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Dynamic response of a Ta2O5-gate pH-sensitive field-effect transistor
ELSEVIER
Sensors and Actuators B 32 (1996) 115-119
ClW~B~AL
Dynamic response of a Ta2Os-gate pH-sensitive field-effect transistor Hirokazu Hara, Tatsuya Ohta Department of Chemistry, Facultyof Education. Shiga University, Otsu, Shiga 520, Japan Received 21 April 1995; revised 13 November 1995; accepted 17 November 1995
Abstract The dynamic response of a Ta2Os-gate pH-sensitive field-effect transistor (pH-ISFET) has been investigated using an activity step method. The measurement set-up can produce the activity step of pH within time-frames of several tens of milliseconds. The response time, toss, is found to be 0.25-0.30 s, which is almost independent of various factors including the flow rate and the direction of the pH change. The measured response times are slower than those previously reported. The applicability of the existing theories to interpret the dynamic response curves is discussed. Keywords: Field-effect transistors ; pH sensors ; Tantalum oxide
1. Introduction The advantage of a pH-sensitive ISFET over conventional glass electrodes is its very fast response [ 1-3]. In particular, Ta2Os is recognized as a very good and stable insulator for fast pH sensing. Klein reported that the response time, ts~_ 95~, of Ta2Os-gate pH-ISFETs was about 5 ms even when the pH value was changed over 12 decades from the alkaline to the acidic region, which was measured with a specially designed experimental set-up [ 3 ]. However, a detailed examination of the dynamic response characteristics of Ta2Os-gate pH-ISFETs has scarcely been published. We have developed a potentiometric analysis system for realizing a high-speed analysis [4]. This system utilized the fast response of a solid-state chloride ion-selective electrode when an instantaneous activity step was applied on the electrode surface. The analysis time for one sample was reduced to 1.2--1.3 s. The object of this paper is to examine the basic dynamic response characteristics of Ta2Os-gate pH-ISFETs in order to determine the applicability of the sensor to a high-speed analysis system.
trial Standard [5]. Their compositions are 0.05 M KH 3 (C204)2.2H20, 0.05 M C6H4 (COOH) (COOK), 0.025 M KI-I2PO4+0.025 M Na2HPO4, 0.01 M Na2B407 and 0.025 M NaHCO3 + 0.025 M Na2COs, respectively. The solutions of 0.2 M HCI + 0.2 M KCI, 0.01 M NaOH and 0.1 M NaOH were also used as sample solutions of pH 1, 12 and 13, respectively. The accurate pH of these solutions was determined using a pH glass electrode beforehand.
3. Measurement set-up Fig. 1 depicts the measurement set-up. Buffer solutions were delivered using a roller pump (Cole-Partner, Chicago, IxY-plot
ter]
I a.lcompu t e r i-- I
-lion
] meter ]
2. E x p e r i m e n t a l
2.1. Reagents The pH standard buffers (pH ffi 1.68, 4.01, 6.86, 9.18 and 10.02 at 25 °C) were prepared according to the Japan Indus0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved Pll S 0 9 2 5 - 4 0 0 5 ( 9 6 ) 0 1 8 7 7 - 1
Pump ( s i d e view) Buffer Fig. i. Measurement set-up for recording the potential-time responsecurves: SV, solenoid valves; PIO, programmable I / 0 interface; IMB, interface and memory board.
