Development and Improvement of Carbon Nanotube ... - IEEE Xplore

IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 12, NO. 2, MARCH 2013

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Development and Improvement of Carbon Nanotube-Based Ammonia Gas Sensors Using Ink-Jet Printed Interdigitated Electrodes Pattamon Teerapanich, Myo Tay Zar Myint, Claire M. Joseph, Gabor L. Hornyak, and Joydeep Dutta, Senior Member, IEEE

Abstract—Gas sensors have been widely used in many applications including environmental monitoring, industrial control, and detection in warfare or for averting security threats. High sensitivity, selectivity, and fast response time are required for application in real-time monitoring and detection of toxic gases. Single-walled carbon nanotubes (SWCNTs) provide large specific surface area beneficial for gas adsorption thereby increasing sensor sensitivity. In this paper, ammonia (NH3 ) gas sensors based on SWCNTs were developed using interdigitated silver electrodes printed with nanoparticulate ink on alumina substrates. Simple and inexpensive methods including shaking and dispersion in appropriate solvents were used to debundle SWCNTs for improving sensor response. The fabricated sensors showed a maximum response of 27.3% for 500 ppm NH3 at room temperature. Detection limit of the sensor devices at room temperature were estimated to be ∼ 3 ppm. Index Terms—Ammonia, gas sensor, ink-jet printer, interdigitated silver electrode (IDE), single-walled carbon nanotubes (SWCNTs).

I. INTRODUCTION AS sensors play a vital role in environmental monitoring and detection of toxic gases like nitrogen oxide (NOx ), ammonia (NH3 ), and carbon monoxide (CO) particularly in the manufacturing industries and in the agricultural sector. Exposure to low concentrations (25–150 ppm) of ammonia can cause lung and eye irritation or skin irritation due to its corrosive alkaline property. On the other hand, exposure to extremely high ammonia concentrations (above 5000 ppm) severely affects hu-

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Manuscript received May 29, 2012; revised August 14, 2012; accepted August 31, 2012. Date of publication January 30, 2013; date of current version March 6, 2013. This work was supported in part by the National Nanotechnology Center, Thailand, in party by the National Science & Technology Development Agency, Royal Thai Government, in part by the Center of Excellence in Nanotechnology, Asian Institute of Technology, Thailand, in part by the Ministry of Science and Technology, Thailand, and in part by the HM King scholarship. The review of this paper was arranged by Associate Editor J. Li. P. Teerapanich, C. M. Joseph, and G. L. Hornyak are with the Asian Institute of Technology, Klong Luang, Pathumthani 12120, Thailand (e-mail: [email protected]; [email protected]; glhornyak@gmail. com). M. T. Z. Myint and J. Dutta are with Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, Al-Khod 123, Oman (e-mail: myotayzar. [email protected]; [email protected]). This paper has supplementary downloadable material available at http://ieeexplore.ieee.org. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2013.2242203

