Anal. Chem. 1997, 69, 558-562
Amperometric Sensor for Monitoring Ethylene Larry R. Jordan† and Peter C. Hauser*,†
Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand George A. Dawson
Industrial Research Limited, P.O. Box 2225, Parnell, Auckland, New Zealand
Sensing electrodes were fabricated by chemical deposition of gold onto a Nafion membrane. These electrodes, with suitable choice of real surface area, applied potential, supporting electrolyte, and gas flow rate, showed a detection limit of 40 ppb ethylene for a signal-to-noise ratio of 3. A linear response was observed up to at least 500 ppm. The choice of flow rate was especially influential as it was found to affect the mechanism of the sensor response, which is kinetically controlled even at relatively low flow rates. No cross-sensitivity was found to ethanol and acetaldehyde which might be present in certain applications of the sensor, such as in horticulture.
A sensor for monitoring ethylene in produce stores is desired because of the important role this gas plays in the postharvest physiology of horticultural produce.1 Ethylene is given off by produce and acts as a ripening hormone. The influence of ethylene depends on the product, but concentrations as low as several tens of parts per billion will cause a response in some fruit.2 Hence, in storage facilities, the level of ethylene should be kept low by either venting2 or using catalytic combustion methods to scrub ethylene.3,4 However, since produce is stored under refrigeration, this is energy consuming and therefore costly. The situation is even more difficult when controlled atmospheres with elevated carbon dioxide and humidity and reduced oxygen levels are employed. Monitoring ethylene will allow a more economical control of ethylene levels in produce storage facilities and lead, one hopes, to better quality produce at the market. Gas chromatography (GC) is still the most selective and sensitive method used routinely for ethylene determination at biologically active concentrations.1 An automated microprocessor GC system has been developed using an alumina column and a flame ionization detector for the monitoring of fruit and vegetables in storage.5 Other GC detectors for ethylene, including tin oxide semiconductor, photoionization, and photoacoustic devices, have been described.1 Even though gas chromatography can be automated, its use for the routine monitoring of ethylene in
produce storage seems limited due to its complexity, high power dissipation, and requirement for compressed gases, with resulting low portability. A chemiluminescence method for the monitoring of ethylene has been described6 in which ethylene is reacted with ozone to produce a chemiluminescent signal. Although the detection limit of such a chemiluminescence device covers the low-ppb to ppm ethylene range of interest, it is not suitable for all applications. A sensor based on a platinum thin metal oxide semiconductor field effect transistor (Pt TMOSFET) was found to have a detection limit of ∼1 ppm ethylene.7 However, the sensor was almost twice as sensitive to ammonia and ethanol compared to ethylene and responded strongly to acetic acid, acetone, and ethyl acetate. The response to alcohol would be a particular problem since ethanol can be produced by fruit in storage.1 The oxidation of ethylene on Pt,8,9 Au,10,11 and Pd12 metal electrodes in aqueous acid electrolyte has been reported in the literature. The oxidation of ethylene on a Pt electrode in ethylenesaturated 0.5 M H2SO4 produces CO2 as a reaction product. In contrast to platinum, gold is inert to the chemisorption of ethylene, resulting in partial oxidation to acetaldehyde.10 A residual current due to corrosion is observed on Pd electrodes,13 making it not ideal as a catalyst for a chemical sensor. The deposition of precious metals on solid polymer electrolytes (SPE) by chemical deposition allows the construction of efficient electrodes for the oxidation or reduction of gases directly in the vapor phase. Such electrochemical cells have for example been demonstrated for the reduction or oxidation of alcohol vapors,14,15 carbon dioxide,16,17 and carbon monoxide.18 The use of SPE bonded electrodes for amperometric sensors has resulted in a number of catalyst and membrane config-
† Present address: Department of Chemistry, The University of Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland. (1) Abeles, F. B.; Morgan, P. W.; Salviet, M. E. Ethylene in Plant Biology, 2nd ed.; Acadamic Press: San Diego, CA, 1992. (2) McDonald, B.; Snowball, D. New Zealand Department of Scientific and Industrial Research, Auckland Industrial Development Division, Publication No. G.143, 1982. (3) McDonald, B.; Harman, J. E. Sci. Hortic. 1982, 17, 113-23. (4) Eastwell, K. C. Plant Physiol. 1978, 62, 723-6. (5) Inaba, A.; Kubo, Y.; Nakamura, R. J. Jpn. Soc. Hortic. Sci. 1989, 58, 4438.
