Anal. Chem. 1987, 59, 1426-1432
1426
Microelectrochemical Transistors Based on Electrostatic Binding of Electroactive Metal Complexes in Protonated Poly(4-vinylpyridine): Devices That Respond to Two Chemical Stimuli Daniel BBlanger and Mark S. Wrighton* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
This artkie reports the fabrlcation and characterlzatlon of a microelectrochemical trandstor derlved from coating and connecting two closely spaced ( 0 ) to a significant extent at VG = E", VG',because only then is there a significant concentration of oxidized and reduced sites. At V,-Jsignificantly (>0.2 V) more negative, VG*,or positive, VcS,of Eo ' only the reduced or oxidized sites are present, respectively, and the device is off (ID = 01, giving rise to the ZD-VGcharacteristic shown. conductivity of the conventional redox polymers is very low, as was emphasized in connection with a microelectrochemical diode based on two different conventional redox polymers (17). Lower conductivity yields smaller peak ID Further, it is known that conventional redox polymers will yield devices having slow switching speed (In, owing to poor effective
diffusion coefficients for charge transport. The microelectrochemical transistor described below, and sketched in Scheme I, illustrates (1)the ID-VG characteristic that can be expected for devices based on conventional redox polymers, (2) the consequences of small DCT and large source-drain separation, namely, sluggish switching (17)and
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ANALYTICAL CHEMISTRY, VOL.
59,
NO.
10, MAY 15, 1987
small ID(peak), and (3) a multistimulus response device, namely, a device that turns on only below a certain pH, when electroactive anions are bound in the (4-VPyH+), and when Vc is close to the E"' of the electrostatically bound redox system.
EXPERIMENTAL SECTION Microelectrode Arrays. Arrays of eight Au microelectrodes each -80 pm long, 2.3 pm wide, and 0.1 pm thick and spaced 1.7 pm apart were used in the work described in this article. Fabrication of the microelectrode arrays on p-Si/Si02/Si3N4substrates has been described previously (17,18,20). Prior to use, arrays of microelectrodes were cleaned by an rf O2 plasma etch to remove residual photoresist, followed by cycling the potential of each electrode between -1.5 and -2.0 V vs. SCE in 0.1 M aqueous K2HP04at 200 mV/s, to evolve H,. The Au microelectrodes were then "tested" for their electrochemical response to a 5 mM solution of Ru(NH3):+ in 0.1 M LiCl. Such a test reveals, for a "good" microelectrode, a sigmoidal current-voltage curve with a plateau current of -25 nA. Figure 1shows similar curves for the reduction of Fe(CN),,- at a "naked" Au microelectrode. Where required, Au microelectrodes were modified by the electrochemical deposition of Pt onto their surface to reduce the interelectrode gap (17,18,20). The Pt was deposited from aqueous 0.1 M K2HP04containing 2 mM K2PtC14. Two different arrangements of platinized electrodes were used. One arrangement consists of the heavy platinization of electrodes 2,4,6,and 8 while leaving electrodes 1,3,5,and 7 naked. In this procedure electrodes were platinized one at a time by holding all electrodes not to be plated at +0.2 V vs. SCE while the one to be plated was held at a negative potential to deposit Pt. The arrays thus fabricated had a spacing of -0.5 pm between microelectrodes as determined by scanning electron microscopy. The deposition of the Pt typically involves -5 pC of charge per microelectrode. The second arrangement of platinized microelectrodes involves a gradual increase of Pt deposited on each of three pairs of Au microelectrodes: electrodes 1 and 2 were left naked while 0.5, 1.0, and 2.0 pC were passed to deposit Pt on each electrode of the following pairs of electrodes: 3 and 4, 5 and 6, and 7 and 8, respectively. The separation between the naked electrodes, 1 and 2, was determined to be 1.7 pm, separation for 3 and 4 was 1.2 pm, separation for 5 and 6 was 1.0 pm, and the most heavily platinized, 7 and 8, electrodes were separated by -0.45 pm. Fabrication of Polymer-Coated Microelectrodes. Films of (CVPy), were deposited on a microelectrode array by micropipetting precise volumes (2-3 pL) of a 0.25% (w/v) solution of (4-VPy), and 3.7 mM 1,6-dibromohexane in CH30H onto the array. The solution was allowed to dry at 298 K in a CH30Hsaturated atmosphere for periods of time ranging between 1and 15 h (21). The modified microelectrode array was then heated at 360 K for 30 min. The 1,6-dibromohexane is used as a cross-linking agent by alkylating the pyridine nitrogens. The amount of cross-linking is sufficiently small that electrostatic binding of Fe(CN)6s/&cannot be detected electrochemically unless the nonalkylated nitrogens are protonated. Exposure of (CVPy), coated electrodes to aqueous pH 1.8 solutions of Fe(CN)63-/0.2 M Na[CF3COO]/CF3COOHresults in the incorporation of Fe(CN)63-into the polymer coating. Integration of the cyclic votammogram wave associated with interconversion of bound Fe(CN)63-and Fe(CN)64-can be used to assess coverage of the
(~-VPYH+.'/,F~(CN),~-).. Chemicals. The solvents used were H20 and CHBOH(both Omnisolv). (4-VPy), of molecular weight 200000 was used as received from Reilly Tar and Chemical Co., Indianapolis, IN. All other chemicals were reagent grade and were used as received. Equipment. Electrochemical equipment included a Pine Instruments RDE-4 bipotentiostat for the characterization of microelectrodes. A Kipp and Zonen BD 91 x-Y-Y' recorder was used for recording characteristics of the microelectrodes. A Hewlett-Packard Model 1084-b HPLC was used as the system to deliver chemical signals to the microelectrochemical devices. Microscopy. The microelectrode arrays were examined by electron microscopy on a Cambridge Mark 2A Stereoscan. The arrays were first coated with -20 nm of Au to minimize problems from surface charging. The optical microscopy was done with
CYCLIC VOLTAMMOGRAMS A T GOLD MICROELECTRODES BEFORE AND A F T E R D E P O S I T I O N OF POLY ( 4 - V I N Y L P Y R I D I N E )
.
. C 1 AuIPVFy
FI Au/PVPy
I1 Au/PVP)
t-F-r-lk-r-l-lk-r-r-r 02 04 06 0 0 2 04 06 0 02
0
04
06
P O T E N T I A L , V v s SCE
Figure 1. Cyclic voltammograms at 100 mV/s of naked (A, D, G) and (CVPy), -coated (B, E, H) microelectrodes in Fe(CN),3- solution. Fe(CN),> concentrations were as follows: A,B, 0.1 mM; D,E, 1 mM; and G,H, 5 mM. Curves C, F, and I were recorded in pure supporting electrolyte (0.2 M aqueous Na[CF,COO]/CF,COOH; pH 1.8) after the incorporation of Fe(CN):- in the polymer. The Fe(CN):- coverage (from integration of the cyclic vobnmogram) was 7.3 x 10" mol/cm2.
a Bausch & Lomb MicroZoom microscope equipped with a Polaroid camera.
