2388
Anal. Chem. 1985, 57,2388-2393
be regarded as very preliminary, they do indicate that intermolecular forces may be the limiting factor in ion formation for high molecular weight polymers. The effect of different solvents used for sample preparations on fragmentation and cationization was studied for N.66. An effective solvent for nylons must disrupt the extensive hydrogen bonding which occurs between the amide H and the carbonyl 0 atoms of adjacent chains (dissociation energy cz 8 kcal/mol). Nylon 66 samples were dissolved in trifluoroacetic acid and 10% formic acid. They were prepared as described in the Experimental Section and were run under similar instrumental conditions. The silver ions showed similar intensities (f3%) for both samples. The intensity of the peak for Na+ for the sample using formic acid is approximately equal to that of Ag+ but it is 35% lower than that for the trifluoroacetic acid sample. Fragments smaller than the repeat unit and C,H,Ag+ clusters are virtually the same for both samples. Silver cationized repeat units were recorded for n = 1-9 for trifluoroacetic acid and n = 1-6 for formic acid. The intensities were higher by factors 2-66 and shifted toward higher m / z for trifluoroacetic acid even though the availability of Ag+ for cationization was about the same for both samples. For both samples cationization with Na+ produced clusters for n = 1-8 with intensities higher for trifluoroacetic acid up to a factor of 2. It appears that sample preparation has a significant effect on fragmentation and on cationization. In this case trifluoroacetic acid appears to be the better solvent. In summary, this paper presents the first report of high mass fragments detected for nylons by mass spectrometry. Because the side group of nylons remains intact, it should be possible using TOF-SIMS to determine exactly where the amide bond breaks. The fact that the side chain remains intact on the backbone can be beneficial for characterization of other polymers containing pendant groups, for example,
graft copolymers. Such studies are currently under way in our laboratory, in addition to studies to further evaluate use of TOF-SIMS for characterization of a wide variety of polymers.
ACKNOWLEDGMENT We wish to thank J. H. Magill for providing the nylon 66(ru6)sample. LITERATURE CITED Luderwald, I.; Merz, F.; Rothe, M. Angew. Makromol. Chem. 1978, 6 7 , 193. Luderwald, I.;Merz, F. Angew. Makromol. Chem. 1978, 7 4 , 165. Adams, R. E. Anal. Chem. 1983, 55, 414. Gardella, J. A.; Graham, S. W.; Hercules, D. M. In "Polymer Characterlzation"; Craver, C. D., Ed.; ACS: Washington, DC, 1983; Advances in Chemistry Series 203, pp 635-876. Briggs, D. S I A , Surf. Interface Anal. 1982, 4 , 151. Bahr, U.; Luderwaid, I.; Muller, R.; Schulten, H A . Angew. Makromol. Chem. 1984, 120, 163. Steffens, P.; Nlehuis, E.; Friese, T.; Benninghoven, A. I n "Ion Formation from Organic Solids"; Benninghoven, A., Ed.; Springer-Verlag: Berlln, 1983; Vol. 25, pp 11 1-1 17. Poschenrleder, W. P. Int. J . Mass Spectrom. Ion Phys. 1972, 9 , 357. Rollgen, F. W.; Giessman, U.; Schulten, H.-R. In "Advances of Mass Spectrometry"; Daly, N. R., Ed.; Heyden: London, 1978; Vol. 78, pp 14 19-1424. Goodman, I . J . Polym. Scl. 1955, 17, 587. Strauss, S.; Wall, L. A. J . Res. Natl. Bur. Stand. ( U . S . ) 1958, 6 0 , 39. Kamerbeek, 8.; Kroes, G. H.; Grolle, W. In "Thermal Degradation of Polymers"; S.C.I.: London, 1981; Monograph No. 13, p 357. Gardella, J. A.; Hercules, D. M. Anal. Chem. 1980, 5 2 , 226.
RECEIVED for review March 28,1985. Accepted June 11,1985. This work was supported by the National Science Foundation under Grant CHE8411835 and by the Deutsche Forschungsgemeinschaft. We are particularly grateful to the Alexander Von Humboldt Foundation for providing a senior fellowship for D.M.H. which stimulated this work.
