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Appl. Phys. Lett. 115, 163502 (2019); https://doi.org/10.1063/1.5124525 115, 163502
© 2019 Author(s).
Hydrophobin-functionalized film bulk
acoustic wave resonators for sensitive and
polarity-sensitive sensing of volatile organic
compounds
Cite as: Appl. Phys. Lett. 115, 163502 (2019); https://doi.org/10.1063/1.5124525
Submitted: 15 August 2019 . Accepted: 02 October 2019 . Published Online: 14 October 2019
Jin Tao, Ye Chang, Jingqiu Liang, Xuexin Duan, Wei Pang, Yanyan Wang, and Zefang Wang
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Hydrophobin-functionalized film bulk acoustic
wave resonators for sensitive and polarity-sensitive
sensing of volatile organic compounds
Cite as: Appl. Phys. Lett. 115, 163502 (2019); doi: 10.1063/1.5124525
Submitted: 15 August 2019 .Accepted: 2 October 2019 .
Published Online: 14 October 2019
Jin Tao,
1,2,3
Ye Chang,
3
Jingqiu Liang,
1
Xuexin Duan,
3
Wei Pang,
3
Yanyan Wang,
3
and Zefang Wang
2,4,a)
AFFILIATIONS
1
State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,
Changchun 130033, China
2
College of Life Sciences, Tianjin University, Tianjin 300072, China
3
State Key Laboratory of Precision Measuring Technology and Instruments, College of Precision Instrument and Opto-electronics
Engineering, Tianjin University, Tianjin 300072, China
4
Tianjin International Joint Academy of Biotechnology and Medicine, Tianjin 300457, China
a)
Author to whom correspondence should be addressed: zefangwang@tju.edu.cn
ABSTRACT
Film bulk acoustic wave resonators have demonstrated great potential in the detection of volatile organic compounds owing to their high
sensitivity, miniature size, low power consumption, capacity for integration, and other beneficial characteristics. However, it is necessary to
functionalize the surfaces of these resonators to enhance the adsorption and discrimination of volatile organic compounds. Here, we report a
convenient and reliable method for functionalizing the surfaces of film bulk acoustic wave resonators with hydrophobins via self-assembly to
enable highly sensitive and polarity sensitive detection of volatile organic compounds. Experiments conducted using various concentrations
of five volatile organic compounds possessing different polarities demonstrated that the hydrophobin coating enhanced the responsivity of
the proposed sensor. The obtained results were in good agreement with the Brunauer–Emmett–Teller model of multilayer physisorption,
which suggests that the hydrophobin coating enhanced the sensitivity by improving the monolayer adsorption capacity. Our work demon-
strates that the combination of multifunctional biosurfactants and microelectromechanical devices can permit high-performance gas sensing.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5124525
The detection of volatile organic compounds (VOCs) is of great
significance in the fields of human healthcare, indoor air quality, indus-
trial gas pollution, and forensic science because VOCs are biomarkers
of various diseases and disorders
1,2
and among the most important
substances known to degrade indoor air quality.
3
VOCs can also be
used to determine the causes of crimes
4
and track airborne contami-
nants.
5
Traditional technologies for detecting VOCs are based on gas
chromatography–mass spectrometry and gas chromatography–flame
ionization detection.
6,7
However, spectrometer-based VOC-sensing
systems are generally expensive and suffer from the lack of portability
and high power demand. Optofluidic microsystems are a newly devel-
oped analytical technology integrating photonics and microfluidics
that possess numerous unique characteristics for enhancing the sensing
performance for VOCs.
8–10
Optofluidic VOC detection systems are
typically complex because they have to include light sources, photode-
tectors, pumps, and gas channels. Therefore, it is necessary to develop
miniature, portable, inexpensive, and accurate VOC sensors. To date,
various small, inexpensive, and fast VOC sensors have been developed,
such as chemoresistors,
11,12
metal oxide semiconductor sensors,
13
ther-
mal sensors, hybrid nanostructure sensors,
14,15
mechanical resonator
sensors,
16
and optical sensors.
