822
Anal. Chem. 1991, 63,622-626
Effect of Electrode Substrate on the Morphology and Selectivity of Overoxidized Polypyrrole Films Allan Witkowski, Michael S. F r e u n d , and Anna Brajter-Toth*
Department of Chemistry, University of Florida, Gainesville, Florida 32611-2046
Polypyrrole (PP) films were electropolymerlred on glassy carbon (GC) and rough pyrolytlc graphlte (RPG) electrodes and treated by overoxidation In sodlum hydroxlde and phosphate buffer. The effects of treatment on fllm morphology were Investigated, Including permselective response of treated films. Fllms grown on GC and RPG were also compared, and It was found that the substrate plays a deflnlte role In the properties of the polymer layer. Dlfferences In response to Ionic probes on treated PP electrodes are shown to arlse from the poroslty of the membranes, which affects the electron density withln the fllm due to the spaclng of carbonyl groups Introduced during treatment.
INTRODUCTION Electropolymerized polypyrrole (PP)films have been widely used in recent years as a means to modify electrode surfaces (1-5). Many factors have been found to influence the behavior of these films, including electrode potential (6), solvent (1, 6, 7), counterion (8, 9), and temperature (10) used during polymerization. The drying temperature after polymerization (11)and the potential applied to the conducting polymer (2) also affect these films. Much of this work has addressed changes in film conductivity, and platinum has been the most widely used electrode substrate. But little information about irreversibly overoxidized PP has been reported (121, probably due to the loss of what many see as its desirable characteristics in this state. Also, no structural comparisons of films grown on carbon surfaces have been published to our knowledge. This study examines several aspects of oxidative PP film treatment, including the loss of electroactivity. In addition, the effects on film morphology of two different carbon electrodes, glassy carbon (GC) and rough pyrolytic graphite (RPG), on which PP films were grown are investigated. The results indicate that the electrode substrate plays a definite role in film morphology, and treatment of PP films can introduce additional unique features. The analytical implications of these observations are also discussed. EXPERIMENTAL SECTION Materials. Sodium hydroxide and acetonitrile (MeCN) were obtained from Fisher. Pyrrole and tetrabutylammonium perchlorate (TBAP) were from Kodak, and ferrocene (Fc) was from Arapahoe. Sodium phosphate monobasic and dibasic, potassium ferricyanide,and ascorbic acid were obtained from Mallinckrodt. Hexaaminoruthenium(II1) chloride was purchased from Alfa Products, methyl viologen dichloride hydrate (MV2+)was from Aldrich, and potassium ferrocyanide trihydrate was obtained from Baker. 3-Hydroxytyramine, or dopamine, was purchased from Sigma. All chemicals were used as received. Aqueous solutions were freshly prepared from doubly distilled, deionized water and were purged with nitrogen for at least 5 min prior to use. Apparatus. All electrochemical measurements were made with a Bioanalytical Systems electrochemical analyzer (BAS-loo),and data were downloaded to an IBM PS/2 Model 50 computer. A conventional three-electrode setup was employed, with a carbon working electrode, platinum flag auxiliary, and a saturated calomel electrode (SCE) or Ag wire as the reference. All potentials are
reported versus SCE unless stated otherwise. Electrode Preparation. The GC electrodes were constructed from 3-mm-diameter GC rods (Electrosynthesis). These electrodes were polished before use with Gama1 y-aluminalwater slurry (Fisher) on a microcloth by using an Ecomet 1 polishing wheel (Beuhler). After polishing, the electrodes were ultrasonicated in distilled water for at least 5 min immediately before use. RPG electrodes were made from rectangular rods of RPG (Pfuer) whose exposed square face had sides of ca. 3 mm. These electrodes were resurfaced prior to use with 600-grit Sic paper (Fisher) on a polishing wheel and rinsed thoroughly with distilled water. Polypyrrole Deposition. PP films were electropolymerized on the carbon electrodes at a constant potential