116
H. Ham, 7".Ohm/Sensors and Actuamrs B 32 (1996) !15-119
L ; Masterflex 7553,20) that was equipped with two pump heads and C-flex tubes ( 1.9 mm X 4.9 ram). Solenoid valves (MCV.3R-M6F~ Takasago Electronic, Nagoya) were used to exchange the two buffer solutions. The switchover of the valves was controlled using a microcomputer (NEC PC9801 DX; the CPU was changed with aCylix Cx486SLC; 25 MHz) ~ a n 110 in~rface board (I 0 Data Kiki, Kanazawa; P I O - ~ 2 ) . The Ta2Os-gate ISFETs (51-1314) and the ~meter (50-1400) were purchased from BAS (Tokyo). The ~ n - . s o u r c e voltage and current were adjusted to 5 V and 160 ttA, respectively. The getc--source voltages were measured versus a double-junction.type Ag/AgCi electrode (Orion ~ ) . The recorder output of the ISFET meter was introduced every 10 ms into a microcomputer via an ionmeter (Orion 901) and a digitizer (APC 204, Autonics, Tokyo). The ion-meter was used effectively to reduce the electrical noise. The response-time carve was displayed on a CRT and plotted using an X-F plotter. 4. Meesurement procedure The measurement was performed in 3 s. The exchange of sample solutions that was caused by the switchover of the solenoid valves was programmed to occur at I s. Thus, one data set was composed of 301 pairs of time and output voltage values every 10 ms, including the values at time 0 s. One set of the experimental results at a particular flow rate equated 28 data sets, which could be divided into two groups of 14 data sets. All sets were recorded twice. One half was taken from pH 6.86 to the respective pH (i.e., from pH 6.86 to pH !.00, 1.68, 4.01, 9.18, 10.02, 11.94 or 12.74) and the other half was taken in the other direction. 5, Data mulysls
tOO
20(
-20C
pH Fig. 2. The pH response of a Ta2Os.gate ISFET. Open circles show the equilibrium potential for a batehwise measurement, Closed circles show the initial potentials of the flow measurement. The flow rete was 1.52 m s - i.
(initial output voltages at the respective pH are shown). Using the batch method, the linearity of the sensor was excellent between pH 1 and 13; however, there was no longer linearity at pH 13 using the flow method. The response at pH 13 was very slow (about 5 min) in the batch measurement. The initial output voltage at pH 13 in the flow method may not be at equilibrium, although a stable voltage was apparently observed within ~everal seconds after the flow of a sample covered the sensor surface. 6.2.
Dynamic response
Fig. 3 shows the dynamic response curves. The response time, tgs~, was 0.25-0.31 s in all cases. Little dependence of the response time on the final (left) or the initial (right) pH values was observed.
The time, to~t,when the change of the output voltage first began, was defined by the following inequalities;
6.3. Effect offlow rate
E ( i + 1) -E(I) > 1.5 mV
Table I shows the effect of the flow rates on the response time, t95~. Little dependence on the flow rates or the direction of the pH change was also observed, at least in the range of flow rates between 0.75 and 1.78 m s - i.
and E ( l + 2 ) -E(I) > 3 mV
(1)
where E(i), E(I+ 1) and E ( l + 2 ) were the lth, ( ! + 1)th and ( ! + 2)th output voltage values, respectively. The initial and final output voltages were defined as the mean value of the output voltage values between 0 and 1 s and between 2 and 3 s. The response time, t95st, was defined as the period be,.~,eea te~ and the time when the output voltage first exceeded the 95% response of the difference between the initial and final output voltages.
6, Results
6,1. Responseto ptl Fig. 2 shows the pH responses of the TaaOs-gate ISFET measured using the conventional batch and flow methods
6.4. Effect of otherfactors The solenoid valves used in this study have a different response time for opening ( 15-20 ms) and closing (5-10 ms). The response-time data shown thus far were taken when the solution was exchanged when the valves were closing. Little effect on the response time was observed when the solution was exchanged when the valves were opening. The gate surface was placed parallel to the direction of the flow of a sample solution. Little effect on the response time was also observed when the gate surface was placed perpendicular to the flow. (The perpendicular position was undesirable because the gate surface of the ISFET is so sensitive that
H. Hara. 7". Ohta /Sensors and Actuators B 32 (1996) 115-119
12.74
3oc
S~Xu'
1o[ "~ .?
