The EMOSFET as an oxygen sensor: constant current potentiometry

Sensors and Actuators B 59 Ž1999. 35–41 www.elsevier.nlrlocatersensorb

The E MOSFET as an oxygen sensor: constant current potentiometry J. Hendrikse ) , W. Olthuis, P. Bergveld MESA Research Institute, UniÕersity of Twente, P.O. Box 217, 7500AE Enschede, Netherlands Received 15 September 1998; received in revised form 10 February 1999; accepted 12 February 1999

Abstract In a previous paper, a novel type of potentiometric dissolved oxygen sensor was introduced. The transduction principle of the sensor is based on the modulation of the work function of an iridium oxide film by the ratio of Ir IIIrIr IV oxide in the film. This ratio depends on the oxygen concentration in the solution, so that the work function of the iridium oxide is a measure of the amount of dissolved oxygen. The work function changes are determined by using the iridium oxide film as the gate contact of a MOSFET. Because the threshold voltage of a MOSFET depends on the work function of the gate contact, it can be used as a sensor signal. In the present paper, a reducing current of constant magnitude is in addition externally applied to the iridium oxide film, so that an overpotential is generated. The influence of this overpotential on the sensor signal is studied and it is shown that the sensitivity towards oxygen increases compared to the equilibrium potential. Measurement results are shown and compared to the theory. The applied current leads to sensitivities to oxygen and pH that correspond well to the values that may be expected from the literature. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Oxygen sensor; Field effect transistors; Work function; Constant current potentiometry

1. Introduction Although concentrations of oxygen are generally measured amperometrically, using the Clark cell, potentiometry offers an alternative route to oxygen sensing. Using this method, no oxygen is consumed during the measurement and at low oxygen concentrations the logarithmic sensitivity of potentiometric sensors becomes an advantage over the linear sensitivity of amperometric sensors. Meruva and Meyerhoff w1x showed that the electrode potentials E of copperrcopper oxide and cobaltrcobalt oxide electrodes depend on the level of dissolved oxygen. In these electrodes, the electrons necessary for the reduction of oxygen are released by the ongoing oxidation of the metal. They found that the mixed potential of the cobaltrcobalt oxide electrode was the most stable and developed a catheter-type sensor that contained an oxygen sensor based on this electrode w2x. We proposed an alternative potentiometric sensor based on the oxygen sensitivity of the work function of the

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iridium oxide electrode w3x. When the iridium oxide is used as the gate contact of a MOSFET, the threshold voltage VT of this MOSFET depends on the work function and can be used as a sensor signal. However, it was found that the reaction between the iridium oxide electrode and the dissolved oxygen is slow, probably because of the low exchange current density of the oxygen reduction reaction. This results in a sluggish response that is moreover easily disturbed by competing redox couples. In the literature investigating the oxygen reduction reaction, this problem is solved by the application of a small reducing current to the electrode w4x. Obviously, the equilibrium electrode potential of the oxygen reduction reaction is not determined in this case. However, at a relatively small applied current density, the measured electrode potential approaches the equilibrium electrode potential, as described by the Butler Volmer equation. The application of this method to the previously described sensor system is investigated in this paper. An expression for the electrode potential E and threshold voltage VT as a function of dissolved oxygen concentration, pH and magnitude of the applied current density is derived. Measurements of E and VT as a function of dissolved oxygen concentration at different current densities and pH values are presented and compared to the theory.

0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 1 9 5 - 1

J. Hendrikse et al.r Sensors and Actuators B 59 (1999) 35–41

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2. Theory 2.1. Constant current potentiometry of oxygen The electrochemical reduction of oxygen is a complicated process that involves a large number of intermediate steps. Since some of the steps appear to be rather slow, the overall reaction is characterized by a very low exchange current density, I0 . The most simple description of the redox reaction at an inert electrode in acidic solutions is based on the products before and after the reaction is completed and does not take into account any of the intermediate steps: O 2 q 4eyq 4Hql 2H 2 O

Ž I.

Based on this simple description, the electrode potential of an electrode in equilibrium with the dissolved oxygen, Eeq , can be expressed as Eeq s E 0 q kTr4 q ln Ž aO 2 . q kTrq ln Ž aH q .

