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Sensors and Actuators B 148 (2010) 292–297

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

novel cataluminescence gas sensor based on MgO thin film

ing Tao, Xiaoan Cao ∗, Yan Peng, Yonghui Liunvironmental Science and Engineering Institute, Guangzhou University, No. 230 Wai-Huan West Road, 510006 Guangzhou, PR China

r t i c l e i n f o

rticle history:eceived 17 February 2010eceived in revised form 21 April 2010ccepted 28 April 2010

a b s t r a c t

Using MgO film as sensing material, a cataluminescence sensor was proposed by the determinationof ethylene glycol ethers (2-ethoxyethanol and 2-methoxyethanol). This ethylene glycol ethers sensorshowed high sensitivity and specificity. With detection limits of 1.0 ppm and 1.4 ppm, the linear rangesof cataluminescence intensity versus ethylene glycol ethers concentrations were 2.0–2000 ppm for 2-

vailable online 5 May 2010

eywords:-Methoxyethanol-EthoxyethanolgO film

hemiluminescence

ethoxyethanol and 2.0–1500 ppm for 2-methoxyethanol, respectively. The response time was less than5 s. Foreign substances passed through the surface of MgO film without response, such as ammonia,benzene, ethyl acetate, acetaldehyde, vinyl acetate, methanol, acetone, ethanol, acetic acid, formaldehyde,and isopropyl ether. The sensor could determine 2-ethoxyethanol and 2-methoxyethanol whether theyexisted alone or together in air samples.

© 2010 Elsevier B.V. All rights reserved.

as sensor

. Introduction

Ethylene glycol ethers are frequently used as solvents, deter-ents, and emulsifiers alone or as components in numerousndustrial and domestic products such as paints, varnishes, inks,osmetics, and cleaning agents industrial. There exist extensiveiteratures reporting reproductive, testicular, embryotoxic, ter-togenic and hematologic toxicity of ethylene glycol ethers innimal experiments [1–3]. In the 1990s ethylene glycol ethersere reported be responsible for a higher risk level of miscarriage

mongst women workers in semiconductor factories in USA [4,5].2-Methoxyethanol (EM, CAS No. 109-86-4) and 2-

thoxyethanol (EE, CAS No. 110-80-5) are two typical members ofndustrial solvents collectively known as ethylene glycol ethers.hey are mainly used as solvents, colorant, and stabilizer in dyesndustry. The standard permitted concentrations of EM and EEapors in ambient air are less than 8.8 ppm and 8.9 ppm ruledy the Occupational Exposure Limit for hazardous agents in theorkplace (GBZ 2-2002, China). Short-chained ethylene glycol

thers are phased out due to their health hazards, EE has beenorbidden in Germany. However, EM and EE are still widely usedn most of other countries [6,7]. The ethylene glycol ethers may

eadily enter the body by inhalation as well as dermal uptake. Aapid detecting technique of EM or EE is very important in modernndustry.

∗ Corresponding author. Tel.: +86 20 39366937; fax: +86 20 39366946.E-mail address: caoxiaoan2003@yahoo.com.cn (X. Cao).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.04.043

Primary methods determined EM and EE are based on biologicalmonitoring [8], gas chromatograph equipped with flame ionizationdetector (GC-FID) [9,10], gas chromatography–mass spectrome-try (GC/MS) [11], and UV/vis spectrometer [12]. They possessedhigh performance and sensitivity, however, with difficult oper-ation and no real-time detection. Gas sensors have advantagesof on-site and real-time detection of hazard vapors. There is nospecific sensor for detection of EM and EE. So, ethylene glycolethers sensor was developed to detect gaseous quickly in work-place.

In 1976, Breysse et al. observed that a weak catalytic lumines-cence phenomenon occurred during catalytic oxidation of CO onThO2 surface [13]. This chemiluminescence mode was defined ascataluminescence (CTL). In the 1990s, Nakagawa et al. reportedCTL sensors using �-Al2O3 as catalyst material for determina-tion of ethanol and acetone [14–18]. Zhang et al. developedthe CTL sensors, a porous alumina film with 0.5 mm thick wasused for detecting saccharides [19]. Many nano-materials such asTiO2, ZrO2, BaCO3, SrCO3, ZnO, �-Al2O3, La2O3, �-Al2O3 + Nd2O3,V2Ti4O13, and Y2O3 were applied to detect organic vapors ofethanol, acetaldehyde, pinacolyl alcohol, propane, butane, acetone,ethylene dichloride, benzaldehyde, ethyl ether, and acetic acid withhigh sensitivity and specificity [20–33]. Cao reported that the vinylacetate sensor based on MgO nanoparticles showed good catalumi-nescence characters [34]. New development of CTL array sensors

can distinguish several different gases [35]. CTL gas sensors haveadvantages of on-site and real-time detection, small size, and noneed of lamp-house. Nevertheless, it is still a challenge for scien-tists to obtain better sensitive and specific CTL sensors to some toxicgases since their low level of concentrations in the air.

