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Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America
Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 6718–6722, 2014
www.aspbs.com/jnn
Microcantilever Sensors Coated with a Sensitive
Polyaniline Layer for Detecting Volatile
Organic Compounds
C. Steffens1 2 ∗
, F. L. Leite3
, A. Manzoli2
, R. D. Sandoval1 2
,
O. Fatibello1 4
, and P. S. P. Herrmann1 2 5
1
Department of Biotechnology, Federal University de São Carlos (UFSCar), 13560-000, SP, Brazil
2
National Nanotechnology Laboratory for Agribusiness, Embrapa Instrumentation, São Carlos, 13560-970, SP, Brazil
3
Department of Physics Mathematics and Chemistry (DFMQ), Nanoneurobiophysics Research Group,
Federal University of São Carlos (UFSCar), Sorocaba, 18052-780, SP, Brazil
4
Department of Chemistry, Federal University of São Carlos (UFSCar), São Carlos, 13560-000, SP, Brazil
5
Embrapa Labex Europe, Germany, Forschungszentrum Jülich, Institute of Bio-Geoscience 2,
52428, Jülich, NWF, Germany
This paper describes a silicon cantilever sensor coated with a conducting polymer layer. The
mechanical response (deflection) of the bimaterial (the coated microcantilever) was investi-
gated under the influence of several volatile compounds—methanol, ethanol, acetone, propanol,
dichloroethane, toluene and benzene. The variations in the deflection of the coated and uncoated
microcantilevers when exposed to volatile organic compounds were evaluated, and the results
indicated that the highest sensitivity was obtained with the coated microcantilever and methanol.
The uncoated microcantilever was not sensitive to the volatile organic compounds. An increase in
the concentration of the volatile organic compound resulted in higher deflections of the microcan-
tilever sensor. The sensor responses were reversible, sensible, rapid and proportional to the volatile
concentration.
Keywords: Microcantilever Sensor, Volatile Organic Compound, Sensitive Layer, PANI,
Sensitivity.
1. INTRODUCTION
The working principle of the microcantilever sensor is
based on the adsorption of analytes onto the sensing sur-
face, which results in a differential surface stress and
increases the mass of the microcantilever.1 2
The surface
functionalization of the microcantilevers makes them suit-
able candidates for selectively detecting trace amounts
of compounds. In addition, these sensors could detect
physical, chemical or biological stimuli and have sev-
eral advantages over conventional techniques, including
high sensitivity, low cost, a simple procedure and rapid
responses.3–5
Given its analysis ability and ultra-sensitive
analyte detection, this technology demonstrates promise
for the next generation of miniaturized sensors with highly
sensitive and desirable detection limits. Thus, the sensor
∗
Author to whom correspondence should be addressed.
can be applied in several fields, including safety and secu-
rity, environmental monitoring and the food industry, by
helping to reduce food lost during the harvest and post-
harvest of fresh products and maintaining the sensory char-
acteristics of processed products.
The first work with chemical sensors based on micro-
cantilevers assessed physical and environmental param-
eters such as temperature, pressure and humidity. The
deflection of silicon nitride microcantilevers coated with
4-nm chromium and 40-nm gold, another sensor coated
with 5–13-nm aluminum and an uncoated surface exposed
to varying relative humidity was reported.6
The deflec-
tion was nearly linear and reversible in the gold-coated
microcantilevers, negligible in the uncoated sensors and
extremely sensitive in those coated with aluminum.
The microcantilever surfaces coated with different poly-
mers were used to analyze a mixture of volatile organic
6718 J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 9 1533-4880/2014/14/6718/005 doi:10.1166/jnn.2014.9348

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Steffens et al. Microcantilever Sensors Coated with a Sensitive Polyaniline Layer for Detecting Volatile Organic Compounds
compounds.1 7–11
Exposing the polymers to the gaseous
compounds caused the polymer matrix swell due to the
adsorption of the analyte, which altered the density and
the polymer mass. The minimum frequency resolution and
the mass resolution achieved were limited by the quality
factor of the oscillator system.
The development of an electro-driven device via the
redox activation of polyaniline on one side of the micro-
cantilever surface was evaluated. The deflection response
was controlled by an electrostatic repulsive interaction that
applied a potential between −0.5 and 0.8 V.12
The oxi-
dation of the polymer on the microcantilever caused an
electrostatic repulsion, swelling the polymer chains and
thereby resulting in a deflection. In contrast, reducing the
polymer released the applied tension, allowing the micro-
cantilever to return to its original position.
The mechanical deflection of microcantilevers was eval-
uated using the electrical signal of polyaniline elec-
trodeposited on a gold-coated microcantilever surface.13
A redox reaction was used to stimulate the deflection of
the coated surface with the polyaniline film in the oxidized
state. The deflection occurred on the order of milliseconds
(2 4×10−4
) and was attributed to the swelling and shrink-
ing of the polymer.