from a solution of 50 mM pyrrole and 0.1 M TBAP in MeCN. Films were grown on GC and RPG substrates at + L O and +0.9 V versus Ag wire quasi-reference, respectively. PP films on these surfaces will be referred to as PP-GC and PP-RPG. Growth of the films was controlled based on the amount of charge passed, with 24 mC/cm2 resulting in a film 0.1 pm thick (1). Electrode areas were determined by chronocoulometry prior to deposition of PP using K,Fe(CN),. An alternative PP film was also prepared where Fe(CN),4- was the electrolyte counterion incorporated into the film. The polymerization solution was 50 mM pyrrole and 0.1 M K4Fe(CN),.3HzO in distilled water. Ferrocyanide-PP films were grown on both GC and RPG at +0.8 V, and the thickness was controlled based on lo00 mC/cm2,yielding a f i b 1.6 pm thick (13). The resulting films are referred to as FeCN-PP-GC and FeCN-PP-RPG. Polypyrrole Film Overoxidation. PP films were treated to reduce the large background charging currents by poising the electrodes at +1.0 V (14). The electrodes were treated in 0.5 M NaOH and pH 7.0,0.5 M phosphate buffer (referred to here as buffer) solutions. The current was monitored at the Same potential during the treatment process. Initially, a very large current was passed, but this rapidly decreased as oxidation progressed. After several minutes, the current leveled off and eventually began to fluctuate slightly around one value. This was taken to indicate complete overoxidation. This process usually took about 5 min in NaOH solutions and 7-10 min in buffer, with final electrolysis currents generally below 10 PA. Rotating Disk Electrode (RDE) Experiments. RDE experiments were conducted by using an IBM RDE controller and rotator. The screw-on electrode tip was made from Teflon, with a GC rod heat pressed into one end. Electrical contact was made with a platinum wire. Similarly, an RPG RDE was made by sealing the RPG rod in the tip with nonconducting epoxy (Epoxi-patch, Dexter Corp.). RESULTS Background Reduction and Loss of Conductivity. Background cyclic voltammograms (CV's) were run in buffer at 0.02 V/s from -0.3 to +0.1 V. Figure 1 shows the results for bare RPG, PP-RPG, and buffer treated PP-RPG electrodes. Although initially low on bare RPG, background currents increase dramatically for PP-RPG. The current is reduced to the background level at RPG by oxidative treatment. Rotating disk voltammetry (RDV) was run on bare GC, PP-GC, and buffer treated PP-GC RDEs in order to confirm the loss of conductivity with treatment. This procedure was similar to that of Mao and Pickup (15),who used RDE's to investigate the electronic conductivity of PP. In their ap-
0 199 1 American Chemical Society 0003-2700/91/0363-0622$02.50/0
6.0
4.0
1
ANALYTICAL CHEMISTRY, VOL. 63,NO. 6, MARCH 15, 1991
* I . . . . . . . . . . . .I . . . , . ,
Table I. AEpfor Fe(CN)," and Ru(NHS)t+on Different Electrodes
**
probe"
surface
Fe(CNL3. ."
I/)
E
-s
Ru(NH3)63+
-0.0
-2.0 2 . 0 ~
..
***.....
Background response of different electrodes in 0.5 M phosphate buffer (pH 7.0). u = 0.02 VIS ((---) bare RPG, ( * ) PP-RPG, (-) treated PP-RPG). Figure 1.
-50.0
A v)
RPG GC RPG GC treated PP-RPG
U
P
64 88
64 64 64
aProbes were 1 mM in pH 7.0 phosphate buffer.
C 0 -4.0 L
0.0 50.0
623
I
......
UU
**
a
E - 100.0
%
--150.0
+ C
p -200.0
L
L
3 0 -250.0
-300.0
-350.0 -400
-200
0
200
400
600
potential (mv) Figure 2. Rotating disk voltammograms of 2.0 mM Fc and 0.1 M TBAP in MeCN at RDE's versus Ag wire. w = 1000 rpm; u = 0.01 V/s ((-) bare GC, (---) PP-GC, ( * ) treated PP-GC).
proach, large solution redox species served as the electron source at the polymer/solution interface, while the conducting polymer shuttled electrons to the electrode surface. Rotation rates were chosen to prevent diffusion of the probe in solution from limiting the response. By varying the applied potential, changes in film conductivity could be observed. Figure 2 displays the results obtained for RDV at different electrodes. While conductive PP-GC gives essentially the same response to Fc as the bare GC electrode, the observed current is drastically reduced after treatment. Similar conductivity behavior is seen when RPG is used as the electrode substrate.
