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Fast-response surface acoustic wave humidity sensor based on hematoporphyrin film
Sensors and Actuators B 137 (2009) 592–596
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fast-response surface acoustic wave humidity sensor based on hematoporphyrin film a,c ˇ R. Rimeika a,∗ , D. Ciplys , V. Poderys b , R. Rotomskis b , M.S. Shur c a b c
Department of Radiophysics, Vilnius University, 10222 Vilnius, Lithuania Biophotonics Group, Laser Research Centre, Vilnius University, 10222 Vilnius, Lithuania Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
a r t i c l e
i n f o
Article history: Received 19 September 2008 Received in revised form 19 January 2009 Accepted 3 February 2009 Available online 20 February 2009 Keywords: Humidity sensing Surface acoustic wave Hematoporphyrin
a b s t r a c t The fast-response and high-sensitivity surface acoustic wave sensor based on the hematoporphyrin (Hp) layer–lithium niobate (LiNbO3 ) structure has been investigated. The amplitude and phase of the Hp–LiNbO3 SAW delay line output signal were measured as functions of relative humidity (RH) at the steady-state conditions and as functions of time upon the step-like RH variation. Both the SAW attenuation and velocity strongly depend on the relative humidity (RH) due to water sorption by Hp film. At SAW frequency 86 MHz, the changes of −0.8 dB in SAW transmission loss and −3◦ in phase per 1% of RH were obtained, corresponding to 110 ppm/1% RH in relative SAW velocity change and 1.23 × 10−3 per 1% RH in the change in the SAW attenuation per wavelength. The sensor exhibits fast response to the step jump up in humidity and fast recovery upon the abrupt humidity drop down with characteristic times of about 1 s. The applicability of the sensor for human breath monitoring is demonstrated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors are of great importance in various fields of applications [1,2]. In recent years, a lot of attention has been paid to the development of sensors for medical and health applications [3]. In particular, breath monitoring based on humidity measurements requires sensors with relatively fast response and recovery times on the order of seconds [4]. However, the characteristic times of most humidity sensors to date are in the range of tens of seconds (see e.g. [5]) with very few faster humidity sensors being available [6–9]. Hence, there is a need for humidity sensitive materials with fast response and recovery times. Organic compounds of porphyrins family has attracted much attention in recent years as promising sensing materials and various techniques have been used for implementation of sensors on their basis [10–12]. Surface acoustic waves (SAWs) present a very convenient tool [13] for sensing applications since they allow for implementation of low cost, reliable and compact sensors with good sensitivity and response speed. Recently, we have demonstrated the applicability of porphyrin family materials for fast-response humidity sensing applications [14,15]. The SAW sensors used meso-tetra (4 sulfonatophenyl) porphyrin (TPPS4) nano-strip and monomer structures as active layers. In the present work, we expand our SAW sensing studies to the other mem-
∗ Corresponding author. E-mail address: [email protected] (R. Rimeika). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.02.009
ber of porphyrin family, hematoporphyrin (Hp). (Hematoporphyrin is a tetrapyrrole aromatic macrocycle, derivatives of which have been used clinically in photodynamic therapy, a treatment for cancerous tumors [16]). Fast response (on the order of 1 s) of hematoporphyrin-based optical humidity sensor has been reported [6], but no investigations of Hp-based SAW structures and their applications for humidity sensing were available. In this paper, we report on the high-sensitivity and fast-response SAW humidity sensor based on hematoporphyrin film deposited on LiNbO3 substrate. The choice of the substrate was determined by its high electromechanical coupling coefficient (K2 = 4.5% [17]), allowing for efficient excitation and reception of surface acoustic waves.
2. Experimental 2.1. Sample preparation Fig. 1 shows the schematic diagram of a SAW-humidity sensor under investigation. The device consisted of a SAW delay line incorporating a hematoporphyrin film. A piezoelectric Y-cut, Zpropagation LiNbO3 substrate with a pair of Al-film interdigital transducers (IDTs) and the Hp film deposited between the IDTs formed the SAW delay line. The IDT period was 40 m, the aperture was 2 mm, the number of electrode pairs N = 20, and center-tocenter spacing of IDTs was 12 mm. Dissolving Hp powder (Sigma–Aldrich) in distilled water and further diluting to obtain concentration of 2 × 10−4 mol/l pro-
R. Rimeika et al. / Sensors and Actuators B 137 (2009) 592–596
593
Fig. 1. Hp-on-LiNbO3 sample for humidity sensing by SAW.