Y. Tao et al. / Sensors and Actuators B 148 (2010) 292–297 293

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tific). XPS analysis was using a Mono Al K� (1486.6 eV), X-ray sourceoperating at 150 W. The vacuum of the analysis chamber was betterthan 2 × 10−9 mbar. All spectra were acquired at a fixed analyzerenergy mode. Pass energy of 150 eV was applied to wide scans,

Table 1Chemical composition of the MgO thin film surface.

Name Peak BE (eV) FWHM (eV) Atomic percent (%)

C 1s 284.75 1.83 31.53

Fig. 1. Schematic diagram of the CTL detection system.

In chemistry sensors, film catalysts were used to improve stabil-ty, sensitivity and specificity. ZnO and SnO2 thin film sensors wereeported to enhance the sensitivity of CO and NO2 [36,37]. MgO iswidely used catalysis, its thin film has also been considered as a

uitable material for technological applications. It can be used asensor elements for humidity sensor [38] and CH3OH adsorption39]. However, there are few thin films used in the catalumines-ence sensors. Most of nano-materials in cataluminescence sensorsre in powder state and might easily come off the substrates. Sincehin films could attach firmly and uniformly to the substrate, MgOhin film was synthesized and characterized in the present study. Aew CTL-based gas sensor using MgO thin film for the detection ofthylene glycol ethers vapors was developed. MgO thin film exhib-ted the highest sensitivity of EE and EM and showed good analyticalharacters. To the best of our knowledge, EE and EM sensors haveeen reported in the present work firstly.

. Materials and methods

.1. Apparatus

The schematic diagram of the detection system was shown inig. 1. The catalyst was coated as a layer on the ceramic heatingubes which were coated with nano-size gold. The 5 mm in diam-ter ceramic heating tube was put into a 12 mm inner-diameteruartz tube. The temperature of the catalysts could be adjusted byontrolling the voltage of the heating tube. The air from the pumpas mixed with detecting vapor and flowed through the quartz

ube, a catalytic reaction occurred on the surface of catalysts. TheTL intensity was measured by a BPCL Ultra Weak Chemilumines-ence Analyzer (Biophysics Institute of Chinese Academy of Science,R China).

.2. Synthesis and characterization of thin film materials

All regents were of analytical grade. Indium chlorideInCl3·4H2O) was obtained from Reagent No. 1 Factory of Shanghaihemical Reagent Co., Ltd. Magnesium chloride (MgCl2·6H2O),odium citrate (Na3C6H5O7·2H2O), polyethylene glycol (PEG-000), zirconium nitrate (Zr(NO3)4·3H2O), and oxalic acid wereurchased from Tianjin Damao Chemical regent Factory.

In2O3, MgO and ZrO2 were prepared by sol–gel method. (1)

n2O3 precursor was synthesized as follows: 0.65 g InCl3·4H2O and.0 g PEG-2000 were dissolved in 30 mL distilled water with vig-rous stirring for 30 min. Then 10% ammonia solution was addedntil a white precipitate of In(OH)3 was formed, further additionf ammonia solution resulted in dissolving of the precipitate until

Fig. 2. TFE-SEM photo of MgO thin film.

pH value reached 5. Then stirred for another 1 h. (2) MgO precursorwas synthesized as follows: 2.5 g MgCl2·6H2O and 2.5 g PEG-6000were dissolved in 50 mL distilled water with vigorous stirring for20 min, NaOH solution was added into the mixture until pH valuereached 12. Keep on stirring for 1 h. (3) ZrO2 precursor was synthe-sized as the above method, the molar ratio of zirconium nitrate tooxalic acid was 4.5:1.

