- Email: [email protected]
Environmental temperature-independent gas sensor array based on polymer composite
Sensors and Actuators B 108 (2005) 258–264
Environmental temperature-independent gas sensor array based on polymer composite Seung-Chul Ha∗ , Yoonseok Yang, Yong Shin Kim, Soo-Hyun Kim, Young Jun Kim, Seong Mok Cho Bio MEMS Group, Basic Research Laboratory, Electronics and Telecommunications Research Institute, 161 Kajong-dong, Yusong-gu, Taejon 305-350, Republic of Korea Received 11 July 2004; received in revised form 23 September 2004; accepted 18 October 2004
Abstract This paper reports the effect of gas temperature on the response of sensor array based on polymer–carbon black composite. Temperaturecontrolled chemical vapors of chloroform, methanol, benzene, acetone, and cyclohexane were injected into the sensor array equipped with a micro heater. The temperature of the sample gas was varied (20, 40, and 60 ◦ C) and the resistance change was measured with the sensor films thermostatted at the temperature of 25 and 45 ◦ C, using embedded micro heater. The maximum relative differential resistance of polymer composite was severely affected by the gas temperature, and the response pattern of the sensor array also changed with the gas temperature. Consequently, the chemical vapors controlled at the temperature of 20 and 40 ◦ C were recognized differently in principal component space, even though the same concentration of chemical vapor was injected. However, when the polymer detector was maintained at a temperature of 45 ◦ C, radial-response patterns for the gas temperature of 20 and 40 ◦ C showed similar shape and were recognized identically in the projected principal component space. © 2004 Elsevier B.V. All rights reserved. Keywords: Gas temperature; Sensor array; Polymer–carbon black composite; Partition coefficient
1. Introduction As the ability to monitor and detect various chemical gases has been important for many applications [1–4], researches for various types of gas sensors have gained increasing focus. Of these gas sensors, polymer based sensors, such as carbon black–polymer composite [5–7], conducting polymer composite [8], polymer-coated quartz crystal microbalances (QCM) [9], and polymer-coated surface acoustic wave (SAW) devices [10,11], have advantage that these sensors are operable at room temperature with low electric power consumption. Due to its low power consumption, polymer composite sensor has benefit to be used as a detector of portable chemi∗
Corresponding author. Tel.: +82 428601474; fax: +82 428606836. E-mail address: [email protected] (S.-C. Ha).
0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.10.044
cal sensor array. For example, Cyrano Science has produced hand held e-nose based on carbon black–polymer composite sensors. Polymer composite sensor can also readily modulate selectivity through not only choosing different polymers as matrix but also adding plasticizer [12]. The magnitude of interaction with chemical vapors of these sorption-based sensors relies primarily on the partition coefficient of the gaseous analyte into the polymer. Sensor array, in which each sensor was formed with a chemically different polymer, has been demonstrated to allow discrimination among various chemical vapors based on the differences in response patterns produced by the sensor array [5,6]. Moreover, polymer composite sensor is inexpensive, easily controlled, and robust in many different environments. In spite of such advantages, polymer composite sensor has drawback as a detector of portable sensor array because its response and resistance is significantly changed by the environmental temperature,
S.-C. Ha et al. / Sensors and Actuators B 108 (2005) 258–264
as the temperature strongly affects the partition coefficients for polymer–vapor interactions. In the previous work [13], we reported a new structure suitable for sensor array based on polymer composite. The structure consisted of 16 separate sensors; each sensor was equipped with an interdigitated electrode, integrated micro heater and micromachined well that supplied reservoir to reproducibly contain the polymer composite–solvent solution in a designed position. Using the structure of sensor array, we tested the effect of sample temperature on the response of polymer composite based sensor array, and found the way to eliminate the effect of environmental temperature.
