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Fabrication of Functional Nanofibrous Ammonia Sensor Kalayil Manian Manesh, Anantha Iyengar Gopalan, Kwang-Pill Lee, Padmanabhan Santhosh, Kap-Duk Song, and Duk-Dong Lee

Abstract—A nanofibrous sensor for ammonia gas is fabricated by electrospinning the composite of poly(diphenylamine) (PDPA) with poly(methyl methacrylate) (PMMA) onto the patterned interdigit electrode. The composite electrospun membrane shows interconnected fibrous morphology. Functional groups in PDPA and the high active surface area of the fibrous membrane make the device detect a lower concentration of ammonia with a good reproducibility. The sensing capability of the device is studied by monitoring the changes in resistance of the membrane with different concentrations of ammonia. The changes in resistance of the membrane shows linearity with the concentration of ammonia in the limit of 10 and 300 ppm. UV-visible spectroscopy reveals the mechanism of sensing ammonia by the membrane. Index Terms—Gas detectors, membrane, resistance, sensitivity, spectroscopy.

I. INTRODUCTION

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EVELOPMENT of techniques on detection of ammonia NH gas is receiving importance for monitoring the environmental pollution and to know the trace amounts of NH in industrial effluents and breath analysis. Olfactory limit of detection of NH gas is 55 ppm and the maximum limit is 25 ppm for 8 h exposure in a workplace [1]. NH gas is corrosive and very toxic. Several sensing materials like semiconducting metal oxides [2], [3], silicon [4], [5], organic materials [6], [7], carbon black-polymer composites [8], CuBr, tellurium thin films [9], [10], nanoporous anodized alumina [11], acrylic acid doped polyaniline [12], CU S films [13], and polypyrrole films [14] have already been reported for the detection of NH gas. However, the sensors developed with the above materials have a drift in the sensitivity and also shorter lifetimes. Therefore, fabrication of a reproducible and low concentra50 ppm detectable NH sensor is a demanding task. tion Manuscript received July 14, 2006; revised March 26, 2007. This work was supported by the Korean Research Foundation under Grant KRF-2006-J02402. The review of this paper was arranged by Associate Editor Y. Lu. K. M. Manesh and P. Santhosh are with the Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea. A. I. Gopalan is with the Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea, and also with the Nano Practical Application Center, Daegu 704-230 South Korea. K.-P. Lee is with the Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea, and also with the Nano Practical Application Center, Daegu 704-230, South Korea (e-mail: [email protected]. kr). K.-D. Song and D.-D. Lee are with the School of Electronic and Electrical Engineering, Kyungpook National University, Daegu 702-701, South Korea. 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.2007.903918

The most promising approach for the development of NH sensors is based on “molecular recognition.” The basic idea in fabricating a sensor is to use the functional (to sense NH gas) and active (high contact area) material to preferentially detect the species. The applicability of these materials as sensor is based on their selectivity for detection of low concentration of analyte. The present study focuses its attention in this direction. Conducting polymers (CPs) are susceptible to modifications in their chemical structures and such modified functional materials can be used for practical applications. CPs have -electron backbone structure that is responsible for their unusual electronic properties such as electrical conductivity, low energy optical transitions, low ionization potential, and high electron affinity. Sensors based on CPs commonly rely on the changes in resistance and electronic property when they are exposed to a specific analyte. In recent years, sensors based on CPs have been developed for various organic vapors [15], hazardous gases [16], humidity [17], and warfare agent stimulants [18]. Among the CPs, polypyrrole [19] and polyaniline (PANI) [20] were used as the sensing material for the detection of NH gas. PANI exhibits a larger sensitivity for NH gas with a shorter response time (1–2 min) in comparison to polypyrrole. Kanazawa et al. [21], Nylander et al. [22], and Gustafsson et al. [23], [24] have demonstrated that the resistance of PANI film changes when it is exposed to NH gas. There were variations in the film resistance even for the small changes of NH concentrations. Recently, research interest has been focused on the N-substituted PANI derivatives, namely, poly(diphenylamine) (PDPA). Like PANI and many other CPs, PDPA has a simple and reversible acid/base doping/dedoping chemistry and by altering the doping state it is possible to control properties such as free volume, solubility, electrical conductivity, and optical activity [25], [26]. It is widely known that sensitivity of a sensor towards a specific analyte is enhanced by increasing the surface area per unit mass of the sensing component. Also, it is expected that larger surface area can provide unusual high sensitivity and fast response time for the sensor. Therefore, it is important to have a larger surface area for the sensing component while fabricating a sensor device. Electrospinning of polymer offers such a possibility. Electrospinning can result in polymer fibers with nanoscale diameters [27]. Electrospun nanofibrous membranes have larger surface area (approximately 1–2 orders of magnitude larger than the continuous films) and can be modified as a sensor membrane having augmented sensor characteristics [28]. This forms the basis of the present investigation.