i
0
12.74
11.94
tl.94
2oll
l 17
10.02
20(
tO.Qe
9.1B
tOO
9.1D
6.06
6.06
~ -too:
~ -I00
&
4.01
4.0t
-~o
-200 1.§S
!.58 -300
I.OO -4ol 0.OO
I
i
t.OO
2.OO
1.00 i 3.OO
Time/s
-40C ).00
I 2.00
I 1.oo
I 3.00
Timo/s
Fig. 3. Time-responsecurves of a TaaOs-gate. ISFET. Left: the pH of the two b;;fferzolutions was changed from pH 6.86 to pH valuesshown on each graph. Right: vice versa. The flow rate was i. 52 m s- '. Table I Effect of flow rate on the response time. tgs~ of a Ta2Osgate ISFET F]ow rate
Response time, tgs.~(s)
(ms -I)
pHtniaat> PHa.-, (mean, SD)
0.75 1.14 1.52 !.78
0.28--0.02 (a= 0.28+0.02 (n= 0.27 5=0,01 (n= 0.265=0.02 (u=
pHt,aeal< pHi..1
(mean. SD) 14) 14) 14) 14)
0.305=0.02 (u= 0.295=0.01 (n= 0.285=0.01 (n= 0.295=0.04 (n=
14) 14) 14) 14)
(2) The response time was scarcely dependent on various factors including the flow rate or the direction of the pH change. (3) The response time curves from pH A to B and from pH B to A were almost symmetrical if the sign of the change of the output voltage was ignored. W e shall discuss the slow response time and the theoretical interpretation of the transient response curves.
7.3. Slow response time
it may be easily damaged by small particles in a sample solution under the high flow rate.)
7. Discussion and conclusions Z 1. Response to p H
Bousse et al. found that in alkaline solutions a Ta2Os-gate pH-ISFET has a slightly lower pH response [6]. This was interpreted in terms of a surface-site model, the ion-exchange reaction between the hydrogen and sodium ions at the surface. In our result, the sensor response was l i n e ~ up to pH 13 in the batch measurement, although the response at pH 13 was rather slow. The origin of the slow response at pH 13 may be ascribed to the interference from the sodium ion [7]. Z 2. Dynamic response characteristics
The main features of the dynamic response of the Ta205gate ISFET were summarized as follows: (1) The response time, tgs~, was approximately 0.25-0.30 s, which was slower than the response time data reported so far [2,3].
Bousse and Bergveld discussed the response time of inorganic-gate ISFETs and stated that it was probably determined by the speed of the pH variation itself and not by the ISFET [2]. In their opinion, the intrinsic response times should be of the order of one millisecond or faster. The response times observed in this study were, in contrast to their opinion, at least two orders of magnitude slower. If the response times that were observed in this study reflect the speed of the pH variation itself, they should be more significantly affected by the flow rate. (The buffer capacities of the pH standard solutions were in the range 0.01 to 0.3, which was so high that the pH variation on the ISFET surface may occur through the exchange process between the initial and final pH buffer solutions rather than through their reaction.) It is worth mentioning that the response-time data may be affected not only by the response character of the sensor itself but also by the whole measurement set-up. The relatively slow responses might be attributed in part to the measurement set-up. However, our results can be regarded to reflect the real response characteristics of the sensor because the same
118
H. Ham, 7", Ohto /Sensors and Actuators B 32 (1996) 115-119
symmetrical character of the transient response curves as was reported by Klein [3] was also observed in this study.
250
7.4. Interpretation of the transient response curves
21111
pH
The dynamic response of the ion-selective electrodes(ISEs) has been extensively studied [8]. Several models have been devised in order to explain the transient response curve of ISEs [8,9]. The question is, which model is most appropriate to explain the dynamic response characteristics of the Ta2Os-gate ISFETs previously described? Klein [3] stated that the response-time curve could be explained with the following equation:
,®i
E(t)=£n.,j+$1og[l-(l-aJaf)exp(-t/r)]
2~ . . . . . . . . . . . . . . . . . . l i p H g .