duced the solution used for film fabrication. Such a concentration is sufficiently high for the hematoporphyrin molecules arranging themselves in a slightly offset face-to-face arrangement (Haggregates) [18]. A 10–20 l amount of the prepared solution was dropped on the substrate surface and dried at room temperature in ambient air. The width of Hp film along SAW propagation was 3 mm and its thickness was in micron range and much less than the acoustic wavelength. The surface of the Hp film was imaged with the scanning probe microscope diInnova (Veeco Instruments, Inc.) in atomic force microscopy tapping mode using RTESPA (Veeco Instruments, Inc.) tips. Measurements were performed in ambient air at room temperature. The image of the film is shown in Fig. 2. The fractal structures formed during the drying process can be clearly seen. We identify these structures as the macro-aggregates of hematoporphyrin, which are separated by spaces covered with smaller Hp aggregates or monomeric molecules. 2.2. Measurement setup The impact of ambient humidity on the SAW propagation characteristics in Hp–LiNbO3 structures was investigated. Fig. 3 shows the setup of the measurement system. A sample was placed into a closed chamber formed by the glass hood of about 5 l volume. The air humidity under the hood slowly varied by inserting a small piece of wet paper (for humidity increase) or dry paper (for humidity decrease). The digital thermo-hygrometer RH411 (Omega Engineering, Inc.) monitored variations in relative humidity (RH). Step changes in the air humidity were accomplished by fast (of about 0.1 s duration) manual removal of the hood, resulting in an abrupt humidity drop or jump from the actual value under the hood to that of the surrounding air in the laboratory room. In this case, only the initial and final steady-state values of relative humidity could be measured by the hygrometer RH411 because its response time was much longer than the transient humidity process. The net-
Fig. 2. Scanning probe microscope image of hematoporphyrin film.
Fig. 3. Experimental setup: (1) sample on holder, (2) glass hood, (3) hygrometer probe, (4) hygrometer, (5) network analyzer.
work analyzer E5062A (Agilent Technologies, Inc.) measured the SAW delay-line transmission characteristics (loss and phase shift) as functions of relative humidity at the steady-state conditions and as functions of time upon the step-like RH variation. The measurements were performed at the room temperature. The temperature stability during a particular measurement session was verified by measuring the lithium niobate-based SAW delay-line oscillator frequency [4]. As RH under the hood varied from 10% to 80%, the relative frequency variation did not exceed 6 ppm, which corresponds to 0.1 K temperature variation (temperature coefficient of frequency for YZ LiNbO3 is 94 ppm/K [17]). 3. Results and discussion 3.1. Static measurements Fig. 4 shows the dependencies of SAW transmission loss and phase on relative humidity measured at the IDT center frequency 86.3 MHz. As seen, both the amplitude and the phase of the transmitted SAW signal decrease as the relative humidity increases. We attribute this to the increase in SAW attenuation and to the decrease of the SAW velocity due to the water sorption by the Hp layer. Several physical mechanisms – the mass loading, the acoustoelectric interaction, and the visco-elastic effect – are known to be responsible for externally induced changes in SAW parameters [13]. Since the mass loading leads only to the change in the SAW velocity, whereas the other two mechanisms affect both the velocity
Fig. 4. SAW transmission loss and phase shift in Hp–LiNbO3 structure versus relative humidity in steady-state conditions at IDT center frequency.
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Fig. 5. Time variation of SAW transmission parameters upon abrupt humidity jump from 13% to 26%: transmission loss (a) and phase (b) in Hp–LiNbO3 structure, and transmission loss in bare LiNbO3 substrate (c). Here and in Fig. 6, vertical dashed line shows the start of hood removal.
and attenuation, they are most likely responsible for the observed SAW transmission loss and phase variations. However, available experimental data are insufficient to evaluate quantitatively their contributions. The shapes of the loss and phase curves are very similar: their steepness significantly grows with increasing humidity. Assuming the amplitude of a SAW traversing the Hp layer of length L1 varies as U = U0 exp(−˛x), where ˛ is the SAW attenuation per unit length along the propagation direction x, the change in SAW attenuation per wavelength, = ˛, is given by = −
1 A 8.686 L1
(1)
where A is the measured transmission loss variation in dB. Above RH = 25%, the amplitude versus RH curve in Fig. 4 can be approximated by the linear dependence with the slope of −0.8 dB/%RH. Eq. (1) yields = 1.23 × 10−3 per 1% RH. The relative variation in the SAW velocity V/V can be found from the SAW phase shift = − 2f/V x along the propagation direction, V = , V L1 360
(2)
where is the measured variation in the SAW phase at the delayline output, in degrees. For RH > 25%, the dependence of phase on RH can be approximated by the linear one with the slope −3◦ /%RH. From Eq. (2), one finds the relative change in the SAW velocity per unit humidity change V/V = 110 ppm/%RH. These and V/V values are much higher than those obtained in our previous work with Meso-tetra (4 sulfonatophenyl) porphyrin (TPPS4 ) layers on LiNbO3 : = 7 × 10−5 per 1% RH and V/V = 11 ppm/%RH [4]. For comparison, the recently reported humidity sensor based on a SAW delay-line oscillator with camphor sulfonic acid doped polyaniline (PANI) nanofibers [19] exhibited the 4.58 kHz drop in frequency per cent of RH at the fundamental frequency of 145 MHz. This corresponds to the sensitivity V/V = 32 ppm/%RH, which (according to [19]) is amongst the best values reported to date for SAW humidity sensors. 3.2. Dynamic measurements Next, the response of the Hp–LiNbO3 SAW delay line to fast humidity changes was investigated. The abrupt humidity step was obtained by a fast (of about 0.1 s duration) removal of the hood, exposing the sample to the surrounding room atmosphere. Fig. 5 shows the time variations of the SAW transmission loss and of the phase shift when the humidity jumps up from 13% in the closed
Fig. 6. Time variation of SAW transmission loss (a) and phase (b) in Hp–LiNbO3 structure upon abrupt humidity drop from 33% to 26%.
