Detection of Ammonia Using Shear Horizontal Surface Acoustic Wave Resonators

Abstract

A shear horizontal surface acoustic wave (SH-SAW) sensor coated with the polyaniline (PANI) film was investigated in this study. The frequency shift of the SH-SAW was measured for detecting ammonia. In addition, the temperature effect was discussed in order to find a suitable temperature of operation. The SH-SAW sensor in this study could immediately respond the concentration of ammonia gas and showed a linear response. This sensor presented a sensitivity of 0.21 ppm/ppm at 23degC.

Full text

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Shear Horizontal Surface Acoustic Wave Resonators
for Ammonia Detection

CHI-YEN SHEN, CHENG-LIANG HSU, and JIN-SHING JENG
Department of Electrical Engineering
I- Shou University
1, Section 1, Hsueh-Cheng Rd., Ta-Hsu Hsiang, Kaohsiung County
TAIWAN


Abstract: This Shear horizontal surface acoustic wave (SH-SAW) devices coated with L-glutamic acid
hydrochloride have been fabricated and used as ammonia sensors in this work. The humidity effect on the
ammonia sensors was also studied. The sensitivity of the ammonia sensor was 4.6 ppm/ppm and the frequency
shift was negative at 50°C in dry air. As the humidity rose in the environment, the sensor exhibited a positively
increasing frequency shift. It proved that the humidity in the environment interfered with the detection to
ammonia. This work would focus on estimating the cross-sensitivity of the humidity interference.

Key-Words: Shear horizontal surface acoustic wave, L-glutamic acid hydrochloride, Ammonia, Sensitivity,
humidity, Cross-sensitivity.

1 Introduction
The detection of ammonia gas is an important task
in many circumstances such as food technology,
industrial processes, environmental protection, and
medical diagnosis, because ammonia presents in
ambient polluting aerosols and may cause disease in
humans. The chemical interfaces for ammonia
detection have metal oxide, metal film, and polymer.
SnO2 and group-III-element-doped zinc-oxide were
successfully used to detect ammonia at 350°C [1, 2].
Hohkawa et al. [3, 4] studied the characteristics of
surface acoustic wave (SAW) sensors based on
porous alumina with Pt or Co catalyst to ammonia.
Palladium metal-oxide-semiconductor (Pd-MOS)
structures were also proved to be useful for
ammonia detection [5]. D’Amico et al. [6] employed
a SAW delay line coated with selectively sorbent
platinum (Pt) film to sensitively detect ammonia.
Penza et al. [7-9] employed the polypyrrole film,
prepared by the Langmuir-Blodgett (LB) technique,
to selectively and sensitively detect ammonia gas
from 46 ppm to 10000 ppm.
In this study, L-glutamic acid hydrochloride was
as the chemical interface for detecting ammonia.
The sensor was the shear horizontal surface acoustic
wave (SH-SAW) resonator. We continuously studied
the detection properties of SAW sensors based on
L-glutamic acid hydrochloride to ammonia [10-15].
It was proved that the SAW delay line based on
L-glutamic acid hydrochloride had high sensitivity,
selectivity, reversibility, and repeatability to
ammonia. The detection limit of L-glutamic acid

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hydrochloride for ammonia has been found to be
less than 0.90 ppm ammonia in dry air. In general,
the humidity in the air seriously interferes with the
detection of ammonia sensors. The previous report
[11] showed the gas detecting properties of
L-glutamic acid hydrochloride deposited on 128°
YX-LiNbO3 SAW delay lines were interfered with
increasing humidity. All previous studies related to
the humidity effect focused on the concentration of
ammonia above 1 ppm. In fact, it is very important
to realize the humidity effect on the detection of ppb
concentration of ammonia for biosensor application.
In
36oYX-LiTaO3 resonators coated with L-glutamic
acid hydrochloride were investigated to demonstrate
this study, the SH-SAW sensors on
the humidity effect on the detection of ppb
ammonia.


2 Experimental
The SH-SAW resonators were fabricated on a
36oYX-LiTaO3 substrate by lift-off methods, using
aluminum 1200 Å metallization. The characteristics
of the SH-SAW resonator were measured by
network analyzer (E5071A, Agilent, USA).
L-glutamic acid hydrochloride was the chemical
interface. A known quantity of L-glutamic acid
hydrochloride (Aldrich, USA) was weighed and
dissolved in a known volume of deionized water at
75°C, to a concentration of 0.1 mg/ml. Prior to the
coating layer being applied, the surface of the
SH-SAW resonator had been cleaned in acetone, and
dried in an oven (Rendah, Taiwan) at 80oC. Then, a
coating of L-glutamic acid hydrochloride was
deposited on the surface of the SH-SAW resonator
by air brushing.
The SH-SAW resonators were introduced into a
SH-SAW sensing system (Nenogram balance, ftech,
Taiwan), which applied a dual-device configuration.
The operating frequency was 148 MHz. The period
of interdigital transducer (IDT) was 28 µm. Each
IDT had eight finger-pairs and the acoustic aperture
was 750 µm. The center-to-center spacing between
the two IDTs was 1659 µm. The RF electronic
oscillator circuit was employed to generate RF
signals in SH-SAW sensing system. The improved
circuit and precise temperature-controller were used
to ensure temperature stability with ± 0.01oC. A
gaseous ambience was controlled by the mass flow
controller (Sierra, USA) on the flow rate of
110ml/min. All detections were preceded at 50°C.
Before testing gas exposure, the sensor was exposed
in dry air for 30 min to stabilize the initial SH-SAW
signal. Lastly, a frequency counter monitored the
frequency shifts in SH-SAW sensing system, which
was connected to a computer system via a RS-232
interface board.