Filar Electrodes: Steady-State Currents and Spectroelectrochemistry at Twin I nterdigitated Electrodes Douglas G. Sanderson and Larry B. Anderson* Department of Chemistry, T h e Ohio State University, Columbus, Ohio 43210
A method Is descrlbed for generatlng steady-state currents between two closely spaced, coplanar electrodes and slmultaneously measuring the absorbance of the electroactlve specles. The design, conslsllng of two lnterdlgltated electrodes, Is constructed by etchlng a gold film, vapor deposlted on a quartz window. Semlemplrlcal equatlons are presented whlch quantltatlvely explain data from steady-state electrochemical and spectroelectrochemlcal experlments on model redox couples. The electrodes used have a spacing of 100 pm between anode and cathode and reach virtual steady state In 1 mln. Steady-state electroanalysls Is performed on 70-100 fiL of solutlon at 0.1 mM concentratlons.
Steady-state methods of electroanalysis can provide improved signal-to-noise behavior because, after application of an electrical excitation, sufficient time is allowed for charging
and background currents to reach low values before measuring the electroanalytical response. If the steady-state current is achieved between closely spaced twin electrodes, an additional improvement in the signal to noise ratio is obtained because one of the two electrodes can be maintained at a fixed potential while the other is scanned to produce the steady-state signal. Two methods that have been suggested for accomplishing twin-electrode steady-state electrochemistry, rotating ring-disk (1) and twin-electrode thin layer (2, 3), illustrate some of the difficulties of achieving the potential improvements of steady-state analysis. The rotating ring-disk greatly enhances the analytical current by forced convection of the analyte in a hydrodynamically stable manner. However, the apparatus employed is complex and relatively expensive. Furthermore, the limiting noise, due principally to mechanical vibration, is commonly greater than 0.5% and collection efficiencies for electrochemical products at the ring electrode are less than 50% ( I ) .
0003-2700/S5/0357-2388$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985 B*
A
1
F*
2389
I
Flgure 2. Exploded view of the thin-layer cell employing the optically transparent. filar electrode: BW. back window: E, filar electrode, F W front window; GL. gold leaf contact: I, injection port: T, Teflon spacer.
BACK WINDOW
I
0
I
I
12
6
I
18
I
W
-X
Flgure I. Twin filar electrode: (A) top view of electrode design and (8)sdremanc steady-state proflle In me X-z plane. Solid lhes lndkate lines of wnstant flux as a percentage ofmaximum flux at steady state.
In thin-layer methods ( 2 , 4 , 5 ) ,very difficult mechanical problems must be overcome in preparing planar, parallel surfaces separated by a thin layer of analyte solution of several micrometers (6). In addition, a combination of edge effects and iR drop across the face of the electrodes inherently limits the transient response of the current measurement., We propose an alternative approach to steady-state electrolysis at stationary solid electrodes. Two coplanar, interdigitated gold electrodes, called "filar" electrodes here because of their threadlike geometry (see Figure la), are etched in a gold film vapor-deposited on an optically flat insulating substrate hy standard photolithographic methods. This coplanar design enables the potential of each electrode to be controlled independently. Thus, an electroactive species generated by a potential excitation at one electrode diffuses across the thin-layer gap and reacts electrochemically at the opposite electrode. The current response generated by this reaction is independently measured as a function of the potential imposed at the excitation electrode. Figure l h is a cross-sectional profile of the filar electrode in a thin-layer cell showing the steady-state flux of electroactive species hetween the two closely spaced filaments (cathode and anode). Separations of 20 pm between electrodes have been achieved and much narrower spacings can he anticipated. At 20 pm the time constant for transport of a molecule or ion from anode to cathode is on the order of 1 s and a steady-state current is achieved after several seconds. This paper presents a description of wplanar, closely spaced fh electrodes, their fabrication, and the steady-state current behavior of typical redox couples. The electrode is constructed of gold vapor-deposited on quartz blanks and may also he used as an optically transparent electrode to monitor the products of electrode reactions by UV-vis spectrometry.