chamber to 26% in the open air. Fig. 6 shows the variations of the same parameters when the humidity drops down from 33% in the closed chamber to 26% in the open air. The reference hygrometer RH411 used to measure the initial and final steady-state RH values was too slow to follow the rapid RH variations between these values. Meanwhile, the Hp–LiNbO3 structure exhibited fast variation in transmitted SAW amplitude and phase. We use the conventional definition of the response time as the time required to a measured quantity to vary from its initial value to 90% of the final value. Fig. 5 yields the response time 0.8 s for the SAW phase change and 1.7 s for the amplitude change. Similarly, the characteristic response times determined from Fig. 6 are 0.7 s for the SAW phase change and 2 s for the amplitude change. Hence, the recovery time of the structure (when the humidity drops down) was of the same order as the response time (when the humidity jumps up). This is an important advantage in comparison with sensors, which have short sorption but long desorption times (e.g. [20]). All the curves presented in Figs. 5 and 6 exhibit a very fast initial variation of the measured quantity (it takes parts of a second to attain about 70% of the total change) followed by comparatively slower variation towards the final value. Similar kinetics was observed and discussed in Ref. 7. This feature might be attributed not only to the sensor properties, but also to the actual shape of the temporal humidity variation. One may conclude that the Hp–LiNbO3 based SAW structure is capable to respond to RH changes with a characteristic time of about 1 second. To confirm that the SAW response was due to hematoporphyrin, we repeated the experiment using the YZ-LiNbO3 substrate without Hp layer. No significant SAW response was observed upon hood removal in this case as seen from curve c of Fig. 5 showing the behavior of SAW transmission loss for the bare substrate. From Figs. 5 and 6 we determine the noise level of 0.1 dB and 1.4◦ in the SAW amplitude and phase signals, respectively. Using the above determined sensitivities 0.8 dB/%RH and 3◦ /%RH, we estimate the limit of detectable change in relative humidity at the signal to noise ratio 2:1 to be 0.3%RH from the SAW loss measurements and 0.9% RH from the SAW phase measurements. It should be noted that these values are valid for the range of steeper SAW parameter variation with RH from 25% to 33%, and the limits of detection increase at lower RH values. 3.3. Breath monitoring Due to the fast response capability, the Hp-based SAW humidity sensor is very suitable for monitoring the human respiration by detecting the moisture contained in the breathed out air stream. We have compared the responses to breathing of the commercial ref-
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to 1 s. We have demonstrated that the Hp–LiNbO3 SAW structure allows for implementation of a sensor for human breath monitoring operating at dual (amplitude and phase) mode, which provides more flexibility and reliability of measurements. Acknowledgments The work at Vilnius University was partially supported by the Lithuanian State Science and Studies Foundation. The work at Rensselaer Polytechnic Institute was supported by ONR (under MURI, Project monitor Dr. Paul Maki). References
Fig. 7. Time variation of SAW transmission loss (a) and phase (b) in Hp–LiNbO3 sensor subjected to human respiration. Dashed line (c) shows the response of hygrometer RH411 to respiration.
erence hygrometer and of our Hp-based SAW structure (Fig. 7). For this purpose, the relevant sensor was placed at the distance of several centimeters against the volunteering person’s nostrils, and the response signal was recorded during many cycles of breathing. The hygrometer RH411 does not recover after each single breath and integrates the humidity effect over consecutive breathing cycles leading to monotonously growing response function. In contrast, the Hp–LiNbO3 SAW sensor is fast enough to detect an individual breath-in and breath-out cycles. Fig. 7 shows that both the SAW amplitude and phase vary considerably due to breathing. In agreement with Figs. 5 and 6, the abrupt decrease in the SAW amplitude and phase correspond to the breath out, and the more gradual increase of these quantities takes place during the breath in. At the nostril-sensor distance of 10 cm, the average cyclic variation in SAW transmission loss was about 8 dB with the phase shift about 20◦ . These quantities increased to 12 dB and 45◦ , respectively, when the distance was reduced to 5 cm. In both cases, the respiration frequency was 0.28 Hz (≈17 breaths per min). 