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Flow-through pH-ISFET detector for flow-injection analysis
Analytica Chimica Acta, 222 (l989) 373-317 Elsevier Science Publishers B.V., Amsterdam -
373 Printed in The Netherlands
Short Communication
FLOW-THROUGH pH-ISFET INJECTION ANALYSIS
DETECTOR FOR FLOW-
S. ALEGRET, J. ALONSO, J. BARTROLI and J. DOMENECH Departament de Quimica, Universitat Autonoma de Barcelona, 08193 Bellaterra (Spain) N. JAFFREZIC-RENAULT*
and Y. DUVAULT-HERRERA
Luboratoire de Physicochimie des Interfaces, Ecole Centrale de Lyon, UA CNRS 404, B.P. 163, 69131 Ecully Ce’den (France) (Received 4th January 1989)
Summary. A simply constructed flow-through pH-ISFET detector suitable for flow-injection analysis is described. Its response characteristics are presented and discussed. The calibration plot is linear in the pH range 2-12; the slope is 53 mV per decade.
Much attention has been devoted to the development of ion-sensitive fieldeffect transistors (ISFETs) owing to the advantage which they offer in comparison with ion-selective electrodes (ISEs) [l-3]. ISFETs are small, have a low impedance output signal and fast response and no need for conditioning. Further, these chemically sensitive electronic devices can be made using the same technology as is available for the production of integrated circuits with all the possible advantages, such as low cost, high reproducibility and largescale production. All these advantages make them suitable for exploring new applications such as in continuous monitoring systems. A shortcoming of these sensors is their high drift rate. However, this can be circumvented by using them in flow-injection systems. These systems, which are very suitable for continuous automatic monitoring [4,5], allow discrete sequential analysis in near real time. The surface sensor is continuously cleaned by the carrier solution, thus re-establishing the base line after each sample injection, the drift being neglected in the computation of the analytical signal (peak height) [6]. There are serious difficulties in adapting ISFET sensors in flow cells because encapsulating the sensor hinders the formation of a good stagnant layer [ 7,8].
0003-2670/89/$03.50
0 1989 Elsevier Science Publishers B.V.
374
However, the recent availability of ISFETs with rear-side contacts [9] is promising in this respect because this configuration could in principle facilitate the construction of compact flow-through ISFET sensor systems. This reports the implementation of a pH-ISFET with rear-side contacts in a flow-injection system by means of a specially designed flow cell; the results obtained for the sensor response under dynamic flow conditions are presented. Experimental Instrumental aspects. The Al,O, pH-ISFET with rear-side contacts was supplied by the University of Neuchhtel, Switzerland. A description of the manufacturing process has been published elsewhere [ 91. This A1203 ISFET shows a pH sensitivity of about 55 mV pH_’ and static conditions [9] and a drift rate of less than 0.2 mV h-l [lo]. Each chip (1.5~ 1.5 mm) includes one ISFET with rear-side holes for the electrical connection. The size of these holes is 80x80 pm at the bottom and 480 x 480 pm at the opening. The leads were glued into the holes with conductive epoxy (Epo-Tek 417; Epotecny, Velizy, France). The chip encapsulated in epoxy resin was mounted in a plastic holder (disposable lOOO-~1 Eppendorf pipette tip) and fitted into a specially designed flow-through cap assembly whose void volume is very small (see Fig. 1). The flow cell was incorporated in the conduits of a flow-injection system as shown schematically in Fig. 2. The injection valve (Reodyme 5020) was provided with exchangeable sample loops. The carrier and sample streams were 8
8
I t
5
T
1
6
Fig. 1. Designof the flow-throughpH-ISFET detector. (A) pH-ISFET (1) with rear-side contact wires (2 ) encapsulated in epoxy resin (3) and fitted into a plastic pipette tip (5 mm o.d. ) (4). (B) Block with the inserted pH-ISFET active surface. (C) Lid with inlet/outlet flow channels (5,6). An O-ring (7) ensures tight contact and defines a very small detection chamber. Pieces B and C are squeezed together by screws through the appropriate holes (8).
