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Analytica Chimica Acta xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
A miniature chemiresistor sensor for carbon dioxide Sira Srinives 1, Tapan Sarkar 2 , Raul Hernandez, Ashok Mulchandani * Department of Chemical and Environmental Engineering, University of California-Riverside, Riverside, CA 92521, USA
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
Poly(ethyleneimine) functionalized polyaniline nanothin film miniature chemiresistor CO2 sensor. Detection by CO2–amines reactions acid–base, dissolution and of base-catalyzed hydration. pH change from acids and carbamates produced and detected by polyaniline nanothin film. Sensor was accurate, precise, highly sensitive and selective. Suitable for portable multi-analyte breath analysis and environmental monitoring detector.
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 September 2014 Received in revised form 4 March 2015 Accepted 12 March 2015 Available online xxx
A carpet-like nanostructure of polyaniline (PANI) nanothin film functionalized with poly(ethyleneimine), PEI, was used as a miniature chemiresistor sensor for detection of CO2 at room temperature. Good sensing performance was observed upon exposing the PEI–PANI device to 50–5000 ppm CO2 in presence of humidity with negligible interference from ammonia, carbon monoxide, methane and nitrogen dioxide. The sensing mechanism relied on acid–base reaction, CO2 dissolution and amine-catalyzed hydration that yielded carbamates and carbonic acid for a subsequent pH detection. The sensing device showed reliable results in detecting an unknown concentration of CO2 in air. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Conducting polymer Polyaniline Poly(ethyleneimine) Gas sensor Carbon dioxide
1. Introduction Growing concerns on environmental conditions have motivated researcher worldwide to focus attention in developing effective methods for environmental monitoring and efficient devices that
* Corresponding author. Tel.: +1 951 827 6419. E-mail address: [email protected] (A. Mulchandani). 1 Current address: Chemical Engineering Department, Mahidol University, 25/25 Puttamonthon 4 Rd., Nakorn pathom 73170, Thailand. 2 Current address: School of Chemical Technology, Guru Gobind Singh Indraprastha University, Sector 16C, Dwarka, New Delhi 110078, India.
run on low-power and are portable for on-site measurement. In particular, CO2 detection in ambient air has continued to be a challenge due to stability of the compound and interferences from several species, such as nitrogen dioxide (NO2) and carbon monoxide (CO). Further, a CO2 sensor can be greatly beneficial to a wide range of applications, including breath and blood analysis for medical diagnosis [1–3], portable gas detector for personal protection and gas monitoring for climate control [4]. Current approaches for CO2 detection, including spectrophotometry [4,5], solid electrolyte electrochemical sensors [6] and semiconductive metal oxide based sensors [7], are limited by their cost, bulky size or high energy consumption, leaving the
http://dx.doi.org/10.1016/j.aca.2015.03.020 0003-2670/ ã 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Srinives, et al., A miniature chemiresistor sensor for carbon dioxide, Anal. Chim. Acta (2015), http://dx.doi. org/10.1016/j.aca.2015.03.020
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Severinghaus electrode [4,6] as the most prominent option for a miniature sensor. The Severinghaus principle relies on CO2 dissolution in aqueous electrolyte, generating carbonic acid that can be monitored via a potentiometric pH electrode. The issue regarding the Severinghaus principle concerns base line shift in the Ag/AgCl reference of the pH sensing unit, necessitating a regular maintenance to keep the device well calibrated. Hence, a selective, highly sensitive and low-powered CO2 sensor with an ability to be installed in a compact area or implemented in a sensor array is still needed. Following the Severinghaus’ approach, certain kind of alkyl amines, such as poly(ethyleneimine) [1,8] and 3-aminopropyltrimethoxysilane [9,10], have been employed as CO2 recognition layers, producing carbamates [11] and carbonic acid that contribute to the pH change in the amine phase. Depending on types of the amine sorbents (primary, secondary or tertiary) temperature and relative humidity, CO2 reaction/interaction occurs through three possible paths (Fig. 1): acid–base reaction, dissolution, and base-catalyzed hydration of CO2 [12]. The primary and secondary amines are favored for the acid–base reaction generating carbamates while the tertiary amine is a preferred catalyst in CO2 dissolution accelerating carbonic acid formation via the base-catalyzed hydration. When considering reaction rate, the base-catalyzed hydration is slower than the acid–base reaction; however, it captures more CO2 (1 mol CO2 per 1 mol amine) and regenerates faster (lower heat of regeneration) in comparison [12]. Thus, a practical/preferred approach is to employ the PEI, containing primary, secondary and tertiary amines, as the CO2 recognition layer combining advantages and balancing disadvantages from all different amine types. In addition, a branched-structure of the PEI offers steric effect that inhibits acid–base reaction channeling the amines for the base-catalyzed hydration and resulting in a high CO2 loading capacity and a good amine recovery [12]. Polyaniline (PANI) is an attractive material for sensor applications owing to its excellences in mechanical flexibility, reductant/ oxidant reactivity, acid/base sensitivity, and huge range of tunable conductivity (>10 orders of magnitude). When applied for a pH sensing, PANI is induced by acid/base conditions that cause protonation/deprotonation in the polymer chains leading to a reversible transformation between the conductive state emeraldine salt (PANI-ES) and the insulating state emeraldine base (PANI-EB) [13,14]. Our group has previously demonstrated a highly-sensitive chemiresistive gas sensor based on 2-D nanostructure PANI nanothin film [15]. The pristine PANI film device exhibited an excellent sensing performance to NH3 and NO2 gases, which was comparable to that of the 1-D nanostructures, as a result of nanometer-thin film and carpet-like surface morphology that helped enhancing the surface reaction. The objective of this work was to fabricate a CO2 chemiresistor sensor utilizing CO2 recognition ability of the PEI and pH sensitivity of the PANI
