Anal. Chem. 2004, 76, 4583-4588
Use of Room Temperature Ionic Liquids in Gas Sensor Design Marisa C. Buzzeo,† Christopher Hardacre,‡ and Richard G. Compton*,†
Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, United Kingdom, and School of Chemistry, Queen’s University, Belfast, Belfast, Northern Ireland, BT9 5AG, United Kingdom
The attainable steady-state limiting currents and time responses of membrane-covered and membrane-independent gas sensors incorporating different electrode and electrolyte materials have been compared. A new design comprising a membrane-free microelectrode modified with a thin layer of a room temperature ionic liquid is considered. While the use of ionic liquid as electrolyte eliminates the need for a membrane and added supporting electrolyte, the slower diffusion of analyte within the more viscous medium results in slower time responses. Such sensors do, however, have potential application in more extreme operating conditions, such as high temperature and pressure, where traditional solvents would volatise. Several amperometric electrochemical sensors have been developed over the past fifty years to measure target gases such as oxygen and carbon dioxide.1-7 These sensors have varied in their design, specifically in electrode dimension, electrode material, and choice of membrane, to improve performance, lifetime, and reliability, while reducing the cost and size. Herein we will discuss four such archetypal gas sensors and compare and contrast their relevant properties, namely, the attainable steadystate currents and time responses, as the electrode size, membrane material, and electrolyte composition are varied. Early gas sensor models involve working macroelectrodes, while later designs incorporate electrodes on the micrometer scale. A gaspermeable membrane is usually employed to separate the sample from electrolyte, and the properties of this layer have significant implications on the rate of transport of analyte to the electrode surface. We propose, however, a gas sensor that incorporates a room temperature ionic liquid as electrolyte, eliminating the need for a membrane in the sensor design. The first model, B1, resembles the original design proposed by Clark in 1956 and comprises a container that houses both the * To whom correspondence should be addressed. Tel: 01865 275413. Fax: 01865 275410 E-mail:
[email protected]. † University of Oxford. ‡ Queen’s University, Belfast. (1) Clark, L. C. Trans. Am. Soc. Artif. Intern. Organs 1956, 2, 41. (2) Carritt, D. E.; Kanwisher, J. W. Anal. Chem. 1959, 31, 5. (3) Jensen, O. J. J. Electroanal. Chem. 1978, 87, 203. (4) Albery, W. J.; Barron, P. J. Electroanal. Chem. 1982, 79. (5) Hahn, C. E. W. Analyst 1998, 123, 57R. (6) Cao, Z.; Buttner, W. J.; Stetter, J. R. Electroanalysis 1992, 4, 253. (7) Chang, S. C.; Stetter, J. R.; Cha, C. Talanta 1993, 40, 461. 10.1021/ac040042w CCC: $27.50 Published on Web 07/03/2004
© 2004 American Chemical Society
electrodes and electrolyte, with a gas-permeable membrane separating the electrolyte from the gaseous sample (see Figure 1).1 This membrane frequently consists of poly(tetrafluoroethylene) (PTFE, Teflon) or polyethylene and typically ranges from 1 to 20 µm in thickness. In this arrangement, the diffusion layer of the electrode completely overlaps the membrane and significant concentration gradients exist in both the membrane and the electrolyte. The second sensor, B2, consists of a similar arrangement, but the working electrode is now of micrometer dimensions, resulting in a partial overlap of the diffusion layer with the membrane.3 The third design, A2, contains a microelectrode situated inside a thin-layer chamber, with reference and counter electrodes in adjacent compartments and a flow path for delivery of the target gas.8 The size of the microelectrode employed must be sufficiently small so that its diffusion layer does not encroach on the membrane. As such, the signal observed only reflects the transport of the electroactive species within the thin layer of electrolyte, not the membrane. Finally, we now propose a membrane-free gas sensor, A1, which consists of a two-electrode cell design, whereby the surface of the working microelectrode is modified with a thin layer of a roomtemperature ionic liquid (RTIL), which serves as the nonvolatile electrolyte and excludes the need for any membrane. RTILs are compounds consisting entirely of ions that exist in the liquid state around 298 K and below. They possess several properties, including negligible vapor pressure, wide potential windows, high thermal stability, and good intrinsic conductivity, which render them attractive alternative electochemical solvents9 and, more specifically, potentially advantageous in the development of stable and robust gas sensors. As the viscosity of an ionic liquid is typically 1-2 orders of magnitude larger than tranditional organic solvents,10-12 mass transport is significantly slower in the former, and several studies have been done to investigate the voltammetry and slower diffusion of electroactive species observed within this medium,13-20 as well as in polymer electrolytes, which can be considered predecessors to RTILs.21-28 (8) Lawrence, N. S.; Jiang, L.; Jones, T. G.; Compton, R. G. Anal. Chem. 2003, 75, 2499. (9) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, 2003. (10) Sun, J.; Forsyth, M.; MacFarlane, D. R. J. Phys. Chem. B 1998, 102, 8858. (11) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. J. Phys. Chem. B 1999, 103, 4164. (12) Dzyuba, S. V.; Bartsch, R. ChemPhysChem 2002, 3, 161.
