1. Microcantilever Sensors Coated With Doped Polyaniline for the
Detection of Water Vapor
C. STEFFENS,1,2
F.L. LEITE,3
A. MANZOLI,2
R.D. SANDOVAL,1,2
O. FATIBELLO,1,4
AND P.S.P. HERRMANN
1,2
1
Department of Biotechnology, Federal University de Sa˜o Carlos (UFSCar), SP, Brazil
2
National Nanotechnology Laboratory for Agribusiness, Embrapa Instrumentation, Sa˜o Carlos, SP, Brazil
3
Department of Physics, Chemistry, and Mathematics, Nanoneurobiophysics Research Group (GNN), Laboratory of
Nanoneurobiophysics (LNN), Federal University of Sa˜o Carlos, Sorocaba, SP, Brazil
4
Department of Chemistry, Federal University of Sa˜o Carlos (UFSCar), Sa˜o Carlos, SP, Brazil
Summary: In the present work, PANI (polyaniline)
emeraldine salt (doped) and base (dedoped) were used
as the sensitive layer of a silicon microcantilever, and
the mechanical response (deflection) of the bimaterial
(coated microcantilever) was investigated under the
influence of humidity. PANI in the emeraldine base
oxidation state was obtained by interfacial synthesis and
was deposited on the microcantilever surface by spin-
coating (dedoped). Next, the conducting polymer was
doped with 1 M HCl (hydrochloric acid). A four-
quadrant AFM head with an integrated laser and a
position-sensitive detector (AFM Veeco Dimension V)
was used to measure the optical deflection of the coated
microcantilever. The deflection of the coated (doped and
undoped PANI) and uncoated microcantilever was
measured under different humidities (in triplicate) at
room pressure and temperature in a closed chamber to
evaluate the sensor’s sensitivity. The relative humidity
(RH) in the chamber was varied from 20% to 70% using
dry nitrogen as a carrier gas, which was passed through a
bubbler containing water to generate humidity. The
results showed that microcantilevers coated with
sensitive layers of doped and undoped PANI films
were sensitive (12,717 Æ 6% and 6,939 Æ 8%, respec-
tively) and provided good repeatability (98.6 Æ 0.015%
and 99 Æ 0.01%, respectively) after several cycles of
exposure to RH. The microcantilever sensor without a
PANI coating (uncoated) was not sensitive to humidity.
The strong effect of doping on the sensitivity of the
sensor was attributed to an increased adsorption of water
molecules dissociated at imine nitrogen centers, which
improves the performance of the coated microcantilever
sensor. Moreover, microcantilever sensors coated with
a sensitive layer provided good results in several cycles
of exposure to RH (%). SCANNING 36:311–316, 2014.
© 2013 Wiley Periodicals, Inc.
Key words: microcantilever sensor, relative humidity,
sensitive layer, sensitivity
Introduction
The growing interest in the development of nano-
devices is due to the great potential for application in
several areas, such as environmental control, monitoring
of chemical vapors (Deisingh et al., 2004), humidity
detection (Then et al., 2006), pH detection (Hu et al.,
2004), pharmacological research (Koev et al., 2009),
precision agriculture (Nugaeva et al., 2005), biotechno-
logy (Zhang et al., 2007), and agribusiness (Steffens
et al., 2012b). The market for microelectromechanical
systems(MEMS)generatedarevenueofU.S.$6.9billion
in 2009, and it is estimated that the market will reach U.S.
$8 billion in 2010, with a growth of 13% over the next
5 years (Ding, 2012). Thus, the market for nanosensors is
expanding, and the technological development potential
is great.
One of the advantages to using MEMS, which fit
microcantilevers used in atomic force microscopy
(AFM), is the ability to tailor the size and structure of
the device (Lang et al., 2010). A sharp tip located at
the free end of a flexible cantilever scans over a surface.
The reflection of a laser beam is focused at the
backside of the cantilever, and the reflected beam is
directed to a photodiode, which provides a voltage. The
microcantilever beam is made of silicon or silicon nitride
and possesses micrometer dimensions, with a length of
Contract grant sponsor: FAPESP; contract grant numbers: 2009/08244-0,
2007/05089-9; contract grant sponsor: INCT-NAMITEC; contract grant
number: CNPq 573738/2008-4.