4. Conclusion We have investigated the impact of ambient humidity on the SAW propagation characteristics in hematoporphyrin layer–LiNbO3 substrate structure in the range of relative humidities from 13% to 33% at frequency 86 MHz. With increasing RH, the SAW attenuation considerably increases, and the SAW velocity decreases. These effects are attributed to the changes in hematoporphyrin layer properties due to water sorption. Further investigations are required to elucidate the role of conductive and/or visco-elastic mechanisms responsible for humidity-induced changes in SAW parameters. Both the amplitude and phase at the output transducer of the Hp–LiNbO3 SAW delay line can be used as a sensor signal. The loss and phase dependencies on RH have a strongly non-linear shape: their steepness significantly grows with increasing humidity. In the relative humidity range from 25% to 33%, the sensor exhibits high sensitivity of 1.23 × 10−3 per 1% RH in SAW attenuation per wavelength and 110 ppm/1% RH in relative SAW velocity change. Such sensitivities are by the order of magnitude higher than those obtained, in our previous work, for SAW structures based on Meso-tetra (4 sulfonatophenyl) porphyrin (TPPS4 ) layers on the same substrate. The corresponding limits of detectable change in relative humidity are 0.3%RH and 0.9% RH from the SAW loss and phase measurements, respectively. However, the sensitivity is reduced at lower humidity values. The sensor exhibits a high speed of operation with both the response and recovery times close
[1] R. Fenner, E. Zdankiewicz, Micromachined water vapor sensors: a review of sensing technologies, IEEE Sens. J. 1 (2001) 309–317. [2] T.L. Yeo, T. Sun, K.T.V. Grattan, Fibre-optic sensor technologies for humidity and moisture measurement, Sens. Actuators A 144 (2008) 280–295. [3] C. Laville, J.Y. Deletage, C. Pellet, Humidity sensors for a pulmonary function diagnostic microsystem, Sens. Actuators B 76 (2001) 304–309. ˇ [4] R. Rimeika, D. Ciplys, V. Poderys, R. Rotomskis, S. Balakauskas, M.S. Shur, Subsecond-response SAW humidity sensor with porphyrin nanostructure deposited on bare and metallised piezoelectric substrate, Electron. Lett. 43 (2007) 1055–1057. [5] D. Patil, Y.K. Seo, Y.K. Hwang, J.S. Chang, P. Patil, Humidity sensitive poly (2.5dimethoxyaniline)/WO3 composites, Sens. Actuators B 132 (2008) 116–124. [6] M. Morisawa, H. Uematsu, S. Muto, Optical humidity sensor with a fast response time using dye-adsorbed Langmuir–Blodgett films, Jpn. J. Appl. Phys. 31 (1992) L1202–L1205. [7] A. Tetelin, C. Pellet, C. Laville, G. N’Kaoua, Fast response humidity sensors for a medical microsystem, Sens. Actuators B 91 (2003) 211–217. [8] J.J. Steele, A.C. van Popta, M.M. Hawkeye, J.C. Sit, M.J. Brett, Nanostructured gradient index optical filter for high-speed humidity sensing, Sens. Actuators B 120 (2006) 213–219. [9] X.Q. Fu, C. Wang, H.C. Yu, Y.G. Wang, T.H. Wang, Fast humidity sensors based on CeO2 nanowires, Nanotechnology 18 (145503) (2007) 1–4. [10] C. Di Natale, D. Salimbeni, R. Paolesse, A. Macagnano, A. D’Amico, Porphyrinsbased opto-electronic nose for volatile compounds detection, Sens. Actuators B 65 (2000) 220–226. [11] A.K. Jain, S.M. Sondhi, S. Rajvanshi, A PVC based hematoporphyrin IX membrane potentiometric sensor for zinc (II), Electroanalysis 14 (2002) 293–296. [12] C. Caliendo, P. Verardi, E. Verona, A. D’Amico, C. Di Natale, G. Saggio, M. Serafini, R. Paolessse, S.E. Huq, Advances in SAW-based gas sensors, Smart Mater. Struct. 6 (1997) 689–699. [13] H. Wohltjen, R. Dessy, Surface acoustic wave probe for chemical analysis, Anal. Chem. 51 (1979) 1458–1464. ˇ [14] R. Rimeika, R. Rotomskis, V. Poderys, D. Ciplys, A. Sereika, A. Selskis, M.S. Shur, Surface acoustic wave interaction with humidity sensitive TPPS4 nano-strip structure, Ultragarsas (Ultrasound) 1 (2006) 13–15. ˇ [15] R. Rimeika, R. Rotomskis, V. Poderys, A. Sereika, A. Selskis, D. Ciplys, M.S. Shur, Humidity-sensitive SAW devise based TPPS4 nanostrip structure, in: Proc. 5th IEEE Conf. on Sensors, Daegu, Korea, 2006, pp. 97–100. [16] T.J. Dougherty, S.L. Marcus, Photodynamic therapy, Eur. J. Cancer 28A (1992) 1734–1742. [17] C.C. Campbell, Surface Acoustic Wave Devices for Mobile and Wireless Communications, Academic Press, San Diego, 1998, p. 31. [18] S.B. Brown, M. Shillcock, P. Jones, Equilibrium and kinetic studies of the aggregation of porphyrins in aqueous solution, Biochem. J. 153 (1976) 279–285. [19] T.T. Wu, Y.Y. Chen, T.H. Chou, A high sensitivity nanomaterial based SAW humidity sensor, J. Phys. D: Appl. Phys. 41 (085101) (2008) 1–3. [20] C. Laville, C. Pellet, Comparison of three humidity sensors for a pulmonary function diagnosis microsystem, IEEE Sens. J. 2 (2002) 96–101.