EXPERIMENTAL SECTION Electrode Fabrication. By w e of standard vapor-deposition techniques, quartz optical flats (3.0 X 1.7 X 0.3 cm) were coated in vacuo with a 200-400 A layer of chromium followed by a 1000-2000 A layer of gold. Adhesion of the metallic layers was
improved by subsequent annealing in a muffle furnace at 3CC-400 "C for 1-2 h. Figure l a illustrates the geometry of the filar electrode used. In this work, the overall electrode dimensions were 0.5 cm X 0.5 cm (not counting the contact pads which were covered with an insulating layer during active electrolysis). Each filament was 0.5 cm long and 50 pm wide, separated from the adjacent fdaments by a gap of 50 pm on each side. This gap, which consists of the surface of the underlying quartz plate is optically transparent and can be utilized as a window to observe the ahsorhance of the solution separating the anode and cathode. The pattern shown in Figure l a was etched onto the gold surface by standard photolithographic techniques (7). A thin layer of Kodak (KPR) photoresist was spin-coated on the gold surface and the pattern was transferred by contact exposure to near-UV light through a negative photographic mask. The image was developed in Kodak KPR developer and washed with dry acetone. After drying, the coated quartz plate was etched from 10 to 60 s in a 4 1 dilution of a standard iodine etch (I00 g of I,, 400 g of KI, and 400 mL of distilled water) (8). Etching was halted when observation of the pattern under a microscope indicated complete separation of the electrode filaments. The residual photoresist was hurned off by a second heating at 3 W 4 0 0 "C. On properly constructed filar electrodes,the resistance was measured, in air, to be greater than 100 MO between the two interleaved electrodes. Figure 2 shows a schematic diagram of the filar electrode incorporated into a Beckman Model UV-10 spectrophotometric cell. This cell confines a thin film of solution between a standard front quartz window and a quartz flat imprinted with the gold filar design. The outside of the electrode window was partially masked with black enamel paint to allow light to pass only through the filament-covered region where active electrolysis occurs. A IO-pm Teflon spacer separated the two windows. Independent electrical contacts were made to the two electrode halves by placing a strip of gold leaf onto each electrode pad prior to assembly. When the spacer and top window were clamped into place with the quick-tightened screws, intimate contact between the gold leaf and gold electrode pads was assured. The secured Teflon spacer prevented the cell liquid from wicking out over the gold pads and leaf contact. The cell was filled and washed without disassembly by injection of solutions through predrilled holes in the cell frame and front window. Instrumentation and Procedure. Electrochemical experiments were performed with a Bioanalytical Systems (BAS) Model CV-1B potentiostat to control the potential and measure the current at the fixed potential electrode (FPE). An Ag/AgCl electrode was used as the reference electrode ( E ' ' k , ~ l =0.199 V vs. NHE, [CT] = 3 M). A potential WBS imposed on the second or stepped potential electrode (SPE) hy using a hattery and potentiometer as described previously (2). Solution contact with auxiliary and reference electrodes was made to the two working electrodes at the solution injection port shown in Figure 2. Single-electrodespectroelectrochemistry,analogous to minigrid experiments (91, was accomplished by connecting both the FPE and SPE to the single working electrode input of the BAS potentiostat. When a potential step was applied, the thin layer of solution was electrolyzed until the oxidized to reduced species ratio was in equilibrium with the applied potential. Spectrophotometric measurements (see below) then gave quantitative information on the thin-layer contents. Twin-electrode spectroelectrochemistry was accomplished by applying a potential difference between the two independent Nar electrodes (see above). Absorbance measurements of the elec-
2890
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
6OOrnV
450mV
0
W X-
Y
.-Lr
Figure 4. Schematic diffusion profile at limiting steady-state between closely spaced electrodes.
25mV
0
E
16
32
24 Time
40
48
56
(mid
Figure 3. Steady-state currents produced by potential step excitation of a twin, filar electrode ( W = 100 pm) containing 0.50 mM BHMF In 10 mM phosphate buffer (pH 7.0).
troactive species in the diffusion layer between the electrode filaments were directly related to the concentration of oxidized and reduced species at the electrode surfaces (see Results and Discussion). All spectrophotometric measurements were made with an Aminco Model DW-2a spectrophotometer used in the single-beam, dual-wavelength mode. The cell was mounted in the light path at the exit port of the sample compartment. The collimated light beam illuminated virtually the entire filar electrode and was masked elsewhere as described above. Spectra of dichlorophenolindophenol (DCPIP) were recorded by fixing the reference wavelength at 500 nm where the oxidized and reduced species show little or no absorbance. After application of a potential excitation to the electrode(s),sufficient time was allowed for the current to reach steady state before the spectra were recorded. Reagents. 1,l’-Bis(hydroxymethy1)ferrocene (BHMF) was obtained through the generosity of T. Kuwana and was used without further purification. Dichlorophenolindophenol(DCPIP) was purchased from Sigma Chemical Co. Other chemicals were of reagent grade and used without further purification. All solutions were prepared from doubly distilled water and buffered at pH 7.0 using 10 mM phosphate buffer and sufficient NaCl to bring the ionic strength to approximately 0.11 M.
