gas also affects the reaction rate of the cataluminescence reaction, and was studied in the range of 200–800 mL min−1 , as shown in Fig. 6. As a result, the maximum of CTL intensity and S/N were separately obtained at 300 and 400 mL min−1 . The CTL intensity of triethylamine increased
gradually with the increasing of ﬂow rate in the range of 200–300 mL min−1 . However, at higher ﬂow rate (>300 mL min−1 ), the triethylamine was diluted by carrier gas and the reaction was not sufﬁcient, so the CTL intensity and S/N get decreased with the increasing of ﬂow rate. Taking these two factors into consideration, the ﬂow rate of 360 mL min−1 was chosen for the determination of triethylamine. 3.3. Selectivity Under the optimum working conditions, a series of common foreign substances such as methanol, ammonia, methylbenzene, carbon tetrachloride, phenylamine, butylamine, ethylacetate, ethanol, acetone, acetaldehyde and diethylamine, with the same concentration of 25 ppm were investigated. As shown in Fig. 7, triethylamine had a signiﬁcantly higher CTL intensity than others. Ethanol, acetone and acetaldehyde exhibited slight interference
Fig. 3. TEM (transmission electron microscope) photo of (a) LaF3 and (b) LaF3 –CeO2 .
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Fig. 4. CTL spectra emission on LaF3 –CeO2 and S/N (signal/noise) ratio. Working temperature: 200 ◦ C and air ﬂow rate: 300 mL min−1 .
Fig. 6. Air ﬂow rate dependence of the CTL intensity and S/N ratio. Working temperature: 205 ◦ C and wavelength: 490 nm.
and diethylamine could cause interference by about 20%. This phenomenon might be attributed to the good catalytic ability and the high luminescence efﬁciency of the catalyst.
3.4. CTL response proﬁles of triethylamine on LaF3 –CeO2
Fig. 5. Temperature dependence of the CTL intensity and S/N ratio. Air ﬂow rate: 300 mL min−1 and wavelength: 490 nm.
The CTL response proﬁle was investigated by injecting triethylamine vapor of different concentrations at a carrier gas rate of 360 mL min−1 and temperature of 205 ◦ C and with a band pass ﬁlter of 490 nm. As shown in Fig. 8, curves a, b and c represented the CTL response of different concentrations of 12, 24 and 48 ppm, respectively. The results showed that the proﬁles of CTL emissions were similar to each other, and the signal sharply increased and reached the maximum immediately after injection. The CTL signal increased from the baseline to maximum value with less than 3 s, and the decay time for the ﬁrst two curves was 25 s and for the third one was 28 s, respectively. It indicated a rapid response of this sensor to triethylamine of different concentrations.
Fig. 7. Selectivity of triethylamine sensor on LaF3 –CeO2 . Wavelength: 490 nm, temperature: 205 ◦ C, and air ﬂow rate: 360 mL min−1 .
L. Xu et al. / Sensors and Actuators B 169 (2012) 261–266
Fig. 8. Time proﬁles of CTL emission. Triethylamine concentration: (a) 12 ppm, (b) 24 ppm, and (c) 48 ppm. Wavelength: 490 nm, temperature: 205 ◦ C, and air ﬂow rate: 360 mL min−1 .
Fig. 10. Typical results obtained from six replicate injections of triethylamine at optimal conditions (working temperature: 205 ◦ C, wavelength: 490 nm, and air ﬂow rate: 360 mL min−1 ).
3.7. Possible mechanism 3.5. Analytical characteristics Under the optimized conditions, the CTL intensity is proportional to the concentration of diethylamide in the range of 0.9–54 ppm with a detection limit (S/N = 3) (signal-to-noise) of 0.2 ppm. As shown in Fig. 9, the linear regression equation was Y = −934 + 5911X (correlation coefﬁcient R = 0.9987), where Y was the CTL intensity and X was the concentration of triethylamine. As shown in Fig. 10, the relative standard deviation (R.S.D., n = 6) was 4.3% for 36.5 ppm triethylamine.
