Surface Plasmon Resonance-Based Fiber Optic ... - IEEE Xplore

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 33, NO. 13, JULY 1, 2015

Surface Plasmon Resonance-Based Fiber Optic Chlorine Gas Sensor Utilizing Indium-Oxide-Doped Tin Oxide Film Satyendra K. Mishra and Banshi D. Gupta

Abstract—Fabrication and characterization of a surface plasmon resonance (SPR)-based fiber optic chlorine gas sensor are carried out. The fiber optic probe is fabricated by depositing a thin layer of indium-oxide-doped tin oxide over a silver-coated unclad core of the fiber. The SPR spectra of the chlorine gas for its different concentrations are obtained. It is observed that the resonance wavelength increases as the concentration of the chlorine gas increases and appears to saturate for higher concentrations of the gas. The sensitivity of the sensor depends on the thickness and the doping concentration of the indium-oxide-doped tin oxide film. The optimum thickness and the atomic weight percent doping concentration of the film are found to be 12 nm and 6 at. wt.%, respectively. To compare the performance, experiments are also carried out on probes coated with indium oxide and tin oxide layers over silver coated unclad core of the fiber. The performance of both the probes is found to be inferior to the one coated with indium-oxide-doped tin oxide layer. Further, the indium oxide doped tin oxide layer based probe is highly sensitive to chlorine gas for low concentrations. The sensor has low response time and is reversible. The proposed probe has advantages of online monitoring and remote sensing. Index Terms—Chlorine gas, indium oxide (In2 O3 ), optical fiber sensor, surface plasmon, sensor, tin oxide (SnO2 ).

I. INTRODUCTION remendous amount of work has been reported in the literature on the sensing of various gases using different techniques. Most of the gases have industrial applications but are also hazardous to environment and health. Chlorine is one of such gases. It is highly toxic and can affect the nervous system of human body if inhaled. However, it is used in industrial processes as well as in the purification of water. Further, it has a very low toxic limit and hence the detection of low concentration of chlorine in environment is very important. Usually, chlorine gas is detected by chemical detecting tubes or gas chromatography. These devices are very bulky and therefore there is a high demand of a compact chlorine gas sensor. Solid state electrochemical sensors for the detection of chlorine gas using metal chlorides have been reported in the literature [1]–[3]. However, these sensors are incapable of detecting chlorine gas

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Manuscript received December 3, 2014; revised February 17, 2015; accepted March 10, 2015. Date of publication March 11, 2015; date of current version May 22, 2015. This work was supported by the Council of Scientific and Industrial Research, India. The authors are with the Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India (e-mail: [email protected]; [email protected]). 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/JLT.2015.2412615

with concentration lower than 100 ppm. In recent years there is a growing interest in metal oxide materials for the sensing of gases [4]. Most of the metal oxide based gas sensors can operate in landfill, automotive industry for combustion emission control and many other industrial fields [5]–[6]. Although metal oxide based gas sensors have considerable applications but there are some problems related to their performance like reproducibility, stability, sensitivity and selectivity which must be overcome. If these or at least some of these shortcomings can be overcome then their utilities will improve accordingly [4]. Tin oxide and indium oxide doped tin oxide powders are used in many fields/applications such as microelectronics, solar cells and electroluminescence. In addition to these applications, tin oxide, indium oxide and indium oxide doped tin oxide also find applications in the sensing of various gases like CH4 , H2 , CO, C2 H5 OH, etc., [7]–[15]. Surface plasmon resonance (SPR) technique has been widely used for the sensing of refractive index and various chemical and biological analytes [16]–[18]. Surface plasmons are basically the free electron charge density oscillations at the metal-dielectric interface. Due to these oscillations a transverse magnetically polarized electromagnetic wave, also called surface plasmon wave, propagates along the metal-dielectric interface with maximum field at the interface which decays exponentially in both the media. For the excitation of the surface plasmons, p-polarized light is used with propagation constant equal to the propagation constant of the surface plasmon wave. Due to the negative real part of the complex dielectric constant of the metal the propagation constant of the surface plasmon wave is always greater than the propagation constant of the direct excitation light. Hence, to achieve the resonance condition, the propagation constant of the excitation light is increased using Kretschmann configuration in which the base of a high refractive index prism is coated with a thin metal layer and the dielectric medium to be studied or sensed is kept in contact of the other side of the metal layer. The light is incident on the metal-dielectric interface through one of the faces of the prism and the intensity of the reflected light is measured as a function of angle of incidence. When the two propagation constants (one of the excitation evanescent wave at the prismmetal interface and the other of the surface plasmon wave at the metal-dielectric interface) become equal a maximum transfer of power from excitation light to surface plasmons occurs resulting in the decrease in the intensity of the reflected light. This occurs at a particular angle of incidence called resonance angle which depends on the dielectric constant/refractive index