man health that can eventually lead to death. The main source of ammonia gas in the atmosphere originates from agricultural activity where it is introduced into the soil in the form of ammonium ions from fertilizer [1]. Additionally, the decomposition of manure, leaves, and other organic matter produces large amount of ammonia gas in the atmosphere (20–35 Tg/yr) [2]. Another source of ammonia emission is the combustion in chemical plants (e.g., during production of fertilizer and refrigeration systems) and road transportation that generate gaseous ammonia at a rate of 2.1–8.1 Tg/yr [2]. According to the National Institute for Occupational Safety and Health (NIOSH), 300 ppm NH3 presents immediate danger to life and health (IDLH). The short-term (15 min) exposure limit (STEL) defined by the American Conference of Industrial Hygienists (ACGIH) is 35 ppm and 25 ppm for time-weighted average (TWA) over 8-h-period exposure [3]. In order to detect such small ammonia concentration, nanotechnology has become an increasingly attractive avenue for fabrication of devices with high sensitivity and selectivity [4]. In particular, carbon nanotubes (CNTs) have become a practical choice for use as a material for gas sensing applications due to their unique electrical properties and huge surface area— both traits that are beneficial for gas adsorption and measurement. Adsorption behavior of various gas molecules like nitrogen dioxide (NO2 ), oxygen (O2 ), NH3 , nitrogen (N2 ), carbon dioxide (CO2 ), methane (CH4 ), moisture (H2 O), hydrogen (H2 ), and argon (Ar) on individual single-walled carbon nanotubes (SWCNTs) and its bundles has been studied extensively including the evaluation of adsorption energy, charge transfer, and electronic band structures [5]. CNTs have been used to detect small concentrations of various gases such as H2 [6], [7], NH3 [8], [9], NOx [10], [11], and CO [12]. NH3 gas sensors with conventional metal oxides like tin dioxide (SnO2 ) [13], titanium dioxide (TiO2 ) [14], indium oxide (In2 O3 ) [15], tungsten oxide (WO3 ) [16], and zinc oxide (ZnO) [17] have been reported that show fast response recovery and good selectivity. However, these NH3 gas sensors using metal oxides operate effectively only at higher temperatures (150 ◦ C and 400 ◦ C). A distinct advantage of CNT-based gas sensors is that they are capable of operating at low temperatures—ca. 80 ◦ C which promotes safety, reduces power demand, and removes complexity in sensor design. CNT gas sensors detect changes in the electrical resistance of the nanotubes upon the adsorption of gas molecules. Adsorption of electron donor species such as NH3 and H2 on p-type CNT

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surfaces (hole carriers) results in the transfer of electrons from the gas molecules to the valence band of CNTs reducing the number of hole carriers. The hole depletion phenomena in turn causes a reduction in electrical conductivity of the nanotubes and a concomitant increase in electrical resistance is observed [18], [19]. In contrast, adsorption of electron acceptor species like O2 and NO2 on the CNT surface leads to electron transfer from the valence band of CNTs to the gas molecules. This action results in an increase in the number of hole carriers in the valence band, thereby enhancing the electrical conductivity of the nanotubes [20]. Many research groups have used individual nanotubes in field-effect transistor-based sensors [21], [22] and nanotube bundles or thin film of nanotubes in resistive gas sensors [11], [23], [24]. However, in order to fabricate these sensors, highly sophisticated fabrication equipments and expensive techniques like micromachining are required. Additionally, CNTs agglomerate into bundles due to van der Waals interactions resulting in the reduction of overall surface area of the nanotubes, hence reducing sensor sensitivity. Therefore, debundling of nanotubes is a crucial challenge for the development of high performance gas sensors. Various types of metal nanoparticles such as Pd [7], [25], Pt [12], [26], [27], and Au [28], [29] are used to functionalize the nanotubes in order to enhance the sensor performances in terms of sensitivity and selectivity due to catalytic and metal-CNT synergistic effects. In this paper, CNT-based sensors for the detection of ammonia at room temperature were developed using simple and inexpensive methods including ink-jet printed interdigitated electrodes (IDE). IDE provides better electrical contact between the nanotubes and the electrodes resulting in an amplification of sensor signals that result in improved response. Silver nanoparticles were synthesized and used for the fabrication of IDE by a customized ink-jet printer, as described elsewhere [29], [30]. CNT suspensions were deposited on silver patterned IDEs fabricated on robust alumina substrates. In order to improve sensor sensitivity, vortex shaking was utilized to debundle SWCNTs and the nanotubes were subsequently sonicated in dimethylformamide (DMF) (dispersing agent). This approach offers a simple fabrication process for mass production of low-cost, high-performance sensors. Comparative study of sensor response of NH3 at room temperature between bundled and debundled nanotubes was performed and is reported here. II. EXPERIMENTAL SETUP A. Fabrication of Interdigitated Electrode Colloidal silver nanoparticle ink was synthesized by the reduction of silver nitrate (AgNO3 ) precursor with sodium borohydride (NaBH4 ) in a basic medium, at room temperature [31]. Molar ratio (MR) of AgNO3 to NaBH4 of 10 was maintained for silver nanoparticle synthesis with polyacrylic acid (PAA) as the capping agent. The as-synthesized silver ink was stable in the pH range of 10–12. In order to obtain conductive silver lines for IDE and to prevent clogging of ink-jet printer nozzle, proper amounts of metal loading were required. The details of ink-jet printing are provided as supplementary information [Fig. S1(a) and (b)].

