Fabrication and characterization of chemical sensors made from nanostructured films of poea prepared with different doping acids

Fabrication and characterization of chemical sensors made from
nanostructured films of poly(o-ethoxyaniline) prepared
with different doping acids
E.D. Brugnollo a,b
, L.G. Paterno c,⁎, F.L. Leite a,b
, F.J. Fonseca c
,
C.J.L. Constantino d
, P.A. Antunes d
, L.H.C. Mattoso a
a
EMBRAPA Instrumentação Agropecuária, CP 741, CEP 13560-970, São Carlos, SP, Brazil
b
Instituto de Física de São Carlos, USP, CP 369, CEP 13560-970, São Carlos, SP, Brazil
c
Departamento de Engenharia de Sistemas Eletrônicos, EPUSP, CEP 05508-900, São Paulo, SP, Brazil
d
Departamento de Física, Química e Biologia, FCT-UNESP, CEP 19060-900, Presidente Prudente, SP, Brazil
Received 8 February 2007; received in revised form 20 July 2007; accepted 10 August 2007
Available online 29 August 2007
Abstract
Chemical sensors made from nanostructured films of poly(o-ethoxyaniline) POEA and poly(sodium 4-styrene sulfonate) PSS are produced and
used to detect and distinguish 4 chemicals in solution at 20 mM, including sucrose, NaCl, HCl, and caffeine. These substances are used in order to
mimic the 4 basic tastes recognized by humans, namely sweet, salty, sour, and bitter, respectively. The sensors are produced by the deposition of
POEA/PSS films at the top of interdigitated microelectrodes via the layer-by-layer technique, using POEA solutions containing different dopant
acids. Besides the different characteristics of the POEA/PSS films investigated by UV–Vis and Raman spectroscopies, and by atomic force
microscopy, it is observed that their electrical response to the different chemicals in liquid media is very fast, in the order of seconds, systematical,
reproducible, and extremely dependent on the type of acid used for film fabrication. The responses of the as-prepared sensors are reproducible and
repetitive after many cycles of operation. Furthermore, the use of an “electronic tongue” composed by an array of these sensors and principal
component analysis as pattern recognition tool allows one to reasonably distinguish test solutions according to their chemical composition.
© 2007 Published by Elsevier B.V.
Keywords: Chemical sensors; Electronic tongue; Poly(o-ethoxyaniline); Nanostructured films; Atomic force microscopy
1. Introduction
Chemical sensors using thin films of conjugated polymers as
sensing element have been rapidly developed in the last decades
[1,2]. This type of sensor is basically comprised by a suitable
electrode coated with the polymeric film, whose electrical
responses change accordingly to the physicochemical char-
acteristics of the environment surrounding them, either gas or
liquid phase. Such characteristics may be related to the presence
of a specific chemical compound (analyte), pH, ionic strength,
dielectric constant, and so on. Additionally, conducting
polymers are relatively cheap, and can be processed into thin
films. These advantages are highly desirable when considering
the development of low cost technologies.
Because conjugated polymers are sensitive to several classes
of compounds, chemical sensors made with these materials are
usually non-selective and non-specific [1,2]. For that reason
they have found great utility in multi-sensor systems, including
especially “electronic nose” [3] and “electronic tongue” [4].
These sensor systems use the non-specificity of several different
sensors as a way to produce unique response patterns (finger-
prints) for the samples they are designed to analyze. These
patterns are later recognized by statistical or computational tools
which allow the identification and classification of the sample
according to the information provided during sensor training.
Available online at www.sciencedirect.com
Thin Solid Films 516 (2008) 3274–3281
www.elsevier.com/locate/tsf
⁎ Corresponding author. Departamento de Engenharia de Sistemas Eletrôni-
cos, EPUSP, CEP 05508-900, São Paulo, SP, Brazil. Tel.: +55 11 3091
5256r215; fax: +55 11 3091 5585.
E-mail address: paterno@lme.usp.br (L.G. Paterno).
0040-6090/$ - see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.tsf.2007.08.118

Therefore, for such a kind of application, the sensors must be
stable and chemically different since specificity and selectivity
are not required.