nanothin film. The film was electropolymerized on a prefabricated microelectrodes and further functionalized with the PEI via a dip coating technique. The sensing device was employed for CO2 detection in the operation range of 50–5000 ppm, and also tested against other selected analytes including ammonia (NH3), methane (CH4), carbon monoxide (CO), and nitrogen dioxide (NO2) to investigate cross reactivity, i.e. sensor selectivity. Reliability of the sensor device was validated by comparing a CO2 in compressor air sensing readout to that of the nondispersive infrared spectrometry (NDIR). The effects of humidity and temperature on CO2 sensing ability were also studied. 2. Experimental 2.1. Device fabrication Unless stated otherwise, all compounds were reagent grade and used with no further treatment. All solutions were prepared using deionized water (Milli-Q water), and, if required, were deoxygenated by purging with nitrogen stream. The details regarding PANI nanothin film fabrication and characterization can be referred to our previous work [15]. In brief, the film device was prepared on a pair of gold microelectrodes (100 mm wide 200 mm long) that was separated by a 3-mm gap. The microelectrodes were defined on heavily doped p-type silicon substrate coated with silicon oxide dielectric layer using standard photolithography. The chip with microelectrodes was cleaned in piranha solution (1.5 mL H2O2 + 3.5 mL H2SO4), rinsed with deionized water and dried under nitrogen stream. Next, the dried substrate was silanized by incubating in 2% (v/v) octadecyltrichlorosilane (OTS) in toluene, followed by rinsing with fresh toluene and ethanol, sequentially. The purpose of silanization was to functionalize the SiO2 surface with long hydrocarbon chains making the SiO2 portion of the substrate highly hydrophobic. Hydrophobicity of the OTS-silanized substrate helped attracting aniline monomer and enhancing electropolymerization rate of PANI film in lateral direction [16]. Monomer accumulation as a result of hydrophobicity of the substrate was the key to a realization of the nanothin film nanostructure. Next, the chip was placed in a standard electrochemical cell along with Ag/AgCl reference and platinum strip counter electrodes and submerged in the aniline monomer solution (0.5 M aniline + 1 M perchloric acid). The electropolymerization was performed by applying constant potentials of 0.6 V and 0.8 V versus the Ag/AgCl of the two gold electrodes while electrochemical currents were plotted versus operating time creating chronoamperometric curves of the two electrodes. During electropolymerization, chronoamperometric curves bifurcated at a certain point indicating a film bridging across the 3-mm gap channel. The result was a uniform and nanometerthick PANI film (9–20 nm [15]) with a carpet-like structure (Fig. 2). Functionalization of the PANI with PEI was achieved by dip coating the device in a pH controlled 0.5% (w/v) PEI solution (PEI: Mw 750,000; P3143 FLUKA; pH 7), followed by lowvacuum drying in a desiccant chamber. After functionalization, a thin layer of PEI was observed under optical microscope as transparent gel on top of the PANI nanothin film. 2.2. Gas detection/sensing
Fig. 1. Paths for amines–CO2 reactions [12].
In order to investigate the CO2 sensing performance of the device, the PEI–PANI nanothin film was mounted into a custommade gas sensing system that provided a temperature controlled chamber with mass flow controlled gas streams (mass flow controllers, MFCs: Alicat Scientific Inc., AZ, USA). Electrical resistance of each device was acquired via a FieldPoint module (National Instrument, TX, USA) by continually measuring current
Please cite this article in press as: S. Srinives, et al., A miniature chemiresistor sensor for carbon dioxide, Anal. Chim. Acta (2015), http://dx.doi. org/10.1016/j.aca.2015.03.020
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Fig. 4. Dynamic responses of pristine (a) and PEI-functionalized (b) PANI devices upon CO2 exposures. The inset shows sensing response of the PEI–PANI at lower CO2 concentration range.
Fig. 2. SEM image of the PANI nanothin film showing a carpet-like structure of the film surface.
that passed through the sensor at a bias potential of 1 V. All recording, displaying and controlling were performed by a LabVIEW1 interface, which was installed on a personal computer. Multiple PEI–PANI devices were allowed to stabilize under humid air before introduction of CO2 or other selected analytes. Background medium was prepared by mixing dry air and water vapor-saturated air flowing at a total flow rate of 100 standard cm3 min1 (sccm). Various concentrations of CO2 were generated by diluting a stream of 10,000 ppm CO2 (1% v/v) from a certified cylinder with dry air. For the purpose of study, exposure time and recovery time were set constant at 15 and 20 min, respectively, while the sensing responses were presented in term of normalized resistance [(R R0)/R0] versus experimental time.
