Amperometric detection of gaseous ethanol and acetaldehyde at low

Amperometric detection of gaseous ethanol and acetaldehyde at low concentrations on an Au–Nafion electrode P. Jacquinot,a A. W. E. Hodgson,a B. Müller,b B. Wehrlib and P. C. Hauser*a a b

The University of Basel, Department of Chemistry, Spitalstrasse 51, 4056 Basel, Switzerland EAWAG, The Limnological Research Center, CH-6047 Kastanienbaum, Switzerland

Received 11th December 1998, Accepted 9th March 1999

An amperometric sensor capable of detecting mixtures of ethanol and acetaldehyde in the low ppb range without the need for prior separation is described. The electrochemical cell was based on a Au sensing electrode chemically deposited onto one side of a Nafion membrane with 1 M NaOH internal electrolyte solution. The detection was achieved by applying two potentials, 2450 and 2290 mV vs. a saturated mercury–mercurous sulfate electrode (MSE), at which ethanol and acetaldehyde react at different rates. Under the conditions investigated, acetaldehyde oxidation was mass transport limited at both potentials, whereas the anodic current due to the oxidation of ethanol was 40% lower at the more cathodic potential. Detection limits of 2 and 1 ppb (S/N = 3) were determined for ethanol and acetaldehyde respectively when the analyte species were detected individually. Poisoning of the working electrode was not observed for concentrations of ethanol in the ppb range. Acetaldehyde oxidation was found not to affect the sensing electrode condition, even at concentrations of several tens of ppm.

Introduction Gas monitoring plays an important role in the storage of fruit and vegetables. During the ripening process fruit and vegetables produce, besides CO2, organic volatile compounds such as ethylene, ethanol, acetaldehyde, acetone and ethyl acetate. 1–3 Amongst the most important ripening indicators are ethylene, ethanol and acetaldehyde. Monitoring of these gases enables the exact control of the ripening stages and thus an accurate regulation of the oxygen and carbon dioxide content of the storage atmosphere, which results in the enhancement of the lifetime and quality of the perishable goods. Currently these indicator gases are analysed by gas chromatography,4,5 although the technique is known to have inherent disadvantages such as expense, need for experienced personnel and lack of possibility for online monitoring. There is a strong demand for cheap and easy-to-use electrochemical sensors able to detect parts per billion concentrations of the different indicator gases on line. An amperometric sensor for ethylene based on a Au–Nafion electrode with a detection limit of 40 ppb was recently developed.6–8 In this sensor design, a solid polymer electrolyte (SPE), e.g. Nafion®, separates the electrolyte compartment from the gas phase and simultaneously acts as an ionconducting support for the working electrode facing the gas phase. The electrode is, therefore, directly exposed to the gas phase and the gaseous analyte is not required to diffuse through the membrane, as in the case of Clark electrodes, or to diffuse through a porous Teflon layer, as in the case of gas diffusion electrodes, prior to undergoing electron transfer. This metal– SPE electrode arrangement has shown faster responses, higher sensitivities and a higher performance than other amperometric gas sensor designs.9–11. For the electrochemical detection of methanol and ethanol in the gas phase, similar sensor arrangements have been described.12–15 In all of these sensors, Nafion, a perfluorinated cation exchange membrane, was employed and the sensors differed in the working electrode material and electrolyte solution composition. When platinum was used as electrode material,15 rapid fouling of the electrode was observed due to strongly adsorbed reaction products. On gold electrodes, on the

other hand, the fouling of the electrode was observed to a lesser extent.16 Promising results were obtained from the use of gold as the working electrode and a strong basic internal electrolyte solution.17–19 To prevent fouling of the working electrode, the application of a pulsed amperometric detection technique was found necessary.20,21 By combining the Au–Nafion 1 M NaOH cell arrangement with a pulsed amperometric detection technique, Schiavon et al. achieved a detection limit of 1ppm for ethanol,13 which is sufficient for the determination of ethanol in breath. A common problem encountered in all amperometric ethanol sensors described is the strong interference of acetaldehyde oxidation, which has not been overcome as yet. In the fruit storage industry this problem is of particular importance as the atmosphere contains both ethanol and acetaldehyde in comparable concentrations. In addition, sensors for ethanol monitoring were never tested for concentrations in the low ppb range, a requirement for monitoring in many fruit storage atmospheres. In this article we show that it is possible to exclude the mutual interference of acetaldehyde and ethanol by exploiting their different oxidation rates at two potentials. By selecting two suitable potentials we demonstrate that it is possible to distinguish ethanol and acetaldehyde electrochemically and to detect them without prior separation even in the low ppb range.