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MISHRA AND GUPTA: SURFACE PLASMON RESONANCE-BASED FIBER OPTIC CHLORINE GAS SENSOR UTILIZING INDIUM-OXIDE-DOPED

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of the dielectric medium. Due to the number of shortcomings of prism based SPR sensors, the prism is replaced by the core of an optical fiber. Number of sensors have been reported that use optical fiber in collaboration with SPR technique for the sensing of various chemical and biological analytes [19]–[23]. In this paper, we report the fabrication and characterization of a SPR based fiber optic chlorine gas sensor using coating of indium oxide doped tin oxide thin film over silver coated optical fiber core. Although both indium oxide and tin oxide are capable of sensing various gases, however, if these two oxides are mixed in a certain ratio then the doped material becomes sensitive to one particular gas due to the creation of defects and formation of active sites in the doped material. In the proposed sensor, the doping concentration is fixed to sense chlorine gas with maximum sensitivity. For sensing and calibration of the probe, a polychromatic source is used to launch light in the optical fiber probe and the SPR spectrum of the transmitted light is recorded for different concentrations of the chlorine gas. The resonance wavelength of the SPR spectrum increases as the concentration of the chlorine gas increases. The shift in the resonance wavelength occurs due to the interaction of the gas with the indium oxide doped tin oxide film resulting in the change in the dielectric constant of the film. Further, it is observed that the doping concentration of the indium oxide in tin oxide and its thickness affect the sensing properties of the film towards chlorine gas. The sensitivity of the sensor is studied for various gases and it is found that the probe is highly sensitive towards chlorine gas. II. EXPERIMENTAL DETAILS A. Fabrication of Probe To fabricate the probe plastic clad silica fiber of core diameter 600 μm and numerical aperture 0.40 was used. Out of 24 cm total length of the fiber used 1 cm length from the middle was uncladed using sharp blade. The unclad core of the fiber was cleaned by acetone, methanol and ion bombardment before coating with sensing layers. The cleaned core was coated with silver layer of thickness 40 nm using thermal evaporation coating machine. Number of such fibers with coating of silver layer were prepared. The silver coated fibers were then coated with indium oxide doped tin oxide thin film of different doping concentrations. In addition to these, for one particular doping concentration, the indium oxide doped tin oxide film of different thicknesses were coated over silver coated fiber optic probes using thermal evaporation coating machine. The film thicknesses were measured by inbuilt quartz crystal thickness monitor fixed in the thermal evaporation vacuum coating unit. The thicknesses were also confirmed using thickness profilometer. B. Experimental Set Up The experimental setup of the chlorine gas sensor is similar to that given in ref. [21] except that for the present study the chlorine gas pipe was connected to the nitrogen gas assembly for the purging of the gas. The concentration range of the chlorine gas in the chamber was varied from 10 to 100 ppm and for

