An air monitoring system incorporating this absorbent was readily constructed by simple modification of a common FID hydrocarbon monitor, with addition of a commercially available valve oven, switching valve, and timer. This system, except for the adjusted response time, had virtually the same operating specifications as the original hydrocarbon analyzer. Short-term studies indicated the system was practical, reliable, and accurate. Air-monitoring data obtained for Cincinnati indicated that the method could provide significant new information about air quality.
LITERATURE CITED (1) A. P. Altshuller, W. A. Lonnernan, F. D. Sutterfleld. and S. L. Kopczynski, Environ. Sci. Techno/.,5, 1009 (1971). (2) S. L. Kopczynski, W. A. Lonneman, F. D. Sutterfield, and P. E. Darley. Environ. Sci. Techno/..6, 342 (1972). (3) W. A. Glasson, and C. S. Tuesday, Environ. Sci. Techno/., 4, 916 ( 1970). (4) B. Dirnltrlades. Environ. Sci. Techno/.,6, 253 (1972). (5) T. A. Hecht and J. H. Seinfeld, Environ. Sci. Techno/.,6, 47 (1972). (6) S. K. Friedlander and J. H. Selnfeld, Environ. Sci. Techno/., 3, 1175 (1969).
(7) J. M. Heuss and W. A. Glasson, Environ. Sci. Techno/.,2, 1109 (1968). (8) Environmental Protection Agency, "National Primary and Secondary Ambient Air OuaHty Standards," Fed. Regis., 36, 8186-8201 (April 30, 1971). (9) A. P. Altshuller, G. C. Ortrnan, and B. E. Saltzrnan, J. Air Pollut. Control ASSOC.,16, 87 (1966). (10) R. K. Stevens and A. E. O'Keefe, Anal. Chem., 42 (2), 143A (1970). (11) W. A. King, Jr.. Environ. Sc;. Techno/.,4, 1136 (1970). (12) D. L. Klosterrnan and J. E. Sigsby, Environ. Sci. Techno;., 1, 309 (1967). (13) W. B. Innes, W. E. Barnbrick, and A. J. Andreatch, Anal. Chem., 35, 1198(1963). (14) R. H. Groth and V. A. Zaccardi, J. Air Pollut. Control Assoc., 22, 696 (1972). (15) A. P. Altshuller, J. Air Pollution Control Assoc., 16, 257 (1966). (16) A. P. Altshuller and I. R. Cohen, Int. J. Air Water Polluf., 7, 787 (1963). (17) A. P. Altshuller and J. J. Bufalinl, Environ. Sci. Techno/. 5, 39 (1971). (18) National Air Pollution Control Adrnin., Publication Ap-64, "Air Quality Criteria for Hydrocarbons." pp 3-8.
RECEIVEDfor review March 31, 1975. Accepted July 28, 1975. This work was supported in part by the Center for the Study of the Human Environment under U.S. Public Health Service Grant ES 00159, and in part by the Environmental Protection Agency under Research Grant R800869.
Potassium Ion-Sensitive Field Effect Transistor Stanley
D. Moss, Jifi Janata,' and Curtis C. Johnson
Department of Bioengineering, University of Utah, Salt Lake City, Utah 84 172
The construction and theory of operation of a potassiumsensitive field effect transistor Is described, and Its performance is characterized both as a solid-state field-effect device and as an electrochemical sensor. The performance of this device is comparable with the correspondlng PVC-type ion selective electrodes. The transistor operates satisfactorliy in the presence of proteins and it has been used for determination of potassium ion concentration in blood serum.