Experimental Instrumentation and experimental set-up The experimental set-up is shown in Fig. 1. The electrochemical cell has been previously described,6 and has a geometric electrode area of 0.79 cm2. Different ethanol and acetaldehyde concentrations in the gas phase were prepared by mixing certified gas standards (Carbagas, Basel, Switzerland) of 100 ppm or 5 ppm ethanol and 100 ppm acetaldehyde (v/v, balanced in dry nitrogen) with nitrogen of different flow rates, whilst maintaining the total flow rate constant. Control of the gas streams was achieved via the use of mass flow controllers Analyst, 1999, 124, 871–876

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(MFC) with maximum flow rates of 10, 20 and 100 cm3 min21 (type 1179A and 1159B, MKS Instruments Inc., M¨unich, Germany). Acetaldehyde concentrations in the low ppb range were obtained by employing a two-step dilution. A first dilution step produced a constant concentration of 5 ppm which was then fed to a second dilution system in which the concentrations could be varied. For the preparation of acetaldehyde–ethanol mixtures a further mass flow controller was connected in parallel. Moisturising was achieved by passing defined fractions of dry nitrogen through a Dreschel bottle partially filled with deionised water. To prevent humidity condensing at the working electrode due to possible temperature gradients in the tubing and at the gas entry compartment, the cell was held at 298 K via the use of a thermostat (TropiCooler, model 260014, CLF Analytische Laborger¨ate GmbH, Emersacker, Germany) whilst maintaining the Dreschel bottle at a slightly lower temperature by using a water bath. A BAS100B/W Potentiostat (Bioanalytical Systems, West Lafayette, IN, USA) was used to control the working electrode potential and to measure the resulting currents. All potentials are given with respect to a saturated mercury–mercurous sulfate electrode (MSE; XR200, Radiometer Analytical, Lyon, France) with a potential of +640 mV vs. a standard hydrogen electrode (SHE).

tetrachloroauric acid (2 mM instead of 10 mM) and the solution of sodium borohydride (12 mM instead of 1 M) in 1 M NaOH. The reaction time allowed was approximately 3 h. After the deposition step, the coated membranes were boiled in distilled water to remove residues, cut into suitable pieces and mounted into the sensor with the gold coated side of the membrane facing the gas phase. The freshly prepared electrodes were allowed to equilibrate in the sensor with the sensor housing filled with 0.5 M H2SO4 electrolyte solution, prior to measurements. The working electrode was then cycled between 2700 mV vs. MSE and +1000 mV vs. MSE, from hydrogen to oxygen evolution, until the cyclic voltammograms showed no further decrease in the charges associated with gold oxide formation and reduction and hence no further decrease in the active surface area of the deposited gold. The real surface area of the Au–Nafion electrodes were estimated by comparing the charges due to the reduction of a gold oxide monolayer with the reported value of 420 3 1026 C cm22 (ref. 23) for the reduction of a monolayer of oxide on a clean surface of Au. Real surface areas ranging from 30 to 120 cm2 were obtained. After the determination of the real surface area, the acid electrolyte was first replaced with water and then with 1 M NaOH.

Electrode preparation

Results and discussion

All chemicals used were of reagent grade quality and were used without further purification. Milli-Q water was used to prepare solutions and to rinse the Au–Nafion electrodes after preparation. Nafion® 117, the solid polymer electrolyte (SPE) membrane was purchased from Aldrich (Buchs, Switzerland). Gold was chemically deposited onto one side of rectangular shaped pieces of Nafion membrane as described elsewhere,22 but with significantly decreased concentrations for both the solution of