Ž 1.

where k, T and q have their usual meaning and aO 2 and a H q are the activities of dissolved oxygen and protons, respectively. The standard electrode potential E 0 for this reaction is equal to 1032 mV vs. AgrAgCl. According to Eq. Ž1., the sensitivity of the electrode potential to pH changes is y59 mVrdecade at room temperature while the sensitivity to oxygen concentration variations is 15 mVrdecade. However, neither these dependencies nor the true value of E 0 can be verified experimentally using zero current potentiometry, because at the extremely low current density I0 of the oxygen reduction reaction, impurity levels as low as 10y1 0 M are enough to disturb the measurement w4x. Instead, Eeq can been determined by measuring E while a small current density I is applied to the electrode. By variation of I, a Tafel plot showing log < I < as a function of E can be constructed and Eeq can be determined w5x. Fig. 1 shows a number of Tafel plots for different magnitudes of I0 . The overpotential h is defined as h ' E y Eeq . The Tafel plot construction was developed to determine Eeq at a given oxygen concentration and pH, but it can easily be adapted to make an oxygen sensor. When a small current density is applied to the electrode, the measured value of E does not depend only on the factors determining the equilibrium potential Eeq as described by Eq. Ž1., but also on the magnitude of the applied current density I. For a potentiometric oxygen sensor this means that an extra term is introduced in the oxygen sensitivity of the electrode potential: dE s d ln Ž aO 2 .

d Eeq d ln Ž aO 2 .

dh q d ln Ž aO 2 .

Ž 2.

For simple reaction mechanisms of the type Ox q eyl Re, the relation between the overpotential and the applied

Fig. 1. Sketch of log < I < as a function of the overpotential h ŽTafel plot. and as a function of I0 .

current density is described by the Butler Volmer ŽBV. equation: I s I0 Ž eya n hq r kT y eŽ1ya .n hq r kT .

Ž 3.

where I0 is the exchange current density, n is the number of electrons that is being transferred and a is the transfer coefficient of the reaction, which is often close to 0.5. I0 does not only depend on the electrochemical reaction and on the catalytic properties of the surface, but also on the concentrations of the species taking part in the reaction. For the simple reaction Ox q e l Re, I0 can be written as w6x I0 s Fk 0 w Ox x

1y a

w Re x

a

Ž 4.

where F is Faraday’s constant and k 0 is a reaction constant. Eqs. Ž3. and Ž4. can be used to illustrate qualitatively the influence of the oxygen concentration on the electrode potential while a current density is applied. In Fig. 1, logŽ< I <. is presented as a function of h and logwOxx for a value of a s 0.5 and n s 1. In the bottom plane of the figure, a number of curves connecting points that have the same current density are projected. This projection shows h as a function of logwOxx at constant I. At h < 0, reaction I proceeds from the left to the right only, and the last exponential term of Eq. Ž3. can be neglected, so that the straight left hand part of the curves

J. Hendrikse et al.r Sensors and Actuators B 59 (1999) 35–41

in Fig. 1 has a constant slope of ya qrkT. This means that for h < 0 ykT

hs

qa

ln

I

ž /

Ž 5.

I0

Substitution of Eq. Ž4. in Eq. Ž5. leads to the following expression for the overpotential as a function of wOxx at constant current density I and h < 0: ykT

hs

qa

ln Ž I . q

kT qa

ln Ž Fk 0 w Ox x

1y a

w Re x

a

Ž 6.

.

The exchange current density I0 of the oxygen reduction reaction may be expected to depend in some fashion on the activities of the products before and after reaction I: O 2 , Hq and H 2 O. The exact dependence will be determined by the kinetics of the rate determining step, which depends on the pH of the solution and the electrode surface. In Section 5, our measurement results will be compared to reaction schemes that are known from the literature. For now, it is assumed that aO 2 is equal to wOxx, substitution of Eqs. Ž1. and Ž6. into Eq. Ž2. can be used to obtain an indication of d ErdlnŽ aO 2 .: dE s d ln Ž aO 2 .

d Eeq d ln Ž aO 2 .

dh q d ln Ž aO 2 .

kT 1 y a

kT s

q 4q

q

a

Ž 7. Especially when a is smaller than 0.5, the impact of the last term can be very large. At high oxygen activities andror low current densities, the applied current density will become of the order of I0 , so that h will be close to zero and less dependent of the oxygen concentration, as indicated by the curvature of the projected curves in Fig. 1. It should be emphasized that, since the BV equation is a simplification, care should be taken when this relation is used in any quantitative way. However, published measurements of log Ž I . as a function of h generally show a straight line over a substantial range of h w5x, and the BV equation can be used to summarize those measurement results. 2.2. The E MOSFET The analysis in the previous section is in principle independent of the electrode material, although values of I0 and a depend strongly on the catalytic properties of the electrode material used. In this paper, iridium oxide is used as an electrode material for a specific reason. Iridium oxide is an electron conducting oxide that can exist as a mixture of Ir III and Ir IV oxide. The electrode potential E of the electrode depends on the relative amounts of Ir III and Ir IV so that E rises when the electrode is oxidized and falls when it is reduced. Moreover, the work function of iridium oxide is known to depend on the relative amounts of Ir III and Ir IV w7x. Because the threshold voltage VT of a MOSFET depends on the work function of its gate contact