RESULTS AND DISCUSSION Semiempirical Analysis of Steady-State Currents. The steady-state current, i,, is defied BS the time invariant current produced when the two filar electrodes in Figure l b are held at different potentials. The electrochemical oxidation product of potential excitation at one filar electrode (the SPE) diffuses across the space between the filaments and is reduced at the opposing electrode (the FPE) where the resulting current is measured. If both product and reactant are soluble in the solution phase and chemically stable, the flux at each filar electrode will asymptotically approach a true steady-state value at long times. The detailed solution of the boundary value problem describing the transient current produced by this mass transport problem will be the subject of a subsequent communication (IO). At steady state
where subscripts I and I1 denote the cathode and anode, respectively, W is the center filament to center filament separation, and k is a dimensionless constant dependent only on the geometry of the electrode filaments. Experimental
observation of the current at a filar electrode with W of 100 bm indicates that a virtual steady-state current is obtained within a few minutes of an application of a step potential excitation at the SPE (Figure 3). If the potentials at the two filar electrodes are held so that C ~ =J CR,II = 0 (see Figure 4), the current reaches a limiting steady-state value (written for the FPE)
or =
jlim 88
2nFADCT (3)
kW
where
CT =
(c@+ c&I)/2
and the reduced diffusion coefficient is
n = ( DODoDR -k DR
)
(4)
Our choice of pattern dimensions for the design illustrated in Figure l a involved an empirically determined balance among analytical requirements and a number of restrictions imposed by the fabrication process. The analytical signal, i& in eq 3, is directly proportional to the electrode area and inversely proportional to the gap between the electrode filaments. As illustrated in Figure lb, we have chosen to make the width of each filament equal to W/2 as well as the gap between filaments equal to W/2. Obviously, an increase in the filament width could increase the area and decrease the separation between the closest edges of the twin electrodes resulting in an increase in the analytical signal. However, as the edges of the filaments approach one another, an adventitious error in photolithography is much more likely to cause a short somewhere along the extended common borders of the twin, interdigitated electrodes. Furthermore, as the edges of the widened filaments approach each other, a marked nonuniformity in current density across the width of the filaments would develop, complicating interpretation of the data. Our choice of 25 filaments for each twin electrode is also arbitrary but reasonable. Similar interdigitated designs are used as strain gauges, where a balance between sensitivity and complexity must also be achieved. One practical method to change the electrode separation, W/2, is to photoreduce a single master pattern while maintaining the proportions described above. This process also reduces the projected area. For each of the 50 filaments, A = 50W(W/2), or for the 25 filaments of one of the twin electrodes in Figure 1, A = 2 5 2 w . Thus eq 3 becomes
(
iip = 1250nF W )DCT
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985 I O
I.0
1
I
2391
I
I
09
08 08
I
0 6 q 07
.f.04 -2 OE _ier
0.2
I
;:
i
05
0 1.0
0
2.O
04
[ F ~ ( C N ) ~ - ]m, g
Flgure 5. Limiting steady-state current for solutions contalning 2 mM BHMF and varying concentratlons of ferrocyanlde In 0.1 M KCI. E,, = 0 mV vs. Ag/AgCI; E,, = 600 mV vs. AgIAgCI.
I
O?
