- Email: [email protected]
Anion-sensitive field effect transistors based on polyion complex bilayers
Ana&icu Chimica AC& 252 (1991) 41-46 Elsevier Science Publishers B.V., Amsterdam
41
Anion-sensitive field effect transistors based on polyion complex bilayers Keiichi Kimura Chemical Process Engineering, (Received
*, Masataka
Matsute and Masaaki Yokoyama
Faculty of Engineering.
Osaka Unioersity,
28th March 1991; revised manuscript
Yamada-oka,
Suita, Osaka 565 (Japan)
received 29th May 1991)
Abstract Polyion complex bilayers made of dioctadecyldimethylammonium poly(styrene sulphonate) were tested for their usefulness as anion-sensing membranes of ion-sensitive field effect transistors (ISFETs). Doping of the polyion complex with the corresponding quaternary ammonium chloride enhanced the sensitivity of the anion ISFET. Anion ISFETs based on the ammonium-doped polyion complex showed a near-Nemstian response in the Cl- activity range 0.1-l x 10m4 M. Annealing of bilayer membranes at a temperature higher than its crystal-to-liquid crystal transition temperature improved to some extent the selectivity for Cl- with respect to other anions. Doping of the polyion complex with the corresponding quatemary ammonium perchlorate also afforded ClO;-sensitive membranes for ISFETs. Keyworak
Potentiometry;
Anion-sensing
membranes;
Ion-sensitive
Since the pioneering work of Bergveld [l] and Matsuo’s group [2] on ion-sensitive field effect transistors (ISFETs), a large number of inorganic and organic membranes have been tried as ionsensing layers [3]. Plasticized poly(viny1 chloride) (PVC) membranes containing active materials are applicable to ISFETs [4]. Nevertheless, as PVC membranes are liable to peel off from FET gates, some efforts have been made to enhance the membrane stability [5-g]. Also, several other organic membranes have been tested for their applicability to ISFETs [lo-141. Polyion complexes between quaternary ammonium ions and polyanions such as poly(styrene sulphonate) and poly(viny1 sulphonate) are known to form stable films that retain ordered bilayer structures [15]. Polyion complex bilayers are therefore promising membrane materials, which have been applied to sensing membranes for piezoelectric sensors [16], ion-selective electrodes [17] and impedance sensors [18]. Attempts have been made COO3-2670/91/$03.50
field effect transitors;
Polyion
complex
bilayers
to use polyion complex bilayers of dioctadecyldimethylammonium poly(styrene sulphonate) to anion-sensing membranes for ISFETs and also to dope the polyion complex with the corresponding quaternary ammonium salts in order to improve the sensitivity of the anion-sensitive ISFETs. This paper is concerned with the characteristics of anion-sensitive ISFETs based on polyion complex bilayers.
EXPERIMENTAL
Materials
The polyion complex, dioctadecyldimethylammonium poly(styrene sulphonate), was prepared [15] as follows. Dioctadecyldimethylammonium chloride (3.5 mmol) was dispersed in water (about 200 ml) at 7O”C, using an ultrasonic cleaning bath. The ammonium salt-dispersed solution and an aqueous solution (10 ml) of sodium poly(styrene
0 1991 - Elsevier Science Publishers D.V. All rights reserved
K. KlibflJRA
42 (3 mmol in the unit, MW = 70000) werr: poured into a beaker at 70 o C with vigorous stirrink:. The heating and stirring were continued for a further 1 h. Cooling the solution to room temperature resulted in a precipitate of polyion complex, which was purified by repeated reprecipitation from chloroform in acetonitrile. The ratio of ammonium and sulphonate units in the resulting polyion complex, calculated from its elemental analysis data, was 1.06. Ammonium salts for doping the polyion complex and various potassium salts were of analytical-reagent grade.
sulphonate)
ISFET fabrication FET devices, illustrated in Fig. 1, were kindly supplied by Shindengen Electric. The gate size was 10 x 370 pm. An aliquot (2 ~1) of a chloroform solution of the polyion complex (10 mg ml-‘) was placed on the gate surface of the FET device by using a magnifier and a microsyringe and the device was air-dried at room temperature overnight. The thickness of the polyion complex film on the FET gate was estimated to be 5-10 pm by microscopy. The polyion complex-coated FET devices were annealed in a water-bath at 70” C for 30 min. Potential measurements Potentiometric measurements were made at room temperature using an ISFET pH/mV meter (Shindengen Electric). The source-drain voltage
ET AL.