375
W
Fig. 2. Schematic diagram of the flow-injection system. A, Carrier solution; B, sample solution; C, pump; D, pulse suppressor; E, injection valve; F, coil; G, grounding electrode; H, flow-throughpHISFET detector; I, reference electrode inserted in a flow-through cap; J, ISFETmeter; K, recorder; W, waste.
pumped with a Gilson Minipuls 2. The pulse suppressor, grounding electrode and reference electrode were constructed and positioned as described previously [ 111. All the flow connections were made with PTFE tubing of 0.7 mm i.d. The potentials were measured at room temperature with an amplifier ISFETmeter (Solea-Tacussel, France). The source voltage ( VS) was measured while the drain current (In) and drain voltage (V,) were kept constant, the source and substrate being connected; V, was 0.5 V and 1o 100 ,uA. The voltages were measured against an Ag/AgCl reference electrode (Orion 90-02-00 double-junction reference electrode). Reagents and solutions. All reagents were prepared from analytical-reagent grade chemicals. Doubly distilled, deionized water was used throughout. The buffers used for testing the sensor were standard buffers prepared as described [ 121, to which hydrochloric acid or sodium hydroxide was added in order to obtain pH values in the range 2-12. The experiments were carried out with a carrier solution buffered at pH 7, and the injected solutions were buffer solutions with a pH within the above-mentioned range. Results and discussion The performance of the flow system was tested by varying the injection volume, the flow-rate and the length of the tube between the injection valve and the detector. The results are shown in Fig. 3. As can be seen, both flow-rate and tube length have little influence on peak height. Moreover, the peak height did not depend on the injection volume above 25 ,ul, in contrast to what happens with a similar flow cell which includes a planar ion-selective microISE [ 131, in which the injection volume is a critical parameter. The use of ISFETs in combination with flow-injection systems makes it possible to minimize dispersion in the detector because of the small sensitive surface area exposed to the flow, as has been observed previously [ 71 for other types of flow-through ISFET devices. With the selected experimental conditions corresponding to a flow system
376 PH.12
I
53mV
pH=9
pHr4.3
Fig. 3. Effect of the different flow-system parameters on the analytical signal (potential height, V). From top to bottom: Flow-rate, V, ; injection volume, Vi ; tube length, L.
peak
Fig. 4. Strip-chart recording showing the pH-ISFET detector response in the flow-injection system for replicates of samples of different pH. Flow-rate, 3.5 ml min-‘; injection volume, 25 ~1; tube length, 30 cm.
of low dispersion i.e., with a tube length which is as short as possible and a high flow-rate, the system shows near-Nernstian response. Between pH 2 and 12, the calibration graph was linear with a regression coefficient of 0.9987 and a slope of 53 mV pH-l. A calibration recording is shown in Fig. 4. The reproducibility of the peak heights evaluated from repeated injections (n = 4) of the different pH buffers was 0.7%. The time elapsed between sample injection and the maximum peak height was about 5 s and the time required for return to the baseline was about 8 s. This value indicates that ca. 360 samples can be processed per hour. According to these results, flow-through pH-ISFET cells such as that presented here seem to be ideal tools for continuous pH monitoring by flow-injection systems, because their response time is very short and their housing assemblies are robust and small, especially in comparison with those designed for glass pH electrodes. Further, an attractive feature is the possibility of multiion determination, because the contribution of an ISFET array to the total sample dispersion zone is negligible.
377
Financial support for the exchange of researchers received through FrenchSpanish Integrated Action is acknowledged. Part of the work was supported by CICyT, Madrid.
REFERENCES
10
11
12 13
J. Janata and R.J. Huber, Solid State Chemical Sensors, Academic, Orlando, 1985, Chap. 2. E. Leroy, P. Gareil and R. Rosset, Analusis, 10 (1982) 351. R.G. Kelly and A.E. Owen, J. Chem. Sot., Faraday Trans. I, 82 (1986) 1195. B.H. van der &hoot, P. Bergveld, M. Bos and L.J. Bousse, Sens. Actuat., 4 (1983) 267. C.B. Ranger, in D.P. Manka (Ed.), Automated Stream Analysis for Process Control, Vol. 1, Academic, New York, 1982, pp. 39-67. W.E. van der Linden, Anal. Chim. Acta, 179 (1986) 91. A.V. Ramsing, J. Janata, J. Ruzicka and M. Levy, Anal. Chim. Acta, 113 (1980) 45. C. Hongbo, E.H. Hansen and J. Ruzicka, Anal. Chim. Acta, 169 (1985) 209. H. van den Vlekkert, B. Kloeck, D. Prougne, J. Berthoud, B. Hu and N.F. de Rooij, Sens. Actuat., 14 (1988) 165. C. Arnoux, R. Buser, M. Decroux, H. van den Vlekkert and N.F. de Rooij, in Proceedings of the 4th International Conference on Solid-State Sensors and Actuators (Transducers ‘87), Tokyo, June 2-5,1987, pp. 751-754. S. Alegret, J. Alonso, J. Bartroli, A. Machado, J. Lima and J.M. Paulis, Quim. Anal., 6 (1987) 278. D.D. Perrin and B. Dempsey, Buffers for pH and Metal Ion Control, Chapman and Hall, London, 1974. S. Alegret, J. Alonso, J. Bartroli, J. Lima, A. Machado and J.M. Paulis, Anal. Lett., 18 (1985) 2291.