0.2
The alternative choice, changing W while maintaining A constant, would require redrawing the master pattern to include either more or fewer filaments within a fixed electrode area, a forbidding prospect for routine application. Perhaps others would find a different balance point more favorable for their own applications. Since many processes which give rise to nonfaradaic background currents, such as adsorption and double-layer charging, are directly proportional to the electrode area ( W ) (11),the steady-state current to background ratio should increase approximately linearly with a decrease in W. As a test and application of eq 3 we have used the limiting steady-state current of a mixture of BHMF and Fe(CN)g in 0.1 M KC1 to estimate the reduced diffusion coefficient of the BHMF molecule in terms of the known diffusion coefficients of ferro- and ferricyanide (12). When the cathode is maintained a t 0.00 V and the anode at 0.600 V, a limiting steady-state current is observed, which depends directly on the concentration of both BHMF and Fe(CN)6"
i!,m = nlFA(DC)BHMF
kW
+
n2FA(DC)Fe(CN)6'
kW
01
1 -240
The plot of:!i vs. [Fe(CN)G'+]shown in Figure 5 has a slope equal to 2nFADF,(cN,6d/k W. When values of the fer&/ ferrocyanide diffusion coefficients of Do = 0.650 x cm2 s-l and DR = 0.763 X cm2 s-l (12) are inserted, the value of 2nFAlk W is determined to be 0.159 pA mM-l. Taking A as the projected area of one filar electrode, 6.25 X cm-2, and n equal to one, we estimate the effective separation, k W, to be 0.665 pm for this thin-layer filar geometry (and an intercept at [Fe(CN):-] = 0 of 0.415 PA). Substitution into eq 6 yields an estimate of the reduced diffusion coefficient for BHMF, DBHMF, equal to 7.5 X lo4 cm2 s-l. Steady-State Voltammetry. Voltammograms are obtained readily at the filar electrodes with a variety of redox species. Figure 6 shows a current-voltage curve obtained with 0.5 mM BHMF. In a solution containing only reduced BHMF, the common potential of both electrodes was fiied initially at least 120 mV negative of Eo 'BHMF, with the exact value of the initial potential chosen to assure the background current was a minimum. The potential of electrode I1 (the SPE) was then stepped to a more positive potential and time allowed for the current response at electrode I (the FPE) to reach a steady-
-00
I 80
0
1 160
I 240
n(L-G)-pln% R ( r n ~ ~
Flgure 6, Twin-electrode steady-state voltammetry of 0.50 mM BHMF in 10 mM phosphate buffer (pH 7.0). The s o l i line was calculated from eq 8. Open circles represent actual €-I data with D o = DR,n = 1, and E o lBHMF = 0.264 V vs. Ag/AgCI.
state value. For a Nernstian electrode reaction
E =EO'Op
+ RT In
[ $1
(7)
with boundary conditions tss=
(6)
-100
&ADR
kW (CR,I - CR,II) =
and material balance 2CT = CR,I + CR,II
nFADoCo,11 kW
+ C0,II = C@ + C&l
eq 8 describes the relationship between the limiting steadystate current and the surface concentrations
When these equations are solved, the following potentialcurrent relationship is obtained:
Figure 6 presents the measured current response at the FPE to a series of potential steps at the SPE. After each step sufficient time was allowed for the FPE current to reach a steady-state value (see, for example, Figure 3). The steadystate current, i,, follows closely the behavior predicted by eq 9 for a Nernstian electrode process, with a formal potential of 0.464 V vs. NHE (13). Steady-State Absorbance-Voltage Relations. When the filar electrode system is used as an optically transparent
2382
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
electrode, the absorbance of the solution observed through the spaces between the electrode filaments provides a direct measure of the average concentrations of the electroactive species in the thin solution layer. For a two-component system, this absorbance is given by the following equation: A = q b o R 4- c2bCo t 10) where c1 and are the molar absorptivities of reduced and oxidized species, respectively, b is the cell path length normal to the electrode plane, and C R and Co are the average concentrations of the reduced and oxidized species, respectively, in the diffusion layer visible through the optical window. Two distinctly different types of spectroelectrochemical experiments can be carried out with the filar electrode system: a minigrid-type titration of the contents of the spectroelectrochemical cell (9) and dynamic steady-state twin-electrode voltammetry with spectrochemical monitoring of the oxidized to reduced concentrations in the diffusion layer. Minigrid Experiment. When both electrodes are maintained at the same potential, and that potential is varied through the range of potentials near E" 'oIR, the average concentrations, C R and 00,in the light path are equal to the bulk solution concentrations, CR and Co, reached when the solution has come to equilibrium with the electrode($. Then, in addition to eq 7 and 10 A O I = ElbCT
