Sensor for volatile organic compounds using an interdigitated gold electrode modified with a nanocomposite made from poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) and ultra-large graphene oxide
Amirhossein Hasani a †
, Hamed Sharifi Dehsari b †
, Jaber Nasrollah Gavgani b†
, Elham
Khodabakhshi Shalamzari b
, Alireza Salehi a
, Farmarz Afshar Taromi* b
, Mojtaba Mahyari c a) Department of Electrical Engineering, K.N. Toosi University of Technology,
P.O.Box 16315-1355, Tehran, Iran b) Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, P.O.Box 15875-4413, Tehran, Iran c) Department of Chemistry, Shahid Beheshti University, G. C., P.O. Box 19396-
4716, Tehran, Iran
*Corresponding author. Tel.: +98 2164542401, Email address: afshar@aut.ac.ir
Conductivity
Fig. S1 The conductivity change tendency of PEDOT-PSS/UL-GO nanocomposite gas sensors with various UL-GO concentration.
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It can be observed that PEDOT-PSS/UL-GO nanocomposite gas sensors has much lower initial resistance than PEDOT-PSS reference gas sensor (Fig. S1). From the results, the conductivity changing behaviors of PEDOT-PSS/UL-GO nanocomposite sensing films on interdigitated gold electrode is considerably increased and the highest value is obtained with 0.04 wt% UL-GO. The conductivity of PEDOT-PSS is increased by more than a factor of 1.5 after the addition of 0.04 wt% UL-GO.
Response and recovery times
Table S1 . The response and recovery times of the PEDOT-PSS and PEDOT-PSS/UL-GO nanocomposite gas sensors with various UL-GO contents.
Concentration of methanol
(ppm)
35
70
150
250 t
PEDOT-PSS response
(s)
32
30
28
28 t recovery
(s)
86
88
124
136
PEDOT-PSS / UL-GO
0.02 wt% t response
(s) t recovery
(s)
4.8
4.8
4.6
4.6
20
22
24
28
PEDOT-PSS / UL-GO
0.04 wt% t response
(s) t recovery
(s)
3.2
3.2
3
3
16
16.5
18
18.5
PEDOT-PSS / UL GO / UL-GO
0.06 wt% 0.08 wt% t response
(s) t recovery
(s) t response
(s) t recovery
(s)
5.6
5.2
5
5
24
25
28
32
28
28
25
22
76
82
94
110
The response time, defined as the time to reach 95% of the maximum total resistance change, and recovery time of the PEDOT-PSS and PEDOT-PSS/UL-GO nanocomposite gas sensors are presented in Table S2. By adding UL-GO into PEDO-PSS, the response and recovery times of the formed PEDOT-PSS/UL-GO nanocomposite gas sensors is remarkably decreased, and lowest values around 3.2 s and 16 s, respectively is obtained with 0.04 wt% UL-GO. Althou gh, the response and recovery times increase when the amount of UL-GO is further increased.
Moreover, the resistance of PEDOT-PSS/UL-GO nanocomposite gas sensors could recover nearly to the initial value within the pure nitrogen exposure times of 40 s, while the PEDOT-PSS reference gas sensor shows undesirable resistance drift. Thus, PEDOT-PSS/UL-GO nanocomposite gas sensors exhibit relatively short response and recovery times compared with
PEDOT-PSS one. The PEDOT-PSS reference gas sensor presented a maximum response time of
32 s and a maximum recovery time of 136 s. On the other hand, the sensivity, response and recovery times of the optimized PEDOT-PSS/UL-GO (0.04 wt%) nanocomposite gas sensor is
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11.3 %, 3.2 s, and 16 s, respectively which is 110, 32, and 6 times higher than that of pristine
PEDOT-PSS gas sensor even under low methanol vapor concentration of 35 ppm.
