Sensors and Actuators B 147 (2010) 37–42
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Detection of volatile organic compounds by an interferometric sensor a,∗ ˜ Carlos Martínez-Hipatl a , Severino Munoz-Aguirre , Georgina Beltrán-Pérez a , Juan Castillo-Mixcóatl a , Javier Rivera-De la Rosa b a b
Facultad de Ciencias Físico-Matemáticas, Benemérita Universidad Autónoma de Puebla, Av. San Claudio y 18 Sur, Col. San Manuel CU, Puebla, Pue. México C.P. 72570 Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, Monterrey, N. L., Mexico
a r t i c l e
i n f o
Article history: Received 11 June 2009 Received in revised form 6 March 2010 Accepted 10 March 2010 Available online 17 March 2010 Keywords: Polydimethylsiloxane Gas sensor Optical sensing
a b s t r a c t In the present work the development of an interferometric gas sensor to detect volatile organic compounds (VOCs) is presented. A polydimethylsiloxane (PDMS) sensing ﬁlm was deposited on a glass substrate. PDMS has the property of swelling and/or change its refractive index when it interacts with VOCs. This causes a fringe shift in a Pohl interferometric arrangement. Such fringe shift was measured by a conventional photodetector. The results showed that the sensor response was almost linear in the concentration range of 0–24,000 ppm for ethanol. The sensor responses for other VOCs were also measured. From measurements and calculations it was found that the sensor response characteristics are determined essentially by the swelling effect more than the refractive index change. Therefore it could be said that the sensor response is related to the volume of the VOC molecule. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The detection and quantiﬁcation of volatile organic compounds (VOCs) is important in a variety of scientiﬁc and industrial ﬁelds, such as food or beverage control quality  or even in the diagnosis of some diseases like caries or cancer , besides of the environmental monitoring . In some ﬁelds, such as food or beverages industries, quality control is performed by a panel of humans; however recently the use of electronic systems based on sensor arrays which are called electronic noses has become common . Therefore, an exhaustive research on gas sensors is required. Although there are a variety of gas sensors, it is well known that the effectiveness of a sensor array depends upon the variety of parameters used to perform the recognition . Thus, there are systems based on just one type of sensors, while others combine different types of sensors. As is well known, it is possible to employ a variety of physical parameters that can be used for sensors, such as conductivity, mass loading effect and optical signals, among others [6,7]. The optical sensors have some advantages such as the low noise level, the possibility of being integrated and the fact that they are inert to electromagnetic noise. Among the optical methods, interferometry has been used in a variety of ﬁelds such as the measurement of strengths, refractive index , distance or speed evaluation and recently applications to the detection of organic vapors have been reported [9–12].
Zhang et al. proposed an interferometric sensor based on a zeolite sensing ﬁlm deposited on the tip of an optical ﬁber . The refractive index change of the sensing ﬁlm owing to its interaction with vapor molecules was measured as the sensor response. There are shown results for propanol detection in different concentrations. The sensor shows response saturation at approximately 1500 ppm, which is quite low. On the other hand, Gauglitz et al. studied a sensor based on spectral interferometry . They used white light to illuminate the substrate with the deposited ﬁlm and analyzed the correspondent interference spectrum. The measurement of sensor responses to various VOC was performed. However, their setup and analysis are quite complicated and the results are not directly obtained since it is necessary to perform the information extraction. Moreover, there was not analyzed the effect of the variation of sensing ﬁlm refractive index related to the interaction with gas molecules. In the present work, the development of a sensor based on interferometry that operates in the so-called quadrature point of an interference pattern with a direct linear response is presented. The used setup was quite simple and it has the possibility of being extended to be used with optical ﬁber, used to transport light and to receive the interference fringe pattern. Moreover, an analysis of the effect of the refractive index variation on the fringes pattern caused by the interaction of the ﬁlm with the vapor molecules is presented. 2. Sensor theory
∗ Corresponding author. Tel.: +52 222 229 5500x7588; fax: +52 222 229 5636. ˜ E-mail address: [email protected]
(S. Munoz-Aguirre). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.03.042
The operation principle of the sensor based on interferometry, which uses a Pohl interferometer arrangement , is shown in Fig. 1.
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Fig. 1. Pohl interferometer arrangement used as a VOC sensor.
