Microelectrochemical multitransistor devices based on electrostatic

Chem. , 1993, 65 (20), pp 2740–2746. DOI: 10.1021/ac00068a007. Publication Date: October 1993. ACS Legacy Archive. Cite this:Anal. Chem. 65, 20, 274...
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Anal. Chem. 1993, 65, 2740-2746

Microelectrochemical Multitransistor Devices Based on Electrostatic Binding of Electroactive Anionic Metal Complexes in Protonated Poly(4-vinylpyridine): Devices That Can Detect and Distinguish up to Three Species Simultaneously Jian Huang and Mark 5. Wrighton' Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Microelectrochemical multitransistor devices based on the reversible electrostatic incorporation of electroactive anionic metal complexes, such as IrCls2-,MO(CN)~~-, and Fe(CN)s4-,into protonated poly(4-vinylpyridine)[(VPyH+),] can detect and differentiate between these species. Arrays of closely spaced, individually addressable, band microelectrodesare connected by (WyH+),. Pairs of these microelectrodes are operated as independent microelectrochemicaltransistors. Each transistor shows high drain current only when its gate potential corresponds to the redox potential of a metal complex bound to the (VPyH+),, and this serves as the means of detecting and identifying the metal complex. Binding of anionic metal complexes in (VPyH+),is reversible,and therefore, the devices can be demonstrated to respond continuously to a flow of electrolyte in which the number and identity of the metal complexes is varied. We report the fabrication and characterization of microelectrochemicalmultitransistor devices based on the reversible electrostatic incorporation of anionic metal complexes into protonated poly(4-vinylpyridine) [(VPyH+),l and on metal complex-mediated charge propagation through (VPyH+),. Two such devices are described, one able to simultaneously detect and differentiate between two anionic electroactive metal complexes and the other able to detect and differentiate between up to three metal complexes simultaneously. The electrochemistry of anionic metal complexes bound in (VPyH+)n, (VPyH+.l/xM*-),, has been widely studied on macroscopic electrodes'-7 and offers two distinctive features which allow us to demonstrate the devices described herein. (1)Koet, K. M.; Bartak, D. E.; Kazee, B.; Kuwana, T. Anal. Chem. 1990,62, 151. (2)(a) Oh, S. M.; Faulkner, L. R. J . Electroanul. Chem. Interfacial Electrochem. 1989,269,77.(b) Oh, S.M.; Faulkner, L. R. J . Am. Chem. SOC.1989,111,5613. (3)Folster,R. J.;Kelly,A. J.;Voe, J.G.;Lyons,M.E.G. J.ElectroanaL Chem. Interfacial Electrochem. 1989,270,365. (4)(a) Lindholm, B. J. Electroanal. Chem. Interfacial Electrochem. 1988, 250, 341. (b) Lindholm, B.; Sharp, M.; Armstrong, R. D. J . Electroanul. Chem.InterfaciolElectrochem.1987,235,169.(c)Lindholm, B.; Sharp, M. J. Electroanul. Chem. Interfacial Electrochem. 1986,198, On 01.

(5)Andrieux, C. P.;Haas, 01;Saveant, J. M. J . Am. Chem. SOC.1986, 108,8175. (6)Oyama, N.; Yamaguchi, S.; Nishiki, Y.; Tokuda, K.; Matauda, H.; Anson, F. C. J. Electroanul. Chem. Interfacial Electrochem. 1982,139, 371. (7)(a) Doblhofer, K.; Lange, R. J. Electroanal. Chem. Interfacial Electrochem. 1987,229,239.(b) Doblhofer, K.; Braun, H.; Lange, R. J . Electroanal. Chem. Interfacial Electrochem. 1986,206,93. (c) Lange, R.;Doblhofer, K. J . Electroanal. Chem. Interfacial Electrochem. 1987, 216,241. (d) Niwa, K.; Doblhofer, K. Electrochim. Acta 1986,31,549. 0003-2700/93/0385-2740$04.00/0

