A selective DMSO gas sensor based on nanostructured conducting polypyrrole doped with sulfonate anion

Sensors and Actuators B 168 (2012) 303–309

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A selective DMSO gas sensor based on nanostructured conducting polypyrrole doped with sulfonate anion Sajad Pirsa, Naader Alizadeh ∗ Department of Chemistry, Factually of Science, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 16 November 2011 Received in revised form 29 March 2012 Accepted 7 April 2012 Available online 17 April 2012 Keywords: Conducting polymers Polypyrrole Nanostructure DMSO Gas sensor

a b s t r a c t A selective dimethyl sulfoxide (DMSO) gas sensor based on nanostructured conducting polypyrrole doped with sulfonate anion was constructed. Five sulfonate anions, including HSO3 − , para toluene sulfonate (PTS), dodecyl benzene sulfonate (DBS), dodecyl sulfonate (DS) and 5-sulfo salicylate (SS) were used to dope polypyrrole film. Sulfonated doped polypyrrole samples (PPy-S) have been prepared by polymerization of pyrrole on surfaces of the polyester fibers in the presence of an oxidizing agent. The presence of the dopants in the conducting polymer matrix was verified by FT-IR spectroscopy and the morphology of the resulting PPy-S was analyzed by scanning electron microscopy (SEM). The effects of the dopant type and concentrations on the conductivity and response patterns of the PPy-S as a gas sensor to the different gases were studied. The PPy-S gas sensors had demonstrated fast response time (<1 s). The responsivity of the PPy-S gas sensors for various volatile organic compounds (VOCs) was also reported. The PPy-HSO3 exhibit a lowest detection limit to the DMSO and could also be successfully applied as a highly selective sensor for detection of DMSO (DL = 30 ng). Linearity of calibration curve was observed at the range of 0.1–500 ␮g. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Conducting polymers, such as polypyrrole (PPy), polyaniline (PAni), polythiophene (PTh) and their derivatives, have been used as the active layers of gas sensors since early 1980s [1]. Previous reports revealed that conducting polymers, such as PAni, PPy and polyacetylene (PAc), have great advantages of higher sensitivity toward VOC’s gases, lower detectable limit and great potential to operate at room temperature [2]. Of conducting polymers, PPy stands out as an excellent one due to its high conductivity and good environmental stability and hitherto a large variety of application potential, such as sensors, actuators and electric devices [3–18]. Therefore, to obtain PPy with excellent chemical and physical characteristics becomes more and more attractive. For this purpose, polymerization of pyrrole in different surfactant systems has been developed quickly, because surfactants can induce the pyrrole to grow in certain manners and hence result in PPy with ordered morphology, which will show superior properties to that from conventional aqueous solution [19,20]. Among the conventional surfactants, cetyltrimethyl-ammonium bromide (CTAB) is proved to be an effective one to guide for the formation of PPy nanofibers

∗ Corresponding author. Fax: +98 21 82883455. E-mail address: [email protected] (N. Alizadeh). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.04.027

[21,22]. Zhang et al. further investigated the controllable synthesis of PPy nanostructures by using different kinds of surfactants, including CTAB, dodecyl tri methyl ammonium, octyltrimethyl ammonium, poly(ethylene glycol) mono-p-nonylphenyl ether and sodium dodecyl sulfate (SDS). They showed that using the former two and ammonium persulfate (APS) as oxidant could obtain PPy with ribbon wire-like structure, but the others could only generate sphere-like structure despite of the oxidant (FeCl3 ·6H2 O or APS), except that when SDS was used, PPy without geometrical nano feature would be formed [23]. Grady et al. have reported the formation of nanostructured PPy with controlled morphologies on atomically flat surfaces using adsorbed surfactant molecules as templates [24]. Shuangxi Xing and Guoku Zhao have also used sodium dodecyl benzene sulfonate (SDBS) comprehensively for the polymerization of pyrrole [25]. Dimethyl sulfoxide (DMSO) is known as an organic solvent, but it is also a natural product. DMSO has also been found in foods and beverages, such as wine, coffee, and tea, at the micromolar level [26,27]. Dimethyl sulfide (DMS), the reductive product of DMSO, is unfavorable for foods or beverages because it has an offensive smell. DMSO was measured as a DMS potential source in their quality control. Conventionally, a purge and trap-GC method was used for determination of DMSO [28]. Also, electrochemical enzymatic biosensor was reported as an analytical method for aqueous DMSO samples [29–31]. According to our knowledge, there has

