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Anal. Chem. 1987, 59, 2842-2846
a TMFE in conjunction with the Metexchange solution is very low in comparison to that obtained with a NCTMFE in the hydrochloric acid medium employed in this study. Oehme and Lund also found low sensitivity for lead in whole blood with the Metexchange reagent at a hanging mercury drop electrode (31). Moreover, the Metexchange reagent does not solve the problem of electrode fouling, and the sensitivity decreased during repetitive scans (Figure 2). These results emphasize the point that the Metexchange approach for determination of lead in whole blood only works well in conjunction with the patented ESA Trace Metals Analyzer with its special working electrode, consisting of a tubular, waximpregnated graphite electrode in which a cone-shaped stirrer is placed (9). The major advantages of the NCTMFE approach for determination of lead in blood are therefore that the only reagent required in large quantities is a mineral acid readily available in high purity and that the analysis can be performed on any electrochemical instrument with ASV facilities. Also, the strongly acidic medium eliminates interference from chelating agents such as penicillamine and EDTA, whereas special precautions must be taken with the Metexchange approach if such compounds are present in the sample (10). Registry No. C, 7440-44-0; Pb, 7439-92-1; Cu, 7440-50-8; Nafion 125, 65506-90-3.
LITERATURE CITED (1) Brezonik, P. L.; Brauner, P. A.; Stumm, W. Water Res. 1978, IO, 605-6 12. (2) Bond, A. M.; Reust, R. 6. Anal. Chim. Acta 1984, 162, 389-392. (3) Batley, G. E.; Florence, T. M. J . Nectroanal. Chem. 1978, 72, 121-126. (4) Wang, J. J . Electroanal. Chem. 1982, 139, 225-232. (5) Jagner, D.; Josefson, M.; Westlund, S.; Aren, K. Anal. Chem. 1981, 53, 1406-1410. (6) Jagner, D.: Josefson, M.; Westlund. S. Anal. Chim. Acta 1981, 128, 155-161.
(7) Jagner, D.; Danielson, L. G.; Aren, K. Anal. Chim. Acta 1979, 106, 15-21. (8) Morrell, G.; Giridhar, G. Clin. Chem. (Winston-Salem, N.C.)1978. 22,221-223. (9) U.S. Patent No. 4 201 646. (IO) The Model 30 IOA Trace Metals Ana/yzer Methods Manual; ESA: Bedford. MA. 1984. (11) Stoeppier, MT; Mohl, C.; Ostapczuk, P.; Goedde, M.; Roth. M.; Weidmann, E. Fresenius' 2.Anal. Chem. 1984, 317,486-490. (12) Boone, J.; Hearn, T.; Lewis, S. Clin. Chem. (Winston-Salem, N.C.) 1979, 25,389-393. (13) Lee, S. W.; Meranger. J. C. Am. J . Med. Technoi. 1980, 4 6 , 853-857. (14) Boeckx, R. L. Anal. Chem. 1988, 58, 275A-286A. (15) Hoyer, 6.; Florence, T. M.; Batley, G. E. Anal. Chem. 1987, 59, 1608- 1614. (16) Martin, C. R.; Rhoades, T. A.; Ferguson, J. A. Anal. Chem. 1982, 5 4 , 1639-1641. (17) Stauber, J. L.; Florence, T. M. in Proceedings of the Ninth Symposium on Analytical Chemistry, Sydney, April 27, 1987. (18) Moore, R. 6.; Martin, C. R. Anal. Chem. 1988, 58, 2569-2570. (19) Handbook on the Toxicolagy of Metals; Frlberg, L., Nordberg, G. F., Vouk, V. B., Eds.; Elsevler/North Holland: Amsterdam, 1979. (20) Lund, W.; Eriksen, R. Anal. Chim. Acta 1979, 107,37-46. (21) Copehnd, T. R.; Christie, J. H.; Osteryoung, R. A,; Skogerboe, R. K. Anal. Chem. 1973. 4 5 , 2171-2174. (22) Franke, J. P.; de Zeeuw, R. A. J . Anal. Toxicol. 1977, 1 , 291-295. (23) Hoyer, 6.; Kryger, L. Talanta 1988, 33,883-888. (24) Florence, T. M., unpublished results. (25) Stauber, J. L.; Florence, T. M. Sci. TofalEnviron. 1987, 60, 263-271. (26) Wang, J.; Luo, D. Taknfa 1984, 31, 703-707. (27) Henry, R. J.; Cannon, D. C.; Winkelman, J. W. Clinical Chemistry, 2nd ed.,Harper and Row: New York, 1974. (28) Underwood, E. J. Trace Nements in Human and Animal Nutrition ; Academic: New York, 1977. (29) Florence, T. M. Anal. Chim. Acta 1980, 119, 217-223. (30) PDV 2000 Instruction Manual; Chemtronics: Bentiey, Western Australia, 1986. (31) Oehme, M.; Lund, W. fresenius' 2. Anal. Chem. 1979, 298, 260-268.