and current (Ids) were kept at 5 V and 100 PA, respectively. The reference electrode was a double-junction type Ag/AgCl electrode (DKK 4083-0.65C) with 3 M potassium chloride as the internal solution and 1 M lithium acetate as the external solution. The anion activities were changed by injection of high-concentration solutions to testing solutions (an incremental method). All the anion sources were their corresponding potassium salts. The selectivity coefficients for Cl- with respect to other anions were determined by a mixed solution method (FIM). The background anion concentrations were 1 x 10B4 M for ClO; and SO:-, 5 x 10s4 M for Br- and NO,, 0.2 M for CH,COOand 0.5 M for CO:-.
(V,,)
Other measurements Differential scanning calorimetry
RESULTS AND DISCUSSION
Dioctadecyldimethylammonium poly(styrene sulphonate) was employed as a polyion complex for the ion-sensing layer of ISFETs. A typical response of polyion complex-based ISFETs to KC1
Fig. 1. Schematic diagram and cross-section of FET device tip. (1) Gate: (2) diain (n+-Si); (3) source (n+-Si); (4) channel stopper; (5) Si3N,; (6) SiO,; (7) p-55.
ANION-SENSITIVE
FIELD
EFFECT
43
TRANSISTORS
complex formation. That is, the excess ammonium ions, especially, in the vicinity of the membrane surface, are able to exchange anions. However, there seems to be only a small number of ammonium ions active to exchange of anions. This is probably the reason why the anion ISFET shows such moderate sensitivity, i.e., sub-Nernstian response. Table 1 summarizes anion selectivities for Clwith respect to several other anions. The anion selectivity sequence for the polyion complex-based ISFET follows the Hofmeister lyotropic series [19], again indicating that the potential response to anion concentration changes in the polyion compIex is derived from the conventional ion-exchange mechanism. This type of polyion complex bilayer generally undergoes a phase transition from crystalline to liquid crystalline on heating [15]. The polyion complex employed here shows a phase transition temperature (T,) of 41° C, which was supported by a distinct endothermic peak in the DSC trace, as demonstrated in Fig. 3. Annealing of the polyion complex-based ISFET at a temperature higher than the T, was therefore attempted. By annealing, the selectivities for Cl- were enhanced more or less with respect to all the other anions (Table l), although no significant change was found in the sensitivity, that is, the slope of the Cl- calibration graph. When the polyion complex was annealed in water at a temperature higher than the T,, its DSC peak became more intense. This means that annealing promotes crystalline phase formation in the polyion complex bilayer. The substantial formation of a crystalline phase was also sup-
,\ 1OOmV .%
543210 -lay 1x-1 Fid. 2. Calibration graphs for anion ISFETs based on polyion complex biiayers. (0) Cl- with 10 wt.% dioctadecyldimethylammonium chloride doping; (a) Clwithout doping; (A) CIO; with 10 wt.% dioctsdecyldimethylammonium perchlorate doping.
concentration changes is depicted in Fig. 2. The decrease in potential with increasing KC1 concentration in test solutions clearly indicates the anion response of the ISFET. The calibration graph exhibits linearity with a slope of about 50 mV per decade in the Cl- activity range 5 x 10-4-0.1 M. If the polyion complex employed consisted only of 1: 1 ion pairs of the ammonium and sulphonate units, the resulting ISFETs would never respond to anion activity changes in test solutions owing to the extremely low mobility of the polysulphonate anions. Elemental analysis data showed that several percent of the quatemary ammonium units in the polyion complex do not form ion pairs with the sulphonate units, being free from polyion TABLE 1 Selectivity coefficients for Cl- with respect to other anions dioctadecyldimethylammonium chloride doping Polyion complex
(k&)
in ISFETs
based
on polyion
complex
with
and without
log kE; ClO,-
NO;
Br-
ca,coo-
so,‘-
co,‘-
Undoped Unannealed Annealed
1.90 1.73
1.08 0.93
0.60 0.38
- 1.10 - 1.62
- 1.92 - 2.05
- 2.85 - 3.30
Doped (10 wt.%) Unannealed Annealed
1.91 1.71
1.07 0.93
0.56 0.46
- 1.14 - 1.58
-1.80 - 2.90
- 2.85 -3.22
K. WMURA
.o E
?il
/-
ET At,
r
afterannealing
AZ
i5 u
4
.I.