Biographies Romualdas Rimeika received PhD degree in physics from Vilnius University in 1993. He is currently an assistant professor and senior researcher in the Department of Radiophysics at Vilnius University. His areas of research are surface acoustic wave and guided optical devices and their applications for sensing and signal control. ˇ iplys graduated from Vilnius University in 1967, received PhD degree Daumantas C from the A.F. Ioffe Institute of Physics and Technology, St. Petersburg, Russia, in 1974, and habilitation from Vilnius University in 2005. He is currently a professor and superior researcher at Vilnius University. During 1999–2007 he also periodically worked at Rensselaer Polytechnic Institute, Troy, NY, USA as Visiting Scientist. His scientific interests are focused on the acoustic, acousto-optic, and acousto-electronic effects in solids and layered structures. Vilius Poderys received BSc degree in modern technology management and the MSc in Biophysics at Vilnius University. He is currently a PhD student in the Biophotonics
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group of Quantum Electronics Department at Vilnius University where his primary research focus is the self-assembling nanostructures and characterization of their properties. Riˇcardas Rotomskis is a professor in the Biophotonics group of Quantum electronics department at Vilnius University and a Head of the Biomedical physics laboratory in the Institute of oncology at Vilnius University. His research interests are photodynamic tumor therapy, nanoparticles and their application in medicine, optical diagnostics methods. Michael S. Shur received MSc in electrical engineering degree from St. Petersburg Electrotechnical Institute, St. Petersburg, Russia, in 1965, PhD and D. Sc. degrees in physics and mathematics from the A.F. Ioffe Institute of Physics and Technology, St.
Petersburg, in 1967 and 1992, respectively, and the Honorary Doctorate degree from Saint Petersburg State Technical University, St. Petersburg, in 1994. He is currently Director of the Center for Integrated Electronics and of the Center for Broadband Data Transport Science and Technology, professor in the Department of Electrical, Computer, and Systems Engineering and in Physics, Applied Physics, and Astronomy at Rensselaer Polytechnic Institute, Troy, NY, USA. He has authored over 1000 technical publications; given more than 300 plenary, keynote, and invited talks and conference presentations; authored, co-authored, or edited 38 books and 29 book chapters and holds 33 patents on solid-state devices. His research interests include novel semiconductor devices, such as those using wide band gap semiconductors, high power transistors, visible and ultra violet light emitting diodes, and surface acoustic wave devices.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fast-response surface acoustic wave humidity sensor based on hematoporphyrin film a,c ˇ R. Rimeika a,∗ , D. Ciplys , V. Poderys b , R. Rotomskis b , M.S. Shur c a b c
Department of Radiophysics, Vilnius University, 10222 Vilnius, Lithuania Biophotonics Group, Laser Research Centre, Vilnius University, 10222 Vilnius, Lithuania Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
a r t i c l e
i n f o
Article history: Received 19 September 2008 Received in revised form 19 January 2009 Accepted 3 February 2009 Available online 20 February 2009 Keywords: Humidity sensing Surface acoustic wave Hematoporphyrin
a b s t r a c t The fast-response and high-sensitivity surface acoustic wave sensor based on the hematoporphyrin (Hp) layer–lithium niobate (LiNbO3 ) structure has been investigated. The amplitude and phase of the Hp–LiNbO3 SAW delay line output signal were measured as functions of relative humidity (RH) at the steady-state conditions and as functions of time upon the step-like RH variation. Both the SAW attenuation and velocity strongly depend on the relative humidity (RH) due to water sorption by Hp film. At SAW frequency 86 MHz, the changes of −0.8 dB in SAW transmission loss and −3◦ in phase per 1% of RH were obtained, corresponding to 110 ppm/1% RH in relative SAW velocity change and 1.23 × 10−3 per 1% RH in the change in the SAW attenuation per wavelength. The sensor exhibits fast response to the step jump up in humidity and fast recovery upon the abrupt humidity drop down with characteristic times of about 1 s. The applicability of the sensor for human breath monitoring is demonstrated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors are of great importance in various fields of applications [1,2]. In recent years, a lot of attention has been paid to the development of sensors for medical and health applications [3]. In particular, breath monitoring based on humidity measurements requires sensors with relatively fast response and recovery times on the order of seconds [4]. However, the characteristic times of most humidity sensors to date are in the range of tens of seconds (see e.g. [5]) with very few faster humidity sensors being available [6–9]. Hence, there is a need for humidity sensitive materials with fast response and recovery times. Organic compounds of porphyrins family has attracted much attention in recent years as promising sensing materials and various techniques have been used for implementation of sensors on their basis [10–12]. Surface acoustic waves (SAWs) present a very convenient tool [13] for sensing applications since they allow for implementation of low cost, reliable and compact sensors with good sensitivity and response speed. Recently, we have demonstrated the applicability of porphyrin family materials for fast-response humidity sensing applications [14,15]. The SAW sensors used meso-tetra (4 sulfonatophenyl) porphyrin (TPPS4) nano-strip and monomer structures as active layers. In the present work, we expand our SAW sensing studies to the other mem-
∗ Corresponding author. E-mail address: [email protected] (R. Rimeika). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.02.009
ber of porphyrin family, hematoporphyrin (Hp). (Hematoporphyrin is a tetrapyrrole aromatic macrocycle, derivatives of which have been used clinically in photodynamic therapy, a treatment for cancerous tumors [16]). Fast response (on the order of 1 s) of hematoporphyrin-based optical humidity sensor has been reported [6], but no investigations of Hp-based SAW structures and their applications for humidity sensing were available. In this paper, we report on the high-sensitivity and fast-response SAW humidity sensor based on hematoporphyrin film deposited on LiNbO3 substrate. The choice of the substrate was determined by its high electromechanical coupling coefficient (K2 = 4.5% [17]), allowing for efficient excitation and reception of surface acoustic waves.