2. Experiment The substrate for sensor array used in the experiment is shown in Fig. 1(a). The sensor array was fabricated in a fourmask process of double side-polished (1 0 0) silicon wafer. Using lift-off process, interdigitated electrode (Au) and micro heater (Pt) was fabricated. To evaluate membrane structure, anisotropic wet etching process was carried out on rear
259
side of 5 in. silicon wafer by using 5 wt.% tetra methyl ammonium hydroxide (TMAH) solution. A sensor array chip consisting of 16 separate sensors is 30 mm × 14 mm. As shown in Fig. 1(b), the micromachined well, formed by the anisotropic wet etching, acted as a container of dropping polymer composite–solvent solution. The fabrication process of sensor array and the effect of micromachined well on the dispensing process as well as the formation of sensor film were described in detail [13]. Pt microheater embedded in sensor structure, as shown in Fig. 1(c), proved typical heating characteristic of resistance heater. The micro heater was operated at constant current of 1.6 mA to heat up the sensor film. The micro heater located on membrane consumed about 7 mA to maintain the temperature of 45 ◦ C. Polymers used in sensor array were described in Table 1. Polymers were purchased from Aldrich Inc. Black Pearls 2000 or 700 carbon black, depending on the resistance of composite film and dispersion property, from Cabot Corp. was used to fabricate polymer composite sensor. Eighty-four milligrams of one of the polymers listed in Table 1 was dissolved in 10 ml of chloroform or tetrahydrofuran (THF), depending on the solubility. The solution was agitated in an ultra-sonic bath. Once the polymer was fully dissolved, 16 mg carbon black was added to the solution and agitated further to improve uniform dispersion. Well-dispersed solution of polymer composite-solvent was dispensed on the interdigitated electrode formed on the membrane. An automated micropipette, produced by Matrix Technologies Corp. and capable of dispensing sub-microliter of volumes reproducibly, was used to control the dispensed volume of 1.5–3.5 l. Fig. 2 shows the schematic diagram of dilution flow system that was consisting of LabVIEW software, a laptop computer, water bathes, and electronically controlled solenoid valves and mass flow controllers. A carrier gas was passed through a bubbler filled with the chemical of choice, and diluted with a dilution gas. Both gases used for all experiments were ultra-pure air (99.999%) obtained from a general gas refinery. Water bath 1 was used to control vapor pressure of the chemical in the bubbler. Temperature of sample gas, carried into the detection chamber, was adjusted through the water bath 2. As shown in Fig. 2, gas line before the detection chamber was fully immersed in water. The length of gas Table 1 Polymers used to make polymer composite sensors
Fig. 1. Photographs of sensor array consisting of 16 separate sensors with an interdigital electrode, micro heater, and micro-machined membrane: (a) front side; (b) rear side; and (c) micromachined well of sensor array.
Sensor ID
Polymers
S1 S2 S3 S4 S5 S6 S7
Cellulose acetate Poly (caprolactone) Poly (vinyl butyral)-co-vinyl alcohol-co-vinyl acetate Hydroxypropyl cellulosea Ethyl cellulose Poly (vinyl stearate) Polystyrene-black-polyisoprene-black-polystyrene
a
Plasticized with 50% di (ethylene glycol) dibenzoate by mass.
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Fig. 2. Schematic diagram of the dilution flow system for measuring gassensing property.
line immersed in water is long enough to make the temperature of the sample gas adjustable to the temperature of water contained in the water bath 2. Using thermometer, we measured the temperature of sample gas and confirmed that the temperature of sample gas was well controlled by changing the temperature of water in the water bath 2. The temperature of sample gas was varied to 20, 40, and 60 ◦ C in this experiment to find the effect of environmental temperature on the response of sensor array based on the polymer composite, and micro heater temperature was varied between 25 and 45 ◦ C. The resistance change of polymer composite film was monitored in response to the incorporation of chemical vapor. The resistance was amplified by 20 times and recorded every 0.1 s. Measurement was performed after the sensor array was placed into the chamber and the signal of resistance was stabilized. Each measurement consisted of three steps of stabilization, exposure, and purge. The following chemicals were tested: acetone, benzene, chloroform, cyclohexane, and methanol.
3. Results and discussion The sensor time series resistances were normalized by subtracting the baseline resistance value R0 from each data point R in the time series and then dividing by R0 [(R − R0 )/R0 ] × 100 This normalization is typically used for gas sensor based on polymer composite to set off the difference of initial resistances. Fig. 3 shows the maximum relative differential resistance of the sensor array maintained at the micro heater temperature of 25 and 45 ◦ C, when the sensor array was exposed to the sample gas of different temperature (20, 40, and 60 ◦ C). Two thousand parts per million of (a) chloroform and (b) methanol were used as sample gases. The maximum resistance (Rmax ) is equilibrated resis-
Fig. 3. Maximum relative differential resistance of sensor array exposed to 2000 ppm of: (a) chloroform; and (b) methanol controlled at the temperature of 20, 40, and 60 ◦ C. Measurements were performed at the micro heater temperature of 25 and 45 ◦ C.