RESULTS AND DISCUSSION a. Modification and Electrochemical Behavior of Microelectrodes with (4-VPy),. Figure 1 shows the electrochemical response of a single microelectrode to 0.1, 1, and 5 mM K,Fe(CN), in pH 1.8 aqueous electrolyk solution. The response of the naked microelectrode to Fe(CN)63-is essentially independent of pH. The microelectrode response shows a sigmoidal current-voltage curve characteristic of microelectrodes (22), because the width of the microelectrodes, -2 pm, is sufficiently small that the unusual diffusion properties applicable to microelectrodes obtain. A microelectrode array, consisting of eight individually addressable, Au microelectrodes, can be coated with (4-VPy), as described in the Experimental Section. At high p H the (4-VPy),-coated microelectrodes show no response to solution Fe(CN)63-/4-. At low pH, the (4-VPy), can be protonated and will electrostatically bind Fe(CN)63-. Figure 1includes electrochemical characterization of a (kVPy),-coated microelectrode in the pH 1.8 solutions containing 0.1, 1, or 5 mM Fe(CN)63-. As shown by the cyclic voltammetry scans B, E, and H in Figure 1,the (4-VPy),-coated microelectrode shows a well-developed cyclic voltammetry wave, not a sigmoidal current-voltage curve, in the pH 1.8 solution containing the 0.1, 1, or 5 mM Fe(CN),,-. The larger peak current and lack of sigmoidal current-voltage curve (in comparison to the naked electrode) for the sweep from +0.6 to 0 V vs. SCE in the case of 0.1 mM Fe(CN)63- indicates that the electrochemical response is dominated by Fe(CN)63-bound in the (4-VPyH+),. At 1 and 5 mM, sigmoidal current-voltage curves are also not found for the (4-VPyH+),-coated electrode and the limiting cathodic current a t 0.0 V vs. SCE is not as large as that found for the naked electrode. The results a t 1 and 5 mM Fe(CN)63-indicate that the (4-VPyH+.1/,Fe(CN)63-)n-coated microelectrode actually inhibits the reduction of the solution Fe(CN)63-. The reduction of the solution Fe(CN):- is known to be retarded by sufficiently thick coatings of (4-VPyH+.'/,Fe(CN),"), and is a consequence of low values of DcT in the polymer which includes slow diffusion of Fe(CN)63- inside the polymer. In any event, the (4-VPyH+.1/3Fe(CN)63-),-coated microelectrode appears to show a limiting current of -2 nA, a current density of -1 mA/cm2. This is in rough accord with expectation associated with the coverage of -7 X mol of Fe-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987 :YCLIC VOLTAMMOGRAMS A T A D J A C E N T MICROELECTRODES 4ODIFIED W I T H A F E R R I C Y A N I D E LOADED POLY ( 4 - V I N Y L ’YRIDINE) F I L M
GENERATION AND COLLECTION AT MICROELECTRODES MODIFIED WITH A FERRICYANIDE-LOADED POLY (4-VINYLPYRIDINE) FILM
-
H20/0.2M
1429
H 2 0 / 0 . 2 M CF3COONa; pH’1.8
CF3COONa; p H ~ 1 . 8
IOmVls
500mV/s
WIRESA
WIRE B
WIRE A
I-
13.5 -.E4 EL5= 0 . 4 V vs.SCE
z W LT
a
ia %Eq
3
b 7 - l - l - k Q2
0.4
Q6 0
0.2
0.4
0.6 0
0.2 0.4
‘1.7 vr.E4 E, ,,:0.4 V vs. SCE
E 2 , 6 * 0 4 V %SCE 14 VI.
Eq
I4VI.E4
E2,6 0 . 4 V vc. SCE
u
0
‘2,6 vr’E4
E 1 , 7 * 0 . 4 Vv i S C E
0.6
P O T E N T I A L , V vs. SCE
Flgure 2. Cyclic voltammograms of adjacent microelectrodes coated with (4-VPyH+-‘/3Fe(CN)63-), as a function of scan rate. The sum of the areas under the cycllc voltammograms for the individual electrodes A and B is the area found when the microelectrodes A and B are externally connected together and driven as a single electrode. The electrolyte composition is 0.2 M Na[CF,COO]/CF,COOH, pH 1.8,and the Fe(CN),* coverage is 3.1 X 10“ mol/cm*.