Analytical Chemistry, Vol. 76, No. 15, August 1, 2004 4583
Figure 1. Diagram of the four different types of amperometric gas sensor: models (A) B1, (B) B2, (C) A2, and (D) A1.
Recently, the use of ionic liquids as solvents for the electrochemical detection of oxygen, carbon dioxide, and ammonia was reported.20,29-33 Cai et al.34 have developed a sulfur dioxide gas (13) Hultgren, V. M.; Mariotti, A. W. A.; Bond, A. M.; Wedd, A. G. Anal. Chem. 2002, 74, 3151. (14) Zhang, J.; Bond, A. M. Anal. Chem. 2003, 75, 2694. (15) Zhang, J.; Bond, A. M.; Belcher, W. J.; Wallace, K. J.; Steed, J. W. J. Phys. Chem. B 2003, 107, 5777. (16) Compton, D. L.; Laszlo, J. A. J. Electroanal. Chem. 2002, 520, 71. (17) Kosmulski, M.; Osteryoung, R. A.; Ciszlowska, M. J. Electrochem. Soc. 2000, 147, 1454. (18) Lagrost, C.; Carrie, D.; Vaultier, M.; Hapiot, P. J. Phys. Chem. A 2003, 107, 745. (19) Evans, R. G.; Klymenko, O. V.; Hardacre, C.; Seddon, K. R.; Compton, R. G. J. Electroanal. Chem. 2003, 556, 179. (20) Buzzeo, M. C.; Klymenko, O. V.; Wadhawan, J. D.; Hardacre, C.; Seddon, K. R.; Compton, R. G. J. Phys. Chem. A 2003, 107, 8872. (21) Geng, L.; Reed, R. A.; Kim, M.-H.; Wooster, T. T.; Oliver, B. N.; Egekeze, J.; Kennedy, R. T.; Jorgenson, J. W.; Parcher, J. F.; Murray, R. W. J. Am. Chem. Soc. 1989, 111, 1614. (22) Geng, L.; Reed, R. A.; Longmire, M.; Murray, R. W. J. Phys. Chem. 1987, 91, 2908. (23) Parcher, J. F.; Barbour, C. J.; Murray, R. W. Anal. Chem. 1989, 61, 584. (24) Wooster, T. T.; Longmire, M.; Zhang, H.; Watanabe, M.; Murray, R. W. Anal. Chem. 1992, 64, 1132. (25) Poupart, M. W.; Velazquez, C. S.; Hassett, K.; Porat, Z.; Haas, O.; Terrill, R. H.; Murray, R. W. J. Am. Chem. Soc. 1994, 116, 1165. (26) Hatazawa, T.; Terrill, R. H.; Murray, R. W. Anal. Chem. 1996, 68, 597. (27) Long, J. W.; Velazquez, C. S.; Murray, R. W. J. Phys. Chem. 1996, 100, 5492. (28) Williams, M. E.; Masui, H.; Long, J. W.; Malik, J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 1997. (29) AlNashef, I. M.; Leonard, M. L.; Kittle, M. C.; Matthews, M. A.; Weidner, J. W. Electrochem. Solid-State Lett. 2001, 4, D16. (30) AlNashef, I. M.; Leonard, M. L.; Matthews, M. A.; Weidner, J. W. Ind. Eng. Chem. Res. 2002, 41, 4475. (31) Giovanelli, D.; Buzzeo, M. C.; Lawrence, N. S.; Hardacre, C.; Seddon, K. R.; Compton, R. G. Talanta 2004, 62, 904. (32) Buzzeo, M. C.; Giovanelli, D.; Lawrence, N. S.; Hardacre, C.; Seddon, K. R.; Compton, R. G. Electroanalysis 2004, 16, 888. (33) Buzzeo, M.; Klymenko, O. V.; Wadhawan, J. D.; Hardacre, C.; Seddon, K. R.; Compton, R. G. J. Phys. Chem. B 2004, 108, 3947.