Cyclic Voltammetry of Fe(CN)l- and Ru(NH,),~+at Bare and Polymer-Coated Electrodes. CV of Fe(CN)63and Ru(NH3)2+was run a t different electrodes to determine the effect of the electrode substrate on probe kinetics. CV's of 1 mM Fe(cN)63-in buffer were obtained a t bare GC and RPG electrodes. A buffer-treated PP-RPG surface was also used to record CV's of 1 mM Ru(NH3),,+ in buffer. The potential window used for Fe(CN),,- was 0.0-0.4 V, while Ru(NH&~+was run from -0.1 to -0.5 V. The scan rate was 0.02 V/s for both probes. After correction for buffer backgrounds, the values of hE, for the probes were measured from the CV's. These results are shown in Table I. The values of Upfor Fe(CN),,- on RPG and GC are significantly dif-
ferent. Apparent rate constants (kapp)at these electrodes were calculated by using Nicholson's method (16),yielding rates of 2 x cm/s for Fe(CN),3- on bare RPG and and 4 X GC, respectively. Ru(NH,),~+displays the same hEpa t both RPG and GC as Fe(CN)l- has on RPG, indicating it also has relatively fast kinetics. The fact that the lowest hEpwe could observe was 64 mV and this was the same for Ru(NH,),~+a t RPG and GC indicates that the error in our values is about 5 mV.
Determination of Apparent Diffusion Coefficients. The rate of diffusion of redox species through a polymer film is determined by the probe's membrane partition coefficient ( a )and its molecular diffusion coefficient (D,) within the fii. Therefore, an apparent diffusion coefficient (aD,) is generally measured. In order to determine the values of CUD,for different probes in PP films, RDE experiments were conducted. The probes used were MV2+, Ru(NH&~+,and Fe(CN)63-. MV2+ was employed because its reduction potential is sufficiently negative to ensure that untreated PP films are in the totally reduced, nonconductive state (1, 2, 15). Thus, differences in aDm for MV2+on the PP electrodes are due to structural properties of the treated films and not conductivity. Ru(NH&~+and Fe(CN),,- were chosen because they have similar size and fast kinetics on GC and RPG but are oppositely charged. These ions were used to investigate the interactions between charged probes and treated PP films. The limiting currents at different rotation rates were used to calculate CUD,based on the method of Gough and Leypoldt (17). For a membrane-covered RDE, the membrane diffusion limited current, id, is given by
id = nFAP,Cb
(1)
where A is the electrode area, P, is the permeability of the membrane, and Cb is the bulk concentration of the probe. Furthermore,
P m = aDm/L = D a p p / L
(2)
where 6, is the membrane thickness. Thus, by substituting eq 2 into eq 1 and rearranging, one obtains
C~D,= iddm/nFACb
(3)
With RDE, at infinite rotation rates, the effect of diffusion of the probe in solution becomes negligible, and id depends simply on diffusion through the film. So the intercept of a plot of id-' versus w-1/2 (Koutecky-Levich plot) gives id-' a t the film diffusion limit. The reciprocal of this intercept gives the value of id to use in eq 3. For the probes used here, n = 1,F = 96484 C/mol, 6 , = 0.1 Km, Cb = 5 mM, A is in cm2, and id is in A. Thus, aD,'s calculated from eq 3 have units of cmz/s. Table 11 lists the values of CUD,obtained for the probes at different electrodes. The precision of the aD, values is limited to f20% by the uncertainty in film thickness (18). It should be noted that when the fresh PP-GC RDE was placed in the Fe(CN):- solution and RDV was run, no response was observed. A CV with this electrode in the same solution did not show any current over background. This electrode was then
624
ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
100.0
Table 11. Apparent Diffusion Coefficients of MVZt, Ru(NHg):+, and Fe(CN),'- at Different Electrodes
surface
treatment
PP-GC PP-GC PP-GC PP-GC PP-GC PP-RPG PP-RPG
none buffer
probe"
aD,, X10" cm2/s
MVZ+ MV2+ MVZ+
NaOH buffer buffer
Ru(NH,),~+ Fe(CN):RU(NH,),~+ Fe(CN):-
buffer buffer
n
3.2 2.3 2.5 2.8 5.2b 33 5.8
"All probes were 5 mM in pH 7.0 phosphate buffer.