First, (VPyH+J/,M*-), exhibits redox conduction only in a narrow region of potential centered a t the redox potential of the bound metal complex.8 Second, the incorporation of the anionic metal complexes is reversible in acidic electrolytes. Closely spaced microelectrodes connected by electrochemically active materials can be configured to function much like a conventional solid-state transisto$ and, in principle, can be tailor-made to respond to particular chemical signals. Redox-active materials that have been used in such devices include various organic conducting polymers? metal oxides,10 and conventional redox polymers.'' Previous work has also demonstrated a microelectrochemical transistor based on (VPyH+.1/3Fe(CN)63-)n,which is sensitive to two chemical stimuli: H+ and Fe(CN)63-.12 The operation of microelectrochemical transistors is illustrated in Scheme I. When a fixed small drain voltage, VD, is maintained between two closely spaced band microelectrodes connected by an electroactive material, the drain current, ID,flowing between the two microelectrodes, will be controlled by the electrochemical potential of the gate, VG. As VG is changed, the electroactive material changes from insulating to conducting or conducting to insulating, and as a result, IDis turned on or off. In so-called conventional redox polymers, including (VPyH+.l/xMX-)n[Mx-= IrCl&, MO(CN)~P, Fe(CN)&], conductivity is concentration gradient-driven. At VGclose to the redox potential of the metal complex, a small VDproduces a concentration gradient of the oxidized and reduced forms of the metal complex between the two microelectrodes, and ID flows between the two microelectrodes. When VGis far from the redox potential of the metal complex, all the redox centers at both microelectrodes are in the same oxidation state, conductivity is small, and ID becomes small.12 Our new multitransistor devices consist of arrays of closely spaced band microelectrodes connected by (VPyH+),. Pairs of these microelectrodes are operated as independent transistors. VGof each transistor is fixed at the redox potential of a particular redox-active anionic metal complex which can be reversibly electrostatically bound in the cationic polymer. For a given reversibly electroactive metal complex, (VPyH+J/,M*-), is only conducting in a narrow potential ( 8 ) (a) N a b , M. J.; Wrighton, M. S.Prog. Inorg. Chem. 1989,37,391. (b) Wrighton, M.S.; Thackeray, J. W.; Natan, M. J.; Smith, D. K.; Lane, G. A.; Belanger, D. Philos. Tram. R. SOC.London 1987,B316, 13. ( c ) Wrighton, M. S. Comments Inorg. Chem. 4 , 1986,4 , 269. (9)(a) Thackeray, J. W.; White, H. S.; Wrighton, M. S.J . Phys. Chem. 1985,89,5133.(b) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J . Am. Chem. SOC.1984,106,7389. ( c ) Chao, S.;Wrighton, M. S. J . Am. Chem. SOC.1987,109,6627. (10)Schloh, M. 0.;Leventis, N.; Wrighton, M. S. J. Appl. Phye. 1989, 66,965. (11)Kittlesen, G. P.;White, H. S.; Wrighton, M. S. J. Am. Chem. SOC. 1986,107,7373. (12)Belanger, D.; Wrighton, M. S. Anal. Chem. 1987,59,1426.

0 1993 Amerlcan Chemical Soclety

ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993

Scheme I. Microelectrochemical Transistor Device.

Electroactive Material in its

Device "Off'

Electroactive Material in its

Device "on"

Turns on (ID > 0) when the gate voltage, Vc, is set to p, where the electroactive material is conducting, and turns off = 0) when VG is set to Vi,where the electroactive material is insulating.

(f~

region centered at the redox potential of the metal complex.12J3 Therefore, each transistor only shows high ID when a metal complex with redox potential corresponding to VG of the transistor is incorporated into (VPyH+),. The identity and number of the metal complexes in the electrolyte determine which and how many of the transistors on the device are turned on. EXPERIMENTAL SECTION Chemicals. Poly(4-vinylpyridine)of molecular weight 200 OOO was used as received from Reilly Tar and Chemical,Indianapolis, IN. I 1

1

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0.4 0.6 0.0

POTENTIAL, V v s . S C E

Flgure 2. Cyclic voltammogram of a (VPy),&oated microelectrode in 0.2 M CF3COOH/NaOH(pH 9) electrolyte Containing 5 mM MO(CN)~~. The microelectrode Is the same one used In Figure 1. The absence of a response to MO(CN)~~ shows that the neutral(VPy), film Is plnholefree.

be operated in a flow system where the metal complexes in the electrolyte are being changed successively, vide infra. However, the displacement of one metal complex bound to (VPyH+), by another is aslow process (1min to 2 h, depending on the thickness of the polymer and the concentration of the