tance value when the sensor is sufficiently exposed to sample gas. The maximum resistance change of sensor array decreased as the micro heater temperature increased from 25 to 45 ◦ C. Higher responses of polymer composites at the lower temperature are due to an increasing tendency for condensation as a result of the larger chemical potential of the molecules in the polymer phase compared with the gas phase. When the micro heater temperature was maintained at 25 ◦ C, the maximum response of sensor array dramatically changed with the gas temperature. The response changes of sensors with the gas temperature were divergent to one another, which means that the response pattern of sensor array would be changed with the temperature of sample. Consequently, when the gas temperature is changed, it will be recognized as a different gas even though the sensor array is exposed to the same concentration of the target. On the other hand, when the polymer detector was heated to the temperature of 45 ◦ C, the maximum resistance changes of sensors were little affected by the temperature of sample gas in both the cases: (a) chloroform and (b) methanol, as shown in Fig. 3. Figs. 4 and 5 show the radial-response pattern with the variation of the gas temperature when the sensor array is exposed to 2000 ppm of chloroform (Fig. 4) and methanol
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Fig. 4. Radial-response patterns of sensor array when the sensor array was exposed to 2000 ppm chloroform vapor of temperature 20, 40, and 60 ◦ C. Sensor array was thermostatted to the temperature of 25 and 45 ◦ C by using an embedded micro heater.
(Fig. 5). When the polymer detector was not heated, the shape of radial-response pattern of the sensor array changed with the increase in gas temperature. However, radial-response pattern of the sensor array measured at the micro heater temperature of 45 ◦ C showed similar shape for the gas temperature of 20 and 40 ◦ C, but radial-response pattern for sample gas of 60 ◦ C showed different shape from the pattern for the gas temperature of 20 and 40 ◦ C. This means that the gas temperature little affects the response pattern of the sensor array when polymer detectors are maintained at a constant temperature above that of sample gas. The bulky solubility of molecules from the gas phase in the polymer composite is dominated by the partition coefficient: fp/g =
i cpolymer i cgas
(1)
which is defined as the gas of i concentration dissolved in a polymer composite divided by the concentration of the same volume in a gas phase. When the polymer detector is thermostatted at a constant temperature of 45 ◦ C, the partition coefficient seems to be independent of gas temperature over the specific operating range of 0–45 ◦ C, because the detector temperature is the dominant temperature for thermodynamics of the polymer detector–vapor interaction. Though the gas temperature does not affect the partition coefficient for detector–vapor interactions when the gas temperature is lower than that of detector, sample temperature can affect the concentration of the sample, which seems to be the cause of the little difference in the shapes of response patterns for the gas temperature of 20 and 40 ◦ C. A more quantitative approach to evaluate the effect of gas temperature on the sensor–vapor interaction was provided
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Fig. 5. Radial-response patterns of sensor array when the sensor array was exposed to 2000 ppm methanol vapor of temperature of 20, 40, and 60 ◦ C. Sensor array was thermostatted to the temperature of 25 and 45 ◦ C by using an embedded micro heater.
by principal component analysis (PCA), as shown in Fig. 6. At first, the sensor array maintained at the temperature of 25 ◦ C was exposed to 2000 ppm of five different chemical vapors controlled at the temperature of 20 ◦ C: acetone, benzene, chloroform, cyclohexane, and methanol. And the chemical vapors of chloroform (a) and methanol (c) adjusted to the temperature of 40 ◦ C were injected to the sensor array. As shown in Fig. 6(a) and (c), when the sensor array was not heated to the temperature higher than that of the sample gas, the response pattern was significantly affected by the gas temperature and recognized as different chemical in projected principal component space. On the other hand, when the sensor array was thermostatted at a temperature of 45 ◦ C, as shown in Fig. 6(b) and (d), the effect of gas temperature was seemingly negligible. Data measured at different gas temperatures were represented in same group in princi-
pal component space, which means that the sensor array can recognize the sample gas even though the gas temperature was changed. Using micro heater embedded in the structure of sensor array, we developed a portable sensor array that could recognize chemicals regardless of environmental temperature. Problem still exists in case of the mixture sample. For single component gas, sample temperature only affects total concentration, and changes in total concentration can typically be corrected by normalization of the maximum responses. However, for mixtures sample temperature affects composition as well as total concentration. Since composition of mixture is changed with the temperature, database developed at one sample temperature cannot be valid for another temperature. Therefore, the database that includes all sample temperatures, which depends on the application, is necessary to recognize mixture sample.
