Anal. Chem. 2004, 76, 3612-3618
Scanning Electrochemical Microscopy with a Band Microelectrode: Theory and Application Catherine Combellas,* Adrien Fuchs, and Fre´de´ric Kanoufi
Laboratoire Environnement et Chimie Analytique, UMR 7121, 10 rue Vauquelin, 75231 Paris Cedex 05, France
Scanning electrochemical microscopy (SECM) is described using a band microelectrode tip. Numerical calculations allow the determination of approach curves of an insulating or a conductive substrate, and the numerical analysis is compared to experimental curves. Natural convection provides a steady-state current at the band microelectrode at an infinite distance from the substrate, and the band tip may be used in the SECM configuration as easily as the tip of a disk. Owing to the millimetric dimension of the band microelectrode, the substrate has an influence on the current at much longer distances than with the disk. Finally, the advantage of SECM with a band microelectrode is observed with the fast electrochemical modification of a fluoropolymer surface. In a typical scanning electrochemical microscopy (SECM) experiment, a disk microelectrode is held above a surface and moved toward it while generating a stable redox-active species. The microelectrode feedback current depends on the rate of the electron transfer at the solution-substrate interface. Owing to its experimental configuration, the SECM provides a current efficiency of 11 and then allows the estimation of a wider range of kinetics than the double-band systems. Indeed, higher and lower feedback currents can be detected by SECM, which corresponds to faster and slower kinetics, respectively. Nowadays, SECM is widely used as a convenient tool for the investigation of heterogeneous charge-transfer reaction kinetics,2-5 although, originally, its major use was for nano- or microfabrication by electrochemically induced modification of a substrate.5,6 However, owing to the slowness of the chemical reaction or of the diffusion of the reacting species, local surface modification by such scanning devices generally proceeds at a speed lower than a few micrometers per second. Therefore, SECM is generally too slow a technique for surface modification compared to gas-phase lithographies. However, SECM presents the advantage of allowing the * Corresponding author. E-mail:
[email protected]. (1) Treichel, D.; Mirkin, M. V.; Bard, A. J. J. Phys. Chem. 1994, 98, 57515757. (2) Bard, A. J.; Fan, F.-R. F.; Kwak, J.; Lev, O. Anal. Chem. 1989, 61, 132138. (3) Bard, A. J.; Denuault, G.; Lee, C.; Mandler, D.; Wipf, D. O. Acc. Chem. Res. 1990, 23, 357-363. (4) Bard, A. J.; Fan, F.-R. F.; Mirkin, M. V. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, pp 243-373. (5) Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001. (6) Mandler, D. In Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001; pp 593-627.
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controlled electrochemical or chemical tuning of surfaces that is hardly achieved by classical gas-phase lithographies. For example, we have recently demonstrated that SECM permits the localized reduction of fluoropolymers by the radical anion of an electrogenerated redox mediator. Reduction of fluoropolymers results in their carbonization,7-9 and we have used SECM to investigate the reduction mechanism10 or to induce local chemical functionalization of poly(tetrafluoroethylene) (PTFE).11,12 In these experiments, the microelectrode was disk shaped and so was the carbonized zone obtained by the reduction of the polymeric material. Our next objective in this field is to selectively modify fluoropolymers along 2-D patterns by SECM.13 This could be achieved by moving the disk electrode of a SECM above the surface of the substrate at a constant disksubstrate distance and will be developed in future studies. The major drawback of such a method is that it is very slow and requires electrolysis times of hours that are too lengthy for solution aging and industrial applications. An alternative is to design a metallic electrode with the desired pattern and to use it in the SECM configuration. Such a metallic pad could be obtained by beam