A new type of electrochemical sensor, an ion-sensitive field-effect transistor (ISFET), was introduced when Bergveld removed the metal gate from a metal oxide semiconductor field-effect transistor (MOSFET) and exposed the silicon oxide gate insulator to a measured solution ( I ) . A similar approach was followed later by Matsuo and Wise ( Z ) , and this new subject area has been recently reviewed by Zemel ( 3 ) .In the broader sense of chemically sensitive field-effect transistors, one sensitive to molecular hydrogen has also been reported ( 4 ) . The ISFET is a result of the integration of two technologies: ion-selective electrodes and solid state microelectronics. This development opens several new possibilities, such as miniaturization, development of multiprobes, all solidstate design and in situ signal processing. Because of its small size, it presents a difficult encapsulation and packaging problem which is, however, amply offset by the elimination of electrical pick-up noise by in situ impedance conversion and on site signal amplification. Bergveld did not modify the ion-sensitive layer in any way although he considered introducing impurities in order to render the device ion selective. In this paper, we introduce a class of devices having a chemically-sensitive layer placed over the gate region, and we report our results with valinomycin/plasticizer/poly(vinylchloride) membrane Present address, Corporate Laboratory ICI, Ltd., The Heath, Runcorn, Cheshire, England. 2238
which is selectively sensitive to potassium ion ( 5 ) .Performance of this ISFET is examined both from the point of view of a solid state field effect device and as an electrochemical sensor.
EXPERIMENTAL Integrated Circuit. A photomicrograph of the specially-designed integrated circuit chip is shown in Figure 1. The circuit was processed on a 2-inch diameter, P-type, silicon wafer having a (111) crystal orientation and 12 to 15 ohm-cm resistivity. The chip is approximately 1.28 mm X 2.13 mm and contains two ISFET transistors and two metal gate control devices. Each transistor is an N-channel depletion-mode (normally on) device with a source to drain spacing of approximately 20 gm. The transistors were intentionally made large (400 pm wide) for experimental purposes. The gate insulator consists of a thermally grown Si02 layer approximately 400 A thick and a Si3N4 layer approximately 500 A thick. The N-diffused source and drain regions are approximately 1 to 2 pm in depth and 150 pm wide. The field oxide is approximately 10,000 %, thick. Contact holes are etched in the insulating layers to allow electrical connection to be made to the source and drain diffusions. Aluminum conductors approximately 1 micron thick are formed on the surface by evaporation and photo-etching techniques. The IC chip is die-attached and wire-bonded to a 10-pin TO-5 type IC header. The chemical membrane is next applied by a solution casting technique (6). Once the membrane has cured, a quarter-inch long piece of flexible tubing is placed over the IC header. The IC header is then placed in a holding fixture at an angle of approximately 45', and encapsulant is injected into the lower portion of the IC header-tubing volume. The encapsulant used is Tra Cast 3012 (Tra-Con, Inc., Medford, Mass.) having a volume resistivity of 4.5 X 10'j ohm-cm. Once the encapsulant has cured, the IC header is rotated 90° and the encapsulant injection procedure is repeated. This process continues for a total of 4 cycles until the bonding wires and scribe lanes of the chip are totally covered by the encapsulant. Leads are then soldered to the existing pins on the TO-5 header, after which the entire unit is placed in a short glass tube and sealed with additional applications of epoxy encapsulant. Chemicals. Ion-exchange membranes were cast from T H F solutions in the following compositions: 10.0 mg valinomycin (Sigma Chemical Company), 0.89 ml dioctyladipate (K & K Laboratories,
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
I
l
L I
i
Figure 2. Cross section of a conventional MOS field-effect transistar The threshold voltage for MOSFET is defined by the following expression:
VT = a,
--+co QS,
24f--
QB
co
where a, = metakemiconductor work function difference, $,, = fixed surface-state charge density per unit area a t the SiOz-Si interface, 4f = Fermi potential of the suhstrate, and QB = Surface depletion region charge per unit area. When the metal gate is replaced with a chemical membrane, the threshold voltage is defined as:
Figure 1. Photomicrograph of 1.28 mm X 2.13 mm ISFET chip Inc.), 0.33 poly(vinylch1oride) (Polysciences,Inc.), 13.0 ml tetrahydrofurane (vacuum distilled and stored under nitrogen). Testing solutions were prepared from analytical grade chemicals and distilled de-ionized water was used for the preparation. Bovine serum albumin, Cahn fraction Y (Sigma Chemical Company) and Wellcomtrol serum two (Wellcome Research Laboratories) were used for the study of ISFET performance in the presence of prateins. Activity coefficients were calculated according to the formula, -log/ = 0.5115Z2V%l + 4). Selectivity coefficients Kx+me+, were calculated from expression KK+,w~+ = ( I K + / ~ N ~where + DN*+ was the activity of Na+ and K+ as obtained from the intercept in Figure 5. Apparatus. A double junction silver/silver chloride (saturated KC1) reference electrode with 1M NaCl salt-hridge was used in all measurements. The drain voltage, VD, was set at C5.0 volts and was supplied from the PAR 110 electrochemical instrument which was also used for measuring the drain current ID.All measure ments except the temperature dependence of IDwere carried ou at 25.00 i 0.05 OC. Solutions were stirred magnetically. A 6.~01 dry cell connected BCXOSS a 10K ten-turn potentiometer served as i variable gate voltage source Vco. This voltage was measured wit1 B digital voltmeter. THEORY O F O P E R A T I O N The conventional MOSFET is produced by placing I metal gate (M) over the thinned gate insulator (I) as s h o w1 in Figure 2. A potential is applied to the drain (D) with re spect to the source (S),and the resultant current flowin( through the channel (C) is controlled hy the magnitude o f the voltage applied to the gate metal. Current-voltage characteristics equations have been de rived by several authors (7, 8) for two principal regions o operation. When the drain-to-source voltage, Vo, is smal with respect to the absolute value of the difference betweer the gate voltage, VG, and the threshold voltage, VT, the de vice is said to operate in the non-saturated (Triode) regioi and the drain-to-source current is:
where LY = ~ C ~ W V D IfiL = , surface carrier mobility, CO = gate capacitance per unit area, W = channel width, and L = channel length. Our device is biased such that it operate:3 in the non-saturated condition.
where Eref = potential of the reference electrode, 4c.1 = chemical memhrane-insulator work function difference, and
VG= V
(4)
G O i E
where VGO= the potential applied externally t o the reference electrode and E is the Nernst potential. Combining Equations 1, 3, and 4 gives the relationship between drain current and ion activity in the non-saturated region of operation:
K + VGOf - RT lnai nF where K = V*T - ‘h VD + E o
1
(5)
The temperature coefficient of the MOS transconductance (gain) has been shown to he negative (8).However, the temperature coefficient of drain current a t a selected gate bias voltage, Vco, can be either positive or negative (9). The change in drain current as a function of temperature can, therefore, be in excess of &10 Fa per degree centigrade depending on the bias voltage, VGO. RESULTS Field Effect Device Characteristics. The leakage current characteristics of the ISFET has been measured. A transducer was fabricated with a potassium ion-sensitive membrane. The transducer was placed in a TRIS-buffer solution ar.d connected to a reference electrode and biased with a positive 2.5 volts with respect to the substrate. Leakage current through the reference electrode was monitored with a Keithley model 616 electrometer. The total leakage current was approximately 3 X A a t the initiation of the test and increased fairly linearly to a value of approxiA over a 25-day test period. The total mately 600 X surface area exposed to the aqueous solution was approximately 6.6 X lo5 hm2. Therefore a maximum leakage curA/umz was obrent density of approximately 9 X The relationship of the experimental data to the theoretical characteristics fok. the K+ ISFET is shown in Figure 3. Equation 5 describes the operation of our device in the
AIrlALYT1CAL CHEMISTRY. VOL. 47. NO. 13, NOVEMBER 1975
2239
g [ 175
-
-
I
I
Cj
I
150-
130 75 50 -
1
1
Figure 4. Diagram of ISFET testing circuit
125
-8
1
-6
-4
-2
0
2
+1
i
10
YG (‘dolts)
Figure 3. Comparison of theoretical with experimental ISFET characteristics The non-saturated region of operation lies to the right and the saturated region to the left of the vertical dashed line.