(6) Quickert, N.; Findlay, W. J.; Monkman, J. L. Sci. Total Environ. 1975, 3, 323-8. (7) Winquist, F.; Lundstro ¨m, I. Anal. Chim. Acta 1990, 231, 93-100. (8) Triaca, W. E.; Rabockai, T.; Arvia, A. J. J. Electroanal. Chem. 1979, 126, 218-26. (9) Piovano, S. M.; Chialvo, A. C.; Triaca, W. E.; Arvia, A. J. J. Appl. Electrochem. 1987, 17, 147-55. (10) Pastor, E.; Schmidt, V. M. J. Electroanal. Chem. 1995, 383, 175-80. (11) Schmidt, V. M.; Pastor, E. J. Electroanal. Chem. 1994, 376, 65-72. (12) Otsuka, K.; Shimizu, Y.; Yamanaka, I. J. Electrochem. Soc. 1990, 137, 207681. (13) Dahms, H.; Bockris, J. O. M. J. Electrochem. Soc. 1964, 111, 728-36. (14) Enea, O. J. Electroanal. Chem. 1987, 235, 393-401. (15) Liu, R.; Fedkiw, P. S. J. Electrochem. Soc. 1992, 139, 3514-23. (16) Cook, R. L.; MacDuff, R. C.; Sammells, A. F. J. Electrochem. Soc. 1990, 137, 187-9. (17) Komatsu, S.; Tanaka, M.; Okumura, A.; Kungi, A. Electrochim. Acta 1995, 40, 745-53. (18) Kita, H.; Nakajima, H. Electrochim. Acta 1986, 31, 193-200.
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urations.19-24 To maximize the current and hence the sensitivity of an amperometric sensor, particular consideration must be given to the real surface area of the electrode (the surface roughness) and the diffusion rate of analyte to the electrode. Sensors for ozone23 and hydrogen sulfide24 with the noble metal catalyst vapor deposited directly onto the surface of the Nafion membrane have been described. The remarkable sensitivities achieved have been attributed to the direct access of analyte gas to the reaction interface due to the avoidance of any membrane permeation step.23,24 The fabrication of gold-Nafion electrodes is not as common as platinum-Nafion electrodes, which are well documented in the literature due to the importance of platinum-catalyzed electrochemical reactions for applications such as fuel cells25 and water electrolyzers.26 Previous gold-Nafion sensor electrodes have utilized vapor deposition techniques.23 The efficiencies of these electrodes, however, is expected to be low compared to electrodes formed by chemical deposition as the gold is in less intimate contact with the Nafion. A noble metal-Nafion configuration is described in this paper which is suitable for the amperometric monitoring of ethylene in produce storage atmospheres with regard to the high sensitivity and selectivity required. EXPERIMENTAL SECTION Platinum Electrode. For the fabrication of platinum-Nafion electrodes, the impregnation-reduction procedure described in the literature27,28 was used. The first step utilizes the cation exchange properties of the Nafion membrane to impregnate the membrane with a cationic platinum salt [tetraammineplatinum(II) chloride]. Next, the platinum salt is reduced to platinum metal using sodium borohydride as the reducing agent. Being an anionic compound, BH4- has limited penetration of the Nafion and therefore the platinum salt is reduced close to the Nafionreducing solution interface, producing a thin, porous catalyst layer which is in intimate contact with the Nafion but also exposed to the surface allowing contact to a current collector. Gold Electrode. Two chemical deposition methods were investigated. The first procedure was an impregnation-reduction method analogous to the one used for platinum deposition using a cationic gold salt [tetraamminegold(III) nitrate], which was synthesized29 from tetrachloroauric acid. The second method has been described previously.16 In this procedure, tetrachloroauric acid, which is in contact with only one side of the membrane, is reduced to gold metal using a reducing agent that diffuses through from the opposite face. The impregnation-reduction method produced gold deposits which were unfortunately not localized near the Nafion surface (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)