To measure and detect VOCs (volatile organic com-
pounds), compounds that vaporize at ambient temperature
and pressure, with higher precision, microcantilever sen-
sors are important because they can detect acids, alcohols,
ketones, amines, and aromatics, among other compounds
with high sensitivity. The qualitative and quantitative
detection of VOCs are necessary for monitoring emission
and exposure limits11
from diverse sources of these vapors
including the environment (air pollution, greenhouse gas
emissions, and organic matter decomposition), medicine
(urine and odors of the breath and skin), the food industry
(coffee, fish, meat, flavor control during wine fermenta-
tion, bacteria identification, and fruit ripening).14
In this study, we evaluate the performance of coated and
uncoated microcantilever sensors utilizing the vapors of
various volatile organic compounds with different polari-
ties. The sensitive polyaniline layer in the doped state was
deposited onto the microcantilever surface, which had been
cleaned via plasma. The sensitivity, response time and the
detection limit of this sensor were evaluated.
2. EXPERIMENTAL DETAILS
Commercially available (NT-MDT), rectangular silicon
microcantilevers with the following dimensions were used
in this study: length = 350 m, width = 30 m and
thickness = 0 5–1 5 m. The microcantilever surfaces
were cleaned via plasma sputtering under high vacuum. An
argon gas pressure of less than 0.1 mbar and a background
pressure of 0.1 mbar were used. A 40-kHz radio frequency
at a power of 150 W at a temperature of 130 C was
applied during the treatment. The microcantilevers were
subsequently dried in an oven at 50 C for 10 hours and
were stored in a vacuum desiccator.
To render the cantilever sensitive to vapors, one side
was coated with a conducting polymer using PANI. The
polymer was obtained in the emeraldine base oxidation
state by an interfacial synthesis followed by a chemical
route to obtain the PANI nanofibers.15–17
The polymer was
then deposited on the microcantilever surface using a spin-
coating technique (dedoped). After spinning at 500 rpm
for 8 seconds, 3.0 L of the PANI solution was deposited
onto the microcantilever surface. The spinning rate was
then increased to 1000 rpm for 10 seconds and finally to
3000 rpm, which was maintained for 1 minute. The exper-
iments were performed at room temperature and ambient
humidity (25 ± 2 C). Afterwards, the coated microcan-
tilever sensors were dried in a vacuum desiccator for
12 hours at room temperature, and the sensitive PANI layer
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 micro-
cantilever. The voltages obtained in the photodiode were
converted to nanometers to yield the deflection. The deflec-
tions of the coated (doped and undoped sensitive PANI
layer) and uncoated microcantilevers were measured at
different volatile concentrations (in triplicate) at ambient
pressure and temperature in a closed chamber to evaluate
the sensitivity.
2.1. Evaluation of the Microcantilever Sensor
Response to Volatile Organic Compounds
The sensor response to various VOCs was investigated
using different VOC concentrations from 0 to 1000 ppmv
(0, 100, 250, 500, 750 and 1000 ppmv), and the sensor
exposure to the volatiles was ordered according to polar-
ity (polar to nonpolar) because the non-polar compounds
could degrade the sensitive layer and interfere with the
sensor performance. The volatile compounds used were
methanol, ethanol, acetone, propanol, dichloroethane, hex-
ane and toluene and were of analytical grade, purchased
from Sigma Aldrich, PA. All tested VOCs were liquids
and easily vaporized at ambient temperature and pressure.
The deflection of the coated (doped and undoped sensi-
tive PANI layer) and uncoated microcantilevers were mea-
sured in triplicate in a closed chamber to evaluate their
sensitivity. The experiments were performed at a constant
temperature of 20 ± 0 2 C, which was maintained using
an ultrathermostatic bath (Nova Ética, model 521/2D), and
the temperature was remained stable for over two hours
in the chamber. A baseline was initially obtained for three
minutes. Then, the analyte vapor was introduced into the
chamber with a syringe (Hamilton), and the sensor deflec-
tion was observed over three minutes. To remove this
volatile concentration, the chamber was purged with a
0.1 L/min flow of dry nitrogen gas (with analog flow mass
controllers). This experimental sequence was repeated for
J. Nanosci. Nanotechnol. 14, 6718–6722, 2014 6719

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Microcantilever Sensors Coated with a Sensitive Polyaniline Layer for Detecting Volatile Organic Compounds Steffens et al.
all evaluated concentrations. The relative humidity inside
the chamber was monitored using a commercial sensor
(Sensirium TM).
After each volatile measurement, the materials in con-
tact with the vapors (the chamber, syringe and tubes) were
cleaned and rinsed with acetone, isopropyl alcohol and
distilled water.