lithography. The main experimental difficulty to circumvent should then be to ensure parallelism between the pad and the substrate. This can be achieved by adding to the system two (or three, depending on the dimension of the system -quasi 1D or 2D) independent and remote metallic disks and by checking the constancy of the ratios of currents at the two (or three) disks when moving the assembly toward the substrate. The simplest electrode of this type is a microband (band with one micrometric dimension). Microband electrodes in solution have been extensively studied.14-22 Their electrochemical behavior is similar to that of hemicylinders;14,15 their current at long time (7) (a) Kavan, L. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1991; Vol. 23, pp 71-171. (b) Kavan, L. Chem. Rev. 1997, 97, 3061-3082. (8) Brewis, D. M.; Dahm, R. H. Int. J. Adhes. Adhes. 2001, 21, 397-409. (9) Jansta, J.; Dousek, F. P. Electrochim. Acta 1973, 18, 673-674. (10) Combellas, C.; Ghilane, J.; Kanoufi, F.; Mazouzi, D. J. Phys. Chem. B, in press. (11) Combellas, C.; Kanoufi, F.; Mazouzi, D.; Thie´bault, A. J. Electroanal. Chem. 2003, 556, 43-52. (12) Combellas, C.; Fuchs, A.; Kanoufi, F.; Mazouzi, D.; Nunige, S. Polymer, in press. (13) McLaughlin, W. Microfluidic and biosensor applications of fluoropolymer films. Ph.D. thesis, Stanford, CA, 2001. (14) Kovach, P. M.; Caudill, D. G.; Peters, D. G.; Wightman, R. M. J. Electroanal. Chem. 1985, 185, 285-295 (15) Szabo, A.; Cope, D. K.; Tallman, D. E. J. Electroanal. Chem. 1987, 217, 417-423. (16) Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1987, 225, 19-32. 10.1021/ac049752s CCC: $27.50
© 2004 American Chemical Society Published on Web 05/11/2004
Figure 1. Schematic representation of the band + disks assembly: (a) in perspective; (b) bottom view.
reaches a quasi-steady state and not a steady state, due to the existence of a nonmicrometric dimension. Different expressions of this quasi-steady current are given in the literature, which differ only by the R coefficient:
1 i∞(t) ) 2πnFDc*l ln(R(4Dt/w2))
(1)
where n is the number of electrons transferred at the electrode, F is the Faraday constant. D is the diffusion coefficient of the electroactive species, c* its bulk concentration, l is the electrode length, w is its width, and t is time. The theoretical absence of a steady state may complicate experiments with a microband electrode in the SECM configuration. Besides disk microelectrodes (or micropipets23), SECM has also been described with ring,24 ring-disk assembly,25 and hemispherical or conical microelectrodes26 but not yet with a microband. In this paper, we present numerical calculations of approach curves of a microband in the SECM configuration toward an insulating and a conductive substrate since those approach curves are essential for the control of the band electrode positioning over a substrate. We compare them to the experimental curves and also to the approach curves with disk microelectrodes. We also demonstrate that such band electrodes can be used for the fast and easy local carbonization of PTFE along a line. EXPERIMENTAL SECTION Chemicals and Materials. Insulating and conductive substrates were studied in ultrapure water solution that was obtained in the laboratory (Milli-Q grade). (17) Delahay, P. New Instrumental Methods in Electrochemistry; Interscience: New York, 1954. (18) Amatore, C.; Fosset, B.; Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1986, 215, 49-61. (19) Amatore, C.; Fosset, B.; Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1987, 225, 33-48. (20) Amatore, C.; Combellas, C.; Sella, C.; Thie´bault, A.; Thouin, L. Chem. Eur. J. 2000, 6, 820-835. (21) Mirkin, V.; Bard, A. J. J. Electroanal. Chem. 1992, 323, 29-51. (22) Coen, S.; Cope, D. K.; Tallman, D. E. J. Electroanal. Chem. 1986, 215, 2948. (23) Shao Y.; Mirkin, M. V. J. Phys. Chem. B 1998, 102, 9915-9921. (24) Lee, Y.; Amemiya, S.; Bard, A. J. Anal. Chem. 2001, 73, 2261-2267. (25) Liljeroth, P.; Johans, C.; Slevin, C. J.; Quinn, B. M.; Kontturi, K. Anal. Chem. 2002, 74, 1972-1978. (26) Mirkin, M. V.; Fan F.-R. F.; Bard, A. J. J. Electroanal. Chem. 1992, 328, 47-62.