2. Experimental 2.1. Sample preparation Fig. 1 shows the schematic diagram of a SAW-humidity sensor under investigation. The device consisted of a SAW delay line incorporating a hematoporphyrin film. A piezoelectric Y-cut, Zpropagation LiNbO3 substrate with a pair of Al-film interdigital transducers (IDTs) and the Hp film deposited between the IDTs formed the SAW delay line. The IDT period was 40 m, the aperture was 2 mm, the number of electrode pairs N = 20, and center-tocenter spacing of IDTs was 12 mm. Dissolving Hp powder (Sigma–Aldrich) in distilled water and further diluting to obtain concentration of 2 × 10−4 mol/l pro-
R. Rimeika et al. / Sensors and Actuators B 137 (2009) 592–596
593
Fig. 1. Hp-on-LiNbO3 sample for humidity sensing by SAW.
duced the solution used for film fabrication. Such a concentration is sufficiently high for the hematoporphyrin molecules arranging themselves in a slightly offset face-to-face arrangement (Haggregates) [18]. A 10–20 l amount of the prepared solution was dropped on the substrate surface and dried at room temperature in ambient air. The width of Hp film along SAW propagation was 3 mm and its thickness was in micron range and much less than the acoustic wavelength. The surface of the Hp film was imaged with the scanning probe microscope diInnova (Veeco Instruments, Inc.) in atomic force microscopy tapping mode using RTESPA (Veeco Instruments, Inc.) tips. Measurements were performed in ambient air at room temperature. The image of the film is shown in Fig. 2. The fractal structures formed during the drying process can be clearly seen. We identify these structures as the macro-aggregates of hematoporphyrin, which are separated by spaces covered with smaller Hp aggregates or monomeric molecules. 2.2. Measurement setup The impact of ambient humidity on the SAW propagation characteristics in Hp–LiNbO3 structures was investigated. Fig. 3 shows the setup of the measurement system. A sample was placed into a closed chamber formed by the glass hood of about 5 l volume. The air humidity under the hood slowly varied by inserting a small piece of wet paper (for humidity increase) or dry paper (for humidity decrease). The digital thermo-hygrometer RH411 (Omega Engineering, Inc.) monitored variations in relative humidity (RH). Step changes in the air humidity were accomplished by fast (of about 0.1 s duration) manual removal of the hood, resulting in an abrupt humidity drop or jump from the actual value under the hood to that of the surrounding air in the laboratory room. In this case, only the initial and final steady-state values of relative humidity could be measured by the hygrometer RH411 because its response time was much longer than the transient humidity process. The net-
Fig. 2. Scanning probe microscope image of hematoporphyrin film.
Fig. 3. Experimental setup: (1) sample on holder, (2) glass hood, (3) hygrometer probe, (4) hygrometer, (5) network analyzer.
work analyzer E5062A (Agilent Technologies, Inc.) measured the SAW delay-line transmission characteristics (loss and phase shift) as functions of relative humidity at the steady-state conditions and as functions of time upon the step-like RH variation. The measurements were performed at the room temperature. The temperature stability during a particular measurement session was verified by measuring the lithium niobate-based SAW delay-line oscillator frequency [4]. As RH under the hood varied from 10% to 80%, the relative frequency variation did not exceed 6 ppm, which corresponds to 0.1 K temperature variation (temperature coefficient of frequency for YZ LiNbO3 is 94 ppm/K [17]). 3. Results and discussion 3.1. Static measurements Fig. 4 shows the dependencies of SAW transmission loss and phase on relative humidity measured at the IDT center frequency 86.3 MHz. As seen, both the amplitude and the phase of the transmitted SAW signal decrease as the relative humidity increases. We attribute this to the increase in SAW attenuation and to the decrease of the SAW velocity due to the water sorption by the Hp layer. Several physical mechanisms – the mass loading, the acoustoelectric interaction, and the visco-elastic effect – are known to be responsible for externally induced changes in SAW parameters [13]. Since the mass loading leads only to the change in the SAW velocity, whereas the other two mechanisms affect both the velocity
Fig. 4. SAW transmission loss and phase shift in Hp–LiNbO3 structure versus relative humidity in steady-state conditions at IDT center frequency.
