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Sensors and Actuators B 210 (2015) 297–301

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

novel ultra-high frequency humidity sensor based on aagnetostatic spin wave oscillator

. Matataguia,∗, O.V. Kolokoltseva, N. Qureshia, E.V. Mejía-Uriartea, J.M. Sanigerb

Fotónica de Microondas, CCADET, Universidad Nacional Autónoma de México (UNAM), MexicoMateriales y Nanotecnología, CCADET, Universidad Nacional Autónoma de México (UNAM), Mexico

r t i c l e i n f o

rticle history:eceived 19 October 2014eceived in revised form9 December 2014ccepted 30 December 2014vailable online 6 January 2015

a b s t r a c t

This paper presents a robust, simple and highly sensitive method to detect vapour compounds using amagnetostatic spin wave (MSW) tunable oscillator based on a yttrium iron garnet (YIG) epitaxial thin filmcoupled to a coplanar waveguide resonator. In this work, the device has been tested as relative humiditysensor. For this purpose, the coplanar waveguide resonator was coated with polyvinylpyrrolidone (PVP)polymer and used as a humidity probe. A dielectric change in the PVP due to a change in the ambienthumidity introduces a fast phase shift in the probe, which causes a frequency shift in the MSW oscillator.

eywords:agnetic resonanceagnetostatic spin waveumidity sensorVPleep apnea

The humidity sensing behavior of this device was investigated at room temperature over a range of12.5–95% relative humidity (RH). Furthermore, the device was used to monitor human breath as a non-invasive sensor, showing great adaptability and ease of use in a real application. In conclusion, the sensorexhibited reproducibility, accuracy, high sensitivity, and fast response and recovery times. It also has theadvantage of being simple and cost-effective.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

In recent decades, research has given rise to new, low-cost sen-ors to detect vapors in the environment, based on capacitive effects1,2], resistive effects [3,4], optical fibers [5,6], field effect transis-ors (FETs) [7,8], surface acoustic waves (SAWs) [9,10] and quartzrystal microbalances (QCM) [11,12]. Many of the aforementionedevices have also been applied as air humidity sensors [13–20],nd have been combined with different sensitive materials, suchs porous ceramics [21,22], semiconductor materials [23,24] andolymers [25,26] in order to obtain a large response to changes inumidity. In general, the sensitive materials used to detect humid-

ty are hygroscopic, which means that they attract and hold waterolecules from the surrounding environment.Research in improved humidity sensors is motivated by a need

or humidity control in many industrial applications. In microelec-ronics, dry conditions are required in the processing of siliconafers in a clean room. In agriculture, humidity affects seed qual-

ty, leaf growth, photosynthesis, pollination, occurrence of diseases,

nd, subsequently, economic yield. Additionally, humidity is crucialn many other fields, such as food storage, high-tech instruments,

∗ Corresponding author. Tel.: +52 55 56 22 86 02/19.E-mail address: daniel.matatagui@ccadet.unam.mx (D. Matatagui).

ttp://dx.doi.org/10.1016/j.snb.2014.12.118925-4005/© 2015 Elsevier B.V. All rights reserved.

pharmaceutical and biomedical applications, etc. Thus, interest inhumidity detection is growing rapidly.

On the other hand, tunable magnetostatic spin wave (MSW)oscillators based on yttrium iron garnet (YIG) have been usedfor more than fifty years in microwave instruments. In par-ticular, magnetostatic surface wave (MSSW) oscillators havebeen researched for more than twenty years [27,28]. The mainadvantages of these oscillators over other magnetic oscillatorssuch as the YIG sphere oscillator, is their planar configurationwhich makes them easier to integrate into circuits, and the factthat they work as delay lines. Among their many properties,it is interesting to point out their low propagation losses atmicrowave frequencies, their high loaded Q value, their smalldimensions and their tuneability (from 0.2 GHz to 8 GHz). Theoscillation frequency can be tuned by changing the magnitudeof an applied magnetic field, while the wavelength remainsconstant. Consequently, numerous novel devices and applica-tions based on MSW oscillators have been recently investigated[29–31].