373 Printed in The Netherlands
Short Communication
FLOW-THROUGH pH-ISFET INJECTION ANALYSIS
DETECTOR FOR FLOW-
S. ALEGRET, J. ALONSO, J. BARTROLI and J. DOMENECH Departament de Quimica, Universitat Autonoma de Barcelona, 08193 Bellaterra (Spain) N. JAFFREZIC-RENAULT*
and Y. DUVAULT-HERRERA
Luboratoire de Physicochimie des Interfaces, Ecole Centrale de Lyon, UA CNRS 404, B.P. 163, 69131 Ecully Ce’den (France) (Received 4th January 1989)
Summary. A simply constructed flow-through pH-ISFET detector suitable for flow-injection analysis is described. Its response characteristics are presented and discussed. The calibration plot is linear in the pH range 2-12; the slope is 53 mV per decade.
Much attention has been devoted to the development of ion-sensitive fieldeffect transistors (ISFETs) owing to the advantage which they offer in comparison with ion-selective electrodes (ISEs) [l-3]. ISFETs are small, have a low impedance output signal and fast response and no need for conditioning. Further, these chemically sensitive electronic devices can be made using the same technology as is available for the production of integrated circuits with all the possible advantages, such as low cost, high reproducibility and largescale production. All these advantages make them suitable for exploring new applications such as in continuous monitoring systems. A shortcoming of these sensors is their high drift rate. However, this can be circumvented by using them in flow-injection systems. These systems, which are very suitable for continuous automatic monitoring [4,5], allow discrete sequential analysis in near real time. The surface sensor is continuously cleaned by the carrier solution, thus re-establishing the base line after each sample injection, the drift being neglected in the computation of the analytical signal (peak height) [6]. There are serious difficulties in adapting ISFET sensors in flow cells because encapsulating the sensor hinders the formation of a good stagnant layer [ 7,8].
0003-2670/89/$03.50
0 1989 Elsevier Science Publishers B.V.
374
However, the recent availability of ISFETs with rear-side contacts [9] is promising in this respect because this configuration could in principle facilitate the construction of compact flow-through ISFET sensor systems. This reports the implementation of a pH-ISFET with rear-side contacts in a flow-injection system by means of a specially designed flow cell; the results obtained for the sensor response under dynamic flow conditions are presented. Experimental Instrumental aspects. The Al,O, pH-ISFET with rear-side contacts was supplied by the University of Neuchhtel, Switzerland. A description of the manufacturing process has been published elsewhere [ 91. This A1203 ISFET shows a pH sensitivity of about 55 mV pH_’ and static conditions [9] and a drift rate of less than 0.2 mV h-l [lo]. Each chip (1.5~ 1.5 mm) includes one ISFET with rear-side holes for the electrical connection. The size of these holes is 80x80 pm at the bottom and 480 x 480 pm at the opening. The leads were glued into the holes with conductive epoxy (Epo-Tek 417; Epotecny, Velizy, France). The chip encapsulated in epoxy resin was mounted in a plastic holder (disposable lOOO-~1 Eppendorf pipette tip) and fitted into a specially designed flow-through cap assembly whose void volume is very small (see Fig. 1). The flow cell was incorporated in the conduits of a flow-injection system as shown schematically in Fig. 2. The injection valve (Reodyme 5020) was provided with exchangeable sample loops. The carrier and sample streams were 8
8
I t
5
T
1
6
Fig. 1. Designof the flow-throughpH-ISFET detector. (A) pH-ISFET (1) with rear-side contact wires (2 ) encapsulated in epoxy resin (3) and fitted into a plastic pipette tip (5 mm o.d. ) (4). (B) Block with the inserted pH-ISFET active surface. (C) Lid with inlet/outlet flow channels (5,6). An O-ring (7) ensures tight contact and defines a very small detection chamber. Pieces B and C are squeezed together by screws through the appropriate holes (8).
375
W
Fig. 2. Schematic diagram of the flow-injection system. A, Carrier solution; B, sample solution; C, pump; D, pulse suppressor; E, injection valve; F, coil; G, grounding electrode; H, flow-throughpHISFET detector; I, reference electrode inserted in a flow-through cap; J, ISFETmeter; K, recorder; W, waste.