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Fig. 5. Time variation of SAW transmission parameters upon abrupt humidity jump from 13% to 26%: transmission loss (a) and phase (b) in Hp–LiNbO3 structure, and transmission loss in bare LiNbO3 substrate (c). Here and in Fig. 6, vertical dashed line shows the start of hood removal.
and attenuation, they are most likely responsible for the observed SAW transmission loss and phase variations. However, available experimental data are insufficient to evaluate quantitatively their contributions. The shapes of the loss and phase curves are very similar: their steepness significantly grows with increasing humidity. Assuming the amplitude of a SAW traversing the Hp layer of length L1 varies as U = U0 exp(−˛x), where ˛ is the SAW attenuation per unit length along the propagation direction x, the change in SAW attenuation per wavelength, = ˛, is given by = −
1 A 8.686 L1
(1)
where A is the measured transmission loss variation in dB. Above RH = 25%, the amplitude versus RH curve in Fig. 4 can be approximated by the linear dependence with the slope of −0.8 dB/%RH. Eq. (1) yields = 1.23 × 10−3 per 1% RH. The relative variation in the SAW velocity V/V can be found from the SAW phase shift = − 2f/V x along the propagation direction, V = , V L1 360
(2)
where is the measured variation in the SAW phase at the delayline output, in degrees. For RH > 25%, the dependence of phase on RH can be approximated by the linear one with the slope −3◦ /%RH. From Eq. (2), one finds the relative change in the SAW velocity per unit humidity change V/V = 110 ppm/%RH. These and V/V values are much higher than those obtained in our previous work with Meso-tetra (4 sulfonatophenyl) porphyrin (TPPS4 ) layers on LiNbO3 : = 7 × 10−5 per 1% RH and V/V = 11 ppm/%RH [4]. For comparison, the recently reported humidity sensor based on a SAW delay-line oscillator with camphor sulfonic acid doped polyaniline (PANI) nanofibers [19] exhibited the 4.58 kHz drop in frequency per cent of RH at the fundamental frequency of 145 MHz. This corresponds to the sensitivity V/V = 32 ppm/%RH, which (according to [19]) is amongst the best values reported to date for SAW humidity sensors. 3.2. Dynamic measurements Next, the response of the Hp–LiNbO3 SAW delay line to fast humidity changes was investigated. The abrupt humidity step was obtained by a fast (of about 0.1 s duration) removal of the hood, exposing the sample to the surrounding room atmosphere. Fig. 5 shows the time variations of the SAW transmission loss and of the phase shift when the humidity jumps up from 13% in the closed
Fig. 6. Time variation of SAW transmission loss (a) and phase (b) in Hp–LiNbO3 structure upon abrupt humidity drop from 33% to 26%.
chamber to 26% in the open air. Fig. 6 shows the variations of the same parameters when the humidity drops down from 33% in the closed chamber to 26% in the open air. The reference hygrometer RH411 used to measure the initial and final steady-state RH values was too slow to follow the rapid RH variations between these values. Meanwhile, the Hp–LiNbO3 structure exhibited fast variation in transmitted SAW amplitude and phase. We use the conventional definition of the response time as the time required to a measured quantity to vary from its initial value to 90% of the final value. Fig. 5 yields the response time 0.8 s for the SAW phase change and 1.7 s for the amplitude change. Similarly, the characteristic response times determined from Fig. 6 are 0.7 s for the SAW phase change and 2 s for the amplitude change. Hence, the recovery time of the structure (when the humidity drops down) was of the same order as the response time (when the humidity jumps up). This is an important advantage in comparison with sensors, which have short sorption but long desorption times (e.g. [20]). All the curves presented in Figs. 5 and 6 exhibit a very fast initial variation of the measured quantity (it takes parts of a second to attain about 70% of the total change) followed by comparatively slower variation towards the final value. Similar kinetics was observed and discussed in Ref. 7. This feature might be attributed not only to the sensor properties, but also to the actual shape of the temporal humidity variation. One may conclude that the Hp–LiNbO3 based SAW structure is capable to respond to RH changes with a characteristic time of about 1 second. To confirm that the SAW response was due to hematoporphyrin, we repeated the experiment using the YZ-LiNbO3 substrate without Hp layer. No significant SAW response was observed upon hood removal in this case as seen from curve c of Fig. 5 showing the behavior of SAW transmission loss for the bare substrate. From Figs. 5 and 6 we determine the noise level of 0.1 dB and 1.4◦ in the SAW amplitude and phase signals, respectively. Using the above determined sensitivities 0.8 dB/%RH and 3◦ /%RH, we estimate the limit of detectable change in relative humidity at the signal to noise ratio 2:1 to be 0.3%RH from the SAW loss measurements and 0.9% RH from the SAW phase measurements. It should be noted that these values are valid for the range of steeper SAW parameter variation with RH from 25% to 33%, and the limits of detection increase at lower RH values. 3.3. Breath monitoring Due to the fast response capability, the Hp-based SAW humidity sensor is very suitable for monitoring the human respiration by detecting the moisture contained in the breathed out air stream. We have compared the responses to breathing of the commercial ref-
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to 1 s. We have demonstrated that the Hp–LiNbO3 SAW structure allows for implementation of a sensor for human breath monitoring operating at dual (amplitude and phase) mode, which provides more flexibility and reliability of measurements. Acknowledgments The work at Vilnius University was partially supported by the Lithuanian State Science and Studies Foundation. The work at Rensselaer Polytechnic Institute was supported by ONR (under MURI, Project monitor Dr. Paul Maki). References
Fig. 7. Time variation of SAW transmission loss (a) and phase (b) in Hp–LiNbO3 sensor subjected to human respiration. Dashed line (c) shows the response of hygrometer RH411 to respiration.