In this paper we demonstrate the use of a tunable MSSWoscillator as a humidity sensor. A single stub in the form of acoplanar waveguide is connected to the oscillator circuit and is

coated with a sensitive material to act as a vapour sensor. We usepolyvinylpyrrolidone (PVP) as a sensitive material, which is highlyhygroscopic and whose relative swelling in a humid atmosphere isdocumented in the literature [32]. In this way we show that changes

298 D. Matatagui et al. / Sensors and Act

Fig. 1. (a) Scheme representing MSSW oscillator based on a two-port YIG delaylwR

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ine. (b) 3D scheme representing the MSSW oscillator coupled with the coplanaraveguide. (c) Front view of the probe and example of the PVP swelling for differentH.

n atmospheric humidity result in a significant frequency shift in thescillator, giving rise to a highly sensitive humidity sensor.

. Materials and methods

.1. Magnetostatic surface wave oscillator

The MSW oscillator was based on a 5 mm × 2 mm rectangu-ar ferromagnetic sample, composed of a 7.3 �m thick YIG filmn a 0.5 mm thick gallium gadolimuim garnet (GGG) substrate. Awo-port delay line whose insertion loss was approximately 10 dB

etween 0.2 GHz and 8 GHz, was formed by placing two anten-as over the YIG film. This YIG delay line was introduced into the

eedback loop of a solid-state amplifier, satisfying the criteria forscillation: the total phase shift in the loop is 2�n (n = integer) and

Fig. 2. Scheme of the instrumentation and experimenta

uators B 210 (2015) 297–301

the gain over the closed loop is 1 (Fig. 1a). In this case a bias mag-netic field applied perpendicular to the wave propagation directionand parallel to the YIG film plane, gives rise to magnetostatic sur-face wave (MSSW) propagation. This bias field was of 200 Oe andresulted in an oscillation frequency of about 1.2 GHz. In order tomeasure this frequency, a third antenna was placed in the feed-back loop, providing an output signal of approximately 10 mW ofpower. Using a spectrum analyzer the linewidth of this signal wasmeasured to be less than 1 kHz. The oscillator was tunable withina range of 0.2–3 GHz limited by the solid-state amplifier. However,with careful control over the bias magnetic field and with a suitableamplifier, the oscillation frequency can be extended to 8 GHz.

2.2. Coplanar waveguide probe

The coplanar waveguide consisted of a 0.5 mm thick substrateof alumina, coated with a 0.05 mm thick copper layer, in which astripline with a width of 0.2 mm and a length of 50 mm was readilyfabricated using low-cost lithography; the gap between the lineand the ground was 0.1 mm. PVP is a water-soluble non-conductingpolymer which has excellent wetting properties and readily formsfilms. This makes it suitable for use as a coating or an additiveto coatings in a wide variety of fields such as medicine, pharma-ceutical, cosmetic and industrial production. It is also used as asensing material to detect VOCs in gas sensors. Here, pure PVP(Mw ∼360,000 g/mol) powder (Sigma–Aldrich) was dissolved inisopropanol to make a PVP solution with a concentration of 4% w/v.The probe was then spin coated with the PVP solution at 2000 rpm.The MSSW oscillator was coupled with the coplanar waveguidethat worked as a humidity probe (Fig. 1b). Since PVP absorbs watermolecules in the environment, the thickness of the polymer filmschanges significantly, modifying the permittivity in the gap of thecoplanar waveguide, producing a perturbation in the confined elec-trical field. This causes a phase shift in the probe, and the phasevariation in the probe induces a frequency change in the oscillatorcircuit, which we use to detect humidity. Fig. 1c presents differentviews of the probe: a top view shows the shape and dimensions ofthe probe, and a front view illustrates the relative swelling of PVPcaused by absorption of water molecules when it is exposed to dif-ferent relative humidities, resulting in different thicknesses of thePVP layer which affect the permittivity in the gap of the coplanarwaveguide.

2.3. Experimental setup

The detection system consisted of a test chamber containingthe humidity probe, coupled to the MSSW oscillator circuit whoseoutput frequency was measured by means of a frequency counter.

Since the magnetic field produced by the magnet is very sensitiveto temperature, the magnet was kept at 25 ◦C, using a proportionalintegral derivative (PID) system that includes a platinum resistance(Pt100) and a Peltier device.

l setup used for the data acquisition in real time.

D. Matatagui et al. / Sensors and Actuators B 210 (2015) 297–301 299

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period and to the room environment in the inhalation semi-period.The range of human breathing rate is very wide, usually between12 and 20 exaltation per minute. However, for some respiratorydisorders, such as sleep apnea, a person has one or more pauses in

Fig. 3. Phase vs. frequency of the probe before and after PVP deposition.