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Fig. 6. Data obtained from the injection of chemical vapors (acetone, benzene, chloroform, cyclohexane, and methanol) controlled at the temperature of 20 ◦ C into the sensor array, maintained at the temperature of 25 ◦ C (a and c) and 45 ◦ C (b and d) were represented in principal component space, and the responses of sensor array for the chloroform (a and b) and methanol (c and d) heated to the temperature of 40 ◦ C were represented together in principal component space.
4. Conclusion We tested the effect of gas temperature on the response of sensor array based on polymer composite using chemical vapor of chloroform and methanol. Gas condensation process from the gas phase into the polymer was significantly affected by the gas temperature, and maximum relative differential resistances of the sensors were dramatically changed with the gas temperature. Response pattern of the sensor array was also changed with the gas temperature, and so chemical vapors with the temperature of 20 and 40 ◦ C were recognized differently, even though concentrations of both samples were maintained identically. However, when the polymer detector was thermostatted at the temperature of 45 ◦ C, radial-response patterns for the chemical-vapor, controlled at the temperature of 20 and 40 ◦ C, showed similar shape and were recognized identically in projected principal component space. The partition coefficient was independent of gas temperature over the specific operating range when the polymer
detector was thermostatted at a temperature higher than the temperature of operating range, because the detector temperature is dominant temperature for the thermodynamics of the polymer detector–vapor interaction.
Acknowledgements This work has been supported in part by the Ministry of Information and Communication of Korea and in part by the Ministry of Science and Technology of Korea through the NRL program.
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Biographies Seung-Chul Ha received his BSc degree (1997) in materials science and engineering from the Pohang University and an MSc degree (1999) in materials science and engineering from the Seoul National University. He had researched various thin film processes and devices in Hynix Semicondutor Co. Ltd. (1999–2002). He is currently a research engineer in
Electronics and Telecommunications Research Institute (2002–current). His research interests are mainly focused on the fabrication of intelligent sensors including monolithic electronic nose and nanoporous materials for gas sensor. Yoon Seok Yang received the BS degree in control and instrumentation engineering from Seoul National University, Korea and MS degree in interdisciplinary course (biomedical engineering major) from Yonsei University, Korea. He received the PhD degree in Medical Electronics Laboratory (MELab) of the Seoul National University on the study of ultrasonic interrogation techniques in medical equipments, in 2002. Then he joined the BioMEMS group in Electronics and Telecommunications Research Institute (ETRI), Korea, where the major research activities are development of portable electronics and intelligent algorithms for medical and biological applications, such as biosensors. His research interests include DNA and molecular biology. Yong Shin Kim received a PhD degree in chemistry from Korea Advanced Institute of Science and Technology (KAIST), in 1997 with a study on the chemical dynamics of photodissociation and photoionization of fragmented halogen atoms. After obtaining his degree, he has worked as a research senior member at the Electronics and Telecommunications Research Institute (ETRI). Now his research activities are focused on the miniaturized and smart chemical sensor arrays based on MEMS technology and nanomaterials. Soo-Hyun Kim received the BS degree in metallurgical engineering from Seoul National University, Seoul, Korea, in 1997 and the MS and PhD degrees in materials science and engineering from Seoul National University, Seoul, Korea, in 1999 and 2003, respectively. Currently, he is a member of technical staff in memory research and development division in Hynix Semiconductor Inc., Korea. The focus of his current research is on the development of metallization process for advanced DRAM, which includes the silicides contact, diffusion barrier, interconnect and interfacial reaction in metallization system of DRAM. Young Jun Kim received his PhD in polymer science from the University of Akron, Akron Ohio, US in 1997. He, as a research associate, had been involved with the controlled synthesis of block and functional polymers, and their applications in surface modifications related with biocompatible surfaces and nanostructured materials at the University Connecticut and Princeton University until 2000. Since then at Electronics and Telecommunications Research Institute (ETRI) he has been involved with nanoencapsulating memory molecules and synthesizing nanoclusters for memory media and biosensors. Seong Mok Cho received the BS, MS, and PhD degree in materials engineering from Pohang University of Science and Technology (POSTECH), Korea, in 1992, 1994, and 2001, respectively. After getting MS degree, he had joined Samsung Electronics, Korea, in 1994, where he had worked for the development of lithography technology for 256M and 1G DRAM until 1996. He joined Electronics and Telecommunications Research Institute (ETRI), Korea, in 2001 as a senior researcher in the Basic Research laboratory. His current research interests are mainly focused on the sensor technologies such as infrared image sensor and electronic nose.