pumped with a Gilson Minipuls 2. The pulse suppressor, grounding electrode and reference electrode were constructed and positioned as described previously [ 111. All the flow connections were made with PTFE tubing of 0.7 mm i.d. The potentials were measured at room temperature with an amplifier ISFETmeter (Solea-Tacussel, France). The source voltage ( VS) was measured while the drain current (In) and drain voltage (V,) were kept constant, the source and substrate being connected; V, was 0.5 V and 1o 100 ,uA. The voltages were measured against an Ag/AgCl reference electrode (Orion 90-02-00 double-junction reference electrode). Reagents and solutions. All reagents were prepared from analytical-reagent grade chemicals. Doubly distilled, deionized water was used throughout. The buffers used for testing the sensor were standard buffers prepared as described [ 121, to which hydrochloric acid or sodium hydroxide was added in order to obtain pH values in the range 2-12. The experiments were carried out with a carrier solution buffered at pH 7, and the injected solutions were buffer solutions with a pH within the above-mentioned range. Results and discussion The performance of the flow system was tested by varying the injection volume, the flow-rate and the length of the tube between the injection valve and the detector. The results are shown in Fig. 3. As can be seen, both flow-rate and tube length have little influence on peak height. Moreover, the peak height did not depend on the injection volume above 25 ,ul, in contrast to what happens with a similar flow cell which includes a planar ion-selective microISE [ 131, in which the injection volume is a critical parameter. The use of ISFETs in combination with flow-injection systems makes it possible to minimize dispersion in the detector because of the small sensitive surface area exposed to the flow, as has been observed previously [ 71 for other types of flow-through ISFET devices. With the selected experimental conditions corresponding to a flow system
376 PH.12
I
53mV
pH=9
pHr4.3
Fig. 3. Effect of the different flow-system parameters on the analytical signal (potential height, V). From top to bottom: Flow-rate, V, ; injection volume, Vi ; tube length, L.
peak
Fig. 4. Strip-chart recording showing the pH-ISFET detector response in the flow-injection system for replicates of samples of different pH. Flow-rate, 3.5 ml min-‘; injection volume, 25 ~1; tube length, 30 cm.
of low dispersion i.e., with a tube length which is as short as possible and a high flow-rate, the system shows near-Nernstian response. Between pH 2 and 12, the calibration graph was linear with a regression coefficient of 0.9987 and a slope of 53 mV pH-l. A calibration recording is shown in Fig. 4. The reproducibility of the peak heights evaluated from repeated injections (n = 4) of the different pH buffers was 0.7%. The time elapsed between sample injection and the maximum peak height was about 5 s and the time required for return to the baseline was about 8 s. This value indicates that ca. 360 samples can be processed per hour. According to these results, flow-through pH-ISFET cells such as that presented here seem to be ideal tools for continuous pH monitoring by flow-injection systems, because their response time is very short and their housing assemblies are robust and small, especially in comparison with those designed for glass pH electrodes. Further, an attractive feature is the possibility of multiion determination, because the contribution of an ISFET array to the total sample dispersion zone is negligible.
377
Financial support for the exchange of researchers received through FrenchSpanish Integrated Action is acknowledged. Part of the work was supported by CICyT, Madrid.
REFERENCES
10
11
12 13
J. Janata and R.J. Huber, Solid State Chemical Sensors, Academic, Orlando, 1985, Chap. 2. E. Leroy, P. Gareil and R. Rosset, Analusis, 10 (1982) 351. R.G. Kelly and A.E. Owen, J. Chem. Sot., Faraday Trans. I, 82 (1986) 1195. B.H. van der &hoot, P. Bergveld, M. Bos and L.J. Bousse, Sens. Actuat., 4 (1983) 267. C.B. Ranger, in D.P. Manka (Ed.), Automated Stream Analysis for Process Control, Vol. 1, Academic, New York, 1982, pp. 39-67. W.E. van der Linden, Anal. Chim. Acta, 179 (1986) 91. A.V. Ramsing, J. Janata, J. Ruzicka and M. Levy, Anal. Chim. Acta, 113 (1980) 45. C. Hongbo, E.H. Hansen and J. Ruzicka, Anal. Chim. Acta, 169 (1985) 209. H. van den Vlekkert, B. Kloeck, D. Prougne, J. Berthoud, B. Hu and N.F. de Rooij, Sens. Actuat., 14 (1988) 165. C. Arnoux, R. Buser, M. Decroux, H. van den Vlekkert and N.F. de Rooij, in Proceedings of the 4th International Conference on Solid-State Sensors and Actuators (Transducers ‘87), Tokyo, June 2-5,1987, pp. 751-754. S. Alegret, J. Alonso, J. Bartroli, A. Machado, J. Lima and J.M. Paulis, Quim. Anal., 6 (1987) 278. D.D. Perrin and B. Dempsey, Buffers for pH and Metal Ion Control, Chapman and Hall, London, 1974. S. Alegret, J. Alonso, J. Bartroli, J. Lima, A. Machado and J.M. Paulis, Anal. Lett., 18 (1985) 2291.