3. RESULTS AND DISCUSSION
Microcantilever sensors are applicable in many fields and
can be used for chemical and food quality control, air
quality monitoring and process control. Sensors with high
detection limits for solvent vapors are necessary for safe
process technologies during the storage and transport of
large solvent quantities; they are also useful in evaluating
food contamination and the ripening of fruits and veg-
etables. Thus, we investigated the use of microcantilever
sensors for detecting different VOCs (methanol, ethanol,
propanol, acetone, dichloromethane, toluene and hexane).
The behavior of these sensors was investigated by analyz-
ing these VOCs in decreasing order of their polarity, eluent
strength and dielectric constant.
The response of the microcantilever sensors to the VOCs
was measured at room temperature (20.00 C ± 0 02),
atmospheric pressure (0.908 atm) and 50% relative humid-
ity. The VOC concentration in the chamber was calculated
according to the following equation:
Yi ppm =
VVOC ∗ VOC
MW VOC
R∗T
P ∗V chamber
(1)
in which Vvoc is the VOC volume, is the density, MW is
the molecular weight, R is the gas constant, T is the tem-
perature, P is the pressure, and V is the chamber volume.
To verify the sensitivity of the sensor, the coated and
uncoated (reference) microcantilevers were inserted into a
gas line and exposed to various concentrations of volatile
compounds (0, 100, 250, 500, 750 and 1000 ppmv). All
VOC concentrations were measured in triplicate. The num-
ber of molecules inserted into the chamber at each con-
centration (ppmv) was the same for all VOCs.
Figure 1 shows that the polyaniline-coated microcan-
tilever sensors exhibited a deflection response to the
different VOC concentrations evaluated. The uncoated
microcantilevers (reference) exhibited no visible deflection
response to the VOCs.
The sensitivity of the sensors was obtained through the
angular coefficient of the concentration versus the deflec-
tion, and a good correlation coefficient (R2
) was obtained.
The deflection of the sensors to the VOCs could be cor-
related to the differences in the chemical structures of
volatile compounds and to the chain length.18
The coated microcantilever demonstrated the great-
est sensitivity to methanol, which can be related to the
small size of the volatile molecule that facilitates its
interaction with and diffusion into the polymer matrix.19
The higher polarity, dielectric constant and eluent strength
0 200 400 600 800 1000
0
1000
2000
3000
4000
5000
Deflection(nm)
Concentration (ppm)
Methanol
Ethanol
Propanol
Acetone
Dichloromethane
Toluene
Hexane
Reference
Figure 1. Responses of the microcantilever both uncoated and coated
with a sensitive layer of polyaniline to the VOC molecules.
of methanol enhance its interaction with the nitrogen
atoms in the sensitive layer of the polyaniline, causing a
further expansion or swelling of the polymer chains, which
can increase the sensor’s sensitivity.
The sensitivity of the coated microcantilever to the
VOCs was shown to increase with polarity, except for the
volatile acetone. The sensitivity to acetone was greater
than that of propanol, despite acetone’s lower polarity
because acetone has a higher vapor pressure.
The sensitivity can also be correlated to the dielectric
properties, which provide an indication of the polarity of
the volatile compound. Solvents with a dielectric constant
below 15 are considered non-polar. Volatile substances
with differing dielectric properties have distinct effects on
the conductivity of a polymer film because a change occurs
in the electronic transition between the polymer chains and
any defects present in the polymer.20
Figure 2 demonstrates the sensitivity differences in the
coated microcantilever sensors to the VOCs and confirms
that the highest sensitivity was achieved for methanol,
which could be due to the interaction between the polyani-
line and the analyte molecules. The cantilever response for
methanol, ethanol, 1-propanol and 1-butanol was evaluated
using a microfabricated cantilever array coated with a
Methanol lonahtE Acetone lonaporP
enahteorolhciD
Toluene enaxeH
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Sensitivity(nm/ppm)
Figure 2. Sensitivity of the coated microcantilever sensors to the VOCs.
6720 J. Nanosci. Nanotechnol. 14, 6718–6722, 2014

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Steffens et al. Microcantilever Sensors Coated with a Sensitive Polyaniline Layer for Detecting Volatile Organic Compounds
polymer layer (2–3 m thickness) and verified that the
largest and most rapid response in the static mode occurred
with methanol.9
Methanol is the smallest of the molecules
investigated, and this small size promotes a decreased dif-
fusion time.