Tierney, M. J.; Kim, H.-O. L. Anal. Chem. 1993, 65, 3435-40. Yan, H.; Lu, J. Sens. Actuators B 1989, 19, 33-40. Yan, H.; Liu, C.-C. Sens. Actuators B 1994, 17, 165-8. Otagawa, T.; Madou, M.; Wing, S.; Rich-Alexander, J.; Kusanagi, S.; Fujioka, T.; Yasuda, A. Sens. Actuators B 1990, 1, 319-25. Schiavon, G.; Zotti, G.; Bontempelli, G.; Farnia, G.; Sandona, G. Anal. Chem. 1990, 62, 293-8. Schiavon, G.; Zotti, G.; Toniolo, R.; Bontempelli, G. Anal. Chem. 1995, 67, 318-23. Paik, W.-K.; Springer, T. E.; Srinivason, S. J. Electrochem. Soc. 1989, 136, 644-9. Millet, P.; Alleau, T.; Durand, R. J. Appl. Electrochem. 1993, 23, 322-31. Millet, P.; Durand, R.; Dartyge, E.; Tourillon, G.; Fontaine, A. J. Electrochem. Soc. 1993, 140, 1373-80. Liu, R.; Her, W.-H.; Fredkiw, P. S. J. Electrochem. Soc. 1992, 139, 15-23. Skibsted, L. H.; Bjerrum, J. Acta Chem. Scand. 1974, 28, 740-6.
Figure 1. Schematic diagram of the electrochemical cell and gas flow fitting; (WE) working or sensing electrode; (RE) reference electrode; (CE) counter electrode.
as revealed in transmission electron micrographs of cross sections. This is possibly due to the kinetics of the gold(III) reduction by borohydride being too slow. Such an effect was reported to be the case for Ir-Nafion electrodes prepared by the reduction of [Ir(H2O)6]3+ incorporated into a Nafion membrane.26 The goldNafion electrodes prepared by the impregnation-reduction procedure did not produce high currents in cyclic voltammetry experiments when exposed to ethylene gas and so were not suitable as sensing electrodes. The second method in which HAuCl4 was reduced resulted in gold deposits on the surface of the Nafion. The real surface areas of the Au-Nafion electrodes were determined from the charge due to the reduction of the oxide monolayer in the cathodic sweep of the cyclic voltammogram based on a reported charge density18 of 420 µC/cm2. Electrode areas of up to 460 cm2 were obtained. The geometric surface area of the electrode was 0.79 cm2, giving a roughness factor of up to 580, indicating high real surface area gold electrodes comparable to those reported for platinum electrodes which have been chemically deposited in Nafion membranes.28 Electrode Assembly. The electrode arrangement is shown schematically in Figure 1. The electrochemical cell consists of a Plexiglas cell which contains the 0.5 M H2SO4 electrolyte and the electrodes. The cell enables contact of the metal-deposited side of the sensing electrode (the metal-Nafion electrode) with a flowing gas stream and the back side with the electrolyte. The exposed geometric working electrode area to the gas stream was 0.79 cm2. The sensing electrode and a gold or platinum foil current collector with a 1 cm diameter hole cut to expose the Nafion electrode to the gas stream were fixed in place between the cell and a gas manifold and the assembly secured in place with M3 stainless steel screws. The reference electrode used was a mercury/mercurous sulfate electrode (MSE; REF-601, Radiometer Analytical, Lyon, France) with a 0.5 M H2SO4 filling solution. All potentials reported here have been converted to the standard hydrogen scale (SHE) by adding 640 mV. Wire of the same metal as the sensing electrode was used as the counter electrode, 99.99% purity (Advent Research Materials Ltd., Suffolk, U.K.). Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