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sensor resembling the Clark model that employs an ionic liquid as the electrolyte, while Wang and co-workers35,36 have very recently reported a supported ionic liquid membrane-coated oxygen sensor, incorporating 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2MIM][BF4]) into a polyethylene membrane. The remarkably low volatility and ability of ionic liquids to endure temperatures upward of 180 °C would potentially allow a sensor to perform under conditions where conventional electrolytes would fail. The hygroscopic nature of RTILs, however, can complicate the interpretation of the voltammetric response since absorbed moisture decreases the viscosity, which in turn affects the diffusion coefficient and results in larger limiting currents.37-40 Purging the system with an inert gas or in vacuo is therefore necessary prior to experimental measurements to ensure removal of atmospheric moisture and other gaseous interferences, and this requirement would have to be considered carefully in sensor applications and/ or design. EXPERIMENTAL SECTION Chemical Reagents. The following RTILs were synthesized from the corresponding halide salt via a metathesis reaction in aqueous lithium, as described by Bonhoˆte et al.:37 1-Butyl-3(34) Cai, Q.; Xian, Y.; Li, H.; Yang-ming, T.; Jin, T. Huadong Shifan Daxue Xuebao 2001, 3, 57. (35) Wang, R.; Hoyano, S.; Ohsaka, T. Chem. Lett. 2004, 33, 6. (36) Wang, R.; Okajima, T.; Kitamura, F.; Ohsaka, T. Electroanalysis 2004, 16, 66. (37) Bonhoˆte, P. A.; D.; Papageorgiou, N.; Armand, M.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (38) Koch, V. R.; Nanjundiah, C.; Appetecchi, B.; Scrosati, B. J. Electrochem. Soc. 1995, 142, L116. (39) Schroder, U.; Wadhawan, J. D.; Compton, R. G.; Marken, F.; Suarez, P. A. Z.; Consorti, C. S.; de Souza, R. F.; Dupont, J. New J. Chem. 2000, 24, 1009. (40) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem, in press.
methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4MIM][N(Tf)2]), 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4DIM][N(Tf)2]), 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8MIM][N(Tf)2]), 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C10MIM][N(Tf)2]). and 1-pentyl-3-methylimidazolium trifluorotris(pentafluoroethyl)phosphate, ([C5MIM][[FAP]) were supplied by Merck KgaA. Ferrocene (Aldrich), tetrabutylammonium perchlorate (TBAP, Fluka), and acetonitrile (MeCN, Fisher Scientific) were used directly without further purification. Impurity-free oxygen and nitrogen (BOC, Guildford, Surrey, U.K.) were used for electrochemical experiments as described below. Instrumentation. A commercial potentiostat, PGSTAT 20 (Eco Chemie Utrecht, The Netherlands) was used for the electrochemical experiments in conjunction with a Pentium-based PC. The 5-µm-diameter Au microdisk working electrode (Cypress Systems) was carefully polished before each experiment using a 1.0-µm alumina slurry (Buehler, Lake Bluff, IL), followed by a 0.3µm alumina suspension (Buehler). The electrode was then polished on a clean, damp cloth (Microcloth, Buehler), immersed in 10% nitric acid solution to remove any adventitious adsorbates, and then rinsed with MeCN. The diameter of the microdisk electrode was calibrated electrochemically using 2 mM ferrocene in 0.1 M TBAP/MeCN, using a value for the diffusion coefficient of 2.3 × 10-9 m2 s-1 at 20 °C.41 A trimmed disposable micropipet tip was fitted to the end of the microdisk electrode to provide a cavity into which a small sample of ionic liquid (∼15 µL) was delivered. All experiments were carried out in an electrochemical cell designed to sustain vacuum conditions, similar to that reported previously.20,42 Prior to the addition of gases, the complete cell arrangement was placed under vacuum (Edwards high vacuum pump, model ES 50) until the baseline showed no trace of atmospheric oxygen (typically ∼60 min). Oxygen and nitrogen were introduced to the electrochemical cell in desired ratios using a Wo¨sthoff triple gasmixing pump (Bochum), accurate to (1%. Gas mixtures were passed through the cell for at least 30 min before final measurements were taken to ensure that equilibration of the gas and the ionic liquid had been established.
