Universidade de São Paulo
Biblioteca Digital da Produção Intelectual - BDPI
Departamento de Física e Ciências Materiais - IFSC/FCM
Artigos e Materiais de Revistas Científicas - IFSC/FCM
2010-05
Detection of volatile organic compounds using
a polythiophene derivative
Physica Status Solidi A,Weinheim : Wiley-VCH Verlag,v. 207, n. 7, p. 1756-1759, July 2010
http://www.producao.usp.br/handle/BDPI/49252
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Phys. Status Solidi A 207, No. 7, 1756–1759 (2010) / DOI 10.1002/pssa.200983723
a
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applications and materials science
Detection of volatile organic
compounds using a polythiophene derivative
1
2
1
V. C. Gonçalves , B. M. Nunes , D. T. Balogh , and C. A. Olivati*
,3
1
Instituto de Fı́sica de São Carlos, IFSC/USP, CP 369, CEP 13566-590, São Carlos/SP, Brazil
Depto. de Fı́sica, IGCE/UNESP, CP 178, CEP 13500-970, Rio Claro/SP, Brazil
3
Depto de Fı́sica, Quı́mica e Biologia, FCT/UNESP, CP 467, CEP: 19060-900, Presidente Prudente/SP, Brazil
2
Received 4 June 2009, revised 15 March 2010, accepted 15 April 2010
Published online 28 May 2010
Keywords conjugated polymers, electrical properties, sensors
* Corresponding
author: e-mail [email protected], Phone: þ55 18 32295355, Fax: þ 55 18 32295355(30)
Conjugated polymers have been subject of great interest in
the recent literature from both fundamental point of view
and applied science perspective. Among the several types
of conjugated polymers used in recent investigations, polythiophene and its derivatives have attracted considerable
attention over the past 20 years due to their high mobility
and other remarkable solid-state properties. They have
potential applications in many fields, such as microelectronic
devices, catalysts, organic field-effect transistors, chemical
sensors, and biosensors. They have been studied as gas and
volatile organic compounds (VOCs) sensors using different
principles or transduction techniques, such as optical
absorption, conductivity, and capacitance measurements.
In this work, we report on the fabrication of gas sensors
based on a conducting polymer on an interdigitated gold
electrode. We use as active layer of the sensor a polythiophene
derivative: poly (3-hexylthiophene) (P3HT) and analyzed its
conductivity as response for exposure to dynamic flow of
saturated vapors of six VOCs [n-hexane, toluene, chloroform,
dichloromethane, methanol, and tetrahydrofuran (THF)].
Different responses were obtained upon exposure to all VOCs,
THF gave the higher response while methanol the lower
response. The influence of moisture on the measurements
was also evaluated.
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction The growth of population and industrial development gave rise to unprecedented air pollution that
can cause harm to humans and the environment. Efforts to
reduce this pollution include recognition of the problem,
collection of information, definition of sources and causes, and
the selection and implementation of the appropriate solutions
[1]. In order to control the emission levels of pollutants,
portable devices using arrays of gas sensors have been
developed over recent years [2–4]. These so-called electronic
noses (e-nose) are mainly based on selective layers made from
metal oxide semiconductors [3, 5–7]. However they have been
shown some drawbacks, which include their poor selectivity
and sensitivity, high operation temperatures, and instability
because of response to humidity [8].
Conjugated polymers have been proposed as alternative
materials to improve these properties. The easy synthesis, the
diversity, and the sensitivity at room temperature are the
main advantages of these polymers over inorganic materials
[7–10]. Among the several types of conjugated polymers
used in recent investigations, polythiophene and its
derivatives have attracted considerable attention due to their
good environmental and thermal stability, their high
mobility, and other remarkable solid-state properties [11].
Polythiophenes can be obtained from chemical or
electrochemical polymerizations and have been used as
sensing materials for different gases and vapors like NOx [12,
13] and volatile organic compounds (VOCs) [14–16]. VOCs
include a variety of chemicals that significantly evaporate
under normal conditions [1]. They have been contributed to
air pollution and they are suspected to cause cancer in
humans. Therefore, the control of VOCs emission levels is
currently of wide interest.