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Fig. 1. Schematic diagram describing the processes in fabrication of sensor device using PDPA-PMMA as functional sensing material. (a) Sensing electrode (front side). (b) FNFM sensor.

We present here the fabrication of a sensor device for NH gas using electrospun functional nanofibrous membrane as the sensor material. The composite of PDPA with PMMA was electrospun to obtain the functional nanofibrous membrane (FNFM). PDPA doped with -napthalene sulfonic acid (NSA) is the functional component in the electrospun membrane for sensing NH gas and PMMA provides the fibrous matrix. The fabrication of the NH sensor device with FNFM as sensing materials, sensor characteristics of the device toward NH gas, and mechanism of sensor action are detailed. II. EXPERIMENTAL A. Fabrication of Sensing Substrate Fig. 1 describes the steps involved in the fabrication of sensor device. An interdigit (IDT) electrode (10 mm 7 mm 1.2 mm) was prepared by dc sputtering of platinum on alumina substrate, screen printed using silver paste, and further heated to high temperature. This pattern was used as substrate to load the sensing material [Fig. 1(a)]. The incorporation of functional electrospun membrane in the sensor device is described below. B. Fabrication of Sensor Device With Electrospun Functional Nanofibrous Membrane (FNFM) PDPA was prepared by the oxidative polymerization of diphenylamine (50 mM in 1 M NSA) with ammonium persulphate (0.2 M in 1 M NSA) at 5 C. The green colored precipitate (NSA doped PDPA) was filtered, washed with 1 M NSA, and dried in a vacuum oven. Adequate amounts of PMMA and PDPA were dissolved in DMF/acetone mixture (7 : 3 v/v). The composite solution was taken in the syringe and delivered with a flow rate of 10 mL/h. A potential difference of 25 kV was applied. A distance of 15 cm was kept between the syringe tip and collector. A metering pump was used to control the flow rate of composite solution to obtain the fibrous morphology. FNFM was electrospun on the pattern encoded over IDT electrode to obtain the sensor device [Fig. 1(b)]. The morphology of the fibrous membranes was examined by field emission scanning electron microscope (FESEM) using

Hitachi S-4300 with a field emission gun operated at 200 kV. Fourier transform infrared (FTIR) spectra were recorded using a Bruker IFS 66v FTIR spectrophotometer for studying the intermolecular interactions between the functional group in PMMA and PDPA. Thermogravimetric analysis (TGA) was made using under a nia Dupont 9900/2100 at a heating rate of 10 C trogen atmosphere over a temperature range of 25 C to 800 C. NH sensing characteristics of the FNFM sensor device were determined in a reaction chamber having controlled temperature. The electrical signal of the sensor was collected and calculated through a DAQ board (National Instrument Co.). Resistance of the sensing electrode was followed on injection of NH gas at various concentrations and at various temperatures. The concentration of NH gas was precisely controlled by mass flow controller. UV-visible (Varian, Cary Winuv) spectra were recorded to understand the mechanism of NH sensing by FNFM. III. RESULTS AND DISCUSSION A. Membrane Characterization It is well known that the morphology of a sensing material significantly influence the sensor characteristics. Electrospun FNFM shows interconnected fibrous morphology (Fig. 2). The interfiber twisting in the composite membrane provides high surface area and contact of functional groups in PDPA to NH . The membrane is adherent to IDT electrode and showed dimensional stability during sensor action. The FESEM image in Fig. 2(a) informs that the average diameter of PMMA fiber 400 nm. The fibers are nearly in the membrane is about straight and have regular structure. On the other hand, the PDPA-PMMA membrane has larger interconnectivities and pores [Fig. 2(b) and (c)]. The average diameter of the FNFM having 5% and 2%-wt. of PDPA is found to be 200 nm and 250 nm, respectively. Based on this, it is inferred that the diameter of nanofibers changes with the content of PDPA. It is concluded that the surface area of FNFM increases with increasing content of PDPA. Interconnected fibrous morphology for the FNFM is expected to arise from the probable intermolecular interactions

MANESH et al.: FABRICATION OF FUNCTIONAL NANOFIBROUS AMMONIA SENSOR

Fig. 2. FESEM images of electrospun: (a) PMMA, (b) PMMA-PDPA (2%), and (c) PMMA-PDPA (5%) fibrous membrane.

