Response of a SAW Sensor Array based on Nanoparticles for
Measuring Ammonia in the Environment
D. Matatagui
1
, I. Gràcia
2
and M. C. Horrillo
1
1
SENSAVAN, Instituto de Tecnologías Físicas y de la Información (ITEFI), CSIC, Serrano 144, 28006 Madrid, Spain
2
Instituto de Microelectrónica de Barcelona (IMB), CSIC, Campus UAB, 08193 Bellaterra, Spain
Keywords: Sensor Array, Surface Acoustic Wave, Ammonia, Nanoparticles, Gas Sensors, Environment.
Abstract: Four surface acoustic waves (SAW) sensors based on sensitive layers of Fe
2
O
3
nanoparticles, pure and
combined with noble metals nanoparticles, composed an array sensor to measure ammonia in the
environment. The sensor array was tested with nanostructured sensitive layers, which detected the changes
of the elastic properties induced by ammonia interaction. The sensor with pure Fe
2
O
3
nanoparticles exposed
to 50 ppm of ammonia showed no significant effect, however the sensors with Fe
2
O
3
nanoparticles
combined with Au, Pt and Pd nanoparticles responded to these concentrations of this gas, which is so
dangerous for the environment and the health, with a high sensitivity.
1 INTRODUCTION
The ammonia is mainly an industrial gas with
tremendous effects for the environment and for the
health, above all to high concentrations. With
respect to environment, due to its acidity, this
compound is one of the most important acid
pollutants, since its deposit can cause great damage
to natural ecosystems sensitive to acidification:
fauna, flora and quality of air.
Ammonia is a precursor compound of particulate
material, and therefore contributes to the health
effects caused by PM10 and PM2.5 particles.
High concentrations of ammonia are a great
damage to the human health. The lower limit of
human ammonia perception by smell is tabulated to
be around 50 ppm (Budarvari, 1996). However, even
below this concentration, ammonia is irritating to the
respiratory system, skin and eyes. Immediate and
severe irritation of the nose and throat occurs at 500
ppm. High ammonia concentrations, 1000 ppm or
more, could cause pulmonary edema; and higher
concentrations, 5000-10000 ppm, could be already
lethal within 5-10 min (Timmer, 2005).
The Occupational Safety and Health
Administration (OSHA) established an exposure
limit of 25 ppm for ammonia in workplace air during
an 8-hour day and a 35 ppm limit for a short period
of 15 minutes. The National Institute for
Occupational Safety and Health (NIOSH)
recommends that the ammonia level in the
workplace air should not exceed 50 ppm during a 5-
minute exposure period.
Therefore, efficient, reliable, low cost, sensitive,
small sensors and easy to handle are need to control
the air pollution and to replace the conventional
techniques of gas analysis. There are many works
where the efficiency of SAW sensors for gas sensing
at room temperature has been demonstrated in the
last decades, but in almost all of them, the sensitive
layers used have been polymers deposited by drop or
spray coating, being therefore difficult to get
repeatability and homogeneity of the sensitive layers
(Reibel, 2000, Bender, 2003; Sunil, 2015). In
addition, the polymer coatings are degraded and
suffer the swelling effect over time (Matatagui,
2005), due to the gas exposures and this leads to a
great loss of stability and sensitivity to the gases. To
cover these disadvantages, for some years, it has
begun to work with thin coatings of metal oxides,
and more recent with nanostructured coatings of
these materials (Grate, 1994), since in addition to
offering a great long-term stability they also have a
much greater surface area for sensing, and therefore
the sensitivity to gases is increased.
In this work, coatings of iron oxide nanoparticles
have been prepared by spin coating and besides have
been functionalized with Au, Pd and Pt in order to
Matatagui, D., Gràcia, I. and Horrillo, M.
Response of a SAW Sensor Array based on Nanoparticles for Measuring Ammonia in the Environment.
DOI: 10.5220/0008918600930096
In Proceedings of the 9th International Conference on Sensor Networks (SENSORNETS 2020), pages 93-96
ISBN: 978-989-758-403-9; ISSN: 2184-4380
Copyright
c
2022 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
93
compare the sensitivity to low concentrations of
ammonia through Love wave devices.
2 MATERIALS AND METHODS
2.1 Surface Acoustic Wave Sensor
Array
The group of micro-electromechanical systems
(MEMS) includes SAW devices, which consist
essentially of a piezoelectric substrate with two
interdigital transducers (IDT) used to generate and
receive the acoustic waves, obtaining a two-port
delay line (DL). The type of the propagated wave
depends on the selected configuration of the device.
In the present work shear horizontal (SH) guided
waves were used that are called Love wave (LW).
Our LW device was based quartz substrate with
200 nm thick of aluminium IDTs. The wavelength
(λ) was 28 µm, the center-to-center separation
between both IDTs (Lcc) was 150λ and the acoustic
aperture (W) was 75λ that is the length of the IDT
strips (Fig. 1a). A film of SiO
2
with a thickness of 3.5
µm was deposited on the piezoelectric by plasma
enhanced chemical vapour deposition (PECVD). The
synchronous frequency of the Love wave multi-
guiding layer was around 160 MHz (Fig. 1).
Figure 1: 3D scheme representing Love-wave device.