Fig. 1. (a) SPR spectra of fiber optic probe having Ag (40 nm) and In2 O3 (6 at. wt.%) doped SnO2 (12 nm) coatings for different concentrations of chlorine gas, and (b) variation of resonance wavelength with the concentration of chlorine for the same probe.

each concentration the resonance wavelength was determined from the SPR spectrum. These data were used to determine the sensitivity of the sensor which is defined as the shift/change in the resonance wavelength per unit change in the concentration of the gas. The same procedure was followed for all the probes fabricated. III. RESULTS AND DISCUSSION A. Sensor Characterization Fig. 1(a) shows the SPR spectra for different concentrations of chlorine gas varying from 10 to 100 ppm around the fiber optic probe fabricated with silver film of 40 nm thickness and indium oxide doped tin oxide thin film of 12 nm thickness. The doping concentration of indium oxide in tin oxide film was 6 at. wt.%. It is observed that the wavelength corresponding to dip in the transmitted spectrum, called as resonance wavelength, depends on the concentration of the chlorine gas around the probe.

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As the concentration of the chlorine gas increases the resonance wavelength increases. All the SPR spectra shown in Fig. 1(a) were recorded after 1 minute of inserting gas in the chamber. However, the SPR spectra were also recorded at different times (greater than 1 min) but no change was observed implying that the response time of the probe is less than 1 min. It may be noted that SPR spectrum for 0 ppm concentration is absent in Fig. 1(a). This is because to find the SPR spectrum for a given concentration of the gas the transmission spectrum of 0 ppm was used as a reference and all the SPR spectrum were determined by dividing their transmission spectra by the transmission spectrum of 0 ppm concentration. Thus if we show the SPR spectrum of 0 ppm concentration it will be just a horizontal line at 1.0 normalized transmitted power. To see the variation more clearly, the resonance wavelength is plotted as a function of concentration of the chlorine gas in Fig. 1(b). It may be noted from the figure that as the concentration of chlorine gas increases the resonance wavelength increases nonlinearly. The increase is fast in the beginning but slows down for higher concentrations of the gas and around 100 ppm concentration it appears to saturate. It means that the sensor is applicable for low concentrations of the chlorine gas. It may also be noted from Fig. 1(a) that, apart from shift in resonance wavelength, the depth of the each SPR dip increases as the concentration of the chlorine gas increases. This implies that both the real and imaginary parts of the dielectric constant of the indium oxide doped tin oxide film change with the change in the chlorine gas concentration. The increase in the depth of the SPR spectrum is due to the change in the imaginary part of the dielectric constant while the shift in resonance wavelength is due to the change in the real part of the dielectric constant. Both, the real and the imaginary parts of the dielectric constant, increase up to a certain values and after that no change occurs on increase in the concentration of the chlorine gas. These kinds of changes occur because the indium oxide doped tin oxide film has a fixed number of active sites for the chlorine gas molecules to react/interact. For low concentration of the gas, the number of active sites available per chlorine molecule is large. As the concentration of the gas increases this ratio decreases and hence the shift in resonance wavelength does not increase linearly. For the gas concentration above 100 ppm, a fixed number of chlorine molecules interact with the active sites and therefore no further increase in the resonance wavelength is observed for concentration higher than 100 ppm due to unavailability of the active sites. To see whether dopant is important for sensing of chlorine gas, experiments were carried out on fiber optic SPR probe fabricated with coatings of silver and tin oxide layers over unclad core of the fiber. Fig. 2(a) shows the SPR spectra for different concentrations of the chlorine gas around this probe. Similar to Fig. 1(a) SPR spectrum shifts to higher wavelength as the concentration of the chlorine gas increases. The variation of resonance wavelength with the concentration of the chlorine gas is shown in Fig. 2(b). It may be noted that the variation is similar to that shown in Fig. 1(b) but the shift in resonance wavelength is smaller than that obtained in Fig. 1(b). The reason of shift in resonance wavelength is the same as was for indium oxide doped tin oxide coated probe, i.e., the interaction of the chlorine gas with the tin oxide layer resulting in the change in