(CN),”/&/crn2. The coverage of Fe(CN)t- can be assessed by integration of the cyclic voltammetry wave, recorded a t slow scan speed, associated with the Fe(CN),” e Fe(CN),” interconversion. Curves C, F, and I in Figure 1 show cyclic voltammetry for a microelectrode coated with (4-VPy), and loaded with Fe(CN)6“ from 0.1, 1, and 5 mM Fe(CN),,- solutions, respectively. Within experimental error, the scans c, F, and I in pH 1.8 solution containing no Fe(CN)63-show the same amount of Fe(CN)6”/’ electrostatically bound. The conclusion is that loading the (4-VPyH+), from 0.1 mM Fe(CN)63-is just as effective as loading from 5 mM Fe(CN)63-. However, the time required to load the (4-VPyH+), with Fe(CN),” does depend on the concentration; approximately 15 min is required to ensure fully loaded (4-VPyH+), at 0.1 mM, whereas 1 mM Fe(CN)2- gives the ultimate electrochemical response in about 1 min. Figure 2 shows cyclic voltammograms for two adjacent microelectrodes of an eight-electrode array coated with (4VPyH+J/,Fe(CN)t-), at a coverage of -3 X mol of Fe(CN)63-/4-/cm2. The bound Fe(CN),’-/‘- redox couple is persistently attached in the aqueous pH 1.8 electrolyte solution in the absence of added Fe(CN),”/&. The chemically derivatized microelectrodes show similar electrochemical response for several hours, and detailed studies of the electrochemical behavior are possible. Figure 2 shows the cyclic voltammograms as a function of sweep rate for two of the eight microelectrodes of an array. All eight microelectrodes show the same cyclic voltammogram when addressed separately, establishing that the array is uniformaly modified with (4-VPyH+.’/,Fe(CN)t-),. For the sweep rates shown, the integral of the cyclic voltammetry wave of n electrodes driven together as one electrode is equal to the sum of the integrals for the n electrodes driven individually. This result means that the microelectrodes are not electrically connected on the time scale of the slowest sweep.
t - 0.2 l - r0.4l l 0.6- l l0 l l 0.2l l -0.4
0
0.6
0
02
0.4
0.6
P O T E N T I A L , V vs. SCE
Flgure 3. Generation/collection experiment with (4-VPyH+.’/,Fe(CN),”),-coated microelectrodes in 0.2 M Na(CF,COO] /CF,COOH; pH 1.8. The lower cyclic voltammograms are for the generator electrode 4 as its potential was swept between 0 and +0.6 V vs. SCE at 10 mV/s while the potential of the collector electrodes was held at +0.4 V vs. SCE. Symmetrical pairs of collector electrodes (33;2,6;and 1,7)were used. The Fe(CN),” coverage was 7.2 X lo-’ mol/cm*. Cf. Scheme I I I a for a sketch of the microelectrode array.
Only the Fe(CN)63-near a given microelectrode is actually detected, though for very slow rates there is the expectation that all of the Fe(CN)6” would be accessible from any one of the microelectrodes. The inability to rapidly access all of the polymer-bound Fe(CN),” using a single microelectrode is a manifestation of the low value of DCT for the polymer-bound Fe(CN),,-l4- system (3-6). In contrast to the conventional redox system illustrated in Figure 2, when an adjacent pair of microelectrodes is connected with a conducting polymer, such as polyaniline (12),all of the polymer can be accessed by addressing only one microelectrode even for sweep rates faster than those illustrated in Figure 2. Experiments described in the following section do establish, though, that the (4-VPyH+J/,Fe(CN)t-), film does provide a current path between the coated microelectrodes. Charge Transport from One Microelectrode to Another via Polymer-Bound Fe(CN),”/+. Charge transport from one microelectrode to another of a (4-VPyH+s1/,Fe(CN)63-),-coated array does occur for coverages >1 X mol/cm2. The evidence for this conclusion comes from electrochemical experiments that are analogous to generation and collection experiments at a rotating ring/disk, as recently shown for poly(viny1ferrocene)-coatedmicroelectrode arrays (17). Figure 3 shows representative data for an array like that illustrated in Scheme IIIa. Electrode 4 of the array is regarded as the “generator” (analogue of the disk) and the pairs 3,5; 2,6; and 1,7 are regarded as “collector” (analogue of the ring) electrodes. The potentials of the collector electrode are held constant at +0.4 V vs. SCE while the potential of the generator electrode is swept, linearly in time, from 0.6 V vs. SCE to 0.0 V vs. SCE. The polymer-bound species is thus initially in the
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987 TEMPERATURE DEPENDENCE OF GENERATION AND COLLECTION AT MICROELECTRODE MODIFIED WITH A F E R R I C Y A N I D E - L O A D E D POLY ( 4 - V I N Y L P Y R I D I N E I F I L M
Scheme 111. Arrangement of (4-VPy),-Coated Microelectrodes Used for Experimentation in This Work"
r 2 0 / 0 Z h j CF3COONa 2 3 rnV/i
1
(a)
1
3
2
4
5
6
7
1
E !