Environmental temperature-independent gas sensor array based on polymer composite Seung-Chul Ha∗ , Yoonseok Yang, Yong Shin Kim, Soo-Hyun Kim, Young Jun Kim, Seong Mok Cho Bio MEMS Group, Basic Research Laboratory, Electronics and Telecommunications Research Institute, 161 Kajong-dong, Yusong-gu, Taejon 305-350, Republic of Korea Received 11 July 2004; received in revised form 23 September 2004; accepted 18 October 2004
Abstract This paper reports the effect of gas temperature on the response of sensor array based on polymer–carbon black composite. Temperaturecontrolled chemical vapors of chloroform, methanol, benzene, acetone, and cyclohexane were injected into the sensor array equipped with a micro heater. The temperature of the sample gas was varied (20, 40, and 60 ◦ C) and the resistance change was measured with the sensor films thermostatted at the temperature of 25 and 45 ◦ C, using embedded micro heater. The maximum relative differential resistance of polymer composite was severely affected by the gas temperature, and the response pattern of the sensor array also changed with the gas temperature. Consequently, the chemical vapors controlled at the temperature of 20 and 40 ◦ C were recognized differently in principal component space, even though the same concentration of chemical vapor was injected. However, when the polymer detector was maintained at a temperature of 45 ◦ C, radial-response patterns for the gas temperature of 20 and 40 ◦ C showed similar shape and were recognized identically in the projected principal component space. © 2004 Elsevier B.V. All rights reserved. Keywords: Gas temperature; Sensor array; Polymer–carbon black composite; Partition coefficient
1. Introduction As the ability to monitor and detect various chemical gases has been important for many applications [1–4], researches for various types of gas sensors have gained increasing focus. Of these gas sensors, polymer based sensors, such as carbon black–polymer composite [5–7], conducting polymer composite [8], polymer-coated quartz crystal microbalances (QCM) [9], and polymer-coated surface acoustic wave (SAW) devices [10,11], have advantage that these sensors are operable at room temperature with low electric power consumption. Due to its low power consumption, polymer composite sensor has benefit to be used as a detector of portable chemi∗
Corresponding author. Tel.: +82 428601474; fax: +82 428606836. E-mail address: [email protected] (S.-C. Ha).
0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.10.044
cal sensor array. For example, Cyrano Science has produced hand held e-nose based on carbon black–polymer composite sensors. Polymer composite sensor can also readily modulate selectivity through not only choosing different polymers as matrix but also adding plasticizer [12]. The magnitude of interaction with chemical vapors of these sorption-based sensors relies primarily on the partition coefficient of the gaseous analyte into the polymer. Sensor array, in which each sensor was formed with a chemically different polymer, has been demonstrated to allow discrimination among various chemical vapors based on the differences in response patterns produced by the sensor array [5,6]. Moreover, polymer composite sensor is inexpensive, easily controlled, and robust in many different environments. In spite of such advantages, polymer composite sensor has drawback as a detector of portable sensor array because its response and resistance is significantly changed by the environmental temperature,
S.-C. Ha et al. / Sensors and Actuators B 108 (2005) 258–264
as the temperature strongly affects the partition coefficients for polymer–vapor interactions. In the previous work [13], we reported a new structure suitable for sensor array based on polymer composite. The structure consisted of 16 separate sensors; each sensor was equipped with an interdigitated electrode, integrated micro heater and micromachined well that supplied reservoir to reproducibly contain the polymer composite–solvent solution in a designed position. Using the structure of sensor array, we tested the effect of sample temperature on the response of polymer composite based sensor array, and found the way to eliminate the effect of environmental temperature.