The sensor consists of a glass substrate with a deposited polymer ﬁlm such as polydimetylsiloxane (PDMS). This material has the property of swelling and/or change its refractive index (optical path length change) when it interacts with organic vapor molecules. A laser is used as the light source I0 , which produces different reﬂections in the interfaces air–substrate I1 , substrate–polymer I2 and polymer–air I3 , respectively. The interference of the light beams I1 , I2 and I3 produces an interference fringes pattern on a screen located in front of the substrate. The swelling and/or change of refractive index of the polymer ﬁlm would cause the shift of the fringes pattern, which can be measured with a conventional photodetector. For simplicity, the glass substrate and the PDMS ﬁlm are considered ﬂat and their surfaces are parallel. However, in practice it is not true since there must exist a small angle between the surfaces, which makes possible the interference of the beams at certain ﬁnite distance. In this case, the angle is considered small enough to be negligible in the calculations of optical path length difference and it does not affect the ﬁnal results. According to the ﬁlm-on-substrate geometry as is shown in Fig. 2 and from Fresnel equations, the light intensity I of the light beams reﬂected at the different interfaces were calculated. The used refractive index values were n1 = 1, n2 = 1.5 and n3 = 1.404 for air, glass and PDMS ﬁlm, respectively. The incident angle was set to be 0.275 rad (12.5◦ ), the glass substrate and ﬁlm thicknesses were d = 6 mm and d1 = 8 m, respectively. The resulting intensities were I1 = 0.043I0 , I2 = 0.001I0 and I3 = 0.028I0 , where I0 is the intensity of the incident beam. The intensities of beams I1 and I3 are of the same order, while the beam I2 is much smaller; then only the interference of the former beams was considered. The initial phase difference between I1 and I3 was also calculated from geometric considerations. The result is given by Eq. (1). =
4 4 1/2 1/2 d(n22 − n21 sin2 i ) + d1 (n23 − n21 sin2 i ) 0 0
where d and d1 are the thickness of the glass substrate and the sensing ﬁlm, respectively. As it was already mentioned above, n1 , n2 and n3 are the refractive indices, i is the incident angle of the light beam and 0 is the wavelength of the incident light. Considering the interference of two waves linearly polarized, the interference intensity of light beams I1 and I3 was calculated by
Fig. 2. Operation principle of the sensor based on the interference of a light beam reﬂected on a polymer sensing ﬁlm deposited on a glass substrate.
Fig. 3. Fringe pattern calculated for a PDMS sensing ﬁlm deposited on the glass substrate (continuous line). Fringe pattern shift caused by: swelling (dotted line) and refractive index change (dashed line) of PDMS ﬁlm.
I = I1 + I3 + 2
I1 I3 cos(ı)
with: ı = 2yksin −
where y is the position of the fringe on the screen, k is the wavenumber of the light and is the interference angle between light beams I1 and I3 . The interference fringes pattern calculated from Eq. (2) in function of the position on the screen y (cm) is shown in Fig. 3 (continuous line). For a given ﬁxed value of the interference angle and a constant value of the fringes will remain static when they are projected on a screen with the intensity changing with the position y. However, a change in the thickness d1 and/or refractive index n3 of the sensing ﬁlm will produce a phase variation inside the cosine function argument ı, which physically would be observed as a shift of the interference fringe pattern. The variation of d1 can be used since it is supposed that the substrate area change is negligible. Using the above obtained equations and the mentioned parameters, the calculation of the effects of d1 and n3 variation on the interference fringes pattern was performed. From Eq. (1), it can be observed that is a non-linear function of d1 and n3 because the d1 n3 product is involved. However, if the changes in d1 (d1 ) and n3 (n3 ) are considered small enough, it is possible to linearize the effects of both parameters, which means that they could be analyzed separately and then add the results to ﬁnd the total effect. The ﬁrst calculations were performed taking into account only the swelling effect produced by ethanol molecules. The concentration of ethanol in the PDMS ﬁlm was calculated from the partition coefﬁcient K, which is the relationship between the concentration of gas molecules in the sensing ﬁlm to the one in the vapor phase . The value of K for ethanol in PDMS is 180 as has been calculated from linear solvation energy relationships (LSER) . The swelling of the PDMS ﬁlm was calculated supposing an ethanol concentration in air of 24,000 ppm, which was the maximum value used for real measurements, as it will be discussed later. The obtained swelling d1 was approximately 67 nm, which means a swelling of about 0.8% for a sensing ﬁlm thickness d1 = 8 m. The interference fringes pattern calculated with the optical path change due to the swelling is shown in Fig. 3 (dotted line). This curve is shifted to the left compared to that of static fringes (continuous line). The refractive index effect was calculated according to the Maxwell–Garnett theory , which states that when two substances A and B (A = sensing ﬁlm, B = gas molecules) are mixed, the resulting refractive index must take a value between those of both substances, depending on the mixture proportion. The refractive
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Fig. 4. Experimental setup for volatile organic compounds detection.