For the PTFE reduction in a DMF solution, tetrabutylammonium tetrafluoroborate was used as the electrolyte; it was synthesized from ammonium tetrafluoroborate and tetrabutylammonium chloride (Fluka) and recrystallized in petroleum ether.27 DMF (puriss, Fluka) was used as received. The other chemical reagents, phthalonitrile, 2,2′dipyridyl, sodium ferrocyanide, and sodium chloride, were purchased from Aldrich (Saint-Quentin Fallavier, France) and used as received. The insulating substrate was PTFE and the conductive substrate a glassy carbon electrode (20 × 20 mm2). The PTFE samples were plaques (diameter, 2 cm; thickness, 3 mm) supplied by Goodfellow. Before treatment, they were polished first on abrasive paper (P4000, Presi) and then on a wet cloth (DP-Nap, Struers, France) until a shiny “mirror” surface was obtained. Thereafter, they were rinsed in water and then acetone under sonication for 5 min and dried in an oven (80 °C, overnight). Electrodes. The band microelectrode consisted of (i) a gold sheet (0.057 × 3.95 mm2) and (ii) two independent platinum disks (radius, 0.025 mm) that were placed and stuck with standard epoxy resin between two glass plaques (microscope slides, width 1 mm) as shown in Figure 1. The assembly was polished and beveled so that its total thickness (glass + gold) was wg (wg ≈ 10w). A schematic representation of the band is given in Figure 2. A platinum counter electrode (radius, 0.5 mm) and a reference electrode Ag/AgCl (radius, 0.5 mm) were used. Platinum wires and the gold band were at least of 99.9% purity (Goodfellow). Operation. The band assembly was moved by a vertical translation stage (speed, 1 or 2 µm/s) driven by an electrical microstep motor (resolution, 0.1 µm) piloted by a computer (CMA motor driven by an ESP300). The substrate, which was located at the bottom of a glass cell, was held on a tangential plate equipped with a motorized goniometer in order to adjust its parallelism with the band assembly. The band assembly, the platinum counter electrode, and the reference electrode were placed in 10 mL of electrolytic solution. Potentials were imposed and currents measured by a bipotentiostat/galvanostat (CHI720A, CH Instruments). In the case of PTFE, the solution was degassed with nitrogen for 10 min before use, and the whole device was kept under nitrogen in a polyethylene bag (glovebag, Aldrich) during the (27) Amatore, C.; Azzabi, M.; Calas, P.; Jutand, A.; Lefrou, C.; Rollin, Y. J. Electroanal. Chem. 1990, 288, 45-63.
Analytical Chemistry, Vol. 76, No. 13, July 1, 2004
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Figure 2. Schematic representation of the band electrode: (a) in the (x,y) plane; (b) in the (x,z) plane.
experiment. The humidity in the plastic bag was maintained as low as possible, RH < 0.3 with molecular sieves. NUMERICAL RESULTS AND DISCUSSION Formulation of the Problem. We have studied chronoamperometry at a band microelectrode in the SECM configuration by numerical methods.We consider the oxidation of R (bulk concentration, c*) at the electrode into O, which diffuses toward the substrate. Two cases are found depending on the insulating or the conductive character of the substrate. In the first case, O does not react at the substrate. In the second case, when O reaches the substrate it is reduced to R at the surface of the substrate. We assume that reactions at the electrode or at the substrate are fast enough so that reaction rates do not need to be taken into account and the process is only controlled by diffusion. Let w be the metal bandwidth, wg the (metal + glass) width, and d the electrode-substrate separation distance. Diffusion along the y axis was neglected, and the space along the x axis was limited to wg/2 (Figure 2). We have assumed that for x > wg/2 the concentration of R was constant and equal to c*. When the SECM operates with disk microelectrodes, this assumption is valid if wg g 10w.5 Due to the symmetry of the system, calculations were restricted to x > 0. The diffusion equation of the system was then the following:
[
]
∂c ∂2c ∂2c )D 2+ 2 ∂t ∂x ∂z
(2)
with the boundary conditions
t)0
z>0
t>0
all z
t>0
z)0
t>0 t>0 t>0
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z)d
-∞ 10. The band microelectrode was then connected to the potentiostat and poised at the same potential of 0.5 V for which the oxidation of ferrocyanide was mass-transfer limited. The chronoamperogram obtained in a solution Fe(CN)64- is reported in
Table 1. Parameters of the Analytical Expressions 20 and 21 of the Approach Curves toward, Respectively, an Insulating and a Conductive Substrates substrate
insulating
conductive
tip form
diska
bandb
diska
bandb
A B C D E F
0.4572 1.460 0.4313 -2.351 -0.1454 5.577
0.2463 1.077 0.08396 -13.61 -2.224 2.981
0.7203 0.7513 0.2665 -1.621
1.174 2.021 -0.3256 -0.4533
a
From ref 31. b Relative error, 0.4%.