Although stability may be critical, conducting polymers of
different chemical structures (and properties) can be easier
obtained when compared to other sensoactive materials [2]. A
simple strategy for that is doping the polymer with different
acids. Polyaniline and its derivatives undergo doping by a
typical acid-base reaction, in which the H+
from the dopant acid
protonates the nitrogen atoms of the polymeric chains [5,6]. At
the same time, the counterions are associated to the protonated
chain in order to maintain its electroneutrality. It is observed that
the nature of the associated counterions plays significant roles
on the morphology and electrical properties of the doped
polymers [5–8].
In the present contribution, we report on the fabrication of
chemical sensors based on nanostructured films of a conjugated
polymer, poly(o-ethoxyaniline) POEA, and study on their
electrical performance in an “electronic tongue” system. The
sensors are comprised by POEA films deposited through the
layer-by-layer technique (LbL) at the top of interdigitated
microelectrodes. Doping of POEA solutions with different
protonic acids allows the fabrication of sensors with distinct
surface morphologies, and distinct electrical responses. The
chemical structure and morphology of the POEA films are
investigated by UV–Vis and Raman spectroscopy, and by
atomic force microscopy (AFM). The performance of the
chemical sensors made with POEA films is studied by electrical
capacitance measurements. Sucrose, sodium chloride, caffeine,
and hydrochloric acid diluted solutions are used as test samples.
2. Experimental details
Poly(o-ethoxyaniline), POEA (Mw 14,000), was chemically
synthesized according to method described elsewhere [9]. Poly
(sodium 4-styrenesulfonate) PSS (Mw 70,000) and the doping
acids hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid
(HNO3), p-toluenesulfonic acid monohydrate (TSA), and 10-
camphorsulphonic acid (CSA), were all purchased from Sigma-
Aldrich, and used as received. For sensor analysis, we prepared
test solutions using sodium chloride (Sigma-Aldrich), hydro-
chloric acid (Sigma-Aldrich), sucrose (Mallinkrodt), and
caffeine (Malinkrodt). These substances were used in order to
mimic the 4 basic tastes recognized by humans. All water used
in this work was purified by a Milli-Q system (Millipore Co.),
resistance 18 MΩ cm−1
.
All films were deposited onto optical glass slides
(1×10×30 mm) previously cleaned using a piranha solution
followed by RCA solution (NH4OH/H2O2/H2O 1:1:5, v/v). For
Fig. 1. UV–Vis spectra of films containing 10 POEA/PSS bilayers fabricated
with different dopant acids, as indicated. Solution conditions: Polymeric
concentration 0.1 g L− 1
; pH 3.
Fig. 2. Resonance Raman spectra (λo =785 nm) of an undoped POEA film (10 layers) and POEA/PSS films (10 bilayers) fabricated with different acids as indicated.
3275E.D. Brugnollo et al. / Thin Solid Films 516 (2008) 3274–3281

the electrical measurements, films were deposited onto glass
slides containing gold interdigitated microelectrodes. For film
deposition, solutions of each polymer (0.1 g L−1
, pH 3) were
prepared in ultra-pure water, except that POEA was initially
dissolved in dimethyl acetamide and diluted in water after-
wards. Both solutions were filtered prior to use. The pH of
solutions was adjusted by adding the acids abovementioned or
NH4OH. Film depositions were carried out at room temperature
(25 °C) by using the layer-by-layer technique [10], briefly
described as follows: the glass substrate was immersed in the
doped POEA solution for 3 min followed by rinsing in an
aqueous solution of same pH and same acid during 30 s, and
then dried with a gentle flow of high purity gaseous N2. A
subsequent PSS layer was adsorbed in the same way, followed
by rinsing and drying. These steps generate a film containing 1
bilayer of POEA/PSS, and films containing up to 10 bilayers
were fabricated by repeating these steps successively. The same
procedure was adopted for the deposition of films on the
interdigitated microelectrodes.
The chemical structure of POEA/PSS films was investigated
by UV–Vis (Hitachi U2001 model), and Raman spectroscopies
(micro-Raman spectrograph Renishaw, 785 nm laser line,
power at the sample was ca. 50 W). The morphological
characterization was conducted in an atomic force microscope
Fig. 3. AFM images of an undoped POEA film (5 layers) and POEA/PSS films
(5 bilayers). The images refer to a POEA layer on top. A) undoped POEA film,
B) POEA.HCl/PSS, C) POEA.CSA/PSS, D) POEA.TSA/PSS, E) POEA.HNO3/
PSS, F) POEA.H2SO4/PSS. Maximum Z scale: 10 nm.