--

1 2 " 4 0 2 0 4 06 0 8

metal complex in the electrolyte),and this limits the usefulness of the (VPyH+),-based device in very dilute metal complex solutions. In basic solution containing Mo(CN)&, the cyclic voltammogram shows no faradaic current (Figure 2), indicating that the deprotonated (VPy). expels all the anionic metal complex. Thus, (VPy), is a pinhole-free coating, preventing anionic metal complexes in basic solution from reaching the electrodes. Figure 3 shows cyclic voltammetry, in pure electrolyte, of Mo(CN)&, Fe(CN)&, and IrCh" bround in (VPyH+), for a single microelectrode of a (VPyH+),-coated microelectrode array. For the scan rates shown, the peak currents correlate linearly with the square root of the scan rate. This resemblance to a semiinfinite diffusion process is consistent with both the coverage of the polymer (10-7 mol/cm2, derived by integration of the anodic peak of the cyclic voltammetry wave a t 10mV/s), and the slow charge transport in (VPyH+J/,Mx-), (DCT N 10-9 cm$/s).12 The sluggish charge transport in (VPyH+JI,MX-),can be demonstrated by driving two adjacent microelectrodes separately and together, as Figure 4 shows for (VPyH+J/4Mo(CN)&),. For all three metal complexes tested, the sum of the integrals of the cyclic voltammetry

ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993

2743

a C 4-

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0.2 0.4 0.6 0.8 0.4 0.6 0.8 1.0 POTENTIAL, V vs. SCE

--

Flgurr 3. Cyclic voltammograms of Fe(CN)sC,Mo(CN)aC, and IrCias- bound in (VPyH+), at a single microelectrode of a (VPyH+),,-coated array at various sweep rates: (1) 500, (2) 200, (3) 100, (4) 50, and curve (5) 20 mV/s in 0.2 M CF3COONa/CF3COOH(pH 1.7) aqueous solutions. (D-F) show the linear relationship between the peak anodic current and the square root of the sweep rate. I

l

l

1

A . Electrode A

3. Electrode B

;. Electrode A

a. Fe(CN)i' 1.5

ond B together

%=IoomV *\b=50mV VD=20mV

0.5

V u u 0.4 0.6 0.8 1.0 0.2 0.40.6 0.8 0 0.2 0.4 0.6 POTENTIAL, V vs. SC E Flgure 4. Cyclic voltammograms of M O ( C N ) ~bound ~ in (VPyH'),

at two adjacent electrodes of a (VPyH+),,-coated array at various sweep rates: (1) 500, (2) 200, (3) 100, (4) 50, and (5) 20 mV/s In pure supporting electrolyte (CF3COONa/CFSCOOH,pH 1.7). (A) Cyclic voltammograms of microelectrode A. (B) Cyclic voltammograms of microelectrode B. (C) Cyclic voltammograms of microelectrodes A and B driven together. Note the different current scale in (C). These data conflrm slow charge transport through the polymer fllms. waves of the two adjacent microelectrodes driven separately equals the integral of the cyclic voltammetry wave of the two adjacent microelectrodes driven together as one electrode. This is a consequence of the low values of DCT for (VPyH+J/,MX-),,and indicates that, on the time scale of sweep rates as low as 10 mV/s, the average diffusion distance of

1.0

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, 0.2

0.4

0.6

0.8

Gate Voltage, VG, V vs. SCE Flgwr 5. I D vs V, characteristics for mlcroeiectrochemicaltransistora based on (VPyH+), into which anionic metal complexes are bound. One of the electrodes of the transistor is coated with Pt to narrow the gap between the two adjacent electrodesto -0.5 pm. ID is recorded at the other microelectrode.

charge in the polymer is shorter than the interelectrode gap. The slow charge transport in (VPyH+*'/,M'-)n ensures that transistors in the multitransistor devices based on (VPyH+)ncoated microelectrode arrays do not interfere with each other. Panels a-c of Figure 5 show the ID vs VG characteristics for microelectrochemical transistors based on (VPyH+)nincorporating Fe(CN)&, Mo(CN)&, and I r C P , respectively. In order to increase the magnitude of ID, the interelectrode gap

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ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993

I 1

Scheme 11. Two-Transistor Device Based on Poly(4-vinylpyridine). Potentiostat I

/-----I

/

Reference Electrode

1 /Conducting Region I-

g

s

I

I

-1 u

Transistor I

I +

VI = 0.25V,V2= 0.35V

2

t7-l c. IrCIB-

I/

Insulating Region

-

C

Counter Electrode

Transistor I1 V7= 0.45V,Vs= 0.55V

Potentiostat Reference Electrode

I

Counter Electrode

1.