Address for reprints: C. Steffens, Department of Food Engineering,
URI – Campus de Erechim, 1621, Av. Sete de Setembro, Erechim,
99700-000, RS, Brazil
E-mail: claristeffens@yahoo.com.br
Received 1 April 2013; Accepted with revision 31 May 2013
DOI: 10.1002/sca.21109
Published online 1 July 2013 in Wiley Online Library
(wileyonlinelibrary.com).
SCANNING VOL. 36, 311–316 (2014)
© Wiley Periodicals, Inc.

2. 100–500 mm and a thickness of 0.5–5 mm. Microcanti-
lever beams may be V- (triangular) or T-shaped
(rectangular) (Carrascosa et al., 2006).
The sensitive layer of microcantilevers is one of the
most important parameters of a sensor (Steffens
et al., 2012c). Thus, a major challenge is finding low-
cost, sensitive layers that make the microcantilevers
more sensitive. Among conductive polymers, polyani-
line (PANI) is advantageous as a sensitive layer in
microcantilevers for the detection of analytes. PANI is
easy to synthesize, inexpensive, and presents a more
rapid vapor adsorption/desorption rate (Lahav et al.,
2001; Ostwal et al., 2009).
PANI is a conducting polymer, and the doping level
of PANI controls its conductivity (Mattoso et al., ’95;
Ostwal et al., 2009). When exposed to humidity, a
change in its electrical conductivity (S/cm) and
oxidation state occurs due to the dissociation of
adsorbed water molecules at imine nitrogen centers,
which migrates a positive charge through the polymer.
In most cases, imines units of PANI are enveloped in the
polymer coil, and only those on the surface can come
into contact with water molecules, which changes their
sensitivity to humidity (Li et al., 2004; Steffens
et al., 2009). Thus, the present work aimed to use
PANI emeraldine salt (doped) and base (undoped) as the
sensitive layer of a silicon microcantilever and investi-
gate the mechanical response (deflection) of the
bimaterial under the influence of water vapor. To
investigate the doping process, we analyzed the polymer
using ultraviolet–visible spectroscopy (UV–Vis). The
microcantilever sensors used in the current study were
either uncoated or coated to evaluate the sensitivity and
reversibility under different humidity levels.
Experimental
Commercially available (NT-MDT), rectangular
silicon (tip) microcantilevers with aluminum reflective
coating were used in the present study by the following
dimensions: length ¼ 350 mm, width ¼ 30 mm, and
thickness ¼ 0.5–1.5 mm. The microcantilever surfaces
were cleaned via plasma sputtering under high vacuum.
The argon gas pressure was less than 0.1 mbar, and the
background pressure was 0.1 mbar. A radio frequency
of 40 kHz, a power of 150 W and a temperature of 130˚
C were applied in the treatment. Subsequently, the
microcantilevers were dried in an oven at 50˚C for 10 h
and were stored in a vacuum desiccator.
PANI was obtained in the emeraldine base oxidation
state through an interfacial synthesis, according to the
chemical route reported by Huang and Kaner (2004),
which was used to form to PANI nanofibers. Subse-
quently, PANI was deposited on the microcantilevers
surface via spin-coating (dedoped) using a spinner.
After spinning at 500 rpm for 8 s, 3.0 ml of PANI
solution was deposited on the microcantilever surface.
The spinning rate was increased to 1,000 rpm for 10 s
and 3,000 rpm for 1 min. The experiments were
performed at room temperature and humidity
(25 Æ 2˚C). Afterwards, the coated microcantilever
sensors were dried in a vacuum desiccator for 12 h at
room temperature. Next, the sensitive layer of PANI was
doped with 1 M HCl (hydrochloric acid). Doped PANI
was evaluated via UV–Vis using a Shimadzu spectro-
photometer. All measurements were performed in the
range of 400–1,000 nm.
The hydrophobicity and hydrophilicity of the silicon
microcantilever surface with and without PANI film
(doped and dedoped) was determined by evaluating the
contact angle with a drop of water (Milli-Q1
, surface
tension of 72.7 mJ/m2
) using a contact angle meter
(KSV Instruments). The measurements were performed
in triplicate at 25˚C and 45% humidity, depositing
approximately 4.0 ml of water with a Hamilton syringe.
