Response of a SAW Sensor Array based on Nanoparticles for

Measuring Ammonia in the Environment

D. Matatagui

, I. Gràcia

and M. C. Horrillo

SENSAVAN, Instituto de Tecnologías Físicas y de la Información (ITEFI), CSIC, Serrano 144, 28006 Madrid, Spain

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

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

nanoparticles exposed

to 50 ppm of ammonia showed no significant effect, however the sensors with Fe

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

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

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

)

nanoparticles (Sigma 544884, average size smaller

than 50 nm) were prepared. One of them, S1, was

prepared exclusively from (Fe

) 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

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

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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