Lithography by Scanning Electrochemical Microscopy with a

May 20, 2010 - ... electrode arrays for scanning electrochemical microscopy experiments. Catherine Adam , Frédéric Kanoufi , Neso Sojic , Mathieu Et...
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Anal. Chem. 2010, 82, 5169–5175

Lithography by Scanning Electrochemical Microscopy with a Multiscaled Electrode Fre´de´rique Deiss,† Catherine Combellas,‡ Christian Fretigny,§ Neso Sojic,*,† and Fre´de´ric Kanoufi*,‡ Groupe Nanosyste`mes Analytiques, Institut des Sciences Mole´culaires, CNRS UMR 5255, Universite´ Bordeaux 1, ENSCPB, 16 avenue Pey-Berland, 33607 Pessac, France, Physico-Chimie des Electrolytes, des Colloïdes et Sciences Analytiques, ESPCI ParisTech, CNRS UMR 7195, and Physico-chimie des Polyme`res et Milieux Disperse´s Sciences et Inge´nierie de la Matie`re Molle, ESPCI ParisTech, CNRS UMR 7615, 10 rue Vauquelin, 75231 Paris Cedex 05, France A multiscaled electrochemical probe is presented for Scanning Electrochemical Microscopy (SECM) experiments. It is fabricated by wet chemical etching followed by sputter-coating of an ordered optical fiber bundle. Owing to the optical fiber bundle preparation, the global electrode may present different shapes. After the chemical etching step, each one of these shapes is conserved and finally decorated with 6000 nanotips. Numerical simulations and approach curves are used to study the probe properties and the influence of the global shape and of the nanotips. The numerical simulations show that the approach curves do not depend on the shape of the electrode but rather on the total height of the protuberance of its electroactive part. Such new SECM probes are then used to pattern a Teflon surface. Indeed, by controlling the time scale of the applied potential pulses, the thickness of the reaction layer is confined at each nanotip, and the nanotip pattern is electrochemically transferred onto the non-conductive surface. Both scales (i.e., global electrode shape and nanotip array) thus show distinct and complementary features for positioning the probe and for the subsequent electrochemical patterning. Scanning Electrochemical Microscopy (SECM) involves the use of an ultramicroelectrode (UME) moving above the investigated substrate while the amperometric or potentiometric signal reflects the local concentration of the redox analyte. Typically, micrometric disk-shaped electrodes are scanned over the substrate at a constant height to map the local electrochemical or (bio)chemical reactivity or kinetics. Approach curves recorded on different points of a surface also give information about local chemical differences of the substrates as well as about the UME itself. The UME is usually made from a metal wire insulated with glass, epoxy, or an electrophoretic paint. Even though the precise disk geometry ensures well-defined conditions for the diffusion of the redox analyte, SECM can be adapted to UMEs with different * To whom correspondence should be addressed. E-mail: frederic.kanoufi@ espci.fr (F.K.), [email protected] (N.S.). † Institut des Sciences Mole´culaires, Universite´ Bordeaux 1. ‡ Physico-Chimie des Electrolytes, des Colloïdes et Sciences Analytiques, ESPCI ParisTech. § Physico-chimie des Polyme`res et Milieux Disperse´s Sciences et Inge´nierie de la Matie`re Molle, ESPCI ParisTech. 10.1021/ac100399q  2010 American Chemical Society Published on Web 05/20/2010