(I)
a
E
T
0.0
.. ....'* -
L
3
0
'-
-150.0
\
E"
-%
1 -400
0
-200
200
400
600
potential (mv)
\
Flgure 4. Cyclic voltammograms of different electrodes in pH 7.0 phosphate buffer. Y = 0.02 V/s ((0untreated FeCN-PP-RPG, (-) buffer treated FeCN-PP-RPG, (- -) buffer-treated FeCN-PP-GC).
80.0
-
40.0
c
r
. .
1
-100.0
n
.- _ -
....
+ bR~-
.-
:
W
(NH3)ztmediated; see text.
120.0
3
0.0
3 0
-40.0
-80.0
1
can be seen, most of the Fe(CN):- is retained after buffer treatment of PP f i i s as the background currents are reduced. There is a definite difference between the response of treated FeCN-PP-RPG and treated FeCN-PP-GC electrodes. When NaOH was used to treat the electrodes, complete expulsion of Fe(CN):- was seen with FeCN-PP-RPG, while only slight retention of the anion occurred in FeCN-PP-GC.
1
'9:.
!
& , -600 -800
-400
-200
0
200
400
potential (mv) Figure 3. Rotating disk voltammograms of pH 7.0 phosphate buffer solutions containing 5 mM Fe(cN)63-cf Ru(NH,)z+ at R E ' S . w = 1000 rpm, v = 0.01 VIS. Fe(CN)t- on treated PP-RPG (-) and treated PP-GC (Ru(NH,)t' mediated; see text) (---). Ru(NH,),,+ on treated PP-RPG ( * ) and treated PP-GC (A).
placed in the R u ( N H ~ ) ~ solution, '+ and RDE data were collected to determine CUD,. When the electrode was returned to the Fe(CN),3- solution, it gave a response in the Ru(NH3)63+ potential window. This apparent mediation of Fe(CN)63-by Ru(NH3)i3+is shown in Figure 3. Since response to Fe(CN)63was now seen on the R u ( N H ~ ) impregnated ~~+ PP-GC RDE, the anion's uDm could then be obtained. A CV of this electrode in buffer in the range of -0.1to -0.5 V gave no response, indicating that very little Ru(NH3):+ was retained in the film. As shown in Table 11, the CUD,for MV2+ decreases slightly with treatment, but this appears to be independent of the type of solution used during overoxidation. The CUD,for Ru(NH3)63+ on treated PP-RPG is over an order of magnitude greater than on treated PP-GC, and although initially excluded from treated PP-GC, Fe(CN)63-has about the same CUD,on treated PP-RPG and R u ( N H ~ ) mediated ~~+ treated PP-GC. The ND,'s for R u ( N H ~ ) and ~ ~ +Fe(CN)63-on treated PP-RPG show that even if anions can penetrate the film, their ability to move through the membrane is much lower than that of cations. FeCN-PP Film Response. PP films were polymerized in the presence of Fe(CN)64-to incorporate this species as the counterion. Although incorporation of Fe(CN):- into PP films has been previously reported (13,19,20),none of these studies examined the effect of electrolyte on retention of Fe(CN)64during overoxidation. In order to investigate this aspect of film treatment, FeCN-PP films were deposited on both GC and RPG, and the electrodes were overoxidized in buffer and NaOH. The results of cyclic voltammetry using untreated and buffer treated FeCN-PP films are shown in Figure 4. As
DISCUSSION Although much work has been done on PP films, considerably less information about overoxidized PP has been reported (12). It has been observed that film electroactivity decreases after exposure to high potentials, and this has been attributed to loss of conductivity (6) and/or degradation of the film (5,11,21). It can be seen in Figure 1that the typicdy high background charging currents on PP films (18) are returned to the level of bare electrodes after overoxidation. We attributed this to a loss in conductivity, which was confirmed by the RDE data for Fc at a GC RDE. Figure 2 shows that untreated PP-GC gives a limiting current essentially equal to that for the bare electrode. This occurs because the PP film is conducting and electron transfer to the rather large Fc molecules takes place mainly at the polymer/solution interface. However, the current after treatment is drastically reduced and represents only the contribution of Fc reduction a t the GC substrate after diffusion through the membrane. Thus, the film is no longer electronically conductive after overoxidation. However, no drastic changes occur in the film structure during treatment. This can be seen by comparing the aD, for MV2+on untreated and treated PP-GC (Table 11). If the films were extensively damaged by overoxidation, the aD, should greatly increase and even approach the solution diffusion coefficients after treatment. But the CUD,actually decreases slightly, which would result from slight changes in the structure of the film. The major effect of overoxidation is the introduction of carbonyl groups (12),whose electron density can cause increased interaction with the I"-?+dication and slow down its movement through the film. The correlation between loss of electroactivity and carbonyl group introduction was also reported by Ferreira et al. (7), and an increase in anionic sites has been shown to reduce the conductivity of PP films (8, 9). These results also indicate that film morphology can be controlled by the choice of substrate. Comparing the behavior of Ru(NH3),3+at buffer-treated electrodes (Table 11),the aD,
