Anal. Chem. 1995, 67, 4586-4588
Miniaturized Reference Electrode Based on a Perchlorate-SelectiveField Effect Transistor Wilhelm POtter, Christa Dumschat,* and Karl Cammann lnstitut fur Chemo- und Biosensorik, Westfalische-Wilhelms-Universii'at Munster, Mendelstrasse 7, 13-48749 Mclnster, Germany
A new type of reference field effect transistor (REFET) based on a perchlorate-selectivefield effect transistor has been developed. The top of the ion-selective layer is coated with another layer containing solid potassium perchlorate. Potassium perchlorate is not readily soluble in aqueous solutions; therefore, the perchlorate activity in the aqueous surface layer is equal to that of a saturated solution. The constant activity leads to a constant potential of the REFET. The stability of this REFET potential was examined in solutions of different salt concentrations and different pH values. The iduence of the pH value on the REFET potential was compared to that of a commercially available Ag/AgCl electrode. According to this comparision, the perchlorate reference electrode proved to be very well suited for an integration on an ISFET chip. Compared with ion-selective electrodes (ISEs),the ion-selective field effect transistor (ISFW is small and physically robust and can be produced by microelectronic methods, with a future prospect of lowcost bulk production. However, until now, ISFETs were used mostly in combination with conventional reference macroelectrodes. This reduces considerably the benefits gained from the miniaturization of the sensor. Attempts to miniaturize reference macroelectrodes of the second kind'J resulted in a serious reduction of the electrode lifetime due to the reduced electrolyte quantity. Therefore, reference electrodes without a reference electrolyte appear to be advantageous. It can be shown that a decreasing the number of hydroxy groups at the gate surface of an ISFET will lead to an insensivity of the ISFET toward the pH values3 To achieve this goal, insensitive polymer membranes have been used as the gate surface.j,j However, Collins described the practical limits of these reference field effect transistors (REFETs), which suffer from the difficulty in defining the interface between the insensitive membrane and sample.6 A REFET based on a fluoride-selective ISFET with CaF2 as internal reference electrolyte and a polymer membrane which reduces its diffusion has also been described.' The sensor (1) Smith, R.; Scott, D. C. IEEE Trans. Biomed. Eng. 1986,33,83-90. (2) van den Berg, A: Griesel, A,; van den Wekkert, H. H.: de Rooij, N. F. Sens. Actuators B 1990,1, 425-432. (3) van den Berg, A; Bergveld, P.; Reinhoudt, D. N.: Sudholter, E. J. R. Sens. Actuators 1985,8, 129-148. (4) Matsuo, T.; Nakajima. H. Sens. Actuators 1984,5,293-305. (5) Skowronska-Ptasinska, M.; van der Wal, P.; van den Berg, A,; Bergveld, P.; Sudholter, E.; Reinhoudt, D. Anal. Chim. Acta 1990,230, 67-73. (6) Collins. S. D. Sens. Actuators B 1993,10, 169-178. (7) Lisdat. F.; Montz, W. Sens. Actuators B 1993,15, 228-232. 4586
Analytical Chemistry, Vol. 67, No. 24, December 75. 7995
potential is influenced by fluoride and calcium ion concentrations in the sample and by varying d&ssion potentials at the interface of the internal electrolyte and sample because of the low electrolyte concentration within a saturated CaFz solution. A further REFET with reference electrolyte has been described by Comte and Janata.* A stable potential is obtained using buffered agarose on top of a pH-selective ISFET, with a glass capillary as liquid junction. Nevertheless, a significant influence of the pH on the REFET potential due to the capillary tip potential remains. To combine the benefits of a longer lifetime with an excellent potential stability, we have sucessfully developed a new perchlorate reference electrode? In this reference electrode, a perchlorate-sensitive sensor is in contact with a saturated solution of KC104and Cas04 over solid KC104 and CaS04. The perchlorate activity in this solution is determined by the solubility of KC104 (0.148 mol/L at 25 OC),'O and the corresponding constant perchlorate activity leads to a constant electrode potential. The perchlorate-sensitive sensor and the solution are located in a cavity, with a liquid junction to the analyte solution. A prolonged lifetime compared with those of Ag/AgCl reference electrodes is achieved by adjusting of the perchlorate activity, considering by the solubility of KC104, which is much lower than that of KC1. Furthermore, interferences are low due to the high selectivity of the perchlorate-sensitive membrane. In addition, no influence of redox couples is expected. EXPERIMENTAL SECTION