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Chemicals and Instrumentation. A BAS-100B/W potentiostat (Bioanalytical Systems, West Lafayette, IN) was used for applying fixed potentials and performing voltammetric experiments. The sampled current voltammetry technique was used for some initial experiments instead of cyclic voltammetry as, with suitably long step times, it discriminates against the oxidation of ethylene adsorbed at a lower potential on the electrode surface. This was found to be especially important in the case of ethylene reactions on platinum where ethylene can be adsorbed, but not oxidized, in the “double-layer only” region. The resulting voltammogram therefore shows only the ethylene reactions expected for fixed potential amperometry. Mass flow controllers type 1159B (MKS Instruments Inc., Andover, MA) were used to provide gas mixtures of known concentration by blending of a calibration gas mixture and pure nitrogen or air (BOC, Auckland, New Zealand). The mass flow controllers were connected to the gas cylinders via 1/4 in. outer diameter Nylon tubing using 1/4 in. Swagelok fittings. Swagelok T piece was used. to mix the gases from the mass flow controllers. An adapter from 1/4 in. Swagelok to 1/4 in. × 28 fittings (built in-house) was used for connection to the sensor via Teflon tubing with a 1 mm internal diameter (Alltech, Auckland, New Zealand). RESULTS AND DISCUSSION Reaction of Ethylene at SPE Electrodes. (1) Platinum. In Figure 2 sampled current voltammograms for ethylene on PtSPE and Au-SPE electrodes are shown. As evidenced in Figure 2a, oxidation of ethylene could not be observed on platinum. It is thought that the adsorption of ethylene on platinum is hindered by the formation of an oxide layer in the potential region where its oxidation would occur at room temperature. Therefore it was not possible to use oxidation on platinum in an amperometric method. The electrochemical reduction process seen at low potentials in Figure 2a can also not be exploited for amperometric sensing in real atmospheres since oxygen reduction occurs in the same potential range. (2) Gold. As opposed to platinum, the oxidation of ethylene on gold occurs prior to oxide layer formation since the potential for gold oxide monolayer formation is ∼400 mV more anodic than for the corresponding platinum oxides.30,31 This makes amperometric detection possible as evidenced by the anodic current in the presence of ethylene in the sampled current voltammogram displayed in Figure 2b. It is also shown in Figure 2b that the high overpotential required for the oxygen reduction reaction on gold23 enables the detection of ethylene in the presence of oxygen over a wide potential range. The decrease in the ethylene oxidation current seen in Figure 2b above ∼+1.35 V coincides with the gold oxide formation which is seen in the anodic sweep of the cyclic voltammogram of Figure 3. The potential just below gold oxide monolayer formation is therefore the highest potential suitable for monitoring ethylene. When the 0.5 M H2SO4 electrolyte was replaced with 0.5 M K2SO4 at pH 5 the oxidation of ethylene was inhibited. This is consistent with the findings of Dahms and Bockris,13 who reported that the potential at which ethylene oxidation on gold commences is not dependent on pH. Due to the negative shift in potential for (30) Bard, A. J. Encyclopedia of Electrochemistry of the Elements; Marcel Dekker Inc.: New York, 1975; Vol. IV. (31) Bard, A. J. Encyclopedia of Electrochemistry of the Elements; Marcel Dekker Inc.: New York, 1976; Vol. VI.