ANALYTICAL CHEMISTRY, VOL. 63, NO. 6 , MARCH 15, 1991
on PP-RPG was more than an order of magnitude larger than on PP-GC. Similarly, PP-RPG electrodes do respond to Fe(CNI6>,while these anions are excluded from PP-GC films. These differences in response can be attributed to the porosity of the two films. Previous work in our group (22) has shown that GC and RPG have greatly different surface activities, with nucleation and growth occurring much more rapidly on RPG. This was also seen here during electropolymerization, since films on RPG electrodes had to be grown at a lower potential in order to prepare thin films in a controlled manner. Even so, polymerization on RPG at +0.9 V versus Ag still proceeded faster than on GC. The rate of growth can then account for the porosity differences, since the slower growing PP-GC films can pack better and become more dense, resulting in narrower pores. A similar explanation was put forth by Carlin et al. (23) in reference to copolymers of vinylferrocene and (vinylcyclopentadieny1)manganesetricarbonyl, and the relationship between porosity and transport rates has been established by several groups (24-27). Leddy and Vanderborgh have shown that diffusion through even relatively small pores occurs very rapidly (26). Thus, it is not the size of the pores alone that causes the difference in response of these substrates. Instead, the narrower GC-based pores have more tightly packed carbonyl groups, resulting in higher electron density in these pores. As the electron density increases, cationic probes interact more strongly and their diffusion is retarded, while anionic species are repelled and become unable to enter the film. Further evidence for porosity differences induced by the underlying electrodes was obtained by incorporation of Fe(CN);- into PP films. Figure 4 shows the response of FeCN-PP-RPG and FeCN-PP-GC films. Buffer treatment of FeCN-PP-RPG results in a reduction of background current with no significant change in peak shape. Although the peak potentials have shifted slightly, movement of ions in the film occurs relatively easily and peak tailing due to thin-layer behavior is seen. The shift toward more positive peak potentials, indicating an increase in difficulty of oxidation, may result from increased interactions after treatment. However, the peak shape for FeCN-PP-GC films shows that in this case transport is significantly hampered by the narrower pores and, therefore, diffusional tailing characteristic of a slower process is observed. The values for Upobtained from CV (Table I) reveal several interesting points. Ru(NH3)2+displays the same AE, a t both RPG and GC as Fe(CN):- has on RPG, but the AE, for Fe(CN)63-shows that GC is a slower surface. However, R U ( N H & ~ +is so fast that the difference in surface activity is not seen within the resolution of our experiment. Since these ions have similar diffusion coefficients (7.2 X lo4 cm2/s for Fe(CN)63-and 5.5 X lo4 cm2/s for Ru(NH3)63+)(28),the standard rate constant ( k , ) for R u ( N H ~ ) must ~ ~ + be significantly greater than that of Fe(CN)63-on RPG. Since Ru(NH3)63+has the same hEpon RPG, GC, and buffer-treated PP-RPG, the type of substrate electrode or presence of PP films does not noticeably affect its kinetics. Therefore, the 12-fold difference in CUD,for R u ( N H ~ ) on ~ ~PP-RPG + and PP-GC must be due solely to variations in morphology of the films, because the kinetics of R u ( N H ~ )at ~ ~the + underlying GC and RPG are the same. The exclusion of anions from PP films is also dependent on the electrode substrate, as seen from the RDE experiments. Initially Fe(cN)63-was excluded from PP-GC films, but a response was observed after the aD, of R u ( N H ~ ) ~was ~+ measured. Since PP-GC films are less porous, the spacing of carbonyl groups in these films leads to the observed anion exclusion. It is interesting to note that the aD, for Fe(CN)ton PP-RPG agrees rather nicely with that for Nafion films
625