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Fig. 1. SEM images of the PPy-S on the surfaces of the fibers with different sulfonate anions: (a) 5000×, scale bar 5 ␮m and (b) 15 000×, scale bar 1000 nm.

not been any report on the fabrication of the DMSO selective gas sensor. We found in this study that, when using the in situ doping polymerization method to prepare PPy sulfonate anion dopants (PPy-S) in the polymerization strongly affected the conductivity, morphology and sensory behavior of PPy-S. Also the effect of oxidant/sulfonate anion dopant and concentration of hydrochloric acid in solution on the resistance of sensor was investigated. In this paper, the basic characteristics and the fabrication of the DMSO gas sensor are described.

2. Experimental 2.1. Chemicals Pyrrole (Fluka, Switzerland) was distillated and stored in a refrigerator in dark prior to use. Ferric chloride (FeCl3 ) as oxidant, NaHSO3 , para toluene sulfonate (PTS), sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS) and sodium 5-sulfo salicylate (SSS) was used as dopant from Aldrich. All organic compounds used for responsivity tests were purchased from Merck.

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Table 1 Preparation conditions of the polymerization of pyrrole in the presence of different sulfonate anion dopants in aqueous solution at 0 ◦ C. No.

[Compound] (M)

PPy-DS

PPy-DBS

PPy-HSO3

PPy-PTS

PP-SS

1

[FeCl3 ] [Pyrrole] [Sulfonate anion] [HCl] Resistance (k) [FeCl3 ] [Pyrrole] [Sulfonate anion] [HCl] Resistance (k) [FeCl3 ] [Pyrrole] [Sulfonate anion] [HCl] Resistance (k)

0.05 0.1 0.05 0 H.R.a 0.1 0.1 0.05 0 H.R. 0.2 0.1 0.05 0 0.8

0.05 0.1 0.05 0.02 H.R. 0.1 0. 1 0.05 0.02 H.R. 0.2 0.1 0.05 0.02 0.8

0.05 0.1 0.05 0.05 H.R. 0.1 0.1 0.05 0.05 H.R. 0.2 0.1 0.05 0.05 0.25

0.05 0.1 0.05 0. 5 H.R. 0.1 0.1 0.05 0. 5 H.R. 0.2 0.1 0.05 0. 5 0.25

0.05 0.1 0.05 1 H.R. 0.1 0.1 0.05 1 H.R. 0.2 0.1 0.05 1 0.12

2

3

a

Fig. 2. FT-IR spectra of the PPy-S obtained from the systems with different sulfonate anions as dopant and compared with PPy-Cl: (a) PPy-DS, (b) PPy-PTS, (c) PPy-Cl, (d) PPy-DBS, (e) PPy-HSO3 , (f) PPy-SS.

H.R.: High Resistance (≥100 M).

stirred for 1 h. PPy-S layer was prepared on the polyester substrate by using the dip coating method. After the polymerization and coating, the substrate was taken from the reaction solution and abundantly washed with deionized water for 5 min, and finally dried for 1 h at 70 ◦ C in oven. Different PPy-S sensors were prepared for the gas sensing by repeating the procedure in different FeCl3 /sulfonate anion dopant/HCl contents for variation of the conductivity, gas sensing behavior and morphology of polymer. All experiments were carried out at same temperature. The total volume of the solution was 10 mL and the detailed preparation conditions were listed in Table 1. In all experiment, 0.1 M of pyrrole in the presence of 0.05 M sulfonate anion dopants was selected to compare with other preparation conditions because apparent difference could be observed when the conditions were changed accordingly. The conductivities of the PPy-S films were measured by using a standard two-probe method at room temperature.