index of the mixture can be calculated from Eq. (4). εAB = εA
3f (εA − εB ) 1+ 2εA + εB + f (εA − εB )
Fig. 5. Optical power proﬁle of the interference pattern for a laser with a Gaussian beam, the inset shows the fringe pattern obtained on a screen.
where εAB , εA , εB , and f are the dielectric constants of the mixture A–B, the substance A, the substance B and the volume fraction of substance B, respectively. The relationship between the dielectric constant and the refractive index is ε = n2 . For our case, since it has been considered that the area of the substrate does not change, then the volume fraction can be approximated as f = d1 /d1 , which was around 0.8% as it was already mentioned above. The resulting refractive index variation was n3 = 0.00035. This value was used to calculate the shift interference fringes pattern and the obtained result is shown in Fig. 3 (dashed line), where it can be noted that the fringes pattern remains almost the same as that of the ﬁlm with only PDMS. Therefore, it could be said that the change in the optical path is due essentially to swelling and the effect of refractive index change is negligible. From the above discussion, it could be expected that the sensor sensitivity would be directly related to the molecular volume of the VOC sample, since the swelling is proportional to this parameter. 3. Experimental The used sensing ﬁlm was PDMS (Sigma–Aldrich Co. Ltd.) deposited on a commercial glass (6 mm thickness) by the spin coating method. The glass substrates were previously polished in order to obtain high quality ﬂat surfaces in both faces (∼/4). The PDMS ﬁlm thickness was calculated from the weight difference before and after the ﬁlm deposition. Such difference was measured with a balance with a resolution of 0.1 mg. Since the area of the substrate was 16 cm2 , the calculated ﬁlm thickness was approximately 8.2 m with a reproducibility error of approximately 25%. The sensing ﬁlms were used for approximately 8 h continuously without any perceptible degradation of the interference pattern. The setup used for the gas sensor response measurement is shown in Fig. 4. The light source was a power stabilized He–Ne laser (Spectra Physics 117A) with = 632.8 nm and an output power of 4.5 mW. In order to expand the laser beam a microscope objective lens with an 8 mm effective focal length (EFL) was used and to collimate it a lens with 250 mm EFL was used. The collimated beam was reﬂected in the sensor, which was located in a measurement Teﬂon chamber whose volume was approximately 1.5 l. The interference fringes shift was measured with a power meter (Thorlabs PM100), which has a resolution of 5 pW. The data were stored in a personal computer at a rate of 4 points per second. The photodetector was placed on a movable stage in order to be able to set the position on the fringe pattern with a resolution of 20 m per division. The photodetector was covered by a mask with a 0.5 mm pinhole in the center in order to allow
Table 1 Some physical properties of the measured samples: molecular weight (MW), boiling point (bp) and refractive index (n). Sample Methanol Ethanol Propanol Methyl acetate Propyl acetate Heptane Octane
Me Et Pp MA PA Hep Oct
bp (◦ C)
32.04 46.07 60.1 74.08 102.1 100.2 114.2
65.0 78.5 97 57 102 98 120
1.3284 1.3614 1.3776 1.3610 1.3840 1.3876 1.3914
just a small portion of the fringe to reach it. Therefore, the fringe shift would produce a change in the measured intensity through the pinhole just in one dimension. The best point to perform the sensor response measurements was determined obtaining the fringe proﬁle by a scan of the fringe pattern with the photodetector moved by the stage. For this case, the fringe pattern proﬁle has a Gaussian distribution, since it is the beam proﬁle of the laser as is shown in Fig. 5. Therefore, the best point to perform the measurements is determined by the most brilliant fringe of the pattern. As it was already mentioned above, the sensor is set in the chamber and a scan of the fringes with the photodetector is performed by means of the stage looking for the most brilliant fringe of the pattern. The sensor is set to operate around the so-called quadrature point Q, which is the linear region of the sinusoidal function that represents the fringe pattern. For small values of it can be considered sin ≈ , therefore the intensity power variations caused by the fringe shift would be almost linear. This region is found to be almost 25% of the most brilliant fringe intensity above and below the Q point; such region covers a total of 50% (AB segment) of the fringe as is shown in Fig. 5. The VOC measured were of three different functional groups, alcohols (methanol (Me), ethanol (Et), propanol (Pp)), esters (methyl acetate (MA), propyl acetate (PA)) and alkanes (heptane (Hep) and octane (Oct)). Some of their physical properties such as molecular weight (MW), boiling point (bp) and refractive index (n) are summarized in Table 1. For each measurement, the liquid sample was injected into the chamber and let it evaporate until the steady state was reached while the optical power was monitored. These samples were chosen in order to perform a comparison inside the same functional group and among functional groups. For instance, propanol, propyl acetate and heptane have quite similar boiling point, which is one of the parameters that determine their volatility.