(VPyI-I+.1/4Mo(CN)g4'),

I

I 100

200

300

Transistor I VI= 0.25V,V2= 0.35V I,= 0

400

Drain Voltage, mV

Flgure 6. Relationship between the peak drain current and the drain voltage of microelectrochemical transistors based on (VPyH+), into which metal complex is bound.

has been narrowed by plating one of the microelectrodes of the transistor with Pt. The drain current is a t a maximum when VGis at the redox potential of the metal complex bound in the polymer, with significant drain current only observed for VGwithin hO.1 V of the redox potential of the metal complex. This ID-VGcharacteristic is different from that for other kinds of microelectrochemical transistors based on conducting polymers8 or metal oxides? which show a broad range of VGover which they are turned on, but is consistent with the known characteristics of charge transport of (VPyH+.1/,Mx-)..12113 The important point which Figure 5 illustrates is that the regions of VGin which each of the three different (VPyH+J/,M*-),-based transistors has high ID are narrow and well-separated from each other, and this is essential to the function of the multitransistor devices described herein. The peak ZD in Figure 5 has an ohmic relation with VD for VD < 0.1 V and approaches a limiting value at VD > 0.2 V (Figure 6). This is because ZD is concentration gradient driven, and the concentration gradient can not increase beyond the point a t which all redox sites at one microelectrode are reduced and all sites at the other are oxidized. The configuration of a two-transistor device is shown in Scheme 11. Two bipotentiostats were used to control four microelectrodes, which are needed for two transistors. The two transistors are separated in space by four microelectrodes, and because of the low DCTof (VPyH+J/,M+-),, this distance ensures that they do not interfere with each other. The VD of each transistor is set to 0.1 V. The VGof transistor I corresponds to the redox potential of Fe(CN)&. The VGof transistor I1corresponds to the redox potential of Mo(CN)&. Figure 7 shows the current at each source and drain electrode of the device in acidically buffered electrolytes containing Fe(CN)& or Mo(CN)&. The ID is the difference in current measured at source and drain. In 0.2 mM Fe(CN)& electrolyte, transistor I shows significant ID, because ita VG is set at the redox potential of the Fe(CN)e3-/& couple. Transistor I1 shows no ID because its VGis far from the Fe(CN)e3-IC redox potential. Likewise, in 0.2 mM Mo(CN)& electrolyte, transistor I1 shows significant ID and transistor I does not. In an electrolyte containing both Fe(CN)& and Mo(CN)&, both transistors are turned on. The magnitudes

Transistor I1

V7 = 0.45V,Vs I:0.55V I,>O 0 Transistor I involves electrodes 1and 2 as source and drain.

Transistor I1 involves electrodes 7 and 8 as source and drain. The potentials of these four electrodes are controlled by two bipotentiostata at 0.25,0.35,0.45, and 0.55 V, respectively. The two transistors can be turned on and off alternately by changing the metal complex in the electrolyte.

-y li 1

a. 0.2 mM K4Fe(CN)6

b. 0.2inMKqMo(CN)~

4

7

3 0.3

0.4

0.5

Electrode Potential, V vs. SCE

Current at each source and drain electrode of a microelectrochemical,two-transistor device (cf.Scheme I I) to electrolytes (CF&OONa/CF&OOH; pH 1.7) containing (a) 0.2 mM Fe(CN)ec, (b) 0.2 mM M O ( C N ) ~and ~ , (c)0.4 mM Fe(CNkC/O.1 mM MO(CN)~~. Points 1-4 represent the currents and voltages of electrodes 1, 2 and 7, 8. Points 1 and 2 are source and drain transistor I. Points 3 and 4 are source and drain of transistor 11. The drain current in each case is the difference between source and drain. Figure 7.

of the drain currents for the two transistors depend on the concentrations of the two metal complexes inside the polymer. Because the equilibrium process represented by eq 2 favors

ANALYTICAL CHEMISTRY, VOL. 65, NO. 20,OCTOBER 15, l9g3

274S

0.1 mM Mo(CN1;-

+.

Transistor

U Transistor I1

0.I mM MdCNI:.

12min

M

c

O.lmMFe(CNIe4-

TIME

Flgure 8. Draln currents of a microelectrochemical two-transistor devlce (cf. Scheme 11) In a flow system (cf. Scheme 111). The flow rate Is 1 mL/mln. The electrolyte Is CF3COONa/CF3COOH(pH 1.7). The metal complex In the electrolyte Is alternated between 0.1 mM Fe(CN),,& and 0.1 mM Mo(CN)*&.

Scheme 111. Flow System Used To Alter Electrolyte Composition for Multitransistor Experiments.