non-saturated condition where the gate voltage is equal to or more position than -1.5 V. The temperature response of the device was measured in the same solution as was used for measurement of the dependence of drain current on gate voltage. The temperature was varied between f 1 5 and $40 “C in 5-degree steps. A linear dependence of drain current on temperature was obtained with a 27.6 pAPC slope. The calibration of this device produced a 0.167 pA per millivolt change in gate voltage. Chemical Characteristics. A diagram of the testing circuit is shown in Figure 4. A normal testing procedure was as follows: 20.00 ml of either 0.01 or 0.15 M NaCl solution was placed in the thermostated vessel and allowed to reach thermal equilibrium. The ISFET was placed in the solution and electrical connections were made as shown in Figure 4. A constant gate voltage (VGO)was applied, typically between f0.100 and +0.500 volts and the drain current I D was recorded as a function of time. After a short period of recording VGOwas first increased by 60.0 mV then decreased by 60.0 mV and finally returned to its original value. The corresponding change in drain current was recorded. This measurement provided a calibration of the ISFET current change in terms of gate voltage change. This same calibration procedure was done at the end of each chemical test. The transducer response was obtained by the addition of increments of KC1 solutions of known concentration which were also 0.01M or 0.15M in NaC1. The results of such a typical test (test No. 11, Table I) is shown in Figure 5. Stability, Nernst Response, and Selectivity Constants. These three parameters which are considered to be most important in characterization of ion selective electrodes were monitored over a period of seventeen days, stability continuously, Nernst response and selectivity periodically. During this period, the time response, temperature dependence and the effect of proteins were also studied. 2240
7
6
5
4
-LOG
3
2
1
OK+
Figure 5. Dependence of current on activity of potassium ion (Test 11). Activity of sodium ion is 0.108M
Figure 6 shows the stability characteristics of our ISFET (membrane thickness of approximately 270-300 pm). It can be seen that drift, Nernst response, and selectivity changed with time and were also affected by the history of the device. Although data in Figure 6 and Table I are presented for one device, the same general pattern of behavior was obtained with other devices. Our original intention was to use thin membranes. We found, however, that such an ISFET (membrane thickness of approximately 25-30 pm) gave a weak Nernst response within the first operating hour and became inactive after five hours in aqueous solution. The normal response did not return after dry storage. It was suspected that the change of the membrane characteristics may be due to leaching out of valinomycin and/or dioctyladipate from the membrane. To test this assumption, a disc of 950-pm diameter cut out of 250-pm thick membrane was placed in 3.0 ml of 5.0 X 10-3M KC1, 0.15M NaCl solution, and the UV spectrum was repeatedly recorded over a period of 47 hours. The absorbance of the solution increased steadily with time (Figure 7) but no characteristic absorption bands were formed. Spectrum of dioctyladipate shows a strong absorption maximum at 213 nm and two weaker maxima at 279 and 281 mm, respectively. Saturated aqueous solution of valinomycin gave the broad maximum of 265 nm. Shape of spectrum of solution with membrane in it indicates that both dioctyladipate and valinomycin are being slowly leached out of the polymer. Time Response. Two types of time response were measured; the “electrical” response of drain current to a step change of voltage applied to the reference electrode from the source, VGO,and the “chemical” response. The latter was evaluated from the change of drain current following injection of potassium chloride solution into the rapidly
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
Table I. Summary of Chemical Tests Test So.