NII__-*xCy-
3
2
4
5
311
7
6
4.6 8
8
5i3N4 .~
'1.3.5.7 "'.E2
PH I8
5'C
17'C
23T
27'C
33-C
38'C
2
8
Si3N4
(C)
Au
1
Platinized
2
3
4
5
6
7
8
S'3N4 Y
"(A) Array of eight Au microelectrodes. (B)Array of Au microelectrodes with increasing amount of platinization. (C)Array of eight microelectrodes with every other one platinized. Fe(CN)63-state. When the generator is capable of reducing Fe(CN)63-to Fe(CN)6dr,charge passes to the collectors via the polymer-bound Fe(CN)63-/4-redox system. Note that the maximum steady-state current at the collectors occurs for a generator potential just negative of the Eo' for the polymerbound Fe(CN)t-/& redox couple, +0.26 V vs. SCE (from the average position of the anodic and cathodic current peaks in Figure 2). Thus, the maximum steady state current occurs when [Fe(CN)63-]= 0 at the generator and [Fe(CN)64-]= 0 a t the collectors, maximum rate of charge transport occurs at the maximum concentration gradient. Additional application of voltage difference by moving the generator more negative does not increase the steady-state current because the concentration gradient does not increase appreciably. The second feature of importance in the data in Figure 3 is that the maximum current at the collectors falls as the collectors are moved geometrically farther from the generator. This result is consistent with a steady-state charge transport rate that is limited by the redox polymer. The expectation is that the steady-state current would be inversely proportional to the distance across which charge must be transported. The final feature of importance in the data presented in Figure 3 is that the hysteresis of the curves (at fixed sweep rate) increases as the distance between generator and collector increases. Similarly a scan rate dependence study of generation and collection at microelectrodes (at fixed separation) shows that the hysteresis of the collector current curves increases as the scan rate increases. For example, for sweep rates greater than 100 mV/s, the scan rate is too fast to achieve a steady-state collector current. The qualitative results conveyed in Figure 3 can be dealt with quantitatively. A redox polymer-coated microelectrode array has recently been demonstrated to be a way of measuring DCTfor the polymer (17,23). Using the treatment recently presented (231, we find a value of DcT = 5 X lo+' cm2/s for charge transport via the Fe(CN)63-/e system bound in the (4VPyH+), at 298 K pH 1.8,0.2 M Na[CF,COO]/CF,COOH. This result is in reasonable agreement with the DCT = 3 X lo4 cmz/s for this system on macroscopic electrodes (6). The charge transport via the polymer-bound Fe(CN)63-/4-is due
to self-exchange between oxidized and reduced sites and physical diffusion of the redox species (6) and is an activated process. Figure 4 illustrates the temperature dependence for the generator/collector experiment showing that the current for the steady-state charge transport increases significantly for higher temperatures. A plot of the logarithm of the limiting collector current vs. 1/T is approximately linear for the range of T shown. The slope of In current vs. 1/T plot gives an Arrhenius activation energy of 50 kJ/mol, higher than the values of -33 kJ/mol reported for other (4-VPyH+),-bound electroactive anions (24). The higher E, value reported in the present study might be due to the higher loading of the 4-VPy film with Fe(CN)63-(electrostatic cross-linking) and the small extent of cross-linking of (4-VPy), by the l,6-dibromohexane. Since the temperature dependence of DCTis not likely to be sensitive to the geometry of the generator and collector, the microelectrode arrays are probably more useful in measuring the temperature dependence of DCT than in DcT itself which is deduced from the geometry-dependent steady-state current. The "open-face" arrangement of the microelectrodes is certainly the best arrangement for measuring the medium dependence of the generator/collector currents, as will be developed below for the transistor configurations. The quantitative aspects of steady-state charge transport between one microelectrode and another for an array as in Scheme IIIa are unexceptional: the mechanism for charge transport is redox conduction via