PANI (conversion of PANI-EB to PANI-ES). The acids generated during CO2 exposure decomposed when purged with CO2-free medium, causing pH increase and leading to resistance recovery in the sensing device (conversion of PANI-ES to PANI-EB). At the operating conditions of 50% RH and 25 C the sensor sensitivity, defined as slope of linear region of the plot of sensing responses versus the CO2 concentrations (Fig. 5(a)), was determined to be 7.14 103% ppm1 CO2. The sensing signal reached 90% of its maximum response in 7.34 5.21 min and achieved 90% of its total recovery in 10.01 4.86 min. The limit of detection (LOD), 3 times the sensing response for the blank (i.e. air
3. Results and discussion 3.1. CO2 detection PEI is an amine-rich molecule that contains primary, secondary, and tertiary amines (46% primary; 23% secondary; and 31% tertiary amines, Fig. 3). As stated earlier, combination of the three amine types and the branched structure yields the base-catalyzed hydration of CO2 that enhances the acid production rate upon CO2 exposure and provides good amine recovery in absence of CO2. Fig. 4 shows dynamic responses of pristine and PEI-functionalized PANI film devices to different concentrations of CO2 at 50% relative humidity and 25 C. As illustrated in the figure, the PEI-functionalized PANI sensor exhibited a good response to CO2. A decrease in device resistance indicated pH reduction in the PEI recognition layer as a result of carbamates and carbonic acid formation that potentially doped and increased conductivity of
Fig. 3. Molecular structure of the poly(ethyleneimine) used in our experiment.
Fig. 5. Sensing responses, (DR/R0) (%), of the PEI–PANI versus CO2 concentrations (ppm), as a function of humidity and temperature: (a) 25 C; 50% RH (&), 25 C; 35% RH (), and 25 C; 10% RH (4) and (b) 35 C; 50% RH ( ), and 50 C; 50% RH ( ). Data points represent average of responses from 3 sensors and error bars represents 1S.D.
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Table 1 Comparison of analytical performance of different electrochemical sensors for CO2 detection. Sensing element
Fabrication method
- Bipotentiostatic electropolymerization on an electrode - Chemical polymerization of PANI - Dip coating of PANI + PVA thin film on an electrode rGO [18] - Chemical exfoliation of graphite to graphene oxide (GO) - Reduction of GO using hydrogen plasma - Drop casting of rGO solution to a thin film - CVD grown single wall carbon PEI/starch-coated nanotubes carbon nanotubes - Dip coating of PEI + starch on the [1] sensor PEI-functionalized - In situ polymerization of PEDOT PEDOT film [19] - Drop basting of PEI layer on PEDOT film LaOCl-functionalized - Thermal deposition of SnO2 SnO2 nanowires [20] nanowires - Drop casting of SnO2 nanowires and LaCl3 solutions on the sensor electrode
PEI-functionalized PANI nanothin film (this study) PANI–PVA composite film [17]
Operating conditions
CO2 sensitivity (% ppm1)
Limit of detection (LOD)
Dynamic range (CO2 concentration)
Selectivity
25 C; 50% RH
7.14 103
40 ppm CO2
500–5000 ppm
Room temperature; 0.4 104 various humidity levels to 3 104
Not reported
Not reported
Not sensitive to 50 ppm CO, 5 ppm NH3, 50,000 ppm CH4, and 50 ppb NO2 Not reported
23 C; 37% RH
1.18 102
Not reported
350–1500 ppm
Not reported
Room temperature; 80% RH; operated in FET mode
1.01 104
Not reported
500– 100,000 ppm
Expected to be cross-sensitive to acid gases (NOx and SO2, etc.)
Room temperature; 95% RH
2.91 103
Not reported
Not reported
Not reported
300 C; 0% RH
3.33 102
Not reported
250–4000 ppm
Interfered by 100 ppm CO, 20 ppm NH3, and 10 ppm NO2
without CO2) to the sensor sensitivity, was computed to be 40 ppm CO2. The sensing operation at lower CO2 concentrations (50–500 ppm), Fig. 4 (inset), revealed distinguishable sensing responses at the concentration close to the LOD value. The CO2 detection experiment on a pristine PANI nanothin film device at 50% relative humidity and 25 C (Fig. 4(a)) showed no significant change in device resistance upon exposure to 500–5000 ppm CO2. This was expected since PANI reacted poorly with CO2 and could not generate sufficient acid for the pH detection [17]. The sensing results from this study were compared with those from other electrochemical sensors with different sensing elements (Table 1), including carbon nanostructures such as reduced graphene oxide (rGO) and carbon nanotubes; modified conjugated/conducting polymers, i.e. polyaniline–poly(vinyl alcohol) (PANI–PVA) composite and PEI-functionalized poly(3,4-ethylene-dioxythiophene) (PEDOT); and metal oxide – tin oxide (SnO2) nanowires functionalized with lanthanum oxychloride (LaOCl). While the sensitivity of the PEI–PANI sensor in this work was better or similar to others [1,17–20] it had advantages of improved selectivity when compared to rGO [18] and carbon nanotubes [1] and room temperature operation, thus lower power consumption, as compared to metal oxide [20]. 3.2. Dependence on humidity and temperature Dependence on water and heat production of CO2–PEI reaction prompted an investigation to the humidity and temperature effects on the CO2 detection. In order to observe these effects, the PEI–PANI film devices were prepared and mounted in the sensing system as described above. The sensor was stabilized at a fixed value of humidity and temperature. Various concentrations of CO2 were introduced within the detection range of the device. As shown in Fig. 5(a) and (b), sensing response was a direct function of the relative humidity and an inverse function of the temperature. These results were in accordance with the three reaction paths for CO2–PEI reaction. As shown in Fig. 1, the dissolution and
base-catalyzed hydration of CO2 required water, making them strongly dependent on humidity. Decrease in the gas humidity reduced the water content of the PEI layer as well as the nanothin film, slowing down reaction rates in the dissolution and base-catalyzed hydration. On the other hand, the acid–base reaction was independent of water, however, it was easily reversed by excess heat. Further, the increase in temperature could also hinder CO2 adsorption on the PEI layer causing deterioration in magnitude of the sensing response. The dependence of sensor response on temperature and humidity, while may seem to be problem, can be addressed by humidifying/dehumidifying the sample to the desired level and generating calibration plots for different temperatures to determine CO2 concentration at the desired temperature. 3.3. Selectivity To evaluate selectivity of the PEI–PANI, the sensor device was tested against 50 ppm CO, 5 ppm NH3, 50,000 ppm CH4, and 50 ppb NO2 at 50% RH and 25 C. No significant response was observed against any of these gasses (Fig. S1(a) and (b)) indicating no cross reactivity and adequate selectivity of the PEI–PANI sensor to CO2. The concentrations of the above species tested were either occupational safety relevant concentrations (CO, NH3, and CH4) or environmental protection agency regulation (NO2). To validate the newly developed sensor, CO2 was measured in laboratory compressed air and compared to the measurement from NDIR system (LI-840A CO2/H2O Analyzer). Before analysis by both methods, the laboratory compressed air was prepared by passing through HEPA filter followed by dry molecular sieves packed in an HPLC column and was collected in a Teflon bag. The collected air was diluted by half (50% v/v) and introduced to the PEI–PANI sensor that was stabilized at 50% relative humidity and 25 C. Real-time responses of the PEI–PANI sensor to the unknown (Fig. 6(left)) and 500 ppm CO2 from the certified cylinder (Fig. 6(right)) were comparable. The CO2 concentration in the compressor air was determined by the PEI–PANI device to be
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2015.03.020. References
Fig. 6. Real-time sensing results from the PEI–PANI device to an unknown concentration of CO2 (I) and 500 ppm CO2 (II).