geometries. Experimental and theoretical SECM examples of UMEs with various shapes are reported (cone,1-3 sphere,2,4-6 sphere-cap,7 band,8-10 ring or ring-disk,11,12 slightly recessed discs13). More complex structures, such as deformable soft-stylus microelectrodes14 or AFM-cantilever probes15,16 were characterized in the SECM configuration. SECM is also a surface modification technique, and it was used in different configurations to fabricate at the micrometric or submicrometric scale various patterns on a substrate.17-21 SECM is indeed a very promising tool as it can be used to decorate surfaces with various organic functionalities. For example, it was successfully devoted to the local deposition22-27 or etching28,29 (1) Mirkin, M. V.; Fan, F.-R. F.; Bard, A. J. J. Electroanal. Chem. 1992, 328, 47. (2) Fulian, Q.; Fisher, A. C.; Denuault, G. J. Phys. Chem. B 1999, 103, 4387. (3) Zoski, C. G.; Liu, B.; Bard, A. J. Anal. Chem. 2004, 76, 3646. (4) Fulian, Q.; Fisher, A. C.; Denuault, G. J. Phys. Chem. B 1999, 103, 4393. (5) Selzer, Y.; Mandler, D. Anal. Chem. 2000, 72, 2383. (6) Mauzeroll, J.; Hueske, E. A.; Bard, A. J. Anal. Chem. 2003, 75, 3880. (7) Lindsey, G.; Abercrombie, S.; Denuault, G.; Daniele, S.; De Faveri, E. Anal. Chem. 2007, 79, 2952. (8) Combellas, C.; Fuchs, A.; Kanoufi, F. Anal. Chem. 2004, 76, 3612. (9) Xiong, H.; Gross, D. A.; Guo, J.; Amemiya, S. Anal. Chem. 2006, 78, 1946. (10) Amatore, C.; Fosset, B. Anal. Chem. 1996, 68, 4377. (11) Lee, Y.; Amemiya, S.; Bard, A. J. Anal. Chem. 2001, 73, 2261. (12) Liljeroth, P.; Johans, C.; Slevin, C. J.; Quinn, B. M.; Kontturi, K. Anal. Chem. 2002, 74, 1972. (13) Sun, P.; Mirkin, M. V. Anal. Chem. 2007, 79, 5809. (14) Corte´s-Salazar, F.; Tra¨uble, M.; Li, F.; Busnel, J.-M.; Gassner, A.-L.; Hojeij, M.; Wittstock, G.; Girault, H. H. Anal. Chem. 2009, 81, 6889. (15) Holder, M. N.; Gardner, C. E.; Macpherson, J. V.; Unwin, P. R. J. Electroanal. Chem. 2005, 585, 8. (16) Burt, D. P.; Wilson, N. R.; Janus, U.; Macpherson, J. V.; Unwin, P. R. Langmuir 2008, 24, 12867. (17) Wittstock, G.; Burchardt, M.; Pust, S. E.; Shen, Y.; Zhao, C. Angew. Chem., Int. Ed. 2007, 46, 1584. (18) Wittstock, G.; Hesse, R.; Schuhmann, W. Electroanalysis 1997, 9, 746. (19) Alpuche-Aviles, M. A.; Baur, J. E.; Wipf, D. O. Anal. Chem. 2008, 80, 3612. (20) Bard, A. J.; Denuault, G.; Lee, C.; Mandler, D.; Wipf, D. O. Acc. Chem. Res. 1990, 23, 357. (21) Turyan, I.; Matsue, T.; Mandler, D. Anal. Chem. 2000, 72, 3431. (22) Zhou, J.; Wipf, D. O. J. Electrochem. Soc. 1997, 144, 1202. (23) Simeone, F. C.; Albonetti, C.; Cavallini, M. J. Phys. Chem. C 2009, 113, 18987. (24) Turyan, I.; Mandler, D. J. Am. Chem. Soc. 1998, 120, 10733. (25) Kranz, C.; Ludwig, M.; Gaub, H. E.; Schuhmann, W. Adv. Mater. 1995, 7, 38. (26) Wuu, Y.-M.; Fan, F.-R. F.; Bard, A. J. J. Electrochem. Soc. 1989, 136, 885. (27) Marck, C.; Borgwarth, K.; Heinze, J. Chem. Mater. 2001, 13, 747. (28) Combellas, C.; Ghilane, J.; Kanoufi, F.; Mazouzi, D. J. Phys. Chem. B 2004, 108, 6391.