RECEIVED for review May, 18,1987. Accepted August 19,1987. A research fellowship (to Boy Hoyer) from the Faculty of Sciences at Aarhus University, Denmark, and a research grant from the Danish Science Research Council (no. 11-5773) are gratefully acknowledged.
Ion-Channel Sensors Masao Sugawara, Koichi Kojima, Hiroyuki Sazawa, and Yoshio Umezawa* Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan
A new chemical sensor that mimics blologlcal Ion channels
Is descrlbed. Synthetlc Hpld multilayer membranes coated on an underlylng glassy carbon (GC) electrode by the Langmulr-Blogdett method function as an ion channel whlch can be swHched on by the anatyte. The channel is opened reverslbly by a stimulant (analytekmembrane lnteractlon and a great deal of marker Ions are allowed to permeate across the membrane. This change in permeability of the marker Ions Is monitored at the underlying GC electrode, whlch is a dlrecl but much ampl#led measwe d the amount of analytes. Ca(II), Ci04-, and K+ ions were chosen as illustrative examples of analytes (stimulants). The analyte-stlmulated on/off switching of ion channels was ascribed to a conformational change in the lipid membranes and/or electrostatic Interaction between the lipid molecules and marker ions. The present study demonstrates the potentiality of this type of sensor for sensitive detection of various types of analytes in solutions If an appropriate comblnatlon of channel-forming amphiphiles and receptor compounds is made.
The ion channel is a very general system in biological ac-
tivities, e.g., conversion of extracellular events into intracellular signals via hormones, transfer of information in nerve systems, and others. The unique feature of ion channels in biological cell membranes is a selective recognition of substrates and the following amplification of its information by channel switchings: The selective binding of substrates with receptors triggers the opening of an ion-specific channel which allows the permeation of the great amount of ions across the membrane following an electrochemical potential gradient. Therefore, it seems natural to take advantage of this principle as a new philosophy of chemical sensors. In the present study, we have tried to develop an ionchannel sensor using Langmuir-Blodgett monolayers on solid electrodes. This sensor has a function of channel switching by the following sequence. (i) In the absence of a stimulus (analyte), the channel is closed and, therefore, marker ions (monitoring ions) cannot permeate through the membrane (Figure la). (ii) With the stimulus present, the channel is opened, and a large amount of marker ions are allowed to permeate through the membrane, which are immediately detected electrochemically a t an underlying electrode. The amount of marker ions thus detected is a direct but much amplified measure of the amount of analyte (Figure lb). (iii)
0003-2700/87/0359-2842$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
8 ;M a r k e r
ion
0; Stimulus
@; Quenching
reagent
Figure 1. Schematic representations for ion-channel sensors: (a) channel closed; (b) channel opened; (c) reversible channel closing by a quencher.
Figure 2. Synthetic liplds as channel-formlng materials: (1) didodecyl phosphate; (2) dlmethyldioctadecylammonium bromide: (3) 1,3dltetradecylglycero-2-phosphocholine.