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Figure 2. Sampled current voltammogram of (a) platinum-Nafion electrode and (b) gold-Nafion electrode: sweep rate 5 mV/s; hold time 8 s; sample time 50 ms; electrolyte 0.5 M H2SO4; gas flow rate 100 cm3/min. Nitrogen, ]; air, +; ethylene (100%), 0
the gold oxide layer formation by ∼300 mV (60 mV per pH unit)30 but no corresponding cathodic shift in the potential at which ethylene is oxidized, the available overpotential at pH 5 is expected to be very small, resulting in low observed currents. Lowering the pH by using more concentrated sulfuric acid would move the formation of the gold oxide layer in the anodic direction by 60 mV per unit pH,30 thereby increasing the available overpotential that can be applied to the sensing electrode before gold oxide formation commences. However, increasing the H2SO4 concentration from 0.5 to 5 M would shift the oxide formation by only 60 mV. Use of concentrated acid was therefore not investigated in this work. Sensor Performance. The dynamic response of the goldNafion amperometric sensor to ppm levels of ethylene is shown in Figure 4. A potential of +1.25 V, which is close to the highest potential at which ethylene is oxidized, was applied to the sensing electrode. The gas side of the sensing electrode was exposed to a gas stream with a constant flow of 100 cm3/min, and on the reverse side a 0.5 M H2SO4 electrolyte was used to contact the membrane. A detection limit of 40 ppb is obtainable based on a signal-to-noise ratio of 3. The relative standard deviation between measurements was found to be 0.5% as determined by taking five
Figure 3. Cyclic voltammogram of gold-Nafion electrode with nitrogen flow on gas side: sweep rate 50 mV/s; electrolyte 0.5 M H2SO4.
Figure 5. Response of sensor to 10 ppm ethylene as a function of flow rate (0-100 cm3/min) and applied potential. Real electrode surface area 245 cm2.
Figure 4. Dynamic response of sensor to ethylene (0-10 ppm): gas flow 100 cm3/min-1; real electrode surface area 245 cm2; applied potential +1.25 V vs SHE.
consecutive measurements of a 10 ppm ethylene standard over a 0.5 h period. The response was found to be linear up to at least 500 ppmsalthough at this concentration the signal was found to decay rapidly but to recover when the cell was flushed with nitrogen or air. A similar phenomenon has been reported for electrochemical fuel cell sensors,32 where the probable cause has been reported to be the slow removal of the polar reaction products, acetaldehyde and acetic acid, which can hydrogen bond to the electrode surface. It is most likely, since the partial oxidation of ethylene on gold produces acetaldehyde, that the slow removal of this reaction product is the cause of the signal decay. In the application of produce storage the ethylene concentration of interest is normally below several ppm. However, if high concentrations of ethylene were being measured, it may be possible to avoid loss of sensitivity, due to buildup of the reaction
product, by simply reducing the overpotential at which ethylene is measured to a value where product accumulation does not become rate limiting. The sensor response to 10 ppm ethylene showed no shortterm loss of sensitivity. After several days of continuous operation a decrease in sensitivity was noted which could, however, be reversed by applying a brief anodic cleaning potential to the sensing electrode. Room temperature fluctuations appeared to affect the sensitivity of the sensor but by not worse than a 2% change in signal per degree C. Effect of Real Electrode Surface Area. The Faradaic signal due to oxidation of ethylene was found to increase with the real surface area of the electrode. However, at higher surface areas the increase in current was limitedsprobably due to mass transport of gas through the pores of the gold deposit limiting the current. The background noise also increased with increasing electrode surface area but at a rate less than that of the signal. The best detection limit was found for a surface area of ∼200 cm2. It is expected that high surface area electrodes are not necessarily ideal for all gas sensing applications since, if the kinetic reaction rate is fast enough, the sensitivity should be diffusion limited and dependent only on the geometric area of the electrode. High surface area electrodes are therefore most useful when the kinetic reaction rate is slow, such as in the oxidation of hydrocarbons.33 In the case of ethylene oxidation on gold, the reaction kinetics are relatively slow according to thermodynamic calculations13 and also the overpotential that can be applied is limited by the gold oxide monolayer formation. Influence of Flow Rate. The flow rate dependence of the sensor response to 10 ppm ethylene is shown in Figure 5. In this experiment, fixed potentials between +0.95 and +1.25 V were applied and for each potential, after a stable residual current had been attained, the flow rate of the ethylene standard was systematically varied. The sensor response was found to be highly flow rate dependent at low flow but to be relatively constant at higher flow rates. The effect is most noticeable at lower applied
(32) Bahari, S. M.; Criddle, W. J.; Thomas, J. D. R. Analyst 1992, 117, 701-6.