(2)
where ai and ar are the activities of the solutions before and after the exchange and ~'is a time constant. This equation was based on the first-order chemical kinetics and the consecutive reaction model devised by Lindner et al. [ 10]. The equation was similar to that derived from the model based on diffusion through a stagnant layer. The main feature derived from Eq. (2) was the clear dependence of the response time on the direction of the activity change for sensed ions. Namely, the response time is shorter for activity increase than for activity decrease. This dependence comes from the factor ( 1 - ajar) in Eq. (2). The activity dependence clearly conflicts with the symmetrical character of the observed response-time curves. Fig. 4 shows the theoretical curves obtained using Eq. (2). The parameter ~-was determined so as to minimize the root mean square of the difference between the measured aud calculated voltage values during 1 s from to~. Apparently, the theoretical curve fitted the measured response curve only in the case of activity decrease. From this result, it can be concluded that Eq. (2) (and the equation based on the diffusion model) inadequately explains the response-time curve of the TaaOs-gate ISFET owing to the activity dependence. The model based on the energy-barrier concept was first derived by Rechnitz and Hameka [7,8,11 ]. Their model predicts an exponential-type response-time curve. However, it is independent of the activity level of the sample solutions as shown by the following equation: E(t) = Ei,~¢ + ( E r ~ - Ei~,~.~) [ 1 - exp( - kt) ]
(3)
The time constant, k, depends on the final concentration of the activity step, temperature, etc. According to the model, the transient response curve depends on the stepped activity value, namely the final pH, and not on the initial pH. Unfortunately, this conclusion seems not to be in accordance with the independence of the response time on the final pH as well as the initial pH. Thus, a new model may be necessary to explain the dynamic response characteristics of the sensor that have been summarized thus far.
5.86 -> 9.18
150
0.~
I 2,O0
1.00
I 3.00
T|II~/S
18 -> 6.86
20O
150
100 0.00
I 1.00
I 2.00
I 3.00
Tzas/s
Fig. 4. Comparison between the measured (dotted) and the corresponding theoretical curves for increasing (upper) and decreasing (lower) pH values.
Acknowledgements This work was supported by a Grant-in-Aid for scientific research from the Ministry of Education, Science, Sports and Culture of Japan (No. 05640681 ).
References [ 1] B,H. van der Scheot, P. Bergveld, M, Bos and LJ. Bousse, The ISFET in analytical chemistry, Sensors and Actuators. 4 (1983) 267-272. 12] L. Bousse and P. Bergveld, The role of buried OH sites in the response mechanism of inorganic-gate pH-seasidve ISFETs. Sensors and Actuators. 6 (1984) 65-78. [ 3l M. Klein, Time effects ofion-sansitive field-effect transistors. Sensors and Actuators. i7 ( 1989) 203-208. 14] H, Ham. N, Ishio and K. Takahashi, High-speed potentiometric analyzer equipped with a, ion-selective electrode detector, Anal. Chim. Acta. 281 (1993) 45-51. [5] Japan IndasUialStandard 7,8802 (1984). [6] L. Boasse, S. Mostershed. B. van der Schoot and N.F. de Rooij, Comparison of the hysteresis of Ta2Os and SisN4 pH-seasing insulators, Sensors and Actuators B, 17 (1994) 157-164. 17] P.V. Bobrov, Ye,A. Tarantov, S. Kraese and W. Mofitz, Chemical sensitivity of an ISFET with Ta2Os membrane in strong acid and alkaline solutions, Sensors and Actnators B, 3 ( 1991 ) 75-81. 18] E. IAndner. K. T6th and E. Pungor. Dynamic Characteristics of IonSelective Electrodes. CRC Press, Boca Raton, FL. 1988. [9] A. Shatkay, Transient potentials in ion-specific electrodes. Anal. Chem., 48 (1976) 1039--1050.
H. Hara, 7". Ohm~Sensors and Actuators B 32 (1996) 115-119
[ !0] E. Lindner, K. T6lh and E. Pungor. Dynamic Characteristics of ionSelective Electrodes, CRC Press, Boca Raton. FL. 1988. pp. 62-64. [ 1! l G.A. Rechnitz and H.F. Hameka. A theory of glass electrode response. Fresenius" Z Anal. Chem., 214 (1965) 252-257.
Biographies Hirokazu H a r a was born in 1952. He received the degree o f Doctor of Science from Kyoto University in 1981. He
119
joined Shiga University as an assistant professor and is now a professor in the Faculty o f Education. His research interests are in the preparation of electrochemical sensors and their application to environmental analysis. Tatsuya Ohta was an undergraduate student o f Shiga University and graduated from the junior high school teachers' training course in 1995. He was a m e m b e r o f the research group o f Professor H. Hara and studied mainly the dynamic response of ISFET sensors.
