Research article Received: 3 June 2014,
Revised: 25 August 2014,
Accepted: 7 September 2014
Published online in Wiley Online Library: 28 October 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2795
Development of a cataluminescence sensor for detecting benzene based on magnesium silicate hollow spheres Yuhuai Wang,a* Bo Li,b Qihui Wanga,c and Zhouxiang Shoua ABSTRACT: A novel and sensitive gas sensor was developed for the determination of benzene based on its cataluminescence (CTL) by oxidation in air on the surface of hollow magnesium silicate spheres. Luminescence characteristics and optimum conditions were investigated. Results indicated that the as-prepared magnesium silicate hollow spheres exhibited outstanding CTL properties such as stable intensity, high signal/noise values, and short response and recovery times. Under optimized conditions, benzene exhibited a broad linear range of 1–4500 ppm, with a correlation coefficient of 0.9946 and a limit of detection (signal-to-noise ratio (S/N) = 3) of 0.6 ppm, which was below the standard permitted concentration. The relative standard deviation (RSD) for 100 ppm benzene was 4.3% (n = 6). Furthermore, the gas sensor system showed outstanding selectivity for benzene compared with nine other common volatile organic compounds (VOCs). The proposed gas sensor showed good characteristics of high selectivity, fast response time and long lifetime, which suggested the promising application of magnesium silicate hollow spheres as a novel highly efficient CTL sensing material. The mechanism for the improved performance was also discussed based on the experimental results. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: benzene; cataluminescence; gas sensor; magnesium silicate; volatile organic compounds
Introduction
Luminescence 2015; 30: 619–624
* Correspondence to: Y. H. Wang, Qianjiang College, Hangzhou Normal University, Hangzhou, 310016, People’s Republic of China. E-mail: [email protected] a
Qianjiang College, Hangzhou Normal University, Hangzhou, 310016, People’s Republic of China
b
School of Information Engineering, East China Jiaotong University, Nanchang, 330013, People’s Republic of China
c
School of Aeronautics and Astronautics, Zhejiang University, Hangzhou, 310027, People’s Republic of China Abbreviations: CL, chemiluminescence; CTL, cataluminescence; PMT, photomultiplier tube; RSD, relative standard deviation; VOC, volatile organic compounds; XRD, X-ray diffractometer.
Copyright © 2014 John Wiley & Sons, Ltd.
619
Volatile organic compounds (VOCs) are ubiquitous in the air we breathe and can cause short- or long-term adverse health effects (1). Exposure to VOCs may have serious consequences, such as sensory irritation and acute or chronic disease. Determination of the best method to detect and identify VOCs is one of the most important subjects in the chemical sensing field, but remains an enormous challenge. Benzene, one of the commonly used VOCs, has received much attention in recent years due to its wide application as an organic solvent (2), cleanout and drycleaning reagent (3). The increasing demand for benzene in industry leads to a pollution via gaseous and liquid wastes, therefore a good selective determination method for benzene vapor has been eagerly sought. Various methods have been established for benzene determination, such as gas chromatography (4), spectrophotometry (5), and high performance liquid chromatography (6). Although these are powerful and sensitive techniques for trace benzene determination, they also have some inherent disadvantages for real-time monitoring at the actual contamination site. In addition, their long response time and nonlinear response detection has greatly limited their practical application (7). In view of this situation, there is an increasing demand for a simple, portable sensor to determine low concentrations of benzene. In recent years, there has been a growing interest in cataluminescence (CTL), mainly due to its high sensitivity, rapidity, simplicity and low instrumentation and maintenance costs (8,9). CTL was first observed by Breysse et al. (10) in 1976 during the catalytic oxidation of carbon monoxide on the surface of thoria (thorium dioxide), which could produce a weak chemiluminescence (CL) emission. CTL, resulting from the interaction
between gases and solid surfaces, has been investigated as a novel and promising transduction principle for gas sensors (11). CTL-based sensors have significant advantages over traditional sensors, such as stable and durable intensity, high signal/noise (S/N) ratio, and rapid response (12). Persistent efforts have been made for many years to develop a CTL analytical method for the detection of VOCs such as acetaldehyde (13), benzaldehyde (14), formaldehyde (15), ethanol (16), ether (11) and acetone (17). In the current work, magnesium silicate hollow spheres were successfully synthesized, and the CTL of benzene was investigated based on these nanomaterials. A strong CTL emission was observed when benzene vapor passed through the surface of nano-sized magnesium silicate in air. A CTL-based sensor device for benzene was designed and performance aspects of the sensor, such as sensitivity, selectivity and stability, were
Y. Wang et al. investigated in detail. The study showed that the as-prepared hollow spheres had excellent specificity as a sensing material for benzene by testing another nine types common possible VOCs. The possible mechanism of the CTL resulting from catalytic oxidation of benzene on the magnesium silicate sphere surface was also discussed briefly.