Electrochemical oxidation of ethanol and acetaldehyde at Au–Nafion electrodes Overlaid cyclic voltammograms recorded at a Au–Nafion electrode when fed with nitrogen, ethanol (10 ppm in N2) and acetaldehyde (10 ppm in N2) gas streams humidified to 80% are shown in Fig. 2. In the anodic sweep, there are two main features to be noted in the oxidation of the two analytes. Firstly, the oxidation current for acetaldehyde is comparable to the current for the oxidation of ethanol in the potential region prior to gold oxide formation, even though the ratio of electrons transferred per molecule should be 2 and 4, if acetic acid were the only oxidation product according to the reported reactions:24,25 +2 e2

CH3–CHO + H2O —? CH3–COOH + 2 H+ +4 e2

CH3–CHO2OH + H2O —? CH3–COOH + 4 H+

Fig. 1 Schematic representation of the experimental set-up with predilution, dilution, humidifying part and design of the sensor (MFC: mass flow controller).

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(1) (2)

Fig. 2 Cyclic voltammograms of the Au–Nafion electrode with 1 M NaOH as internal electrolyte solution for nitrogen (—), 10 ppm ethanol (2) and 10 ppm acetaldehyde (5) in N2. The scan rate is 50 mV s21 and the gases are humidified to 80% of relative humidity (298 K). The total flow rate of the gas streams is 100 cm3 min21.

Secondly, the potential dependence of the oxidation current is different for the two analyte species. Whilst the reaction rate for acetaldehyde oxidation is independent of the applied potential in the potential region up to gold oxide formation, the reaction rate for ethanol oxidation is strongly potential dependent. The rate of electron transfer, in fact, increases with increasing potential and reaches its maximum oxidation rate at 2290 mV vs. MSE, close to the onset for gold oxide formation. From the potential dependence of the oxidation rate of ethanol and the smaller currents compared to acetaldehyde oxidation it can be assumed that the current is kinetically controlled for the oxidation of ethanol over the whole potential region, even down to 2290 mV vs. MSE. For acetaldehyde, on the contrary, the overall current appears unaffected by slow kinetics. The current versus time profile recorded at a constant potential of 2290 mV vs. MSE for 25 ppb step increases of ethanol and acetaldehyde respectively are shown in Fig. 3. The response times are quite different for the two gases. Acetaldehyde reaches a steady state current within a second, whereas for ethanol oxidation, approximately 30 s are required for a plateau to be established. These characteristics also add to the deduction that the oxidation of acetaldehyde and ethanol is affected by different rate limitations at 2290 mV vs. MSE, namely mass transfer control for acetaldehyde and partial kinetic control for ethanol oxidation. In Fig. 3, it can also be seen that the current response for ethanol does not show signs of fouling of the electrode surface due to poisoning effects, as reported previously by Schiavon et al.13 The absence of electrode fouling is very likely to be due to the low ethanol concentration range studied here. Only small amounts of analyte are oxidised at the sensing electrode of high surface area (90 cm2) in the ppb concentration range and thus the effects of poisoning due to the product species are minimal. Potential pulsing, as described for ethanol,13,20,21,24 is therefore not required. With the Au–SPE electrode investigated in this work, poisoning effects could only be seen for concentrations of ethanol in the ppm range, whereas the concentration of acetaldehyde could be increased up to several tens of ppm without visible passivation of the signal. Ethanol/acetaldehyde monitoring Monitoring of two gaseous analyte species with one sensor is possible if the species undergo electron transfer at the electrode surface at distinct potential ranges. Ideally, each analyte reacts in one distinct potential range and does not undergo electron transfer within the potential limits where the other analyte

Fig. 3 Current–time profile for acetaldehyde (a) and ethanol (b) in 25 ppb (v/v) steps from 0 to 150 ppb. The sensor consists of a chemically deposited Au electrode on Nafion (estimated real surface area: 90 cm2) with 1 M NaOH as internal electrolyte solution. The applied potential is 2290 mV vs. MSE for both ethanol and acetaldehyde. Total flow rate is 100 cm3 min21, relative humidity of the gas stream (298 K) is 80%.

reacts. This is not the case for ethanol and acetaldehyde, which both react over a large common potential range. It is possible, however, to describe the overall current, i1 and i2, at two distinct potentials, E1 and E2, within the range of interest: i1 = sx1 cx + sy1 cy