2. Experiment The substrate for sensor array used in the experiment is shown in Fig. 1(a). The sensor array was fabricated in a fourmask process of double side-polished (1 0 0) silicon wafer. Using lift-off process, interdigitated electrode (Au) and micro heater (Pt) was fabricated. To evaluate membrane structure, anisotropic wet etching process was carried out on rear
259
side of 5 in. silicon wafer by using 5 wt.% tetra methyl ammonium hydroxide (TMAH) solution. A sensor array chip consisting of 16 separate sensors is 30 mm × 14 mm. As shown in Fig. 1(b), the micromachined well, formed by the anisotropic wet etching, acted as a container of dropping polymer composite–solvent solution. The fabrication process of sensor array and the effect of micromachined well on the dispensing process as well as the formation of sensor film were described in detail [13]. Pt microheater embedded in sensor structure, as shown in Fig. 1(c), proved typical heating characteristic of resistance heater. The micro heater was operated at constant current of 1.6 mA to heat up the sensor film. The micro heater located on membrane consumed about 7 mA to maintain the temperature of 45 ◦ C. Polymers used in sensor array were described in Table 1. Polymers were purchased from Aldrich Inc. Black Pearls 2000 or 700 carbon black, depending on the resistance of composite film and dispersion property, from Cabot Corp. was used to fabricate polymer composite sensor. Eighty-four milligrams of one of the polymers listed in Table 1 was dissolved in 10 ml of chloroform or tetrahydrofuran (THF), depending on the solubility. The solution was agitated in an ultra-sonic bath. Once the polymer was fully dissolved, 16 mg carbon black was added to the solution and agitated further to improve uniform dispersion. Well-dispersed solution of polymer composite-solvent was dispensed on the interdigitated electrode formed on the membrane. An automated micropipette, produced by Matrix Technologies Corp. and capable of dispensing sub-microliter of volumes reproducibly, was used to control the dispensed volume of 1.5–3.5 l. Fig. 2 shows the schematic diagram of dilution flow system that was consisting of LabVIEW software, a laptop computer, water bathes, and electronically controlled solenoid valves and mass flow controllers. A carrier gas was passed through a bubbler filled with the chemical of choice, and diluted with a dilution gas. Both gases used for all experiments were ultra-pure air (99.999%) obtained from a general gas refinery. Water bath 1 was used to control vapor pressure of the chemical in the bubbler. Temperature of sample gas, carried into the detection chamber, was adjusted through the water bath 2. As shown in Fig. 2, gas line before the detection chamber was fully immersed in water. The length of gas Table 1 Polymers used to make polymer composite sensors
Fig. 1. Photographs of sensor array consisting of 16 separate sensors with an interdigital electrode, micro heater, and micro-machined membrane: (a) front side; (b) rear side; and (c) micromachined well of sensor array.
Sensor ID
Polymers
S1 S2 S3 S4 S5 S6 S7
Cellulose acetate Poly (caprolactone) Poly (vinyl butyral)-co-vinyl alcohol-co-vinyl acetate Hydroxypropyl cellulosea Ethyl cellulose Poly (vinyl stearate) Polystyrene-black-polyisoprene-black-polystyrene
a
Plasticized with 50% di (ethylene glycol) dibenzoate by mass.
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Fig. 2. Schematic diagram of the dilution flow system for measuring gassensing property.
line immersed in water is long enough to make the temperature of the sample gas adjustable to the temperature of water contained in the water bath 2. Using thermometer, we measured the temperature of sample gas and confirmed that the temperature of sample gas was well controlled by changing the temperature of water in the water bath 2. The temperature of sample gas was varied to 20, 40, and 60 ◦ C in this experiment to find the effect of environmental temperature on the response of sensor array based on the polymer composite, and micro heater temperature was varied between 25 and 45 ◦ C. The resistance change of polymer composite film was monitored in response to the incorporation of chemical vapor. The resistance was amplified by 20 times and recorded every 0.1 s. Measurement was performed after the sensor array was placed into the chamber and the signal of resistance was stabilized. Each measurement consisted of three steps of stabilization, exposure, and purge. The following chemicals were tested: acetone, benzene, chloroform, cyclohexane, and methanol.
3. Results and discussion The sensor time series resistances were normalized by subtracting the baseline resistance value R0 from each data point R in the time series and then dividing by R0 [(R − R0 )/R0 ] × 100 This normalization is typically used for gas sensor based on polymer composite to set off the difference of initial resistances. Fig. 3 shows the maximum relative differential resistance of the sensor array maintained at the micro heater temperature of 25 and 45 ◦ C, when the sensor array was exposed to the sample gas of different temperature (20, 40, and 60 ◦ C). Two thousand parts per million of (a) chloroform and (b) methanol were used as sample gases. The maximum resistance (Rmax ) is equilibrated resis-
Fig. 3. Maximum relative differential resistance of sensor array exposed to 2000 ppm of: (a) chloroform; and (b) methanol controlled at the temperature of 20, 40, and 60 ◦ C. Measurements were performed at the micro heater temperature of 25 and 45 ◦ C.