A four-quadrant AFM head with an integrated laser
and a position-sensitive detector (AFM Veeco Dimen-
sion V) was used to measure the optical deflection of the
microcantilever. After the laser beam alignment on the
end of the cantilever, the calibration was performed
through achieving the spring constant of the cantilever.
The voltages obtained in the photodiode were converted
to nanometers to obtain the deflection. The deflection of
coated (doped and undoped sensitive layer of PANI) and
uncoated microcantilevers was measured at different
humidity levels (in triplicate) at room pressure and
temperature in a closed chamber to evaluate the
sensitivity. The relative humidity (RH) in the chamber
was varied from 20% to 70% using dry nitrogen (Analog
flow mass controllers) at a fixed total flow rate of 0.1 L/
min as a carrier gas, which was passed through a gas
bubbler containing water to generate humidity. The
experiments were performed at a constant temperature
of 20 Æ 0.2˚C, which was maintained using an ultra-
thermostatic bath (Nova E´tica, Vargem Grande Paulista,
SP, Brazil, 521/2D model).
The sensitivity (S) and reversibility (h) of the
microcantilever sensors at different humidities were
calculated from Equations (1) and (2) (Feng and
MacDiarmid, ’99; Steffens et al., 2010):
S ¼
D À D0
D0
 100 ð1Þ
h ¼
D À Df
D À D0
 100 ð2Þ
where D0 is the initial deflection of the microcantilever
sensor obtained at 20% of humidity, D is the deflection
after exposure to a flow of dry nitrogen and Df is the
minimum deflection after exposure to wet nitrogen
using a water bubbler. The initial deflection of the
cantilever sensors was 10 nm during the three cycling
312 SCANNING VOL. 36, 3 (2014)

3. evaluated. The reversibility was calculated to assess the
ability of the sensors vary its state of deflection under the
action of humidity and return to its initial state when the
humidity left to act on the sensitive coating. The
sensitivity of the microcantilever was the ability to
change the deflection sensor is that the exchange of
humidity in each cycle.
The response of coated microcantilever sensors with
doped PANI was measured as a function of the storage
time after 0, 90, and 180 days by measuring the
deflection of the functionalized cantilever at different
humidities. During this time, the sensors were stored in a
vacuum desiccator. Prior to the measurements, AFM
equipment was stabilized for 2 h to thermal drift in the
system. After a stable baseline was obtained at 20% RH,
wet and dry nitrogen gas were cycled to obtain different
relative humidities in the chamber (20–70%).
Results and Discussion
Ultraviolet–Visible Spectroscopy (UV–Vis)
Measurement
UV–Vis spectroscopy was used to evaluate the
absorption bands of the electronic transitions of doped
and dedoped PANI. The investigation of micro-
structural changes is interesting because anions have a
strong effect on the conductivity and electroactivity of
polymer films, which may influence its response when
exposed to an analyte (Steffens et al., 2012a). Figure 1
shows the optical spectra of doped and dedoped PANI
obtained by interfacial synthesis. The spectra were
obtained with PANI solutions (dedoped) used to coat the
microcantilever surface, which were treated with 1 M
HCl to adjust the pH to 2.
In the dedoped PANI spectra, a band was observed at
600 nm, which is related to the charge transfer of
benzenoids ring to quinoid rings. The band corresponding
to this transition does not appear in the spectrum
obtained for doped PANI because it is converted into a
poly(semiquinone radical cation) (Rodrigues et al.,
2005).
With PANI doping, chemical changes occur due to
oxidation and/or reduction reactions, and the counter-
ion (dopant) remains in the polymer matrix. The
molecular structure of the emeraldine base form of
PANI consists of two alternating units, including
reduced amino groups (benzenoid ring) and oxidized
imine groups (quinoid ring) (Epstein, ’97). The
bipolaronic coil in the doped material is unstable and
spontaneously converts to a polaronic coil through a
redox reaction, followed by the separation of polarons
due to electrostatic repulsion of charges, resulting in an
emeraldine salt. Polaronic bands were observed in
Figure 1 at 420 and 800 nm. The doped state was formed
by a poly(semi-quinone) radical cation (Galvo
et al., ’89), which formed a conduction band in the
middle of the polar on energy band, accounting for the
high conductivity of the polymer (Alves et al., 2010;
Leite et al., 2008).