17
Among these methods, resonator sen-
sors are most competitive in terms of sensitivity and resolution owing
to the high quality factor of electromechanical resonators.
16
Film bulk acoustic wave resonators (FBARs), as piezoelectric
transducers operating in the gigahertz regime, have been exploited for
VOC detection based on the mass loading effect
18–25
and chemical
reaction.
26
These sensors feature high sensitivity, miniature size, and
low power consumption. FBAR arrays have also been used as elec-
tronic noses to achieve discrimination of VOCs.
22–24
AsingleFBAR
can also function as a virtual sensor array with temperature modula-
tion to detect and discriminate VOCs.
25
The surfaces of FBARs are
typically functionalized with polymers,
18
supramolecular mono-
layers,
25
or self-assembled monolayers
23,24
to realize selectivity and
improved adsorption. Surface functionalization of the sensors plays a
Appl. Phys. Lett. 115, 163502 (2019); doi: 10.1063/1.5124525 115, 163502-1
Published under license by AIP Publishing
Applied Physics Letters ARTICLE scitation.org/journal/apl
key role in recognition, sensitivity, selectivity, stability, and reversibil-
ity. Polymer coating is the most common surface functionalization
method for electronic noses. However, such modification processes
are complex and exhibit poor selectivity. In addition, some VOCs can
penetrate the polymer bulk, which increases the recovery time and can
even cause the sensor to malfunction. Self-assembled monolayers can
afford high selectivity and fast adsorption/desorption although the
modification process is complex and these monolayers are not robust
over prolonged periods.
In our previous work, we demonstrated that hydrophobins are
excellent materials for surface functionalization.
27–29
Hydrophobins
exhibit remarkable physicochemical properties, such as self-assembly,
amphiphilicity, stability, and surface activity.
30–34
Owing to these
properties, hydrophobins display a variety of functions at interfaces,
including adhesion, formation of coatings, reduction in the surface
tension of water, and modification of the hydrophilic/hydrophobic
properties of the interfaces. Hydrophobins can also form membranes
via self-assembly at air/water or water/solid interfaces.
30
Therefore,
hydrophobins hold great potential for applications involving emulsifi-
cation,
35
separation,
26,27
and interfacial modification.
36–41
It has also
been demonstrated that hydrophobins are safe for use in food prod-
ucts
42,43
and exhibit no cytotoxicity, possibly extending their use to
personal care and biomedical applications.
14
Moreover, hydrophobins
are promising materials for surface modification and as surface mem-
branes in biosensing and electrochemical applications.
44
In this work, we used HFBI, a hydrophobin from Trichoderma ree-
sei,
45,46
to modify the surface of an FBAR and achieve a highly sensitive,
stable, and polarity sensitive VOC sensor. HFBI formed a stable and
uniform film at the surface of the FBAR via self-assembly. The resulting
HFBI film was stable and attractive to polar VOCs. Five VOCs possess-
ing different polarities and concentrations were tested experimentally.
Our results were in good agreement with the Brunauer–Emmett–Teller
(BET) model of multilayer physisorption, which explains the sensing
mechanism of the proposed sensor.
FBARs generally consist of a piezoelectric thin film (e.g., AlN or
ZnO) sandwiched between top and bottom electrodes as shown in
Fig. 1(a). The detailed fabrication process of FBARs was reported pre-
viously.
22
Figure 1(a) presents a cross-sectional view of the FBAR
structure coated with an HFBI film layer. Numerous researchers have
attempted to exploit FBAR devices for sensor applications with
remarkable results.
47
FBAR sensors, which function as mass trans-
ducers, are very similar to the well-known quartz crystal microbalance.
When a mass (Dm) is added to the device surface, the resonant fre-
quency (f
0
)shifts(Df)as described in the following equation:
47
Df¼2f2
0
Affiffiffiffiffiffi
ql
pDm;(1)
where Ais the surface area of the FBAR and qand lare the density
and shear modulus of the piezoelectric material, respectively. The
mass sensitivity (Df=Dm) can be derived from Eq. (1) to be
2.5 10
6
Hz/ng.