The synthesized colloid was de-stabilized by using concentrated acetic acid (CH3 COOH) and subsequently the flocculates containing silver nanoparticles were dried for 2 h at ambient temperature. Redispersion of Ag nanoparticles was carried out in 1% (wt) ammonia solution (NH4 OH). Approximately, 5 weight% of silver was loaded in the ink suspension used to print IDEs. The morphology of colloidal silver nanoparticle ink was examined by using transmission electron microscope (TEM) [JEOL JEM 2010] operated at 120 kV [see the characterization of silver nanoparticles in supplementary information: Fig. S2(a) and (b)]. In this paper, alumina substrate (12 mm length × 5.6 mm width × 1.0 mm thickness) was used due to the robustness, excellent dielectric properties, and thermal stability. Importantly, alumina was chosen because of its lower cost and porosity that facilitates the adhesion of silver ink during ink-jet printing, as well as the attachment of CNTs. Prior to the ink-jet printing, the substrates were cleaned to remove organic contaminants and residues on the surface. Substrates were activated by soaking in concentrated nitric acid (HNO3 ) at room temperature, thoroughly rinsed in deionized water and dried at 95 ◦ C overnight in an oven. After the deposition of the electrodes, the substrates were annealed at 300 ◦ C. The electrodes were then subjected to nickel (Ni) plating in order to improve electrical conductivity and corrosion resistance. Ni plating electrolyte solution composed of 0.2 M NiSO4 ·6H2 O, 0.12 M Ni(C2 H3 O2 )2 ·4H2 O, and 0.075 M (H3 BO3 ). The pH of the solution was approximately 6.85 and the solution was kept at 60 ◦ C. The applied potential of 3 V (dc) was employed between working electrodes (printed silver electrode) and counter electrode (glassy carbon electrode) while the plating time was varied from 1.5 to 2 min. The geometry of finished electrode contained a finger width of 0.18 ± 0.01 mm, finger length of 1.97 ± 0.01 mm, and interelectrode distance of 0.50 ± 0.01 mm [see the schematic diagram of the IDE fabrication process in supplementary information: Fig. S1(a)]. B. Sensor Fabrication Commercial high purity (>90%) SWCNTs were obtained from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Science, China. The nanotubes contain 40% metallic and 60% semiconducting SWCNTs with outer diameter ranging from 1 to 2 nm. Additional purification was done by using acid and thermal treatment to remove metal catalyst and amorphous carbon impurities [32]. Purified SWCNTs (3.4 mg/L) were subsequently dispersed in DMF by sonication. SWCNTs/DMF suspension (10 μL) was then drop deposited on the sensing area on the IDE heated to 80 ◦ C. Finally, the sensors were dried in an oven at 95 ◦ C for 5 h in the ambient to form the SWCNTs networks across the electrode fingers. The SWCNTs were debundled in a vortex mixer for 50 h prior to sonication in DMF. Subsequent sonication in DMF promotes SWCNT dispersion and maintains the charge on nanotube surfaces leading to higher available surface area for gas adsorption, thereby enhancing sensor performance [23]. We found that debundled-nanotubes by shaking formed a highly stable suspension in DMF compared to the as-received nanotubes. The SWCNT/DMF suspension was

TEERAPANICH et al.: DEVELOPMENT AND IMPROVEMENT OF CARBON NANOTUBE-BASED AMMONIA GAS SENSORS

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Fig. 2. Scanning electron micrograph images of a single-walled nanotube network draped across an interdigitated silver electrode. (a) Low magnification showing the sensing area and interelectrode distance of 520 μm. (b) High magnification.

