A wireless, remote query ammonia sensor

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Sensors and Actuators B 77 (2001) 614±619

A wireless, remote query ammonia sensor Qing Y. Cai, Mahaveer K. Jain, Craig A. Grimes* Materials Research Institute and Department of Electrical Engineering, The Pennsylvania State University, 204 Materials Research Lab., University Park, PA 16802, USA Received 8 December 2000; received in revised form 20 February 2001; accepted 1 March 2001

Abstract This paper presents a wireless, remote query ammonia sensor comprised of a free-standing magnetoelastic thick-®lm coated with a polymer, poly(acrylic acid-co-isooctylacrylate), that changes mass in response to atmospheric ammonia concentration. The mass of the polymer layer modulates the resonant frequency the ferromagnetic magnetoelastic substrate, hence by monitoring the frequency response of the sensor, atmospheric NH3 concentration can be determined remotely, without the need for physical connections to the sensor or speci®c alignment requirements. The effect of copolymer composition, polymer ®lm thickness, and relative humidity level (RH) on the sensitivity of the sensor were investigated. The sensor linearly tracks ammonia concentration below 0.8 vol.%, and tracks higher concentrations logarithmically; within the linear calibration range, a 0.02 vol.% change in NH3 concentration can be detected. # 2001 Elsevier Science B.V. All rights reserved. Keywords: NH3 sensor; Polymer; Magnetoelastic; Magnetostrictive; Wireless

1. Introduction Our interest is in development of inexpensive, wireless, passive, remote query sensor platforms [1,2] that can be used for in situ or in vivo monitoring applications. Magnetoelastic sensors are one such platform [3], application of which to sensing ammonia is described. Magnetoelastic thick ®lm sensors mechanically deform when subjected to a magnetic ®eld impulse, launching elastic waves within the sensor the magnitude of which are greatest at the mechanical resonant frequency of the sensor. Since the magnetoelastic material is also magnetostrictive, as the ®lm mechanically deforms it generates magnetic ¯ux that can be remotely detected by use of a pickup coil; the mechanical vibrations also serve to generate an acoustic wave which can be monitored using a microphone or hydrophone [4]. For a thin, ribbon-shaped sensor of length L, the fundamental resonant frequency of the longitudinal vibrations is given by [5]: s E p f ˆ (1) 2 r…1 s † L

* Corresponding author. E-mail address: [email protected] (C.A. Grimes).

where E is Young's modulus of elasticity, s Poisson's ratio, r the density of the sensor material, and L is the long dimension of the sensor. An additional mass Dm to the mass M of the sensor corresponds to an increase of its density by a factor 1 ‡ Dm=M, which in turn changes the resonant frequency by a factor …1 ‡ Dm=M† 0:5. If the mass change is small compared to the mass of the sensor, from Eq. (1), the shift in the resonant frequency is given by: Df ˆ

f Dm 2 M


The frequency shifts linearly downward with increasing mass, Df ˆ fmass loaded f < 0. Therefore, small changes in mass can be detected by monitoring the shift in the resonant frequency of the sensor. As the signal is transmitted by magnetic and/or acoustic waves, no direct physical connections to the sensor are needed, nor any special alignment requirements. Furthermore, since the sensors operate in response to the interrogation ®eld, there are no life-time issues with associated battery drain. Fig. 1 is a schematic drawing illustrating the wireless, remote query nature of the sensor platform. Sensors based upon application of magnetoelastic, amorphous thick-®lm ribbons have been developed to measure physical parameters, such as temperature, pressure, and viscosity [3]; in combination with a chemically responsive,

0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 7 6 6 - 3

Q.Y. Cai et al. / Sensors and Actuators B 77 (2001) 614±619

Fig. 1. Schematic drawing illustrating the remote query nature of the magnetoelastic sensor platform. The sensors can be monitored through use of a pickup coil to detect the magnetic flux emitted from the sensor, a microphone to detect the acoustic energy emitted by the sensor, or through the amplitude modulation of an optical beam reflected from the sensor.