Fe(CN)63-/4-self-exchange and/or physical diffusion of the electroactive species themselves with an effective DCT of - 5 x cm2/s to give very small maximum steady-state generator-collector currents, A. For microelectrode arrays coated with so-called conducting polymers (e.g., poly(3-methylthiophene) (23)) the analogous generator/collector experimentation would show steady-state currents in excess of A, more than 5 orders of magnitude larger than for the conventional redox polymer. Clearly, increased temperature will increase the maximum generator-collector current, Figure 4. Likewise it can be expected that smaller microelectrode spacing should result in larger steady-state currents, based on the data in Figure 3. Deposition of Pt onto Au microelectrodes has been demonstrated to be a way to reduce the microelectrode spacing. Scheme 111, parts b and c, shows two arrangements of platinized Au microelectrodes that have been coated with (4VPyH+.'/3Fe(CN)6s)), to investigate the effect of platinhation on steady-state generator-collector current. For an array like that in Scheme IIIb, the maximum steady-state generator-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987
1431
SOLUTIONS Vg = 100 mV
FLOW RATE
S.Dml/mln
TIME
-
Flgure 8. Change in I, with time for a (+VPyH+),-based tweterminal device placed in contact with various solutions: sdution (A) pH 3,0.065 M KNO,/HNO,/K[C,H,(COOH)(COO)] : solution (B) 5 mM Fe(CN):-’&, pH 9,0.065M KNO,/KOH; solution (C) 5 mM Fe(CN)6”’4; pH 3, 0.065 M KN0,/HN0,/K[C,H4(COOH)(COO)]; solution (D) 5 mM Co(CN),+, pH 3, 0.065 M KN03/HN03/K[C,H,(COOH)(C00)]. Solutions A and B were changed manually while solutions C and D were continuously kept flowing alternately with a HPLC. The Fe(CN),+ coverage was 4 X 10“ mol/cm2. GATE VOLTAGE (VG 1, V
VS
SCE
Figure 5. Drain current, I,, vs. gate voltage, V,, for various draln voltage, V , (20, 50, 100 mV) for a (4-VPyH+.’/,Fe(CN),3-),,-based microelectrochemicaltransistor. I , values are steady-state data obtained after equilibrium was reached (- 1-2 mln). An interdigitated array was used with electrodes 2, 4, 6 and 8 being heavily platinized and Au electrodes 1, 3, 5, and 7 naked (Scheme IIIc). The Fe(CN),% coverage was 1 x io-’ mol/cm2. The eiectrochemicai circuitry used in this experlment Is that of Scheme I.
collector current at platinized Au electrodes (7 and 8) is about 1order of magnitude larger than when none of the electrodes is platinized. Scanning electron microscopy shows the spacing of such a platinized array to be -0.6 pm. The fact that the steady-state current increases more than the expected factor of about 3 (from the initial spacing of 1.7 pm to a spacing of 0.5 pm) signals that the microelectrode array is not an “ideal” geometry, particularly after being platinized. The change in the electrode geometry upon platinization precludes a detailed analysis of the distance dependence. The point is that platinizing the microelectrodes is a useful way to increase steady-state currents, but the effect of platinization is not the equivalent of changing the polymer thickness sandwiched between two planar electrodes. It is worth noting that the distance dependence of the steady-state current from the experiment summarized in Figure 3 is approximately consistent with a (distance)-’ dependence and corresponds to a situation where the geometry of the electrodes is fixed. In practical terms,the low Dm of the polymer-bound Fe(CN)63-/k signals low steady-state rates of charge transport from one microelectrode to another. The data in Figure 3 show that significant increases in currents should be possible for significantly smaller microelectrode spacing. Two Stimulus-Response Microelectrochemical Devices. Figure 5 illustrates the steady-state electrical characteristics of a (4-VPyH+.1/3Fe(CN)63-)n-based microelectrochemical transistor. The microelectrode array used is like that shown in Scheme IIIc where the Au microelectrodes 1, 3, 5, and 7 are regarded as the “source” and the platinized microelectrodes 2,4,6, and 8 are regarded as the “drain” of the transistor. By use of four microelectrodes as source and four microelectrodes as drain, the gate width of the transistor is increased significantly to give a larger maximum value of ID. As discussed in the background section of this paper, the microelectrochemical transistor shows only a narrow region of VG where the device is “on”. Figure 5 shows the ID-VG