2.2. Preparation of PPy-S 2.3. Characterization In polymerization in solution method PPy-S polymers were synthesized by chemical polymerization at 0 ◦ C under atmospheric condition by dip coating. Sulfonate salt aqueous solutions and FeCl3 as oxidant with certain concentration were mixed with different concentration of hydrochloric acid (0–1 M). Pyrrole was dissolved in the above solution under stirring the mixture. The reaction was

The conductivity of fibers was measured by using a standard two-probe method by an assembled computer interface (RS232) for data processing and signal acquisition, which were automatically performed on a microcomputer. The morphology of the fiber surfaces was analyzed by scanning electron micrographs (SEM) using a Philips XL30 scanning electron microscopy (Holland). FT-IR spectra of the samples were recorded at 4 cm−1 resolution with a Nicolet 100 Fourier transform infrared (FT-IR) spectrometer. 2.4. Fabrication of PPy-S gas sensors

Fig. 3. The response behavior of PPy-S sensors to 100 ␮g of different gases which prepared by various sulfonate dopant in solution polymerization method. Condition: injector temperature, 150 ◦ C; detector temperature, 100 ◦ C; flow rate, 100 mL min−1 .

The PPy-S gas sensors consisting of a fiber of 20 mm length having a diameter of ca. 0.5 mm was put in a copper tube of 4 mm inner diameter. The gas sensors were tested with organic solvents using a gas chromatography (Shimadzu GC-4C) system. The detected materials, having the liquid phase at atmospheric pressure, were vaporized by using injection port of GC. A sensor was directly connected to the outlet of the column (without coating), and the whole part was placed in a copper tube. The copper tube is surrounded with a silicone rubber. Sensor and the column temperature are regulated by a thermistor and temperature control circuit. The carrier gas was nitrogen with flow rate of 100 mL min−1 and passed through a 13× molecular sieve (Fluka) trap to remove water vapor and other possible contaminations before entering into the sensor system. After the injection of organic solvents to the chromatography system, the materials are converted to volatile species which

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Table 2 Figure of merits of PPy-S sensors for some compounds at 100 ◦ C. Sensor PPy-DBS

PPy-DS

PPy-HSO3

PPy-PTS

PPy-SS

a

Sample Acetone Methanol Benzene Chloroform Benzaldehyde Benzyl alchole Acetonitrile DMF DMSO Pyridine Acetone Methanol Benzene Chloroform Benzaldehyde Benzyl alchole Acetonitrile DMF DMSO Pyridine Acetone Methanol Benzene Chloroform Benzaldehyde Benzyl alchole Acetonitrile DMF DMSO Pyridine Acetone Methanol Benzene Chloroform Benzaldehyde Benzyl alchole Acetonitrile DMF DMSO Pyridine Acetone Methanol Benzene Chloroform Benzaldehyde Benzyl alchole Acetonitrile DMF DMSO Pyridine

DL (␮g) 100 1.5 200 300 6.0 1.0 3.0 1.0 0.5 0.6 18 15 90 80 20 100 15 0.8 0.5 0.4 5.0 0.8 300 100 1.5 1.0 0.8 0.2 0.03 0.2 10 3.0 200 150 4.0 1.5 2.0 1.5 0.3 0.5 100 1.5 NDa ND 3.0 3.0 1.5 0.3 0.1 0.2