mass-changing layer, the magnetoelastic sensor platform can be extended for use in chemical sensing [6,7]. Furthermore, as seen from Eq. (1) chemical sensors can also be made by combination of the magnetoelastic substrate with a chemically responsive elasticity-changing ®lm. If both the mass and the elasticity of the chemically responsive layer change in response to the analyte of interest, the sensor would need to be calibrated prior to use. In this work, a wireless, passive ammonia-sensitive sensor is described, fabricated by dip-coating a carboxylic-functionalized polymer onto a 38 mm  13 mm  28 mm Metglas [8] 2826MB magnetoelastic ®lm, composition Fe40Ni38Mo4B18; Fig. 2 is an SEM cross-sectional image showing the polymer ®lm coated upon the magnetoelastic sensor. The polymer, synthesized from acrylic acid and isooctyl acrylate, demonstrates reversible and high absorption capacity to ammonia at room temperature, and excellent adhesion to the magnetoelastic thick ®lm. Response pro®les and sensitivity of the ammonia sensor were determined, and the effect of humidity and ®lm thickness on the sensitivity investigated.


Wireless, remote query measurement of ammonia by an inexpensive, disposable sensor would ®nd great utility in a variety of applications including, for example, industrial process control. Metal-oxide sensors [9±11] have been used to accurately monitor ammonia based on the change in electrical conductance caused by adsorption, desorption, or reaction of the target gas with the metal oxide ®lm; application of these sensors, however, is limited by their need to operate at elevated temperatures. Optical sensors [12,13] have been used for monitoring ammonia based on changes in refractive index, however, these sensors cannot be used for monitoring conditions inside sealed, opaque containers. Quartz micro-balance sensors have also been coated with mass-changing polymers for monitoring of gasphase species [14,15], however, application is limited by the necessity of providing direct physical connections to the sensor precluding most in-situ monitoring applications. Combining these sensor platforms with an internal power source and telemetry means to enable remote monitoring adds considerable expense and bulk to the devices. 2. Experimental 2.1. Materials Acrylic acid, 2,20 -azobisisobutyronitrile (AIBN), and isooctylacrylate were purchased from Aldrich. Acrylic acid and isooctylacrylate were distilled under reduced pressure prior to use to remove the inhibitor. Other chemicals were used as received. A 28 mm thick ribbon of commercially available Metglas [8] alloy 2826MB, Fe40Ni38Mo4B18 was used as received. The sensors were laser-cut from a continuous ribbon into 39 mm  12:7 mm rectangles, with a weight of 104.0 mg. The resonant frequency of the un-coated sensor in air is 55.0 kHz; a 20 mg polymer coating (10 mg each side) results in an approximate 5% decrease in the resonant frequency. 2.2. Polymer preparation and characterization Poly(acrylic acid-co-isooctylacrylate) was synthesized by free radical copolymerization of acrylic acid and

Fig. 2. An SEM view of sensor cross section, comprised of polymer layer atop a magnetoelastic thick film substrate.