Fig. 1. Schematic diagram of gas sensor setup for performance testing and the deposition of CNT suspension on interdigitated silver electrodes.

stable for more than one week without signs of any agglomeration. The fabrication of sensors with debundled SWCNTs was similar to the procedure described earlier. It is noted that only purified SWCNTs were used to fabricate the sensors in this paper. C. Sensor Performance Testing Apparatus and Procedure Sensor response to different concentrations of ammonia were carried out in a customized gas sensor test bench described elsewhere [33]. Compressed air was used as the carrier gas as well as for dilution in this paper. Relative humidity (RH) of 56% was maintained during all the measurements. The flow rates of air and ammonia gas (NH3) were controlled by using two mass flow controllers (MFC) (KOFLOC, 8300). The test chamber was connected to a heater with temperature control capability. The change in sensor resistance during performance testing was monitored by a Keithley 617 electrometer linked to the computer via a general purpose interface bus (GPIB) port. LabVIEW software by National Instruments was used for signal control and data acquisition. The chamber was purged with air for about 1 h (1 bar pressure) until the baseline of sensor resistance was found to stabilize. Ammonia vapor was subsequently introduced into the test chamber by mixing with the carrier gas (air in our case). Sensor response was monitored as the change in sensor resistance over time. The test-bench for determination of sensor performance is schematically represented in Fig. 1. Following each measurement, the chamber was purged with a high air flow (2000 sccm) keeping the sensor between 50 and 80 ◦ C for short periods of time to assist desorption of gas molecules from the CNT surface. D. Experimental Design Three standard parameters were used to compare the sensor performances in this paper, including sensor response (S), sensitivity (%S), and response time. The sensor response (ΔR, resistance change upon exposure to NH3 ) was normalized by the initial resistance in air (Ra ) measured prior to exposure to

Fig. 3. Current–voltage (I–V) characteristics of a typical sensor device fabricated from SWCNTs bundles on IDEs. Inset: A typical sensor device with an initial sensor resistance of 7.4 kΩ.

ammonia [(Rg –Ra )/Ra , where Rg is the sensor resistance in the presence of ammonia gas and Ra is the sensor resistance in the carrier gas (air)]. Sensor sensitivity S (%) was calculated from 100 × (Rg –Ra )/Ra . Response time was determined from the time taken by the sensor to undergo the resistance change from 10% to 90% of the final value upon exposure to ammonia gas. III. RESULTS AND DISCUSSION Purified SWCNTs dispersed in DMF forms a network across the fingers of the electrodes (IDEs) as shown in Fig. 2(a) and (b). DMF is used to disentangle nanotube bundles because the amide group attaches readily to the surface of the nanotubes via π–π interaction thereby providing a highly uniform SWCNT suspension. Typical average interelectrode distance of 0.520 mm was observed [see Fig. 2(a)] while nanotube bundles of 30– 50 nm were found to be dispersed between the fingers [see Fig. 2(b)]. The initial resistance of the sensor devices ranged from tens of kΩ to several MΩ depending on the concentration of CNTs dispersed onto the electrodes. The devices showed a good ohmic contact of the nanotube networks to the electrodes, as in Fig. 3.

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Fig. 4. Variation of the sensor resistance at different temperatures in air, showing the behavior of semiconducting nanotubes. Inset: Variation of sensor resistance of as-received CNTs for varying temperatures.

The dependency of sensor resistance on temperature was studied to evaluate the electrical behavior of the devices. Sensors fabricated from as-received and purified SWCNTs without vortex treatment were gradually heated to 300 ◦ C in air and the resistance was monitored at different temperatures as shown in Fig. 4. Three distinct regions of resistance changes can be observed with the variation of the temperature. From room temperature to 50 ◦ C, the sensor resistance decreased due to the semiconducting behavior of CNTs leading to the formation of a greater number of electron hole pairs in the nanotubes. The second region is observed for temperatures ranging from 75 to 100 ◦ C where the sensor resistance became relatively stable. This stability is attributed to desorption of moisture from CNT surfaces. The resistance of the sensor decreased rapidly when the temperature was raised from 100 to 300 ◦ C, and the observed behavior can be attributed to the semiconducting properties of the nanotubes. Sensor resistance gradually increased during cooling due to the readsorption of moisture (data not shown here) from the ambient atmosphere. This observed electrical behavior is consistent with previous studies [9]. Additionally, it can be observed that the electrical behavior of as-received and purified SWCNTs showed similar trend. In all subsequent experiments reported here, purified debundled SWCNTs were selected due to their high exposed surface area for gas adsorption after the purification and debundling steps. In order to determine the operating temperature of the sensors, the devices with purified and debundled carbon nanotubes were exposed to 62.5 ppm NH3 in air at different temperatures ranging between 24 and 150 ◦ C. After each cycle, the chamber was heated at a temperature higher than the previously recorded temperature during data acquisition. Postexperiment heating was carried out to promote and ensure complete desorption of any remaining NH3 on nanotube surfaces, while under high air flow (2000 sccm) that could influence subsequent measurements. A typical sensor behavior is plotted as a function of working temperature (see Fig. 5). Typically, the sensing mechanism