Conformations of PEDOT-PSS/UL-GO nanocomposite
The sensitivity enhancement of PEDOT-PSS/UL-GO nanocomposite sensing films can chiefly be attributed to the separation of PEDOT and PSS chains. Also, the conductive network was formed between the separated PEDOT chains and UL-GO sheet (Fig. S2). It is confirmed that addition of excess PSS into original PEDOT-PSS aqueous solution enable the stable dispersion of hydrophobic PEDOT chains in water. The PSS segments as well as attached PEDOT chains have a coiled conformation as blobs to prevent the contact of PEDOT from water. Meanwhile, the PSS segments serve as the strings between blobs due to the coulombic repulsions among the
PSS (Fig. S2a) [1,2]. Furthermore, addition of DMSO into the pristine PEDOT-PSS solution
cause to the separation of PEDOT and PSS chains. It is confirmed that the PEDOT and PSS chains are surrounded mainly by DMSO and water, respectively. Therefore the coulombic attraction between PEDOT and PSS chains is effectively weakened, so that both PEDOT and
PSS chains change to the linear and extended-coil conformation (Fig. S2b). The stronger interchain interactions of PEDOT chains and the limited overlap between PEDOT chains due to the coulombic repulsions among PEDOT chains, affects the formation of more conductive pathways.
The small functional groups (such as – COOH and – OH) on UL-GO sheets can effectively separate PEDOT and PSS chains and can further decrease the coulombic attraction between
PEDOT and PSS chains and enhance the formation of the linear and extended-coil conformation
of PEDOT and PSS chains [1-3]. The negatively charged PSS chains dissolve in water due to the
negatively charged carboxyl groups on UL-GO sheets, while positively charged insoluble
PEDOT chains will bond with UL-GO sheets through coulombic attraction. Moreover, as shown in Fig. S2c, the PEDOT chains chiefly change to the linear conformation due to the coulombic repulsions among themselves. Therefore the conductivity of PEDOT-PSS/UL-GO nanocomposite sensing film with a small amount of UL-GO is increased due to the bonding of
UL-GO sheets and linear PEDOT chains. The PEDOT-rich grains show a long stretched network at the amount of 0.04 wt% UL-GO. However, more functional groups on UL-GO sheets can bond with PEDOT chains by coulombic attraction when the amount of UL-GO increased up to
0.04 wt%. Therefore, a less amount of PEDOT chains which distributed on each UL-GO sheets
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due to decrease of the coulombic repulsions among the positive charges in the PEDOT chains, and UL-GO segments together have a coiled conformation as blobs (Fig. S2d). These PEDOT blobs and more isolated functional groups on UL-GO sheets will reduce the conductive network and lower charge transport which decrease the sensivity performance of PEDOT-PSS/UL-GO nanocomposite gas sensors towards VOCs vapors.
Fig. S2 (a) Conformations of pristine PEDOT-PSS (a) before and after mixing with (b) DMSO and (c) 0.04 wt%, and (d) 0.06 wt% GO.
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UV–visible spectrometry
The UV–visible spectrometry was used to verify the π–π* transitions in UL-GO and PEDOT-
PSS as shown in Fig. S3. A broad absorption band in the range of 225-240 (centered at ~ 233
nm) of UL-GO is attributed to the π–π* transitions of aromatic C-C bonds [4]. The shoulder at
around 300 nm can be ascribed to the n-π* transition of carbonyl groups. In case of PEDOT-PSS, the UV–visible absorption spectrum exhibits one peak at 225 nm corresponding to typical
absorption band of the aromatic rings in PSS [5]. In case of optimized PEDOT-PSS/UL-GO
nanocomposite, the absorption peaks of PSS aromatic rings are red-shifted (243nm) and new band (centered at ~ 296 nm is observed. These results suggest the blending of UL-GO in the
PEDOT-PSS composite and π–π electron donor–accepter interaction between UL-GO and
PEDOT-PSS [6].
Fig. S3 UV–visible spectra of UL-GO, pristine PEDOT-PSS and optimized PEDOT-PSS/UL-
GO (0.04 wt%) nanocomposite.
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Sensivity vs Temperature
It can be seen that the optimized PEDOT-PSS/UL-GO nanocomposite gas sensor shows low sensitivity to methanol with increase of temperature (Fig. S4). Therefore, the optimal temperature of the optimized PEDOT-PSS/UL-GO nanocomposite gas sensor for VOC detection was found to be room temperature. Since the interaction between optimized PEDOT-PSS/UL-
GO nanocomposite sensing film and VOC vapors is exothermic, the activation energy of desorption is larger than that of the adsorption of methanol molecules of nanocomposite sensing film. Therefore, the low response to methanol at high temperatures demonstrated that optimized
PEDOT-PSS/UL-GO nanocomposite gas sensor exhibits higher desorption rate to methanol at concentration 35 ppm.