The HFBI film was deposited simply by droplet casting. The
HFBI solution was prepared by dissolving HFBI (0.50 mg) in phos-
phate buffer (1 ml, pH 7.2), and 1 ll of this solution was added drop-
wise onto the surface of the FBAR using a micropipette and allowed to
stand for 5 min to undergo self-assembly. Next, the device was washed
with pure water and dried under vacuum. Figures 1(b) and 1(c) pre-
sent images of the bare and HFBI-coated FBARs, respectively.
To check the uniformity of the HFBI film, an atom force micro-
scope was used to measure the smoothness and roughness of the surfa-
ces of the bare FBAR and HFBI-coated FBAR as shown in Figs. 1(d)
and 1(e). The surface roughnesses (Ra) of the bare FBAR and HFBI-
coated FBAR were found to be 1.11 and 1.56 nm, respectively, indicat-
ing that a uniform HFBI film was formed on the FBAR surface.
Figure 1(f) shows the impedance phases of FBAR sensors with
(blue curve) and without (red curve) the HFBI coating. The frequency
shift was 1.83 MHz, which corresponds to a mass increase of 0.732 ng.
A VOC detection system with two gas channels was established.
One channel produced saturated VOC vapor by bubbling pure nitro-
gen gas (99.999%) through liquid VOCs. The other channel carried
pure nitrogen gas to dilute the VOC vapor. The flow velocities were
monitored and controlled using a mass flow controller. Various con-
centrations of VOCs were obtained by adjusting the flow velocity ratio
FIG. 1. (a) Cross-sectional view of the FBAR structure coated with an HFBI film
layer. Optical microscopy images of a bare FBAR (b) and an HFBI-coated FBAR
(c). (d) and (e) AFM images of a bare FBAR and an HFBI-coated FBAR surface.
(e) AFM image of the surface. (f) The resonant frequency of the bare FBAR sensor
was 1.15272 GHz (red curve). The HFBI coating caused a downward frequency
shift of 1.83 MHz (blue curve), corresponding to a mass increase of 0.732 ng.
Applied Physics Letters ARTICLE scitation.org/journal/apl
Appl. Phys. Lett. 115, 163502 (2019); doi: 10.1063/1.5124525 115, 163502-2
Published under license by AIP Publishing
between the carrier nitrogen and pure nitrogen. The different concen-
trations of VOCs were quantified as the ratio (P/P
0
)ofthevaporpar-
tial pressure (P) to its saturation vapor pressure (P
0
), which was
controlled by adjusting the velocities of the two gas channels. The con-
centration of the VOCs in ppm can be calculated as follows:
48
C ppm
ðÞ
¼106Ps
Ptr
rþR
ðÞ
;(2)
where P
s
is the saturation vapor pressure of the VOC, P
t
is the total
pressure (760 mm Hg), and rand Rare the flow rates (in sccm) of the
carrier nitrogen and dilution nitrogen, respectively. Here, r=ðrþRÞis
equal to P/P
0
. Further details of the detection system were reported
previously.
22,24
Figures 2(a) and 2(b) present the real-time responses of the bare
FBAR and HFBI-coated FBAR sensors upon exposure to seven differ-
ent concentrations of ethanol vapor. The sensors were first flushed
with pure nitrogen to obtain a stable baseline. Next, the FBAR sensors
were exposed to ethanol vapor at a constant velocity of 500 sccm. The
resulting negative frequency shift indicates the adsorption of ethanol
molecules to the sensor surface. The response times of the bare FBAR
and HFBI-coated FBAR sensors are 16 and 18 s, respectively, for etha-
nol vapor at a P/P
0
value of 0.7. Next, the vapor flow was switched off,
and pure nitrogen was reintroduced to flush the sensor. The frequency
recovered back to the baseline in several tens of seconds, indicating
that the VOC molecules were removed by the nitrogen gas flow. The
recovery times of the bare FBAR and HFBI-coated FBAR sensors are
32 and 34 s, respectively. These results demonstrated that the fabri-
cated sensors exhibited a quick response and full recovery.
To elucidate the adsorption characteristics, adsorption isotherms
for the frequency shifts of the two sensors are presented in Fig. 2(c).