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Fig. 4. Thermograms of electrospun: (a) PMMA, (b) PMMA-PDPA (2%), and (c) PMMA-PDPA (5%) fibrous membrane recorded at a heating rate of 10 C= min under a nitrogen atmosphere.

Fig. 3. FTIR spectra of electrospun: (a) PMMA, (b) PMMA-PDPA (2%), and (c) PMMA-PDPA (5%) fibrous membrane.

between the functional groups in PMMA and PDPA. Results from FTIR spectroscopy reveal this. FTIR spectrum of pure PMMA and FNFM are presented in Fig. 3. FTIR spectrum of pure PMMA fibrous membrane shows frequency bands at 1475 and 1380 cm and the bands are assigned to CH asymmetric bending and C-CH stretching vibrations of PMMA, respectively. The band with frequencies 1440, 1258, 980, and 748 cm are assigned to CH scissoring, twisting, wagging, and rocking modes of PMMA, respectively. FNFM shows the main bands of PMMA with additional bands of PDPA for stretching vibrations (at the benzenoid and quinoid 1478 and 1580 cm , respectively) [29]. Moreover, it is noted that intensity of C-H stretching band at 2940 cm showed a prominent increase with increasing in PDPA content in FNFM. The variations in band positions and intensities undoubtedly add evidence for the molecular level interactions between groups in PMMA and PDPA of FNFM. Fig. 4 shows the thermograms of electrospun PMMA and FNFM with a loading of 2 and 5 wt% of PDPA. Two steps thermal degradation processes are noticed for FNFM. The first thermal degradation in the range 200 C–300 C is presumably

Fig. 5. Resistance changes of FNFM when exposed to various concentrations of NH gas at room temperature. Inset: Plot of concentration of NH gas Vs resistance of FNFM.

due to the thermal degradation of head-to-head linkages and unsaturated vinyl end groups in the PMMA molecular chains. The second thermal degradation at about 370 C is due to the decomposition of backbone units in PDPA. Furthermore, negligible weight changes are noticed at temperatures exceeding 400 C. B. Sensor Characteristic The sensor behavior of FNFM electrode (Fig. 5) toward NH gas was monitored at room temperature by following the resistance changes in the membrane upon injecting of NH gas into the chamber. A sudden shoot-up in resistance was witnessed on 300 ppm . When NH gas was cut off, the injecting NH gas resistance decreased. The phenomenon of change in resistance was noticed on repeating the process of injecting NH gas and cut off subsequently for several cycles. The results were reproducible and sensing action was quick (short relaxation time). The membrane regained its original state after cutoff. Further, changes in resistance were monitored for the injection of different concentration of NH gas. The inset in Fig. 5 shows the

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Fig. 6. The sensing characteristics of FNFM towards NH gas (50 ppm) in the (a) presence and (b) absence of air. Inset: Variation of the resistance of FNFM with temperature (a) in the presence of air and NH gas (50 ppm) and (b) NH gas (50 ppm) (in the absence of air).

TABLE I A COMPARISON TO THE PERFORMANCE OF FNFM AS AN AMMONIA SENSOR

Fig. 7. Changes in the structure of sensing material (PDPA) on the interaction with NH gas.

sensor behavior of FNFM toward changes in concentration of NH gas. The resistance changes shows linearity between 10 and 300 ppm with a coefficient of sensitivity as 0.9964. Resistance changes were noticed within 1.5 min. The lower detection limit for NH gas at the sensor electrode was found to be 1 ppm. A comparison on the performance of FNFM toward NH gas with other sensors is presented in Table I. Among the CPs based NH sensors [30], [31], [35], FNFM exhibits the combination of lower detection limit and lower response time. However, metal/metal oxide based sensors have lower response times than FNFM but the device fabrication is relatively tedious. The resistance changes in FNFM were followed for an exposure of 50 ppm of NH gas at different temperatures (Fig. 6). Temperature of the sensing element was regulated by means of a microheater. A gradual decrease in the resistance of fibrous material was noticed with increase in temperature (Fig. 6, inset). The resistance of FNFM was higher in the presence of air than in the absence of air.