Fig. 5. Response (%S) of a typical sensor to 62.5 ppm NH3 as a function of working temperature (see normalized sensor responses (ΔR/R a ) over a period of time for different temperatures in supplementary information: Fig. S3)

consists of the adsorption of gas molecules on sensing material surface and the subsequent charge transfer between adsorbed gas molecules and the sensing material, which are both affected by the working temperature of the sensors [34]. The optimum operating temperature for the sensor in terms of sensitivity and response time was observed at room temperature with a sensitivity of 2.9% and response time of 541 s, respectively. This behavior indicated weak physisorption of ammonia gas molecules on the nanotube surface with concomitant small charge transfer— e.g., 6.24×10−21 coulombs per ammonia molecule [35]–[37]. Sensor response (%S) significantly decreased with higher working temperature resulting from the physisorption of ammonia molecules on nanotubes surfaces (see Fig. 5). Nevertheless, the surrounding environment, humidity in particular, plays an important role on the sensor sensitivity and this behavior can be observed at temperatures ranging from 50 to 80 ◦ C as was shown earlier in Fig. 4—the region of constant sensor sensitivity. Response of the devices operating at room temperature was studied for varying concentration of ammonia (250–1750 ppm). After each measurement, the exposed sensors were recovered by purging with a high flow of air without any heating. A typical sensor response (ΔR/Ra ) over a period of time to various ammonia concentrations at room temperature is shown in Fig. 6. Increased sensor resistance upon exposure to NH3 gas can be attributed to the flow of electron from electron lone pair of NH3 molecules (electron donor) to the valence band of CNTs, decreasing number of hole carriers in the CNTs in turn increasing the electrical resistance of the nanotubes. This phenomenon reveals p-type semiconducting behavior of the SWCNTs. The sensor exhibited a maximum response of 14.4% for 1250 ppm NH3 . Response times of the sensor varied from 7 to 25 min depending on NH3 concentration. However, sensor sensitivity was found to reduce for 1750 ppm of NH3 concentration. The undesired saturation state of the sensor might be attributed to an inefficient recovery process (degassing with air) in which available surface sites on SWCNTs for gas adsorption were occupied by ammonia gas from previous cycles, discouraging the


Fig. 6. Sensor (as-received SWCNTs) responses (ΔR/R a ) over a period of time to various NH3 concentrations (250–1750 ppm) at room temperature in air.

Fig. 7. Sensor (as-received SWCNTs) responses (ΔR/R a ) over a period of time to various NH3 concentrations (62.5–750 ppm) at room temperature in air.

adsorption of new gas molecules, thereby reducing sensitivity of the sensor. In separate experiments, sensors were exposed to lower NH3 concentrations ranging from 62.5 to 500 ppm at room temperature in air. Short heating step (3–5 min) at 80 ◦ C simultaneously with air purging was utilized to completely desorb residual NH3 molecules from SWCNT surface after each measurement. Sensor response increased linearly with increasing NH3 concentration achieving 10.2% for 500 ppm NH3 (see Fig. 7) as the heating step assists in regenerating the CNT surfaces. In order to further improve the sensor sensitivity, CNTs were debundled as explained in Section II. Vigorous vortex-induced shaking creates a static charge on nanotube surfaces caused by friction forces thereby contributing to repulsive interactions between nanotubes [38], [39]. Fig. 8 shows the response of sensor fabricated with debundled SWCNTs as a function of NH3 concentration (ranging from 62.5 to 500 ppm at room temperature in air). It should be noted that prior to recording the data, carrier gas was flowed inside the chamber for 1 h until the sensor baseline stabilized. Sensor response increased from 4.82% for 62.5 ppm NH3 to 27.7% for 500 ppm NH3 with a subsequent

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Fig. 8. Sensor (debundled-SWCNTs) responses (ΔR/R a ) over a period of time to various NH3 concentrations (62.5–500 ppm) at room temperature in air.