A 10 ml volume of deionized water was stored in the bubblernd was kept at room temperature in a thermal bath for 30 minheadspace time), so that in the headspace the air was at 100%H. The sensor was characterized in a test chamber, which waslled with a flow of a mixture of wet and dry air, both regulatedy means of mass flow controllers, allowing them to produce con-rolled RH values inside of chamber and guaranteeing a continuousir flow of 200 ml/min into the chamber. Dry air was obtained from

cylinder, while wet air was extracted from the headspace. Thisetup provided the desired humidity, between 12.5% and 95% RH.he experiment control and real-time data acquisition were imple-ented with a PC by means of homemade software. A schematic

iagram of the experimental setup is shown in Fig. 2.

. Results and discussion

.1. Electrical properties

Before testing the device as an RH sensor, the frequencyesponse of the probe was characterized by means of a networkector analyzer, and the parameter S11 was a linear function of thehase with respect to the frequency. A phase equal to 2� was foundt a frequency of ∼1.18 GHz. For this frequency, the highest electriceld intensity is located at the end of the stripline, so that the probe

s most sensitive at this point. However, the probe was PVP coatedver its entire surface, thereby taking advantage its entire sensitiverea. The probe was characterized before and after the depositionf PVP, and a frequency shift of the phase response of 2.51 MHz wasroduced, as shown in Fig. 3.

The oscillator was characterized by connecting its output to apectrum analyzer, and it is important to mention that the oscillatorould be tuned at any frequency between 0.2 and 3 GHz by chang-ng the magnitude of the applied magnetic field. However, whenhe oscillator was coupled with the coplanar waveguide probe, theperating frequencies were limited to the frequency in which theength of the waveguide was a quarter of the wavelength. There-ore, the oscillator coupled with the coplanar waveguide was tunedround 1.18 GHz.

.2. Humidity characterization

The humidity probe was coupled with the tunable MSSW oscil-ator and was placed in the test chamber and exposed to different

H values, while measuring the frequency of the device in real timesing a frequency counter. First, the sensor response was measured,uring cycles of 1 min of wet air (exposure) and 1 min of dry airrecovery), in a range from 12.5% to 95% RH and each cycle was

Fig. 4. Real time response of the sensor under 1 min switching periods between12.5% and 95% RH.

repeated twice. The device showed a fast response to the changesin RH, and the measured changes were repeatable and reversible(Fig. 4). In the case of expositions lower than 75% RH, 1 min wasenough to reach a stable frequency; however, in all other casesmore time was needed to reach the stable frequency. The sensorresponse was very similar in the first and second cycles of eachRH environment, and the maximum frequency shift exhibited anexponential increase with respect to the increase of RH in a rangefrom 12.5% to 95%, The sensor response was measured again overa range from 70% to 90% RH (Fig. 5), but in this particular case thecycles were of 5 min of wet air and 5 min of dry air. Fig. 6 plots themaximum frequency shifts reached after 5 min exposures to differ-ent RH values, and the time required for achieving the maximumfrequency shifts. Both properties exhibited an exponential increasewith increasing RH. The maximum shows that the sensor requiredmore than 1 min to reach the maximum frequency shift for thesecases, however for 70%, 80% and 90% RH the response reached 90%of the maximum frequency shift within the first minute. It mustalso be remarked that for the 95% RH the frequency shift for anexposure of 5 min was 1.92 times that for an exposure of 1 min.

Finally, we demonstrated the ability of the device to operatein real situations as a low-cost, non-invasive and real-time sen-sor, suitable for monitoring human breathing. For this purpose thesensor was exposed to cycles of wet air in a human exhalation semi-

Fig. 5. Real time response of the sensor under 5 min switching periods from 70% to95% RH at room temperature.

300 D. Matatagui et al. / Sensors and Act

Fig. 6. Maximum frequency shifts during 5 min of the sensor’s exposure to differ-ent RH values at room temperature, and the time required to reach the maximumfrequency shift.

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Fig. 7. Real-time response of the sensor to a person’s breath.

reathing while he sleeps, and the breathing pauses can last from aew seconds to minutes. In order to detect respiratory disorders by

eans of humidity variations, the probe was kept at a distance ofbout 3 cm from the mouth. The sensor worked with fast responsend recovery when the humidity changed dramatically due to thereathing.