Fig. S4 Sensivity of the optimized PEDOT-PSS/UL-GO nanocomposite gas sensor to 35 ppm methanol vapor as function of temperature
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Sensivity
Fig. S5a shows the dependence of the optimized PEDOT-PSS/UL-GO nanocomposite gas sensor sensitivity on concentration for methanol, ethanol, and acetone at room temperature. The agreeable linear sensor sensivity to methanol, ethanol, and acetone vapor based on Langmuirhenry-clustering (LHC) electro-sorption model which depends on the vapor chemical nature was
35–1000 ppm. At 1000 ppm concentration, the gas response of optimized PEDOT-PSS/UL-GO nanocomposite gas sensor to methanol, ethanol, and acetone are 189 %, 85 %, and 3 %, respectively. At low concentration (5–50 ppm), the gas response of optimized PEDOT-PSS/UL-
GO nanocomposite gas sensor to methanol is still higher than to ethanol, and acetone. However, the sensivity of optimized PEDOT-PSS/UL-GO nanocomposite gas sensor to ethanol, and acetone are not significant different in the all concentration range. The limit of detection was approximately less than 35 ppm for the optimized PEDOT-PSS/UL-GO nanocomposite gas sensor towards methanol, ethanol, and acetone respectively at the room temperature.
Fig. S5b shows the dynamic responses of the optimized PEDOT-PSS/UL-GO nanocomposite gas sensor towards various concentration of methanol, ethanol, and acetone at room temperature. It indicates that the nanocomposite gas sensor exhibits good repeatability and reversibility of response regardless of vapor structure at room temperature. It can be seen that the optimized
PEDOT-PSS/UL-GO nanocomposite gas sensor shows relatively high response to methanol compared with ethanol, and acetone at room temperature. The resistances of optimized PEDOT-
PSS/UL-GO nanocomposite gas sensor increase upon exposure to VOCs vapor and recover to the initial value upon the removal of VOCs in nitrogen. The response time of the optimized
PEDOT-PSS/UL-GO nanocomposite gas sensor towards 35 ppm methanol, ethanol, and acetone are estimated to be ~ 3.2 s, while the recovery time of the optimized PEDOT:PSS/GO nanocomposite gas sensor towards as mentioned VOCs vapors are ~ 16 s, 13.8 s, and 10.2 s, respectively (Fig. S5c,d). The slower response and recovery times of optimized PEDOT-
PSS/UL-GO nanocomposite gas sensor may be due to low diffusion and short penetration depth of gas molecules on very smooth surface of the optimized PEDOT-PSS/UL-GO nanocomposite sensing film. Therefore, this result may come from the diffusion coefficient (D) of the VOCs vapors, where the ranking of the D for different VOCs vapors is as following: D
methanol
> D
ethanol
> D
acetone
. A higher diffusion coefficient means that relatively large amount of vapors can more easily penetrate into the nanocomposite sensing film and allows them to be adsorbed on the
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surface of nanocomposite sensing film. Therefore, the VOCs vapors showing a better sensitivity, also take more time to be removed out from the surface of nanocomposite sensing film.
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16 s
10.2 s
13.8 s
Fig. S5 (a) Sensivity, (b) dynamic responses of the optimized PEDOT-PPS/UL-GO (0.04 wt%) nanocomposite gas sensor to various concentration of methanol, ethanol, and acetone vapors at room temperature, (c) response time, and (d) recovery time of the optimized PEDOT-PPS/UL-
GO nanocomposite gas sensor towards 35 ppm of methanol, ethanol, and acetone vapors at room temperature.
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Schematic of PEDOT-PSS/UL-GO nanocomposite gas sensor
Fig. S6 (a) VOCs vapors, (b) schematic of PEDOT-PSS/UL-GO nanocomposite gas sensor (c) optimized PEDOT-PSS/UL-GO nanocomposite sensing film, (d) schematic of VOCs vapors sensing at optimized PEDOT-PSS/UL-GO nanocomposite sensing film.
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