Contact Angle Measurement
Contact angle analyses were performed to identify
surface modifications, which determine the wetting
properties of surfaces with and without a PANI film
coating. The surface wettability depends on molecular
interactions between fluids (e.g. liquid and vapor) and
the solid substrate. The microcantilever surface was
cleaned with a plasma and was coated with a sensitive
layer of PANI (doped and dedoped) via spin-coating.
The surface of the uncoated microcantilever showed a
contact angle of 68 (Æ1). A higher contact angle was
obtained for the surface coated with dedoped PANI
(58 Æ 1˚) compared to that of the surface coated with
doped PANI (53 Æ 1˚), indicating that the surface coated
with doped PANI film presented greater hydrophilicity.
The observed increase in the hydrophilicity could be
attributed to the nature of the doping ion because
improved hydrophobicity results from the protonation of
imine nitrogen atoms, generating radical cation poly
(semi-quinones), which transfers both charges and spins
along the polymer chain (Galvo et al., ’89; Leite
et al., 2008), as observed in the UV–Vis spectra. These
results are in agreement with those of an earlier report
by Liu et al. (’94). According to the authors, doped
PANI is more hygroscopic than the dedoped form;
therefore, water absorption on the surface results in
high surface energy and polarity. Also observed values
of contact angle lower in the coated surfaces compared
with the uncoated. This decrease indicates an increase in
the hydrophilicity of the coated surface with the PANI
film.
Fig 1. UV–Vis spectra of dedoped (base) and doped (salt) PANI
obtained by interfacial synthesis.
C. Steffens et al.: Microcantilever Sensors Coated With Doped Polyaniline 313

4. Response of the Microcantilevers Sensors at
Humidity
The deflection measurements of coated and uncoated
microcantilever sensors were performed in triplicate at a
flow rate of 0.1 L/min at 20˚C. The sensitivity and
reversibility of the sensors were evaluated at various
RHs (%) during desiccation and humidification cycles
(wet and dried gas), and the values were obtained using
Equations (1) and (2), respectively. Coated microcanti-
lever sensors showed a sensitivity of 12717.14 Æ 5.78%
and 6939.08 Æ 8.16% for the doped and dedoped film,
respectively, at humidities of 20–70%. However, the
uncoated microcantilever sensor did not show visible
sensitivity at the evaluated RHs (Fig. 2). The coated
sensors were used in several RH cycles, and a
reversibility of 98.60 Æ 0.01% and 99.01 Æ 0.01%
was obtained for the doped and dedoped films,
respectively.
The results obtained in the present study corroborate
those obtained by Sadek et al. (2007), where the authors
evaluated the sensitivity and repeatability of doped and
dedoped polyaniline nanofibers for gas sensors. These
sensors were exposed to various concentrations of
nitrogen gas, and the doped polyaniline nanofiber sensor
showed greater sensitivity and less repeatability than the
dedoped sensor.
Doping PANI alters some of its properties, such as
the volume, conductivity, conformation, morphology,
and hydrophobicity of the material, thus improving the
adsorption/desorption of water vapor and increasing the
sensitivity of the sensors. Therefore, the most promising
microcantilever sensors for RH (%) measurements are
coated with a doped emeraldine salt PANI due to the
greater interaction between the sensitive layer and water
vapor. These features were also observed in the UV–Vis
and contact angle characterizations, where increased
polarity was characteristic of doped PANI.
The sensitive layer deposited on the microcantilever
surface afforded changes in the surface tension,
corresponding to changes in the water vapor. The
results showed that the coated microcantilever sensors
were subjected to tensile and compression stress during
RH changes. Under tensile stress, microcantilevers bend
upward from their reference position due to the
repulsion and swelling of polymer chains, which can
be attributed to the desorption of water vapor from the
sensitive layer. Alternatively, at higher RHs, compres-
sive stress occurs, causing shrinkage in the PANI film,
which bends the microcantilever downward. This
deflection behavior was also observed by Singamaneni
et al. (2007) in cantilevers coated with methacrylonitrile
polymer exposed to humidity variations of 6–66%.
The polymer matrix changing have a greater
influence in the doped state, may interfere in the
electron insertion or attraction, changing the mobility of
charge carriers, and swelling of the polymer matrix,
when compared with the polymer in the dedoped state.