Table 1
Assignment of key bands for films of undoped POEA and doped POEA/PSS
Vibration mode Wavenumber (cm− 1
)
Undoped film Doped film
C–N stretching 1250 1279
C–N+
stretching – 1227
C_N stretching 1481 1513
C–N•+
stretching – 1337
C–C stretching 1600 1625
C–H bending, benzenoid 1199 1199
C–H bending, quinoid 1132 1132
Table 2
Morphological parameters obtained from AFM images of POEA/PSS films
fabricated with different doping acids
Dopant acid Globule mean diameter (nm)a
Mean roughness (nm)b
Undoped 137 1.4
H2SO4 160 3.6
HNO3 149 2.0
HCl 124 1.0
CSA 120 1.4
TSA 108 1.3
a
Standard deviation∼15%.
b
Standard deviation∼7%.
Fig. 4. Electrical capacitance versus time of immersion in a NaCl 20 mM
solution, for sensors made with POEA/PSS films (10 bilayers) fabricated with
different dopant acids, at A) 100 Hz and B) 1 kHz.
3276 E.D. Brugnollo et al. / Thin Solid Films 516 (2008) 3274–3281

(Topometrix Discoverer TMX 2010), using silicon nitride tips
(V-shape) attached to a cantilever of spring constant of 0.09 N
m−1
. All images were obtained in the contact mode at a scan
rate of 2 Hz. The root-mean-square roughness (Rrms) was
calculated using the software provided by the instrument.
Values measured at 3 different regions on film samples were
used to calculate an average Rrms.
The electrical measurements were carried out with a LCR
meter HP 4263A from Hewlett-Packard, in a similar setup
previously used [4]. Each sensor comprising a POEA/PSS film
(10 bilayers) made with different protonic acids and deposited
on interdigitated microelectrodes were immersed in solutions of
NaCl, HCl, sucrose and caffeine at 20 mM and their electrical
capacitance was measured at 100 Hz and 1 kHz. Between each
solution, the sensors were soaked in ultra pure water under
stirring for 5 min for cleaning purposes. The electrical
capacitance of each sensor in ultra pure water was also
measured before and after tested solutions in order to verify
signal reset. In a first set of experiments, the variation of the
electrical capacitance of each sensor as a function of the time of
contact with the test solution was evaluated. This was made in
order to understand the time dependence of the electrical
response of the sensors, and to establish a minimum time for
initiating the measurements. Later, electrical capacitance
measurements for each solution were taken during 5 different
days. In order to avoid experiment artifacts and to confirm
system reproducibility, the sequence in which the solutions
were analyzed was changed every day. Principal component
analysis (PCA) was used as pattern recognition tool. PCA score
plots made by using a pre-designed algorithm and the
MATLAB (version 6.1) software were used in order to verify
the ability of the sensor array to identify and classify the tested
samples. In a second set of experiments, the performance of
sensors was evaluated by carrying out successive measurements
in pure water and NaCl 20 mM solution. Up to 20 cycles of
water/NaCl were performed.
3. Results and discussion
In Fig. 1, UV–Vis spectra of POEA/PSS, 10 bilayers,
fabricated with POEA solutions doped with different acids are
presented where it is clearly seen the presence of the polaronic
band, around 770 nm, typical of the doped polymer [5]. The
position of the polaronic band is slightly shifted to a higher
wavelength (800 nm) in the film made with TSA. Another
important issue is regarded to the dependence of the amount of
POEA adsorbed on the type of acid used for doping. Once the
absorbance is proportional to the concentration of absorbing
material, as stated by the Lambert–Beer equation, one can infer
that the amount of POEA in the films increases according to the
Fig. 5. Electrical fingerprints for different chemicals obtained with sensors made
from POEA/PSS films (10 bilayers) fabricated with different dopant acids, as
indicated. A) 100 Hz, B) 1 kHz.
Fig. 6. Electrical capacitance measured alternately in ultrapure water and NaCl
20 mM solution with sensor containing a POEA/PSS film (10 bilayers)
fabricated with H2SO4 at 100 Hz and 1 kHz. A) 20 cycles, B) 2 cycles.

type of acid in the following order: HClbCSAbTSAbHNO3 -
H2SO4. In a previous contribution [11], we have verified that
organic and bulkiest dopant acids led to a greater amount of
polymer adsorbed in comparison to films fabricated with HCl.