Architecture of the REFET. To build up a REFET/ISFET combination on one transducer, chips with two gate areas and dimensions of 3.0 mm x 3.0 mm (Institute of Microtechnology, University of Neuchltel, Neuchltel, Switzerland) were used." The gate material was pH-sensitive Al203. Only one of these areas was modified with a hollow cylinder made of epoxy resin. This cylinder had a height of 1.5 mm, an outer diameter of 2.0 mm, and an inner diameter of 1.4 mm (Figure 1). The gate area at the bottom of the cylinder was covered with a perchlorate-sensitive acrylate membrane containiig 48% (w/w) acrylate oligomer (Ebecryl600, Radcure, Drogenbos, Belgium), 27% (w/w) 1,Ghexanediol diacrylate, 1%(w/w) 9,lOphenanthrenequinone as photoinitiator, 20% (w/w) 2-nitrophenyl octyl ether as organic solvent, and 4% (w/w) tridodecylmethylammonium nitrate as electroactive component. The membrane (8) Comte, P. A.; Janata, J. Anal. Chim.Acta 1978,101, 247-252. (9) Dumschat. C. German Patent DE 43 02 322 Al. 1993. (10) Noyes, A A: Boggs, C. R.; Farrel. F. S.; Stewart, M. A.J. Am. Chem. SOC. 1911,33,1650-1654. (11) Lfer, S.: Cammann, K. Sens. Actuators B 1992.7, 572-575. 0003-2700/9510367-4586$9OOiO 0 1995 American Chemical Society
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was cast directly on the ISFET surface without an internal electrolyte. It was covalently bound to the surface by 3-(trimethoxysily1)propyl methacrylate which had been heated for 1h at a temperature of 150 "C. M e r photopolymerization with W light for 90 s (Amw = 385 nm, Uvaspot 400/K, Dr. Honle, GmbH, Planegg, Germany), the membrane was conditioned for 24 h in a solution of saturated potassium perchlorate. The space above this membrane was filled with solid KC104and CaS04.2HzO in a mass ratio of lO:l, corresponding to their solubilities in aqueous solutions. CaSOg2HzO was added to "izediffusion potentials at the interface between reference electrolyte and sample. This was covered with an 80 pm thick polyethylene/polyester film (GHQ, Rem, Heubach, Germany). A small hole of -30 pm in this film allowed placement of a liquid junction at the interface between the reference solution and sample. The bond pads of the chip were contacted by a Pt wire of 25 pm diameter, and the whole chip was encapsulated with epoxy resin (Epo-Tek 905, Polytek GmbH, Waldbronn, Germany). Measurements. The ISFET/REFET combination was examined in a constant drain current mode (& = 100 f i ) ,with a corresponding drain voltage (Ids) of 0.5 V. This employed an ISFET amplifier (Institute of Microtechnology, University of NeuchAtel) with an Ag/AgCl electrode as reference electrode (W 02; outer filling solution, 10%(w/w) KNO3, Orion, Boston, MA). Besides the output signals of the two ISFETs, the amplifier supplied the d ~ e r e n c between e these potentials as a third signal. The stability of the REFET potential was observed in different NaC1, KC1, and NH4N03 solutions versus the external reference electrode. The salt concentrationsin these solutions ranged from up to lo-' mol/L. The drift of the sensor was determined in 0.1 M NaCl solutions for a period of 16 h. Furthermore, the influence of the pH value on the REFET potential was investigated. For this purpose, an alkaline buffer solution (0.01 mol/L NaOH and 0.01 mol/L tris(hydroxymethy1)aminomethane) was titrated with different amounts of 1 M HCl. The pH at each point of this measurement was detected with a commercially available glass electrode (HA-265S7, Ingold, Steinbach, Germany) which had been calibrated just before the measurement. The influence of the pH on the REFET potential was also investigated using NBS buffer solutions (for pH 1.68, 3.56, 4.01, 6.86, 9.18, and 12.45, Sensortechnik Meinsberg, Waldheim, Germany) and an additional buffer solution (for pH 10.00, Merck, Darmstadt, Germany). This resulted in another composition of the sample at each pH and greater differences in the ion strengths of these solutions. RESULTS AND DISCUSSION