~ 1 25
0 ( 30
I 35
I 40
I 45
Ammonium
500
TemperatureI% Fig. 3. Differential scanning calorimetric profiles for polyion complex films before and after annealing+
ported by the fact that a semi-transparent film of the polyion complex turned turbid on annealing. Scanning electron micrographs display a clear difference in the surface between the untreated and annealed polyion complex films (Fig. 4). The untreated polyion complex film has a very smooth surface, whereas annealing caused wrinkling of the surface, probably owing to the effective packing of the bilayer. Conceivably, the annealing-induced packing of the polyion complex bilayer rejects
before annealing
5
10
15
20
salt doped I WI%
Fig. 5. Sensitivity enhancement of polyion complex-based ClISFETs by di~tadecyldimethylammonium chloride doping.
large anions, thus increasing the Cl- selectivity over other larger anions. Attempts were made to dope the polyion complex films with dioctadecyldimethylammonium chloride. The doped ammonium salt, if stable enough in the polyion complex bilayer not to leach out io aqueous test solutions, was expected to participate in anion exchange and, therefore, to increase the sensitivity of anion ISFETs. In fact, an increase in the doping amount of ammonium
after annealing
Fig. 4. Scanning electron micrographs for polyion comptex films on ISFETs before and after anneafing.
ANION-SENSITIVE
FIELD
EFFECT
TRANSISTORS
,
I
I
1
’
’
’
I
25
30
35
40
45
50
55
60
Temperature / % Fig. 6. Differential scanning calorimetric profiles for annealed polyion complex films with 5, 10 and 20 wt.% ammonium salt doping.
salt resulted in a considerable sensitivity enhancement of the ISFETs (Fig. 5). An anion ISFET based on a polyion complex with 10 wt.% ammonium doping exhibited a near-Nernstian response to Cl- activity changes. Also, the Clactivity range giving linearity of the calibration graph was extended down to 1 x 10m4 M by doping of the polyion complex with ammonium salt (Fig. 2). Doping with more than 10 wt.% ammonium salt did not improve further the sensitivity and linear activity range of the polyion complex-based anion ISFETs. The ammonium salt doping, however, brought about hardly any significant changes in the Cl- selectivity for the resulting ISFETs (Table 1). An annealing effect on the ion selectivity, i.e., some Clselectivity enhancement, was again found in the anion ISFETs based on the ammonium salt-doped polyion complex. The ammonium salt-doped polyion complexes, when annealed, show strong endothermic peaks on the DSC trace around the T, of the undoped polyion complex (Fig. 6), suggesting that annealing still leads to closely packed bilayer structures even on doping with less than 20 wt.% ammonium salt. The polyion complex was also doped with a corresponding quaternary ammonium perchlorate. The resulting ISFET based on the polyion complex doped with 10 wt.% ammonium perchlorate al-
45
lowed a near-Nemstian response to ClO; activity changes, as demonstrated in Fig. 2. The deviation from the near-Nernstian response at high Cl07 activities can possibly be attributed to some cation interference, because KClO, is easier to distribute to the membrane phase than KCl. Similarly, polyion complex films responsive to various desired anions may be prepared for ISFETs. In conclusion, dioctadecyldimethylammonium poly(stjrrene sulphonate) doped with its ctirresponding ammonium salts are promising for ionsensing layers of anion ISFETs. The polyion complex possesses good adhesive properties, especially without annealing. Moreover, as the polyion complex carries ordered bilayer structures, the polyion complex might therefore be a useful polymer support, in which active materials may be arranged in an orderly manner, for FET sensors. The authors gratefully acknowledge Dr. Shinichi Wakida, Government Industrial Research Institute, Osaka, and Dr. Naotoshi Nakashima, Department of Industrial Chemistry, Faculty of Engineering, Nagasaki University, for helpful discussions.
