A novel way of eliminating drift problems in metal oxide pH sensors is presented. The method employs a FET-structure under the electrode that uses the metal oxide as a gate contact. In addition to the enhanced drift properties, the new sensor has an almost ideal nernstian response. First a theoretical explanation is given, which is then confirmed by measurement

Bergveld, P.

Hendrikse, J.

Olthuis, W.

English

Olthuis, Wouter

Bergveld, Piet

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A drift free nernstian iridium oxide PH sensor

University of Twente Research Information

4C3.02 A DRIFT FREE NERNSTIAN IRIDIUM OXIDE PH SENSOR J. Hendrikse, W. Olthuis and P.Bergveld MESA Research Institute, University of Twente P.O. Box 217,7500AE Enschede, The Netherlands E-mail: j .hendrikse@el.utwente.nl Fax: x-31 534 892 287 SUMMARY A novel way of eliminating drift problems in metal oxide pH sensors is presented. The method employs a FET- structure under the electrode that uses the metal oxide as a gate contact. In addition to the enhanced drift properties, the new sensor has an almost ideal nemstian response. First a theoretical explanation is given, which is then confirmed by measurements. Keywords: pH sensor, drift, FET-structure INTRODUCTION Potentiometric metal oxide pH sensors have been proposed and tested previously [ 11, but the drift problems that are caused by redox couples present in the solution have not been solved. In this paper we present and successfully test a method for the removal of this type of drift. A second problem is the non-nernstian response of most of these sensors. The pH dependence of the sensor we present is almost ideal, even though we chose iridium oxide as an electrode material, which is notorious for its non-nemstian behavior. DRIFT PROBLEMS IN NOBLE METAL OXIDE PH ELECTRODES Electrodes made of hydrated metal oxides MeO(OH), where Me can be Ir,W,Pt, or any other of a number of metals [ 2 ] ,  take part in redox reactions of the type MeO(0H) f) MeO, + H' + e. For the potential of these electrodes with respect to a reference, E, it is easily derived that where k, T and q have their usual meaning, Eo is the standard electrode potential and a, = y i  [ I ]  is the activity of species I, which for the metal oxide molecules has been split into their activity coefficient yi and concentration [I]. The last term on the right-hand side suggests that the use of these electrodes as nernstian pH sensors is possible. In their actual use however, two problems arise. When electrochemically active substances, like oxygen, are present in the solution, they influence the concentrations of MeO(0H) and MeO,. This changes the second term of Eq. (l), thus causing drift. Moreover, the activity coefficients yi may be pH dependent, thus causing a non-nemstian pH response of the electrode [3]. If the change in the activity coefficients is slow after a change in pH, a second type of drift can be identified with this process. In contrast to the frst  type of drift, this type of drift can take place even if there are no electrochemically active substances present in the solution and does not involve the exchange of protons. Drift having these characteristics was reported by Petrii and VitinS. [41 RESOLVING DRIFT PROBLEMS BY MEANS OF AN EMOSFET We propose a new device, denoted the EMOSFET, where the electrode plays the role of the gate contact in a MOSFET-structure, It can be shown theoretically that the threshold voltage of the EMOSFET, V,, depends on the electrode potential, E, and on the pH of the electrolyte.