A selective conductive polymer-based sensor for volatile halogenated organic compounds (VHOC)

Available online at www.sciencedirect.com

Sensors and Actuators B 131 (2008) 646–651

A selective conductive polymer-based sensor for volatile halogenated organic compounds (VHOC) Rosamaria W.C. Li a,∗ , Leonardo Ventura b , Jonas Gruber a , Yoshio Kawano a , Lilian R.F. Carvalho a a

Instituto de Qu´ımica, Universidade de S˜ao Paulo, Caixa Postal 26077, 05513-970 S˜ao Paulo, SP, Brazil b Instituto de Matem´ atica e Estat´ıstica, Universidade de S˜ao Paulo, Brazil Received 31 May 2007; received in revised form 19 December 2007; accepted 20 December 2007 Available online 3 January 2008 Dedicated to Prof. Herman J. Geise, in memory.

Abstract A novel poly(p-xylylene), PPX, derivative bearing alkoxyphenyl side groups was electrochemically synthesized in 87% yield. The polymer, poly(4 -hexyloxy-2,5-biphenyleneethylene) (PHBPE), presented a fraction (92%) soluble in common organic solvents. It showed to be thermally resistant up to 185 ◦ C. UV–vis analysis revealed an Egap of 3.5 eV. Gas sensors made from thin films of 10-camphorsulfonic acid-doped PHBPE deposited on interdigitated electrodes exhibited significant changes in electrical conductance upon exposure to five VHOCs: 1,2-dichloroethane, bromochloromethane, trichloromethane, dichloromethane and tetrachloromethane. The conductance decreased after exposure to tetrachloromethane and increased after exposure to all the other VHOCs. Three-dimensional plots of relative response versus time of half response versus time of half recovery showed good discrimination between the five VHOCs tested. © 2008 Elsevier B.V. All rights reserved. Keywords: Conductive polymers; Gas sensors; Volatile organic compounds; Volatile halogenated organic compounds

1. Introduction Many different types of gas sensors have been employed for the analysis of volatile organic compounds (VOCs). Perhaps the most competitive type is based on the change in electrical dc resistance (or ac impedance) when a semi-conductive material is exposed to a vapor. These sensors are often made from metal oxide semiconductors (MOS) or conducting polymers (CPs) [1]. CP-based sensors demonstrate a number of attractive features including reversible operation at ambient temperature, high sensitivity to a wide range of VOCs, large possibilities of structural variations and relative low cost. The mechanism of the intrinsic CP response is not clear at present but many theories have been suggested [2–4]. Most likely, chemical sensing by CPs may occur either by changes in the extrinsic conductivity due to swelling of polymers by analytes or by changes

∗

Corresponding author. Tel.: +55 11 3091 1103; fax: +55 11 3815 5579. E-mail address: [email protected] (R.W.C. Li).

0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.12.051

in the intrinsic conductivity due to charge-transfer interactions between polymers and analytes [5]. There is a growing concern about the fate of volatile halogenated organic compounds (VHOC) since many of them show acute and chronic toxicity, genotoxicity or carcinogenicity and are suspected to contribute to climate changes and to the destruction of the ozone layer [6]. These compounds include: (i) trihalomethanes (THMs) which are the main by-products of water disinfection performed by chlorination processes, being trichloromethane (or chloroform) the most represented one [7] and (ii) volatile chlorinated hydrocarbons (VCHCs) as, for instance, dichloromethane, trichloromethane, tetrachloromethane and 1,2-dicholoroethane used in a wide variety of industrial and commercial processes [8]. The most commonly applied CPs for gas-sensing purposes have been polypyrrole [9,10], polyaniline [11,12] and polythiophene [13,14]. Recently, studies have been carried out using poly(p-phenylenevinylene)s (PPVs) in gas-sensitive chemoresistors which responded with high selectivity to several organic solvents [15–17]. These studies usually involve several VOCs