Time, lir
0 6 24 36 79 200 216 223 240 246 313 388 407
1 2 3 4 5 6 7
a 9 10 11 12 13
Selectivity ,
Drift, mV:hr
+4 84 +260 +19 +16 0 -5 0 0 0 0 0 + 84 +15
Slope, m V / d e c a d c
-k K K,xa
59.6 58.6 59 .o 42.5 0 62.7 55.3
2.91 3.15 3.03 1.79 0 2.60 3.65
K' serum analysis 53 .O 49.8 60 .O 29.6 27.5
3.13 3.17 2.88 2.36 1.68
%Tedium
0.15.11 NaCl
same same 0.01,11 NaCl same same 0.15.1.1 NaCl horse serum 0.15.11 NaCl same same same same
u
OO
TIME HOURS
Flgure 7. Time dependence of absorbance at 315 nm, 1-cm cell, 3 ml of 0.005M KCI, 0.15M NaCl solution
255-228 hours). This drift could be attributed to a slow desorption of proteins from the membrane surface. I
,
C
'5
1
23
' 3:
' 0:
8 8
5C
'
>
'
-
8:
#
?c
,
, I
105 2 0 0 ~ 1 : 2 2 1
'
1
zi:
ZLC
31237,
38: i:i
~ o ~ n l j
T I M E HOURS
Flgure 6. Stability test of K+ ISFET. Arrows and numbers indicate tests which are summarized in Table I
stirred test solution. Both time responses were found to change with aging of the device. As a rule, both time responses of a dry ISFET immediately after its immersion into solution were relatively long (approximately 50 seconds) but became rapid within 30 t o 45 minutes of hydration. The fully-hydrated ISFET has an electrical time response to a 100-mV step in VGO(either positive or negative) of T95%= 0.6 sec. The chemical time response, T g 5 % , to a change of activity of potassium from 3.0 X 10-4M to 3.2 X 10-3M was 1 sec. The static chemical response was between 2.0 and 2.5 sec. In the terminal period of the life of the ISFET (test 13), both responses became sluggish (Tgj%a t approximately 100 sec.), even though the membrane was completely hydrated. ISFET Performance in Biological Medium. The ISFET was used to analyze potassium ion concentration in horse serum (test 10). A small amount of serum was used to prepare 1.OM KC1 solution which was then added in 10, 25, 50, and 100-pl increments to 10.00 ml of the original serum. The ISFET response to each addition was rapid and stable. Nernst response tests (tests 9 and 11) which were carried out before and after test 10 (see Figure 6) gave Nernst slopes of 55.3 mV and 53.0 mV, respectively. Test 10 was therefore evaluated using Gran plot, and a slope of 54.0 mV. The concentration of potassium on the original serum determined by this procedure was 5.0 mequivfl. Flame emission analysis gave 5.0 mequiv/l. and the Wellcomtrol assay was 5.0-6.0 mequivll. I t was interesting to note that following the serum analysis (test lo), a small negative drift lasting approximately three hours was obtained (Figure 6,
DISCUSSION Both Bergveld ( 1 ) and Zemel ( 3 ) suggested that a reference electrode is not needed to operate the ISFET and claimed this as a major advantage of the ISFET over the ion selective electrode. They based their argument on the known fact that impurities in gate oxide and particularly contamination with Na+ changes the MOSFET device characteristics ( 2 0 ) . There are several explanations for this phenomenon, such as change of conductivity of the oxide layer and/or change of inner electrochemical potential. However, whatever mechanism is conceived, it is still necessary to reference the gate voltage with respect to the substrate. This necessity can be readily seen from Figure 4. If the lead connecting the voltage source VGOto substrate is disconnected, which is equivalent to operation of the ISFET without a reference electrode, then any change of VGOor reference electrode potential or Nernst potential at the membrane/solution interface will have no effect on the drain current. Current will still pass between the drain and source but we will not be able to relate it to any change of voltage or resistance in the disconnected gate input. It is a well known theorem of electrochemistry ( 2 2 ) that potential difference at a single interface cannot be measured. The ISFET being just another device translating change in chemical potential into electrical signal, must also obey this theorem. However, we do agree that impurities a t the solution/membrane interface affect the drain current provided that the gate voltage is properly referenced. There is a possible explanation as to why Bergveld's ISFET worked apparently without a reference electrode. The gate area of the ISFET must be exposed to the solution, yet the rest of the device, Le., contact pads, scribe lane, wires, etc. must be completely isolated from the solution. Bearing in mind the small size of the chip, one can comprehend the difficult encapsulation problem. If there is
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2241
a contact between solution and any substrate connected part of the device, a “reference electrode” is formed. In our earlier device fabrication attempts, we produced, inadvertently, a number of such “self-referenced” devices. They are easily recognized during the VGO-IDtest when a voltage between -5.0 V and +5.0 V is applied between the reference electrode and the substrate. If there is a “leak”, then the drain current becomes erratic because of electrolysis and hydrogen or oxygen evolution depending on the polarity of applied voltage. Gas bubbles produced a t the fault can be observed with a microscope. Another, potentially more serious leakage path is through the silicon oxide layer itself. I t is well known that silicon oxide hydrates, the rate of this process being p H dependent (12). We found that the layer of thermally grown gate oxide will hydrate in a matter of several hours, thicker (field) oxide taking a little longer (6-8 hours). When a gate voltage in excess of f 2 . 