493
Anal. Chem. lS00, 60, 493-490
mentally important, and thermally labile compounds such as Aldicarb, Silvex, and Cyanazine. Detailed studies with these compounds will be reported elsewhere. It should be noted that all the E1 mass spectra illustrated in this paper (Figures 4 and 7-10) are without background subtraction. The low background levels shown are typical for normal MAGIC-LC/MS operation. This is in contrast to the high chemical noise which is found with the moving belt interface, resulting from continuous thermal desorption of phthalate plasticizer from the belt material. Scope of t h e MAGIC-LC/MS Interface. As yet, the full range of compound types accessible with the MAGIC-LC/MS interface has not been fully investigated. In general terms, all compounds that are known to be capable of generating E1 and CI spectra and that have been tried with the interface have generated good quality spectra Thew include carbamate and triazine pesticides, phenyl urea herbicides, polynuclear aromatic hydrocarbons, plant alkaloids, antioxidanta, and EPA Appendix 8 compounds. Compounds known not to generate E1 spectra, such as simple sugars and certain azo dyes, predictably do not generate spectra with the current system. The primary mode of ion formation appears to be through a flash vaporization step in the ion source, followed by E1 or CI ionization, as selected. Any limitation on compound type accessible with MAGIC-LC/MS will be influenced by (1) whether the molecule is functionally capable of generating an E1 spectrum and (2) whether the compound is sufficiently volatile to be capable of forming an adequate vapor pressure in the ion source. This latter property will ultimately represent the interface limitation for generation of E1 and CI spectra,
rather than the molecular weight of the species. Nevertheless, it is possible to generate good searchable E1 spectra with strong molecular ions even for quite involatile species. An example is provided by reserpine, with a molecular weight of 608, which generates a searchable E1 spectrum with a strong molecular ion at m / z 608. Registry No. &-Retinol acetate, 29443-87-6;trans-retinol acetate, 127-47-9;lauric acid, 143-07-7;myristic acid, 544-63-8; palmitic acid, 57-10-3; stearic acid, 57-11-4. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
Arpino, P. J. J. Chromefogr. 1985, 323, 3. Krost, K. J. Anal. Chem. 1985, 5 7 , 763. Vestal, M. L. Int. J . Mass Spectrom. Ion W y s . 1983, 46, 193. Blakley, C. R.; Vestal, M. L. Anal. Chem. 1963, 55, 750. Voyksner, R. D.; Yinon, J. J. Chromafogr. 1966, 354, 393. Willoughby. R. C.; Browner, R. F. Anal. Chem. 1984, 5 6 , 2626. Browner, R. F.; Winkler, P. C.; Perklns, D. D.; Abbey, L. E. Mlcrochem. J . 1966, 3 4 , 15. (8) Browner, R. F.; Boorn, A. W.; Smith, D. D. Anal. Chem. 1962, 54. 1411. (9) Hinds, W. C. Aerosol Technology; Wlley-Interscience: New York, 1982. (IO) Kantrowitz. A.; Grey, J. Rev. Sci. Insfrum. 1951, 22, 328. (11) EPAlNIH Mass Spectral Data Base; Heller, S . R., Milne, G. W. A.; Eds.; U.S. Government Printing Office: Washington, DC, 1978.
RECEIVED for review September 16, 1986. Resubmitted October 29,1987. Accepted November 16,1987. This research was supported by the U S . Department of Energy, Office of Basic Energy Sciences, under Grant No. DE-FG05-85E13435. We are grateful to Hewlett-Packard Scientific Instruments Division, Palo Alto, CA, for the loan of the 5988A mass spectrometer.
Elimination of Neutral Species Interference at the Ion-Sensitive Membrane/Semiconductor Device Interface Xizhong Li, Elisabeth M. J. Verpoorte, a n d D. Jed Harrison*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T 6 G 2G2
Semlconductor electrodes coated with a K+-responslve pdy(vlnyi chiorlde) (PVC)-based membrane, wlth the strucare found to respond to K+ ture n-SI/SlO,/SI,N,/membrane, In a Nernstlan fashion vla a fleld-effect mechanism. Such electrodes respond to CO,, benrolc acid, and ascorbic acld as Interfered. However, lntroductlon of a Ag/AgCI layer to glve n-SI/SIO,/SI,N,/Ag/AgCl/membrane electrodes ellmlnates the interference. Spectroscopic evidence clearly shows that benrolc acld permeates the membrane at low pH. Analysts of the Impedance response of PVCbased K+ membranes demonstrates that all the lnterferents permeate the membrane, resultlng In a reversible reductlon In bulk membrane resistance. Relevance to Ion-sensltlve field-effect transistor devices is discussed.