380 ppm, which was 9% less than the value (350 ppm) determined independently by the NDIR system (Fig. S2). 4. Conclusion In conclusion, we demonstrated a simple, sensitive and selective CO2 chemiresistor sensor incorporating PEI recognition element and pH-sensitive PANI nanothin film transducer. The sensing mechanism relied on the formation of carbamates and carbonic acid during PEI–CO2 reactions; acid–base reaction, CO2 dissolution and base-catalyzed hydration, that caused pH change in the PEI layer. Role of the PANI nanothin film was to monitor the pH change and not to participate directly in the reactions with CO2. The sensor device exhibited a sensitive and selective detection toward 500–5000 ppm CO2 (LOD = 40 ppm) with no significant interferences from other analytes, such as CO, CH4, and NO2. The sensing of an unknown CO2 concentration showed a 9% positive error as compared to the NDIR measurement value. These excellent analytical performances combined with the low power requirement, simple operation and small size open the possibility of using this sensor in a portable multi-analyte detector for breath analysis and environmental monitoring. Acknowledgements A.M. is grateful for support from the W. Ruel Johnson Chair in Environmental Engineering. S.S. and T.S. greatly appreciate financial support from the Thai Government Scholarship (Ministry of Science) and the Government of India, respectively. We would also like to thank Dr. Shusuke Nakao for helping with the NDIR analysis.
[1] A. Star, T.R. Han, V. Joshi, J.C.P. Gabriel, G. Gruner, Nanoelectronic carbon dioxide sensors, Adv. Mater. 16 (2004) 2049–2052. [2] P. Arquint, M. Koudelkahep, N.F. Derooij, H. Buhler, W.E. Morf, Organic membranes for miniaturizes electrochemical sensors – fabrication of a combined pO2, pCO2, and pH sensor, J. Electroanal. Chem. 378 (1994) 177–183. [3] P. Arquint, A. van den Berg, B.H. van der Schoot, N.F. de Rooij, H. Buhler, W.E. Morf, L.F.J. Durselen, Integrated blood-gas sensor for pO2, pCO2 and pH, Sens. Actuators B: Chem. 13 (1993) 340–344. [4] J. Zosel, W. Oelssner, M. Decker, G. Gerlach, U. Guth, The measurement of dissolved and gaseous carbon dioxide concentration, Meas. Sci. Technol. 22 (2011) 72001 (45pp). [5] I.M. Perez de Vargas-Sansalvador, C. Fay, T. Phelan, M.D. Fernandez-Ramos, L.F. Capitan-Vallvey, D. Diamond, F. Benito-Lopez, A new light emitting diode light emitting diode portable carbon dioxide gas sensor based on an interchangeable membrane system for industrial applications, Anal. Chim. Acta 699 (2011) 216–222. [6] S. Herber, J. Bomer, W. Olthuis, P. Bergveld, A. van den Berg, A miniaturized carbon dioxide gas sensor based on sensing of pH-sensitive hydrogel swelling with a pressure sensor, Biomed. Microdevices 7 (2005) 197–204. [7] D.D. Trung, L.D. Toan, H.S. Hong, T.D. Lam, T. Trung, N. Van Hieu, Selective detection of carbon dioxide using LaOCl-functionalized SnO2 nanowires for air-quality monitoring, Talanta 88 (2012) 152–159. [8] C.Y. Chang, B.S. Kang, H.T. Wang, F. Ren, Y.L. Wang, S.J. Pearton, D.M. Dennis, J.W. Johnson, P. Rajagopal, J.C. Roberts, E.L. Piner, K.J. Linthicum, CO2 detection using polyethylenimine/starch functionalized AlGaN/GaN high electron mobility transistors, Appl. Phys. Lett. 92 (2008) 232102–232104. [9] S. Stegmeier, M. Fleischer, A. Tawil, P. Hauptmann, K. Egly, K. Rose, Sensing mechanism of room temperature CO2 sensors based on primary amino groups, Sens. Actuators B: Chem. 154 (2011) 270–276. [10] S. Stegmeier, M. Fleischer, A. Tawil, P. Hauptmann, H.E. Endres, Sensing of CO2 at room temperature using work function readout of (hetero-)polysiloxanes sensing layers, Sens. Actuators B: Chem. 154 (2011) 206–212. [11] D.B. Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, G. Pampaloni, Converting carbon dioxide into carbamato derivatives, Chem. Rev. 103 (2003) 3857–3898. [12] S.A. Rackley, Carbon Capture and Storage, Elsevier, 2010. [13] J. Wang, S. Chan, R.R. Carlson, Y. Luo, G. Ge, R.S. Ries, J.R. Heath, H.