5 V is applied, gas bubbles can be seen evolving first in the gate area and later over the rest of the chip surface (13).I t is obvious that an ISFET with only a Si02 insulator would not be practical because of its short life time. Silicon nitride, on the other hand, resists hydration (14). For this reason, all our devices have a top layer of Si3N4. The ultimate test of integrity of an ISFET is the measurement of leakage current density. Assuming that the level of electrolytic current density in 0.01M electrolyte is A/fim2 our maximum leakage current of A/pm2 shows that we are well below this value. The normal gate current leakage of a standard MOSFET is of the same order of magnitude. This indicates that there is no electrolytic junction and that the only signal return path for our ISFET is through the reference electrode. The temperature coefficient of the chemical systems associated with the ISFET device have been shown to be of the order f l mV/OC. Various parameters can be adjusted both in the design and the fabrication of the field effect device to minimize the overall temperature coefficient of the individual sensors. Metal gate devices can also be employed on the same chip to provide additional temperature compensation. Data in Figure 6 and Table I indicate that there are three distinctive periods in the operating life of an ISFET: preconditioning period (0-100 hours), working period (200322 hours) and terminal period (380-415 hours). During the first and last period, the device is least stable as is demonstrated by large drifts which are observed after any change of experimental conditions (tests 2, 4, and 12). At any time when ISFET is left in solution, the Nernst response and selectivity slowly decreases (tests 4, 5, 10, 12, and 13). On the other hand, dry storage improves both Nernst response and selectivity (tests 1, 6, and 11).The initial large drift could be caused by slow loading of valinomycin with potassium (6, 15). Loss of response and selectivity could be explained by gradual extraction of the electroactive system from the membrane (both valinomycin and DOA) surface. Because diffusion rate inside the polymer must be considerably lower than in the solution, a depletion layer can be formed near the membrane surface. Existence of such a “swollen” layer and “dry” bulk has been suggested by Ruzicka ( 6 ) . This situation would be analogous to a glass electrode membrane; however, unlike glass membranes, ion-exchange sites in the PVC membrane are not part of the matrix and can be, therefore, extracted from the membrane. During dry storage, this layer is replenished from the bulk of the membrane. However, if the membrane is thin ( ~ 2 f0i ) , then there is not enough electroactive material present in the bulk of the membrane and it soon reaches its terminal period. For the thin membranes, the thickness thus becomes a life-determining factor. Our sem2242
iquantitative spectrophotometric study of extraction of valinomycin/DOA from the membrane supports this argument. I t is also known that valinomycin is slightly soluble in water, partitioning coefficient between heptane and water being 160 f 15 ( 1 6 ) . No data on solubility of dioctyladipate in water has been found. I t would be expected that the leaching problem would also affect ion selective electrodes of corresponding types. Unfortunately, the details of storage and details of stability studies are seldom published. Nevertheless, it appears that the Nernst response is generally found to drop with time and that values of reported selectivities for the same type of electrode vary greatly ( 5 ) . Correlation between the amount of plasticizer in the membrane and drift and response characteristics has been noted (17, 18). Drift of potential commonly observed with dipped wire type electrodes has been attributed to the instability of the polymer/metal interface and attempts to remedy this situation have been made (19). Although polymer/metal interfaces are not thermodynamically well defined in terms of reversible potential, it is possible that a more significant portion of drift originates from the membrane solution interface itself. The situation in the ISFET is different; the inner interface is that of an insulator (SisN4)/semiconductor (plasticized polymer) type. Our leakage current measurements show that there is no charge transfer in an electrochemical sense across this interface and that the drain current is affected exclusively by the presence of an electric field in the inversion channel, as was explained earlier. Operationally, this situation is equivalent to the case where an ion-exchange membrane is connected directly to the input transistor of a p H meter with n o conductor in between and the whole assembly immersed in the measured solution. Needless to say, such an arrangement has a minimum electrical noise pick-up. However, leaching of the electroactive systems takes place a t only one side of the membrane which gives rise to drift and associated problems. In the case of ion selective electrodes with an internal liquid reference system ( 5 ) , the drift problem must be greatly reduced by the fact that the disturbing processes take place on both sides and their effects tend to cancel out. Three simple rules can be laid down for the proper use of ISFET: 1) Membrane thickness should not be less than 100 pm. 2) ISFETs should be first preconditioned in 0.1M KCl solution for 24 hours followed by 24 hours of dry storage. 3) When not in use, the device should be stored above rather than in the solution. Our preliminary investigation of the applicability of the potassium ISFET to measurements of biological solutions has been encouraging. A full application study is now being conducted and results of this work as well as results with other types of ISFETs will be reported in the near future.