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Fig. 8. Sensor response in function of the ethanol concentration.
Fig. 6. Sensor response of the sensor to ethanol in function of time.
4. Results and discussion The steady state response of the sensor measured for ethanol in relation to the time is shown in Fig. 6. The ﬁrst 10 min, only the baseline was measured and after that the sample was injected to the measurement chamber. It was observed that the optical power increases, which means that a fringes pattern shift occurs. In all the graphs, the ﬁrst point of the baseline was set to zero in order to observe the absolute response. The response is quite fast at the beginning and turns slower as time runs. This is probably due to various effects such as the evaporation rate of ethanol, the diffusion process of ethanol molecules into the PDMS sensing ﬁlm, among others. However, it would be necessary to perform a more exhaustive analysis to determine the cause of such behavior. Moreover, it can be said that the response is reversible, since when the measurement chamber is purged the response recovers to the initial baseline. There is a drift of the baseline caused probably by the temperature change, however, such drift is not large and it can be neglected. In order to investigate the reproducibility of the sensor response, three measurements for 24,000 ppm of ethanol concentration in different increments were performed. The ﬁrst measurement was performed injecting four increments of 6000 ppm, the second was for two increments of 12,000 ppm and in the third case just one increment of 24,000 ppm was injected. The results are shown in Fig. 7. For equal concentrations the curves almost overlap, which means that the reproducibility is quite high. In this case, the calculated deviation was approximately 6% that means a detection limit
Fig. 7. Sensor responses for different concentration increments (6000, 12,000 and 24,000 ppm) of ethanol.
of approximately 1500 ppm, which is a typical value for polymer sensing ﬁlms. Furthermore, it could be said that the sensor response variation with the concentration is almost linear as is shown in Fig. 8. The response ﬁts quite well to a line with a correlation coefﬁcient of R2 = 0.9819 in the range above mentioned (0–24,000 ppm). This behavior is typical for small concentrations. It is clear that the best ﬁt is not linear; however the measurements ﬂuctuations are in the order of 6% (approximately 5 nW) whereas the maximum deviation between the linear ﬁtting and the measured data is almost of the same order. Therefore is not necessary to perform another ﬁtting. The sensor response was measured for different VOCs, such as methanol, ethanol, propanol, methyl acetate, propyl acetate, heptane and octane. The response pattern for the output optical power is shown in Fig. 9a (the functional groups are shown in different gray scale: white are alcohols, gray are esters and black are alkanes). As can be seen from Table 1, when the molecular weight is larger, the sensor response is higher at a lower concentration. This is probably due to the fact that the molecule with a larger size causes more swelling than a smaller one. In Fig. 9b there is shown the sensor sensitivity (nW/ppm) for the above mentioned samples. The sensitivity is a measure of the power variation caused by each ppm of concentration. It is possible to perform a qualitative comparison between propanol (Pp), propyl acetate (PA) and heptane (Hep) owing to that they have a similar molecular volume and boiling point (around 100 ◦ C), which is an
Fig. 9. (a) Sensor response pattern and (b) sensor sensitivity for the measured volatile organic compounds.