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w8x, VT can be used as a transducer signal. An illustration of such a MOSFET is shown in Fig. 2, together with the potential differences f 1 and f 2 that play a role in the description of E and V T . In a previous paper, it was argued that the electrode potential of an iridium oxide electrode in equilibrium can be separated in two contributions, as illustrated in Fig. 3 w9x. The first contribution is the potential difference f 1 between the iridium oxide and the metal wire connected to it, i.e., across interface 1. The magnitude of this potential difference is determined by the equilibrium between electrons crossing the interface. The second contribution is the potential difference f 2 between the iridium oxide and the solution, i.e., across interface 2. The magnitude of this potential difference is determined by the partition of protons at this interface and turns out to be equal to ykTrq lnŽ a H q .. It is clear that E depends on the sum of f 1 and f 2 , while VT depends on f 1 only. When the electrode is in equilibrium with the solution, it can be shown that w9x V T ,eq s Eeq y

kT q

ln Ž a H q . q C

Ž 8.

where C is a device dependent constant. Measurements have shown that this relation is correct over a large range of pH values and electrode potentials. When an iridium oxide electrode is in equilibrium with an oxygen containing solution, its electrode potential will be described by Eq. Ž1.. By substitution of this equation into in Eq. Ž8., the equilibrium threshold voltage as a function of oxygen activity in the sample is found: VT ,eq s E 0 q C q kTr4 q ln Ž aO 2 .

Ž 9.

Apparently, V T,eq does not depend on the pH of the solution, in contrast to Eeq . This result was to expected since Fig. 2 illustrates that the proton equilibrium at interface 2 does not influence VT . In case a small reducing current density is applied to the iridium oxide electrode, the effect of the overpotential h has to be taken into account. From Fig. 3, it is obvious that h falls across the interfaces 1 and 2, but to our knowledge,

Fig. 2. Sketch of the E MOSFET set-up. The threshold voltage V T is influenced by f 1 , but not by f 2 . See also Fig. 3.

J. Hendrikse et al.r Sensors and Actuators B 59 (1999) 35–41

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Fig. 3. Illustration of the potential as a function of position in a Cu
there is no theory regarding the proportion of h that falls across either of the two. When the portion of h that falls across interface 1, which will be denoted by d , is added to V T,eq , the following expression for the sensor signal under current control can be found: V T s VT ,eq q dh

Ž 10 .

The value of d can be determined by comparison of measured values of E and VT , as will be done in Section 5. Its value will be found to be close to unity for the applied current densities in this system. Following the same line of reasoning that was used to derive Eq. Ž7., it can be shown that dV T

s

d ln Ž aO 2 .

dV T ,eq d ln Ž aO 2 .

dh d ln Ž aO 2 .

d kT 1 y a

kT s

qd

q 4q

nq

a

Ž 11 .

Eq. Ž11. shows how the threshold voltage of an E MOSFET can be used to determine oxygen levels in solutions. Apparently, the sensitivity at constant applied current density is always equal or larger than the equilibrium sensitivity, depending on the value of d in Eq. Ž11.. The cross sensitivity to changes in pH may be influenced by the applied current density, because it is quite possible that I0 depends on the pH. A point that will be touched upon in Section 5.

Fig. 4. Experimental set-up.

counter electrode. The minimum current that the galvanostat was able to apply was 25 nA. Lower current densities were realized by applying the current to a number of devices in parallel. The electrode potential with respect to a Radiometer ref 200 AgrAgCl reference electrode was measured by the galvanostat. At the same time, V T was measured using a circuit that keeps both the drain current and the drain-source voltage at a constant value by adjusting the gate-source voltage VGS using feedback. Thus, any change in VT is reflected by a change in VGS of the same magnitude but of opposite sign. The iridium oxide gate contact is connected to ground and serves as the working electrode of the electrochemical cell. The set-up is shown in Fig. 4. The pH was measured using a GK2401C glass electrode and a PHM80 pH meter, both from Radiometer. The oxygen concentration was measured using an Orion 081010 Dissolved Oxygen probe, which is based on the principle