With elimination of the stimulus, the channel is again closed reversibly (Figure IC). In order to open the channel with the stimulus, some specific interaction of the stimulus with the membrane assemblies is required to change the membrane permeability for marker ions. This may be implemented by incorporating some appropriate receptors into the membrane. Several illustrative examples are presented to discuss the potentiality of this new sensor.
EXPERIMENTAL SECTION Reagents. Didodecyl phosphate, dimethyldioctadecylammonium bromide, and 1,3-ditetradecylglycero-2-phosphocholine shown in Figure 2 are synthetic lipids commercially available from Sogo Yakko Co. (Tokyo, Japan). Tris(2,2’-bipyridine)ruthenium(1I) chloride was synthesized according to the method of Palmer et al. (I). Deionized water was doubly distilled with a Fujiwara (Tokyo, Japan) quartz-made distillation apparatus and used throughout the measurements. Chloroform of G.R grade was used without further purification. Kriptofm 22DD (Merck, D m t a d t , FRG), 4,4’-dioctadecyL2,2’-bipyridyl(Wako, Tokyo, Japan), valinomycin (Boehringer-Manheim, FRG), and bis[ (benzo-15crown-5)-4’-methyl] pimelate (Dojin Laboratories, Kumamoto, Japan) were used as received. Other reagents used were all of analytical reagent grade. Apparatus. The surface pressure isotherms were studied by using a Kyowa Kaimenkagaku (Tokyo, Japan) Teflon-coated trough equipped with a f i i balance. A monolayer was transferred to a glassy carbon electrode (surface area 19.5 mm2) by using a Kyowa Kaimenkagaku surface pressure controller (a horizontal lifting method). Cyclic voltammograms were recorded with a Fuso (Tokyo, Japan) potentiostat, Model 305A, equipped with a Rika Denki (Tokyo, Japan) X-Y recorder, Model RW-11. A threeelectrode configuration was employed: A glassy carbon (GC) electrode coated with Langmuir-Blodgett films as a working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire as an auxiliary electrode. The voltammetric measurements were carried out at a room temperature (25 “C) with a sweep rate of 100 mV s-’. Monolayer Preparation and Langmuir-Blodgett Film Deposition on a Glassy Carbon Electrode. Monolayers were obtained by spreading 1mM chloroform solutions of amphiphiles on the air-water interface of the trough (surface area, 1000 cm2). The subphase was doubly distilled water and thermostated at 7 “C. For complete evaporation of chloroform, the monolayer was
2843
allowed to stand for 2 h. Then the surface was compressed to a constant pressure at a velocity of the barrier movement of 125 mm min-’, Le., 29 A2 molecule-l min-’. The monolayer was transferred to a glassy carbon electrode by lowering the electrode surface placed parallel to the air-water interface until it comes in contact with the interface and then lifting it (a horizontal lifting method) under a constant surface pressure of 45 mN m-l. For the case of the didodecyl phosphate-valinomycin mixture (1:l mol ratio), the monolayer was transferred under a surface pressure of 17.2 mN m-l. Usually three or five monolayers were deposited. The glassy carbon electrode used as a solid support was polished prior to film deposition with wet 0.05-wm alumina powder, rinsed several times with distilled water, and dried in air. The coated substrates were allowed to dry in air for 30 min between each dip.