Experimental Chemical reagents and materials All chemicals (butanol, butanone, methanol, propanol, formaldehyde, benzene, xylene, ammonia, toluene, acetaldehyde) used in the experiment were of high purity analytical grade (≥99.0%) or higher and were purchased from the Shanghai Chemical Reagents Company. Thus they could be used in our experiment without further purification. Synthesis of nanostructured magnesium silicate hollow spheres The porous hollow spherical silicates were prepared by a hydrothermal process in accordance with a previous report with minor modification (18). In a typical procedure, 0.064 g Mg(NO3)2 · 6H2O, 0.025 g silica colloidal spheres and 0.675 g urea were dissolved and stirred in a mixed solvent of ethanol and H2O (volume ratio 1:2) for 0.5 h at room temperature. Then the homogeneous
solutions were transferred into a 23 mL Teflon-lined autoclave and heated at 190 °C for 36 h. After cooling to room temperature, the solid precipitate was collected by centrifugation and washed three times with water and ethanol, respectively. The as-prepared samples were dried at 100 °C for 12 h. Field-emission scanning electron microscopy (FE-SEM; FEI Sirion200), and high-resolution transmission electron microscopy (HRTEM; JEM-2010) analyses were used to study the morphology of nanostructured magnesium silicate hollow spheres. X-ray diffraction (XRD) patterns of the samples were obtained using a Philips X’Pert Pro X-ray diffractometer with Cu-Kα radiation (1.5418 Å). Apparatus Figure 1 shows the schematic diagram of the CTL detection system. A heating rod smeared by nanomaterials was placed in a quartz tube with an inner diameter of 15 mm. A pump with a gas filter provided a steady air flow stream at a controlled flow rate. The pump could not only control the flow rate but also dispersed the tested gas immediately. The gas filter, which is a piece of filterable net, was installed inside the gas input port. The nanomaterials were dispersed in ethanol to form a suspension and placed on the heating rod to form a layer of nanomaterials of 0.5 mm thickness. The working temperature could be adjusted by controlling the voltage of the quartz tube. When the organic vapors flowed through the tube, the catalytic reaction occurred on the surface of the nanomaterials. The CTL intensity was monitored by a photomultiplier tube (PMT) in an Ultra-Weak Luminescence Analyzer.
Results and discussion Estimation of nano-sized materials
Figure 1. Schematic diagram of the sensor system. BPCL, Ultra-weak Chemiluminescence Analyzer system, PMT, photomultiplier tube.
During the synthesis process, silica colloidal spheres slowly dissolved and formed silicate anions in the alkaline solution. Stable alkaline conditions were provided by conversion of urea at high temperature; magnesium cations react with silicate anions and produce magnesium silicate hydroxide hydrate around the surface of the SiO2 spheres. Subsequently, the magnesium silicate core/shell structure was formed with gradual release of the silicate anions from the silica colloidal spheres. Finally, the magnesium silicate hollow spheres were produced in the center of the nanostructure after the remaining silica core dissolved completely (18). The final product synthesized by the above mechanism was characterized by SEM, TEM (Fig. 2) and XRD
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Figure 2. SEM (a), and TEM (b), micrographs of magnesium silicate nanomaterials.
wileyonlinelibrary.com/journal/luminescence
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 619–624
Cataluminescence sensor for detecting benzene
Figure 3. XRD patterns of as-obtained magnesium silicate nanomaterials.