Sensors and Actuators B 32 (1996) 115-119
ClW~B~AL
Dynamic response of a Ta2Os-gate pH-sensitive field-effect transistor Hirokazu Hara, Tatsuya Ohta Department of Chemistry, Facultyof Education. Shiga University, Otsu, Shiga 520, Japan Received 21 April 1995; revised 13 November 1995; accepted 17 November 1995
Abstract The dynamic response of a Ta2Os-gate pH-sensitive field-effect transistor (pH-ISFET) has been investigated using an activity step method. The measurement set-up can produce the activity step of pH within time-frames of several tens of milliseconds. The response time, toss, is found to be 0.25-0.30 s, which is almost independent of various factors including the flow rate and the direction of the pH change. The measured response times are slower than those previously reported. The applicability of the existing theories to interpret the dynamic response curves is discussed. Keywords: Field-effect transistors ; pH sensors ; Tantalum oxide
1. Introduction The advantage of a pH-sensitive ISFET over conventional glass electrodes is its very fast response [ 1-3]. In particular, Ta2Os is recognized as a very good and stable insulator for fast pH sensing. Klein reported that the response time, ts~_ 95~, of Ta2Os-gate pH-ISFETs was about 5 ms even when the pH value was changed over 12 decades from the alkaline to the acidic region, which was measured with a specially designed experimental set-up [ 3 ]. However, a detailed examination of the dynamic response characteristics of Ta2Os-gate pH-ISFETs has scarcely been published. We have developed a potentiometric analysis system for realizing a high-speed analysis [4]. This system utilized the fast response of a solid-state chloride ion-selective electrode when an instantaneous activity step was applied on the electrode surface. The analysis time for one sample was reduced to 1.2--1.3 s. The object of this paper is to examine the basic dynamic response characteristics of Ta2Os-gate pH-ISFETs in order to determine the applicability of the sensor to a high-speed analysis system.
trial Standard [5]. Their compositions are 0.05 M KH 3 (C204)2.2H20, 0.05 M C6H4 (COOH) (COOK), 0.025 M KI-I2PO4+0.025 M Na2HPO4, 0.01 M Na2B407 and 0.025 M NaHCO3 + 0.025 M Na2COs, respectively. The solutions of 0.2 M HCI + 0.2 M KCI, 0.01 M NaOH and 0.1 M NaOH were also used as sample solutions of pH 1, 12 and 13, respectively. The accurate pH of these solutions was determined using a pH glass electrode beforehand.
3. Measurement set-up Fig. 1 depicts the measurement set-up. Buffer solutions were delivered using a roller pump (Cole-Partner, Chicago, IxY-plot
ter]
I a.lcompu t e r i-- I
-lion
] meter ]
2. E x p e r i m e n t a l
2.1. Reagents The pH standard buffers (pH ffi 1.68, 4.01, 6.86, 9.18 and 10.02 at 25 °C) were prepared according to the Japan Indus0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved Pll S 0 9 2 5 - 4 0 0 5 ( 9 6 ) 0 1 8 7 7 - 1
Pump ( s i d e view) Buffer Fig. i. Measurement set-up for recording the potential-time responsecurves: SV, solenoid valves; PIO, programmable I / 0 interface; IMB, interface and memory board.