R.W.C. Li et al. / Sensors and Actuators B 131 (2008) 646–651

as alcohols, diethylether, acetone, toluene, etc., but only one VHOC, either chloroform [12,14] or tetrachloromethane [10]. Thus, as far as we know, there is no CP single sensor capable of detecting several VHOCs described in the literature. In this paper we describe the electrochemical synthesis and full characterization of a novel CP, poly(4 -hexyloxy2,5-biphenyleneethylene) (PHBPE), and its application as the active layer of a selective gas sensor for VHOCs such as dichloromethane, bromochloromethane, trichloromethane, tetrachloromethane and 1,2-dicholoroethane. The reason for designing the PHBPE structure for this purpose is based on the following facts: (a) alkoxylated arylene oligomers have shown excellent sensing properties to VOCs [18]; (b) alkoxy side chains enhance the solubility of CPs in organic solvents, important for film processing [19]; (c) the polymer is a derivative of poly(p-xylylene) (PPX) [20], a highly stable material, compared to conjugated polymers as, for instance, poly(p-phenylenevinylene) (PPV) since there are no double bonds between the arylene moieties that could be oxidized by air/humidity; (d) although there is no extended conjugation through the polymer chain, the presence of biphenylene units (six conjugated double bonds) may ensure doping and electrical conductivity [21]. 2. Experimental 2.1. General methods For the cyclic voltammetry (CV) experiment and the preparative electrolysis, commercial N,N-dimethylformamide (DMF) (Aldrich GPR) was dried over anhydrous CuSO4 for 2 days and then distilled at 44–45 ◦ C (25 mmHg) through a 40 cm vigreux ˚ molecular sieves. column and stored over freshly baked 4 A Commercial grade tetraethylammonium bromide was baked at 150 ◦ C overnight before use. Commercial grade CCl4 was heated under reflux over phosphorous pentoxide for 10 h before distil˚ molecular sieves. All lation. It was stored over freshly baked 4 A other commercially available materials were used as received. 1 H NMR FT spectra (200 MHz) were recorded on a Bruker AC-200 spectrometer using deuteriated chloroform/TMS (Aldrich) as solvent/reference. FTIR spectra were recorded as a KBr disc or in solution (CHCl3 ), on a PerkinElmer 1750 series grating. Only major or important absorptions are given. The UV–vis spectrum was recorded in solution (CHCl3 ) on a Hitachi U-2000 spectrophotometer. CV of the polymeric precursor was carried out using a USP electronics workshopconstructed triangular wave generator/potentiostat with a PAR RE0074 XY recorder. Controlled potential electrolysis was carried out using a potentiostat/galvanostat with an electronic charge integrator constructed in our laboratory [22,23]. Thermogravimetry (TG) experiment was carried out on a TA Instruments Hi Res TGA 2950 thermogravimeter, dynamic air atmosphere (100 mL min−1 ) and heating rate of 20 ◦ C min−1 . Elemental analyses were carried out on a PerkinElmer Elemental Analyser 2400 CHN. Molecular weight determination was made by size exclusion chromatography (SEC) at a flow rate of 1 mL min−1 in THF on a Shimadzu Class-LC10 HPLC equipped with three