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Fig. 10. Comparison of the normalized response patterns for the calculated swelling and the measured sensitivity.
indicator of their volatility. From the comparison, it could be said that the sensor presents a larger selectivity to esters and less selectivity to alcohols. It does not mean that the sensor is not adequate to be used for alcohols, but allow us to know the response pattern that PDMS would present respect to the functional group. As it was already mentioned at the end of Section 2, the sensor response is due basically to the ﬁlm swelling rather than its refractive index change. Therefore, it could be expected a strong dependence between the sensor sensitivity and the molecular volume of the sample under analysis. This means that a certain number of molecules with a given volume would produce a larger swelling than the same number of molecules with a smaller molecular volume. In order to prove the last assumption, the swelling provoked by 1 ppm was calculated. First, the molecular volumes of the samples were calculated using the molecular weight and the density of each sample. The swelling caused by 1 ppm can be evaluated from the molecular volume and the concentrations used in the measurements. To be able to compare both patterns, a normalization of the calculated swelling (nm/ppm) and the measured sensitivity (nW/ppm) were performed for each functional group. The results are shown in Fig. 10 for all the VOC analyzed. It could be noted that both, the sensitivity and swelling patterns are very similar. Therefore it can be thought that, the polymer swelling is provoked by the diffusion of the gas molecules into the sensing ﬁlm and that the sensor response depends upon the molecular volume of the sample under analysis, as it was assumed before. It means that the sensitivity depends linearly on the ﬁlm thickness and that in order to obtain a higher sensitivity it would be necessary a thicker ﬁlm. However, at some thickness, the sensitivity increase would be limited by the saturation effects. This is a quite interesting point that will be analyzed in a future work. Finally, the response of this kind of sensor was not affected by the temperature changes as was observed during a long time (data not shown). A measurement of the interference fringes pattern variation with temperature was performed and it was found that the baseline slightly changed as the room temperature was changing during the day. However such variations were negligible during the time of measurement, which was about 1 h. Therefore, it could be said that these sensors can be used at room temperature. 5. Conclusions A gas sensor based on a PDMS sensing ﬁlm deposited on a glass substrate whose response was measured by means of a Pohl interferometer arrangement was developed. The PDMS sensing ﬁlm was deposited by the spin coating technique and the steady state sensor response measurements were performed for various VOC. The operation principle of the sensor has been proposed as the measurement of the interference fringe shift caused by the sensing ﬁlm swelling and/or refractive index change when it interacts with the vapor molecules (optical path length changes). The fringes shift,
which is used as the sensor response, was measured with a conventional photodetector in the linear region of the power variation of the fringes pattern. From the obtained results it was found that the sensor had a linear response in function of gas concentration. For instance, for the case of ethanol a linear ﬁtting gave a correlation coefﬁcient of R2 = 0.9819 for a concentration range from 0 to 24,000 ppm, which indicates a good linearity. Furthermore, the sensor response was reversible since when the measurement chamber was purged the response recovers its initial baseline and is possible to perform another measurement with a quite good reproducibility (a ﬂuctuation of approximately 6%). According to swelling calculations and its comparison with the measured sensitivities, it was found that the response to different VOCs was in function of the ﬁlm swelling rather than a refractive index change. Such swelling is directly related to the molecular volume of the sample. On the other hand, it was also found that the sensor was slightly selective to esters. This does not mean that it would not be useful to detect other functional groups, but it indicates the response pattern that would be obtained with this sensor. An interesting point was that the effect of temperature changes is negligible, which is important since there would not be necessary to control such parameter. Currently, the improvement of the measurement system and research on new sensing ﬁlms is under progress. Acknowledgments This work was supported by the projects PROMEP/103.5/ 04/1378 and CONACYT Jovenes Investigadores 61126. References  M. Santonico, P. Pittia, G. Pennazza, E. Martinelli, M. Bernabei, R. Paolesse, A. D’Amico, d,D. Compagnone, C. Di Natale, Study of the aroma of artiﬁcially ﬂavoured custards by chemical sensor array ﬁngerprinting, Sens. Actuators B 133 (2008) 345–351.  