Fabrication of the thin films: Ceramic heating tubes which werecoated with gold were used as the solid substrate for film growth.The substrates were washed with hydrochloric acid, ethanol, anddistilled water in sequence and dried in oven at last. Thin film sys-tems were prepared by using the dip coating method from eachcolloidal and sol–gel solution. MgO thin films were synthesizedin the following way: The ceramic substrates were immersed intothe MgO precursor solution for five times, dried at 80 ◦C for 1 hand decomposed to MgO when calcined at 450 ◦C for 1 h. Otherfilms such as In2O3 and ZrO2 were synthesized following the samemethod.

MgO film produced in the present work was characterized forits surface morphology and chemical composition. The microstruc-ture of the film was investigated by Quanta 400 thermal fieldemission environment scanning electron microscope (TFE-SEM).Specimens were observed at accelerating voltages <20 kV. Fig. 2showed the surface morphology of the film. The thickness of thefilm was measured by dektak 150 surface profiler (Veeco Instru-ments Inc.). Subsequently, chemical compositions of the annealedfilms were determined using the X-ray photoelectron spectroscopy(XPS, model: ESCALAB 250; manufacturer: Thermo Fisher Scien-

O 1s 532.52 2.75 44.84Mg 1s 1305.89 2.92 18.36Cl 2p 199.71 2.59 1.61F 1s 685.95 2.12 0.99Na 1s 1073.97 3.98 2.66

294 Y. Tao et al. / Sensors and Actuators B 148 (2010) 292–297

Table 2Comparison of signal-to-noise ratio of six air pollutions on some materials.

Materials Signal-to-noise ratio (S/N) Selective conditions

EM EE Ethanol Formaldehyde Acetone Benzene

3 1.24 Not detectable 296 ◦C, 425 mt detectable 1.08 0.59 279 ◦C, 425 m5 1.06 Not detectable 279 ◦C, 425 m

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Fig. 4. Wavelength dependence of CTL intensity of EE and EM vapors. (1) EE:

In2O3 0.73 0.63 0.7 0.4ZrO2 1.69 Not detectable 1.19 NoMgO 64.52 104.91 0.43 0.5

hile 20 eV was used for narrow region scans. The binding energiesere calibrated with reference to C 1s at 284.8 eV for adventitiousydrocarbon contamination. The analysis area was 500 �m with aypical analysis depth of 5–10 nm.

. Results and discussion

.1. Structure of the thin film material

The surface morphology of MgO thin film was shown in Fig. 2. Itould be seen in Fig. 2 that MgO film has a plate like structure. Thehickness of the films depends on the dig time. And the thicknessesf MgO and In2O3 films are 0.23 �m and 0.20 �m, respectively.

XPS analysis was used to detect the element ingredients of MgOlm. Table 1 summarized the atomic weight ratios of elements andurface chemical compositions of MgO film, which were calculatedrom the experimental data. A single peak of Mg 1s was obtained at305 eV, which indicated that the material was MgO. Air pollutionsaused high atomic weight ratios of elements C and O.

.2. Estimation of the thin film materials for the sensing mode

To select the appropriate sensing materials for ethylene glycolthers sensor, In2O3, ZrO2, and MgO thin films were examined. TheTL responses on the surface of these films were detected whenll the organic vapors were 500 ppm. All detection conditions inable 2 were optimal conditions of using each sensing materialased on the experiments’ results. The data showed that MgO filmas the best material to obtain the highest signal-to-noise ratio.

t could be attributed to a combination of materials’ differencesn (i) diffusivity, (ii) adsorption behavior, and (iii) intrinsic reac-ion rate (restrictions on the size of the transition state) [40,41].

herefore, MgO film was selected for the subsequent study in theresent work. The work on MgO film showed that film at thicknessf 0.2 �m was stable, good reproducibility, and will not come off theubstrate. So, all the thin films were prepared about this thickness.

ig. 3. CTL response profiles of EE and EM. (1) EE: temperature: 279 ◦C, flowate: 300 mL/min, wavelength: 425 nm, and concentration: 500 ppm; (2) EM: tem-erature: 279 ◦C, flow rate: 210 mL/min, wavelength: 425 nm, and concentration:00 ppm.

temperature: 279 ◦C, flow rate: 300 mL/min, and concentration: 500 ppm; (2) EM:temperature: 279 ◦C, flow rate: 210 mL/min, and concentration: 500 ppm.

3.3. CTL response profile of ethylene glycol ethers on MgO film

The CTL response profiles of EE and EM on MgO film were inves-tigated by injecting each vapor into the carrier gas with certain flowrates. Fig. 3 showed the CTL response profiles of samples containingEE or EM at 279 ◦C with a bandpass filter of 425 nm. Curves 1 and 2denoted the results for EE and EM with the same concentration of500 ppm. The CTL response profiles were similar to each other. Thepeak of each curve appeared at about 5 s after sample injection andthe half decay time of each curve was about 45 s, which indicatedthat the EE and EM sensing were fast processes.