116
H. Ham, 7".Ohm/Sensors and Actuamrs B 32 (1996) !15-119
L ; Masterflex 7553,20) that was equipped with two pump heads and C-flex tubes ( 1.9 mm X 4.9 ram). Solenoid valves (MCV.3R-M6F~ Takasago Electronic, Nagoya) were used to exchange the two buffer solutions. The switchover of the valves was controlled using a microcomputer (NEC PC9801 DX; the CPU was changed with aCylix Cx486SLC; 25 MHz) ~ a n 110 in~rface board (I 0 Data Kiki, Kanazawa; P I O - ~ 2 ) . The Ta2Os-gate ISFETs (51-1314) and the ~meter (50-1400) were purchased from BAS (Tokyo). The ~ n - . s o u r c e voltage and current were adjusted to 5 V and 160 ttA, respectively. The getc--source voltages were measured versus a double-junction.type Ag/AgCi electrode (Orion ~ ) . The recorder output of the ISFET meter was introduced every 10 ms into a microcomputer via an ionmeter (Orion 901) and a digitizer (APC 204, Autonics, Tokyo). The ion-meter was used effectively to reduce the electrical noise. The response-time carve was displayed on a CRT and plotted using an X-F plotter. 4. Meesurement procedure The measurement was performed in 3 s. The exchange of sample solutions that was caused by the switchover of the solenoid valves was programmed to occur at I s. Thus, one data set was composed of 301 pairs of time and output voltage values every 10 ms, including the values at time 0 s. One set of the experimental results at a particular flow rate equated 28 data sets, which could be divided into two groups of 14 data sets. All sets were recorded twice. One half was taken from pH 6.86 to the respective pH (i.e., from pH 6.86 to pH !.00, 1.68, 4.01, 9.18, 10.02, 11.94 or 12.74) and the other half was taken in the other direction. 5, Data mulysls
tOO
20(
-20C
pH Fig. 2. The pH response of a Ta2Os.gate ISFET. Open circles show the equilibrium potential for a batehwise measurement, Closed circles show the initial potentials of the flow measurement. The flow rete was 1.52 m s - i.
(initial output voltages at the respective pH are shown). Using the batch method, the linearity of the sensor was excellent between pH 1 and 13; however, there was no longer linearity at pH 13 using the flow method. The response at pH 13 was very slow (about 5 min) in the batch measurement. The initial output voltage at pH 13 in the flow method may not be at equilibrium, although a stable voltage was apparently observed within ~everal seconds after the flow of a sample covered the sensor surface. 6.2.
Dynamic response
Fig. 3 shows the dynamic response curves. The response time, tgs~, was 0.25-0.31 s in all cases. Little dependence of the response time on the final (left) or the initial (right) pH values was observed.
The time, to~t,when the change of the output voltage first began, was defined by the following inequalities;
6.3. Effect offlow rate
E ( i + 1) -E(I) > 1.5 mV
Table I shows the effect of the flow rates on the response time, t95~. Little dependence on the flow rates or the direction of the pH change was also observed, at least in the range of flow rates between 0.75 and 1.78 m s - i.
and E ( l + 2 ) -E(I) > 3 mV
(1)
where E(i), E(I+ 1) and E ( l + 2 ) were the lth, ( ! + 1)th and ( ! + 2)th output voltage values, respectively. The initial and final output voltages were defined as the mean value of the output voltage values between 0 and 1 s and between 2 and 3 s. The response time, t95st, was defined as the period be,.~,eea te~ and the time when the output voltage first exceeded the 95% response of the difference between the initial and final output voltages.
6, Results
6,1. Responseto ptl Fig. 2 shows the pH responses of the TaaOs-gate ISFET measured using the conventional batch and flow methods
6.4. Effect of otherfactors The solenoid valves used in this study have a different response time for opening ( 15-20 ms) and closing (5-10 ms). The response-time data shown thus far were taken when the solution was exchanged when the valves were closing. Little effect on the response time was observed when the solution was exchanged when the valves were opening. The gate surface was placed parallel to the direction of the flow of a sample solution. Little effect on the response time was also observed when the gate surface was placed perpendicular to the flow. (The perpendicular position was undesirable because the gate surface of the ISFET is so sensitive that
H. Hara. 7". Ohta /Sensors and Actuators B 32 (1996) 115-119
12.74
3oc
S~Xu'
1o[ "~ .?