=E ~ ~ C T where AoIand AoII are the absorbances when both electrodes are fixed at a potential >120/n negative to E"'OIR(0absent) and >120/n mV positive of E" 'OIR (R absent) respectively. The resulting potential-absorbance relation is
0O.'7* I
o.6
t I
t
4
0.4
0'31 0.2
0 -100
0
where E is the common potential of both electrodes I and 11. This type of behavior is well-known for spectroelectrochemical observations using a minigrid electrode (9). Figure 7 shows the results of a minigrid experiment using dichlorophenolindophenol (DCPIP) with the filar electrode. The appropriate logarithmic plot, also shown, yields the expected intercept of E" lDCPIP= 0.017 V vs. Ag/AgCl for this wellcharacterized redox couple (14). Because a second step involving DCPIP apparently occurs at potentials more positive than Eo'O/R, the data points at A/Aon > 0.8 begin to diverge significantly from the behavior predicted by eq 11. Twin-Electrode Spectroelectrochemistry. If different potentials are applied to the filar electrodes, the absorbance of the solution visible in the spaces between the electrode filaments is due to three different components: the reduced and oxidized species in the diffusion layer and the reduced species in the bulk of the solution outside the diffusion layer (it is assumed that the oxidized species is initially absent from the cell) A = € l ( b- 6 ) c ~ €16c~ + 96Co (12) where 6 is the Nernst diffusion layer thickness in the z direction parallel to the light path. In addition, the electrode surface concentrations of oxidized and reduced species at steady state are subject to the following boundary and material balance relations: C R = (CRJ + CR,II)/2; C O = c0,11/2
+
2cT =
CR,I
0
100
I
E f m V G Aq/AgCl)
AOII
0
I
-240
I
-160
-80
I
I
I
I
0
80
160
240
nfE-E,O;,i,
mV
Flgure 7. Equilibrlum spectroelectrochemlstry of 0.5 mM DCPIP in 10 mM phosphate buffer (pH 7.0). The solld llne was calculated from eq IO. Open circles represent actual A-€data wlth n = 1 and E" lDcPlp = 0.017 V vs. Ag/AgCI. The Inset is the corresponding logarlthmlc plot showlng the expected Intercept and slope. X = 800 nm; A = 0.00;A "11 = 0.038.
In these relations CR,I and Cn,IIare the steady-state surface concentrations of reduced species at the cathode and anode, respectively, Co,II is the surface concentration of oxidized species at the anode, b is the optical path length normal to the electrode plane, AoI is the initial solution absorbance when both electrodes are negative of Eo and A"!; is the total absorbance of the cell when electrode I1 has been moved to a potential well positive (>120/n mV) of E" 'OIR (Le., when i,, =.):!i If these equations are solved for Co,II and CRJI, and substituted into eq 7, the following voltage-absorbance relation is obtained:
(13) A comparable equation can be written for the experiment where both electrodes are initially held at an anodic potential and the cathode subsequently scanned
+ CRJI + COJI = c&?+ cb%
DR(CR,I - %,Id = DoCo,II; DRCby = DoCby~ A"I = ElbCT AFIm = El(b - 6)CT
+ t16C&/2 + ~26C8'5/2
Figure 8 shows the absorption dependence of the twinelectrode spectroelectrochemical measurement of the DCPIP and BHMF couples along with the appropriate logarithmic dependence predicted by eq 13. Within experimental error
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
I O
09
: 0 DCPIP 0 BHMF
0 8
07
0
ZOO
400/
0.6
A p 0 5
04
Tz 4
03
4 % q 0
02
I
01
2393
/,, ,
-
,
-2 -4 ;1-
-50
E lmVG
50
110
Ag/AgCIl
Figure 8. Steady-state, twin-electrcde spectroelectrochemistry of 0.5 mM DCPIP (open clrcles) and 10 mM BHMF (closed clrcles) in 10 mM phosphate buffer (pH 7.0). The solid ilne was from eq 12. Open and closed circles represent actual A-E data wlth Do = D,, n = 1, E o lDcPIp = 0.017 V vs. Ag/AgCI, and Eo IMF = 0.264 V vs. Ag/AgCI. The insets are the corresponding iogarlthmic plots for DCPIP and BHMF showing the expected intercepts. = 600 nm for DCPIP; = 638 nm for BHMF'; AY;lDCprp= 0.0158 and A oI,DCpIp = 0.00; = 0.0104 and A OI,BHMF = 0.00.