[5] Therefore, V, suffers not only from exactly the same drift problems as E does, but also from the deviation from the nernstian response that is caused by the pH dependent activity coefficients: +-ln(u kT )-A@ (2) 4 H +  where C is a constant andA4; is the potential difference between the metal oxide and the electrolyte solution, which is known to be determined by the pH. If Eq. ( 2 )  is subtracted from Eq. (1) we find: E - V ,  = - C + A 4 f  (3) So, what we actually determine when V, is subtracted from E is, apart from a constant factor, the potential difference between the iridium oxide and the solution. In the literature, potentials across the metal oxide - solution interface have been TRANSDUCERS '97 1997 International Conference on Solid-state Sensors and Actuators Chicago, June 16-10, 1997 0-7803-3829-4/97/$10.00 1  997 IEEE 1367 4C3.02 studied for oxides of for example Si and Al. The pH dependency of these potentials is often lower than the ideal value because the buffer capacity of the oxide surface with regard to protons is not large enough compared to the charge on the double layer capacitance. Tantalum oxide is in this respect an example of nearly ideal behaviour. Since for iridium oxide E is always measured rather than A4f , literature values are unavailable for this oxide. EXPERIMENTAL MOSFET-structures having a gate of 500pm wide and 15pm long were fabricated using standard NMOS processing steps also used in the fabrication of ISFETs. [7] As a gate contact a f 100 mn thick Sputtered Iridium Oxide Film (SIROF) was deposited on top of the gate oxide by sputtering an Ir target in an O,/Ar atmosphere. Iridium oxide films prepared in this way are known to have very small pores resulting in an intimate contact between the film and the electrolyte. The SIROF was shaped as a 2000pm x 100pm rectangle symmetrically placed on top of the gate area by the use of lift-off techniques. In order to insulate the relevant parts of the sensor from the solution, a tantalum oxide film was deposited around the gate area, on top of which a polyimide (PI) film was applied to improve the adhesion of the Hysol encapsulation. The resulting structure is shown in Fig. 1. The devices were then mounted on a piece of printed circuit board and packaged using Hysol epoxy resin. tion Gate Oxide Figure 1: Cross section of the MOSFET-device. The devices were immersed in an aceticiboriciortho phosphatic acid buffer (pH=2.5) at room temperature and both E and V, were determined while the pH was changed by addition of small quantities of 1 .O M NaOH. In order to maximize the drift of the conventional sensor signal, E, during the experiment, the solution was purged alternately with N, and air so that the oxygen concentration was varied. In all experiments analytical grade chemicals (Merck) were dissolved in demineralised water having a specific resistance of more than 5 MQ cm. In order to determine VT, the FET was connected to a circuit keeping both the drain current and the drain-source voltage at a constant value, adjusting the gate voltage by means of feedback. The output voltage of this circuit tracks any change in the threshold voltage, which is up to a constant identical to the flat band voltage. Since Eq. (3) already involves the unknown constant C, this difference is of no importance. In order to determine E, the potential of the SIROF with respect to a Radiometer REF200 Ag/AgCl reference electrode was measured simultaneously using a voltage follower having a 10”Q input impedance. The set-up is shown in Fig. 2 .  Note that the reference electrode is not needed for the VT measurement because the iridium oxide is directly connected to the ground of the amplifier. EMOSFET - -E A N Unity - Figure 2: The measurement set-up. In order to test the drift properties of the new sensor, it was placed in a thermostatted phosphate buffer (T= 300 K, pH=4.85) and both E and V, were monitored for 19 hours using the same set-up. 1368 TRANSDUCERS ’97 1997 lnternational Conference on Solid-state Sensors and Actuators Chicago, June 16-19, 1997 RESULTS As can be seen in Fig. 3, the alternating purging of the solution with N, and Air makes both E and V,, behave in an inpredictable manner, though a sudden change in E can be noticed at the moments when the pH is changed. 0.1 -0.0 9 -0.1 W w -0.2 -0.3 -0.4 c -0.5 -0.6 >' 0 5 10 15 20 Time (min) Figure 3: The electrode potential and threshold voltage as a function of time, while alternately purging with N2 and air. At the same time the pH was changed, as is indicated by the arrows. The difference between the two, E-V,, is presented in Fig. 4, and reacts in a perfectly stepwise fashion to the pH changes. 