The interaction of these analytes with polythiophene
films can cause changes in their color, mass, work function,
and electrical conductivity [7, 12–16]. Of particular interest
is the study of the last property that is relatively easily and
inexpensively to be used in sensors.
In this paper, we report on the use of P3HT for VOCs
electrical detection. The sensing measurements were undertaken on an interdigitated electrode by measuring the current
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Original
Paper
Phys. Status Solidi A 207, No. 7 (2010)
3 Results and discussion Figure 2 show the UV–Vis
spectrum for a P3HT spin-coated film on glass, featuring
lmax at 490 nm, which are related to transitions between
delocalized p–p states. The region below 350 nm was not
recorded since the glass substrate absorbs strongly at this
region, therefore only one peak could be observed in the UV–
Vis spectrum. The PL spectrum of P3HT film displays a zerophonon peak (I0) at 640 nm and no vibronic peaks were
observed probably due to the high optical quality of the film.
These values are close to those reported in the literature for
regiorandom P3HT films [17].
Absorbance
Emission intensity
2 Experimental We use as active layer of the sensor a
polythiophene derivative: poly (3-hexylthiophene) (P3HT),
Fig. 1(A). This material was synthesized via oxidative
polymerization using ferric trichloride. The monomer was
dissolved in nitromethane and solid ferric chloride was
added quickly to the mixture by a powder addition, funnel
under nitrogen atmosphere and magnetic stirring. In the next
step, chloroform was added to the mixture which was
magnetic stirred for 4 h. The polymer was isolated by
precipitation into methanol. The solid was dissolved in
chloroform and insoluble products were removed by
filtration. The soluble fraction was treated with aqueous
ammonia solution and EDTA for dedoping and the dedoped
polymer was precipitated into methanol. The numberaverage molecular weight (Mn) value of dedoped polymer,
estimated by HPSEC in tetrahydrofuran (THF), were around
10.000 g/mol.
P3HT (15 mg mL1 in chloroform) were spin-coated
(1000 rpm, 60 s, 25 8C) onto a glass substrate and a glass
substrate coated with the interdigitated chromium–gold
array. Patterned Au electrodes were prepared by photolithography, where 25 pairs of lines act as electrode contacts. The
length of the electrodes, their thickness, and the distance
between them are 800 mm, 100 nm, and 100 mm, respectively [Fig. 1(B)]. Spin-coated films onto interdigitated array
were about 100 nm thick.
UV–Vis absorption measurements of the film were
carried out in a HITACHI U-2001 spectrophotometer in the
range between 350 and 800 nm. The emission spectra of this
film were recorded in a SHIMADZU 5301RFPC spectrofluorometer. The sample was optically excited at 490 nm.
The response characteristics of the micro gas sensor
films were then investigated against various test gases. The
response behavior of P3HT sensors were characterized by
measuring the current versus time with a fixed applied
voltage (5 V) at room temperature with a Keithley 238 high
voltage source-measure unit, in dark. The sensors were
tested in a homemade test chamber in the presence of volatile
gases such as n-hexane, toluene, chloroform, dichloromethane, methanol, and THF in dynamic flow, dragged by
nitrogen. Prior to data acquisition the sensing units were left
under pure N2 flow for 60 min in the chamber to reach a
stable reading.
350400
500
600
700
800
900
λ (nm)
Figure 2 Absorption (&) and photoluminescence () spectrum for
a P3HT spin-coated film deposited onto a glass substrate.
50
[(It - I600) / I600 ]*100 (%)
versus time with applied voltage at room temperature. The
effect of different VOCs on the electrical properties of
P3HTs is investigated and their potential application as
electrical sensor is discussed.