(3)

i2 = sx2 cx + sy2 cy

(4)

where c is the concentrations and s the sensitivity for the species x and y at potentials E1 and E2. According to these equations, it would appear that the concentration of ethanol and acetaldehyde in a mixture of the two gases could be calculated, provided the current sensitivities for the oxidation of ethanol and acetaldehyde at the two chosen potentials were known. Therefore, substituting the currents measured, i1 and i2, and the current sensitivities into eqns. (3) and (4) and solving the set of simultaneous equations, cx and cy may be deduced. This is only possible, however, if there is a significant change in the ethanol : acetaldehyde sensitivity ratio in going from potential E1 to potential E2. It is evident that the larger the difference in the sensitivity ratio, the higher the possibility of deriving the original concentrations in mixtures of the two gases. In the cyclic voltammogram shown in Fig. 2 a wave due to ethanol oxidation is observed prior to gold oxide formation with a limiting current at about 2290 mV vs. MSE. At more negative potentials the sensitivity for ethanol oxidation decreases whereas acetaldehyde is oxidised at a constant rate over a large potential range, hence, the more negative the potential, the greater the difference in sensitivities between ethanol and acetaldehyde. The choice of potential is, however, restricted on the cathodic end of the range by the onset for oxygen reduction, which may also interfere with the analysis. Hence a potential value of 2450 mV vs. MSE was selected. Calibration plots for ethanol and acetaldehyde in the range 25 to 500 ppb at the two chosen potentials, 2450 mV and 2290 mV vs. MSE are shown in Fig. 4. The sensitivity for acetaldehyde was found to increase by less than 10%, from 4.75 nA ppb21 at 2450 mV to 5.11 nA ppb21 at 2290 mV vs. MSE. Ethanol, on the other hand, showed a significant increase in sensitivity with increasing the potential. Here, the sensitivity increased by more than 70% from 2.79 nA ppb21 at 2450 mV to 4.82 nA ppb21 at 2290 mV. The correlation coefficients of the linear regressions for all calibration curves were equal to or better than 0.9998. Detection limits based on a S/N ratio of 3 were determined at 2290 mV vs. MSE by means of five consecutive measurements of 0 to 25 ppb ethanol and acetaldehyde in N2 respectively. The reproducibility in terms of standard deviation (n = 5) of consecutive measurements of ethanol and acetaldehyde was found to be 3.2 and 1.7 nA,

Fig. 4 Calibration plot for ethanol (2, 5) and acetaldehyde (8,-) at 2290 mV (5, -) and 2450mV(2,8) vs. MSE. The total flow rate of the humidified gas stream (relative humidity 80%, 298 K) is 100 cm3 min21. The sensing electrode is an Au–Nafion SPE electrode (estimated real surface area: 90 cm2) in contact with 1 M NaOH electrolyte solution.

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respectively and these lead to corresponding detection limits of 2 and 1 ppb. The dynamic behaviour of the sensor at each of the two potentials to mixtures of ethanol and acetaldehyde is shown in Fig. 5. The sensor was successively fed with three different ethanol–acetaldehyde mixtures, maintaining the total amount of analyte gas constant: 25 : 75, 50 : 50 and 75 : 25 ppb ethanol and acetaldehyde respectively. The sensor was first allowed to equilibrate at 2290 mV in a nitrogen gas stream of 80% relative humidity at a flow rate of 100 cm3 min21. Having reached a stable ‘zero’ current, the sensor was fed with the first ethanol– acetaldehyde mixture. The total flow rate and the relative humidity of the gas stream were held constant. After each mixture the sensor was fed with nitrogen until the background current was reached, prior to passing the successive mixture to the sensor. The same procedure was then carried out at 2450 mV vs. MSE. In Table 1, the recorded currents for the three mixtures are reported together with the derived concentrations calculated by solving eqns. (3) and (4), using the previously determined current sensitivities. The injected mixtures could be derived with an accuracy of 2 to 3 ppb and a precision of 2 ppb. The precision of the measurement was calculated over an average of 600 points recorded during a sampling time of 150 s. At 2290 mV vs. MSE, the response of the sensor was only slightly affected by changes in the composition of the mixtures because the sensitivities of the two analytes are very close to one another. At 2450 mV vs. MSE, on the other hand, the measured current decreased significantly with increasing ethanol : acetaldehyde ratio in the mixture due to the lower sensitivity for ethanol compared to acetaldehyde at this potential. Potential pulsing was also investigated but due to the high real surface area of the sensing electrode, long pulsing times were necessary