Fig. 7 plots the monitoring of breath in real time, with exhalationnd inhalation times longer than normal in order to demonstratehat the sensor has a very large response after a few seconds ofxposure to high humidity, working properly and maintaining theast response and recovery. The different exposures of the sensor tohe cycles produced a frequency shift of up to 500 kHz (Fig. 7). Theesponse of the sensor exposed for a few seconds to human breaths 4.5 times than to the 95% RH air at room temperature exposedor 1 min. This is because human breath is at about 35 ◦C. Therefore,he sensor can be used to monitor fast and slow respiratory cycles,iagnosing problems in the respiratory system.

. Conclusions

In conclusion, the results have confirmed that the combina-ion of a tunable oscillator based on magnetostatic surface wavesnd a coplanar waveguide probe is a novel, promising, innovative,nd suitable detector for vapors in the environment, and is cost-

ffective. In addition, this device can operate up to 8 GHz, leadingo the development of very small probes with very high sensitiv-ty. To prove the efficiency of the presented sensor, the coplanar

aveguide was coated with polyvinylpyrrolidone in order to work

[

uators B 210 (2015) 297–301

as a humidity probe, and the high sensitivity and repeatability weredemonstrated in a wide range of relative humidity, from 12.5%to 95%. Finally, the results have shown that the MSSW oscilla-tor, when coupled with the coplanar waveguide, is a candidate forconstructing humidity sensors with high performance for variousapplications. The novel sensor presented in this work could formthe basis of an inexpensive, non-invasive and easily removable tool,with the option of fabricating the probe with a flexible polymersubstrate for monitoring human breath and detecting respiratorydisorders.

Acknowledgement

This work was supported by Postdoctoral Grant from CTIC-UNAM and under projects PAPIIT, UNAM, 104513 and PAPIIT,UNAM, IG100314.

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iographies

aniel Matatagui is graduated in physics in 2007, received his M.Sc. in Advancedaterials and Nanotechnology in 2008 and his Ph.D. degree in physics in 2012

uators B 210 (2015) 297–301 301

from the Universidad Autónoma de Madrid. Currently, he is working in the NationalAutonomous University of Mexico (UNAM), in the Microwave Photonics Group atthe CCADET on the development of sensors for volatile-compounds and biosensors.

Oleg Kolokoltsev is a Research Professor at Centro de Ciencias Aplicadas y DesarrolloTecnológico (CCADET) at the National Autonomous University of Mexico (UNAM)and is currently head of the Microwave Photonics Group. He received his Ph.D. inPhysics in 1991 from the Taras Shevchencko National University in Kiev, Ukraine,where he also worked as faculty and head of the Microwave Devices Laboratory.He has been a Royal Society Research Fellow at Salford University in Manchester,UK.

Naser Qureshi is a Research Professor at Centro de Ciencias Aplicadas y Desar-rollo Tecnológico (CCADET) at the National Autonomous University of Mexico(UNAM) and is a part of the Microwave Photonics Group. He received hisB.A. from Princeton University and his Ph.D. from University of California,Santa Barbara in 2002, and has worked as a postdoctoral fellow at the Uni-versity of California, Santa Cruz. His primary interest is in the developmentof novel methods in high-resolution microscopy and their application in thenanosciences.

Elsi Violeta Mejía Uriarte obtained the Ph.D. with Honors in 2003 in MaterialsScience and Engineering from National Autonomous University of Mexico, UNAM,in Mexico City, and in 1998 she was graduated in Master of Science in Physicsfrom National University of Trujillo. She is currently full professor of the Depart-ment of Optics and Microwaves in the Center for Applied Science and TechnologicalDevelopment in the UNAM. Her current interests are luminescent ions in solid,photonic crystal, high-pressure sensor, metal and luminescent nanostructures,optical characterization under high pressure and high temperature and instrumen-tation.

José Manuel Saniger Blesa is a Research Professor at Centro de Ciencias Aplicadas yDesarrollo Tecnológico (CCADET) at the National Autonomous University of Mexico

plutense de Madrid in 1988. He is the founder of the Materials and NanotechnologyGroup of the CCADET. His research areas of interest are molecule/substrate interac-tions, nanostructured and catalytic materials, gas opto electrical sensors and electroceramics.