[N(Tf)2], [C8MIM][N(Tf)2], [C5MIM][FAP]), F is the Faraday constant (96 485 C mol-1), r the radius of the electrode, D the diffusion coefficient, c the concentration of the electroactive species, and t the time. The dimensionless parameter τ* is defined as τ* ) 4Dt/r2 and
THEORY Analysis of Chronoamperometric Transients. To determine the concentration and the diffusion coefficient of oxygen in the ionic liquids, chronoamperometric experiments were performed at the Au microdisk electrode. Theoretical transients were then calculated using a nonlinear curve fitting program available in Origin 6.0 (Microcal Software, Inc.) according to the Shoup and Szabo expression, whereby the current-time curve following a potential step at a microdisk is given by43
where K1 and K2 are modified Bessel functions of the zero and first order, respectively, and xr is a dimensionless radius, defined as
ilim ) 4nFrDcf(τ*)
ilim ) 4nFrDc
(1)
where n represents the number of electrons transferred (n ) 1 for [C10MIM][N(Tf)2], [C4DMIM][N(Tf)2]; n ) 2 for [C4MIM](41) Sharp, P. Electrochim. Acta 1983, 28, 301. (42) Evans, R. G.; Klymenko, O. V.; Saddoughi, S. A.; Hardacre, C.; Compton, R. G. J. Phys. Chem. B 2004, 108, 7878. (43) Shoup, D.; Szabo, A. J. Electroanal. Chem. 1982, 140, 237.
-1/2
f(τ*) ) 0.7854 + 0.8862τ*-1/2 + 0.2416e-0.7823τ*
(2)
Having specified the radius of the microdisk, the computer software optimized the fit between the experimental and theoretical transients by varying the values of the diffusion coefficient and concentration of the electroactive species. Calculation of Transport-Limited Currents of Gaseous Analyte at Different Cell Designs. Model B1. A one-dimensional diffusion model can be used to describe transport of the electroactive species through a membrane and electrolyte layer to a working macroelectrode. The following equation was used to calculate the limiting current5
ilim )
nFπr2ps de/Pe + dm/Pm
(3)
where ilim is the transport-limited current, ps is the partial pressure of oxygen in the sample, de and dm are the thicknesses of the electrolyte and membrane layers, and Pe and Pm are the permeabilities of oxygen through the electrolyte and membrane. The permeability is defined as P ) RD, where R is the solubility of the analyte. Model B2. The following two-dimensional expression is used to approximately describe the steady-state behavior observed at a membrane-covered microelectrode, which accounts for both axial diffusion of the analyte through the membrane and radial diffusion through the electrolyte layer.3
ilim ) nFπr2
[
]
Pm 2 K1(xr) ps 1 + dm xr K2(xr)
xr ) r[(Pm/Pe)dmde]1/2
(4)
(5)
Models A2 and A1. The steady-state current observed at a microdisk electrode can be expressed as
(6)
Calculation of the Time Response to a Step Change in Analyte Concentration. We now consider the time it takes for an electrode to reach steady state following a step change in oxygen concentration at one face of the membrane (or electrolyte, in the case of model A1). Transport via linear diffusion within the membrane is assumed to be rate-determining for model B1, and Analytical Chemistry, Vol. 76, No. 15, August 1, 2004
4585
Figure 2. Theoretical plot of f(τ) vs log τ, where τ ) Dt/d2. See text for definition of f(τ).