REFERENCES 1 P. Bergveld, IEEE Trans. Biomed. Eng., BME-19 (1972)
342. 2 T. Matsuo and K.D. Wise, IEEE Trans. Biomed. Eng., BME-21 (1974) 485. 3 J. Janata and R.J. Huber, Ion-Se!. Electrode Rev., 1 (1979) 31. 4 S.D. Moss, J. Janata and C.C. Johnson, Anal. Chem., 47 (1975) 2238. 5 R.K. Rhodes, IEEE Trans. Biomed. Eng., BME-33 (1986) 91. 6 T. Satchwill and D.J. Harrison, J. Electroanal. Chem., 202 (1986) 75. 7 D.J. Harrison, L.L. Cunningham, X. Li, A. Teclemaiiam and D. Permann. J. Electrochem. Sot.. 135 (1988) 2473. 8 G.J. Moody, J.D.R. Thomas and J.M. Slater, Analyst, 113 (1988) 1703. 9 H. van der Vlekkert, C. Francis, A. Grisel and N. Rooij, Analyst, 113 (1988) 1029. 10 S. Kawakami, T. Akiyama and Y. Ujihira, Fresenius’ Z. Anal. Chem., 318 (1984) 349. 11 S. Wakida, M. Yamane, K. Hiiro, T. Kihara. Y. Ujihira and T. Sugano, Anal. Sci., 4 (1988) 501.
46
12 E.J. Fogt, D.F. Untereker, MS. Norenberg and M.E. MeyerhofF, Anal. Chem., 57 (1985) 1998. 13 T.J. Cardwell, R.W. Cattrall, P.J. Iles and I.C. Hamilton, Anal. Chim. Acta, 219 (1989) 135. 14 M. Battilotti, R. Cohlli, I. Giannini and M. Giongo, Sensors Actuators, 17 (1989) 209. l!j T. Kunitake, A. Tsuge and N. Nakashima, Chem. Lett., (1984) 1783.
K. KIMURA
ET AL.
16 Y. Okahata and H. Ebato, Anal. Chem.. 61 (1989) 2185. 17 T. Ogata and H. Yanagi, High Polym. Jpn., 38 (1989) 212. 18 N. Nakashima, H. Eda, M. Kunitake, 0. Manabe and K. Nakano, J. Chem. Sot., Chem. Commun., (1990) 443. 19 F. Hofmeister, Arch, Exp. Pathol. Pharmakol., 24 (1888) 247.
41
Anion-sensitive field effect transistors based on polyion complex bilayers Keiichi Kimura Chemical Process Engineering, (Received
*, Masataka
Matsute and Masaaki Yokoyama
Faculty of Engineering.
Osaka Unioersity,
28th March 1991; revised manuscript
Yamada-oka,
Suita, Osaka 565 (Japan)
received 29th May 1991)
Abstract Polyion complex bilayers made of dioctadecyldimethylammonium poly(styrene sulphonate) were tested for their usefulness as anion-sensing membranes of ion-sensitive field effect transistors (ISFETs). Doping of the polyion complex with the corresponding quaternary ammonium chloride enhanced the sensitivity of the anion ISFET. Anion ISFETs based on the ammonium-doped polyion complex showed a near-Nemstian response in the Cl- activity range 0.1-l x 10m4 M. Annealing of bilayer membranes at a temperature higher than its crystal-to-liquid crystal transition temperature improved to some extent the selectivity for Cl- with respect to other anions. Doping of the polyion complex with the corresponding quatemary ammonium perchlorate also afforded ClO;-sensitive membranes for ISFETs. Keyworak
Potentiometry;
Anion-sensing
membranes;
Ion-sensitive
Since the pioneering work of Bergveld [l] and Matsuo’s group [2] on ion-sensitive field effect transistors (ISFETs), a large number of inorganic and organic membranes have been tried as ionsensing layers [3]. Plasticized poly(viny1 chloride) (PVC) membranes containing active materials are applicable to ISFETs [4]. Nevertheless, as PVC membranes are liable to peel off from FET gates, some efforts have been made to enhance the membrane stability [5-g]. Also, several other organic membranes have been tested for their applicability to ISFETs [lo-141. Polyion complexes between quaternary ammonium ions and polyanions such as poly(styrene sulphonate) and poly(viny1 sulphonate) are known to form stable films that retain ordered bilayer structures [15]. Polyion complex bilayers are therefore promising membrane materials, which have been applied to sensing membranes for piezoelectric sensors [16], ion-selective electrodes [17] and impedance sensors [18]. Attempts have been made COO3-2670/91/$03.50
field effect transitors;
Polyion
complex
bilayers
to use polyion complex bilayers of dioctadecyldimethylammonium poly(styrene sulphonate) to anion-sensing membranes for ISFETs and also to dope the polyion complex with the corresponding quaternary ammonium salts in order to improve the sensitivity of the anion-sensitive ISFETs. This paper is concerned with the characteristics of anion-sensitive ISFETs based on polyion complex bilayers.