The peculiar interconnected network morphology of electrospun FNFM and presence of functional material PDPA are the reasons for the sensor characteristics of FNFM toward NH gas. PDPA transforms into the reduced form on treatment with NH gas (Fig. 7) and hence resistance of FNFM increases. In the presence of NH gas, protonated imine groups are neutralized to result the less conducting PDPA. The functional activity of PDPA in FNFM is the nucleus for the sensor action. Further, UV-visible spectroscopy was used to explore the mechanism of sensor action of FNFM towards NH gas. UV-visible spectra were collected after exposing the FNFM to various concentrations of NH gas (Fig. 8.). As the concentrations of NH increase, the absorbance of the peak at 740 nm (which corresponds to protonated imine form of PDPA) (Fig. 7) decreases. Simultaneously, the absorbances at 590 nm increase. The peak around 590 nm (Fig. 8) corresponds to deprotonated and reduced form of PDPA (Fig. 7). The changes in resistance as noticed in Fig. 5 are therefore expected to manifest from

MANESH et al.: FABRICATION OF FUNCTIONAL NANOFIBROUS AMMONIA SENSOR

Fig. 8. UV-visible spectra displaying the changes in optical characteristics of FNFM toward NH gas; concentration: (a) 0.5, (b) 1, (c) 10, (d) 100, (e) 200, (f) 300, and (g) 400 ppm.

the transition in the electronic states of PDPA that arise due to interactions with NH gas. IV. CONCLUSION Electrospun functional nanofibrous membrane having PDPA as functional/sensing material exhibits salient characteristics as NH sensor due to the combined contribution of high surface area of the nanofibers and sensing action of PDPA. ACKNOWLEDGMENT The authors would like to thank the Kyungpook National University Center for Scientific Instruments. REFERENCES [1] M. Bendahan, P. Lauque, C. L. Mauriat, H. Carchano, and J. L. Seguin, “Sputtered thin films of CuBr for ammonia microsensors: Morphology, composition and ageing,” Sens. Actuators B, vol. 84, pp. 6–11, Apr. 2002. [2] D. L. Boucher, J. A. Davies, J. G. Edwards, and A. Mennad, “An investigation of the putative photosynthesis of ammonia on iron-doped titania and other metal oxides,” J. Photochem. Photobiol. A: Chem, vol. 88, pp. 53–64, May 1995. [3] Y. Takao, K. Miyazaki, Y. Shimizu, and M. Egashira, “High ammonia sensitive semiconductor gas sensors with double-layer structure and interface electrodes,” J. Electrochem. Soc., vol. 141, pp. 1028–1034, Apr. 1994. [4] H. M. McConnell, J. C. Owicki, J. W. Parce, D. L. Miller, G. T. Baxter, H. G. Wada, and S. Pitchford, “The cytosensor microphysiometer: Biological applications of silicon technology,” Science, vol. 257, pp. 1906–1912, Sep. 1992. [5] A. Mandelis and C. ChristoÞdes, Physics, Chemistry and Technology of Solid State Gas Sensor Devices. New York: Wiley, 1993. [6] W. Qin, P. Parzuchowski, W. Zhang, and M. E. Meyerhoff, “Optical sensor for amine vapors based on dimer-monomer equilibrium of Indium(III) octaethylporphyrin in a polymeric film,” Anal. Chem, vol. 75, pp. 332–340, Jan. 2003. [7] S. Capone, S. Mongelli, R. Rella, P. Siciliano, and L. Valli, “Gas sensitivity measurements on NO sensors based on copper(II) tetrakis (n-butylamino carbonyl) phthalocyanine lb films,” Langmuir, vol. 15, pp. 1748–1753, Nov. 1999.