the effect of the electrolyte layer is not considered, whereas the electrolyte layer limits the response time for the membraneindependent models A2 and A1. The time response for model B2 has not been addressed here. The time-dependent current observed following such a step change is solved for the one-dimensional case and can be expressed by the following equation:44
f(τ) )
i(t) - i0 i∞ - i 0
∞
)1+2
∑ (-1)
m
exp(-m2π2τ)
(7)
m)1
where
τ ) Dt/d2
(8)
and i(t) is the current observed at time t following the step change, i0 is the current immediately before the step change, i∞ is the steady-state current, and D is the diffusion of gaseous analyte through the membrane, electrolyte, or ionic liquid for models B1, A2, and A1, respectively. Similarly, d is the thickness of the membrane, electrolyte, or ionic liquid layer. As the response time is proportional to the square of this thickness, the value of d dictates the length of the electrode’s response. A plot of f(τ) versus log τ was generated (see Figure 2), and values of τ were found to be 0.30 and 0.37 at 90 and 95% of the theoretical curve, (44) Hitchman, M. L. Measurement of Dissolved Oxygen; Wiley: Chichester, 1978.
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respectively. The time responses of the various sensors were then calculated assuming these values for τ. RESULTS AND DISCUSSION Determination of Diffusion Coefficients and Concentrations of Oxygen in Room Temperature Ionic Liquids. As described in the theoretical section, the concentration and the diffusion coefficient of oxygen in five imidazolium-based ionic liquids were determined via the analysis of chronoamperometric experiments performed at the Au microdisk electrode. Transients were recorded for five oxygen concentrations (20, 40, 60, 80, and 100 vol %). The potential was first held for 120 s at 0.0 V (vs Ag), a point corresponding to the passage of no Faradaic current, and then stepped to a potential corresponding to the steady-state region of the one- or two-electron reduction of oxygen (see Analysis of Chronoamperometric Transients). Following a method previously reported, the transient for the lowest concentration was subtracted from all the others to eliminate any contribution to the signal due to background current and trace impurities that could not be removed by purging.42 These corrected data were then fitted according to the Shoup and Szabo expression to determine values of D and c for oxygen in the five RTILs under study.43 In each case, the diffusion of oxygen was found to be an order of magnitude lower than in traditional aprotic media, as expected due to the higher viscosity of ionic liquids, and the determined concentrations were directly proportional to the volume of gas introduced. The averaged results are listed in Table 1, while Table 2 shows previously determined values for the
Table 1. Diffusion Coefficient and Solubility Values of Oxygen in RTILs