The detection limit of the sensors is defined as the min-
imum detectable analyte concentration that can be distin-
guished with a certain level of confidence. In this work,
the detection limit was calculated using the equation rec-
ommended by the International Union of Pure and Applied
Chemistry (IUPAC).21
The coated microcantilever sensors
were found to have a detection limit range of 17–42 ppmv
for the various volatile compounds evaluated. Dong et al.1
estimated that gas sensors developed using microcan-
tilevers coated with different polymers (polyethylene oxide
(PEO), polyvinyl alcohol (PVA) and polyethylene vinyl
acetate (PEVA)) could detect a minimum concentration
of 10 ppm. Si et al.22
coated quartz crystal microbalance
(QCM) sensor surfaces with various thiophene conduct-
ing polymers and observed the following detection limits:
20.2 ppm for toluene, 39.1 ppm for ethanol and 77.6 ppm
for acetone. Therefore, by comparison, the microcantilever
sensors coated with polyaniline exhibited excellent sensi-
tivity and detection limits to the VOCs.
The experiments to evaluate the response time of the
coated microcantilever sensor obtained a baseline over
3 minutes. The VOCs (1000 ppmv) were inserted in
sequence, and the deflection was monitored until no
more variation was noted in their response. Between each
VOC, the measuring chamber was cleaned to analyze any
remaining volatile material.
Figure 3 shows that the response time was relatively
fast at less than 2.1 seconds. Many factors can affect the
response time of the sensors, such as the vapor concen-
tration, the polymer thickness and the crystallinity.23
In
general, large VOC molecules diffuse into the polymers
more slowly than small molecules. The molecular diffusion
0 1 2 3 4 5 6 7
5000
4000
3000
2000
1000
0
1000 ppm VOCs
Baseline
Deflection(nm)
Time (min)
Methanol
Ethanol
Acetone
Propanol
Dichloroethane
Hexane
Toluene
Figure 3. Response time of the polyaniline-coated microcantilever sen-
sors to the VOCs.
0 2 4 6 8 10 12 14 16
0
500
1000
1500
2000 1000 ppm750 ppm500 ppm250 ppm100 ppm
Deflection(nm)
Time (min)
Ethanol
Methanol
Propanol
Acetone
Dichloroethane
Hexane
Toluene
Baseline
VOCs
N2 gas
Figure 4. Response of the coated microcantilever sensors to varying
VOC concentrations.
constant can be reduced by several orders of magnitude
with a relatively small increase in the molecular size.24
The polyaniline-coated microcantilever sensor exhibited
a slight difference in its response times to the VOCs. The
volatile compounds with smaller chains (methanol, ethanol
and propanol) had faster response times (1.9 seconds) than
those with the largest chains (dichloromethane, toluene
and hexane), which had a response time of approximately
2.1 seconds.
The sensor responses to the different vapors were
assessed using the RH (%) and temperature constant after
obtaining a baseline. The deflection data were obtained
over 30 s of exposure to the VOCs, and all measurements
were performed in triplicate. Between each different con-
centration of the same volatile compound, the system was
purged with nitrogen gas to remove the volatile material
from the chamber.
An increase was observed in the response of the micro-
cantilever deflection with increasing volatile concentration
(Fig. 4). After each gaseous concentration was evaluated,
the deflection response returned to its baseline, demon-
strating the reversibility and reproducibility of the sensors.
The coated microcantilever sensors exhibited a compres-
sive deflection for all VOCs. The sensor response to the
volatile compounds was repetitive, rapid and reversible,
and good baseline stability could be obtained for each
volatile compound evaluated. Therefore, polyaniline was
shown to provide an efficient sensitive layer in the micro-
cantilever sensors for detecting VOCs. The polyaniline
structure allowed for the easy diffusion and desorption of
the gases, which resulted in a rapid response time.
4. CONCLUSION
The sensitive coating was able to adsorb the volatile
molecules, as observed by the deflection of the coated
microcantilever sensors. This study shows that the active
polyaniline layer deposited onto the microcantilever sen-
sor was suitable for VOC detection with a detection limit
of 17–42 ppmv.
J. Nanosci. Nanotechnol. 14, 6718–6722, 2014 6721

5. Delivered by Publishing Technology to: UNIVERSIDADE SAO PAULO IF
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Copyright: American Scientific Publishers
Microcantilever Sensors Coated with a Sensitive Polyaniline Layer for Detecting Volatile Organic Compounds Steffens et al.
The sensitivity of the coated microcantilever sensor to
the VOCs increased with increasing polarity, except with
acetone, and the sensors presented a higher sensitivity
to methanol. For all volatile compounds investigated, the
response time was less than 2.1 seconds.
Acknowledgments: The authors would like to thank
Embrapa Instrumentation, which is responsible for the
National Nanotechnology Laboratory for Agribusiness, for
the use of their facilities. They also wish to thank FAPESP
(2009/08244-0 and 2007/05089-9) and INCT-NAMITEC
(CNPq 573738/2008-4) for financial support.
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6722 J. Nanosci. Nanotechnol. 14, 6718–6722, 2014