I---

l r

fl U -

Flowout

Microelectrochemical device

The flow rate is regulated by the height of the electrolyte reservoirsto be 1mL/min. A three-way switch switchesbetween the reservoirs. a

the incorporation of Mo(CN)a4- in (VPyH+),,le the drain current of transistor 11,which responds to Mo(CN)&, is larger than the drain current of transistor I, which responds to Fe(CN)&. Note that there is negligible current for the source and drain of transistor I1 when Fe(CN)& only is present. This shows that mediated oxidation of solution species does not yield significant current response a t the low concentrations of anions. Response of the two-transistor device to change of the anionic metal complex present in acidic electrolyte is shown in Figure 8. Scheme I11shows the experimental setup, which allows the anionic metal complex present in the electrolyte flowing past the device to be switched. When the flow stream contains Fe(CN)&, transistor I is turned on and transistor I1 is turned off. When the flow stream is switched from Fe(CN)& to Mo(CN)&, the ID of transistor I gradually decreases and ID of transistor I1 increases, with this process being reversed as the flow stream is again switched. The switching time of the device is slow because of the slow displacement process of one polymer-bound anionic metal complex by another metal complex from dilute solution. When both Fe(CN),+- and Mo(CN)& are in the flow stream at the same time, both transistors are turned on as shown in Figure 9. The ID of each transistor is smaller than that when only a single transistor is turned on. This is expected, because the concentration of each metal complex bound in (VPyH+),

7nmin

After 2 hours

TIME

Q. Drain currents of a microeiectrochemlcal two-translstor device In a flow system. The flow rate is 1 mL/mln. The electrolyte IsCF3COONa/CF3COOH(pH 1.7).The metal complex In the electrolyte Is changed successively from 0.1 mM Fe(CNb& to 0.1 mM Mo(CNbc to 0.1 mM Fe(CNbC/0.04mM t~lo(CN)~&, and back to 0.1 mM Fe(CNh+. Figure

is lower when both metal complexes are bound in (VPyH+), than when only one metal complex is bound in the polymer. The change in ID of transistor I1 when the electrolyte is switched between containing only Mo(CN)& and containing both Mo(CN)& and Fe(CN)& is smaller than the change in ID of transistor I when the electrolyte is switched between containing only Fe(CN)& and containing both Mo(CN)& and Fe(CN)&. As with the data in Figure 7c, this is a consequenceof the greater affiiity of (VPyH+), for Mo(CN)& than for Fe(CN)&,l6 with the polymer tending to concentrate more Mo(CN)& from electrolytes in which the metal anions are mixed. By analogy to the device in Scheme 11, it is possible to control six independently addressable microelectrodes using three bipotentiostats to operate as a three-transistor device. But Scheme IV shows a more efficient configuration in which four (VPyH+),-coated microelectrodes can be controlled by two bipotentiostats to operate as a three-transistor device capable of detecting and distinguishing between three electrochemically active anionic metal complexes. The potentials of the four electrodes 1-4 are fixed independently at 0.25,0.4, 0.6, and 0.75 V (vs SCE), respectively, by the two bipotentiostats. Electrodes 1 and 2, 2 and 3, and 3 and 4 are the source and drain electrodes for transistors I, 11, and 111, respectively. Panels a-c of Figure 10 show the response of the device to acidic electrolytes containing Fe(CN)&, Mo(CN)&, and IrC&%,respectively. In Fe(CN),& electrolyte, electrode 1 reduces Fe(CN)e% and electrode 2 oxidizes Fe(CN)&, resulting in finite ID between (2) and (1). Electrodes 3 and 4 show a small anodic current due to a small amount of charge diffusion from electrode 1. The main result in the presence of Fe(CN)64-is that transistor I is on and 11 and I11are much less so. In Mo(CN)& electrolyte, transistor I1is turned on, because the redox potential of the Mo(CN)&/% couple is between the potentials of electrodes 2 and 3. ID flows from (3) to (2). Electrode 1 shows a small cathodic current due to a small amount of charge diffusion from electrode 3. Likewise, electrode 4 shows a small anodic current due to a small amount of charge diffusion from electrode 2. In IrCl& electrolyte, transistor I11 is turned on because the redox potential of IrC&” is between the potentials of electrodes 3 and 4, and ZD flows from (4) to (3). Again, electrodes 1and 2 show a small anodic current due to a small amount of charge diffusion from electrode 4. In a flow system, where the metal complex in the flow system changes successivelyfrom Fe(CN)& to Mo(CN)& to IrCl,?, and then back to Fe(CN)& again, the three transistors are turned on and off successively as shown in Figure 11. In this