0.8 0.7 0.6 0.5 F >' I 0.4 0.3 0.2 0.1 i Time (min) Figure 4: The threshold voltage minus the electrode potential as a function of time. PH changes are indicated by arrows Moreover, when the height of the steps in Fig. 4 is determined as a function of the pH, the nemstian response shown in Fig. 5 is the result. Apparently, the buffer capacity of the porous 4C3.02 iridium oxide is quite large compared to the charge on the double layer capacitance, [6] thus resulting in an ideal nernstian response. 0.8 7- 0.6 0.5 - ~ 9 >+ I 0.4- W 0.3 0.2 ~ - n i l  1 1 1 ". I 2  4 6 8 10 PH Figure 5: The electrode potential minus the threshold voltage as a function ofpH. A linear3t gives a slope of -59.8 f 0.3 m V/PH with an r2 value of 0.99983 The drift properties of the sensor while the solution is continuously in contact with air are shown in Fig. 6. While both E and V, suffer from drift, the sensor signal, E-V,, is the same within 13 mV over a period of 19 0.30 0.20 3 0.10 a -0.00 z + c a, 0 w -0.10 hours. I , ,  I l l , ,  , I l l  1 , 1 1 1  l l i  0 5 10 15 20 Time (h) Figure 6: The electrode potential, the threshold voltage and the difference between the two as a function of time. The sensor device was placed in a thermostatted stirred beaker containing a phosphate buffer (pH=4.85). TRANSDUCERS '97 1997 lnternational Conference on Solid-state Sensors and Actuators Chicago, June 16-19, 1997 1369 4C3.02 DISCUSSION AND CONCIdUSION As can be seen in Fig. 5 ,  a pH sensor with a nemstian response was developed by subtracting the threshold voltage of a MOSFET having a noble metal oxide gate contact from the usual potentiometric pH response. Two important problems are thus resolved. Not only is drift arising from oxidizing or reducing agents in the solution eliminated, but also problems due to pH dependent activity coefficients are solved. The method has a clear theoretical background which is proven by measurements. As was observed earlier, the nernstian response of the potential across the metal oxide-solution interface that was found means that the buffer capacity of the oxide surface is very large compared to its double layer capacitance. This phenomenon can be explained in the following way. The pores inside the iridium oxide are so small that the formation of an electrical double layer at the outer Helmholz plane is hindered, whereas the adsorption of protons at the inner Hehnholz plane is still possible. This means that small pores will be able to enhance the buffer capacity of the iridium oxide while the double layer capacitance is more or less constant. The drift properties of the new sensor are considerably better than those of classical potentiometric sensors; compare E and E-V, in Fig. 6. According to the theory, the remaining drift may be caused by changes in the potential across the metal oxide interface or could originate from changes in the “constant” C, possibly due to changes in the FET-structure. ACKNOWLEDGEMENT The authors thank Johan Bomer for technical advice and realization of the devices. REFERENCES A. Fog, R.P. Buck, Sensors and Actuators, 5, 137-146, (1 984) C.G. Granqvist, Handbook of inorganic electrochromic materials, Elsevier 1995 W. Olthuis, M.A.M. Robben, P.Bergveld, M. Bos and W.E. van der Linden, Sensors and Actuators, B4 O.A. Petrii and A. VitinS, Electrokhimiya, 27,461-476, 1991 J.Hendrikse, W. Olthuis and P. Bergveld, submitted to J. Appl. Phys. R.E.G. van Hal, J.C.T. Eijkel and P. Bergveld, Adv. Colloid Interface Sci., 3 1-62, 1996 P. Bergveld and A. Sibbald, Comprehensive Analytical Chemistry Vol. XXIII: Analytical and biomedical applications of ion-selectivejeld-effect transistors, (Elsevier, Amsterdam, 1988) 247-256, 1990 1370 TRANSDUCERS ’97 1997 International Conference on Solid-state Sensors and Actuators Chicago, June 16-19, 1997 

984) C.G. Granqvist, Handbook of inorganic electrochromic materials, Elsevier

Comprehensive Analytical Chemistry Vol. XXIII: Analytical and biomedical applications of ion-selectivejeld-effect transistors,

A drift free nernstian iridium oxide PH sensor

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