characteristic to depend on VD in the sense that the width of the peak is greater for larger VD For VD S 20 mV the width at half-height is -0.1 V with ID(peak) occurring at Eo’ for the polymer-bound Fe(CN)63-/4-.These data are consistent with the narrow potential range where there is significant concentration of both Fe(CN):- and Fe(CN)64-. The small values of V,, < 20 mV, are those that can be used to exploit the intrinsically narrow range of VG where there is a significant value of ID.This does not mean that larger values of VD will not turn on the device. Indeed, the generator/colledor experiments illustrated by data in Figures 3 and 4 are really another way of characterizing the same device. In fact, the generator/collector characterization is one that can be regarded as a characterization where VD and VG are simultaneously varied. Whenever VD exceeds -0.3 V the maximum I D does not increase, because the maximum difference in concentration of the Fe(CN)63-and Fe(CN)64-is no longer influenced by VG. Accordingly, the ID-VG characteristic for the (4-VPyH+.Fe(CN)63-),-based transistor is consistent with expectations for conventional redox conduction. In chemical terms, an interesting aspect of the (4-VPy),coated array is that the transistor can only be turned on when certain chemical criteria are met. To illustrate the concept, several experiments have been done and are summarized by data in Figure 6. First, an array that has been characterized as in Figure 5 was equilibrated in an aqueous electrolyte to deprotonate the polymer and remove the bound Fe(CN)63-. The (CVPy),-coated microelectrode array was then immersed in a 0.065 M KN03/KOH pH 9 solution containing 5 mM Fe(CN)2- and 5 mM Fe(CN)64-. At a potential difference, VD, of 0.1 V, the device was off ( I D = 0) when used as a simple, two-terminal device (electrodes 1, 3, 5, and 7 as source and electrodes 2, 4, 6, and 8 as drain). Without the (4-VPy), coating the microelectrode array used as such a two-terminal device shows ID > 20 nA. The conclusion from this experiment is that the (CVPy), coating effectively blocks the electrodes from the electroactive anions; there are not significant pinholes and the electroactive species do not diffuce through the polymer. In a separate experiment, if the (4-VPy),-coated array is placed in acidic solution, pH 3, 0.065 M KNO,/ HN03/K[C6H4(COOH)(COO)], with no Fe(CN)63-/4-,the device is still off (ID = 0) when used as a simple two-terminal device with VD = 0.1 V (1, 3, 5, 7 as source and 2, 4, 6, 8 as drain). But exposure of the (4-VPy),-coated array to the pH
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987
3 solution with Fe(CN)63-/4-results in turn on, I D > 0, when used as a two-terminal device, VD = 0.1 V. Maintaining the composition of the solution except replacing the Fe(C&%/& with 0.01 M CO(CN)G~results in device turn off, ultimately I D = 0, because C O ( C N ) ~is~not - reversibly electroactive in the same potential regime. Replacing the CO(CN)~% with the Fe(CN)63-/4-results in device turn on. While the response of the device to the exchange of C O ( C N ) ~or~ -Fe(CN):-I4- is not rapid, a steady-state response is achieved in -2 min. Therefore, the concept of the transistor that only turns on when two chemical stimuli are present has been demonstrated. Acid alone or electroactive anion alone is insufficient to turn on the device. When an acid and both halves of an anionic redox couple are present in solution, a two-terminal (4VPy),-coated microelectrode array can be turned on. However, when only either Fe(CN)63-or Fe(CN)64-is present in the solution, even at low pH, the device is off. Since for CO(CN)e3-/*-redox couple only the oxidized half is present in solution, a similar turn off should be expected in these conditions. However this turn off is related instead to the nonelectroactivity of CO(CN)~% as shown by the inset in Figure 6. When the three terminal configuration, Scheme I, is used, there is an additional level of control introduced: the device will turn on