Due to the fact that the counterions from these acids are less
solvated by water, they are more strongly associated to the
protonated nitrogen atoms from POEA, leading to a more
effective charge screening. As a result, intra and intermolecular
electrostatic repulsions between doped POEA chains are
minimized, allowing the approximation of a greater amount of
polymeric segments and also inducing the polymer to assume a
more compact molecular conformation. Both effects contribute
to the adsorption of a greater amount of polymer.
A more detailed characterization of the chemical structure of
undoped POEA and doped POEA/PSS films was carried out
through Raman spectroscopy and the results are presented in
Fig. 2. As it can be observed, the most significant difference
between the spectra is the presence of the radical cation band
(C–N•+
stretching) in doped films, centered in 1337 cm−1
.
Such a band, as expected, does not appear for the undoped
POEA film. The radical cation is formed in the POEA solution
and it is stabilized by the association of acid counterions. As
mentioned before, the protonation of nitrogen atoms in the
polymer chain leads the formation of an electronic state located
in the middle of the gap, which in the case of aromatic
conducting polymers is called polaron [5,6]. In the films, the
radical cations are then stabilized by the formation of ionic
bonds with the sulfonic groups from PSS. As it will be
discussed later, the counterions from dopant acids associated
with the POEA in solution are not included in the film. We can
also identify the quinoid and benzene units, whose bands
intensity is significantly high because of excitation of the
samples at high wavelengths [12,13]. In comparison to different
polyaniline systems already investigated [12,13], we could
identify the most significant bands from the spectra of undoped
POEA and doped POEA/PSS films, as presented in Table 1.
For sensor applications, the surface characteristics of the
sensoactive layer including surface energy, chemical composi-
tion and surface morphology need to be known, once the
interactions between sensor and analyte are mostly occurring at
the surface of the sensoactive layer. In this regard, AFM is an
useful tool to analyze the morphology of nanostructured
materials, mainly because it possess high vertical resolution.
AFM images from undoped POEA and doped POEA/PSS
films, in which the topmost layer is the fifth POEA layer, are
presented in Fig. 3. As it can be noted, the different acids
provide different film morphologies. Although the globules
represent the simplest morphological structures in all films, their
diameters varies significantly according to each type of dopant
acid used, from 108 nm for TSA to 160 nm for H2SO4, as
presented in Table 2. The adsorption of polyanilines in layer-by-
layer films is believed to occur based on a two step mechanism
[14], involving a fast first-order kinetic process with a
characteristic time of few seconds and a slower process
represented by a Johnson–Mehl–Avrami function with a
characteristic time of hundreds of seconds. These two steps
represent the nucleation and growth mechanisms, which in the
case of the above materials usually leads to the formation of
globules [15–19]. Once the AFM images of POEA/PSS films
show the presence of globules, one may conclude that a similar
adsorption mechanism takes place in these systems. In addition,
POEA layers in films alternated with sulfonated lignin
presented similar globular morphology which corroborates
our observations [11,19].
Still on the data presented in Table 2, it can be noted that the
number and size of formed globules depend on the type of
dopant acid used for film fabrication. The globules size of films
fabricated with HNO3 and H2SO4 are comparatively larger than
those found in undoped POEA films. On the other hand, films
of POEA fabricated with HCl, TSA and CSA presented smaller
globules than those found in undoped film. Literature data have
demonstrated that the doping process induces globule swelling
in polyaniline films, due to counterion inclusion [20].
Nonetheless, in the LbL films the complexation process
between polycation and polyanion excludes the counterions
which become part of the supernatant, and therefore, they are
not expected to be found within film structure, except at the
topmost layer [21]. In this sense, the variances on globule size in
POEA films must be related to the influence of different dopant
Fig. 7. PCA plot of an “electronic tongue” composed by a sensor array of five
sensors for liquids of different tastes, sucrose, caffeine, NaCl and HCl. A)
100 Hz, B) 1 kHz.

acids on the polymer adsorption process and not to dopant
counterion inclusion. As a consequence of different globules
size the film surface roughness is also varied depending on the
dopant acid. Films obtained with HCl, TSA and CSA show a
flatter surface when compared to films fabricated with H2SO4
and HNO3. A plausible explanation for such an effect is that as
more POEA is adsorbed rougher is the resulting film. Different
studies [15,22] have shown that the surface roughness of POEA
and POMA LbL films increases with the amount of adsorbed
polymer until it reaches a maximum around 10–15 bilayers,
which corroborates our hypothesis.