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ANALYTICAL CHEMISTRY, VOL. 85, NO. 20, OCTOBER 15, 1993

Scheme IV. Three-Transistor Device Employing Four Microelectrodes and Two BipotentiostatsP

O 2 m M FelCNI$1-

.i

I

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2

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-2 tentiostat

Potentia t a t

TIME

Figure 11. Response of currents at microelectrodes 2 and 3 of a microelectrochemical threetransistor device (cf. Scheme IV) in a flow system. The flow rate Is 1 mL/mln. The electrolyteIs CF3COONa/ CF3COOH (pH 1.7). The metal complex In the electrolyte Is changed successhrely from 0.2 mM Fe(CN)eC to 0.2 mM Mo(CN)aC to 0.2 mM IrCle2- and back to 0.2 mM Fe(CN)8C.

(VPyH+'! / 2 IC$), Insulating Region

Conducting Region

1

positive current as in Figure loa. When Mo(CN)& is in the solution, transistor I1 is turned on, and therefore a negative current is recorded at (2) and a positive current is recorded at (3). When IrCb" is in the electrolyte, transistor I11 is turned on, and the negative current recorded a t electrode 3 is the I D of transistor 111. Electrode 2 shows only a small negative current as in Figure 1Oc. It is clear that the rise and fall of the currents at electrodes 2 and 3 show the successive turning on and off of the three transistors accompanying changes of the metal complex in the electrolyte. In this way, the microelectrochemical three-transistor device is able to detect and distinguish between Fe(CN)&, Mo(CN)&, and IrCh3- in acidic electrolyte.

0

CONCLUSION

Ei VI

v2

"3

"4

The sources and drains of transistors 1-111 are electrodes 1 and 2 , 2 and 3, and 3 and 4, respectively. The potentials of these electrodes 1-4 are controlled by two bipotentiostats at 0.25,0.4, 0.6, and 0.75 V, respectively. The electrochemicalpotentials of the three metal complexes are between the fixed potentials of these four electrodes, as illustrated. The three transistors can be turned on and off successively by changing the metal complex in the electrolyte. a

-1

'

a. 0.2mM K4Fe(CN)6

A

1

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b. 0.2 mM KdMo(CN)

c. 0.2 mM K21rCI6

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1

0

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2

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0.3 0.4

0.5

0.6

0.7

Electrode Potential, V vs. SCE

Flgure 10. Current at each source and draln of a microelectrochemical three-translstor device to electrolytes(CF3COONa/CF3COOH;pH 1.7) Containing (a) 0.2 mM Fe(CNkC, (b) 0.2 mM MO(CN)~~, or (c) 0.2 mM IrCie2-. Points 1-4 represent the currents and voltages of electrodes 1-4, where 1 and 2, 2 and 3, and 3 and 4 are the source and drain of transistors 1-111, respectlvely.

experiment, only the currents a t electrodes 2 and 3 are recorded, because the drain currents of the three transistors can be adequately represented by only recording them at these two electrodes. When Fe(CN)& is in the electrolyte, transistor 1is turned on and the large positive current recorded at (2) is the IDof transistor I. Electrode 3 shows only a small

The (VPyH+),-based microelectrochemicalmultitransistor devices respond to multiple chemical stimuli in acidic electrolyte. They can differentiate between anionic metal complexes having different redox potentials, being able to distinguish between two metal complexes with redox potentials as close to each other as 0.2 V, e.g., M o ( C N ) ~ ~ a IrCL+-. nd The reversible electrostatic incorporation of anionic metal complexes into (VPyH+), enables the (VPyH+),-baseddevices to detect and distinguish between anionic metal complexes in a flow system where the metal complexesare being changed successively. The (VPyH+),-based devices can function in electrolytes containing dilute concentrations of metal complexes because (VPyH+), concentrates the metal complexes. However, the extent of dilution a t which the devices can operate is limited by the slow response time of the devices. Moreover, the devices do not show a simple response to variation of the concentration of the anionic species. Thus, the use, if any, of the multitransistor device would be in the area of analyte identification. The concepts illustrated here, however, are quite general and with appropriate (and different) redox polymer multitransistor devices for analyte identification and measurement of concentration can be envisioned.

ACKNOWLEDGMENT We thank the Defense Advanced Research Projects Agency and the Office of Naval Research for partial support of this research. We appreciatevaluable discussions with Dr. David Ofer. RECEIVEDfor review March 12, 1993. Accepted June 18, 1993.