Four different dispersions of iron oxide (Fe
2
O
3
)
nanoparticles (Sigma 544884, average size smaller
than 50 nm) were prepared. One of them, S1, was
prepared exclusively from (Fe
2
O
3
) nanoparticles
dispersed in water. The other three were decorated
with noble metal nanoparticles of Au, S2, (Metrohm-
Dropsens AUNP-COL), Pt, S3, (Metrohm-Dropsens
PTNP-COL), and Pd, S4, (Metrohm-Dropsens
PDNP-COL). The prepared dispersions were
deposited on the LW device as films at a spin rate of
4000 rpm, and then a 30 min postbake at 150 °C was
carried out in order to fix the nanoparticles on the
surface. In this way, an array of sensors with four
different sensitive layers was obtained (Table 1).
2.2 Experimental Setup
The detection system consisted of the test chamber
with the four Love-wave sensors and a reference LW
device that form the array inside. Each Love-wave
sensor was integrated in an oscillator circuit that
leads the oscillation with a specific frequency, which
was used as an output signal (Fig. 2).
Figure 2: Scheme of the oscillator controlled by the LW-
DL.
The acquisition signal is based on a heterodyne
configuration; each of the four sensor-oscillator
signals is mixed with the signal from the oscillator
based on a reference LW device, obtaining a new
signal from the difference of the two original
frequencies. The sensors worked at room temperature
(24 ºC). The experiment control and data acquisition
in real time were implemented with a PC by means
of software made at home. A scheme of the
experimental setup is shown in Fig. 3.
The sensor array was tested using 50 ppm of
ammonia, which was diluted in synthetic dry air and
stored in a commercial bottle (Praxair). A
computerized flow controller system was used to
obtain the final flow, by mixing the flow of the
samples of the bottles and the synthetic dry air. This
was achieved by using mass flow controllers,
connected to the PC by Modbus protocol, that
provide the desired concentrations. The total constant
flow of the gas was kept at 200 mL·min
1
and the
exposure and the purge times were 5 and 10 min,
respectively. The responses were displayed in real
time and saved for processing and analyzing.
SENSORNETS 2020 - 9th International Conference on Sensor Networks
94
Figure 3: Scheme of the instrumentation and experimental set up used for the data acquisition in real time.
3 RESULTS AND DISCUSSION
3.1 Electrical Characterisation
The sensors were electrically characterized by means
of the vector network analyser which measured the
frequency response. Fig. 4 is an example of the
frequency response (a LW device without sensitive
layer, reference) that shows an attenuation around 18
dB and at the operating frequency around 160 MHz.
The sensitive layers introduced in the new device
insertion losses, reaching up to 30 dB in the case of
the guiding layer based on the combination of iron
oxide and Pd nanoparticles. However, the electronic
nose was made to support SAW sensors with
attenuations up to 38 dB.
Figure 4: Frequency response (attenuation and phase) of a
LW device without sensitive layer.
3.2 Ammonia Characterization
The sensor was characterized in ammonia
environments for concentration of 50 ppm (Fig. 5).
Experimental measurements for gas characterization
showed that sensor response is clearly dependent on
the composition of nanostructured guiding layer.
Therefore, the sensor with pure iron oxide
nanoparticles did not show any evidence of response
for ammonia exposition. On the other hand, the
sensor with sensitive layer based on the combination
of iron oxide and Au nanoparticles showed
maximum response, followed by the sensor with
combination of the sensor with combination of iron
oxide and Pt nanoparticles, and finally by the sensor
with combination of iron oxide and Pd
nanoparticles.
Figure 5: Real time response of a LW sensor array based
on iron oxide nanoparticles for a concentration of 50 ppm
of ammonia.
Response of a SAW Sensor Array based on Nanoparticles for Measuring Ammonia in the Environment
95
The measurement reproducibility was tested
measuring 50 ppm of ammonia in three continuous
exposure-purge cycles, during which a similar
frequency shift was obtained (Fig. 6).
Figure 6: Real time response and recovery of a LW sensor
with senstive layer based on combination of iron oxide
and Au nanoparticles for a concentration of 50 ppm of
ammonia (three exposures and purgue process).
According to the theory (Raj, 2017, Fragoso-
Mora, 2018), the fact that the frequency increased
with gas interaction implied that the velocity of the
wave was highly affected by the elastic properties of
nanoparticle layer, resulting high sensitive to the gas
interaction.
Table 1 shows the stadistic of the response,
sensitivity, standard deviation and limit of detection
(LOD) of the triplicates exposures of the sensors to
50 ppm of ammonia.
Table 1: Sensor Array.
Sensor S1 S2 S3 S4
Noble
Metal NP
--- Au Pt Pd
Response
(Hz)
0 359 314 203
Sensitivity
(Hz/ppm)
0 7.19 6.28 4.06
Standard
deviation
0 18.5 26 16
LOD
(ppm)
--- 4.17 4.77 7.37
4 CONCLUSIONS
The combination of the iron oxide nanoparticles
with noble metal nanoparticles induced an elastic
sensitivity for ammonia.
The results showed that the sensor array was
highly effective in detecting ammonia with high
sensitivity (50 ppm). The nanostructured sensors of
the array showed different sensitivities at room
temperature, good repeatability, fast response and
reversibility, and therefore they are good candidates
to get a wireless sensor network for environmental
applications.
ACKNOWLEDGEMENTS
This work has been supported by the Fundación
General CSIC via Program ComFuturo and the
Spanish Ministry of Science, Innovation and
Universities under the projects RTI2018-095856-B-
C22 (AEI/FEDER) and TEC2016-79898-C6
(AEI/FEDER). This research has used the Spanish
ICTS Network MICRONANOFABS (partially
funded by MINECO).
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