After a detailed characterization of the structure and
morphology of POEA/PSS films, chemical sensors made with
such films were fabricated and their electrical properties were
investigated. In Fig. 4 is shown the variation of the electrical
capacitance of different sensors with the time of immersion in
a NaCl 20 mM solution. As it can be seen, once the sensors get
in contact with the test solution their electrical capacitance are
almost instantly changed, and reached a constant value in the
order of tens of seconds. This behavior was systematical and
reproducible for all tested solutions, and in both investigated
frequencies. For further tests, the sensors were left soaking in
the test solutions for 5 min prior to capacitance measurements.
The response of sensors fabricated with different dopant
acids to solutions of different tastes is illustrated in Fig. 5.
Capacitance values represent an average of 50 measurements
taken 5 min after the immersion of the sensors in the test
solution. In order to verify the reproducibility of responses,
measurements were taken during 5 consecutive days and the
solutions were tested in different sequences to avoid experi-
ments artifacts. Sensors signal is higher at 100 Hz than at 1 kHz,
and it is much higher to NaCl and HCl than that for sucrose and
caffeine, independently on the dopant acid and frequency.
However, the sensor signal to a certain substance depended on
the type of dopant acid used for sensor fabrication. POEA/PSS
sensors fabricated with TSA and CSA presented higher signal to
sucrose and caffeine when compared to other dopants. For NaCl
and HCl, sensors fabricated with TSA and H2SO4 present the
highest signals. Although the limiting concentration of analytes
was 20 mM, the sensors were able to detect them even at 10 μM,
which has been also achieved with other similar systems
[23,24].
It is important to note that sensors signal depended on the
frequency of the applied electrical potential. On the measurements
taken at 100 Hz, capacitance values were higher than those
measured at 1 kHz. Taylor and MacDonald have described in their
work [25] that the electrical characteristics of a metal electrode
coated with a dielectric thin film and immersed in an electrolyte
solution depends decisively on the frequency of the applied
electrical potential. Modeling the system by an equivalent circuit,
and taking into account the contribution of the different interfaces
as circuit elements, they have observed that the electrical
capacitance decreases as the frequency increases. At low
frequency, the electrical impedance receives its largest contribu-
tion from the electrical double layer which is formed at film–
liquid interface. For intermediary values, ranging between 100
and 104
Hz, the presence of the film is manifested, and finally at
higher frequency the impedance is dominated mainly by the
geometric capacitance of the electrodes. Interpreting our data
based on this previous discussion, we can realize that at 100 Hz the
contribution of an electrical double layer could be more significant
than at 1 kHz and, therefore, the capacitance should be higher at
this frequency. As the frequency is increased, the ions will move
according to the oscillating field in such a way that the period of
time they stay in contact with electrodes is short enough to prevent
double layer formation. However, this effect is occurring only for
ions, including NaCl and HCl, but it is not extended for sucrose
and caffeine,once they are non-electrolytes. This may also explain
the low signal of all sensors to these two substances.
Another important characteristic of these sensors is that their
signal increased with the amount of POEA adsorbed in the
films. POEA/PSS sensors fabricated with TSA and H2SO4
exhibited higher signal than other sensors regardless the
substance on study. From UV–Vis data we have found that
these sensors contain more POEA than others. In a particular
study, Stussi et al. [26] have found that the sensitivity of
evaporated polypyrrole films to toluene vapor decreased as the
film thickness increased. They have attributed this behavior to
the gas sorption into the film, which should be easier achieved
for thinner films instead of thicker ones. Nonetheless, sorption
phenomena must be completely different in liquid media,
considering that the interactions between analyte and sensing
layer must be stronger. Moreover, we have previously showed
that film roughness, which may be a representative of surface
area, increases with the amount of POEA adsorbed [24].
Therefore, one could also expect that sensors signal must
increase with the amount of POEA, since signal and sensitivity
usually increase with the surface area of the sensoactive layer.
However, no additional relationship between globules size and
sensor signal was found for the studied sensors.
The performance of these sensors in continued operation was
evaluated by measuring their electrical capacitance when they
were alternately immersed in ultra-pure water and NaCl 20mM
solution. Fig. 6A shows the response of sensor containing a
POEA/PSS film, 10 bilayers, fabricated with H2SO4 during 20
cycles of successive operation at 100 Hz and 1 kHz. An
amplification of the response of this sensor is presented in Fig. 6B.