EXPERIMENTAL
Materials
The polyion complex, dioctadecyldimethylammonium poly(styrene sulphonate), was prepared [15] as follows. Dioctadecyldimethylammonium chloride (3.5 mmol) was dispersed in water (about 200 ml) at 7O”C, using an ultrasonic cleaning bath. The ammonium salt-dispersed solution and an aqueous solution (10 ml) of sodium poly(styrene
0 1991 - Elsevier Science Publishers D.V. All rights reserved
K. KlibflJRA
42 (3 mmol in the unit, MW = 70000) werr: poured into a beaker at 70 o C with vigorous stirrink:. The heating and stirring were continued for a further 1 h. Cooling the solution to room temperature resulted in a precipitate of polyion complex, which was purified by repeated reprecipitation from chloroform in acetonitrile. The ratio of ammonium and sulphonate units in the resulting polyion complex, calculated from its elemental analysis data, was 1.06. Ammonium salts for doping the polyion complex and various potassium salts were of analytical-reagent grade.
sulphonate)
ISFET fabrication FET devices, illustrated in Fig. 1, were kindly supplied by Shindengen Electric. The gate size was 10 x 370 pm. An aliquot (2 ~1) of a chloroform solution of the polyion complex (10 mg ml-‘) was placed on the gate surface of the FET device by using a magnifier and a microsyringe and the device was air-dried at room temperature overnight. The thickness of the polyion complex film on the FET gate was estimated to be 5-10 pm by microscopy. The polyion complex-coated FET devices were annealed in a water-bath at 70” C for 30 min. Potential measurements Potentiometric measurements were made at room temperature using an ISFET pH/mV meter (Shindengen Electric). The source-drain voltage
ET AL.
and current (Ids) were kept at 5 V and 100 PA, respectively. The reference electrode was a double-junction type Ag/AgCl electrode (DKK 4083-0.65C) with 3 M potassium chloride as the internal solution and 1 M lithium acetate as the external solution. The anion activities were changed by injection of high-concentration solutions to testing solutions (an incremental method). All the anion sources were their corresponding potassium salts. The selectivity coefficients for Cl- with respect to other anions were determined by a mixed solution method (FIM). The background anion concentrations were 1 x 10B4 M for ClO; and SO:-, 5 x 10s4 M for Br- and NO,, 0.2 M for CH,COOand 0.5 M for CO:-.
(V,,)
Other measurements Differential scanning calorimetry
RESULTS AND DISCUSSION
Dioctadecyldimethylammonium poly(styrene sulphonate) was employed as a polyion complex for the ion-sensing layer of ISFETs. A typical response of polyion complex-based ISFETs to KC1
Fig. 1. Schematic diagram and cross-section of FET device tip. (1) Gate: (2) diain (n+-Si); (3) source (n+-Si); (4) channel stopper; (5) Si3N,; (6) SiO,; (7) p-55.