Effect of temperature. Temperature also plays an important role in the catalytic oxidation reaction of benzene on the surface of magnesium silicate hollow spheres. Due to weak catalytical activity under low temperature conditions, CTL performance for gas determination had not been observed at room temperature, and external heat was needed overcome the energy barriers (19). As shown in Fig. 5, CTL intensity for benzene was increased as working temperature increased, which should be attributed to the higher conversion rate of benzene on magnesium silicate nanomaterials at higher temperatures. However performance was unsatisfactory at very high temperatures when taking S/N ratios into account. A significant decrease in the S/N ratio was seen at about 220 °C due to increasing noise. In view of these results, a temperature of 220 °C was finally chosen for further study. The chosen method at this temperature showed superior analytical performance compared with other benzene determinations (20). Effect of CTL wavelength. The CTL emission wavelength of benzene on the surface of magnesium silicate hollow spheres was firstly investigated with optical filters at 400–555 nm (400, 430, 450, 480, 490, 535 or 555 nm). Figure 6 shows that the CTL intensity of benzene vapor changed at different wavelengths at a flow rate of 240 mL/min and temperature of 220 °
Figure 4. Air flow rate dependence of CTL intensity and S/N ratio. Temperature: 220 °C; concentration: 100 ppm; and wavelength: 430 nm.
(Fig. 3). As shown in Fig. 2, the magnesium silicate was composed of uniform spherical particles and the surface was rough and porous with a large amount of thin lamellae. The average size of the spheres was uniformly about 400 nm and the shell was about 40 nm in thickness. As shown in Fig. 3, all peaks showed the characteristics of magnesium silicate (reported data: Joint Committee on Powder Diffraction Standards no. 03-0174) from the relative XRD pattern and could be identified as representing a talc structure (19).
Figure 5. Relative CTL intensity and S/N ratio of benzene on magnesium silicate nanomaterials at different temperatures. Concentration: 100 ppm; flow rate: 240 mL/min; wavelength: 430 nm.
Cataluminescence application
Luminescence 2015; 30: 619–624
Figure 6. CTL spectra emission on magnesium silicate nanomaterials. Temperature: 220 °C; air flow rate: 240 mL/min; and concentration: 100 ppm.
Copyright © 2014 John Wiley & Sons, Ltd.
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621
Optimization of air flow rate. The influence of the flow rate of air on CTL intensity was studied in the rate range of 80– 420 mL/min at 220 °C with a band-pass filter of 430 nm. The results (Fig. 4) showed that CTL intensity and S/N ratio increased when air flow rate increased from 80 to 240 mL/min, and reached a maximum at 240 mL/min. However, when the flow rate was greater than 240 mL/min, the CTL intensity and the S/N ratio reduced, probably because the reaction time between benzene and magnesium silicate spheres was not sufficient. Therefore, a flow rate of 240 mL/min was chosen for the determination of benzene.
Y. Wang et al. C. It was clear that the maximal emission of benzene was at 430 nm at maximum S/N ratio. Therefore, 430 nm was chose as the optimal wavelength for quantitative detection in subsequent work. Analytical characteristics. Further study on CTL performance using benzene was carried out under optimal conditions. As shown in Fig. 7, the regression of CTL intensity versus benzene concentration was linear in the range 1–4500 ppm and the detection limit was about 0.6 ppm (S/N = 3), which was below the standard permitted concentration. The linear regression equation value was Y = 60.1X + 5761.7 (correlation coefficient R = 0.9946), where Y was the average relative CTL intensity of replicate tests at the same concentration level, X was the concentration of benzene, and R was the regression coefficient. Relative standard deviation (RSD, n = 6) was 4.3% for 100 ppm benzene, as shown in Fig. 8.
Figure 9. Temporal profiles of CTL emission. Concentration of benzene: (a) 50 ppm; (b) 100 ppm; and (c) 500 ppm. Wavelength: 430 nm; temperature: 220 °C; and air flow rate: 240 mL/min.
CTL response profiles of benzene on magnesium silicate. The CTL response profiles of benzene vapor on the surface of magnesium silicate were investigated by injecting the vapor at different concentrations into a carrier gas at 240 mL/min and 220 °C with a band-pass filter of 430 nm. Figure 9 shows the CTL response curves at different concentrations (50, 100 or 500 ppm). CTL emission profiles were similar to each other, and
Figure 10. The selectivity towards benzene of the prepared CTL sensor system based on magnesium silicate. Concentration: 100 ppm; wavelength: 430 nm; temperature: 220 °C; flow rate: 240 mL/min. 1, butanol; 2, butanone; 3, methanol; 4, propanol; 5, formaldehyde; 6, benzene; 7, xylene; 8, ammonia; 9, toluene; 10, acetaldehyde.
Figure 7. Calibration curve between CTL intensity and benzene concentration. Wavelength: 430 nm; temperature: 220 °C; and air flow rate: 240 mL/min.