LR (␮g) 100–1000 2–1000 200–6000 500–6000 20–1000 2–1000 20–1000 2–1000 0.2–1000 0.2–1000 20–1000 20–1000 100–3000 100–3000 20–1000 200–1000 20–1000 2–500 2–500 0.2–500 20–1000 2–1000 500–3000 300–1000 2–1000 2–1000 0.2–500 2–1000 0.1–500 0.2–1000 20–1000 20–1000 500–6000 500–6000 20–1000 2–1000 20–1000 2–1000 2–1000 2–1000 100–3000 20–3000 – – 20–1000 20–1000 10–3000 20–500 0.1–500 0.2–500

Calibration sensitivity (1/␮g) −6

3.0 × 10 1.0 × 10−5 1.0 × 10−6 1.0 × 10−6 5.2 × 10−6 2.0 × 10−5 1.0 × 10−5 2.5 × 10−5 6.0 × 10−5 5.8 × 10−5 2.2 × 10−6 2.0 × 10−6 3.5 × 10−7 3.9 × 10−7 1.5 × 10−6 9.0 × 10−7 2.0 × 10−6 4.0 × 10−5 6.0 × 10−5 7.0 × 10−5 6.0 × 10−6 3.5 × 10−5 1.0 × 10−6 3.0 × 10−6 1.0 × 10−5 3.0 × 10−5 4.0 × 10−5 1.3 × 10−4 9.9 × 10−4 1.9 × 10−4 3.0 × 10−6 1.0 × 10−5 4.0 × 10−7 5.0 × 10−7 9.0 × 10−6 1.9 × 10−5 1.7 × 10−5 2.0 × 10−5 7.7 × 10−5 7.0 × 10−5

2 × 10−6 1.51 × 10−5 – – 1.0 × 10−5 1.5 × 10−5 2.0 × 10−5 8.4 × 10−5 3.8 × 10−4 1.6 × 10−4

R2 0.997 0.997 0.996 0.992 0.991 0.996 0.997 0.998 0.994 0.998 0.990 0.995 0.990 0.992 0.990 0.996 0.991 0.997 0.991 0.990 0.995 0.992 0.990 0.997 0.994 0.998 0.992 0.990 0.998 0.997 0.996 0.994 0.990 0.997 0.994 0.993 0.999 0.990 0.990 0.990 0.992 0.996 – – 0.990 0.993 0.992 0.991 0.997 0.994

No detection.

leave the column and the resistance change of sensor was measured. The electrical resistance change of PPy-S due to the exposure of VOCs was monitored every 0.5 s with Escort 3145 Multimeter and an automatic data acquisition system. The injection temperature of system was 150 ◦ C and sensor temperature 100 ◦ C at during the measurements. The average response of three replicate injections was measured for each sample in sensor testing. All technique and conditions for fabrication of gas sensors were reported in our previous works [13,14]. 3. Results and discussion 3.1. Conductivity Table 1 shows the conductivity of PPy-S synthesized in solution at different compound percents. In order to avoid the produce of excess precipitates of Fe-sulfonate anion, the molar ratio of the anion dopant to the oxidant was selected at 1:4 (Table 1, No. 3). We can see also that the No. 3 (Table 1) is the best polymerization

conditions and the PPy-S synthesized in this condition has the highest conductivity. We can find a higher value of resistance when the sample has an oxidant/pyrrole mole ratio <2. 3.2. Morphology The influence of type of S-dopants on the morphology of PPy-S is presented in Fig. 1. It has been demonstrated that morphology of PPy-S depends on the preparation method and conditions [32]. The counter ions also have influence on morphology of PPy-S, as shown in Fig. 1. The morphology of PPy-S doped with DS− and PTS− is cabbage like (∼70–150 nm). However, PPy-S doped with DBS− , HSO3 − and SS− is typically spherical (∼60–120 nm). This means that the morphology of PPy is induced by S-dopants during the in situ doping polymerization of pyrrole. 3.3. FT-IR spectra The FT-IR spectra of all the PPy-S samples were collected and the results indicate the typical characteristics of sulfonate polymer