Figure 7. Approach of a band microelectrode tip toward an insulating substrate. Numerical curves (s): bold line for the band and light line for the disk. Experimental curve (4) normalized by, i∞ ) 0.51 µA, the current at t ) 200 s, [Fe(CN)64-] ) 2.0 mM, and [NaCl] ) 0.1 M.
Figure 8. Approach of a band microelectrode tip toward a conductive substrate. Numerical curves (s): bold line for the band and light line for the disk. Experimental curve (]) normalized by i∞ ) 0.97 µA, the current at t ) 150 s, [Fe(CN)64-] ) 3.8 mM, and [NaCl] ) 0.1 M.
Figure 6. Experimental chronoamperometry for the oxidation of a 2 mM solution of Fe(CN)64- + 0.1 M NaCl, at a band microelectrode of w ) 55 µm and l ) 3.95 mm held at 1 mm from the bottom of the cell. Representation of i(T)/i∞ as a function of T-1/2 ) (4Dt/w2)-1/2. The solid line represents the analytical curve according to ref 15.
Figure 6, together with the theoretical response expected by Szabo et al for a line.15 A good agreement was obtained for time shorter than 30 s. This leads to the extraction of D, knowing w ) 55 ( 2 µm, to D ) 6.6 × 10-6 cm2/s, which is in good agreement with the known value of 7 × 10-6 cm2/s for ferrocyanide. For longer time, a steady state was reached. It reflects the influence of natural convection on the development of the hemicylindrical diffusion layer.20,30 The effective length of natural convection ∆conv ) 0.92w ≈ 50 µm ensues from eq 19 and the steady-state value of the current at the microband. This situation is comparable to that observed by numerical calculations at L ) 15, and validates our numerical analysis of the problem. After 50 s, the band assembly was moved at a constant speed of 1 or 2 µm/s toward the substrate. No change in the current at the band was observed, which indicates that the speed of the displacement of the electrode yielded a convective length greater than that of natural convection. The experimental approach curves obtained for an insulating or a conductive substrate are represented with the corresponding numerical curves, in Figures 7 and 8, respectively. The experimental curves were normalized by the current at an infinite distance and time (d > 500 µm) and for a time long enough (t > 150 s) to provide control by natural convection at the band electrode. As can be seen in Figures 7 and 8, there is a good fit between experimental and numerical curves, which indicates that our assumptions, mainly pure diffusion and no convection resulting
from the movement of the electrode, are valid enough to describe the phenomenon. The approach of the conductive substrate shows that there is a physical contact between the electrode assembly and the substrate for L ) 0.65 and consequently for d ) 18 µm (numerical point outside the experimental curve). This average separation distance at contact point represents only 0.25% of the global length of the electrode assembly. It is quite small; it would comparatively correspond to the approach, without physical contact, of a disk microelectrode of RG ) 10 at a separation distance as small as L ) 0.025 from a substrate. PTFE. For PTFE, the electrolytic solution was DMF + 0.1 M NBu4BF4. The solution contained also two redox mediators, phthalonitrile (M1) and 2,2′-dipyridyl (M), where [M1] < 0.1[M]. Phthalonitrile was chosen so that it did not induce the polymer carbonization. It was used to position the band by recording its reduction current, i, as a function of the fluoropolymer plaquemicroelectrode distance, d.10 During this first approach curve, the band microelectrode potential was set on the reduction plateau of phthalonitrile and its current followed the approach curve of an insulating substrate (same as Figure 7, not shown). The tip was then positioned so that i/i∞ ∼ 0.5, which corresponded to d ∼ 50 µm. The microelectrode potential was then set on the reduction plateau of 2,2′-dipyridyl (M) for 150 s, which ensured the diffusion of M•-, the electrogenerated radical anion of M, into the solution between the microelectrode band tip and the PTFE substrate. Consequently, PTFE was reduced by M•- according to
-(CF2-)n + (2 + δ)n M•- + (2 + δ)n Cat+ f [-(Cδ--)n, δn Cat+] + 2n[Cat+, F-] + (2 + δ)n M with Cat+ the electrolyte cation (tetrabutylammonium). This resulted in a carbonized PTFE zone. Analytical Chemistry, Vol. 76, No. 13, July 1, 2004
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Figure 9. Photograph of a part of the PTFE surface carbonized by the band. The scale bar represents 200 µm: (a) one of the hemidiskshaped extremities; (b) carbonized band over a scratched region.