3.6. Lifetime of LaF3 –CeO2 sensor The lifetime of the sensor was examined by continuously introducing a concentration of 36.5 ppm triethylamine into the chamber under the optimized conditions. There was no signiﬁcant change of the catalytic activity of the sensor for 60 h over 6 days, with an R.S.D. of less than 5%. We characterized the material by XRD after CTL reaction, and found that the component of the material did not change after the CTL reaction. However the particles partly fell from the ceramic rod after 6 days’ reaction, which might lead to a short lifetime of the sensor.
According to widely accepted theory about CL reaction, excited intermediates would be formed during the catalytic oxidation of TEA on the surface of LaF3 –CeO2 particles. The luminescence emission could be obtained due to the excited intermediates falling to ground state. In this experiment, the results of GC–MS demonstrated that vinylamine was produced. In addition, we found that the exhaust gas from the catalytic oxidation made the CuSO4 turn blue, indicating that water was created in such process. In study of the oxidation of TEA, De la Fuente et al. [3,33] proposed a stepwise mechanism of electron–proton–electron transfer involving the formation of radical ion pair, dehydrogenated neutral radical of the amine and iminium cation. Thus the possible mechanism of oxidation of TEA on the surface of LaF3 –CeO2 particles could be described as follows: TEA + O2 /2 → O•− + TEA•+
O•− + TEA•+ → OH• + Et2 N C• (H) CH3
OH• + Et2 N C• (H) CH3 → OH− + Et2 N+ CHCH3
Et2 N+ CHCH3 ↔ Et2 N CH CH2 + H+
OH + H = H2 O
TEA + O2 /2 → Et2 N CH CH2 + H2 O
Under the help of catalytic materials, LaF3 –CeO2 particles, TEA ﬁrstly reacted with O2 to form the positive and negative free radicals TEA•+ and O•− (1). The TEA cation radical donated a proton to the radical anion of oxygen to generate the dehydrogenated neutral radical of the amine (2). Then an electronic was transferred from Et2 N C• (H) CH3 to OH• (3), and the product Et2 N+ CHCH3 lost H+ to form Et2 N CH CH (4). Finally the OH− and H+ combine into water (5). Therefore vinylamine and water are the main products of TEA oxidation (6). However, it was difﬁcult to conﬁrm the exact luminescence substances, which might be the products, the intermediates or recombination radiation species. The full details of the reaction mechanism call for further study. 4. Conclusion
Fig. 9. Calibration curve between CTL intensity and triethylamine concentration at optimal working conditions (working temperature: 205 ◦ C, wavelength: 490 nm, and air ﬂow rate: 360 mL min−1 ).
In summary, a new gas sensor for triethylamine was developed based on its cataluminescence emission on the surface of nanosized LaF3 –CeO2 which was synthesized via a simple route. The present sensor has the advantages of good sensitivity, satisfactory stability, high selectivity, fast response, and low LOD. This work
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may be useful for the development of a portable sensor for the determination of triethylamine in food and environment.
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Biographies Xu Liu received her B.S. degree in 2009 from Sichuan University, Chengdu, China. Now she is an M.S. candidate of analytical chemistry of Sichuan University. Her research interest concerns the application of nanosized materials as luminescencebased gas sensors. Hongjie Song received her M.S. degree in 2007 from Shaanxi Normal University, Xian, China. Now she is a Ph.D. candidate of analytical chemistry of Sichuan University. Her research interests include preparation and characterizations of nanostructured composites and their utilization for gas sensing. Jing Hu received her Ph.D. degree in 2011 from Sichuan University, Chengdu, China. Her research interests include luminescence-based sensors and advanced materials for analytical chemistry. Yi Lv received his Ph.D. from Southwest China Normal University (the current Southwest University, Chongqing, China) 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 nanosized materials for analytical chemistry. Kailai Xu received her Ph.D. degree in 2004 from Sichuan University, Chengdu, China. Now she is an associate professor of College of Chemistry at Sichuan University. Her major research interests focus on optical detectors and computational chemistry.