tance value when the sensor is sufficiently exposed to sample gas. The maximum resistance change of sensor array decreased as the micro heater temperature increased from 25 to 45 ◦ C. Higher responses of polymer composites at the lower temperature are due to an increasing tendency for condensation as a result of the larger chemical potential of the molecules in the polymer phase compared with the gas phase. When the micro heater temperature was maintained at 25 ◦ C, the maximum response of sensor array dramatically changed with the gas temperature. The response changes of sensors with the gas temperature were divergent to one another, which means that the response pattern of sensor array would be changed with the temperature of sample. Consequently, when the gas temperature is changed, it will be recognized as a different gas even though the sensor array is exposed to the same concentration of the target. On the other hand, when the polymer detector was heated to the temperature of 45 ◦ C, the maximum resistance changes of sensors were little affected by the temperature of sample gas in both the cases: (a) chloroform and (b) methanol, as shown in Fig. 3. Figs. 4 and 5 show the radial-response pattern with the variation of the gas temperature when the sensor array is exposed to 2000 ppm of chloroform (Fig. 4) and methanol
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Fig. 4. Radial-response patterns of sensor array when the sensor array was exposed to 2000 ppm chloroform vapor of temperature 20, 40, and 60 ◦ C. Sensor array was thermostatted to the temperature of 25 and 45 ◦ C by using an embedded micro heater.
(Fig. 5). When the polymer detector was not heated, the shape of radial-response pattern of the sensor array changed with the increase in gas temperature. However, radial-response pattern of the sensor array measured at the micro heater temperature of 45 ◦ C showed similar shape for the gas temperature of 20 and 40 ◦ C, but radial-response pattern for sample gas of 60 ◦ C showed different shape from the pattern for the gas temperature of 20 and 40 ◦ C. This means that the gas temperature little affects the response pattern of the sensor array when polymer detectors are maintained at a constant temperature above that of sample gas. The bulky solubility of molecules from the gas phase in the polymer composite is dominated by the partition coefficient: fp/g =
i cpolymer i cgas
(1)
which is defined as the gas of i concentration dissolved in a polymer composite divided by the concentration of the same volume in a gas phase. When the polymer detector is thermostatted at a constant temperature of 45 ◦ C, the partition coefficient seems to be independent of gas temperature over the specific operating range of 0–45 ◦ C, because the detector temperature is the dominant temperature for thermodynamics of the polymer detector–vapor interaction. Though the gas temperature does not affect the partition coefficient for detector–vapor interactions when the gas temperature is lower than that of detector, sample temperature can affect the concentration of the sample, which seems to be the cause of the little difference in the shapes of response patterns for the gas temperature of 20 and 40 ◦ C. A more quantitative approach to evaluate the effect of gas temperature on the sensor–vapor interaction was provided
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Fig. 5. Radial-response patterns of sensor array when the sensor array was exposed to 2000 ppm methanol vapor of temperature of 20, 40, and 60 ◦ C. Sensor array was thermostatted to the temperature of 25 and 45 ◦ C by using an embedded micro heater.
by principal component analysis (PCA), as shown in Fig. 6. At first, the sensor array maintained at the temperature of 25 ◦ C was exposed to 2000 ppm of five different chemical vapors controlled at the temperature of 20 ◦ C: acetone, benzene, chloroform, cyclohexane, and methanol. And the chemical vapors of chloroform (a) and methanol (c) adjusted to the temperature of 40 ◦ C were injected to the sensor array. As shown in Fig. 6(a) and (c), when the sensor array was not heated to the temperature higher than that of the sample gas, the response pattern was significantly affected by the gas temperature and recognized as different chemical in projected principal component space. On the other hand, when the sensor array was thermostatted at a temperature of 45 ◦ C, as shown in Fig. 6(b) and (d), the effect of gas temperature was seemingly negligible. Data measured at different gas temperatures were represented in same group in princi-
pal component space, which means that the sensor array can recognize the sample gas even though the gas temperature was changed. Using micro heater embedded in the structure of sensor array, we developed a portable sensor array that could recognize chemicals regardless of environmental temperature. Problem still exists in case of the mixture sample. For single component gas, sample temperature only affects total concentration, and changes in total concentration can typically be corrected by normalization of the maximum responses. However, for mixtures sample temperature affects composition as well as total concentration. Since composition of mixture is changed with the temperature, database developed at one sample temperature cannot be valid for another temperature. Therefore, the database that includes all sample temperatures, which depends on the application, is necessary to recognize mixture sample.