-R. Tseng, Electrochemically fabricated polyaniline nanoframework electrode junctions that function as resistive sensors, Nano Lett. 4 (2004) 1693–1697. [14] A.K. Wanekaya, M.A. Bangar, M. Yun, W. Chen, N.V. Myung, A. Mulchandani, Field-effect transistors based on single nanowires of conducting polymers, J. Phys. Chem. C 111 (2007) 5218–5221. [15] S. Srinives, T. Sarkar, A. Mulchandani, Nanothin polyaniline film for highly sensitive chemiresistive gas sensing, Electroanalysis 25 (2013) 1439–1445. [16] M. Nishizawa, Y. Miwa, T. Matsue, I. Uchida, Surface treatment for electrochemical fabrication of ultrathin patterned conducting polymers, J. Electrochem. Soc. 140 (1993) 1650–1655. [17] M. Irimia-Vladu, J.W. Fergus, Suitability of emeraldine base polyaniline–PVA composite film for carbon dioxide sensing, Synth. Met. 156 (2006) 1401–1407. [18] S.M. Hafiz, R. Ritikos, T.J. Whitcher, N.M. Razib, D.C.S. Bien, N. Chanlek, H. Nakajima, T. Saisopa, P. Songsiriritthigul, N.M. Huang, S.A. Rahman, A practical carbon dioxide gas sensor using room-temperature hydrogen plasma reduced graphene oxide, Sens. Actuators B: Chem. 193 (2014) 692–700. [19] C.J. Chiang, K.T. Tsai, Y.H. Lee, H.W. Lin, Y.L. Yang, C.C. Shih, C.Y. Lin, H.A. Jeng, Y.H. Weng, Y.Y. Cheng, K.C. Ho, C.A. Dai, In situ fabrication of conducting polymer composite film as a chemical resistive CO2 gas sensor, Microelectron. Eng. 111 (2013) 409–415. [20] D.D. Trung, L.D. Toan, H.S. Hong, T.D. Lam, T. Trung, N. Van Hieu, Selective detection of carbon dioxide using LaOCl-functionalized SnO2 nanowires for air-quality monitoring, Talanta 88 (2012) 152–159.
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Analytica Chimica Acta xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
A miniature chemiresistor sensor for carbon dioxide Sira Srinives 1, Tapan Sarkar 2 , Raul Hernandez, Ashok Mulchandani * Department of Chemical and Environmental Engineering, University of California-Riverside, Riverside, CA 92521, USA
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
Poly(ethyleneimine) functionalized polyaniline nanothin film miniature chemiresistor CO2 sensor. Detection by CO2–amines reactions acid–base, dissolution and of base-catalyzed hydration. pH change from acids and carbamates produced and detected by polyaniline nanothin film. Sensor was accurate, precise, highly sensitive and selective. Suitable for portable multi-analyte breath analysis and environmental monitoring detector.
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 September 2014 Received in revised form 4 March 2015 Accepted 12 March 2015 Available online xxx
A carpet-like nanostructure of polyaniline (PANI) nanothin film functionalized with poly(ethyleneimine), PEI, was used as a miniature chemiresistor sensor for detection of CO2 at room temperature. Good sensing performance was observed upon exposing the PEI–PANI device to 50–5000 ppm CO2 in presence of humidity with negligible interference from ammonia, carbon monoxide, methane and nitrogen dioxide. The sensing mechanism relied on acid–base reaction, CO2 dissolution and amine-catalyzed hydration that yielded carbamates and carbonic acid for a subsequent pH detection. The sensing device showed reliable results in detecting an unknown concentration of CO2 in air. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Conducting polymer Polyaniline Poly(ethyleneimine) Gas sensor Carbon dioxide
1. Introduction Growing concerns on environmental conditions have motivated researcher worldwide to focus attention in developing effective methods for environmental monitoring and efficient devices that
* Corresponding author. Tel.: +1 951 827 6419. E-mail address: [email protected] (A. Mulchandani). 1 Current address: Chemical Engineering Department, Mahidol University, 25/25 Puttamonthon 4 Rd., Nakorn pathom 73170, Thailand. 2 Current address: School of Chemical Technology, Guru Gobind Singh Indraprastha University, Sector 16C, Dwarka, New Delhi 110078, India.