Fig. 9. Comparison of average sensitivity of sensor with debundled-SWCNTs and as-received SWCNTs with respect to NH3 concentration at room temperature measured in air.

increase in sensitivity from 8.4 to 27.3%, respectively. The average sensor response to various NH3 concentrations at room temperature was compared between debundled and as-received but purified SWCNTs as shown in Fig. 9. Response time however increased from 580 to 620 s when ammonia concentration was increased from 62.5 to 500 ppm. This might be attributed to higher competition between gas adsorption and desorption for higher gas concentrations lead the sensor signal to reach equilibrium slowly thereby increasing the response time of the sensor. Repeatability of sensitivity (ΔR × 100/Ra ) measurements of a typical sensor determined from three cyclical experiments as a function of NH3 concentration are plotted in Fig. 10. The sensors showed good repeatability and for all the experiments it exhibited a linear relationship with NH3 concentrations ranging from 62.5 to 500 ppm. Importantly, the sensor showed consistent sensitivity of 0.049 ± 0.001 over the whole range of NH3 concentration, defined by the slope or (ΔR/Ra ×100)/Δconcentration.

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definition [41], equation (3) describes the sensor signal that is considered to be the true signal when the signal-to-noise ratio is equal to 3: DL(ppm) =

Fig. 10. Sensitivity of the sensor (debundled-SWCNTs) as a function of NH3 concentration at room temperature in air for three experimental cycles conducted on different days showing a good linear relation and consistent response.

3RMS . slope

(3)

Therefore, the detection limit (ppm) of the sensor fabricated with (debundled-SWCNTs) can be estimated to be 3 ppm from the average sensor response and average noise level of three repeatable cycles for each sensor (see Fig. 11). Sensors fabricated with simple and inexpensive approach could detect low ammonia concentrations (3 ppm) at room temperature and exhibited a good repeatability. SWCNTs are good candidates as the sensing component in gas sensor devices for ammonia detection due to high available surface area for gas adsorption and the ability to detect gas molecules at room temperatures. For further development of SWCNT-based gas sensors, sensitivity and selectivity of the sensor can be improved by utilizing surface chemistry approach to functionalize SWCNTs for multiple gas detection, and electrode material and geometry can be optimized for better sensor performance. IV. CONCLUSION

Fig. 11. Calibration curve of the sensor response (debundled-purified CNTs on IDE electrode) for ammonia. The ammonia vapor was evaporated using a bubbler with a carrier gas (air) at room temperature and ammonia stream was diluted by air.

Due to the limitation of our experimental apparatus, the lowest detectable concentration of the sensor was 62.5 ppm. Theoretically, the detection limit of the sensor can be extrapolated from the calibration curve of the sensor response with respect to ammonia concentration (see Fig. 11). Sensor noise was calculated from (1) by the variation of the baseline sensor resistance using root mean square deviation (RMSD) [40]: (yi − y)2 (1) Vchisq = where yi is the measured data point and y is the corresponding value calculated from the curve-fitting equation. Subsequently, the root mean square noise (RMSnoise ) can be calculated from (2): Vchisq (2) RMSnoise = N where N is the number of data points used in curve fitting. In our case, 10 data points of the sensor baseline (see Fig. 8) were used to calculate RMSnoise . The RMSnoise of 0.000 532 for sensor with debundled-SWCNTs was found. Referring to the IUPAC