erence hygrometer and of our Hp-based SAW structure (Fig. 7). For this purpose, the relevant sensor was placed at the distance of several centimeters against the volunteering person’s nostrils, and the response signal was recorded during many cycles of breathing. The hygrometer RH411 does not recover after each single breath and integrates the humidity effect over consecutive breathing cycles leading to monotonously growing response function. In contrast, the Hp–LiNbO3 SAW sensor is fast enough to detect an individual breath-in and breath-out cycles. Fig. 7 shows that both the SAW amplitude and phase vary considerably due to breathing. In agreement with Figs. 5 and 6, the abrupt decrease in the SAW amplitude and phase correspond to the breath out, and the more gradual increase of these quantities takes place during the breath in. At the nostril-sensor distance of 10 cm, the average cyclic variation in SAW transmission loss was about 8 dB with the phase shift about 20◦ . These quantities increased to 12 dB and 45◦ , respectively, when the distance was reduced to 5 cm. In both cases, the respiration frequency was 0.28 Hz (≈17 breaths per min). 4. Conclusion We have investigated the impact of ambient humidity on the SAW propagation characteristics in hematoporphyrin layer–LiNbO3 substrate structure in the range of relative humidities from 13% to 33% at frequency 86 MHz. With increasing RH, the SAW attenuation considerably increases, and the SAW velocity decreases. These effects are attributed to the changes in hematoporphyrin layer properties due to water sorption. Further investigations are required to elucidate the role of conductive and/or visco-elastic mechanisms responsible for humidity-induced changes in SAW parameters. Both the amplitude and phase at the output transducer of the Hp–LiNbO3 SAW delay line can be used as a sensor signal. The loss and phase dependencies on RH have a strongly non-linear shape: their steepness significantly grows with increasing humidity. In the relative humidity range from 25% to 33%, the sensor exhibits high sensitivity of 1.23 × 10−3 per 1% RH in SAW attenuation per wavelength and 110 ppm/1% RH in relative SAW velocity change. Such sensitivities are by the order of magnitude higher than those obtained, in our previous work, for SAW structures based on Meso-tetra (4 sulfonatophenyl) porphyrin (TPPS4 ) layers on the same substrate. The corresponding limits of detectable change in relative humidity are 0.3%RH and 0.9% RH from the SAW loss and phase measurements, respectively. However, the sensitivity is reduced at lower humidity values. The sensor exhibits a high speed of operation with both the response and recovery times close
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Biographies Romualdas Rimeika received PhD degree in physics from Vilnius University in 1993. He is currently an assistant professor and senior researcher in the Department of Radiophysics at Vilnius University. His areas of research are surface acoustic wave and guided optical devices and their applications for sensing and signal control. ˇ iplys graduated from Vilnius University in 1967, received PhD degree Daumantas C from the A.F. Ioffe Institute of Physics and Technology, St. Petersburg, Russia, in 1974, and habilitation from Vilnius University in 2005. He is currently a professor and superior researcher at Vilnius University. During 1999–2007 he also periodically worked at Rensselaer Polytechnic Institute, Troy, NY, USA as Visiting Scientist. His scientific interests are focused on the acoustic, acousto-optic, and acousto-electronic effects in solids and layered structures. Vilius Poderys received BSc degree in modern technology management and the MSc in Biophysics at Vilnius University. He is currently a PhD student in the Biophotonics
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group of Quantum Electronics Department at Vilnius University where his primary research focus is the self-assembling nanostructures and characterization of their properties. Riˇcardas Rotomskis is a professor in the Biophotonics group of Quantum electronics department at Vilnius University and a Head of the Biomedical physics laboratory in the Institute of oncology at Vilnius University. His research interests are photodynamic tumor therapy, nanoparticles and their application in medicine, optical diagnostics methods. Michael S. Shur received MSc in electrical engineering degree from St. Petersburg Electrotechnical Institute, St. Petersburg, Russia, in 1965, PhD and D. Sc. degrees in physics and mathematics from the A.F. Ioffe Institute of Physics and Technology, St.
Petersburg, in 1967 and 1992, respectively, and the Honorary Doctorate degree from Saint Petersburg State Technical University, St. Petersburg, in 1994. He is currently Director of the Center for Integrated Electronics and of the Center for Broadband Data Transport Science and Technology, professor in the Department of Electrical, Computer, and Systems Engineering and in Physics, Applied Physics, and Astronomy at Rensselaer Polytechnic Institute, Troy, NY, USA. He has authored over 1000 technical publications; given more than 300 plenary, keynote, and invited talks and conference presentations; authored, co-authored, or edited 38 books and 29 book chapters and holds 33 patents on solid-state devices. His research interests include novel semiconductor devices, such as those using wide band gap semiconductors, high power transistors, visible and ultra violet light emitting diodes, and surface acoustic wave devices.