Fig. 2. (a) SPR spectra of fiber optic probe having Ag (40 nm) and SnO2 (12 nm) coatings for different concentrations of chlorine gas, and (b) variation of resonance wavelength with the concentration of chlorine for the same probe.

the dielectric constant of the tin oxide layer. However, in this case, the change in the refractive index of the film is small in comparison to indium oxide doped tin oxide film. Similar kinds of experiments were also carried out on indium oxide coated film over silver coated fiber optic probe. The SPR spectra obtained for different concentrations of the chlorine gas for indium oxide coated probe are shown in Fig. 3(a). Again the SPR spectrum shifts to higher wavelength as the concentration of the chlorine gas around the probe increases. The actual shift in resonance wavelength can be observed from Fig. 3(b). Again the shift in the resonance wavelength is small in comparison to the indium oxide doped tin oxide coated probe. Thus the fiber optic SPR sensor prepared with indium oxide doped tin oxide film over silver coated fiber optic core is highly sensitive in comparison to indium oxide or tin oxide coated film over silver coated fiber optic probe. B. Optimization of Parameters The calibration curve of the sensor fabricated using indium oxide doped tin oxide film shown in Fig. 1(b) corresponds to


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Fig. 3. (a) SPR spectra of fiber optic probe having Ag (40 nm) and In2 O3 (12 nm) coatings for different concentrations of chlorine gas, and (b) variation of resonance wavelength with the concentration of chlorine for the same probe.

Fig. 4. Variation of shift in the resonance wavelength for the change in chlorine gas concentration from 10 to 100 ppm with (a) In2 O3 doping concentration in SnO2 , and (b) the thickness of In2 O3 doped SnO2 film.

6 at. wt.% doping concentration and 12 nm thick film of indium oxide doped tin oxide. To optimize these parameters for the best performance of the sensor, first the experiments were carried out on probes prepared with different doping concentrations and 8 nm thickness of the film. Fig. 4(a) shows the shift/change in the resonance wavelength for the chlorine gas concentration change from 10 to 100 ppm as a function of the doping concentration of the indium oxide in tin oxide film coated over the silver coated fiber optic core. The figure shows that as the doping concentration increases first the shift/change in the resonance wavelength increases and after attaining a maximum value it starts decreasing. The maximum shift occurs for 6 at. wt.% doping concentration. In the same way the thickness of the indium oxide doped tin oxide film was optimized. Fig. 4(b) shows the variation of the shift/change in the resonance wavelength for the chlorine gas concentration change from 10 to 100 ppm as a function of indium oxide doped

tin oxide film thickness for 6 at. wt.% doping concentration. In this case also, as the thickness of the film increases the shift in the resonance wavelength increases up to a certain thickness and then decreases on further increase in the thickness of the film. The maximum shift and hence the sensitivity is obtained around 12 nm thickness of the indium oxide doped tin oxide film. Thus, 6 at. wt.% doping concentration and 12 nm film thickness of the indium oxide doped tin oxide over silver coated unclad core of the fiber are the optimized parameters of the indium oxide doped tin oxide film for the best performance of the chlorine gas sensor. The results shown in Fig. 4(a) can be explained on the basis of active sites or defects in the film. This is because the total shift in resonance wavelength depends on the active sites available on the indium oxide doped tin oxide film which depends on the doping concentration. As the doping concentration increases the defects in doped film increases and reaches a maximum

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Fig. 5. Shift in the resonance wavelength for the change in concentration from 10 to 100 ppm, and 10 to 30 ppm for different gases around In2 O3 doped SnO2 film coated probe.

value at a particular value of the doping concentration and then starts decreasing on further increase in the doping concentration. The maximum shift in resonance wavelength corresponds to maximum number of active sites or maximum defects. As far as indium oxide doped tin oxide is concerned, it is a high refractive index material and it is known that a high refractive index film over metal layer in SPR based sensor increases the field at the interface of the high index layer and sensing medium. The field also depends on the thickness of the high index film and is maximum for a particular thickness of the indium oxide doped tin oxide film. Therefore, the shift is maximum for particular values of the doping concentration and the ITO film thickness.