3 Results and Discussion
Most applications of the chemical sensor utilize a
chemical interface, which acts as a selective sink, on
the surface of the SH-SAW. The responses of the
sensor respond to the changes of properties of the
chemical interface due to the absorption of target.
L-glutamic acid hydrochloride is a stiff and
non-conductive material, so the perturbation for
L-glutamic acid hydrochloride after absorption can
be described as follows [16],
()
()
)
(





+
+
2
−+=∆
''2
0
'''
2
02
2
021
4
v
µλ
µλµ
ρ
hhfkfkkf
f
, (1)
where
1k and
2 k are negative constants of the
substrate material, h is the film thickness,
f
ρ is
film density, and
'λ ,
'u are the shear modulus and

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Lame constants of the chemical interface. The two
terms on the right of Eq. (1) indicate the
contributions of change in mass loading and elastic
effect, respectively, to total changes in the SH-SAW
frequency shift. The frequency shift is negative
when the mass loading dominates the perturbation,
whereas the frequency shift is positive as the elastic
effect is dominant. The responses of sensors at the
various concentrations of ammonia in dry air at 50oC
were measured and shown as Fig. 1. It shows that
the frequency shift linearly increased as the
ammonia concentration increased from 40 ppb to
400 ppb. The sensitivity of the sensor at 50°C in dry
air was 4.6 ppm/ppm. Moreover, the negative
frequency shift presents the mass loading is
dominant during detecting ammonia in dry air at
50°C.

Fig.1 Responses of sensor coated with L-glutamic acid
hydrochloride at the various concentration of ammonia in dry air
at 50°C.

The frequency shift only due to the absorption
of humidity was measured and shown as Fig. 2(a).
The positive frequency shift rapidly increased as
humidity increased until 40%RH. Then the
frequency shift gradually increased and saturated at
60%RH. It can be found that the absorption of water
molecules could change the modulus of L-glutamic
acid hydrochloride and made the elastic effect to be
the primary perturbation. The further increasing
humidity above 60%RH did not significantly change
the modulus and resulted in the saturation of the
response. Fig. 2(b) shows the responses to 40 ppb
ammonia gas as a function of humidity. It illustrates
the similar results to Fig. 2(a) and means that the
contribution resulted from humidity was evident.









Fig. 2 Responses of sensor coated with L-glutamic acid
hydrochloride at 50°C for (a) various relative humidity and (b)
40 ppb ammonia at the various relative humidity.

Therefore, the responses of the sensor can not
be described by a simple linear combination of the
individual sensitivities for dry ammonia and
humidity. A mathematical relation of the response
characteristics to dry ammonia and to humidity can
be described as [17]
()()()( ) (
S
)
OH NHOH NHOH, NH
232323
DSSSS
++=
, (2)
where
()
OH, NH
23
S
is the frequency shift to ammonia
in humid air in Hz,
()
3
NH
S
the frequency shift to
ammonia in dry air in Hz,
()
OH2
S
the frequency
shift to humidity in Hz, and D is the cross-sensitivity.
The third term on the right of Eq. (2) is a cross-term
that describes the mutual dependence of the
response on ammonia and humidity. To explain the
interference from humidity,
()
OH, NH
23
S
,
()
3
NH
S
, and

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()
OH2
S
were separately measured and substituted
into Eq. (2).The cross-sensitivity is shown as Fig. 3.
The cross-sensitivity slightly and positively
increased as the humidity raised up to 30%RH, then
it rapidly turned to negative value and gradually
saturated at 60%RH. This saturation is consistent
with the results of Fig. 2. The cross-sensitivity was
zero when the humidity was 31.4%RH. It suggests
that it can operate the SH-SAW ammonia sensor
below 31.4%RH with negligible humidity
interference.

Fig. 3 Cross-sensitivity of ammonia sensor with various
relative humidity at 50°C.


4 Conclusions
In this work, the humidity effect on the ammonia
sensors coated with L-glutamic acid hydrochloride,
which presented an excellent detection to ammonia
in dry air, was studied. The sensitivity of the sensor
was 4.6 ppm/ppm and the frequency shift was
negative at 50°C in dry air. However, the sensor
positively responded to the humidity in the
environment. It suggests the operation below
31.4%RH is with negligible humidity interference
for ammonia detection.


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