The black line representsthe linear fit of the frequency shift of the bare
FBAR sensor upon exposure to ethanol vapor at partial pressures
(P/P
0
) ranging from 0.1 to 0.7. The adsorption isotherm was almost
linear and fitted well with the Langmuir adsorption theorem,
49
indi-
cating the monolayer adsorption of the ethanol molecules at the bare
FBAR sensor surface in a concentration-dependent manner owing to
weak van der Waals forces between the ethanol molecules and the
FBAR sensor surface. The adsorption of the ethanol molecules to the
FBAR surface led to a downward shift of the resonant frequency. As
van der Waals forces between device surfaces and ethanol molecules
are relatively weak, this interaction was concentration dependent and
reversible. Upon switching off the ethanol flow and flushing the sensor
with nitrogen gas, the resonant frequency of the device recovered to
the baseline.
In contrast, the adsorption isotherm for the HFBI-coated FBAR
sensor upon exposure to ethanol vapor [red curve in Fig. 1(c)]was
more exponential than linear and was well fitted by the BET model of
multilayer physisorption.
24,50
In the BET model, the adsorption of gas
molecules at a solid interface is physical and multilayer. The adsorp-
tion is dependent on the gas concentration and the attractive force
between the gas and solid interface. The BET equation can be
expressed as follows:
m¼mmP=P0
ðÞ
C
1þC2
ðÞ
P=P0
ðÞ
C1
ðÞ
P=P0
ðÞ
2;(3)
where m,mm,andCare the adsorption capacity, monolayer adsorption
capacity, and adsorption energy constant, respectively. The values of
mmand Care typically dependent on the nature of the solid surface
and gas, allowing them to be used as “fingerprints” for the detection
and discrimination of VOCs.
The HFBI coating improved the sensitivity to ethanol by eight-
fold at a P/P
0
value of 0.7. The most plausible explanation for this is
that HFBI is more attractive to ethanol molecules than the bare FBAR
surface (AlN) owing to the negatively charged nature of HFBI. As eth-
anol molecules are also highly polar, the sensitivity enhancement upon
HFBI coating can be attributed to polarity. To confirm this hypothesis,
hexane, a nonpolar VOC, was also investigated as shown in Fig. 3.
The response times of the bare FBAR and HFBI-coated FBAR sen-
sors are 9 and 13 s, respectively, for hexane vapor at a P/P
0
value of 0.7.
TherecoverytimesofthebareFBARandtheHFBI-coatedFBARsen-
sors are 9 and 22 s, respectively. Figure 3(c) shows the adsorption iso-
therms for the bare (black data points and fitting line) and HFBI-coated
(red data points and fitting line) FBAR sensors upon exposure to hexane
vapor. Although the HFBI coating improved the sensitivity toward hex-
ane by approximately twofold, this improvement was smaller than that
FIG. 2. Real-time responses of the (a) bare and (b) HFBI-coated FBAR sensors to eth-
anol vapor concentrations (P/P
0
) ranging from 0.1 to 0.7, where Pand P
0
are the vapor
partial pressure and saturation vapor pressure, respectively, at room temperature. (c)
Adsorption isotherms for the bare (black) and HFBI-coated (red) FBAR sensors.
Applied Physics Letters ARTICLE scitation.org/journal/apl
Appl. Phys. Lett. 115, 163502 (2019); doi: 10.1063/1.5124525 115, 163502-3
Published under license by AIP Publishing
observed for ethanol because hexane molecules are nonpolar, leading to
weaker attractive forces between the hexane molecules and HFBI.
To further evaluate the amplification effect of the HFBI coating,
three additional VOCs (methanol, tetrahydrofuran, and acetone) were
tested. Figure 4(a) compares the observed frequency shifts for various
concentrations of the five VOCs possessing different polarities. Here, we
define the amplification factor as the ratio of the frequency shift for the
HFBI-coated FBAR sensor to that for the bare FBAR sensor upon expo-
sure to a particular concentration of VOC vapor. Figure 4(b) presents
the corresponding amplification factors of five VOCs. It is readily appar-
ent that the HFBI coating improved the sensitivity of the FBAR sensor
and that polar VOCs were adsorbed more strongly than nonpolar
VOCs. The sensing mechanism of the proposed sensor is based on the
BET model of multilayer physisorption as illustrated in Fig. 4(c).