Fig. 6. Shift in the resonance wavelength for the concentration change from 10 to 100 ppm in the case of fiber optic SPR probe coated with (a) In2 O3 , and (b) SnO2 thin film over the silver coated core for different gases.

C. Sensitivity To check the sensitivity of the sensor for other gases, the experiments were carried out on SPR based fiber optic probe prepared using indium oxide doped tin oxide film with 6 at. wt.% doping concentration and 12 nm thickness for gases such as hydrogen sulphide (H2 S), hydrogen (4% H2 in 96% N2 ), nitrogen (N2 ), methane (CH4 ), carbon monoxide (CO) and ammonia (NH3 ) for their two concentrations 10 and 100 ppm. The shift/change in resonance wavelength obtained for the concentration change from 10 to 100 ppm in the case of these gases are shown in Fig. 5. It can be seen that the indium oxide doped tin oxide film interacts with all the above mentioned gases but the maximum shift/change in the resonance wavelength occurs for the chlorine gas which implies that the indium oxide doped tin oxide film is more sensitive for the chlorine gas. The next gas which has more sensitivity is hydrogen sulphide. However, in comparison to chlorine gas, it is five times less sensitive. The similar kinds of results are also shown for the change in gas concentration from 10 to 30 ppm. In this case, the trend is the same but the sensor is nine times more sensitive to chlorine gas than hydrogen sulphide gas. Thus, the indium oxide doped tin oxide film based fiber optic SPR sensor with 6 at. wt.% doping concentration and 12 nm thickness of the film is highly sensitive to chlorine gas for its low concentrations.

The sensitivity of the indium oxide and tin oxide coated thin film over silver coated fiber optic probes were also checked for the same gases that were used for indium oxide doped tin oxide probe. Figs. 6(a) and 6(b) shows the shift/change in the resonance wavelength for these gases for the change in concentration from 10 to 100 ppm in the case of indium oxide and tin oxide coated fiber optic sensor, respectively. It is observed that all the gases interact with indium oxide and tin oxide thin films. However, the sensors based on the coatings of these oxides are only two times more sensitive to chlorine gas in comparison to next sensitive gas (hydrogen sulphide). Further, the shift/change in the resonance wavelength for the change in gas concentration from 10 to 100 ppm in the case of chlorine gas is much larger in the case of indium oxide doped tin oxide film than the indium oxide or tin oxide film based SPR sensor. Hence the sensors utilizing indium oxide and tin oxide thin films are not very sensitive for the chlorine gas if compared with the indium oxide doped tin oxide thin film based fiber optic SPR sensor. D. Sensing Principle Indium oxide and tin oxide are the n-type semiconductors and are used in gas sensing devices because these semiconductors react with most of the gases. These materials when come in