i
0
12.74
11.94
tl.94
2oll
l 17
10.02
20(
tO.Qe
9.1B
tOO
9.1D
6.06
6.06
~ -too:
~ -I00
&
4.01
4.0t
-~o
-200 1.§S
!.58 -300
I.OO -4ol 0.OO
I
i
t.OO
2.OO
1.00 i 3.OO
Time/s
-40C ).00
I 2.00
I 1.oo
I 3.00
Timo/s
Fig. 3. Time-responsecurves of a TaaOs-gate. ISFET. Left: the pH of the two b;;fferzolutions was changed from pH 6.86 to pH valuesshown on each graph. Right: vice versa. The flow rate was i. 52 m s- '. Table I Effect of flow rate on the response time. tgs~ of a Ta2Osgate ISFET F]ow rate
Response time, tgs.~(s)
(ms -I)
pHtniaat> PHa.-, (mean, SD)
0.75 1.14 1.52 !.78
0.28--0.02 (a= 0.28+0.02 (n= 0.27 5=0,01 (n= 0.265=0.02 (u=
pHt,aeal< pHi..1
(mean. SD) 14) 14) 14) 14)
0.305=0.02 (u= 0.295=0.01 (n= 0.285=0.01 (n= 0.295=0.04 (n=
14) 14) 14) 14)
(2) The response time was scarcely dependent on various factors including the flow rate or the direction of the pH change. (3) The response time curves from pH A to B and from pH B to A were almost symmetrical if the sign of the change of the output voltage was ignored. W e shall discuss the slow response time and the theoretical interpretation of the transient response curves.
7.3. Slow response time
it may be easily damaged by small particles in a sample solution under the high flow rate.)
7. Discussion and conclusions Z 1. Response to p H
Bousse et al. found that in alkaline solutions a Ta2Os-gate pH-ISFET has a slightly lower pH response [6]. This was interpreted in terms of a surface-site model, the ion-exchange reaction between the hydrogen and sodium ions at the surface. In our result, the sensor response was l i n e ~ up to pH 13 in the batch measurement, although the response at pH 13 was rather slow. The origin of the slow response at pH 13 may be ascribed to the interference from the sodium ion [7]. Z 2. Dynamic response characteristics
The main features of the dynamic response of the Ta205gate ISFET were summarized as follows: (1) The response time, tgs~, was approximately 0.25-0.30 s, which was slower than the response time data reported so far [2,3].
Bousse and Bergveld discussed the response time of inorganic-gate ISFETs and stated that it was probably determined by the speed of the pH variation itself and not by the ISFET [2]. In their opinion, the intrinsic response times should be of the order of one millisecond or faster. The response times observed in this study were, in contrast to their opinion, at least two orders of magnitude slower. If the response times that were observed in this study reflect the speed of the pH variation itself, they should be more significantly affected by the flow rate. (The buffer capacities of the pH standard solutions were in the range 0.01 to 0.3, which was so high that the pH variation on the ISFET surface may occur through the exchange process between the initial and final pH buffer solutions rather than through their reaction.) It is worth mentioning that the response-time data may be affected not only by the response character of the sensor itself but also by the whole measurement set-up. The relatively slow responses might be attributed in part to the measurement set-up. However, our results can be regarded to reflect the real response characteristics of the sensor because the same
118
H. Ham, 7", Ohto /Sensors and Actuators B 32 (1996) 115-119
symmetrical character of the transient response curves as was reported by Klein [3] was also observed in this study.
250
7.4. Interpretation of the transient response curves
21111
pH
The dynamic response of the ion-selective electrodes(ISEs) has been extensively studied [8]. Several models have been devised in order to explain the transient response curve of ISEs [8,9]. The question is, which model is most appropriate to explain the dynamic response characteristics of the Ta2Os-gate ISFETs previously described? Klein [3] stated that the response-time curve could be explained with the following equation:
,®i
E(t)=£n.,j+$1og[l-(l-aJaf)exp(-t/r)]
2~ . . . . . . . . . . . . . . . . . . l i p H g .