CTL signal increased from baseline to maximum value in
Revised: 25 August 2014,
Accepted: 7 September 2014
Published online in Wiley Online Library: 28 October 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2795
Development of a cataluminescence sensor for detecting benzene based on magnesium silicate hollow spheres Yuhuai Wang,a* Bo Li,b Qihui Wanga,c and Zhouxiang Shoua ABSTRACT: A novel and sensitive gas sensor was developed for the determination of benzene based on its cataluminescence (CTL) by oxidation in air on the surface of hollow magnesium silicate spheres. Luminescence characteristics and optimum conditions were investigated. Results indicated that the as-prepared magnesium silicate hollow spheres exhibited outstanding CTL properties such as stable intensity, high signal/noise values, and short response and recovery times. Under optimized conditions, benzene exhibited a broad linear range of 1–4500 ppm, with a correlation coefficient of 0.9946 and a limit of detection (signal-to-noise ratio (S/N) = 3) of 0.6 ppm, which was below the standard permitted concentration. The relative standard deviation (RSD) for 100 ppm benzene was 4.3% (n = 6). Furthermore, the gas sensor system showed outstanding selectivity for benzene compared with nine other common volatile organic compounds (VOCs). The proposed gas sensor showed good characteristics of high selectivity, fast response time and long lifetime, which suggested the promising application of magnesium silicate hollow spheres as a novel highly efficient CTL sensing material. The mechanism for the improved performance was also discussed based on the experimental results. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: benzene; cataluminescence; gas sensor; magnesium silicate; volatile organic compounds
Introduction
Luminescence 2015; 30: 619–624
* Correspondence to: Y. H. Wang, Qianjiang College, Hangzhou Normal University, Hangzhou, 310016, People’s Republic of China. E-mail: [email protected] a
Qianjiang College, Hangzhou Normal University, Hangzhou, 310016, People’s Republic of China
b
School of Information Engineering, East China Jiaotong University, Nanchang, 330013, People’s Republic of China
c
School of Aeronautics and Astronautics, Zhejiang University, Hangzhou, 310027, People’s Republic of China Abbreviations: CL, chemiluminescence; CTL, cataluminescence; PMT, photomultiplier tube; RSD, relative standard deviation; VOC, volatile organic compounds; XRD, X-ray diffractometer.
Copyright © 2014 John Wiley & Sons, Ltd.
619
Volatile organic compounds (VOCs) are ubiquitous in the air we breathe and can cause short- or long-term adverse health effects (1). Exposure to VOCs may have serious consequences, such as sensory irritation and acute or chronic disease. Determination of the best method to detect and identify VOCs is one of the most important subjects in the chemical sensing field, but remains an enormous challenge. Benzene, one of the commonly used VOCs, has received much attention in recent years due to its wide application as an organic solvent (2), cleanout and drycleaning reagent (3). The increasing demand for benzene in industry leads to a pollution via gaseous and liquid wastes, therefore a good selective determination method for benzene vapor has been eagerly sought. Various methods have been established for benzene determination, such as gas chromatography (4), spectrophotometry (5), and high performance liquid chromatography (6). Although these are powerful and sensitive techniques for trace benzene determination, they also have some inherent disadvantages for real-time monitoring at the actual contamination site. In addition, their long response time and nonlinear response detection has greatly limited their practical application (7). In view of this situation, there is an increasing demand for a simple, portable sensor to determine low concentrations of benzene. In recent years, there has been a growing interest in cataluminescence (CTL), mainly due to its high sensitivity, rapidity, simplicity and low instrumentation and maintenance costs (8,9). CTL was first observed by Breysse et al. (10) in 1976 during the catalytic oxidation of carbon monoxide on the surface of thoria (thorium dioxide), which could produce a weak chemiluminescence (CL) emission. CTL, resulting from the interaction
between gases and solid surfaces, has been investigated as a novel and promising transduction principle for gas sensors (11). CTL-based sensors have significant advantages over traditional sensors, such as stable and durable intensity, high signal/noise (S/N) ratio, and rapid response (12). Persistent efforts have been made for many years to develop a CTL analytical method for the detection of VOCs such as acetaldehyde (13), benzaldehyde (14), formaldehyde (15), ethanol (16), ether (11) and acetone (17). In the current work, magnesium silicate hollow spheres were successfully synthesized, and the CTL of benzene was investigated based on these nanomaterials. A strong CTL emission was observed when benzene vapor passed through the surface of nano-sized magnesium silicate in air. A CTL-based sensor device for benzene was designed and performance aspects of the sensor, such as sensitivity, selectivity and stability, were
Y. Wang et al. investigated in detail. The study showed that the as-prepared hollow spheres had excellent specificity as a sensing material for benzene by testing another nine types common possible VOCs. The possible mechanism of the CTL resulting from catalytic oxidation of benzene on the magnesium silicate sphere surface was also discussed briefly.