Ion-responsive semiconductor devices such as the field-effect coupled diode or ion-sensitive field-effect transistor (ISFET) are the subject of continued research interest (1-3). Recently, it has been reported that pH-sensitive ISFETs coated with poly(viny1chloride) (PVC)-basedK+-sensitivemembranes are subject to a number of unexpected interferences (4). Species 0003-2700/88/0360-0493$0 1.50/0
such as COz, acetic acid, an benzoic acid, which show no interference at ion-selective electrodes (ISE) employing the same membrane, cause significant potential shifts at K+ membrane coated ISFET's (4). Diffusion of neutral species through the membrane to the silicon nitride surface of the device gate, resulting in a pH shift at the gate, is postulated as the cause of this effect (4). In ISFET operation, the potential induced via a field effect in the Si substrate is controlled by the charge state of the silicon nitride/membrane interface (1-3). For fixed K+ concentration in solution the surface silicon nitride charge will not remain fmed if the proton activity varies at the solid surface. Consequently, the standard cell potential of the ISFET/reference electrode system will be dependent on the history of the silicon nitride surface if the membrane is permeable to acids or bases. Solid-state contacts to replace the internal reference solution of a K+-sensitivePVC, dioctyl adipate, valinomycin ISE have been previously proposed (5,6).The most successful solidstate internal contact is the Ag/AgCl reference system. Simon has pointed out (7) that by incorporating KB(C,H,), in the PVC membrane, in addition to the other components, the Ag/AgCl/membrane/solutionstructure is unblocked to charge transfer at each interface and the thermodynamics are not 0 1988 American Chemical Society
494
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
The impedance for an MIS electrode is then approximated by
ZR =
Figure 1. Equivalent circuit representation of a n-Si/insulator/K+ membrane electrode: R,,series resistance of Si substrate and solution; C , insulator capacitance; C,, space charge capacitance of n-Si substrate; C,, R,, geometric capacitance and resistance of the membrane, respectively; CHI double layer capacitance; Ret, charge transfer resistance; W , Warburg impedance.
dependent on charge stored at the interfaces, leading to more reliable solid-state electrode performance. In considering a Ag/AgCl layer as a constituent of an ISFET gate, we note that Buck and Hackleman have previously examined the opencircuit voltage and impedance response of n-Si/SiOz/ AgBr/solution electrodes (8). They established that such electrodes respond to Ag+ or halogen ions via a field-effect mechanism and can serve as a model of ISFET response. In this paper we demonstrate that by rendering the electrode surface insensitive to proton activity changes, it is possible to eliminate the interference of neutral acids at PVC membrane coated solid electrodes. We have studied the K+-responsivemembrane composed of PVC, dioctyl adipate, valinomycin, and KB(CBH&4coated on a semiconductor electrode that serves as a model of the ISFET. The coated n-Si electrode has an interfacial structure of n-Si/insulator/Ag/AgCl/membrane and responds to K+ through a field effect. Such electrodes do not respond to interfering species such as COz or ascorbic acid that cause a potential shift at K+-sensitive,membrane coated ISFETs (4).Ultraviolet (W) spectroscopy of the membranes demonstrates unequivocally that benzoic acid does permeate the K+ membrane at low pH. Finally, initial results show that analysis of PVC membrane impedance response is a valuable tool in studying penetration of these membranes by benzoic acid and other lipophilic species. Membrane Equivalent Circuit Analysis. Analysis of the impedance response as a function of frequency provides greater insight into membrane properties than measurement of open-circuit voltage alone (9,IO). Recent impedance studies of valinomycin-based PVC membranes (11-14)have led to greater understanding of the nature of the potential determining mechanism and the role of various membrane constituents. On the basis of a knowledge of the membrane impedance ( I I) and the semiconductor electrode characteristics, an equivalent circuit of the MIS electrodes we have examined is proposed in Figure 1. The semiconductor electrode introduces a space charge capacitance, C,, and in series, a geometric capacitance of the insulator, Cox. These give a total electrode capacitance, CE = C,C,/(C, + C,). The membrane and solution impedance can be modeled as two lumped impedance networks (11-14). The bulk membrane properties lead to a geometric resistance, R,, and capacitance, C,, in parallel. The solution/membrane interface leads to a network consisting of the double layer capacitance, CH, the charge-transfer resistance a t the interface, Rct, and the diffusional, or Warburg, impedance, W. Series resistance of the solution and the Si electrode, R,, is small (100 Q or less) and compared to membrane impedance (lo6 Q or more) can be neglected in analysis of the circuit. The element9 arising from the membrane/solution interface are observed a t frequencies below 5 Hz (II), indicating a long time constant. This circuit element will show high frequency limiting behavior over the 25 Hz to 40 kHz range used in this study and so is not observed in our data.