(2)
where ai and ar are the activities of the solutions before and after the exchange and ~'is a time constant. This equation was based on the first-order chemical kinetics and the consecutive reaction model devised by Lindner et al. [ 10]. The equation was similar to that derived from the model based on diffusion through a stagnant layer. The main feature derived from Eq. (2) was the clear dependence of the response time on the direction of the activity change for sensed ions. Namely, the response time is shorter for activity increase than for activity decrease. This dependence comes from the factor ( 1 - ajar) in Eq. (2). The activity dependence clearly conflicts with the symmetrical character of the observed response-time curves. Fig. 4 shows the theoretical curves obtained using Eq. (2). The parameter ~-was determined so as to minimize the root mean square of the difference between the measured aud calculated voltage values during 1 s from to~. Apparently, the theoretical curve fitted the measured response curve only in the case of activity decrease. From this result, it can be concluded that Eq. (2) (and the equation based on the diffusion model) inadequately explains the response-time curve of the TaaOs-gate ISFET owing to the activity dependence. The model based on the energy-barrier concept was first derived by Rechnitz and Hameka [7,8,11 ]. Their model predicts an exponential-type response-time curve. However, it is independent of the activity level of the sample solutions as shown by the following equation: E(t) = Ei,~¢ + ( E r ~ - Ei~,~.~) [ 1 - exp( - kt) ]
(3)
The time constant, k, depends on the final concentration of the activity step, temperature, etc. According to the model, the transient response curve depends on the stepped activity value, namely the final pH, and not on the initial pH. Unfortunately, this conclusion seems not to be in accordance with the independence of the response time on the final pH as well as the initial pH. Thus, a new model may be necessary to explain the dynamic response characteristics of the sensor that have been summarized thus far.
5.86 -> 9.18
150
0.~
I 2,O0
1.00
I 3.00
T|II~/S
18 -> 6.86
20O
150
100 0.00
I 1.00
I 2.00
I 3.00
Tzas/s
Fig. 4. Comparison between the measured (dotted) and the corresponding theoretical curves for increasing (upper) and decreasing (lower) pH values.
Acknowledgements This work was supported by a Grant-in-Aid for scientific research from the Ministry of Education, Science, Sports and Culture of Japan (No. 05640681 ).
References [ 1] B,H. van der Scheot, P. Bergveld, M, Bos and LJ. Bousse, The ISFET in analytical chemistry, Sensors and Actuators. 4 (1983) 267-272. 12] L. Bousse and P. Bergveld, The role of buried OH sites in the response mechanism of inorganic-gate pH-seasidve ISFETs. Sensors and Actuators. 6 (1984) 65-78. [ 3l M. Klein, Time effects ofion-sansitive field-effect transistors. Sensors and Actuators. i7 ( 1989) 203-208. 14] H, Ham. N, Ishio and K. Takahashi, High-speed potentiometric analyzer equipped with a, ion-selective electrode detector, Anal. Chim. Acta. 281 (1993) 45-51. [5] Japan IndasUialStandard 7,8802 (1984). [6] L. Boasse, S. Mostershed. B. van der Schoot and N.F. de Rooij, Comparison of the hysteresis of Ta2Os and SisN4 pH-seasing insulators, Sensors and Actuators B, 17 (1994) 157-164. 17] P.V. Bobrov, Ye,A. Tarantov, S. Kraese and W. Mofitz, Chemical sensitivity of an ISFET with Ta2Os membrane in strong acid and alkaline solutions, Sensors and Actnators B, 3 ( 1991 ) 75-81. 18] E. IAndner. K. T6th and E. Pungor. Dynamic Characteristics of IonSelective Electrodes. CRC Press, Boca Raton, FL. 1988. [9] A. Shatkay, Transient potentials in ion-specific electrodes. Anal. Chem., 48 (1976) 1039--1050.
H. Hara, 7". Ohm~Sensors and Actuators B 32 (1996) 115-119
[ !0] E. Lindner, K. T6lh and E. Pungor. Dynamic Characteristics of ionSelective Electrodes, CRC Press, Boca Raton. FL. 1988. pp. 62-64. [ 1! l G.A. Rechnitz and H.F. Hameka. A theory of glass electrode response. Fresenius" Z Anal. Chem., 214 (1965) 252-257.
Biographies Hirokazu H a r a was born in 1952. He received the degree o f Doctor of Science from Kyoto University in 1981. He
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joined Shiga University as an assistant professor and is now a professor in the Faculty o f Education. His research interests are in the preparation of electrochemical sensors and their application to environmental analysis. Tatsuya Ohta was an undergraduate student o f Shiga University and graduated from the junior high school teachers' training course in 1995. He was a m e m b e r o f the research group o f Professor H. Hara and studied mainly the dynamic response of ISFET sensors.