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Fig. 4. Resistance responses and reproducibility of the PPy-HSO3 sensor upon exposure to 10 ␮g DMSO (about 30 ppm) at flow rate of 100 mL min−1 and 100 ◦ C (a); normalized electrical resistance responses calibration curve for DMSO (b).

and compare with the PPy-Cl spectrum (PPy film doped by Cl− was prepared with the similar manner as described above and in the absence of sulfonate anion). As shown in Fig. 2, the peaks at 1540 and 1454 cm−1 are corresponding to the stretching vibration of the C C double bond of PPy and the C N stretching vibration, respectively. The peaks at 1300 and 1176 cm−1 represent the CH in plane vibration [33,34], and the latter can also indicate that the PPy is in doping state [23]. The peak at 1032 cm−1 belongs to SO3 − group of the dopant and the peaks in the range of 700–500 cm−1 correspond to S O, C S stretching modes, both above which imply that the sulfonic acid groups are introduced in the polymer backbone [35]. Further evidence of the presence of this anion in the polymer film is revealed by peaks at about 1600 cm−1 , which may be assigned to SO2 stretch in sulfonates [36]. In the cases of PTS (b) and DBS (e) anions, electron withdrawing of aromatic group caused SO2 stretch in sulfonates shift to the more frequency. The weak peak around 2900 cm−1 is also found, which can further confirm the formation of the doping because this peak can be attributed to the stretching vibration mode of the methylene in the surfactant structure [23,32]. However, with exception of PPy-Cl the FT-IR spectra in Fig. 2 are similar. 3.4. Response behavior of PPy-S sensors toward VOCs In a previous work [13] we have demonstrated that PPy film doped by Cl− (PPy-Cl) exhibit a sensitiveness to the presence of polar than apolar compounds, a fact that could limit their widespread use as active materials in electronic nose instruments. This fact has led us to look for new types of anion dopants that could reveal a higher sensitiveness in their interaction with polar

or apolar substances and therefore could be more appropriate for use as sensors of the presence of this type of compounds. To test the selectivity and sensitiveness of the PPy-S films as sensors, we have exposed these fabricated PPy-S sensors to vapors of some polar and apolar VOCs. Usually, the signal resulting from a sensor–volatile compound interaction is quantified in terms of the relative electrical resistance difference (RRD) of the film used as sensor. The RRD was calculated by (R − R0 )/R0 , where R0 and R denote the initial resistance and real-time resistance (resistance of the sensor when it was exposed to analyte gas). We present the RRDs of the sensors used in this work measured after several second of exposure to each one of the VOCs (for three replicate injections) in Fig. 3. Although the sensors respond to all volatile compounds tested, they do so in varying extents. When compared to the others we can identify an important difference in the behavior of the PPy-S response towards polar aprotic and non polar compounds. The most intense responses are observed for polar aprotic compounds. Fig. 3 shows the RRDs of five PPy-S sensors to different volatile organic compounds that prepared with various sulfonate anion dopants in solution (dopant concentration of 0.05 mol L−1 , pyrrole 0.1 mol L−1 and FeCl3 0.2 mol L−1 ). As can be seen, the responses pattern, gas-sensing abilities, including the calibration sensitivity and selectivity, of the sensor were dependent on anion dopant type. Most of the conducting polymers are doped/undoped by redox reactions; therefore, their doping level can be altered by transferring electrons from or to the analytes. Electron transferring can cause the changes in resistance and work function of the sensing material. The work function of a conducting polymer is defined as the minimal energy needed to remove an electron from bulk