1757
N2
VOCs on
25
VOCs off
N2
0
-25
methanol
dichloromethane
hexane
toluene
chloroform
tetrahydrofuran
water
-50
-75
-100
0
200
400
600
800 1000 1200 1400 1600 1800
Time (s)
Figure 1 (A) Chemical structure of poly(3-hexylthiophene) and
(B) interdigitated chromium gold array (a ¼ 100 nm, b ¼ 800 mm,
and c ¼ 100 mm).
www.pss-a.com
Figure 3 Normalized current responses for the different analytes
tested as function of time. VOCs analytes were introduced in the
chamber for 10 min.
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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V. C. Gonçalves et al.: Detection of volatile organic compounds
Figure 3 shows the normalized current changes for P3HT
sensors upon exposure to different VOCs. Normalization has
been used to analyze and to compare polymers sensors data
with different initial currents [18]. In this case, the normalizations were performed using each sensor current immediately before the VOC exposition (t 600 s). The system was
designed to be always vapor saturated during the exposures.
Considering the effect of moisture on the sensitivity, a test
was carried out using water vapor injection and the results
show that the moisture did not influence the conductivity
of P3HT film.
For others vapors tested it was observed a negative
response, indicating that the conductivity of the
material decreases in the presence of the VOCs, since
the conductivity is directly proportional to current
value. The sensors demonstrated a fast response to all
vapors. The methanol vapor showed the minimum
response and THF vapor the maximum one. For n-hexane
and toluene, which are non-polar and weak solvents
for P3HT, results show intermediary responses (between
methanol and THF).
The sensors current values did not return to the
initial ones after the chamber was flushed for 10 min
with nitrogen, showing that these changes are partially
reversible with different degrees of hysteresis. Most of these
hysteresis can be attributed to irreversible attachment of
VOCs at P3HT and/or irreversible effects of VOCs on the
conduction mechanisms of these films. The nitrogen flow can
also promoted small decreases of P3HT current with the time
(ca. 4% for the analysis period) that can also contribute to
these hysteresis.
To better understand the response effects, conduction
mechanisms of the polymers must be conceived. In the
room temperature regime, polaron hopping conduction is
the most accepted model of transportation mechanism
inside the conjugated polymer [14, 15]. The interaction
between the organic material and gas molecules results
in an increase or decrease of polaron densities on the
band gap of the polymers. Furthermore, exposure of
the conducting polymers to chemical solvent vapors can
cause polymer swelling, resulting a decrease in the
electrical conductivity.
The P3HT responses for some VOCs are the result of
weak physical interactions between analytes and polymers
causing swelling of the polymer films [19]. Swelling of the
polymer matrix due to absorption of organic vapors may
also increase the distance between polymer chains, thereby
decreasing the hopping conduction. This effect is the most
probable mechanism that can explain the electrical behavior
of the sensor.
This swelling effect depends on the interactions
between polymer and vapor and the polymer solubility.
This process can influence the aggregation of the
polymer chains, changing their conformation and sometimes cannot be completely reversed only by passing a
nitrogen flow. The highest response for THF is probably
caused by the swelling effect, since THF is a polar
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
analyte and a strong solvent for P3HT. The analyte
molecules can penetrate into the polymer film and
enlarge the spacing between the polymeric chains,
resulting in a lower conductivity. The poor sensitivity to
methanol, which is not a solvent for P3HT, is probably
caused by the absence of the swelling effect. Similar
effects were observed for optical VOCs detection using
P3HT as active layer. The complete set illustrates the
variety of responses and it was not possible to directly
correlate the responses intensities with any properties
of these analytes, such as dielectric constants or dipole
moments for example [20].
4 Conclusions In summary, we have demonstrated
that a polythiophene derivative, P3HT, is a promising
material for use in VOC sensors. Sensing mechanisms
involving various physical interactions between the polymers and analytes, and further experiments are necessary to
better identify and understand those mechanisms. The
simple preparation approach, which can offer excellent
properties as well as the low-cost interdigitated electrodes,
suggests that this polythiophene derivative film-based
device could be used in the gas sensors field, such as in
electronic noses.
Acknowledgements The authors acknowledge the financial
assistance from FAPESP and CNPq/INEO (Brazil), and are grateful
to LNLS (Brazil) for providing interdigitated electrodes (project
LMF 7832).
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