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Supelco Progel columns (G5000 + G4000 + G3000). The molecular weight is reported relative to narrow dispersity polystyrene standards (2500, 5000, 17,500, 30,000, 50,000, 95,800 and 184,200 g mol−1 ). 2.2. Syntheses 2,5-Dimethylphenylboronic acid (1) was purchased from Aldrich and 1-bromo-4-hexyloxybenzene (2) was prepared according to a literature procedure [24]. 4 -Hexyloxy-2,5-dimethylbiphenyl (3): tetrakis(triphenylphosphine) palladium was prepared, shortly before use, following a literature procedure [25]. To a vigorously stirred solution, under dry nitrogen, of 1-bromo-4-hexyloxybenzene (2) (5.00 g; 19.5 mmol), 2,5-dimethylphenylboronic acid (1) (3.00 g, 20.1 mmol), aqueous sodium carbonate solution (60 mL, 20%) and 1,4-dioxane (60 mL), Pd(PPh3 )4 (0.658 g; 0.570 mmol) was added and the reaction mixture was refluxed for 10 h. After cooling to room temperature, water (60 mL) was added; the mixture was extracted with dichloromethane (3 × 40 mL) and dried over MgSO4 . The solvent was removed under reduced pressure and the crude brown product was purified by column chromatography [Al2 O3 , hexane–dichloromethane (2:1)]. Compound 3 was obtained as a colorless oil (4.52 g; 16.0 mmol; 82%). 1 H NMR (200 MHz; CDCl3 ) δ: 0.91 (t, J = 7.0 Hz, 3H), 1.38 (m, 6H), 1.81 (m, 2H), 2.23 (s, 3H), 2.34 (s, 3H), 3.99 (t, J = 7.0 Hz, 2H), 6.09–7.25 (m, 7H). FTIR (CHCl3 ), cm−1 : 3033 (CH aromatic), 2956, 2930, 2870, 2861 (CH aliphatic), 1593, 1494 (C C aromatic), 1469, 1380 (CH aliphatic), 1244, 1048 (COC), 887, 833, 810 (CH aromatic). 4 -Hexyloxy-2,5-bis(bromomethyl)biphenyl (4): 4 -hexyloxy-2,5-dimethylbiphenyl (3) (1.00 g; 3.55 mmol), NBS (1.30 g; 7.31 mmol) and dibenzoyl peroxide (5.0 mg) were added to dry carbon tetrachloride (7 mL) and heated to reflux for 4 h under illumination of a 500 W halogen bulb, then cooled to room temperature. The insoluble succinimide was filtered off then washed with hot chloroform. The combined filtrate was washed with aqueous sodium chloride and then with water. After drying over anhydrous magnesium sulfate and solvent evaporation, a viscous orange oil was obtained (0.541 g; 0.827 mmol; 49%). 1 H NMR (200 MHz; CDCl ) δ: 0.92 (t, J = 7.0 Hz, 3H), 1.37 3 (m, 6H), 1.82 (m, 2H), 4.00 (t, J = 7.0 Hz, 2H), 4.46 (s, 2H), 4.50 (s, 2H), 6.94–7.54 (m, 7H). FTIR (CHCl3 ), cm−1 : 3036 (CH aromatic), 2954, 2930, 2869, 2858 (CH aliphatic), 1609, 1489 (C C aromatic), 1469, 1391 (CH aliphatic), 1245, 1025 (COC), 835, 823 (CH aromatic), 660 (CBr). Anal. calcd. for C20 H24 Br2 O: C, 54.57; H, 5.49. Found: C, 55.02; H, 5.22. CV: −1.9 V versus Ag/AgBr (−2.2 V versus SCE). Poly(4 -hexyloxy-2,5-biphenyleneethylene), PHBPE, (5):  4 -hexyloxy-2,5-bis(bromomethyl) biphenyl (4) (1.07 g; 2.44 mmol) was electrolyzed at a mercury pool cathode in Et4 NBr (0.1 mol L−1 )–DMF solution (50 mL) at −1.9 V (versus Ag/AgBr) in a divided cell and a graphite anode. The cathode compartment was continually flushed with a slow stream of dry nitrogen. A dark yellow precipitate formed during electrolysis. After ca. 2.1 F mol−1 had passed, the cell current dropped close to the background value. The precipitate was filtered and

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Fig. 1. Top plan view of the interdigitated sensor.

washed several times with water to remove DMF and Et4 NBr, and dried in vacuo. Water was added to the filtrate and another crop of polymer was obtained (DMF-soluble fraction), which was also washed and dried. Yield: 50 mg (0.18 mmol; 7%) of insoluble fraction and 540 mg (1.9 mmol; 80%) of DMF-soluble fraction. 1 H NMR (200 MHz; CDCl3 ) δ: 0.92 (m, 3H), 1.38 (m, 6H), 1.82 (m, 2H), 2.75–2.90 (m, 4H), 4.03 (t, J = 7.0 Hz, 2H), 6.90–7.21 (m, 7H). FTIR (CHCl3 ), cm−1 : 3035 (CH, aromatic), 2953, 2929, 2859 (CH aliphatic), 1609 (C C, aromatic), 1420, 1391 (CH aliphatic), 1243, 1046 (COC), 832 (CH aromatic). TG: 185 ◦ C (m = −2%); 397 ◦ C (m = −46%); ¯w= 491 ◦ C (m = −58%); 600 ◦ C (m = −95%); SEC: M ¯ n = 1.1 × 103 g mol−1 , M ¯ w /M ¯ n = 2.2. 2.4 × 103 g mol−1 , M 2.3. Preparation of the sensors A thin (10–30 ␮m) uniform layer of PHBPE doped with 10camphorsulfonic acid (CSA) was deposited on a sensor substrate by drop-casting a solution containing 4.5 mg of PHBPE, 0.5 mg of CSA and 5.0 mL of chloroform. CSA was chosen as a dopant because it is an organic Lewis acid that presents good miscibility with the polymer. The sensor substrate consisted of a flat 23 mm × 9 mm fiber glass printed circuit board with a pair of tin-coated copper interdigitated electrodes having a gap of ca. 0.2 mm between them (Fig. 1). 2.4. Testing the sensors The sensor’s response and selectivity were evaluated by exposing it, in a 250 mL closed vessel, to air saturated (at 25 ◦ C) with an organic vapor for 2 s, followed by 10 s of exposition to clean dry air (recovery time). The saturated concentration of each organic vapor was (vol%): 1,2-dichloroethane

Fig. 3. Synthetic route to PHBPE.