Silver nanoparticle ink was synthesized for the fabrication of interdigitated silver electrodes for gas sensor device by a customized ink-jet printer. A suspension of vortex-mixed-sonicated SWCNTs in DMF was deposited dropwise on nickel-plated silver-patterned IDEs. The sensor performance for ammonia detection was investigated. The optimum operating temperature of the sensor devices was found to be at room temperature— a trait beneficial for real-time sensing application as well as toward reduction of electrical power consumption. Sensor sensitivity was improved by introducing shaking technique (vortex mixing) to create surface charges on nanotubes as well as using DMF solvent while sonicating to debundle-SWCNTs. The sensor with debundled-SWCNTs showed a maximum response of 27.3% for 500 ppm NH3 . By extrapolation of the results, the sensors were found to be suitable to detect ammonia concentrations down to 3 ppm at room temperature. SWCNT and multiwalled CNTs (MWCNT)-based sensors had been reported to have similar sensitivity toward NH3 at concentrations ranging from 100 to 400 ppm [42]. Detection of NH3 measuring 5 ppm has been reported when pristine SWCNTs were used for gas-sensing in either purified air or in nitrogen atmosphere. These sensors are suitable for detection of ammonia at levels that can immediately cause danger to human health and life (IDLH: 300 ppm) as described by The National Institute for Occupational Safety and Health (NIOSH). In conclusion, gas sensors based on SWCNTs were successfully developed for ammonia detection by using simple and inexpensive approach integrating ink-jet printing for electrode fabrication. REFERENCES [1] B. Timmer, W. Olthuis, and A. v. d. Berg, “Ammonia sensors and their applications-a review,” Sens. Actuators B, Chem., vol. 107, no. 2, pp. 666– 677, 2005.


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Claire Joseph received the B.S. degree in applied physics from the University of the Philippines, Los Ba˜nos, Laguna, Philippines, in 2002, and the M.S. degree in microelectronics and embedded system from the Asian Institute of Technology, Bangkok, Thailand, in 2011. She is currently working as a Packaging Engineer at Texas Instrument, Inc., Baguio City, Philippines. Her research interests include synthesis of silver ink nanoparticles and fabrication of interdigitated electrodes for gas sensor application using inkjet printing technology.

Gabor L. Hornyak received the Ph.D. degree from Colorado State University, Fort Collins, CO, USA, with Charles Martin developing template synthesis methods and studying optical properties of nanogoldporous alumina composite. He is the Director of the Center for Learning, Innovation, and Quality, Asian Institute of Technology, Thailand. He received a postdoctoral appointment at the University of Essen in Germany with Dr. G. Schmid working with gold55 quantum dots; a five-year appointment at the National Renewable Energy Laboratory where he was involved in carbon nanotube technology and synthesis. From 2003 through 2005, he founded and was executive director of the Colorado Nanotechnology Initiative resulting in creation of the Colorado Nanotechnology Alliance, the State’s advocate group for nanotechnology. He coauthored two cutting-edge textbooks titled “Introduction to Nanoscience” and “Fundamentals of Nanotechnology.” He is the editor-in-chief of “Perspectives in Nanotechnology Series” published by the CRC Press.


Joydeep Dutta (M’07–SM’09) received the Ph.D. degree in amorphous silicon solar cells from Calcutta University, Calcutta, India, in 1990. Since October 2011, he is the Chair Professor in nanotechnology at the Sultan Qaboos University, Sultanate of Oman. During this work, he was the Vice President (Academic Affairs) of the Asian Institute of Technology (AIT) and Director of the Center of Excellence in Nanotechnology at AIT, Bangkok, Thailand. His broad research interests include nanomaterials in nanotechnology, self-organization, and application of nanoparticles. He has authored 3 text books and more than 200 research publications, 11 chapters in Science and Technology reference books, 5 patents (+9 patents applications), and has delivered more than 70 invited and keynote lectures. Dr. Dutta is a fellow of the Institute of Nanotechnology and the Society of Nanoscience and Nanotechnology, founding Member of the Thailand Nanotechnology Society, and member of several other professional bodies. He is an award winning author (Choice award for Outstanding Academic title of 2010 from American Library Association) for the book Fundamentals of Nanotechnology.