ANION-SENSITIVE
FIELD
EFFECT
43
TRANSISTORS
complex formation. That is, the excess ammonium ions, especially, in the vicinity of the membrane surface, are able to exchange anions. However, there seems to be only a small number of ammonium ions active to exchange of anions. This is probably the reason why the anion ISFET shows such moderate sensitivity, i.e., sub-Nernstian response. Table 1 summarizes anion selectivities for Clwith respect to several other anions. The anion selectivity sequence for the polyion complex-based ISFET follows the Hofmeister lyotropic series [19], again indicating that the potential response to anion concentration changes in the polyion compIex is derived from the conventional ion-exchange mechanism. This type of polyion complex bilayer generally undergoes a phase transition from crystalline to liquid crystalline on heating [15]. The polyion complex employed here shows a phase transition temperature (T,) of 41° C, which was supported by a distinct endothermic peak in the DSC trace, as demonstrated in Fig. 3. Annealing of the polyion complex-based ISFET at a temperature higher than the T, was therefore attempted. By annealing, the selectivities for Cl- were enhanced more or less with respect to all the other anions (Table l), although no significant change was found in the sensitivity, that is, the slope of the Cl- calibration graph. When the polyion complex was annealed in water at a temperature higher than the T,, its DSC peak became more intense. This means that annealing promotes crystalline phase formation in the polyion complex bilayer. The substantial formation of a crystalline phase was also sup-
,\ 1OOmV .%
543210 -lay 1x-1 Fid. 2. Calibration graphs for anion ISFETs based on polyion complex biiayers. (0) Cl- with 10 wt.% dioctadecyldimethylammonium chloride doping; (a) Clwithout doping; (A) CIO; with 10 wt.% dioctsdecyldimethylammonium perchlorate doping.
concentration changes is depicted in Fig. 2. The decrease in potential with increasing KC1 concentration in test solutions clearly indicates the anion response of the ISFET. The calibration graph exhibits linearity with a slope of about 50 mV per decade in the Cl- activity range 5 x 10-4-0.1 M. If the polyion complex employed consisted only of 1: 1 ion pairs of the ammonium and sulphonate units, the resulting ISFETs would never respond to anion activity changes in test solutions owing to the extremely low mobility of the polysulphonate anions. Elemental analysis data showed that several percent of the quatemary ammonium units in the polyion complex do not form ion pairs with the sulphonate units, being free from polyion TABLE 1 Selectivity coefficients for Cl- with respect to other anions dioctadecyldimethylammonium chloride doping Polyion complex
(k&)
in ISFETs
based
on polyion
complex
with
and without
log kE; ClO,-
NO;
Br-
ca,coo-
so,‘-
co,‘-
Undoped Unannealed Annealed
1.90 1.73
1.08 0.93
0.60 0.38
- 1.10 - 1.62
- 1.92 - 2.05
- 2.85 - 3.30
Doped (10 wt.%) Unannealed Annealed
1.91 1.71
1.07 0.93
0.56 0.46
- 1.14 - 1.58
-1.80 - 2.90
- 2.85 -3.22
K. WMURA
.o E
?il
/-
ET At,
r
afterannealing
AZ
i5 u
4
.I.
~ 1 25
0 ( 30
I 35
I 40
I 45
Ammonium
500
TemperatureI% Fig. 3. Differential scanning calorimetric profiles for polyion complex films before and after annealing+
ported by the fact that a semi-transparent film of the polyion complex turned turbid on annealing. Scanning electron micrographs display a clear difference in the surface between the untreated and annealed polyion complex films (Fig. 4). The untreated polyion complex film has a very smooth surface, whereas annealing caused wrinkling of the surface, probably owing to the effective packing of the bilayer. Conceivably, the annealing-induced packing of the polyion complex bilayer rejects
before annealing
5
10
15
20
salt doped I WI%
Fig. 5. Sensitivity enhancement of polyion complex-based ClISFETs by di~tadecyldimethylammonium chloride doping.
large anions, thus increasing the Cl- selectivity over other larger anions. Attempts were made to dope the polyion complex films with dioctadecyldimethylammonium chloride. The doped ammonium salt, if stable enough in the polyion complex bilayer not to leach out io aqueous test solutions, was expected to participate in anion exchange and, therefore, to increase the sensitivity of anion ISFETs. In fact, an increase in the doping amount of ammonium
after annealing
Fig. 4. Scanning electron micrographs for polyion comptex films on ISFETs before and after anneafing.
ANION-SENSITIVE
FIELD
EFFECT
TRANSISTORS
,
I
I
1
’
’
’
I
25
30
35
40
45
50
55
60
Temperature / % Fig. 6. Differential scanning calorimetric profiles for annealed polyion complex films with 5, 10 and 20 wt.% ammonium salt doping.