run on low-power and are portable for on-site measurement. In particular, CO2 detection in ambient air has continued to be a challenge due to stability of the compound and interferences from several species, such as nitrogen dioxide (NO2) and carbon monoxide (CO). Further, a CO2 sensor can be greatly beneficial to a wide range of applications, including breath and blood analysis for medical diagnosis [1–3], portable gas detector for personal protection and gas monitoring for climate control [4]. Current approaches for CO2 detection, including spectrophotometry [4,5], solid electrolyte electrochemical sensors [6] and semiconductive metal oxide based sensors [7], are limited by their cost, bulky size or high energy consumption, leaving the
http://dx.doi.org/10.1016/j.aca.2015.03.020 0003-2670/ ã 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Srinives, et al., A miniature chemiresistor sensor for carbon dioxide, Anal. Chim. Acta (2015), http://dx.doi. org/10.1016/j.aca.2015.03.020
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Severinghaus electrode [4,6] as the most prominent option for a miniature sensor. The Severinghaus principle relies on CO2 dissolution in aqueous electrolyte, generating carbonic acid that can be monitored via a potentiometric pH electrode. The issue regarding the Severinghaus principle concerns base line shift in the Ag/AgCl reference of the pH sensing unit, necessitating a regular maintenance to keep the device well calibrated. Hence, a selective, highly sensitive and low-powered CO2 sensor with an ability to be installed in a compact area or implemented in a sensor array is still needed. Following the Severinghaus’ approach, certain kind of alkyl amines, such as poly(ethyleneimine) [1,8] and 3-aminopropyltrimethoxysilane [9,10], have been employed as CO2 recognition layers, producing carbamates [11] and carbonic acid that contribute to the pH change in the amine phase. Depending on types of the amine sorbents (primary, secondary or tertiary) temperature and relative humidity, CO2 reaction/interaction occurs through three possible paths (Fig. 1): acid–base reaction, dissolution, and base-catalyzed hydration of CO2 [12]. The primary and secondary amines are favored for the acid–base reaction generating carbamates while the tertiary amine is a preferred catalyst in CO2 dissolution accelerating carbonic acid formation via the base-catalyzed hydration. When considering reaction rate, the base-catalyzed hydration is slower than the acid–base reaction; however, it captures more CO2 (1 mol CO2 per 1 mol amine) and regenerates faster (lower heat of regeneration) in comparison [12]. Thus, a practical/preferred approach is to employ the PEI, containing primary, secondary and tertiary amines, as the CO2 recognition layer combining advantages and balancing disadvantages from all different amine types. In addition, a branched-structure of the PEI offers steric effect that inhibits acid–base reaction channeling the amines for the base-catalyzed hydration and resulting in a high CO2 loading capacity and a good amine recovery [12]. Polyaniline (PANI) is an attractive material for sensor applications owing to its excellences in mechanical flexibility, reductant/ oxidant reactivity, acid/base sensitivity, and huge range of tunable conductivity (>10 orders of magnitude). When applied for a pH sensing, PANI is induced by acid/base conditions that cause protonation/deprotonation in the polymer chains leading to a reversible transformation between the conductive state emeraldine salt (PANI-ES) and the insulating state emeraldine base (PANI-EB) [13,14]. Our group has previously demonstrated a highly-sensitive chemiresistive gas sensor based on 2-D nanostructure PANI nanothin film [15]. The pristine PANI film device exhibited an excellent sensing performance to NH3 and NO2 gases, which was comparable to that of the 1-D nanostructures, as a result of nanometer-thin film and carpet-like surface morphology that helped enhancing the surface reaction. The objective of this work was to fabricate a CO2 chemiresistor sensor utilizing CO2 recognition ability of the PEI and pH sensitivity of the PANI
nanothin film. The film was electropolymerized on a prefabricated microelectrodes and further functionalized with the PEI via a dip coating technique. The sensing device was employed for CO2 detection in the operation range of 50–5000 ppm, and also tested against other selected analytes including ammonia (NH3), methane (CH4), carbon monoxide (CO), and nitrogen dioxide (NO2) to investigate cross reactivity, i.e. sensor selectivity. Reliability of the sensor device was validated by comparing a CO2 in compressor air sensing readout to that of the nondispersive infrared spectrometry (NDIR). The effects of humidity and temperature on CO2 sensing ability were also studied. 2. Experimental 2.1. Device fabrication Unless stated otherwise, all compounds were reagent grade and used with no further treatment. All solutions were prepared using deionized water (Milli-Q water), and, if required, were deoxygenated by purging with nitrogen stream. The details regarding PANI nanothin film fabrication and characterization can be referred to our previous work [15]. In brief, the film device was prepared on a pair of gold microelectrodes (100 mm wide 200 mm long) that was separated by a 3-mm gap. The microelectrodes were defined on heavily doped p-type silicon substrate coated with silicon oxide dielectric layer using standard photolithography. The chip with microelectrodes was cleaned in piranha solution (1.5 mL H2O2 + 3.5 mL H2SO4), rinsed with deionized water and dried under nitrogen stream. Next, the dried substrate was silanized by incubating in 2% (v/v) octadecyltrichlorosilane (OTS) in toluene, followed by rinsing with fresh toluene and ethanol, sequentially. The purpose of silanization was to functionalize the SiO2 surface with long hydrocarbon chains making the SiO2 portion of the substrate highly hydrophobic. Hydrophobicity of the OTS-silanized substrate helped attracting aniline monomer and enhancing electropolymerization rate of PANI film in lateral direction [16]. Monomer accumulation as a result of hydrophobicity of the substrate was the key to a realization of the nanothin film nanostructure. Next, the chip was placed in a standard electrochemical cell along with Ag/AgCl reference and platinum strip counter electrodes and submerged in the aniline monomer solution (0.5 M aniline + 1 M perchloric acid). The electropolymerization was performed by applying constant potentials of 0.6 V and 0.8 V versus the Ag/AgCl of the two gold electrodes while electrochemical currents were plotted versus operating time creating chronoamperometric curves of the two electrodes. During electropolymerization, chronoamperometric curves bifurcated at a certain point indicating a film bridging across the 3-mm gap channel. The result was a uniform and nanometerthick PANI film (9–20 nm [15]) with a carpet-like structure (Fig. 2). Functionalization of the PANI with PEI was achieved by dip coating the device in a pH controlled 0.5% (w/v) PEI solution (PEI: Mw 750,000; P3143 FLUKA; pH 7), followed by lowvacuum drying in a desiccant chamber. After functionalization, a thin layer of PEI was observed under optical microscope as transparent gel on top of the PANI nanothin film. 2.2. Gas detection/sensing
Fig. 1. Paths for amines–CO2 reactions [12].