(12), tetrachloromethane (16), bromochloromethane (20), trichloromethane (26) and dichloromethane (56). The conductance over the contact pairs was continuously monitored with an accurate conductivity meter [26], operating with 80 mV peak-topeak 2 kHz triangle wave ac voltage, and connected via a 10 bit analog to digital converter to a PC in which a software enabled plotting conductance versus time graphs (Fig. 2). 3. Results and discussion 3.1. Syntheses The synthetic route to PHBPE (5) is shown in Fig. 3. The key step is the palladium-catalyzed cross-coupling reaction [27] of boronic acid 1 with aryl bromide 2 to form biphenyl 3. The hexyloxy side-chain is necessary to ensure the solubility of the final polymer in organic solvents. Then, compound 3 was brominated at the benzylic positions leading to the polymer precursor 4. The peak potential observed in single-sweep CV of compound 4 was used for the reduction potential of controlled potential electrolysis [20] leading to polymer 5 in 87% yield. 3.2. Characterization The NMR and IR analytical methodologies used to establish the key features of this electrosynthesized polymer were essentially those used for analogous poly(p-xylylene)s (PPXs) and they have been fully described [28]. Spectroscopic data are given in Section 2. Important observations include the IR absorptions at 1243 and 1046 cm−1 , which confirms the presence of the hexyloxy side-chain, also confirmed by the 1 H NMR signals and integrations at 0.92, 1.38, 1.82 and 4.03 ppm.

Fig. 2. Block diagram of the sensor response measuring system.

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Fig. 4. TG and DTG curves of PHBPE (5), sample weight 3.09 mg. Dynamic air atmosphere, rate 20 ◦ C min−1 .

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Fig. 5. Response of three CSA doped PHBPE sensors to five different VHOCs.

The UV–vis spectrum showed two absorptions at 4.94 and 6.04 eV corresponding to ␲–␲* transitions of biphenyl and of isolated benzene rings, respectively. The optical gap was estimated from the first absorption onset at ∼3.5 eV [29]. Thermogravimetry (TG) (Fig. 4) revealed the polymer to be stable up to 185 ◦ C (2 wt% loss). Above this temperature, it decomposed in three steps with inflection points at 225, 351 and 537 ◦ C. The residual weight above 600 ◦ C was 5% suggesting some unburned carbon residue. Examination by size exclusion chromatography (SEC) in tetrahydrofuran (THF) solution using polystyrene standards ¯ w = 2.4 × 103 , M ¯ n = 1.1 × 103 and for comparison gave M ¯ ¯ Mw /Mn = 2.2.

The presence of molecules of volatile compounds in the polymer matrix may influence intrachain mobility of free charge carriers due to a solvation-induced alteration of the molecular conformation, which may contribute positively or negatively to the conductance, depending on details of the molecular arrangements [4]. This may explain why exposure to tetrachloromethane (nonpolar) led to a decrease in conductance while the other VHOCs led to the opposite effect. In order to analyze the data generated by a set of several tests in which n similar sensors were exposed to the above five VHOCs in random sequences and in different occasions, we define three parameters (see Eqs. (1)–(3) and Fig. 6): the relative response (Ra ), the time for half-response (T1 ) and the time for half-recovery (T2 ).

3.3. Sensor response

Ra =

Firstly, the effect of doping on the electrical conductance was evaluated comparing measurements carried out on sensors prepared with undoped polymer (<5 × 10−10 S) and with doped polymer (5 × 10−5 S). Typical responses of three similar gas sensors, having as active layers CSA-doped PHBPE thin films, to five different volatile halogenated organic compounds are shown in Fig. 5. The only difference between the sensors was the film thickness, being approximately 10 ␮m for sensor 1, 15 ␮m for sensor 3 and 30 ␮m for sensor 2. As can be seen, the sensors exhibited: (i) different responses to all tested compounds. In most cases, the conductance increased, while for tetrachloromethane (the only nonpolar compound of the series) it decreased. Even for those with the same tendency in the conductance change, the shape of the response curve was different in each case. (ii) Excellent response reproducibility between sensors made with different film thickness; (iii) very fast response (2 s) and recovery (≤10 s); (iv) no significant drift of the background conductance after several exposures; (v) very low power consumption (less than 1 ␮W) since the applied voltage was only 80 mV and the average conductance of the sensors was ca. 50 ␮S.