D IbB
1
+ (wR,C,)'
where 2, is the real component of impedance and ZIMis the imaginary component. Equations 1and 2 predict a semicircle in the impedance plane at high frequencies, and when w is sufficiently low, the electrode capacitance, CE,will dominate, leading to a rapidly rising tail a t low w. In the limit where 1 / d E is the dominant term in eq 1 and 2, the imaginary component of the admittance, YIM,is given by Y I M = wCE. Consequently, at sufficiently low frequencies the imaginary admittance will be proportional to the capacitance of the space charge layer and show a functional dependence on voltage. An approximate but more general relation is obtained when (WC,R,)~ 10 to give eq 4, which predicts that admittance is directly proportional
(4) to C, and has become inversely proportional to CE,the semiconductor/insulator impedance. At sufficiently high frequencies, bulk membrane properties will dominate and the equivalent capacitance, Ym/w, will be independent of applied voltage, as long as C, is voltage independent.
EXPERIMENTAL SECTION Semiconductor electrodes were prepared from n-Si wafers, (100) face (Monsanto, 4-in. diameter),onto which 500 A of SOz,followed in some cases by 500 A of silicon nitride (nominallySi3N4),were deposited by chemical vapor deposition by Process Technology, Ltd. (Oromocto, New Brunswick, Canada). Insulating film on the back face was removed by application of etchant; 5 min of 10% HF (Si/Si02)or 5 min of 48% HF (Si/Si02/Si3N4).Ag was sputter deposited under vacuum to -2000 A thickness on the front, polished face of several wafers. Wafers were scribed to give -7 mm X 7 mm pieces. Ohmic contact was made to the back face with Ga/In eutectic (15)and then Ag epoxy contacted the back face to a coiled Cu wire. This assembly was cured (100 "C, -3 h) and then cleaned in H20,CH30H,and trichloroethylene, followed by 30 s in 5% HF. Electrodes were then insulated with epoxy (Clear,Hysol) to give exposed areas -3 mm X 3 mm, cured 24 h at room temperature, and recleaned with H20,CH30H, and trichloroethylene. Ag-coated electrodes were chloridized chemically by immersing in a 1 M NaC1, 16% HN03solution for 3 to 4 min. A white film was observed on the Ag surface following treatment. Under a tetrahydrofuran-saturated atmosphere, membranes were cast by evaporation of 1 to 4 drops of the following solution: 3 mL of tetrahydrofuran (BDH, distilled from LiAlH4),0.15 g of dioctyl adipate (Fluka), 0.075 g of poly(viny1 chloride) (Polyscience,chromatographic grade), 0.0015 g of Valinomycin (Sigma), 25 fig of KB(C,H,), (16). UV spectra were obtained with an HP 8451A diode array spectrophotometer. Free-standing membranes were soaked 90 h in the indicated solution, rinsed with doubly distilled H20,and mounted on quartz slides in the spectrometer. Open-circuit voltage measurements were made by using an Orion 701A ion-selective electrode meter and a sodium chloride saturated calomel electrode (SSCE). A glass electrode (Orion) was used to determine pH. K+ calibration curves were measured at fixed ionic strength by using 0.1 M NaCl as ionic buffer. M KCl, 0.1 M trisodium Interference of acids was examined in
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
4Q5
Table I. Effect of Addition of Interferents on Electrode Potentials
interface structure on Si/SiO," SiBNI Si3N4/membrane membrane Ag/ AgCl/membrane Si3N4/Ag/AgCl/membrane
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citrate adjusted to the desired pH with HC1 or NaOH. A 0.1 or 0.01 M KNOBsolution was used to examine C1- interference by addition of KC1 solution of equal concentration. Impedance measurements and capacitance-voltage (C-V) curves were made with the electrochemicalcell in a Faraday cage. An Amel 521 potentiostat (Amel, Milan, Italy) used in threeelectrode confiiation provided potentiostatic control and current measurement,with an HP 200 AB audio oscillator to provide a 10-mV peak-to-peak sine wave, and a Hi-tek PPRI waveform generator as source of a 5 mV/s dc ramp. A PAR Model 5204 lock-in analyzer gave in-phase (real) and out-of-phase (imaginary) components of the current, with respect to the phase of the applied voltage oscillation. The PAR 5204 output was monitored with an HP 7090A digital recorder/plotter and the digitized signal was transferred to an IBM PC for data analysis. Instrumental phase shifts were compensated by replacing the electrochemicalcell with a capacitor (between 10 pF and 40 nF) and adjusting the in-phase component of current to zero at each frequency. For capacitance-voltage curves the system was calibrated by replacing the cell with a number of capacitors varying from 2 pF to 80 nF in series with a 100-Qresistor. The out-of-phase component is then directly proportional to equivalent capacitance of the cell (17,18). Impedance and C-V curves were acquired in solutions of constant ionic strength either 0.1 M in NaCl or 0.1 M in trisodium citrate buffer adjusted with HCl or NaOH. A SCE with a double junction filled with 0.1 M NH4Cl served as reference electrode, and a large-area Pt mesh (80 mesh) acted as counter electrode. Neither of these electrodes affected the impedance measurements.