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[31] V. V. Chabukswar, S. Pethkar, and A. A. Athawale, “Acrylic acid doped polyaniline as an ammonia sensor,” Sens. Actuators B, vol. 77, pp. 657–663, Jul. 2001. [32] C. N. Xu, N. Miura, Y. Ishida, K. Matuda, and N. Yamazoe, “Selective detection of NH over NO in combustion exhausts by using Au and MoO doubly promoted WO element,” Sens. Actuators B, vol. 65, pp. 163–165, Jun. 2000. [33] H. Mbarek, M. Saadoun, and B. Bessaís, “Screen-printed tin-doped indium oxide (ITO) films for NH gas sensing,” Mater. Sci. Eng. C, vol. 26, pp. 500–504, Mar. 2006. [34] E. Bekyarova, M. Davis, T. Burch, M. E. Itkis, B. Zhao, S. Sunshine, and R. C. Haddon, “Chemically functionalized single-walled carbon nanotubes as ammonia sensors,” J. Phys. Chem. B, vol. 108, pp. 19 717–19 720, Aug. 2004. [35] M. Penza, E. MiMla, M. B. Alba, A. Quirini, and L. Vasanelli, “Selective NH gas sensor based on Langmuir–Blodgett polypyrrole film,” Sens. Actuators B, vol. 40, pp. 205–209, May 1997. [36] B. Ding, J. Kim, Y. Miyazaki, and S. Shiratori, “Electrospun nanofibrous membranes coated quartz crystal microbalance as gas sensor for NH detection,” Sens. Actuators B, vol. 101, pp. 373–380, Jul. 2004. [37] F. Winquist, A. Spetz, and I. Lundstrom, “Determination of ammonia in air and aqeous samples with a gas-sensitive semiconductor capacitor,” Anal. Chim. Acta, vol. 164, pp. 127–138, Mar. 1984. [38] C. Malins, T. M. Butler, and B. D. MacCraith, “Influence of the surface polarity of dye-doped sol-gel glass films on optical ammonia sensor response,” Thin Solid Films, vol. 368, pp. 105–110, Jun. 2000.

Kalayil Manian Manesh received the M.S. degree in industrial chemistry specializing in electrochemistry from Alagappa University, India, in 2004. He is currently working toward the Ph.D. degree at Kyungpook National University, Korea. His research interests include development of newer nanomaterials for device applications.

Anantha Iyengar Gopalan received the M.S. degree in 1979 with specialization in physical chemistry and the Ph.D. degree from Madurai Kamaraj University, Madurai, India. He did postdoctoral work at National Cheng Kung University, Taiwan, R.O.C., and was a Visiting Researcher at Lawrence Berkeley National Laboratory. He has worked in industrial chemistry at Alagappa University, Karaikudi, India, since 1986. He is currently a Visiting Professor at Kyungpook National University, Korea. He is recently focusing his research attention on interdisciplinary topics covering synthesis of conducting polymers, nanostructuring of materials, electrochemistry, and device applications.

Kwang-Pill Lee received the M.S. degree in applied chemistry and the Ph.D. degree from Nagoya University, Japan, in 1985 and 1988, respectively. He did postdoctoral work at the Japan Atomic Energy Research Institute. He was Principal Researcher at Korea Research Institute of Standards and Science and was Visiting Researcher at Lawrence Berkeley National Laboratory. He started his career as an Associate Professor in 1994 and is currently a Professor in the Department of Chemistry Education, Kyungpook National University, Daegu, Korea. His current research activities involve synthesis of nanomaterials and composites, nanofibers and applications of nanomaterials as sensor, battery, separation science, etc.

Padmanabhan Santhosh was born in Erode, India, in 1976. In 2005, he received the Ph.D. degree in industrial chemistry from Alagappa University, India, in the field of polymer electrolytes for lithium batteries. He then moved to Prof. K. P. Lee’s group at Kyungpook National University, Korea, for a postdoctoral stay. He is currently at Max-Planck-Institute for Solid State Research, Stuttgart, Germany. Besides polymer electrolytes, his research interests include the chemical and electrochemical modifications of carbon nanotubes and development of the modified carbon nanotubes for device applications.

Kap-Duk Song received M.S. degrees and the Ph.D. degree from Kyungpook National University, Korea, in 1994 and 2006, respectively. He was an Assistant Professor at Daegu Science College, Korea. Currently he is a Research Professor at the Advanced Display Manufacturing Research Center, Kyungpook National University, Daegu. He has performed research on micro gas sensors and MEMS processes.

Duk-Dong Lee received M.S. degrees from Kyungpook National University, Korea, in 1974 and the Ph.D. degree from Yon-Dei University, Korea, in 1984. He was a Visiting Professor at the School of Electrical Engineering, Cornell University, from 1981 to 1982. Currently he is a Professor at the School of Electronics and Electrical Engineering, Kyungpook National University, Daegu. He has performed research on semiconductor gas sensors since 1978 and in the field of optical sensors. Prof. Lee was the president of Korea Sensors Societies from 2005 to 2006.