3. Experimental 3.1. Measurement set-up MOSFET devices, including the iridium oxide gate, were fabricated in exactly the same way as described before w3x. In order to determine E and VT as a function of the applied current, oxygen concentration and pH, the encapsulated FET-structure was immersed in a solution while a current was applied using a PAR 263A galvanostat. A platinum sheet having a 2 cm2 area was used as a

Fig. 5. VT and E as a function of the dissolved oxygen concentration. A reducing current of 13 nA is applied to the iridium oxide electrode. The pH of the solution is 3.

J. Hendrikse et al.r Sensors and Actuators B 59 (1999) 35–41

Fig. 6. E as a function of the dissolved oxygen concentration for the indicated magnitudes of applied current.

39

Fig. 8. E, VT and EyV T as a function of the pH of the solution. A reducing current of 25 nA is applied to the iridium oxide electrode. The O 2 concentration of the solution is 7.1 mgrl.

of the Clark cell. Because both the reading of this probe, as well as the electrode potential and threshold voltage may be sensitive to stirring, all solutions were characterized at similar stirring conditions.

the solution was 7.1 " 0.3 mgrl over the course of the pH measurement.

3.2. Protocol

4. Measurement results

The O 2 sensitivity of both E and VT was determined in a 100 mM KNO 3 q 1 mM HNO 3 solution made by dissolving analytical grade substances ŽMerck. in demineralized water. The oxygen concentration was varied by bubbling the solution for short periods using O 2 or N2 , depending on the desired direction of the change. The electrolyte solution that was used to determine the pH dependence of both the electrode potential and the threshold voltage consisted of a 0.5 mM aceticrboricrphosphoric acid buffer solution dissolved in demineralized water. The pH of the solution was changed by adding small quantities of 1 M NaOH and the oxygen concentration in

First, E and V T were determined while the oxygen concentration was changed. A constant current of 13 nA was applied and the pH was equal to 3.0. In Fig. 5 both E and V T are shown as a function of the oxygen concentration. A dependence of approximately 300 mVrdecade was found for both E and VT . These experiments were repeated with various applied current densities. The recordings of E as a function of wO 2 x are shown in Fig. 6. The sensitivity to oxygen lies in the same range for the chosen currents, as indicated in the figure. From the values at 10 mg O 2rl of the three curves in Fig. 6 and the applied current, part of the Tafel plot of the oxygen reduction reaction, log < I < vs. E, is constructed. This curve is shown in Fig. 7. From the slope of this curve it follows that d ErlogŽ I . is equal to 610 mVrdecade. A similar value was found for dVTrdlogŽ I .. Subsequently, E, V T and the difference between the two were determined while the pH was changed. A constant current of 25 nA was applied and wO 2 x had a value of 7.1 " 0.3 mgrl. The dependence of E, V T and E y V T on the pH is shown in Fig. 8. E depends on the pH by y53 mVrdecade. In contrast, V T is rather insensitive to the pH. Note that the difference between E and V T as a function of pH has a close to nernstian slope of y57.6 mVrdecade.

5. Discussion and conclusions Fig. 7. Construction of part of the Tafel plot of the reduction of oxygen on iridium oxide. Measurements are derived from Fig. 6.

The value of Eeq of the oxygen redox reaction in this medium can be calculated from Eq. Ž1. using the tabulated