RESULTS AND DISCUSSION Recognition of Analytes by Langmuir-Blodgett (LB) Multilayer and Information Transduction. Cyclic voltammetric (CV) measurements of ferrocyanide ions as a marker (monitoring) ion with a glassy carbon (GC) electrode coated with didodecyl phosphate film are shown in Figure 3. The reversible cv peaks of a Fe(CN)64-/Fe(CN)63-system at an uncoated GC electrode (Figure 3a) are almost completely suppressed by depositing three layers of didodecyl phosphate monolayer on the surface of the GC electrode (Figure 3a). More than three layers of LB film deposition was necessary for complete blocking of the permeation of ferrocyanide anions as a marker ion (Figure 3a). The suppression of the peak current at +0.2 V is due to the electrode reaction which is becoming more irreversible in the presence of the film. The irreversible characteristics of the system are clearly demonstrated by the shift in the peak potential and the marked decrease of the cathodic reduction peak in contrast to the oxidation current. Since the phase transition temperature of the didodecyl phosphate lipid bilayer from solid to liquid crystalline state is known to be 45 “C (2),the f i i of didodecyl phosphate coated on the GC is in the solid crystalline state under the present experimental conditions of 25 “C. Therefore, the suppression of CV peaks of the marker ion by coating the LB film appears to be due to a rigid alignment of the phosphate molecules which functions as a “closed” channel. Relatively bulky ferrocyanide anions are not allowed to permeate through the closed channel toward the underlying electrode. This situation is schematically drawn in Fig. la. The phospholipid bilayer having an acidic phosphate head group interacts with divalent metal ions such as calcium ions and changes the molecular configuration (2). A drastic change in the alignment of molecular assembly of didodecyl phosphate on the GC surface is expected by the interaction with such metal ions. This seems to cause opening of the gate through which the marker ion can permeate to an underlying electrode and to be detected electrochemically as schematically drawn in Figure Ib. Cyclic voltammetric responses of the didodecyl phosphate film coated calcium ion sensor to different concentrations of calcium ions are shown in Figure 3b. The CV peaks of Fe(CN)a- ions as marker ion appear by adding electroinactive calcium ions and the peak current increases with increasing the concentration of calcium(I1). The reversibility of electrode reaction of ferrocyanide ions is also improved by adding higher concentration of calcium ions. When a slight excess amount of ethylenediaminetetraacetate (EDTA) is added to the solution of calcium ions in the neutral media in order to quench the stimulus, calcium ions, the CV peaks of marker ions disappear again (Figure 3c). The important features of these results with a calcium ion responding ion-channel sensor are as follows: (i) an analyte (calcium ion) is chemically recognized by the LB film of the didodecyl phosphate multilayer and this information is quantitatively transduced into a electrochemical signal corresponding to the amount of permeated marker ions (Fe(CN),3-I4-ions), and (ii)
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
a
b
C
fi
-V
+0.8
0.0
-0.2
0.0
~0.8
-0.2
u
+0.8
0.0
-0.2
E(vs. S C E ) / V
E (vs. SCE) / V E ( v s . 5 ~ I~V) Figure 3. Cyclc voltammetrlc detectlon of 1.0 mM Fe(CN)BCas the marker ion with Ca(I1)ion stimulated ionchannel sensors based on didodecyl phosphate monolayers using a supporHng electrolyte of 10 mM NaNO,. (a)Detection with (1) an unmodified GC electrode, (2) a monolayercoated GC electrode, and (3)a three-monolayer-coated GC electrode. (b) Response of the sensor to differentconcentrations of Ca(I1): (1) 0, (2) 0.1, (3) 0.2, (4) 0.4, (5) 1.0, (6) 3.0 mM. (c) Reversible closing of channel by adding a Ca(I1)quencher, EDTA: (1) 3.0 mM Ca(II),( 2 ) (1) 4.0 mM
+
NaA (d). Table I. Characteristics of Some Ion-Channel Sensors membrane materials (Ci2)2P04Hn 3 layers
(Cl&NBrb 5 layers (C14)Z-Gly-PC'
3 layers
(Ciz)zP04Hn valinomycin 4 layers
marker ions Fe(CN)64RU(~PY),~+ Fe(CN)tRu(bpy),2+ Fe (CN) R ~ ~ P3*+Y ) Fe (CN)B4-
permeation of marker ions
close
controlled noncontrolled noncontrolled controlled controlled noncontrolled controlled
-
ion-channel switching open open
Ca(II),Mg(II),Ba(I1)
-
close
EDTA
C1Oc Ca(I1) K+