In the case of disk microelectrodes, we have shown that the modified zone, observed by profilometry, revealed a hemiellipsoid of circular base along the x-y plane while the hemielliptic propagation is observed along vertical cross sections (x-z or y-z planes). The circular base results from the radial propagation of the modification owing to the cylindrical regime of diffusion provided by the disk tip. On the other hand, the elliptic profile in the x (or y)-z plane is in agreement with the slower reduction rate along the z direction compared to that along the lateral one. We have also shown that the dimensions of the carbonization depended on time, concentration, and standard reduction potential of M, nature of the electrolyte, electrode-substrate distance, and size of the electrode. In a forthcoming paper, we will discuss transient results obtained in the SECM configuration with disk microelectrodes and we will depict the reductive growth of the carbonized zone with time when using disk microelectrodes. With the band microelectrode, the surface representing the modified zone was a rectangle ended by hemidisks (shown in Figure 9a), as can be seen in Figure 9. The surface modification is sensitive to the PTFE surface preparation as scratches present on the surface lead to different transformation rates (discontinuities observed on the right part of Figure 9b). The dimensions of the modification could be compared to those of the band: 0.29 × 3.99 mm2 instead of 0.055 × 3.95 mm2 for the band. As expected, the length of the modified zone was about that of the band because of its millimetric dimension; the radius of the carbonized hemi(32) Kanoufi, F.; Combellas, C.; Shanahan, M. E. R. Langmuir 2003, 19, 67116716.
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disks was about half the bandwidth. On the contrary, the width of the modified zone increased a lot, it was ∼5 times that of the band. With the same mediator and a 50-µm-diameter disk microelectrode positioned at the same distance (d ∼ 50 µm, L ) 2), the diameter of the modified zone was ∼1.5 times that of the disk. The latter is comparable to the expansion of the modified zone along the longitudinal axis of the band and would suggest that the edge of the band behaves as a disk electrode. The higher increase along the bandwidth axis should result from lateral diffusion of M•- and confirms, again, that, for a given distance from the PTFE substrate, higher feedback currents are observed with a band rather than with a disk in the SECM configuration. CONCLUSIONS Simulation of the band microelectrode behavior in the SECM configuration could be performed by using the same formalism as that of the disk. The numerical approach curves toward an insulating or a conductive substrate were obtained. The boundary conditions adopted in the theoretical description of the system implied the establishment of a steady-state current. This was a reasonable assumption since natural convection was experimentally shown to limit the expansion of the hemicylindrical diffusion layer around the band electrode. The band microelectrode can then be used as a classical disk microelectrode tip in SECM experiments. Experimental approach curves toward an insulating or a conductive substrate were successfully fitted to the numerical calculations. Compared to the disk, the interest of the band lies in its sensitivity and its adjustable length. We have demonstrated the advantage of using a band microelectrode tip in the SECM configuration for the local chemical modification of surfaces. We have reduced locally PTFE along lines, millimeters large, in less than 200 s. The hydrophilic character of reduced PTFE as opposed to the hydrophobic and inert character of PTFE32 and the speed of the surface modification process should be convenient for the use of locally reduced fluoropolymer surfaces in microfluidic applications. Received for review February 13, 2004. Accepted April 12, 2004. AC049752S