Table 3. Comparison of Limiting Currents
RTIL
D/10-10 m2 s-1
c/mM
model
electrolyte
radius/ µm
membrane thickness/µm
limiting current/nA
[C2MIM][N(Tf)2] [C4DIM][N(Tf)2] [C8MIM][N(Tf)2] [C10MIM][N(Tf)2] [C5MIM][FAP]
2.5 2.1 2.8 2.5 3.5
10.8 ( 0.6 7.2 ( 0.8 8.3 ( 0.8 13.0 ( 0.9 9.7 ( 0.2
B1 B1 B2 B2 A2 A2 A1 A1 A1
H2O DMSO H2O DMSO H2O DMSO [C2MIM][N(Tf)2] [C10MIM][N(Tf)2] [N6222][N(Tf)2]
500 500 5 5 2 2 2 2 2
5 5 5 5 n/aa n/a n/a n/a n/a
930 240 2.8 1.2 6.4 3.4 2.2 2.5 1.3
Table 2 solventa
D/10-10 m2 s-1
c/mM
(a) Previously Reported Diffusion Coefficient and Solubility Values of Oxygen H2O 16 1.3 DMSO 21 2.1 DMF 47.5 4.8 MeCN 110 8.1 [C2MIM][N(Tf)2] 7.3 3.9 [N6222][N(Tf)2] 1.5 11.6 [C4MIM][BF4] 7.0 [C4MIM][PF6] 2.2 3.6 [Py14][N(Tf)2] 5.2b 6.1b 7.5b 6.0b [P14,666][N(Tf)2] [P14,666] [FAP] 6.1b 7.8b
ref
45 46, 47 46, 47 46, 47 20 20 49 29 42 42 42
(b) Previousyly Reported Diffusion Coefficient and Solubility Values of Carbon Dioxide H2O 24 33 51 DMSO 10 125 53 DMF 35.6 198 47, 54 MeCN 383 280 47, 54 [N6222][N(Tf)2] 2.3 55 33 [C4MIM][BF4] 30 49 [C4MIM][PF6] 90.2 55 Py14 ) N-methyl-N-butylpyrrolidinium. P14,666 ) tris(n-hexyl)tetradecylphosphonium. b Measurements taken at 35 °C. a
diffusion coefficients and solubility of oxygen and carbon dioxide in various other solvents for comparison.20,29,33,42,45-55 Transport-Limited Currents. A comparison of the obtainable limiting currents for the reduction of oxygen in each of the four sensor models incorporating various membrane thicknesses (1, 5, and 10 µm) and electrolytes (H2O, DMSO, [C2MIM][N(Tf)2], [C10MIM][N(Tf)2], [N6222][N(Tf)2]) is shown in Table 3. The following assumptions were made for models B1 and B2: Pe ) 2.7 × 10-9 m2 s-1 atm-1, Pm ) 8.0 × 10-11 m2 s-1 atm-1, de ) 5 µm, and ps ) 0.2 atm.45 For the case of B1, linear diffusion of oxygen through the membrane and electrolyte layers to the (45) Evans, N. T. S.; Quinton, T. H. Respir. Physiol. 1978, 35, 89. (46) Sawyer, D. T.; Chiericato, G.; Angells, C. T.; Nanni, E.; Tsuchlya, T. Anal. Chem. 1982, 54, 1720. (47) Wadhawan, J. D.; Welford, P. J.; McPeak, H. B.; Hahn, C. E. W.; Compton, R. G. Sens. Actuators, B 2003, 88, 40. (48) Wadhawan, J. D.; Welford, P. J.; Maisonhaute, E.; Climent, V.; Lawrence, N. S.; Compton, R. G.; McPeak, H.; Hahn, C. E. W. J. Phys. Chem. B 2001, 105, 10659. (49) Husson-Borg, P.; Majer, V.; Gomes, M. F. C. J. Chem. Eng. Data 2003. (50) Roberts, J. L., Jr.; Sawyer, D. T. J. Electroanal. Chem. 1966, 12, 90. (51) Gertz, K. H.; Loeschcke, H. H. Z. Naturforsch. 1956, 11b, 61. (52) Welford, P. J.; Brookes, B. A.; Wadhawan, J. D.; McPeak, H. B.; Hahn, C. E. W.; Compton, R. G. J. Phys. Chem. B 2001, 105, 5253. (53) Gennaro, A.; Isse, A. A.; Viannello, E. J. Electroanal. Chem. 1990. (54) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2002, 106, 7315. (55) Kamps, A. P.; Tuma, D.; Xia, J.; Maurer, G. J. Chem. Eng. Data 2003, 48, 746.
a
n/a, not available.