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[Benzoate] (MI Figure 2. Open-circult voltage of a n-Si/insulator/membrane electrode as a function of benzoic acid concentration, in lob3M KCI, 0.1 M trisodium citrate, at pH 4.5 (') and pH 8.6 (+). The response of a n-Si/SiO,/Si,N, electrode with no membrane coating is also shown, pH 4.5 (0).
80 mV as the benzoic acid concentration increases from 0 to 30 mM in a M K+, 0.1 M citrate, pH 4.5 solution, Figure 2. At pH 4.5 about 33% of the benzoic acid is in the neutral, protonated form. However, at pH 8.6, where the benzoate ion is the dominant form (>99.9%), the same MIS electrode shows no change in V , with increasing [benzoate], Figure 2. In RESULTS AND DISCUSSION contrast, a naked n-Si/SiOz/Si3N4electrode does not respond to benzoic acid at either pH (pH 4.5 shown, Figure 2), indiWe have examined the response of a number of K+ sensitive membrane coated semiconductor electrodes to K+ and the cating the Si3N4surface has no specific response to benzoic possible interferent species ascorbic, benzoic, and carbonic acid at a fixed, buffered pH. Bicarbonate shows a similar acids (4). Electrodes with an n-Si/SiOz/Si3N4/membrane effect at the MIS electrodes we have examined to that reported structure have been used as a model of the exposed gate of ( 4 ) previously for K+ responsive ISFET's, Table I. AddiSi3N4coated ISFET devices and are referred to here as MIS tionally, the data in Table I also show that ascorbic acid interferes at MIS electrodes. Both ascorbic and carbonic acid (membrane/insulator/semiconductor)structures. The open circuit voltage, V,, of MIS electrodes responds linearly to K+ show a much stronger "memory" effect at MIS electrodes than does benzoic acid, although they generally induce a smaller concentration over the range lo-' to M with a slope of shift in V, than does benzoic acid. Electrodes prepared with 57.5 mV. With 0.1 M Na+ present a selectivity coefficient, the interfacial structure n-Si/SiO,/membrane also respond kK,Na,of 2 X lo4 is observed, which is somewhat poorer than the selectivity (kK,Na= 0.8 X lo4) of ion-selective electrodes to the neutral acids at low pH, Table I. Taken together these results support the hypothesis (4) that the neutral acid can with the standard solution/membrane/solution structure. In examining interference due to neutral acids, we have also enter the membrane and that it is the ability of the acid to prepared electrodes coated with a Ag/AgCl layer to give nchange the internal pH at the Si3N4/membraneinterface that results in a potential response. Si/Si02/Si3N4/Ag/AgCl/membrane structures, referred to as MAgIS electrodes (membrane/Ag/insulator/semiconducThe n-Si substrate is coupled to the membrane layer via tor). The MAgIS electrodes also show a Nernstian response a field effect across the insulator. This is clearly demonstrated to K+ over the range 10-1-10-4 M K+ ion with a slope of 58 by measuring, as a function of applied voltage, the equivalent capacitance of MIS electrodes at low frequencies where the mV, indicating the Ag/AgCl layer does not affect response response is dominated by the space charge layer of the semof the assembly to K+. iconductor. The apparent value of the flat band voltage, Em, A number of acids, neutral in their protonated form, have the applied voltage at which the energy bands in the semibeen shown by Fogt et al. ( 4 ) to cause a response at Si3N4 conductor are flat, can be determined from the capacitanceinsulated ISFET's coated with K+-sensitivePVC membranes. voltage (C-V) curves. The C-V curves may be analyzed by We have examined ascorbic, benzoic, and carbonic acids, and using the model developed to describe the impedance of find that all of them act as interferents at MIS electrodes at pH 4.5. The V, of an MIS electrode shifts positive by about metal/oxide/semiconductorand electrolyte/oxide/semicon-
496
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4 -