this logarithmic behavior for DCPIP is identical with the equilibrium experiment shown in Figure 7, as would be expected if Do and DR were very similar (the relatively large DCPIP molecule carries a small net charge). We are aware of the close relationship between the filar electrodes described here and the ultramicrovoltammetric electrodes recently described in the literature (15,16).The steady-state currents reported in those studies were several nanoamperes and represent a balance between the rate of electron transfer at a very small disk electrode and the spherically symmetric flux of electroactive species toward the microelectrode surface. If a filar electrode were made with W = 1 pm, the electrode as a whole would be only 50 pm across and would behave much as do the microvoltammetric devices. However, the steady-state current generated between twin filar electrodes is significantly different. The opposing electrode acts to amplify the flux by defining the maximum diffusion layer thickness as the distance between the filaments. In
addition, the extended surface area provided by the unique filar design provides a total current that is large enough to be easily measured by ordinary operational amplifier circuitry. The twin-electrode thin-layer properties of this cell design come without necessarily paying any penalty for serious resistance problems, This is true for two reasons; iR drop due to current between the auxiliary (external) and working electrodes is small because the surface areas are small and the large steady-state currents occur between electrodes positioned only a few micrometers from each other. Troublesome edge effects are also diminished in this design because the large steady-state current rapidly swamps out any current due to diffusion into the thin-layer volume from the edges. In the cell design described here, a closely spaced opposing quartz window is employed principally to restrict the cell volume and shorten the time required to reach true steadystate currents. In other experiments (IO)we have generated pseudo-steady-state currents between filar electrodes exposed to an open (semiinfinite) solution volume. In this configuration the solution resistance problem virtually vanishes and the edge effect becomes a component part of the overall mass transport problem in two dimensions.
ACKNOWLEDGMENT The assistance of Joel Elhard and Roy Tucker in preparing the vapor deposited substrates is greatly appreciated. We wish to thank Robert Haima and Steven Bibyk for advice on the photolithographic preparation of the gold electrodes. Special thanks to Elizabeth Gross for her support and suggestions. The authors are grateful to The Ohio State University Materials Research Laboratory for use of their facilities. LITERATURE CITED (1) Albery, W. J.; Hitchman, M. L. "Ring-Disc Electrodes"; Oxford Unlverslty Press: London, 1971. (2) Anderson, L. E.; Rellley, C. N. J. Nectroenal. Chem. 1965, IO, 295. (3) McDuffle, E.; Anderson, L. E.; Rellley, C. N. Anal. Chem. 1966, 3 8 , 883. (4) Sluyters, J. H. Red. Trav. Chlm. Pays-Bas 1963,8 2 , 100. (5) Hubbard, A. T.; Anson, F. C. J . Electroanal. Chem. 1970, 4 , 129. (6) Helneman, W. R.; Klssinger, P. T. "Laboratory Technlques In Electroanalytical Chemistry"; Marcel Dekker: New York, 1984;p 109ff. (7) Glang, R.; Gregor, L. V. I n "Handbook of Thin Film Technology"; Malssel, L. I., Glang, R., Eds.; McGraw-HIii: New York, 1971;Chapter 7. (8) Reference 7, pp 7-37. (9) Murray, R. W.; Helneman, W. R.; O'Dom, G. W. Anal. Chem. 1967, 38, 1668. (IO) Sanderson, D. G.; Anderson, L. B.,unpublished results, 1985. (11) Heyrovsky, J.: Kuta, J. "Principles of Polarography"; Academic Press: New York, 1966;p 53. (12) Adams, R. N. "Electrochemistry at Solld Electrodes"; Marcel Dekker: New York, 1969;p 219. (13) Szentrimay, R.; Yeh, P.; Kuwana, T. "Electrochemical Studies of Bio-
logical Systems"; Sawyer, D. T., Ed.; American Chemical Society: Washington, DC, 1976;ACS Symposium Series, no. 38,pp 143-169. (14) Dawson, R. M. C., Elliott, D. C., Elliott, W. H., Jones, K. M., Eds. "Data for Biochemical Research", 2nd ed.; Clarendon Press: Oxford, 1974; p 437. (15) Howell, J. 0.; Wightman, R. M. Anal. Chem. 1084, 56, 524. (16) Robinson, R. S.;McCreery, R. L. J. Electroanal. Chem. 1965, 782,
61.
RECEIVED for review May 2,1985. Accepted June 21,1985. This work was presented in part at the Charles N. Reilley Memorial Symposium, American Chemical Society Southeastern Regional Meeting, Raliegh, NC, Oct 26, 1984.