Q.Y. Cai et al. / Sensors and Actuators B 77 (2001) 614±619

isooctylacrylate in ethanol with an initial mole ratio of 1:1 at an overall monomer concentration of 4.5 mol/l. After 1 h bubbling of nitrogen to deoxygenate, the solution, 0.4 mol% AIBN was added as initiator, and the temperature was slowly raised to 708C to start the polymerization, and maintained for 2 h to complete the polymerization under nitrogen atmosphere. The polymeric residue was washed in hexane repeatedly to remove the un-reacted components and small molecular-weight polymers. The resulting polymer was dried in a vacuum oven at 1208C under reduced pressure (10 Torr) overnight, with a resulting polymer density of 1.02 g/ml. The transition temperature of the polymer (Tg) is 1028C, as determined using a DSC 2920 (TA Instrumental). The polymer was characterized using infrared spectrometry (IR) and 1 N NMR spectrometry. IR spectra was recorded on a Nicolet Magna 560 Infrared Spectrometer. NMR data was recorded on a Varian NOVA Spectrometer at 400 MHz with dimethyl sulfoxide-d6 (99.9 at.% D) as the solvent, with chemical shifts expressed as ppm relative to tetramethylsilane (TMS). The pertinent IR spectrum data (cm 1) are: IR2958.0s, 2933.5s, 2873.1m, 1734.9s, 1711.3s, and around 3200, there is a broad weak peak. 1 H NMR: d, 0.70±0.85 (±CH3), d, 1.1±1.6 (±CH2±), d, 1.74 (=CH±), d, 2.1±2.3 (=CHC(O)±), d, 3.9 (±OCH2±). The composition of the polymer was calculated from the relative intensity of the 1H NMR spectrum signals for the ±OCH2± protons (d, 3.9 ppm, with relative intensity of 10.8) in isooctylacrylate and the =CHC(O)± proton (d, 2.1±2.3, with relative intensity of 10.1) in both acrylic acid and isooctylacrylate. The signals at d, 2.1 and d, 2.3 overlap, hence the ratio of acrylic acid to isooctylacrylate considered to be 1:1, which is consistent with the initial monomer composition. The hydrophobic unit, isooctylacrylate is expected to be randomly distributed along the polymer chain [16]. 2.3. Sensor fabrication Prior to coating, the magnetoelastic sensors were washed using Micro-Cleaning solution (Cat #6731, International Products Co., P.O. Box 70, Burlington, NJ 08016-0070, USA) in an ultrasonic cleaner for 5 min, rinsed with deionized water and acetone, and then dried with a stream of nitrogen. The polymer was applied by dip-coating in a 5 wt.% ethanol solution, coating both sides of the sensor. The polymer-coated sensors were dried in a vacuum oven at 808C under reduced pressure (10 Torr) overnight to remove the solvent. Microscope examination of the ®lms indicated that smooth, uniform ®lms were obtained. The thickness of the polymer layers were determined from the known coated surface area of the sensor, 9.88 cm2, and the known (measured) density and weight of the polymer layer. The mass of the polymer coating was determined by weighing the sensor before and after polymer coating using a microbalance (A-160, Denver Instrumental Co).

Fig. 3. Schematic drawing of test-system layout.

2.4. Calibration and testing The sensor under test was placed within a gas-¯ow controlled plastic test chamber (1:5 cm  4:5 cm  6:5 cm). A 16 ms, 3 Oe pulse was used to interrogate the sensor, generated by passing a current pulse through a 10 cm diameter 20-turn coil. Fig. 3 shows the test-system layout. The test atmosphere was generated by mixing known volumes of 99.999% NH3 and 99.999% N2 directly from a compressed air cylinder. The ¯ow rate of both the test and N2 atmospheres were controlled with a mass ¯ow controller and monitored with ¯ow meters. Part of the N2 atmosphere was passed through a distilled-water bubbler to obtain the desired humidity level, which was monitored with a Digital Thermo-Hygrometer (VWR Scienti®c). Part of the test atmosphere was vented prior to the test chamber to guarantee a constant ¯ow rate of 200 ml/min through the chamber. All experiments were performed at a room temperature of 24  18C. An uncoated sensor was set in the chamber, together with the polymer-coated working sensor, and used as a reference sensor to eliminate extraneous environmental interference, if any (e.g. pressure ¯uctuations, temperature instability, variation in ¯uid ¯ow velocity). The reported frequency response is the difference between the resonant frequency of the ammonia sensor and the un-coated reference sensor. 3. Results and discussion 3.1. Polymer composition effect Optimal performance, with high ammonia sensitivity and low humiditysensitivity, wasachievedatthemole composition of 1:1 acrylic acid to isooctylacrylate. Increasing the acrylic acid composition in poly(acrylic acid-co-isooctylacrylate)

Q.Y. Cai et al. / Sensors and Actuators B 77 (2001) 614±619

Fig. 4. Typical response profile of the magnetoelastic NH3 sensor as it is repeatedly cycled between, initially, an atmosphere of nitrogen and nitrogen with 0.1% NH3, then, between an atmosphere of nitrogen and nitrogen with 1% NH3.