3.4. Optimization of wavelengths

In order to investigate the effect of wavelength on the CTL inten-sity, six bandpass filters (400, 425, 440, 460, 490, and 535 nm)were used to select the optimal wavelength. The noise signal pro-duced from incandescent radiation of the ceramic heater substrateincreased towards longer wavelength, so signal-to-noise ratioswere used to show the CTL intensity. From Fig. 4, the optimal wave-length for CTL was 425 nm to detect EE and EM. This wavelengthwas used for the quantitative analysis because of the lower incan-descent radiation.

3.5. Optimization of working temperature

The effect of working temperature on CTL was shown in Fig. 5.As shown in Fig. 5, the incandescent radiation noise of sub-

strate increased markedly above 320 ◦C, therefore 279 ◦C wasselected as the optimum detection temperature owing to the max-imum signal-to-noise ratio. Moreover, the noise produced fromincandescent radiation was still at a very low level at this tem-perature. Therefore, 279 ◦C was used for the subsequent study.The optimum detection temperature of EM was the same asEE’s.

Y. Tao et al. / Sensors and Actuators B 148 (2010) 292–297 295

Fig. 5. Temperature dependence of CTL intensity of EE and EM vapors. (1) EE:wavelength: 425 nm, flow rate: 300 mL/min, and concentration: 500 ppm; (2) EM:wavelength: 425 nm, flow rate: 210 mL/min, and concentration: 500 ppm.

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ig. 6. Effect of flow rate of carrier air on CTL intensity of EE and EM vapors. Tem-erature: 279 ◦C; wavelength: 425 nm and concentration: 500 ppm.

.6. Optimization of flow rate of carrier gas

The flow rate dependence on CTL intensity was investigatedanged from 50 to 395 mL/min. As illustrated in Fig. 6 (EE), the CTLntensity increased gradually with the increase of flow rate from 50o 300 mL/min. When the flow rate was above 300 mL/min, the CTLntensity deceased slightly. Fig. 6 (EM) showed the similar trend,owever, 210 mL/min was the inflexion. The results showed thathe catalytic oxidation was under a diffusion controlled conditionhen the flow rate was below 300 mL/min or 210 mL/min. The rea-

on for this case might be that the reaction time between EE, EMnd MgO film would not be sufficient when flow rates of carrier gasncreased to above 300 mL/min or 210 mL/min. So, 300 mL/min and10 mL/min were finally chosen for detection.

.7. Analytical characteristics

Under the optimal conditions described above, the calibra-ion curves of CTL intensity versus EE or EM concentrationere linear in the range of 2.0–2000 ppm or 2.0–1500 ppm. The

able 3thylene glycol ethers analysis in artificial samples.

Sample no. Composition Prepared values (ppm)1 EE 1000

EM 250

2 EE 1000Ethanol 200

Fig. 7. CTL responses of different compounds on the sensor. Temperature: 279 ◦C;wavelength: 425 nm; flow rate: 250 mL/min; and concentration: 500 ppm.

linear regression equations of EE and EM were described byI = 7.289C + 2.504 (r = 0.9981) and I = 5.088C − 47.578 (r = 0.9976),respectively. Where I was the relative CTL intensity, C was the con-centration of EE or EM (ppm) and r was the regression coefficient.The detection limit was 1.0 ppm for EE and 1.4 for EM. Since theregression equations of EE and EM were a little different, approx-imate total amounts of ethylene glycol ethers could be obtained ifthey coexisted in one sample (see sample 1 in Table 3).

According to the Occupational Exposure Limit for hazardousagents in the workplace (GBZ 2-2002, China), the maximum allow-able concentrations of EE and EM vapor in air are less than 8.9 ppmand 8.8 ppm. The detection limits of the EE and EM vapors arebelow the standard permitted concentrations, therefore, the sensorcan be used for air quality monitoring of ethylene glycol ethers inworkplace.