In order to investigate the CO2 sensing performance of the device, the PEI–PANI nanothin film was mounted into a custommade gas sensing system that provided a temperature controlled chamber with mass flow controlled gas streams (mass flow controllers, MFCs: Alicat Scientific Inc., AZ, USA). Electrical resistance of each device was acquired via a FieldPoint module (National Instrument, TX, USA) by continually measuring current
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Fig. 4. Dynamic responses of pristine (a) and PEI-functionalized (b) PANI devices upon CO2 exposures. The inset shows sensing response of the PEI–PANI at lower CO2 concentration range.
Fig. 2. SEM image of the PANI nanothin film showing a carpet-like structure of the film surface.
that passed through the sensor at a bias potential of 1 V. All recording, displaying and controlling were performed by a LabVIEW1 interface, which was installed on a personal computer. Multiple PEI–PANI devices were allowed to stabilize under humid air before introduction of CO2 or other selected analytes. Background medium was prepared by mixing dry air and water vapor-saturated air flowing at a total flow rate of 100 standard cm3 min1 (sccm). Various concentrations of CO2 were generated by diluting a stream of 10,000 ppm CO2 (1% v/v) from a certified cylinder with dry air. For the purpose of study, exposure time and recovery time were set constant at 15 and 20 min, respectively, while the sensing responses were presented in term of normalized resistance [(R R0)/R0] versus experimental time.
PANI (conversion of PANI-EB to PANI-ES). The acids generated during CO2 exposure decomposed when purged with CO2-free medium, causing pH increase and leading to resistance recovery in the sensing device (conversion of PANI-ES to PANI-EB). At the operating conditions of 50% RH and 25 C the sensor sensitivity, defined as slope of linear region of the plot of sensing responses versus the CO2 concentrations (Fig. 5(a)), was determined to be 7.14 103% ppm1 CO2. The sensing signal reached 90% of its maximum response in 7.34 5.21 min and achieved 90% of its total recovery in 10.01 4.86 min. The limit of detection (LOD), 3 times the sensing response for the blank (i.e. air
3. Results and discussion 3.1. CO2 detection PEI is an amine-rich molecule that contains primary, secondary, and tertiary amines (46% primary; 23% secondary; and 31% tertiary amines, Fig. 3). As stated earlier, combination of the three amine types and the branched structure yields the base-catalyzed hydration of CO2 that enhances the acid production rate upon CO2 exposure and provides good amine recovery in absence of CO2. Fig. 4 shows dynamic responses of pristine and PEI-functionalized PANI film devices to different concentrations of CO2 at 50% relative humidity and 25 C. As illustrated in the figure, the PEI-functionalized PANI sensor exhibited a good response to CO2. A decrease in device resistance indicated pH reduction in the PEI recognition layer as a result of carbamates and carbonic acid formation that potentially doped and increased conductivity of
Fig. 3. Molecular structure of the poly(ethyleneimine) used in our experiment.
Fig. 5. Sensing responses, (DR/R0) (%), of the PEI–PANI versus CO2 concentrations (ppm), as a function of humidity and temperature: (a) 25 C; 50% RH (&), 25 C; 35% RH (), and 25 C; 10% RH (4) and (b) 35 C; 50% RH ( ), and 50 C; 50% RH ( ). Data points represent average of responses from 3 sensors and error bars represents 1S.D.
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Table 1 Comparison of analytical performance of different electrochemical sensors for CO2 detection. Sensing element
Fabrication method
- Bipotentiostatic electropolymerization on an electrode - Chemical polymerization of PANI - Dip coating of PANI + PVA thin film on an electrode rGO [18] - Chemical exfoliation of graphite to graphene oxide (GO) - Reduction of GO using hydrogen plasma - Drop casting of rGO solution to a thin film - CVD grown single wall carbon PEI/starch-coated nanotubes carbon nanotubes - Dip coating of PEI + starch on the [1] sensor PEI-functionalized - In situ polymerization of PEDOT PEDOT film [19] - Drop basting of PEI layer on PEDOT film LaOCl-functionalized - Thermal deposition of SnO2 SnO2 nanowires [20] nanowires - Drop casting of SnO2 nanowires and LaCl3 solutions on the sensor electrode
PEI-functionalized PANI nanothin film (this study) PANI–PVA composite film [17]
Operating conditions
CO2 sensitivity (% ppm1)
Limit of detection (LOD)
Dynamic range (CO2 concentration)
Selectivity
25 C; 50% RH
7.14 103
40 ppm CO2
500–5000 ppm
Room temperature; 0.4 104 various humidity levels to 3 104
Not reported
Not reported
Not sensitive to 50 ppm CO, 5 ppm NH3, 50,000 ppm CH4, and 50 ppb NO2 Not reported
23 C; 37% RH
1.18 102
Not reported
350–1500 ppm
Not reported
Room temperature; 80% RH; operated in FET mode
1.01 104
Not reported
500– 100,000 ppm
Expected to be cross-sensitive to acid gases (NOx and SO2, etc.)