Experimental Chemical reagents and materials All chemicals (butanol, butanone, methanol, propanol, formaldehyde, benzene, xylene, ammonia, toluene, acetaldehyde) used in the experiment were of high purity analytical grade (≥99.0%) or higher and were purchased from the Shanghai Chemical Reagents Company. Thus they could be used in our experiment without further purification. Synthesis of nanostructured magnesium silicate hollow spheres The porous hollow spherical silicates were prepared by a hydrothermal process in accordance with a previous report with minor modification (18). In a typical procedure, 0.064 g Mg(NO3)2 · 6H2O, 0.025 g silica colloidal spheres and 0.675 g urea were dissolved and stirred in a mixed solvent of ethanol and H2O (volume ratio 1:2) for 0.5 h at room temperature. Then the homogeneous
solutions were transferred into a 23 mL Teflon-lined autoclave and heated at 190 °C for 36 h. After cooling to room temperature, the solid precipitate was collected by centrifugation and washed three times with water and ethanol, respectively. The as-prepared samples were dried at 100 °C for 12 h. Field-emission scanning electron microscopy (FE-SEM; FEI Sirion200), and high-resolution transmission electron microscopy (HRTEM; JEM-2010) analyses were used to study the morphology of nanostructured magnesium silicate hollow spheres. X-ray diffraction (XRD) patterns of the samples were obtained using a Philips X’Pert Pro X-ray diffractometer with Cu-Kα radiation (1.5418 Å). Apparatus Figure 1 shows the schematic diagram of the CTL detection system. A heating rod smeared by nanomaterials was placed in a quartz tube with an inner diameter of 15 mm. A pump with a gas filter provided a steady air flow stream at a controlled flow rate. The pump could not only control the flow rate but also dispersed the tested gas immediately. The gas filter, which is a piece of filterable net, was installed inside the gas input port. The nanomaterials were dispersed in ethanol to form a suspension and placed on the heating rod to form a layer of nanomaterials of 0.5 mm thickness. The working temperature could be adjusted by controlling the voltage of the quartz tube. When the organic vapors flowed through the tube, the catalytic reaction occurred on the surface of the nanomaterials. The CTL intensity was monitored by a photomultiplier tube (PMT) in an Ultra-Weak Luminescence Analyzer.
Results and discussion Estimation of nano-sized materials
Figure 1. Schematic diagram of the sensor system. BPCL, Ultra-weak Chemiluminescence Analyzer system, PMT, photomultiplier tube.
During the synthesis process, silica colloidal spheres slowly dissolved and formed silicate anions in the alkaline solution. Stable alkaline conditions were provided by conversion of urea at high temperature; magnesium cations react with silicate anions and produce magnesium silicate hydroxide hydrate around the surface of the SiO2 spheres. Subsequently, the magnesium silicate core/shell structure was formed with gradual release of the silicate anions from the silica colloidal spheres. Finally, the magnesium silicate hollow spheres were produced in the center of the nanostructure after the remaining silica core dissolved completely (18). The final product synthesized by the above mechanism was characterized by SEM, TEM (Fig. 2) and XRD
620
Figure 2. SEM (a), and TEM (b), micrographs of magnesium silicate nanomaterials.
wileyonlinelibrary.com/journal/luminescence
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 619–624
Cataluminescence sensor for detecting benzene
Figure 3. XRD patterns of as-obtained magnesium silicate nanomaterials.