3.8. Specificity of MgO thin film gas sensor

Eleven kinds of vapors, which might coexist with EE and EM inair, were detected respectively under the optimal conditions. Asshown in Fig. 7, vapors of ammonia, benzene, and ethyl acetate hadno interference with EE or EM. While, acetaldehyde, vinyl acetate,methanol, acetone, ethanol, acetic acid, formaldehyde, and iso-propyl ether caused interferences at levels around 0.77%, 2.01%,0.87%, 0.66%, 0.40%, 0.61%, 0.48%, and 1.93% compared with theresponse of EE. Therefore, the sensor was feasibility for the deter-minations of EE and EM in air owing to significantly specificity toethylene glycol ethers.

3.9. Lifetime of the sensor

The lifetime of the sensor was tested at the optimal conditions.The CTL intensities were measured once per 5 h by continuouslyintroducing 500 ppm of EE for 100 h in the carrier gas through thesensor. No significant decrease of CTL intensity was observed dur-

Measured values (ppm, n = 6) Recovery (%)1370 ± 87 110

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ng the 100 h detection. The relative standard deviations were 2.6%n = 26) for 500 ppm EE, 2.4% (n = 26) for 500 ppm EM. The resultsndicated the durability of the MgO film based sensor.

.10. Determination of ethylene glycol ethers in the syntheticamples

In order to test the feasibility of the sensor, two artificial sam-les containing known concentrations of EE and EM, EE and ethanolere analyzed under the optimal conditions. Since EE and EM

howed a little different sensitivity, approximate total amountsf EE and EM could be obtained if they coexisted in one sample.s shown in Table 3, the recovery of sample 1 was obtained from

he ratio of measured value to aggregate prepared value of EE andM, while the recovery of sample 2 was obtained from the ratiof measured value to prepared value of EE. Satisfactory recoveriesere obtained. As a result of this experiment, EE and EM could beetected separately or when they coexist.

.11. Mechanism

The mechanisms about the oxidation of EE and EM on nano-atalysts have not been studied yet. Lv et al. reported that ethylther would oxidized to acetaldehyde and then to acetic acid, dur-ng which main luminous intermediates CH3CO* were generatednd emitted light [42]. Therefore, possible mechanisms of the cata-uminescence of EM and EE are as the following:

or EM : CH3OCH2CH2OH + O2MgO−→CH3OCH2CHO∗

H3OCH2CHO∗ → CH3CHO + h�

or EE : CH3CH2OCH2CH2OH + O2MgO−→CH3CH2OCH2CHO∗

H3CH2OCH2CHO∗ → CH3CHO + h�

When EM or EE vapors passed through the surface of the MgOhin film, they were catalytically oxidized by O2 in the air. Thelectronically excited methoxy acetaldehyde (CH3OCH2CHO*) orthoxy acetaldehyde (CH3CH2OCH2CHO*) could be produced andbsorbed on the MgO film during the reaction and generated pho-oemission when they returned to their ground states.

. Conclusions

The EE and EM vapors sensor based on MgO thin film had beennvestigated for the first time in this paper. The results showed thathe sensor possessed rapid response, high sensitivity, satisfactoryurability, and excellent specificity to EE and EM among some pos-ible coexistence substances in air. The sensor can detect EE andM whether the gas exist alone or together. It shows the prospector EE and EM determination in industry and environment moni-oring. This paper is also valuable for the future research to developataluminescence sensors based on thin film catalysts.

cknowledgements

The authors gratefully thank for the financial support byhe National Natural Science Foundation of China (20677013),atural Science Foundation of Guangdong Province, China

8151009101000130).

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Biographies

Ying Tao completed her undergraduate studies in the Chemical Technology Insti-tute, Wuhan University of Science and Technology, China. And is currently apostgraduate student at the Environmental Science and Engineering Institute,Guangzhou University, China. Her major is Environmental Chemistry and focus oncataluminescence and the application of the gas sensor.

Xiaoan Cao received her BS degree in 1982 from Department of Chemistry, JiangxiUniversity, China. She is currently working in the Institute of Environmental Scienceand Engineering, Guangzhou University. She has been a professor of GuangzhouUniversity since 2002. Her interest is in the investigation of cataluminescence andthe application to the gas sensor.

Yan Peng received her MS degree in 2001 from East China Geological Institute (now,East China Institute of Technology) China. Now she is a PhD candidate of ChinaUniversity of Geosciences and majors in environmental engineering. Her research

interest is focused mainly on analytical of volatile organic compounds in indoor air.

Yonghui Liu received her PhD degree in 2004 from Dalian University of Science &Technology, China. Now she works in Institute of Environmental Science & Engi-neering at Guangzhou University, China. Her major research interests focus mainlyon VOCs detection and degradation.