ACKNOWLEDGMENT We would like to express our thanks to R. J. Huber for his assistance and consultation in the design and fabrication of microcircuits and to Scott Collins for his help with device evaluation.
LITERATURE CITED P. Bergveld, /E€Trans., BME-19, 342 (1972). T. Matsuo and K. D. Wise, /€ETrans., BME-21, 485 (1974). J. N. Zernel, Anal. Chem.,47, 255A (1975). . . I. Lundstrom, S. Shivaraman, C.Svensson, and L. Lundkvist, Appl. Phys. Lett., 26, 55 (1975). (5) G. J. Moody and J. D. R. Thomas, “Selective Ion Sensitive Electrodes”, Merrow Ltd.. Watford. England 1971. (6) U. Fiedler and J. Rbiikka, Anal. Chim. Acta, 67, 179 (1973). (7) A. S. Grove. “Phvsics and Technoloav of Semiconductor Devices”. John Wiley & Sons: New York, N.Y., 19x7.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
(8) W. N. Carr and J. P. Mize, "MOS/LSI Design and Application", McGrawHill Book Co., New York, N.Y., 1972, (9) L. Vadasz and A. S. Grove, E€€ Trans, ED-13, 893 (1966). (10) S. I. Raider, L. V. Gregor, and R. Flitch, J. Electrochem. SOC., 120, 425 (1973). (1 1) J. O'M. Bockris and A. K. N. Reddy, "Modern Electrochemistry", Vol. 2, Plenum Publishing Company, New York, 1973, p 644. (12) A. Wlkby, J. Electroanal. Chem., 38, 429 (1972). (13) S. D. Moss, J. Janata, and C. C. Johnson, unpublishedresults, 1974. (14) T. E. Burgess, J. C. Baum, F. M. Foukes, R. Holmstrom, and G. A. Shirn, J. Nectrochem. Soc.. 116, 1005 (1969). 115) P. Lauoer. Sclence. 178. 24 (1972). i16j A. A. lev. V. V. Malev,'and'V. V: Osipov. in "Membranes," G. Eisen-
man Ed., Vol. 2, Marcel Dekker, New York N.Y., 1973, Chapter 7, p 482. (17) R. W. Cattrall, S. Tribuzio, and H. Freiser, Anal. Chem., 46, 2223 (1974). (18) H. Freiser, University of Arizona, Tuscon, personal communication, 1975. (19) M. D. Smith, M. A. Genshaw, and J. Greyson, Anal. Chem., 45, 1782, (1973).
RECEIVEDfor review April 28, 1975. Accepted jUly 30, 1975. Support for this research has been provided by the University of Utah.