(291, which are known to have numerous electronegative SOY sites. However, Fe(CN)63-was able to penetrate the PP-GC membrane after the film was used in Ru(NH3)P. Ru(NH3)2+ must have been incorporated during the RDE experiment, possibly via electrostatic interactions with the carbonyl groups. During CV of treated carbon-fiber microcylinder electrodes, Kovach et al. (28) observed significant adsorption of Ru(NH3)63+on the electrode surface, resulting in voltammetric waves due to R u ( N H ~ )for ~ ~over + 15 min after transfer to a buffer solution. But CV's of treated PP-GC electrodes in buffer showed no response after R u ( N H ~ ) was ~ ~ +analyzed, indicating that R u ( N H ~ ) is ~~ not + adsorbed on the underlying GC electrode and the amount of R u ( N H ~ ) retained ~ ~ + in the film must be small. Still, these cations can now partially shield the electron density of the treating groups from incoming anions, allowing the Fe(CN)63-to enter the film. This shielding effect is conceptually similar to that observed by Miller and Majda (30) with octanol and octadecyltrichlorosilane/Nmethyl-N'-octadecyl-4,4'-bipyridiniumassemblies. Experiencing less repulsion, Fe(CN)63-can diffuse toward the GC surface and its aD, in PP-GC now approaches that for PPRPG. Since Ru(NH3),3+has much faster kinetics at GC and moves more readily in the film than Fe(CN)63-,the former is reduced at the electrode surface and then mediates the reduction of Fe(CN):-. In the mediation process, limitations in the rates of charge propagation by Fe(CN)63-/4-or cross reaction between Fe(CN):- and Ru(NH3)2+may occur. Rates of cross exchange between the probes could also account for the displacement in the mediated current versus potential curve (Figure 3), which appears to be a larger background current. Anion exclusion (14) at buffer-treated PP-GC films was also seen using ascorbic acid as a probe. Although no response for a 1 mM solution of ascorbic acid in buffer was obtained at treated PP-GC electrodes, a normal CV was observed for the same solution a t a treated PP-RPG film. Both films responded well to dopamine at and below 1 mM. The magnitude of the values for aDm also indicates that treated PP electrode response is limited by diffusion of the probe through the film rather than partitioning. Both Fe(CN)$- and Ru(NH3),3+exhibit fast electrode kinetics on RPG and GC, and their solution diffusion coefficients are on the order of cm2/s, so neither of these processes limits the aD,'s we obtained. If partitioning of the probe from the solution to membrane phases was the slowest step, our measured aD,'s should have been around lo-" cm2/s (31). Since the aD,'s obtained were about cm2/s, partitioning does not play a major role and response is limited by probe diffusion. The possibility of ultramicroelectrode array (UMEA) behavior of these electrodes was also examined. We proposed that the CV shapes and currents for Ru(NH3),3+a t buffertreated PP-RPG and PP-GC electrodes differed because PP-RPG was more porous and resembled an UMEA. Similarly shaped voltammograms for an UMEA were reported by Armstrong et al. (32). With this model, the PP-GC electrodes were still experiencing radial diffusion, while linear diffusion occurred at PP-RPG on the time scale of the CV's. This was tested by calculating the amount of time required for the diffusion layer to extend beyond the film surface. The diffusion layer began to grow 28.5 mV negative of the peak potential (33) and continued until the switching potential. This difference in potential was converted to time based on the scan rate, resulting in 8.95 s for RPG and 5.75 s for GC. Since the diffusion layer thickness (6) is given by
6 = (Dt)'/2 (4) the time for the diffusion layer to reach the edge of the PP film will be
626
ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
t = 6*/aD, = 1
X
10-’O/aD,
(5) Using the values for CUD,of R u ( N H & ~ +on PP-RPG and PP-GC in eq 5 yields times of 3.03 X and 3.51 X s, respectively. Thus, the diffusion layers for both PP-RPG and PP-GC were well into the solution by the time the switching potential was reached. Therefore, the concept of an UMEA definitely cannot be used to explain the behavior of these electrodes. Treatment in NaOH and in buffer seems to proceed a t different rates, a fact that can be taken advantage of. In NaOH solution, overoxidation is faster than in buffer. Also, in FeCN-PP films treated with NaOH, Fe(CN),4- was completely expelled from RPG-based films and only partial retention of the anion was observed with the GC substrate. This is probably due to the fact that NaOH diffuses readily into the films based on its size and the concentration gradient, and in the process, the smaller anion replaces