only to those anionic redox couples having an Eo’ close (within -0.2 V) of the V,. Thus, in a three-terminal mode redox couples such as Fe(CN)63-/P, Mo(CN)*~-/~-, and Ru(CN)~&/~can be discriminated, for example, because each has a distinct E O ’ . As suggested above, the speed of turn on/turn off of a device like that in Scheme I by alternate exposure to Fe(CN)63-or CC)(CN)~~is controlled by the ion exchange rate. Similar anion exchange of this kind has been previously noted in this laboratory (2.5-28). Essentially complete equilibration requires 2 min exposure to a flowing solution at 5 mM concentration of anions though change in I D begins as soon as the composition of the medium changes. The fact that the rate of the chemistry controls device response is consistent with the time scale for the appearance and disappearance of cyclic voltammetry signals for (4-VPyH+),-bound anions like those illustrated in Figure 1. Further, the speed for electrically driven turn off/turn on by varying V, for the device characterized by data in Figure 6 is on the same time scale of Fe(CN)63-/ CO(CN)6&induced turn on/off. On the other hand, the speed of turn off/turn on by raising and lowering the pH of a solution containing Fe(CN)63-is slow (>30min) and controlled by the rate of equilibration of the state of protonation of the polymer coating with the pH of the ambient solution.
-
CONCLUSIONS A (4-VPy),-coated microelectrode array provides a demonstration cf a chemically responsive microelectrochemical transistor that requires that two chemical criteria be met in order to turn it on. This concept can be easily elaborated, in principle, to produce a variety of new devices. The multistimulus response may prove useful in situations where an array of devices is used in sensor applications. The (4-VPyH+.1/3Fe(CN)63-)n-based transistor is an illustration of microelectrochemical transistor based on a conventional redox material as the active material. The conventional redox material endows the device with a unique ID-VG characteristic, a narrow range of VGwhere ID# 0 with ID(peak)occurring at V, = Eo’of the redox material, for VD Q 20 mV. The specific region of VG where the device is on is again an intrinsic characteristic that may prove useful in sensor applications. However, while the narrow region of V, where the device is turned on is an advantage, the microelectrochemical devices based on conventional redox conduction
have both slow switching speed and small values of ID(peak) in comparison to devices based on so-called conducting polymers. Smaller microelectrodes, more closely spaced are required to make microelectrochemical devices having better electrical characteristics. The intrinsic chemical dependence of the (4-VPy),-coated array is acid-base chemistry and anion exchange. Microelectrodes can be used, albeit not uniquely, to characterize such chemistry. It should be stressed that it is the combination of a microfabricated array and the intrinsic chemical properties of the active materials that allows the demonstration of a set of system functions. The (4-VPy),-coated microelectrodes show functions predictable from the characterization of (4-VPy), on macroscopic electrodes. However, the achievement of such functions involves exploitation of the small dimensions associated with the close spacing of microelectrodes. Chemically responsive transistors will likely never achieve the response time of conventional solid-state transistors (29),but the regime of unique chemical sensor applications will presumably be in situations where the frequency of operation will be many orders of magnitude smaller than for solid-state devices.
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(28) Harrison, D. J.; Daube, K. A,; Wrighton, M. S. J. Nectroanai. Chem. 1984. 163. 93. (29) Sze, S. M. Physics of Semiconductor Devices, 2nd ed.; Wiiey: New York, 1981.
RECEIVED for review October 29, 1986. Accepted February 13, 1987. We thank the Office of Naval Research and the Defense Advanced Research Projects Agency for partial support of this research. Daniel BBlanger wishes to thank le Fonds pour la Formation de Chercheurs et 1’Aide & la Recherche of Quebec for partial support as a postdoctoral fellow, 1986.