Room temperature; 95% RH
2.91 103
Not reported
Not reported
Not reported
300 C; 0% RH
3.33 102
Not reported
250–4000 ppm
Interfered by 100 ppm CO, 20 ppm NH3, and 10 ppm NO2
without CO2) to the sensor sensitivity, was computed to be 40 ppm CO2. The sensing operation at lower CO2 concentrations (50–500 ppm), Fig. 4 (inset), revealed distinguishable sensing responses at the concentration close to the LOD value. The CO2 detection experiment on a pristine PANI nanothin film device at 50% relative humidity and 25 C (Fig. 4(a)) showed no significant change in device resistance upon exposure to 500–5000 ppm CO2. This was expected since PANI reacted poorly with CO2 and could not generate sufficient acid for the pH detection [17]. The sensing results from this study were compared with those from other electrochemical sensors with different sensing elements (Table 1), including carbon nanostructures such as reduced graphene oxide (rGO) and carbon nanotubes; modified conjugated/conducting polymers, i.e. polyaniline–poly(vinyl alcohol) (PANI–PVA) composite and PEI-functionalized poly(3,4-ethylene-dioxythiophene) (PEDOT); and metal oxide – tin oxide (SnO2) nanowires functionalized with lanthanum oxychloride (LaOCl). While the sensitivity of the PEI–PANI sensor in this work was better or similar to others [1,17–20] it had advantages of improved selectivity when compared to rGO [18] and carbon nanotubes [1] and room temperature operation, thus lower power consumption, as compared to metal oxide [20]. 3.2. Dependence on humidity and temperature Dependence on water and heat production of CO2–PEI reaction prompted an investigation to the humidity and temperature effects on the CO2 detection. In order to observe these effects, the PEI–PANI film devices were prepared and mounted in the sensing system as described above. The sensor was stabilized at a fixed value of humidity and temperature. Various concentrations of CO2 were introduced within the detection range of the device. As shown in Fig. 5(a) and (b), sensing response was a direct function of the relative humidity and an inverse function of the temperature. These results were in accordance with the three reaction paths for CO2–PEI reaction. As shown in Fig. 1, the dissolution and
base-catalyzed hydration of CO2 required water, making them strongly dependent on humidity. Decrease in the gas humidity reduced the water content of the PEI layer as well as the nanothin film, slowing down reaction rates in the dissolution and base-catalyzed hydration. On the other hand, the acid–base reaction was independent of water, however, it was easily reversed by excess heat. Further, the increase in temperature could also hinder CO2 adsorption on the PEI layer causing deterioration in magnitude of the sensing response. The dependence of sensor response on temperature and humidity, while may seem to be problem, can be addressed by humidifying/dehumidifying the sample to the desired level and generating calibration plots for different temperatures to determine CO2 concentration at the desired temperature. 3.3. Selectivity To evaluate selectivity of the PEI–PANI, the sensor device was tested against 50 ppm CO, 5 ppm NH3, 50,000 ppm CH4, and 50 ppb NO2 at 50% RH and 25 C. No significant response was observed against any of these gasses (Fig. S1(a) and (b)) indicating no cross reactivity and adequate selectivity of the PEI–PANI sensor to CO2. The concentrations of the above species tested were either occupational safety relevant concentrations (CO, NH3, and CH4) or environmental protection agency regulation (NO2). To validate the newly developed sensor, CO2 was measured in laboratory compressed air and compared to the measurement from NDIR system (LI-840A CO2/H2O Analyzer). Before analysis by both methods, the laboratory compressed air was prepared by passing through HEPA filter followed by dry molecular sieves packed in an HPLC column and was collected in a Teflon bag. The collected air was diluted by half (50% v/v) and introduced to the PEI–PANI sensor that was stabilized at 50% relative humidity and 25 C. Real-time responses of the PEI–PANI sensor to the unknown (Fig. 6(left)) and 500 ppm CO2 from the certified cylinder (Fig. 6(right)) were comparable. The CO2 concentration in the compressor air was determined by the PEI–PANI device to be
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2015.03.020. References
Fig. 6. Real-time sensing results from the PEI–PANI device to an unknown concentration of CO2 (I) and 500 ppm CO2 (II).
380 ppm, which was 9% less than the value (350 ppm) determined independently by the NDIR system (Fig. S2). 4. Conclusion In conclusion, we demonstrated a simple, sensitive and selective CO2 chemiresistor sensor incorporating PEI recognition element and pH-sensitive PANI nanothin film transducer. The sensing mechanism relied on the formation of carbamates and carbonic acid during PEI–CO2 reactions; acid–base reaction, CO2 dissolution and base-catalyzed hydration, that caused pH change in the PEI layer. Role of the PANI nanothin film was to monitor the pH change and not to participate directly in the reactions with CO2. The sensor device exhibited a sensitive and selective detection toward 500–5000 ppm CO2 (LOD = 40 ppm) with no significant interferences from other analytes, such as CO, CH4, and NO2. The sensing of an unknown CO2 concentration showed a 9% positive error as compared to the NDIR measurement value. These excellent analytical performances combined with the low power requirement, simple operation and small size open the possibility of using this sensor in a portable multi-analyte detector for breath analysis and environmental monitoring. Acknowledgements A.M. is grateful for support from the W. Ruel Johnson Chair in Environmental Engineering. S.S. and T.S. greatly appreciate financial support from the Thai Government Scholarship (Ministry of Science) and the Government of India, respectively. We would also like to thank Dr. Shusuke Nakao for helping with the NDIR analysis.
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