contact of the atmosphere, they easily adsorb oxygen molecules. When chlorine gas comes in contact with this kind of thin film it reacts with physisorbed molecules and changes the optical properties (dielectric constant) of the film according to the following reaction: 1 1 2− Cl2 + O(ad) → Cl(ad) + O2 + e− (1) 2 2 1 1 Cl2 + O0 2 −(ad) → Cl0 + O2 + e− . (2) 2 2 Due to this change in the dielectric constant of the indium oxide/tin oxide on exposure to the chlorine gas the change in the resonance wavelength in their SPR spectrum occurs. Indium oxide doped tin oxide thin film is also an n-type semiconductor. The doping of indium oxide in tin oxide increases the defect level in tin oxide which provides the excess active sites for the gases. When gas comes in contact with the indium oxide doped tin oxide thin film the change in the dielectric constant occurs due to (i) adsorbed oxygen molecules and (ii) excess of active sites created due to the increase in the defect level in the structure. Both the reasons are responsible for the shift in the resonance wavelength in the presence of chlorine gas while the second reason is responsible for the larger increase in the resonance wavelength in comparison to indium oxide and tin oxide coated SPR sensors. In other words, the change in the dielectric constant is large in the indium oxide doped tin oxide film in comparison to the indium oxide and tin oxide thin films. E. Reversibility The reversibility of the sensor response was checked by carrying out experiments on 10 and 100 ppm concentrations of the chlorine gas. To perform this experiment, first, all the gases were evacuated from the chamber and then chlorine gas of 10 ppm concentration was inserted into the chamber. The resonance wavelength from its SPR spectrum was measured using the method reported above. These measurements were carried out after every 2 min for three times and the resonance wavelengths were determined. After these measurements, the gas chamber was evacuated and chlorine gas of 100 ppm concentration was inserted in the chamber. The evacuation and insertion took 4 min and again the SPR spectrum was recorded and the resonance wavelength was measured after every 2 min. The chamber was then evacuated and chlorine gas of 10 ppm concentration was inserted into the chamber and resonance wavelength was measured from the SPR spectra recorded after every 2 min. This process was repeated three times. The resonance wavelengths so determined for 10 and 100 ppm concentrations with time are shown in Fig. 7. It may be noted that the response of the sensor is reversible. It may be noted that all the measurements carried out above are in dry atmosphere. This is because the chamber that has been used does not have facility of changing and measuring humidity inside it and hence the calibration curve of the sensor may change in wet environment. Although the adsorption of water molecules on metal oxide surface does not donate electrons to sensing layer, however, it may lower the sensitivity of the

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Fig. 7. Reversibility characteristics of the sensor response between 10 and 100 ppm concentration of chlorine gas in the chamber.

sensor due to some other reasons. The reason can be either the reaction between the water molecules and the surface oxygen or the reduction of surface area and hence the chemisorption of oxygen due to the adsorption of water molecules [24]. The actual amount of effect on the sensor response, in the present study, can be ascertained only after performing the experiment with varying humidity which will be our next study after the modification in the gas chamber. The effect of humidity on tin-oxide film based reducing and combustible gases has been reported in the literature [25]. It has been reported that the incorporation of an organometallic film over the metal oxide layer can dramatically reduce the effect of humidity in the case of carbon monoxide. Therefore, in the case of this study, if humidity affects the sensor response, then something of this kind can be explored to reduce the effect of humidity. Recently we reported a SPR based fiber optic ammonia gas sensor using indium tin oxide and polyaniline layers over unclad core of the fiber [21]. This work and the present study are similar in the experimental setup but are different in terms of fiber optic probes. The probes in the two cases have been designed according to the gases to be sensed. For example in ref [21], the probe has coatings of indium tin oxide (90:10) over unclad core of the fiber with an over layer of polyaniline sensitive to ammonia gas. In the present study, the probe has coating of indium oxide doped tin oxide layer over silver coated unclad core of the fiber. The doping concentration has been adjusted to achieve the highest sensitivity for chlorine gas. As shown in Fig. 5 it is very little sensitive to ammonia gas as compared to chlorine gas. Generally, chlorine gas is detected by chemical detecting tubes or gas chromatography which are very bulky. There are solid state electrochemical sensors for the detection of chlorine gas using metal chlorides [1]–[3]. However, these sensors are incapable of detecting chlorine gas concentration lower than 100 ppm. In addition to these sensors/devices, metal oxide based gas sensors have also been reported [4]. But these sensors have some problems related to their performance like reproducibility,