over 1:1 did not lead to further increases in ammonia sensitivity, but did increase the humidity sensitivity, which was found to be proportional to the acrylic acid content. Reducing the acrylic acid content below 1:1 decreased the ammonia sensitivity of the polymer, indicating that the absorption of ammonia in the polymer is due mainly to acid-base action and hydrogen-bonding between the acrylic acid carboxylic group and ammonia. 3.2. Response characteristics Fig. 4 shows a typical response pro®le for an ammonia sensor comprised of a magnetoelastic ®lm coated with 20 mg of the polymer, equivalent to a 19.8 mm thick layer on each side of the sensor, at room temperature. When switched from nitrogen to an ammonia atmosphere, the sensor shows a sharp drop, within approximately 5 min, in resonant frequency which then recoils to a slightly higher frequency that is stable after about 10 min. When switched from an ammonia atmosphere to nitrogen, the sensor shows the opposite response, i.e. a sharp increase that then recoils to a slightly lower frequency. This recoil appears to be due to changes in the elasticity of the polymer associated with polymer swelling. The measured response times are found to be independent of ammonia concentration, but directly dependent on the thickness of the polymer layer. The response of the sensor was completely reversible upon exposure to nitrogen, however, the recovery time in nitrogen is slightly longer than the ammonia response time, due to the strong ammonia absorption characteristics of the polymer. The response pro®le was highly reproducible over replicate exposures, with a relative standard deviation of four replicate measurements at a given concentration typically ranging from 1.4% to 4.5%. The inter-sensor reproducibility was tested by determining the response of four sensors coated with 20 mg poly(acrylic acid-co-isooctylacrylate) to ammonia atmosphere. For each sensor triplicate, responses were measured and the average response was used to calculate the relative standard deviation. The relative


Fig. 5. Relationship between polymer thickness and both sensitivity and response time of the NH3 sensor.

standard deviation determined is 6.7%, indicating that a uniform coating was obtained. A total of 14 measurements of the sensor response to ammonia at a given concentration over a period of 2 months provide a measure of the sensor stability, with a relative standard deviation of 5.3% in the measured response with no trends observed over time. Both the ammonia sensitivity and the response time increase with polymer thickness, as shown in Fig. 5. The sensitivity increases linearly with increasing polymer thickness to 20 mm, and then nonlinearly to 27 mm beyond which the mechanical response of the sensor is over-damped due to too high a total mass load. As shown in Fig. 5, the response time increases linearly with ®lm thickness. In this work a 20 mg polymer coating was standardly used, equivalent to a 19.8 mm thick layer on each side of the magnetoelastic thick ®lm. The effect of relative humidity on the sensor performance was investigated from 0 to 90% RH using a 20 mg-coated sensor. Dry and water-saturated nitrogen were mixed to generate the desired humidity levels. Fig. 6 shows the measured resonant frequency as a function of humidity. Increasing humidity from 0 to 12% results in 0.9 kHz shift in the resonant frequency. Further increasing humidity from 12% to 56% results only in a 0.1 kHz frequency shift, with negligible frequency shift between 40% and 56% RH. At RH

Fig. 6. Effect of the relative humidity level on the resonant frequency of the NH3 sensor in a nitrogen atmosphere.


Q.Y. Cai et al. / Sensors and Actuators B 77 (2001) 614±619

Fig. 7. Calibration curves of NH3 sensor for variable ammonia concentration levels in 50% RH and 0% RH environments.