lead to O2 sensitivity and O2 can permeate the PVC membrane; however, the chemically prepared AgCl layer is sufficiently good to prevent this. Impedance Response. Figure 6 shows a Nyquist plot of the impedance response of an MIS electrode in M KC1, 0.1 M citrate buffer, pH 4.5, as benzoic acid is added. A single semicircle is observed at frequenciesgreater than 100 Hz, with a rapidly rising tail developed at lower frequencies, in agreement with the response predicted by eq 1 and 2. The response of a Pt electrode coated with the same type of thickness of membrane shows a single semicircle, with a similar time constant as the MIS electrode, but no tail is observed at low o. For both electrode configurations the value of R, extrapolated from the real axis intercept of the semicircle at low frequency is a function of membrane thickness, increasing as thickness increases. The capacitance decreases with increasing film thickness, as determined from C = l/R,wmru where urn= is the frequency in the region of the semicircle giving maximum Zm Consequently, the RC element observed is assigned to the bulk membrane components C, and R,, in agreement with previous results (11-14). At frequencies where the bulk-membrane time constant dominates the impedance response, as determined by a Nyquist plot for the same electrode, the C-V curves for an MIS electrode show no voltage dependence. This is consistent with results on Ptlmembrane electrodes or standard ISE's, for which the differential capacitance is independent of applied voltage over the frequency range measured. At low frequencies, where a tail develops for MIS electrodes in a Nyquist plot, the C-V curves are strongly voltage dependent, Figures 3 and 7. In the range of 25 Hz, the results are consistent with observation of the semiconductor space charge capacitance, C, (1g21). At 25 Hz the C-V curves for a MAgIS electrode show a shift in apparent flat-band voltage as a function of K+ concentration, Figure 7. At higher frequencies (>50 Hz) the membrane dominates the response for this electrode and the equivalent capacitance becomes constant with voltage. The potential shift of 50 mV/decade demonstrates that MAgIS electrodes continue to respond to K+ by a field-effect mechanism. More precise measurements of V , show MAgIS electrodes respond with a slope of 58 mV. Figure 6 reveals a significant change in the value of the bulk membrane resistance, R,, as a function of exposure to benzoic acid. At 4 mM benzoic acid, R, decreases by -5% for an MIS electrode, and the membrane capacitance, C,, varies from 160 to 170 pF. The effect on membrane impedance of exposure to benzoic acid is totally reversible; Figure 6 shows the impedance returns to its original value following 15 min in lo4
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gate subsequently coated with a permselective membrane would be an effective and simple means of avoiding such interferences. The sensitivity to halogen ions that could arise as a complication is easily avoided with PVC membranes containing KB(C6HJl that are permselective for a cation such as K'. Anions such as B(C6H5),- are known to act as nearly fixed sites in the PVC matrix, providing Donnan exclusion of most anions from the membrane. Importantly, procedures for deposition and preparation of Ag/AgCl films have been shown to be compatible with planar Si microlithographic technology (23, 24). Thus the approach presented here is readily incorporated into ISFET device fabrication. Further, since the Ag/AgCl layer appears to adequately protect the underlying gate insulator, it may prove possible to eliminate the Si3N4layer on the gate, thus eliminating one photolithographic step in fabrication of AgC1-coated gates. Both spectroscopic data and impedance analysis of the PVC, dioctyl adipate, valinomycin membrane system provide clear evidence that bulk membrane properties are influenced by species such as ascorbic, benzoic, and carbonic acids. Our results demonstrate that these neutral species can permeate the ion-responsive membrane in a reversible fashion and so confirm the model suggested by Fogt et al. ( 4 )for interference at ISFET electrodes. Finally, impedance analysis of ion-selective electrodes provides a much more sensitive probe than open-circuit voltage measurements of the effect of lipophilic sample solutes on membrane characteristics, and further investigations of the effect of a number of other lipophilic interferents on membrane properties are under way.