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to vacuum energy level. This process occurred when a conducting polymer such as PPy, PTh and in some case PAni films exposed in electron donating (DMSO, DMF, NH3 and H2 S) and electron accepting (NO2 and I2 ) gases [37–45]. DMSO is an electron-donor, when PPy exposed with DMSO, its electric conductance dwindles down sharply. However, after washing with carrier gas, the resistance of the sensing layer can be totally or partly recovered. Table 2 gives the response behavior of PPy-S sensors, which prepared in 0.1 mol L−1 pyrrole and 0.2 mol L−1 aqueous FeCl3 solutions and dopants solutions concentration of 0.05 mol L−1 by solution chemical polymerization method. The detection limit, linear ranges, calibration sensitivity, and correlation coefficient of PPy-S sensors are reported in Table 2. The detection limit of the PPy-S sensors, which was calculated as the amount, gives a reading equal to three times the standard deviation (3 b ) of a series of the procedural blank or background signals (response of the sensor, measured in the absence of compound), can be estimated as follows: DL =

3b calibration sensitivity

(1)

where calibration sensitivity is slope of calibration curve The comparison of results of Table 2 shows that the lower detection limits were obtained for DMSO, pyridine and DMF (electron donor molecules). The rapidly interaction of DMSO and sulfate is reported in the literature [46,47], which may cause a selective response of PP-S sensors to DMSO. In contrast, benzene, chloroform and acetone molecules without electron donating groups cannot interact with polymer matrix efficiently like polar ones due to their non-polar nature, they are likely to act as barriers among polymer chains. The detection limit of PPy-S sensors to DMSO decreases in the following order: PPy-DS, PPy-DBS, PPy-PTS > PPy-SS > PPyHSO3 . The PPy-S sensors with multi charge anion (PPy-HSO3 and PPy-SS) show lower detection limits to DMSO (Table 2). This may be explained by the fact that in these cases, most interaction occurs with DMSO. Effects of different sulfonate anion dopants on solubility, electrical and thermal properties of polypyrrole have been reported in the literature [48–51]. It is obvious that the PPy-HSO3 exhibits a lowest detection limit to the DMSO (30 ng). Fig. 4a shows the typical response behavior of the PPy-HSO3 sensor to switching between N2 and DMSO in cycle tests at flow rate of 100 mL min−1 and 100 ◦ C. It is seen that the response of the PPy-HSO3 sensor upon exposure to DMSO gas could completely return to the original value in all the cycle and reversible in subsequent cycle tests. It can be seen that electrical resistance increased upon exposure to analyte gas, and recovered when flushed with nitrogen flow (no significant drift of the background resistance after several exposures). As shown in Fig. 4a, the response of PPy-HSO3 sensor to 10 ␮g of DMSO exhibited good reproducibility with a relative standard deviation (R.S.D.) < 8% and could be used, repetitively. A typical calibration curve relating the normalized electrical resistance vs. the amounts of DMSO is shown in Fig. 4b. 4. Conclusions PPy-S was successfully prepared from the sulfonate anion dopants in aqueous solutions and the conductivity, morphology, structure and gas sensing behavior of the resulting products were all investigated. The results showed that the type of the sulfonate anion dopants would affect the agglomeration of the as-obtained samples. Addition of acid and using FeCl3 as oxidant would both increase the doping ratio and decrease the degree of the agglomeration. The PPy was doped with sulfonate anions resulting in a conductive material, suitable for application in chemiresistor sensors. The effects of the dopant type and concentrations on the

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Biographies

Sajad Pirsa was born on 22 December 1984 and received his B.Sc. in Chemistry from Guilan University (2007), M.Sc. degree in Analytical Chemistry from Tarbiat Modares University (2009). He is currently a Ph.D. student of Tarbiat Modares University in analytical chemistry. His research interests cover chemical sensors and separation study.

Naader Alizadeh received the Ph.D. degree in Analytical Chemistry from Tarbiat Modares University, Tehran, Iran, in 1996. Currently, he is a Professor of Analytical Chemistry at Tarbiat Modarres University. His research interests cover chemical and electrochemical synthesis of nanostructure conducting polymers and its applications in chemical sensors and microextraction methods, exchange kinetics, and complexation of macrocyclic ligands.