levels higher than 56%, the resonant frequency increases, indicating that the polymer performs differently in low and high humidity environments. An array of multiple magnetoelastic sensors [3], each having non-overlapping operational frequency ranges and coated with an analyte-speci®c chemically responding layer, can be used to simultaneously measure multiple environmental parameters, such as humidity and ammonia concentrations. For example, a humidity sensor can be fabricated by coating a magnetoelastic element with a thin ®lm highly responsive to humidity, such as TiO2 or Al2O3 [17]. Using the output of this sensor to calibrate for humidity, the absolute ammonia concentration can be determined from the polymer-coated sensor. The ammonia sensitivity of the sensor is higher in a wet atmosphere than in a dry atmosphere. Fig. 7 shows the calibration curves of a 20 mg poly(acrylic acid-co-isooctylacrylate)-coated sensor to ammonia in a 50% RH and dry atmosphere, respectively, with the ammonia concentration ranging from 0.1 to 60 vol.%. All data points are the average value of triplicate measurements. At low concentrations up to 0.8 vol.%, the response to NH3 is linear with coef®cients of regression (R2) of 0.99 and slopes of 572 Hz/vol.% at 50% RH, and 480 Hz/vol.% at 0% RH. The sensitivity in 50% RH environment is higher by 16% than in dry environment. The sensor shows a logarithmic response from 0.8 to 60 vol.% for both dry and wet environments. De®ning the limit of detection as 3s per sensitivity, where s is the baseline noise, the limit of NH3 detection was calculated to be 0.02% for a 20 mg polymer coated sensor in a 50% RH environment. It was found that carbon dioxide and carbon monoxide did not interfere with determination of the ammonia concentration; the response of pure CO2 and CO on a 20 mg coated sensor was not detectable. 4. Conclusions In combination with a mass-changing, NH3 responsive copolymer comprised of a 1:1 mole ratio of acrylic acid to

isooctylacrylate, a wireless, passive remote query NH3 sensor is described. The performance of the sensor was found to be stable and repeatable over a period of 2 months, with longer stability expected. The response time of the sensor is proportional to the polymer coating thickness; the response time of a NH3 sensor with a 19.8 mm thick polymer layer (each side) is approximately 15 min. Ammonia sensitivity in a 50% RH environment is higher by 16% than in a dry environment. A linear response was achieved up to 0.8 vol.% of NH3, with a logarithmic response obtained for higher concentrations. Within the linear range a change of 0.02 vol.% of NH3 can be detected for a 20 mg coated sensor. This poly(acrylic acid-co-isooctyl acrylate)-coated magnetoelastic ammonia sensor is wireless, requiring no physical connections between the sensor and processing electronics, passive so that there are no battery life-time issues, operates at room temperature, and has a unit material cost low enough to be readily used on a disposable basis. It should ®nd great utility where in-situ or in-vivo ammonia measurements are required. Acknowledgements Support of this work by the National Science Foundation under contracts ECS±9988598 and ECS±9875104 is gratefully acknowledged. References [1] C.A. Grimes, P.G. Stoyanov, Y. Liu, C. Tong, K.G. Ong, K. Loiselle, S.A. Doherty, W.R. Seitz, A magnetostatic-coupling based remote query sensor for environmental monitoring, J. Phys. D-Appl. Phys. 32 (1999) 1329±1335. [2] K.G. Ong, C.A. Grimes, A resonant printed-circuit sensor for remote query monitoring of environmental parameters, Smart Mater. Struct. 9 (2000) 421±428. [3] C.A. Grimes, D. Kouzoudis, Remote query measurement of pressure, fluid-flow velocity, and humidity using magnetoelastic thick-film sensors, Sens. Actuators A 84 (2000) 205±212. [4] M. Jain, S. Schmit, K.G. Ong, C. Mungle, C.A. Grimes, Magnetoacoustic remote query temperature and humidity sensors, Smart Mater. Struct. 9 (2000) 502±510. [5] L.D. Landau, E.M. Lifshitz, Theory of Elasticity, 3rd Edition, Pergamon Press, Oxford, 1986 (Chapter2). [6] C.A. Grimes, K.G. Ong, K. Loiselle, P.G. Stoyanov, D. Kouzoudis, Y. Liu, C. Tong, F. Tefiku, Magnetoelastic sensors for remote query environmental monitoring, Smart Mater. Struct. 8 (5) (1999) 639±646. [7] Q.Y. Cai, C.A. Grimes, A remote query magnetoelastic pH sensor, Sens. Actuators B 71 (2000) 112±117. [8] The Metglas alloys are a registered trademark of Honeywell Corporation. For product information see: http://www.electronicmaterials.com:80/businesses/sem/amorph/page5_1_2.htm. [9] K. Domansky, D.L. Baldwin, J.W. Grate, T.B. Hall, J. Li, M. Josowicz, J. Janata, Development and calibration of field-effect transistor-based sensor array for measurement of hydrogen and ammonia gas mixtures in humid air, Anal. Chem. 70 (3) (1998) 473±481.

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