Table 4. Comparison of Time Responses
model B2 B2 B2
model A2 A2
model A1 A1 A1
(a) Membrane-Covered Sensors membrane thickness/µm t90/s 1 5 10
0.026 0.66 2.65
(b) Membrane-Independent Sensors electrolyte t90/s H2O DMSO
0.0047 0.0036
(c) Membrane-Free Sensors electrolyte t90/s [C2MIM][N(Tf)2] [C10MIM][N(Tf)2] [N6222][N(Tf)2]
0.010 0.030 0.051
t95/s 0.03 0.81 3.26
t95/s 0.0058 0.0044
t95/s 0.013 0.037 0.063
electrode surface is assumed and transport through the membrane is considered to be rate determining. A two-dimensional hemispherical model is used to describe the current observed for model B2; diffusion of analyte through the membrane is axial, while diffusion to the electrode surface through the electrolyte is radial. For models A2 and A1, diffusion of oxygen to the electrode is radial and transport through the electrolyte is rate determining. As the current is a function of electrode area, those currents calculated for a membrane-covered macroelectrode model, B1, are at least an order of magnitude greater than those achieved at the microelectrode sensors. While model A1 does not require use of a membrane, the intrinsically high viscosity of the ionic liquid medium lowers the diffusion coefficient and thus the attainable current. Consequently, although oxygen is more soluble in the ionic liquids as compared to H2O and DMSO, the slower diffusion of analyte through the more viscous media results in limiting currents comparable to those observed in models A2 and B2.20,47,48 Time Response to a Step Change in Analyte Concentration. Table 4 reports the time responses to a step change in oxygen concentration, which depend on the properties of the membrane for model B1 and the nature of the electrolyte for models A2 and A1. For model B1, transport through the membrane is rate-determining; a value of 1.14 × 10-11 m2 s-1 was assumed for the diffusion of oxygen through the PTFE and the membrane thickness was varied between 1 and 10 µm.45 In the case of model Analytical Chemistry, Vol. 76, No. 15, August 1, 2004
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for the diffusion of O2 in H2O and DMSO, respectively.44,47 Finally, for model A1, the time response was evaluated for three different RTILs, [C2MIM][N(Tf)2], [C10MIM]][N(Tf)2], and [N6222][N(Tf)2], which vary significantly in their viscosity as is reflected in the diffusion coefficients of oxygen: 7.3 × 10-10, 2.5 × 10-10, and 1.48 × 10-10 m2 s-1, respectively.20 For all calculations, the electrolyte layer thickness was held constant at 5 µm. A theoretical plot of f(τ) versus log τ (see Figure 2) allowed values of τ to be deduced (where τ ) Dt/d2), which were then used to calculate the time responses of each of the sensors. The values listed in Table 4 represent the time required for the sensors to achieve 90 (t90) and 95% (t95) of the steady-state current following a step change in analyte concentration. When the membrane employed in the macroelectrode design (B1) is thicker than 1 µm, the time response increases greatly as it is proportional to the square of the membrane layer. Typically, the membrane employed in such sensors is between 5 and 20 µm thick. Figure 3 shows the dependence of the response time on the thickness of the membrane or electrolyte, according to sensor model. The slow diffusion of analyte in the more viscous ionic liquids significantly lengthens the response time of model A1 as compared A2; however, for many applications a response time of ∼1 s is acceptable. Still, model A2 proves to have the greatest design advantage over the other three models, reaching steady state on a practical time scale (de ) 5 µm; t95) 4.4 ms in DMSO) and generating membrane-independent, steady-state signals. CONCLUSIONS The transport-limited currents and response times of four different types of amperometric gas sensor, which vary in electrode dimension, electrolyte composition, and choice of membrane, have been compared. While the low volatility of RTILs excludes the need for a gas-permeable membrane in the sensor design (A1), the lower diffusion coefficients of the gaseous analyte in such media lengthen the response time and limit the attainable steady-state currents, thus preventing any significant advantage over the previously reported membrane-independent microelectrode model (A2). The negligible vapor pressure and robust thermal stability of RTILs, however, does render them practical for incorporation into gas sensors operating under more extreme conditions, i.e., high temperature and pressure, such as in the combustion industry where traditional solvents struggle to remain chemically or physically unchanged. ACKNOWLEDGMENT
Figure 3. Time responses (t90 and t95) of the gas sensors as a function of membrane (m) or electrolyte layer (e) thickness: (A) model B1, e ) H2O, (B) model A2, e ) DMSO, (C) model A1, e ) [C2MIM][N(Tf)2], (D) model A1, e ) [C10MIM][N(Tf)2], and (E) model A1, e ) [N6222][N(Tf)2].
A2, the time response reflects diffusion through the electrolyte later and values of 1.6 × 10-9 and 2.1 × 10-9 m2 s-1 were assumed 4588 Analytical Chemistry, Vol. 76, No. 15, August 1, 2004
M.C.B. thanks the Analytical Division of The Royal Society of Chemistry for a studentship and Alphasense for CASE funding. The authors also thank Oleksiy Klymenko for assistance with the figures and Russell G. Evans for useful discussions.
Received for review March 3, 2004. Accepted April 30, 2004. AC040042W