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stability, sensitivity and selectivity. The proposed sensor does not have these kinds of shortcomings. It is low weight, highly sensitive and cheap. In addition, it can be used for online monitoring and remote sensing of the gas. It may be noted that the performance of SPR based fiber optic sensors depend on the ambient temperature. This is because the change in temperature changes the dielectric properties of the materials involved in the probe such as fiber core, silver and indium oxide doped tin oxide. The change is reflected in the resonance wavelength and the sensitivity of the sensor. However, the change is very slow and for the operating temperatures around room temperature hardly any change is observed. IV. CONCLUSION Fabrication and characterization of a highly sensitive and selective surface plasmon resonance based fiber optic sensor for the detection of low concentration of the chlorine gas have been carried out. The sensor probe is fabricated by coating an unclad core of the optical fiber with silver and indium doped tin oxide layers. The increase in the concentration of the chlorine gas around the sensing probe increases the resonance wavelength. To achieve the maximum sensitivity of the sensor, the doped concentration and the thickness of the indium oxide doped tin oxide layer have been optimized. The sensitivity of the probe has also been studied for various other gases and it has been observed that the probe is highly sensitive for chlorine gas. The response time of the sensor is low and its performance is reversible. Since the probe is fabricated on optical fiber it has some additional advantages such as low cost, miniaturized probe, capability of online monitoring and remote sensing. REFERENCES [1] A. Pelloux, P. Fabry, and P. Durante, “Design and testing of a potentiometric chlorine gauge,” Sens. Actuators, vol. 7, pp. 245–252, 1985. [2] H. Aono, E. Sugimoto, Y. Mori, and Y. Okajima, “Potentiometric chlorine gas sensor using BaCl2 -KCl solid electrolyte: The influence of barium oxide contamination,” Chem. Lett., vol. 131, pp. 34–35, 2000. [3] J. Liu and W. Weppner, “Limiting-current chlorine gas sensor based on β -alumina solid electrolyte,” Sens. Actuators B, vol. 6, pp. 270–273, 1992. [4] E. Comini, V. Guidi, M. Ferroni, and G. Sberveglieri, “Detection of landfill gases by chemoresistive sensors based on titanium, molybdenum, tungsten oxides,” IEEE Sens. J., vol. 5, no. 1, pp. 4–10, Feb. 2005. [5] A. K. Prasad and P. I. Gouma, “MoO3 and WO3 based thin film conductimetric sensors for automotive applications,” J. Mater. Sci., vol. 21, pp. 4347–4352, 2003. [6] K. Ihokura and J. Watson. The Stannic Oxide Gas Sensor: Principles and Applications. Boca Raton, FL, USA: CRC Press, 1994. [7] A. Heilig, N. Barsan, U. Weimar, and W. G¨opel, “Selectivity enhancement of SnO2 gas sensors: Simultaneous monitoring of resistances and temperatures,” Sens. Actuators B, vol. 58, pp. 302–309, 1999. [8] N. Barsan, J. R. Stettner, M. Findley, and W. G¨opel, “The temperature dependence of the response of SnO2 -based gas sensing layers to O2 , CH4 and CO,” Sens. Actuators B, vol. 66 pp. 31–33, 2000. [9] T. Becker, S. Ahlers, C. B. Braunmuhl, G. Muller, and O. Kiesewetter, “Micromachined thin film SnO2 gas sensors in temperature-pulsed operation mode,” Sens. Actuators B, vol. 77, pp. 55–61, 2001.