to allow for the charging currents to decay. In addition, the same degree of accuracy as found possible with the use of a single potential could not be reached. Humidity dependence The effect of relative humidity in the gas streams was tested to ensure that the sensor could be used in storehouses where the relative humidity is normally regulated between 75 and 95%. A plot of current sensitivity against relative humidity of the gas stream for acetaldehyde at 2290 mV vs. MSE is shown in Fig. 6. Prior to measurements, the Au–Nafion SPE sensing electrode was allowed to equilibrate at the chosen potential with an 80% humidified gas stream. This was done to ensure that the Nafion membrane was initially sufficiently swollen and would not suffer from long term dehydration. The relative humidity was then gradually increased from 0 up to 80%. For each setting the sensor was allowed to equilibrate for 5 min under the new conditions. The current for a 1 ppm concentration step was subsequently recorded and, after the current had decayed to its initial value under nitrogen, the relative humidity was increased. It can be seen from Fig. 6 that the response of the sensor shows a significant dependence on the humidity of the gas stream. The sensitivity was found to increase with increasing humidity for acetaldehyde and reach a maximum sensitivity for relative humidity values above 80%. For ethanol a similar humidity dependence was observed. This behaviour may be explained by the fact that acetaldehyde and ethanol require water in order to undergo electron transfer [see eqns. (1) and (2)]. Another explanation could be the evaporation of water on the metal/SPE interface at very low relative humidity content, leading to a local dehydration and thereby a reduced size of the pore channels of the membrane. This would result in a hindered transport of molecules and ions to or from the electrode surface with an associated increase of the electrical resistance between working and counter/reference electrodes with decreasing relative humidity. However, no change of the Ohmic resistance of the membrane was observed with varying humidity of the gas stream. Cross sensitivities

Fig. 5 Overlapped current–time profiles of successive expositions of ethanol–acetaldehyde mixtures at 2290 mV (- - - ) and 2450 mV(——) vs. MSE with the following ethanol : acetaldehyde concentration ratios: 25 : 75 ppb (a), 50 : 50 ppb (b) and 75 : 25 ppb (c). The sensor consists of an Au– Nafion SPE electrode with an estimated real surface area of 90 cm2 in contact with 1 M NaOH. The gas streams are humidified to 80% of relative humidity (298 K). The total flow rate of the gas streams is 100 cm3 min21.

All major interfering species coexisting with ethanol and acetaldehyde in fruit storage atmospheres were tested for cross sensitivity. Amongst these are small organic compounds, such as acetone, ethyl acetate and acetic acid, which are produced by fruit and vegetables during their metabolism.1–3 In addition to

Table 1 Recovery study for injected ethanol (EtOH)–acetaldehyde (AcH) mixtures to the sensor consisting of an Au–Nafion SPE electrode (real surface area 90 cm2) with 1 M NaOH as internal electrolyte solution. The sensitivities are 4.82 and 2.79 nA ppb21 for ethanol, 5.11 and 4.75 nA ppb21 for acetaldehyde at 2290 mV and 2450 mV vs. MSE respectively EtOH–AcH mixture (ppb v/v)

Measured current/nA 2450 mV

2290 mV

Determined EtOH/AcH concentration (ppb v/v)

25/75 50/50 75/25

424 ± 2 379 ± 2 326 ± 2

502 ± 1 495 ± 2 491 ± 2

25.2 ± 2.4/74.5 ± 2.0 48.0 ± 3.2/51.6 ± 2.5 77.2 ± 3.2/23.3 ± 2.5

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Fig. 6 Effect of relative humidity on the sensitivity of acetaldehyde oxidation. The sensor consists of an Au–Nafion working electrode with 1 M NaOH as electrolyte solution. The applied potential is 2290 mV vs. MSE. The total flow rate is 100 cm3 min21. The sensitivity of acetaldehyde at 80% relative humidity (r.h.) is set to 100%.