salt resulted in a considerable sensitivity enhancement of the ISFETs (Fig. 5). An anion ISFET based on a polyion complex with 10 wt.% ammonium doping exhibited a near-Nernstian response to Cl- activity changes. Also, the Clactivity range giving linearity of the calibration graph was extended down to 1 x 10m4 M by doping of the polyion complex with ammonium salt (Fig. 2). Doping with more than 10 wt.% ammonium salt did not improve further the sensitivity and linear activity range of the polyion complex-based anion ISFETs. The ammonium salt doping, however, brought about hardly any significant changes in the Cl- selectivity for the resulting ISFETs (Table 1). An annealing effect on the ion selectivity, i.e., some Clselectivity enhancement, was again found in the anion ISFETs based on the ammonium salt-doped polyion complex. The ammonium salt-doped polyion complexes, when annealed, show strong endothermic peaks on the DSC trace around the T, of the undoped polyion complex (Fig. 6), suggesting that annealing still leads to closely packed bilayer structures even on doping with less than 20 wt.% ammonium salt. The polyion complex was also doped with a corresponding quaternary ammonium perchlorate. The resulting ISFET based on the polyion complex doped with 10 wt.% ammonium perchlorate al-
45
lowed a near-Nemstian response to ClO; activity changes, as demonstrated in Fig. 2. The deviation from the near-Nernstian response at high Cl07 activities can possibly be attributed to some cation interference, because KClO, is easier to distribute to the membrane phase than KCl. Similarly, polyion complex films responsive to various desired anions may be prepared for ISFETs. In conclusion, dioctadecyldimethylammonium poly(stjrrene sulphonate) doped with its ctirresponding ammonium salts are promising for ionsensing layers of anion ISFETs. The polyion complex possesses good adhesive properties, especially without annealing. Moreover, as the polyion complex carries ordered bilayer structures, the polyion complex might therefore be a useful polymer support, in which active materials may be arranged in an orderly manner, for FET sensors. The authors gratefully acknowledge Dr. Shinichi Wakida, Government Industrial Research Institute, Osaka, and Dr. Naotoshi Nakashima, Department of Industrial Chemistry, Faculty of Engineering, Nagasaki University, for helpful discussions.
REFERENCES 1 P. Bergveld, IEEE Trans. Biomed. Eng., BME-19 (1972)
342. 2 T. Matsuo and K.D. Wise, IEEE Trans. Biomed. Eng., BME-21 (1974) 485. 3 J. Janata and R.J. Huber, Ion-Se!. Electrode Rev., 1 (1979) 31. 4 S.D. Moss, J. Janata and C.C. Johnson, Anal. Chem., 47 (1975) 2238. 5 R.K. Rhodes, IEEE Trans. Biomed. Eng., BME-33 (1986) 91. 6 T. Satchwill and D.J. Harrison, J. Electroanal. Chem., 202 (1986) 75. 7 D.J. Harrison, L.L. Cunningham, X. Li, A. Teclemaiiam and D. Permann. J. Electrochem. Sot.. 135 (1988) 2473. 8 G.J. Moody, J.D.R. Thomas and J.M. Slater, Analyst, 113 (1988) 1703. 9 H. van der Vlekkert, C. Francis, A. Grisel and N. Rooij, Analyst, 113 (1988) 1029. 10 S. Kawakami, T. Akiyama and Y. Ujihira, Fresenius’ Z. Anal. Chem., 318 (1984) 349. 11 S. Wakida, M. Yamane, K. Hiiro, T. Kihara. Y. Ujihira and T. Sugano, Anal. Sci., 4 (1988) 501.
46
12 E.J. Fogt, D.F. Untereker, MS. Norenberg and M.E. MeyerhofF, Anal. Chem., 57 (1985) 1998. 13 T.J. Cardwell, R.W. Cattrall, P.J. Iles and I.C. Hamilton, Anal. Chim. Acta, 219 (1989) 135. 14 M. Battilotti, R. Cohlli, I. Giannini and M. Giongo, Sensors Actuators, 17 (1989) 209. l!j T. Kunitake, A. Tsuge and N. Nakashima, Chem. Lett., (1984) 1783.
K. KIMURA
ET AL.
16 Y. Okahata and H. Ebato, Anal. Chem.. 61 (1989) 2185. 17 T. Ogata and H. Yanagi, High Polym. Jpn., 38 (1989) 212. 18 N. Nakashima, H. Eda, M. Kunitake, 0. Manabe and K. Nakano, J. Chem. Sot., Chem. Commun., (1990) 443. 19 F. Hofmeister, Arch, Exp. Pathol. Pharmakol., 24 (1888) 247.