40

J. Hendrikse et al.r Sensors and Actuators B 59 (1999) 35–41

value of E 0 and the values of aO 2 and a H q that can be found assuming that their concentrations are equal to their activities. It follows from E 0 s 1032 mV vs. AgrAgCl a H q s 10y 1.68 s 0.021, and a O 2 s 5.6)10y 6 r32 s 1.75)10y7 that Eeq s 571 mV. The E values that are shown on the x-axis of Fig. 7, are much smaller than this value of Eeq . As was mentioned before the low I0 value of the oxygen reduction reaction makes that a large overpotential h is needed to obtain an observable current. Because of its dependence on aO 2 , a H q and h , it is hard to compare this value with values that are found in the literature. Generally speaking, only at negative values of E can oxygen reduction current be observed. In Fig. 5, E and V T are shown as a function of the oxygen concentration. The sensitivity is 300 mVrdecade of oxygen concentration. When the sensitivity of E ŽEq. Ž7.., is compared to the sensitivity of V T ŽEq. Ž11.. they are found to be almost equal. This means that the value of d is very close to unity. When the value of 300 mVrdecade is substituted in the left hand side of Eq. Ž7. or Eq. Ž11., a can be calculated and a value of 0.14 is found. In Fig. 6 it can be seen that sensitivities of the same order are found for higher and lower current densities. From the offset at 10 mgrl O 2 , part of the Tafel curve can be reconstructed, as is shown in Fig. 7. The sensitivity of E with regard to I, d ErdlnŽ I ., is found to be y610 mVrdecade. Using Eq. Ž5., a value of a of 0.10 is found. The values found are in the same range and point to a very asymmetrical oxygen reduction reaction on iridium oxide electrodes, resulting in a sensitivity which is far larger than expected from the equilibrium sensitivity ŽEq. Ž1... Fig. 8 shows the pH sensitivity of both E and VT . As predicted by Eq. Ž12., the pH sensitivity of V T is negligible compared to the sensitivity of E. However, a substantial transient response to a stepwise change in pH was observed Žnot shown.. Apparently the steady state overpotential is not very pH sensitive either. In the literature, it is acknowledged that the theoretical current density of the oxygen reduction reaction depends on the activity of protons, oxygen and water as follows w10x: a Ž1y a .r4 a r2 I0 A a1y aH 2 O H q aO 2

Ž 12 .

Regarding the fist term of Eq. Ž12., the general trend found in measurements of h as a function of pH points towards an exchange current density that is independent of a H q w10x. This discrepancy between theory and measurement shows that the reaction mechanism is not fully understood yet and is the reason why the introduction of Eq. Ž12. has been postponed until the discussion. The findings in this paper that h is independent of pH are in accordance with the commonly measured, rather than the theoretical trend. The second term of Eq. Ž12. shows that the sensitivity of a r4 I0 towards oxygen is equal to a1y , rather than to aO 2 . O2

This difference leads to a slightly modified value for a , but is of minor importance otherwise. The dependence of I0 on aH 2 O is not relevant to the sensing applications described here, because a H 2 O will always be close to unity when the sensor is applied in aqueous solutions. Concluding, it follows from the measurements presented here that when oxygen is reduced at an iridium oxide electrode, the threshold voltage of an underlying E MOSFET can be used as a potentiometric signal to determine the oxygen concentration. It is found that a major part of the overpotential difference falls across the interface between the iridium oxide and its metal connection, whilst the potential difference across the iridium oxide
Acknowledgements The authors are grateful to Johan Bomer for the fabrication and encapsulation of the FET-devices described in this paper.

References w1x R.K. Meruva, M.E. Meyerhoff, Potentiometric oxygen sensor based on mixed potential of cobalt wire electrode, Analytica Chimica Acta 341 Ž1997. 187–194. w2x R.K. Meruva, M.E. Meyerhoff, Catheter-type sensor for potentiometric monitoring of oxygen, pH and carbon dioxide, Biosens. Bioelectron. 13 Ž1998. 201–212. w3x J. Hendrikse, W. Olthuis, P. Bergveld, The E MOSFET as a potentiometric transducer in an oxygen sensor, Sensors and Actuators B 47 Ž1998. 1–8. w4x A. Damjanovic, Mechanistic analysis of oxygen electrode reactions, in: J.O’M. Bockris, B.E. Conway, R.E. White ŽEds.., Modern Aspects of Electrochemistry V, Chap. 5, Plenum, New York, 1969. w5x A. Damjanovic, A. Dey, J.O’M. Bockris, Kinetics of oxygen evolution and dissolution on platinum electrodes, Electrochim. Acta 11 Ž1966. 791–814. w6x A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Chap. 3, Wiley, New York, 1980.

J. Hendrikse et al.r Sensors and Actuators B 59 (1999) 35–41 w7x E.R. Kotz, ¨ H. Neff, Anodic iridium oxide films, an UPS study of emersed electrodes, Surf. Sci. 160 Ž1985. 517. w8x S.M. Sze, Semiconductor Devices, Chap. 5, Wiley, New York, 1985. w9x J. Hendrikse, W. Olthuis, P. Bergveld, Characterization of the

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E MOSFET, a novel one electrode chemical transducer for redox measurements, J. Electroanal. Chem. 485 Ž1–2. Ž1998. 23–29. w10x A.J. Appleby, Electrocatalysis of aqueous dioxygen reduction, J. Electroanal. Chem. 357 Ž1993. 117–179.