Didodecyl phosphate. Dimethyldioctadecylammonium bromide. 1,3-Ditetradecylglycero-2-phosphocholine. the calcium ion stimulated "on/off" switching of the gate function is reversibly made. Cyclic voltammetric response characteristics of some ionchannel sensors constructed here using different types of amphiphiles as channel-forming materials are summarized in Table I. Both ferrocyanide anions and ruthenium@) complex cations were examined as marker ions. With the didodecyl phosphate film based calcium ion sensor, ferrocyanide anions were useful as the marker ion as described above. However, ruthenium(II) complex cations did not work well as the marker ion, namely, no control of permeation by the phosphate lipid film for the ruthenium(II) complex cations was achieved. With the perchlorate ion sensor based on dimethyldioctadecylammonium bromide, ferrocyanide anions could not be used as the marker ion, because the sensor showed no on/off switching of the gate for permeation of the marker anion even though five monolayers of the lipid quaternary ammonium salt were deposited. These results imply that permeation of marker ions through the LB film is regulated to some extent by electrostatic interaction between the head group of lipids and marker ions. Cyclic voltammetric response of a perchlorate anion stimulated ion-channel sensor based on dimethyldioctadecylammonium bromide to different concentrations of perchlorate anions is shown in Figure 4. The CV peaks of the ruthenium(I1) complex cation as the marker ion appear at almost the same potential value as those observed at an uncoated GC
W I
l
.I20
I
l
I
l
+ 0 8 5 +075 E(vs.SCE)/
v
/
+I 20
/
I
l
l
4085
,
+075
E(vs S C E ) I V
Cyclic voltammetric response of (310,- ion stimulated ionchannel sensor based on dlmethyldloctadecylammonium bromide monolayers using 0.5 mM Ru(bpy)&I, as the marker salt and 10 mM NaCl as a supporting electrolyte. (a)Detection of 0.5 mM Ru(bpy),CI, with (1) an unmodified GC electrode and (2)the five-monolayer-coated GC electrode. (b) Response of the sensor to different concentrations of NaCIO,: (1) 0, (2)0.2,(3)0.6,(4) 1.2,(5) 2.2 mM. Flgure 4.
electrode. The intensity of the peak current increases with the concentration of perchlorate anions (Figure 4b). In the absence of the perchlorate ion stimulus, the channel is in a closed configuration due to a rigid alignment of the lipid quaternary ammonium salt (phase transition temperature of
ANALYTICAL CHEMISfRY, VOL. 59, NO. 24, DECEMBER 15, 1987
2845
C
- - - k l '
0
1.0
2 .o
3.0
[: C a 2 + ]/mM
Flgwe 5. Amplification with a Ca(I1)ion stimulated bn-channel sensw based on three layers of didodecyl phosphate monolayers: concentrations of Fe(CN),+ marker ions, (1)1.0,(2) 2.0, and (3) 3.0 mM; Supporting electrolyte, 10 mM NaNO,.
45 "C (3)). The channel is opened upon addition of perchlorate anions which form an ion-association compound with the lipid quaternary ammonium cations in the LB molecular assembly. Another example of the ion-channel sensor is based on the synthetic phosphocholine (Figure 2c) similar to a typical biological phospholipid, lecithin. The phosphocholine having a quaternary ammonium cation head group was unable to control premeation of the ruthenium complex cations (Table I). The observed calcium ion stimulated switching of the gate in the lecithin monolayers, which allowed permeation of ferrocyanide anions, may be mainly due to a change in the conformation of the phosphocholine multilayer (Table I). Amplification with Ion-Channel Sensors. The ionchannel sensors seem to have the ability to amplify concentration of an analyte through chemical recognition of the analyte by the receptor at the membrane surface whose information is transduced into the change in permeability of electroactive marker ions. In ordir to guarantee the amplification, it necessitates by definition that permeated amounts (moles) of marker ions through channels should be greater than that of the stimulus (analyte) bound to receptor sites a t the membrane surface where the opening of ion channels occurs. Figure 5 shows response currents of the calcium ion sensor based on the didodecyl phosphate film as a function of concentration of the analyte (Ca(I1) ions) using different concentrations of marker ions. The results show that the current intensity, consequently the permeated amount of marker ions (ferrocyanide ions), increases in proportion to the concentration of the marker ion in solution when the concentration of calcium ions is constant. The moles of ferrocyanide ions permeated through the channel were estimated from the cyclic voltammogram of 2 mM ferrocyanide ions in the presence of 3 mM Ca(I1) stimulus by a graphical integration of the oxidation peak with respect to time. The result of integration gives about 3.5 X C of electricity passed through the sensor, which corresponds to 3.6 X mol of ferrocyanide ions oxidized to ferricyanide ions. The number of didodecyl
0
0.8
-0.2
E (vs. S C E ) / V
Figure 6. Cyclic voltammetric detection of 1 rnM Na,Fe(CN), with K+ Ion stimulated ion-channel sensor based on didodecyl phosphate monolayers and valinomycin (1:l mol ratio): detection with (1)an unmodified GC electrode, (2) the four-monolayer-coated GC electrode, and (3) the four-monolayercoatedGC electrode after addition of 4 mM KCI as the stimulus; supporting electrolyte, 50 mM (CH,),NCI.