these, there are the components of the artificially regulated storage atmosphere, such as nitrogen (80–90%), oxygen (0.5–10%) and carbon dioxide (1–5%). Ethylene is both produced by the fruit and added to the atmosphere because of its function as a ripening hormone.26 In order to test the cross sensitivity of the sensor, each copresent species was fed in turn from either a gas bottle, as for CO2, C2H4 and O2, or by bubbling dry nitrogen through a Dreschel bottle containing the undiluted solution (acetic acid, acetone and ethyl acetate). The concentrations of each gas tested were 1% for CO2 and O2, 50 ppm for ethylene, 1.3% for ethyl acetate, 0.6% for acetone and 0.4% for acetic acid. The concentrations of acetic acid, acetone and ethyl acetate in the gas streams were calculated using the partial pressure at 298K. The degree of interference was investigated at both 2450 mV and 2290 mV vs. MSE and the data obtained are reported in Table 2 in terms of relative response with respect to the sensitivity for acetaldehyde oxidation. It can be seen that only acetone and ethyl acetate interfere to a small degree. No detectable current was found for the other gases. The ‘less than’ ( < ) values where obtained by considering the 0.1 nA as the minimum detectable current and the respective concentrations of the gases employed for the measurement as given above. Carbon dioxide and acetic acid are not electroactive at the potentials investigated, but their acidity was found to cause a local pH change in the vicinity of the Au–Nafion SPE electrode. This was observed as a sudden positive potential shift of the onset for gold oxide formation. This may lead to the slow consumption of the basic electrolyte solution. To extend the lifetime of the sensor in atmospheres with high contents of acidic components a filter would, therefore, be recommended. O2, on the other hand, was not found to interfere with the measurement of ethanol or acetaldehyde, thus enabling the determination of these species in air.

work the two potentials were applied individually, one after the other, resulting in two constant potential experiments. In a real application the use of two sensors in parallel may be envisaged. Potential pulsing was also investigated but long pulsing times were found necessary and a decrease in the accuracy of the derived concentrations was observed. In contrast to ethanol sensors previously described in the literature, potential pulsing was not required, as the sub ppm range investigated did not appear to poison the electrode surface. Electrode fouling was not observed in the case of acetaldehyde either, even in the higher ppm concentration range. The sensors were employed over a period of a few months in the time-scale of which, no dramatic changes in the sensitivity were noted. Detection limits of 1 ppb acetaldehyde and 2 ppb ethanol were found for a S/N ratio of 3 when the analytes were detected individually. Several mixtures of the two analytes were measured and the injected concentrations could be deduced with an accuracy of 2 to 3 ppb and a precision of 2 ppb.

Acknowledgements The authors would like to thank the Swiss National Science Foundation for Research for providing financial support (Grant No. 21-49342.96) and Dr M. Meyberg of DMP AG, Zurich, Switzerland for his helpful advice and information.

References 1 2

Conclusion

3

Ethanol and acetaldehyde oxidise on Au in the same potential range, rendering a selective determination of each of the species difficult without further separation. In this work we have shown that it is possible to detect mixtures of ethanol and acetaldehyde in the ppb range at an Au–Nafion SPE electrode with 1 M NaOH as internal electrolyte solution by applying two suitable potentials, 2450 mV and 2290 mV vs. MSE. This is possible thanks to the different reaction rates that ethanol and acetaldehyde oxidation display at the two chosen potentials. Acetaldehyde shows a constant current sensitivity whilst ethanol has significantly different current sensitivities at the two potentials. The difference in the ethanol : acetaldehyde current sensitivity ratio at the two potentials is the basis upon which a simultaneous measurement of the two species is possible. In our

4

Table 2 Cross sensitivities of the sensor consisting of an Au–Nafion sensing electrode with 1 M NaOH as internal electrolyte solution. The cross sensitivities are given in percent relative to the sensitivity of acetaldehyde at the corresponding potential vs. MSE. The flow rate of the humidified gas stream is 100 cm3 min21

5 6 7 8 9 10 11 12 13 14 15 16 17

Cross sensitivity relative to acetaldehyde sensitivity

18

Gas

2450 mV

2290 mV

19

Acetic acid Acetone Carbon dioxide Ethyl acetate Ethylene Oxygen

< 5 3 1028 3 3 1024 < 2 3 1028 5 3 1024 < 4 3 1026 < 2 3 1028

< 5 3 1028 2 3 1023 < 2 3 1028 1 3 1023 < 4 3 1026 < 2 3 1028

20 21 22 23

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