(33) Chang, S. C.; Stetter, J. R.; Cha, C. S. Talanta 1993, 40, 461-77.
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Table 1. Response of Sensor to Other Gases Relative to That of Ethylene gas measured
rel sensor response
gas measured
rel sensor response
acetylene ethanol acetaldehyde CO
7.4 0 0 0
NO NO2 SO2 ethylene
1.8 0.3 2.5 1.0
potentials. At high overpotential and low flow rates, the current becomes independent of potential. These findings are thought to be due to a change in response mechanism: At high flow rate, the stagnant air boundary layer at the gas-electrode interface is reduced to an extent that the reaction is limited by the kinetic reaction rate even for the highest overpotential possible. When the flow is reduced, the stagnant boundary layer is increased and a diffusion-limited contribution to the response is seen at higher overpotentials. A principle advantage of a diffusion-limited response is that the signal is relatively independent of the applied potential so that accurate control of the potential is not required.33 However, careful control of the flow rate is required and the sensitivity of the sensor may be compromised. The kinetically limited response achieved at higher flow rates is independent of flow although in this case accurate control of the applied potential is necessary. Kinetic control of the electrode process is not expected at such low flow rates in gas diffusion electrodes employing Teflon-bound noble metal catalysts where the Teflon membrane acts as a barrier to mass transport.34 The lower rate of diffusion observed for such gas electrodes would limit the obtainable sensitivity, and therefore, they may not be suitable for the monitoring of low levels of ethylene. Selectivity. The sensor response to some other electroactive compounds relative to the response of ethylene is shown in Table 1. The cross-sensitivity to acetaldehyde (100 ppm) and ethanol (100 ppm) was tested because these species are potential interferants as they can occur in fruit storage facilities at relatively high concentrations1 and are electroactive compounds.33 Both of these compounds showed no response, which is consistent with the reported selectivity of gold catalysts to only unsaturated organic compounds.11 Acetylene (10 ppm) was investigated due to its reported reactivity on gold in the same potential region (34) Bay, H. W.; Blurton, K. F.; Sedlak, J. M.; Valentine, A. M. Anal. Chem. 1974, 46, 1837-9.
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where ethylene oxidizes.11 The oxidation current for acetylene was found to be ∼7 times that for ethylene, which is consistent with its complete oxidation to CO2. However, this species is not expected to be present in fruit stores. The reducing combustion products SO2, NO2, and NO showed a significant response although they too are not expected to be present in a storage atmosphere. Unlike ethylene SO2, NO2 and NO were found to be reactive above the gold oxide monolayer formation potential, indicating the possibility of determining these interferants and ethylene by difference. Oxidising gases such as ozone and chlorine are not expected to interfere due to the positive potential applied to the electrode. No cross-sensitivity was seen to CO (100 ppm). CONCLUSION Gold has been shown to be a suitable noble metal catalyst for the sensing electrode of an amperometric ethylene sensor. This is in a way unexpected as gold is a relatively poor catalyst compared with platinum and only 2 electrons are produced per ethylene molecule on gold compared with 12 produced during complete oxidation on platinum. However, because gold is a poor catalyst for organic compounds in general and only reacts with unsaturated organic compounds, which can weakly (in the case of ethylene) and strongly (in the case of acetylene) interact with the gold surface via double and triple bonds, good selectivity is obtained. Even if platinum could oxidize ethylene at room temperature, compounds such as ethanol, which are well-known to react on platinum,15,33 would be potential interferants in a platinum amperometric sensor. High real surface electrodes produced by chemical deposition make up for the limitations of the relatively slow reaction kinetics of ethylene oxidation on gold and together with fast diffusion due to the absence of any membrane permeation result in a highly sensitive amperometric sensor for ethylene monitoring. ACKNOWLEDGMENT The authors thank the Foundation for Research Science and Technology (New Zealand) for funding this work and G. A. Wright of the University of Auckland for his friendly participation in part of it. Received for review October 2, 1996. Accepted December 11, 1996.X AC9610117 X
Abstract published in Advance ACS Abstracts, January 15, 1997.