G2 − G1 G1

(1)

T1 = T(G1 +G2 )/2 − TG1

(2)

T2 = T(G2 +G3 )/2 − TG2

(3)

Fig. 6. Parameters used for calculating T1 and T2 .

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References

Fig. 7. Three-dimensional plot of Ra vs. T1 vs. T2 . There are eight plots for each VHOC, which may not be always visible due to overlapping.

A three-dimensional plot of Ra versus T1 versus T2 obtained from up to eight CSA-doped PHBPE sensors exposed to five VHOCs is shown in Fig. 7. It is possible to see the discriminating power of these sensors, which can perfectly distinguish all the tested compounds. The effect of humidity on the response of the sensor was checked exposing several sensors to air with relative humidity ranging from 30 to ∼100%. No changes in conductance could be detected. This is probably due to the hydrophobic nature of the polymer. Finally, it is worth to mention that several tests consisting of 20 repetitive exposure/recovery cycles of a sensor to a VHOC have shown good reproducibility. Four sensors have been tested for over five months and still respond perfectly to these VHOCs. This fact becomes even more important considering the total cost of a sensor, which is less than US$ 1.00. 4. Conclusions Poly(4 -hexyloxy-2,5-biphenyleneethylene), PHBPE, was synthesized electrochemically and was doped with CSA resulting in a conductive material, suitable for application in chemiresistor sensors. The sensors, obtained by drop-casting a doped polymeric film on interdigitated electrodes, exhibited good selectivity to five volatile halogenated organic compounds (VHOCs): 1,2-dichloroethane, bromochloromethane, trichloromethane, dichloromethane and tetrachloromethane. The sensors are easy to make, fairly cheap and operate at room temperature with very low power consumption. Acknowledgements The authors would like to thank FAPESP and CNPq for their financial support. Thanks are due to Prof. Luiz H. Catalani and Vania A.B. Bueno for the SEC analysis.

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Biographies Rosamaria Wu Chia Li has a BSc in chemistry, an MSc and a PhD in organic chemistry from Instituto de Qu´ımica, Universidade de S˜ao Paulo, Brazil. Now, she is doing a post-doctoral research supported by Fundac¸a˜ o de Amparo a` Pesquisa do Estado de S˜ao Paulo (FAPESP). Her current interests include synthesis of conducting polymers and their application in e-noses for detection of volatile organic compounds.

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Leonardo Ventura is an undergraduate student of applied mathematics at the Instituto de Matem´atica e Estat´ıstica da Universidade de S˜ao Paulo, Brazil. His current research interests include pattern recognition applied to e-noses. Jonas Gruber has a BSc in chemistry, an MSc and a PhD in organic chemistry from Instituto de Qu´ımica, Universidade de S˜ao Paulo (IQ-USP), Brazil. Then he did a post-doctoral work at Queen Mary College, University of London, UK. He is currently an assistant professor at IQ-USP and his research interests include synthesis of conducting polymers and their application in gas sensors and in magnetic and optoelectronic devices. Yoshio Kawano has a BSc in physics from Instituto de F´ısica, Universidade de S˜ao Paulo, an MSc and a PhD in physical chemistry from Instituto de Qu´ımica, Universidade de S˜ao Paulo (IQ-USP), Brazil. Then he did post-doctoral works at the University of Tokyo and at Duke University. He is currently full professor at IQ-USP and his research interests include structural determination of polymers by Raman and IR spectroscopy and their thermal characterization by DSC and TG. Lilian Rothschild has a BSc in pharmaceutical sciences from Faculdade de Ciˆencias Farmacˆeuticas, Universidade de S˜ao Paulo, an MSc and a PhD in inorganic chemistry from Instituto de Qu´ımica, Universidade de S˜ao Paulo (IQUSP), Brazil. Then she did post-doctoral works at IQ-USP and at the University of California. She is currently associate professor at IQ-USP and her research interests include indoor air pollution and volatile organic compounds.