According to Eq. (2), the concentrations of hexane, tetrahydrofu-
ran, acetone, ethanol, and methanol at the P/P
0
value of 0.7 are found
to be 111656, 133333, 1740042, 40152, and 89222ppm, respectively.
The corresponding frequency shifts of the HFBI-coated FBAR are
0.186, 1.638, 1.160, 1.140, and 2.539MHz, respectively. The noise of
the proposed sensor is estimated to be 0.3 kHz. Therefore, the detec-
tion limits of the five VOCs are found to be 180.2, 24.4, 44.0, 10.6, and
10.5 ppm, respectively, as listed in Table I.
By fitting with the BET equation, the values of mmand Cfor the
five VOCs were determined as listed in Table I. As these parameters
FIG. 3. (a) Real-time responses of the (a) bare and (b) HFBI-coated FBAR sensors
to hexane vapor concentrations (P/P
0
) ranging from 0.1 to 0.7. (c) Adsorption iso-
therms for the bare (black) and HFBI-coated (red) FBAR sensors.
FIG. 4. (a) Frequency shifts and (b) amplification factors for various concentrations
of five VOCs with different relative polarities. (c) Schematics of the monolayer
adsorption of bare FBARs and the multilayer adsorption of HFBI-coated FBARs.
TABLE I. BET fitting results of mmand Cfor the five VOCs.
VOC
Detection limits
(ppm)
Relative
polarity
HFBI-coated
FBAR
Hexane 180.2 610.8 0.009 v
m
65.1 63.4
C3.8 61.1
Tetrahydrofuran 24.4 62.8 0.207 v
m
741.9 6120.1
C1.0 60.4
Acetone 44.0 66.2 0.355 v
m
676.0 6102.8
C0.5 60.1
Ethanol 10.6 60.6 0.654 v
m
614.5 655.0
C0.6 60.1
Methanol 10.5 61.8 0.762 v
m
970.4 667.8
C1.6 60.4
Applied Physics Letters ARTICLE scitation.org/journal/apl
Appl. Phys. Lett. 115, 163502 (2019); doi: 10.1063/1.5124525 115, 163502-4
Published under license by AIP Publishing
are generally different for different VOCs, they can be used as finger-
prints for gas discrimination. The mmvalues for the HFBI-coated
FBAR sensors were considerably larger than those for the bare FBAR
sensors, which indicates that the HFBI surface can adsorb a greater
amount of VOC molecules than that present in the single monolayer
formed on the AlN surface of the bare FBAR sensor. Owing to its
polarity, HFBI preferentially adsorbed polar VOCs. In other words,
the number of VOC molecules adsorbed on the HFBI film increased
with increasing VOC polarity.
In summary, this work demonstrated an HFBI-coated FBAR for
the sensitive and polarity-sensitive sensing of VOCs. The measure-
ment results show that the HFBI coating exerted a remarkable influ-
ence on the performance of the FBAR sensor and greatly enhanced its
sensitivity by two- to eightfold. The adsorption isotherms were deter-
mined for the five VOCs. Fitting with the BET equation afforded the
mmand Cvalues for the five VOCs, which can potentially be used as a
fingerprint to discriminate different VOCs. The proposed HFBI-
coated FBAR sensor possesses several advantages, such as the relatively
simple coating method of droplet casting, the satisfactory stability of
the HFBI coating, improved sensitivity, and polarity-sensitive sensing
owing to the negatively charged nature of the HFBI film. This sensor
holds great potential applications in biosensing owing to the multi-
functional nature of HFBI.
The authors acknowledge the financial support of the Science
and Technology Planning Project of Guangdong Province (No.
2016B010111003), the Science and Technology Development
Plan Project of Jilin Province (Nos. 20180201024GX and
20190302062GX), and the National Key R&D Program of China
(No. 2018YFB1801900). Zefang Wang acknowledges the support of
the National Natural Science Foundation of China (No. 81601593).
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