A. D’Amico, C. Di Natale, R. Paolesse, A. Macagnano, E. Martinelli, G. Pennazza, M. Santonico, M. Bernabei, C. Roscioni, G. Galluccio, R. Bono, E. Finazzi Agrò, S. Rullo, Olfactory systems for medical applications, Sens. Actuators B 130 (2008) 458–465.  S. De Vito, E. Massera, M. Piga, L. Martinotto, G. Di Francia, On ﬁeld calibration of an electronic nose for benzene estimation in an urban pollution monitoring scenario, Sens. Actuators B 129 (2008) 750–757. ˜  S. Munoz-Aguirre, A. Yoshino, T. Nakamoto, T. Moriizumi, Odor approximation of fruit ﬂavors using a QCM odor sensing system, Sens. Actuators B 123 (2007) 816–821.  Sylvie Roussel, Gustaf Forsberg, Vincent Steinmetz, Pierre Grenier, Véronique Bellon-Maurel, Optimisation of electronic nose measurements. Part I. Methodology of output feature selection, J. Food Eng. 37 (1998) 207–222.  M. Haug, K.D. Scherbaum, G. Gauglitz, W. Gopel, Chemical sensors based upon polysiloxanes: comparison between optical, quartz microbalance, calorimetric and capacitance sensors, Sens. Actuators B 11 (1993) 383–391.  J.W. Gardner, P.N. Bartlett, Electronic Noses, Oxford University Press, 1999.  E. Hecht, Optica, 3a Edición, Pearson Addison Wesley, 2003.  Z. Qi, I. Honma, H. Zhou, Fabrication of ordered mesoporous thin ﬁlms for optical waveguiding and interferometric chemical sensing, J. Phys. Chem. B 110 (2006) 10590–10594.  Y. Sun, S.I. Shopova, G. Frye-Mason, X. Fan, Rapid chemical-vapor sensing using optoﬂuidic ring resonators, Opt. Lett. 33 (2008) 788–790.  J. Zhang, M. Luo, H. Xiao, J. Dong, Interferometric study on the adsorptiondependent refractive index of silicalite thin ﬁlms grown on optical ﬁbers, Chem. Mater. 18 (2006) 4–6.  G. Gauglitz, G. Brecht, G. Kraus, W. Nahm, Chemical and biochemical sensors based on interferometry at thin (multi)layers, Sens. Actuators B 11 (1993) 21–27. ˜  S. Munoz, T. Nakamoto, T. Moriizumi, Study of crystal microbalance odor sensing system for apple and banana ﬂavors, IEICE Trans. Electron. E85-C (2002) 1291–1297.  A. Hierlemann, E.T. Zellers, A.J. Ricco, Use of linear solvation energy relationships for modeling responses from polymer-coated acoustic-wave vapor sensors, Anal. Chem. 73 (2001) 3458–3466.  M.M. Braun, L. Pilon, Effective optical properties of non-absorbing nanoporous thin ﬁlms, Thin Solid Films 496 (2006) 505–514.
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crimination. His current interests are optoelectronic sensors and multisensor systems.
Carlos Martínez-Hipatl received his BS from Electronics Faculty at Benemerita Universidad Autonoma de Puebla (BUAP), Mexico, in 2001 and his MS from Facultad de Ciencias Físico Matematicas (FCFM–BUAP) in 2006. Currently, he is a PhD candidate at FCFM, BUAP. His research interests include gas sensors, optical detection of volatile organic compounds and gas sensors signal processing systems.
Georgina Beltrán Pérez received her BS in electronics, from Benemérita Universidad Autonoma de Puebla, México, in 1995. In 1998 she received her MS degree from Centro de Investigación Cientíﬁca y Estudios Superiores de Ensenada CICESE, Mexico. She received her PhD in 2002 from INAOE, Puebla, Mexico. Currently, she is a professor at the Benemérita Universidad Autónoma de Puebla, México. Her research interest area includes optical ﬁber sensors, long-period ﬁber gratings and optical ﬁber lasers.
˜ Severino Munoz Aguirre received his BS from Universidad Veracruzana (Mexico, 1991), his MS from Center of Research and Advanced Studies of the National Polytechnic Institute (Mexico, 1995) and his PhD from Tokyo Institute of Technology (Japan, 1999). Currently, he is a professor at the University of Puebla (Mexico). He has been working with gas/odor sensors and odor sensing systems for gas detection, recognition and dis-
Juan Castillo Mixcóatl received a degree in electronics, from Benemérita Universidad Autónoma de Puebla, México, in 1995. In 1998 he received his MS degree from CICESE, Ensenada Baja California, México. He received his PhD in 2003 from INAOE, Puebla, Mexico. He is currently working at Universidad Autónoma de Puebla, México, as research professor.