Fe(CN)64-. The relative efficiency of Fe(CN)2- displacement on PP-RPG and PP-GC further reflects their difference in porosity. Once OHenters the film, treatment occurs via nucleophilic attack of the base on the pyrrole subunits (12). With buffer treatment, the phosphate ions don’t diffuse into the film as readily and there is less driving force for them to displace Fe(CN)64-. Thus, buffer-treated films retain Fe(CNIG4-,at a level greater than 70% based on coulombic calculations, and treatment occurs by nucleophilic attack of HzO. With either treatment, the hydroxyl groups are then oxidized to carbonyls by continued application of the high potential (12). Since the aD,k obtained for MV2+ were identical regardless of the method of treatment, the final structures appear to be basically the same. NaOH solutions do allow faster overoxidation, but if one is attempting to incorporate a specific anion, buffer should be used. Several alternative explanations for the observed results have also been examined. It is possible to attribute variations in CUD,to changes in film thickness between electrodes. Since film growth was monitored by the total charge passed and not simply a fixed amount of time, film thicknesses should be about the same. Also, the difference in CUD,for Ru(NH3):+ would require PP-GC films to be 12 times thicker (eq 3) than PP-RPG if due to thickness alone, which is highly unlikely given the charges used in polymerization. Film thickness would not account for the anion exclusion on PP-GC, and the denser GC films would be thinner, not thicker, resulting in a higher CUD,if due to thickness. The values of CUD,of Fe(CN)63-in native PP-RPG and R~(NH&~+-mediated PP-GC are nearly equivalent, which implies that film thickness cannot differ drastically. It may also be considered that the lack of anion exclusion on PP-RPG films is due to the presence of pinholes. First, the films are relatively thick as viewed by SEM. Also, the presence of pinholes would result in much higher values of CUD,on PP-RPG electrodes, as diffusion would then be more like that in solution. This was not the case. And thicker RPG films, on the order of 1-5 km, still exhibit the same type of response. Finally, since the RPG films were grown at a different potential than that used for GC, the structural differences may have been thought to arise from polymerization potential and not substrate. However, PP films were also grown on RPG at 1.0 V, and the resulting films were slightly thicker but gave the same type of response as seen with the films prepared a t lower potentials.
to cause any gross changes in PP morphology; however, carbonyl groups are incorporated into the f i i during the process (12). Also, the choice of RPG or GC substrate determines whether the film has a porous or more compact morphology, which in turn affects the response to probe molecules. In the case of PP-GC, the dense films lead to virtual anion exclusion due to the closer spacing of electron-rich carbonyl groups, as seen in the values of CUD,obtained for several probes. This lack of response to anions was also observed when ascorbic acid was tested on PP-GC electrodes. But this effect can be lessened by incorporation of a cation, which occurred in the mediation of Fe(CN):- by Ru(NH3):+ on treated PP-GC. In addition, electroactive anions can be readily incorporated into PP films, and buffer treatment reduces background current without loss of the ion. Although faster, treating with NaOH leads to displacement of the counterion. Thus, coating carbon substrates with PP films results in durable electrodes that do not require resurfacing between samples, and response of the films can be tailored by choice of substrate and treatment. Elimination of the high background currents typical of PP electrodes will also enhance the analytical usefulness of these surfaces, making lower detection limits attainable while rejecting many interferences.
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CONCLUSIONS These results show that polypyrrole can be used to effectively coat carbon electrodes. By overoxidizing the film a t +1.0 V, the conductivity of the film is lost, resulting in a reduction of the background currents associated with charging of conducting polypyrrole. This treatment does not appear
RECEIVED for review August 6,1990. Accepted November 16, 1990. This work was supported, in part, by a grant from the National Institutes of Health (Grant No. GM35341-03A2). A.W. gratefully acknowledges the support of a National Science Foundation Graduate Fellowship.