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Fig. 6. Data obtained from the injection of chemical vapors (acetone, benzene, chloroform, cyclohexane, and methanol) controlled at the temperature of 20 ◦ C into the sensor array, maintained at the temperature of 25 ◦ C (a and c) and 45 ◦ C (b and d) were represented in principal component space, and the responses of sensor array for the chloroform (a and b) and methanol (c and d) heated to the temperature of 40 ◦ C were represented together in principal component space.
4. Conclusion We tested the effect of gas temperature on the response of sensor array based on polymer composite using chemical vapor of chloroform and methanol. Gas condensation process from the gas phase into the polymer was significantly affected by the gas temperature, and maximum relative differential resistances of the sensors were dramatically changed with the gas temperature. Response pattern of the sensor array was also changed with the gas temperature, and so chemical vapors with the temperature of 20 and 40 ◦ C were recognized differently, even though concentrations of both samples were maintained identically. However, when the polymer detector was thermostatted at the temperature of 45 ◦ C, radial-response patterns for the chemical-vapor, controlled at the temperature of 20 and 40 ◦ C, showed similar shape and were recognized identically in projected principal component space. The partition coefficient was independent of gas temperature over the specific operating range when the polymer
detector was thermostatted at a temperature higher than the temperature of operating range, because the detector temperature is dominant temperature for the thermodynamics of the polymer detector–vapor interaction.
Acknowledgements This work has been supported in part by the Ministry of Information and Communication of Korea and in part by the Ministry of Science and Technology of Korea through the NRL program.
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Biographies Seung-Chul Ha received his BSc degree (1997) in materials science and engineering from the Pohang University and an MSc degree (1999) in materials science and engineering from the Seoul National University. He had researched various thin film processes and devices in Hynix Semicondutor Co. Ltd. (1999–2002). He is currently a research engineer in
Electronics and Telecommunications Research Institute (2002–current). His research interests are mainly focused on the fabrication of intelligent sensors including monolithic electronic nose and nanoporous materials for gas sensor. Yoon Seok Yang received the BS degree in control and instrumentation engineering from Seoul National University, Korea and MS degree in interdisciplinary course (biomedical engineering major) from Yonsei University, Korea. He received the PhD degree in Medical Electronics Laboratory (MELab) of the Seoul National University on the study of ultrasonic interrogation techniques in medical equipments, in 2002. Then he joined the BioMEMS group in Electronics and Telecommunications Research Institute (ETRI), Korea, where the major research activities are development of portable electronics and intelligent algorithms for medical and biological applications, such as biosensors. His research interests include DNA and molecular biology. Yong Shin Kim received a PhD degree in chemistry from Korea Advanced Institute of Science and Technology (KAIST), in 1997 with a study on the chemical dynamics of photodissociation and photoionization of fragmented halogen atoms. After obtaining his degree, he has worked as a research senior member at the Electronics and Telecommunications Research Institute (ETRI). Now his research activities are focused on the miniaturized and smart chemical sensor arrays based on MEMS technology and nanomaterials. Soo-Hyun Kim received the BS degree in metallurgical engineering from Seoul National University, Seoul, Korea, in 1997 and the MS and PhD degrees in materials science and engineering from Seoul National University, Seoul, Korea, in 1999 and 2003, respectively. Currently, he is a member of technical staff in memory research and development division in Hynix Semiconductor Inc., Korea. The focus of his current research is on the development of metallization process for advanced DRAM, which includes the silicides contact, diffusion barrier, interconnect and interfacial reaction in metallization system of DRAM. Young Jun Kim received his PhD in polymer science from the University of Akron, Akron Ohio, US in 1997. He, as a research associate, had been involved with the controlled synthesis of block and functional polymers, and their applications in surface modifications related with biocompatible surfaces and nanostructured materials at the University Connecticut and Princeton University until 2000. Since then at Electronics and Telecommunications Research Institute (ETRI) he has been involved with nanoencapsulating memory molecules and synthesizing nanoclusters for memory media and biosensors. Seong Mok Cho received the BS, MS, and PhD degree in materials engineering from Pohang University of Science and Technology (POSTECH), Korea, in 1992, 1994, and 2001, respectively. After getting MS degree, he had joined Samsung Electronics, Korea, in 1994, where he had worked for the development of lithography technology for 256M and 1G DRAM until 1996. He joined Electronics and Telecommunications Research Institute (ETRI), Korea, in 2001 as a senior researcher in the Basic Research laboratory. His current research interests are mainly focused on the sensor technologies such as infrared image sensor and electronic nose.