phosphate molecules deposited on the GC electrode was calculated from the surface area for one molecule of the lipid phosphate (32.3 A2 molecule-1 under a surface pressure of 45 mN m-l) in the monolayer alignment at the air-water interface. It is found that the monolayer on the electrode surface corresponds to about 6 X 1013molecules (1 X mol) of the phosphate. As 2 mol of the phosphate is known to bind to 1 mol of calcium(I1) ions (2), the phosphate head group available for binding calcium(I1) is 5 X lo-" mol for each monolayer. Therefore, even if the maximum amount of Ca(II), i.e., 5 X 10-l' mol, is bound to the outmost monolayer of the membrane in order to open the channel for permeation of 3.6 X mol of ferrocyanide ions, the degree of the information transduction and amplification of the calcium sensor expressed as relative moles of the permeated ferrocyanide ions to Ca(I1) ions bound to the receptor/channel is 7.2. This result indicates that this calcium ion sensor has a built-in ability of amplification of concentration of the analyte (Ca(I1) ions). Of course, what is also important here is whether or not the sensor can detect bulk concentration of the analyte with amplification. With the present calcium ion sensor, the permeated amount of ferrocyanide ions as the marker ion was lower than the amount of Ca(I1) ions in the solution, i.e., 1.5 X lo4 mol in the present cyclic voltammetric time scale. However, it is obvious that the amount of marker ions which permeates through the channel may be significantly increased if the time scale of the measurement is extended by using, for example, some coulometric detection of marker ions. The work is now under way. Selectivity. Responses of the calcium ion channel sensor to some divalent alkaline-earth and alkali-metal cations were examined. The calcium sensor responds to magnesium and barium ions in a manner similar to calcium ions but not to alkali-metal ions such as sodium and potassium ions. In-
2846
Anal. Chem. 1087, 59, 2046-2040
corporation of channel receptors in the LB film membrane which interact selectively with analytes and open channels for permeation of marker ions should improve the selectivity of this type of sensor. We have performed some preliminary studies with receptor compounds such as valinomycin, Kryptofix 22DD, 4,4’-dioctadecyL2,2’-bipyridyl,and bis[(benzo-15-crown-5)-4’-methyl] pimelate. When valinomycin and bis[ (benzo-15-crown-5)-4’-methyl] pimelate, which both are regarded as potassium ion receptors, are incorporated into the didodecyl phosphate monolayers, the channel opening by potassium ions is clearly observed as the permeation of Fe(CN)64-ions (Figure 6 and Table I). Obviously, didodecyl phosphate monolayers without incorporation of such potassium ion receptors exhibited no change in cyclic voltammograms with potassium ion stimulus.
Registry No. C, 7440-44-0;Ca, 7440-70-2; C104-, 14797-73-0; K,7440-09-7;Fe(CN)64-, 13408-63-4;Ru(bpy),Clz, 14323-06-9; didodecyl phosphate, 7057-92-3; dimethyldioctadecylammonium bromide, 3700-67-2;1,3-ditetradecylglycero-2-phosphocholine, 94513-40-3; valinomycin, 2001-95-8. LITERATURE CITED ( 1 ) Palmer, R. A,; Piper, T. S. Inorg. Chem. 1966, 5 , 864-878. (2) Okahata, Y. Acc. Chem. Res. 1986, 79, 57-63. (3) Okahata, Y.; Hachlya, S.;Ariga, K.; Seki. T. d . Am. Chem. Soc. 1986, 708, 2863-2869.