[10] G. G. Mandayo, E. Castano, F. J. Gracia, A. Cirera, A. Cornet, and J. R. Morante, “Strategies to enhance the carbon monoxide sensitivity of tin oxide thin films,” Sens. Actuators B, vol. 95, pp. 90–96, 2003. [11] T. Sako, A. Ohmi, H. Yumoto, and K. Nishiyama, “O3 gas sensor of thin film semiconductor In2 O3 ,” Surf. Coat. Technol., vol. 142–144, pp. 781–785, 2001. [12] N. G. Patel, P. D. Patel, and V. S. Vaishnav, “Indium tin oxide (ITO) thin film gas sensor for detection of methanol at room temperature,” Sens. Actuators B, vol. 96, pp. 180–189, 2003. [13] Y. Wang, X. Wu, Y. Li, and Z. Zhou, “Mesostructured SnO2 as sensing material for gas sensors,” Sensors, vol. 48, pp. 627–632, 2004. [14] S. Elouali, L. G. Bloor, R. Binions, I. P. Parkin, C. J. Camalt, and J. A. Darr, “Gas sensing with nano-indium oxides (In2 O3 ) prepared via continuous hydrothermal flow synthesis,” Langmuir, vol. 28, pp. 1879–1885, 2012. [15] S. K. Mishra and B. D. Gupta, “Surface plasmon resonance-based fiberoptic hydrogen gas sensor utilizing indium–tin oxide (ITO) thin films,” Plasmonics, vol. 7, pp. 627–632, 2012. [16] S. Lal, S. Link, and N. J. Halas, “Nano-optics from sensing to waveguiding,” Nature Photon., vol. 1, pp. 641–648, 2007. [17] W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Efficient unidirectional nano slit couplers for surface plasmons,” Nature, vol. 424, pp. 824–830, 2007. [18] J. Homola, S. S. Yee, and G. Gauglitz, “Theory and modelling of optical waveguide sensors utilising surface plasmon resonance,” Sens. Actuators B, vol. 54, pp. 3–15, 1999. [19] R. C. Jorgenson and S. S. Yee, “A fiber-optic chemical sensor based on surface plasmon resonance,” Sens. Actuators B, vol. 12, pp. 213–220, 1993. [20] B. D. Gupta and R. K. Verma, “Surface plasmon resonance-based fiber optic sensors: Principle, probe designs, and some applications,” J. Sens., vol. 2009, p. 979761, 2009. [21] S. K. Mishra, D. Kumari, and B. D. Gupta, “Surface plasmon resonance based fiber optic ammonia gas sensor using ITO and polyaniline,” Sens. Actuators B, vol. 171–172, pp. 976–983, 2012. [22] S. Singh and B. D. Gupta, “Fabrication and characterization of a surface plasmon resonance based fiber optic sensor using gel entrapment technique for the detection of low glucose concentration,” Sens. Actuators B, vol. 177, pp. 589–595, 2013. [23] R. Tabassum, S. K. Mishra, and B. D. Gupta, “Surface plasmon resonancebased fiber optic hydrogen sulphide gas sensor utilizing Cu–ZnO thin films,” Phys. Chem. Chem. Phys., vol. 15, pp. 11868–11874, 2013. [24] C. Wang, L. Yin, L. Zhang, D. Xiang, and R. Gao, “Metal oxide gas sensors: Sensitivity and influencing factors,” Sensors, vol. 10, pp. 2088– 2106, 2010. [25] D. S. Vlachos, P. D. Skafidas, and J. N. Avaritsiotis, “The effect of humidity on tin-oxide thick-film gas sensors in the presence of reducing and combustible gases,” Sens. Actuators B, vol. 24–25, pp. 491–494, 1995.

Satyendra K. Mishra received the M.Sc. degree in physics from D.D.U. Gorakhpur University, India, in 2008 and the M.Tech. degree in nanotechnology from Jamia Millia Islamia, New Delhi, in 2010. Since July 2010, he has been working toward the Ph.D. degree at I.I.T. Delhi, Delhi, India. He is a Student Member of the OSA.

Banshi D. Gupta received the Ph.D. degree from Indian Institute of Technology Delhi, Delhi, India, in 1979 where he is currently a Professor of physics. He has published more than 110 research papers in journals and 80 papers in conferences. He is authored books entitled Fiber Optic Sensors: Principles and Applications (New Delhi, India: NIPA New Delhi, 2006) and Fiber Optic Sensors based on Plasmonics (Singapore: World Scientific, 2015). His current area of interest is plasmonic sensors.