ACKNOWLEDGMENT We wish to thank the Alberta Microelectronics Center for a gift of the n-Si wafers, and Process Technology, Ltd. (Oromocto, New Brunswick, Canada), for a gift of the deposition of SiOz and Si3N4. We also thank B. Arnold, University of Alberta, for sputter deposition of the Ag film. E. M.J.V. acknowledges the Natural Sciences and Engineering Research Council of Canada for a postgraduate scholarship during part of this work. Registry No. PVC, 9002-86-2; K, 7440-09-7; KB(C6H,),, 3244-41-5; Si, 7440-21-3; SiOz,7631-86-9; Si3N4,12033-89-5;Ag,
7440-22-4;AgCl, 7783-90-6;COz, 124-38-9;dioctyl adipate, 12379-5;valinomycin, 2001-95-8; benzoic acid, 65-85-0; ascorbic acid, 50-81-7; carbonic acid, 463-79-6.
LITERATURE CITED (1) Janata, J. In SolM State Chemlcal Sensors; Janata, J., Huber, R. J., Ed.; Academlc: London, 1985. (2) Wohltjen, H. Anal. Chem. 1084. 56, 87A. (3) Zemel, J. N.; Keramatl, B.; Spivak, C. W.; D'Amlco, A. Sens. Actuators, 1081. l , 427. (4) Fogt, E. J.; Untereker, D. F.; Norenberg, M. S.; Meyerhoff, M. E. Anal. Chem. 1085, 5 7 , 1995. (5) Trojanowlcz, M.; Augustowska, 2.; Matuszewskl, W.; Moraczewska, G.; Hulanlckl, A. Talanta 1082, 2 9 , 113. (6) Baucke, F. G. K. J . Elechoanal. Chem. Interfacial Electrochem. 1078, 6 7 , 277. (7) Ammann. D.; Blsdg, R.; Cimerman, 2.; Fledler, U.; Guggl, M.;Morf, W. E.; Oehme. M.; Osswald, H.; Petsch, E.; Simon, W. In Ion and Enzyme E/ec&c&s in BklogyandMedycne: Kessler, M., et al., Ed.; Unlverslty Park Press: Baltimore, MD, 1976 p 22. (8) Buck, R. P.;Hackleman, D. E. Anal. Chem. 1077, 49, 2315. (9) Sluyters-Rehbach, M.; Sluyters, J. H. I n Elec&asnalytcai Chem/stry; Bard, A. J., Ed.; Marcel Dekker: New York, 1969; Vol. 4, Chapter 1. (10) Buck, R. P.; Sandlfer, R. J . Electroanal. Chem. Interfacial Elechochem. 1974, 56, 385. (11) Armstrong, R. D.; Covlngton, A. K.; Evans, G. P. J . Elechoanal. Chem. Intertacial Electrochem. 1083, 159, 33. (12) Armstrong, R. D.; Nlkltas, P. Elechochim. Acta 1085, 30, 1627. (13) Armstrong, R. D.; Lockhart, J. C.; Todd, M. Electrochim. Acta 1088, 31, 591. (14) Horval, G.; Grlf, E.; Tbth, K.; Pungor, E.; Buck, R. P. Anal. Chem. 1088, 58, 2735. (15) Legg, K. D.; Ellis, A. B.; Bob, J. M.;Wrlghton, M. S. Roc. Natl. Acad. Scl. USA 1077, 74, 4166. (16) Band, D. M.;Kratochvll, J.; Treasure, T. J . Physlol. (London) 1978, 265, 5P. (17) Medou, M. J.; Cardon, F.; Gomes, W. P. J . Electrochem. SOC. 1077, 124, 1623. (18) Thackeray, J. W.; Natan, M. J.; Ng, P.; Wrighton, M. S. J . Am. Chem. Soc. 1086. 108, 3570. (19) Linder, R. Bell Sys. Tech. J . 1082, 4 1 , 803. (20) Grove, A. S.; Deal, B. E.; Snow, E. H.; Sah, C. T. SolM-State Electron. 1065, 8 , 145. (21) Slu, W. M.;Cobbold, R. S. C. I€€€ Trans. Electron. Devices 1070, Ed-26, 1805. (22) Verpoorte, E. M. J.; Harrison, D. J., in preparation. (23) Bousse. L. J.; Bergveld, P.; Geeraedts, H. J. M. Sens. Actuators 1088. 9 , 179. (24) Koudelka, M. Sens. Actuators 1086, 9 , 249.
RECEIVEDfor review July 28,1987. Accepted October 20,1987. This work was supported by the Natural Sciences and Engineering Research Council of Canada.