RECEIVED for review June 1,1987. Accepted August 18,1987. The authors gratefully acknowledge the financial support from the Ministry of Education, Science and Culture and the Suhara Foundation.
Multisensing Ion-Selective Field-Effect Transistors Prepared by Ionophore Doping Technique K l i r a Bezegh,’ A n d r i s Bezegh,2 and Jid Janata*
Center for Sensor Technology, University of Utah, Salt Lake City, Utah 84112 U r s O e s ~ hAiping ,~ X U , a~n d Wilhelm Simon
Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland
An Improved procedure for preparatlon of multlple-gate lonsensitlve fleld-effect transistors ( ISFETs) with polymeric membranes has been developed. Flrst, a blank membrane contalnlng only the polymer and the plastlclzer Is applied to the encapsulated FET chlp. I n the second step the electroactlve components of the membrane (Ionophores and/or other addltlves) are Introduced by locally doplng thls blank membrane. The maln benefit of thls shnple procedure Is the fact that the membrane Is contlnuorrs and therefore it Is mechanlcalty stable and lt Is without electrical shunts between the membrane and the encapsulant. I n thls paper the electroanalytlcal behavior of the ISFETs with membranes prepared by thls procedure Is evaluated and compared to conventlonal macroelectrodes of the same membrane types.
Polymeric ion-selective membranes consist of at least two kinds of components: the polymer, which provides primarily the mechanical support, and the ionophore and other electroactive ingredients, which introduce the desired electrochemical properties. The additional use of suitable plasticizers allows a more subtle adjustment of desired properties of the membrane. Primarily plasticizers serve as solvents, thus providing the required mobility for a sufficient fast kinetics of the ion-exchange/extraction at the phase boundary of the membrane and the sample. To some extent, plasticizers may On leave from the Jlnos Hospitals, 2nd Internal Medicine Department, 1125 Budapest, Hungary. *On leave from the Institute for General and Analytical Chemistry Technical University, 1502 Budapest, Hungary. dPresent address: Ciba-CorningDiagnostics Corp., Medfield, MA 02052. On leave from the Hygiene Department, Nanijng Medical College, Nanijng, People’s Republic of China.
also take part and/or influence the stability of the ionlionophore complex, thus influencing the selectivity pattern of the membrane (1). The usual method of application of this type of membrane involves dissolution of all the materials in an organic solvent, or in a mixture of organic solvents, and then casting a film of a desired thickness (50-500 pm), followed by controlled evaporation of the solvent (2). For macroelectrodes (diameter > 1 mm) a piece of this film is usually cut out and mounted in an electrode holder. This procedure works well for macroscopic membranes (3). For ion-selective field-effect transistors (ISFETs) the casting of the membrane solution is done directly over the gate area of the chip to the final thickness of approximately 100 pm. Although the principle of operation of ISFETs with ion-selective membranes is simple, their preparation is not. This is due mainly to the small size of the active areas, to the planarity of the chip, and to the need to encapsulate rigorously parts of the chip that are in close proximity of the active gate ( 4 ) without damaging the membrane. During the membrane casting, which has to be done under a microscope, the organc solvent rapidly evaporates from the casting solution, which results in clogging of the tip of the dispenser. For this reason the orifice of the casting capillary has to be relatively large, as compared to the area over which the membrane is applied. This makes this preparation step particularly troublesome and, at present, ultimately limits the closeness of the spacing between individual channels on a multisensor chip (5). A possible electrical path between the membrane and the adjacent encapsulant can effectively short out the membrane potential. The undesirable transient type of response of the ISFETs with small membranes covering the gates has been attributed to this leakage (5). The relatively high ratio of exposed area to the length of the boundary between the
0003-2700/87/0359-2846$01.50/0 0 1987 American Chemical Society