Open dots and filled squares represent the 50 measurements
continuously taken with the sensor immersed in the test sample.
The reproducibility of the electrical response for ultrapure water
and NaCl 20 mM can be clearly seen, regardless the frequency.
The initial capacitance value for water is always recovered after
each measurement in NaCl, indicating great reversibility of the
sensor response. Besides, the steady capacitance value is rapidly
reached in both liquids ultrapure water and NaCl. Other sensors
were submitted to the same evaluation and exhibited the same
behavior, which could be reproduced several times. Currently, we
are carrying out experiments in order to verify sensor stability and
aging, after numerous cycles of operation.
Although specificity may be introduced into polymeric
materials allowing them to effectively carry out molecular
recognition processes, literature data have demonstrated that
non-modified conducting polymer films are usually non-
specific, and they may respond to different analytes whatever

it would be their chemical composition [1,2]. From previous
contributions [4,27,28] it was observed that a same polymer
film could respond to different analytes and beverages,
confirming the theory that these materials in their pristine
condition usually do not present a specific response to a
particular substance. Considering an array of sensors made with
different conducting polymer films, each of them will respond
to a same analyte with different sensitivity levels. The
individual responses of each sensor will form together a
fingerprint of the sample analyzed, which will contain the
information used to identify and distinguish one sample from
another. Sensor array technology has gained special interest in
the last years, since the low specificity presented by an
individual sensing unit may be overcome by collecting
responses from a group of sensors instead. Furthermore,
different methods of multivariate data analysis have been
applied to interpret the response of electronic tongues [29].
Using a sensor array composed of POEA/PSS sensors
fabricated with different acids and principal component analysis
(PCA) as a pattern recognition tool, we could distinguish test
solutions of different tastes, as displayed by the PCA plot
showed in Fig. 7. Each kind of taste is at a defined location in
the plot, confirming that this sensor array works according to
the global selectivity concept [4,27,28,30] mimicking the
working of the human tongue at a certain extent. In previous
contributions it has been obtained similar performances using
sensors composed of films from different materials [4,27,28].
Once the sensors fabricated in the present study have low
sensitivity for bitter and sweet substances, the discrimination
between these two groups is difficult to be achieved.
4. Conclusions
Chemical sensors made from nanostructured films of poly(o-
ethoxyaniline) POEA and poly(sodium 4-styrene sulfonate)
PSS were produced and used to detect and distinguish 4
different substances in solution at 20 mM, namely sucrose,
NaCl, HCl, and caffeine. These substances were used in order to
mimic the four basic tastes recognized by humans. The sensors
were produced by the deposition of POEA/PSS films at the top
of interdigitated microelectrodes via the layer-by-layer tech-
nique, using POEA solutions containing different dopant acids.
The amount of POEA adsorbed and the morphology of films
depended on the type of acid used for film fabrication. The
doping of POEA was satisfactorily achieved with all acids and
POEA was found to be doped in all films fabricated, as
demonstrated by UV–Vis and Raman spectroscopy data. AFM
images showed that all films exhibited a typical globular
structure, with globules sizes varying between 100 and 160 nm
in diameter, depending upon the dopant acid used for film
fabrication. Films containing greater amounts of POEA
exhibited greater surface roughness.
The electrical response of the as-prepared chemical sensors
was very fast, in the order of seconds, systematical and repro-
ducible. In general, sensors signal depended on the frequency of
electrical potential investigated and on the amount of POEA
adsorbed in the sensoactive layer. At 100 Hz the sensors signal
was higher than at 1 kHz, possibly due to a contribution from
the electrical double layer. The signal increased with the
amount of POEA in the sensors, indicating that POEA is the
main responsible for chemical detection. We have also found
that the film surface roughness, which depended on the type of
dopant acid, could also influence the sensors signal towards
analytes. The performance of these sensors could be evaluated,
showing that their electrical responses are reproducible and
repetitive after many cycles of operation. Furthermore, the use
of an “electronic tongue” composed by an array of sensors
made up of POEA/PSS films and principal component analysis
as pattern recognition tool allows one to reasonably distinguish
test solutions according to their chemical composition. The
easiness of film fabrication with controlled properties makes
these sensors very attractive when low cost technologies are
pursued.
Acknowledgements
The authors wish to thank the financial support provided by
Brazilian funding agencies CNPq, Rede Nanobiotec/MCT,
CAPES and FAPESP.
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