The measurements in different salt concentrationsas reported above showed small potential steps up to 2 mV/decade at the
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higher salt concentrations (Figure 2). These potential steps can be interpreted as diffusion potentials which appear at the interface between the reference solution and the sample. In electrodes with a reference electrolyte, it is impossible to avoid these diffusion potentials. Therefore,the aim is to minimize these potentials.The small reported diffusion potentials show that this aim has been reached. Furthermore, it must be mentioned that the diffusion potentials are composed of diffusion potentials at the interface between the reference electrolyte and the sample at the REFET and at the Ag/AgCl reference electrode. It is impossible to detect them separately. The drift of the corresponding measurements had a mean value of 0.6 mV/h. Besides the natural drift of an ISFET itself, this drift may also be caused by direct casting of the ion-selective membrane on the ISFET surface, as this interface cannot be defined thermodynamically. A more complete characterization of the new REFET was done by testing a pH-sensitive ISFET versus the REFET. The investigated pH values and the corresponding response curves of the REFET and the pH-sensitive ISFET at the titration are shown in Figure 3. The solid line curve is the difference between the ISFJ3T and the REFET potentials. As expected, there is a large Muence on the ISFET and only low influence on the REFET potential. The relationship between pH and the potential of each electrode suggests a linear relationship between both the ISFET potential Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
4587
Table 1. Results of the Linear Regression of the ISFETBEFET and ISFETIReference Macroelectrode Response Curves In Figure 3
slope residual standard deviation operation standard deviation
pH-FETvs Ag/AgC1
pH-FETvs REFET
-54.6 mV/pH unit 13.0 mV
-54.0 mV/pH unit 13.4 mV
10.055pH unit
f0.062 pH unit
versus the the reference macroelectrode and the ISFET potential versus the REFET potential. The results of the linear regression of the curves shown there are presented in Table 1. Comparing the values for slope, residual standard deviation, and operation standard deviation, it can be seen that they are approximately the same in size and are only slightly better for the reference macroelectrode. The excellent results of this attempt encouraged us to investigate the influence of the pH by using of different buffer solutions. This time, the curves of the ISFET/REFET and the ISFET/reference macroelectrode combination are not as similar as in the case of titration of an alkaline buffer solution, although a linear relationship for both curves is obtained. The greater diffusion potentials of the REFET using different buffer solutions lead to other values at the linear regression. These larger values for residual standard deviation and operation standard deviation are also found for the Ag/AgCl macroelectrode vable 2). In this case, the REF0 and the Ag/AgCl reference macroelectrode both perform well.
4588 Analytical Chemistry, Vol. 67, No. 24, December 15, 7995
Table 2. Results of the Linear Regression of the ISFETIREFET and ISFETIReference Macroelectrode Response Curves Using Buffer Solutions
pH-FETvs Ag/AgC1 slope residual standard deviation operation standard deviation
PH-FETvs REFET
-53.1 mV/pH h5.8 mV
-52.0 mV/pH h5.8 mV
10.109 pH unit
kO.112 pH unit
All measurements reported so far were reproducible within the first 4 days of the REFET lifetime. After this time, the solid electrolyte reservoir was dissolved. CONCLUSIONS The results show that the perchlorate reference electrode is very well suited for integration on an ISFET chip because of its feasibility of being miniaturized, its good reference potential stability, and its prolonged lifetime. The drift of the REFET does not reduce the quality of the sensor because of its small value, which is only a bit larger than the drift of the ISFET itself. In spite of the miniaturization, the stability of the REFET potential is nearly equal to that of a reference macroelectrode. Received for review August 9, 1995. Accepted September
28, 1995. AC9508053 a Abstract published in
Advance ACS Abstracts, November 1, 1995.