Also, the humidity may have strong physical inter-
actions with the sensing coating, involving absorbing or
swelling the polymer. Thus, the coated microcantilever
with doped PANI presenting a greater surface stress
during the adsorption of molecules on one side of the
cantilever resulting in a static bending. A demonstration
Fig 2. Defection response of uncoated and coated microcantilever sensors over time during the adsorption and desorption of water
vapor (%).
314 SCANNING VOL. 36, 3 (2014)

5. of this interaction was performed to schematically
illustrate the deflection of coated microcantilever sensors
in the presence of water vapor compared to that of an
uncoated microcantilever (Fig. 3). The sensitive coating
was deposit in the upper surface, on the opposite side of
the tip.
In the evaluated RH (%) range, a deflection of 388 nm
was observed in the sensor with a doped sensitive layer,
while the sensor with a dedoped layer showed a
deflection of 154 nm for every 1% change in RH (%).
The surface tension for the maximum deflection for each
1% change in RH (%) was 0.18 and 0.072 N/m for
sensors with a doped and dedoped sensitive layer,
respectively. The detection limit of sensors with a
sensitive layer doped was 0.005 Æ 0.00025%, corre-
sponding to 1 ppmv, showing that the coated micro-
cantilever sensor with doped PANI presented a large
detection limit under the experimental conditions and
good resolution.
The deflection of the microcantilever sensors was
measured with a doped sensitive layer in the presence of
humidity as a function of time for 0, 90, and 180 days.
Thus, the durability measurements were performed
every 90 days at 20˚C using a gas flow of 0.1 L/min. The
results displayed in Figure 4 show the deflection of the
sensor during repeated humidification and desiccation
cycles between dry and wet gas (at relative humidities of
20–70%). The responses of the sensor were reproducible
and stable during the first 90 days of storage (Fig. 4), i.e.
after 90 days, the behavior of the sensors was the same as
that obtained after they were manufactured. After
180 days of storage, a delay in the response time and
deflection was observed, which may be indicative of the
degradation of the polyaniline film. Thus, the sensors
showed significant environmental stability and reliable
performance for 90 days stored in a vacuum desiccator.
The response time of the microcantilever sensor
coated with polyaniline doped in the presence of water
vapor was evaluated as a function of time because this
device showed better results. Initially, a baseline was
obtained at 20% of RH, and the sensors were exposed to
water vapor at 20˚C. An increase in the RH from 20% to
50% was observed, and a deflection of 10,560–773 nm
was detected, respectively, as shown in Figure 5. This
sensor displayed a response time of 4 s, and it took
approximately 400 s until the RH reached a constant
value. Chia-Yen and Gwo-Bin (2003) obtained a
response time of 1.10 s at RHs of 20–40% for silicon
cantilevers coated with polyimide film. Therefore, our
sensors showed a rapid response time. The observed
behavior clearly indicates the strong interaction between
water vapor adsorbed on the surface and PANI film
deposited on the cantilever.
Conclusion
The surface of microcantilevers was coated with a
sensitive layer of PANI via spin-coating. As a result, the
microcantilever surface became more hydrophilic,
presenting characteristics suitable for use as a sensitive
layer in the detection of RH (%). Polymer doping
provided the coated sensor with a sensitive layer of
Fig 3. Schematic representation of the working principle of
uncoated and coated microcantilevers sensors.
Fig 4. Response of coated MC sensors with doped PANI as a
function of time. The storage behaviour was measured at 0, 90,
and 180 days.
Fig 5. The response time of microcantilever sensors coated with
doped PANI.
C. Steffens et al.: Microcantilever Sensors Coated With Doped Polyaniline 315

6. doped PANI, yielding a sensitivity that was two times
greater than that of a coated sensor with dedoped PANI.
The detection limit of the microcantilever sensor at RH
(%) was 0.005%, which corresponds to 1 ppmv, at
humidity variations of 20–70%. Thus, the bimaterial
microcantilever showed high sensitivity and can be
applied for the detection of water vapor with high
detection limits. After fabrication, the response of the
microcantilever sensor coated with doped PANI was
stable in repeated humidification and desiccation cycles
over a period of 90 days.
Acknowledgments
The authors would like to thank Embrapa Instrumen-
tation, which is responsible for the National Nanotech-
nology Laboratory for Agribusiness, for use of their
facilities.
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