Effect of temperature. Temperature also plays an important role in the catalytic oxidation reaction of benzene on the surface of magnesium silicate hollow spheres. Due to weak catalytical activity under low temperature conditions, CTL performance for gas determination had not been observed at room temperature, and external heat was needed overcome the energy barriers (19). As shown in Fig. 5, CTL intensity for benzene was increased as working temperature increased, which should be attributed to the higher conversion rate of benzene on magnesium silicate nanomaterials at higher temperatures. However performance was unsatisfactory at very high temperatures when taking S/N ratios into account. A significant decrease in the S/N ratio was seen at about 220 °C due to increasing noise. In view of these results, a temperature of 220 °C was finally chosen for further study. The chosen method at this temperature showed superior analytical performance compared with other benzene determinations (20). Effect of CTL wavelength. The CTL emission wavelength of benzene on the surface of magnesium silicate hollow spheres was firstly investigated with optical filters at 400–555 nm (400, 430, 450, 480, 490, 535 or 555 nm). Figure 6 shows that the CTL intensity of benzene vapor changed at different wavelengths at a flow rate of 240 mL/min and temperature of 220 °
Figure 4. Air flow rate dependence of CTL intensity and S/N ratio. Temperature: 220 °C; concentration: 100 ppm; and wavelength: 430 nm.
(Fig. 3). As shown in Fig. 2, the magnesium silicate was composed of uniform spherical particles and the surface was rough and porous with a large amount of thin lamellae. The average size of the spheres was uniformly about 400 nm and the shell was about 40 nm in thickness. As shown in Fig. 3, all peaks showed the characteristics of magnesium silicate (reported data: Joint Committee on Powder Diffraction Standards no. 03-0174) from the relative XRD pattern and could be identified as representing a talc structure (19).
Figure 5. Relative CTL intensity and S/N ratio of benzene on magnesium silicate nanomaterials at different temperatures. Concentration: 100 ppm; flow rate: 240 mL/min; wavelength: 430 nm.
Cataluminescence application
Luminescence 2015; 30: 619–624
Figure 6. CTL spectra emission on magnesium silicate nanomaterials. Temperature: 220 °C; air flow rate: 240 mL/min; and concentration: 100 ppm.
Copyright © 2014 John Wiley & Sons, Ltd.
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Optimization of air flow rate. The influence of the flow rate of air on CTL intensity was studied in the rate range of 80– 420 mL/min at 220 °C with a band-pass filter of 430 nm. The results (Fig. 4) showed that CTL intensity and S/N ratio increased when air flow rate increased from 80 to 240 mL/min, and reached a maximum at 240 mL/min. However, when the flow rate was greater than 240 mL/min, the CTL intensity and the S/N ratio reduced, probably because the reaction time between benzene and magnesium silicate spheres was not sufficient. Therefore, a flow rate of 240 mL/min was chosen for the determination of benzene.
Y. Wang et al. C. It was clear that the maximal emission of benzene was at 430 nm at maximum S/N ratio. Therefore, 430 nm was chose as the optimal wavelength for quantitative detection in subsequent work. Analytical characteristics. Further study on CTL performance using benzene was carried out under optimal conditions. As shown in Fig. 7, the regression of CTL intensity versus benzene concentration was linear in the range 1–4500 ppm and the detection limit was about 0.6 ppm (S/N = 3), which was below the standard permitted concentration. The linear regression equation value was Y = 60.1X + 5761.7 (correlation coefficient R = 0.9946), where Y was the average relative CTL intensity of replicate tests at the same concentration level, X was the concentration of benzene, and R was the regression coefficient. Relative standard deviation (RSD, n = 6) was 4.3% for 100 ppm benzene, as shown in Fig. 8.
Figure 9. Temporal profiles of CTL emission. Concentration of benzene: (a) 50 ppm; (b) 100 ppm; and (c) 500 ppm. Wavelength: 430 nm; temperature: 220 °C; and air flow rate: 240 mL/min.
CTL response profiles of benzene on magnesium silicate. The CTL response profiles of benzene vapor on the surface of magnesium silicate were investigated by injecting the vapor at different concentrations into a carrier gas at 240 mL/min and 220 °C with a band-pass filter of 430 nm. Figure 9 shows the CTL response curves at different concentrations (50, 100 or 500 ppm). CTL emission profiles were similar to each other, and
Figure 10. The selectivity towards benzene of the prepared CTL sensor system based on magnesium silicate. Concentration: 100 ppm; wavelength: 430 nm; temperature: 220 °C; flow rate: 240 mL/min. 1, butanol; 2, butanone; 3, methanol; 4, propanol; 5, formaldehyde; 6, benzene; 7, xylene; 8, ammonia; 9, toluene; 10, acetaldehyde.
Figure 7. Calibration curve between CTL intensity and benzene concentration. Wavelength: 430 nm; temperature: 220 °C; and air flow rate: 240 mL/min.
CTL signal increased from baseline to maximum value in