Potentiometric Response of the Calcium Selective Membrane Electrode in the Presence of Surfactants Ramon A. Llenado Procter and Gamble Company, Ivorydale Technical Center, Cincinnati, Ohio 4521 7
The response characteristics of the calcium selective membrane electrode were investigated in calcium test solutlons containing linear alkyibenzene sulfonate (LAS) or dilsobutylphenoxyethoxyethyi(dimethyl)benzyiammonium chioride (Hyamine). Both surfactants were found to interfere at the membrane electrode, albeit by different mechanisms. LAS interferes via competing reactions at the organophiilc membrane between LAS and Ca2+ on the one hand, and Ca2+ and the ton-exchanger site on the other. Hyamine interferes through the displacement of ca*+ in the ion-exchanger site. The interference effect of LAS can be overcome by loading the membrane phase with an equilibrium amount of LAS. This procedure desensitizes the sensing eiement from surfactant effects and permits the use of the calcium electrode for Ca*+-surfactant interaction studies. Hyamlne interference patterns cannot be overcome. Seiectivlty studies showed the calcium electrode to be -lo3 times more selective to Hyamlne than to calcium. This, however, permits the use of the electrode for LAS-Hyamine titration analysis and also for the direct potentiometric measurement of Hyamine and analogous species.
The past decade has seen the renaissance of the field of analytical potentiometry with the development of ion-selective membrane electrodes (1-17). During this time period, the field of ion-electrodes has progressed to a point where there are now commercially available about two dozen types and an equal number that are still in the laboratory curiosity stage (1-9). The popularity of membrane electrodes is due to their impressive list of advantages such as ease of measurement, low cost, sensitivity, selectivity, etc. (3, 5 ) . Perhaps, their most important advantage is their ability to measure thermodynamic ionic activity in a direct fashion by potentiometry ( 2 , 3 ) . In the soap and detergent industry, the calcium selective membrane electrode is probably the most useful. To a large extent, its uses include water hardness measurement and calcium-sequesterer interaction studies (18). Other investigations dealt with preparation and theory of operation (19),response times ( 2 0 ) ,response characteristics in highly acid solutions ( 2 1 ) ,and a variety of other studies (22-23). Despite the wealth of information on the calcium electrode, there is considerable variation and oftentimes conflicting
opinions on the suitability of using the calcium electrode for potentiometric measurement in the presence of surfactants. Generally, these opinions are based on theoretical expectations because of the lack of actual experimental data. Two recent papers included descriptions of possible surfactant effects on the potassium (34) and calcium ( 3 5 ) electrodes. In another paper ( 2 8 ) ,the response of the calcium electrode to cyclohexylammonium ion was shown to be Nernstian. Other than these three papers, there is a dearth of information on the surfactant effects at the calcium membrane electrode. In most cases, investigators failed to study further or completely ignored the effects. Because of the importance of the calcium electrode in a variety of detergency studies, experiments were carried out to characterize the effect of surfactants on the potentiometric response of the electrode. This paper describes the result of these experiments. I t will be shown that the effect of a typical anionic surfactant linear alkylbenzene sulfonate (LAS) is different from that of a cationic surfactant, Le., diisobutylphenoxyethoxyethyl(dimethy1)benzylammonium chloride (Hyamine). Explicit examples will also be given wherein advantage is taken of the mechanism of the surfactant contribution to the membrane potential for a variety of studies.
EXPERIMENTAL Apparatus. An Orion model 92-20 calcium electrode was used in conjunction with an Orion model 90-01 single junction reference electrode or a Corning model 476002 saturated calomel reference electrode. Potential measurements were carried out with an Orion model 801 digital meter. The output of the meter was connected to a Sargent-Welch recorder, model DSRLG. Recording of the potentiometric response was necessary to monitor drifts, noise levels, and response times of the electrodes. A special holder, Orion model 92-00-01, that inclines the electrodes about 20° from the normal was also used to minimize the possibility of air bubbles being trapped in the concave space a t the electrode. Reagents. Standard calcium solutions were prepared from Mallinckrodt analytical reagent grade CaC12.2H20 by dissolving 147.02 grams in enough distilled water to make 1 liter of solution. Standardization was carried out with EDTA titration. With careful quantitative techniques, solutions can be prepared to within 1.000 f 0.004M Ca2+ by the above procedure. The stock solution was sequentially diluted to prepare a series of calibrating solutionslo-' to 10-jM ea2+. NaCl solution was used to adjust ionic strength when necessary. For a cationic surfactant solution, 46.61 grams of Hyamine 1622 (98.8% diisobutylphenoxyethoxyethyl(dimethy1)benzylammonium chloride monohydrate) from Rohm and
ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 13, NOVEMBER 1975
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