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of polymers and to the local desorption30-32 or etching33-37 of self-assembled monolayers. More recently, this strategy was illustrated by the patterning of a solid substrate with fluorescent molecules by combining SECM and click-chemistry.38 The use of glass-based UMEs also warrants the easy handling of organic solvents and therefore allows using potentially more reactive etchants. For example, this was illustrated for the local decoration of conducting surfaces with electrogenerated radicals obtained by reduction of onium salts.39,40 With SECM, the pattern is formed by moving the UME over the substrate and no preformed stamp, mold, or mask is used prior or during the electrochemical writing step. Thus, micro- or nanostructures are fabricated sequentially. This feature corresponds to an inherent limitation of such a scanning technique where each structure has to be written individually. Indeed, parallelization of the process has not been reported for the fabrication of high-density arrays by SECM as, for instance, by dip-pen nanolithography.41,42 The ability to pattern surfaces with micro/nanostructures in a parallelized and high-throughput manner is of great importance for applications in biosensing, plasmonics, nanoelectronics, and so forth. Photolithography, microcontact printing, and parallel scanning probe techniques are traditionally used for this purpose. However, the use of “stamp” electrodes such as microelectrode arrays (heptode43,44) or band microelectrodes8-10 in the SECM configuration proves that fast and high-throughput patterning of surfaces by SECM is a reasonable challenge. In this work, we present a new type of multiscaled electrochemical probe for SECM, which consists of a high density electrode array of 6000 nanotips. The working electrode is fabricated by chemical etching of an optical fiber bundle (total diameter: 300 µm) comprising 6000 individually cladded optical cores,45,46 which is sputter-coated by a thin gold layer. The global electrode may present different “bump” shapes, that we divide into three types, cone, ellipsoid, and “volcano”, among which only the first two are geometrically well-defined; each shape is decorated with nanotips (Figure 1A). Thus, the electrodes are structured at different scales. (29) Combellas, C.; Kanoufi, F.; Nunige, S. Chem. Mater. 2007, 19, 3830. (30) Pust, S. E.; Szunerits, S.; Boukherroub, R.; Wittstock, G. Nanotechnology 2009; 075302. (31) Wilhelm, T.; Wittstock, G. Angew. Chem., Int. Ed. 2003, 42, 2248. (32) Wittstock, G.; Schuhmann, W. Anal. Chem. 1997, 69, 5059. (33) Zhao, C.; Witte, I.; Wittstock, G. Angew. Chem., Int. Ed. 2006, 45, 5469. (34) Shiku, H.; Takeda, T.; Yamada, H.; Matsue, T.; Uchida, I. Anal. Chem. 1995, 67, 312. (35) Matrab, T.; Hauquier, F.; Combellas, C.; Kanoufi, F. ChemPhysChem 2010, 11, 670. (36) Hauquier, F.; Matrab, T.; Kanoufi, F.; Combellas, C. Electrochim. Acta 2009, 54, 5127. (37) Slim, C.; Tran, Y.; Chehimi, M. M.; Garraud, N.; Roger, J.-P.; Combellas, C.; Kanoufi, F. Chem. Mater. 2008, 20, 6677. (38) Ku, S.-Y.; Wong, K.-T.; Bard, A. J. J. Am. Chem. Soc. 2008, 130, 2392. (39) Cougnon, C.; Gohier, F.; Be´langer, D.; Mauzeroll, J. Angew. Chem., Int. Ed. 2009, 48, 4006. (40) Matrab, T.; Combellas, C.; Kanoufi, F. Electrochem. Commun. 2008, 10, 1230. (41) Salaita, K.; Wang, Y.; Fragala, J.; Vega, R. A.; Liu, C.; Mirkin, C. A. Angew. Chem., Int. Ed. 2006, 45, 7220. (42) Mirkin, C. A. ACS Nano 2007, 1, 79. (43) Ufheil, J.; Borgwarth, K.; Heinze, J. Anal. Chem. 2002, 74, 1316. (44) Sklyar, O.; Ufheil, J.; Heinze, J.; Wittstock, G. Electrochim. Acta 2003, 49, 117. (45) Pantano, P.; Walt, D. R. Rev. Sci. Instrum. 1997, 68, 1357. (46) Chovin, A.; Garrigue, P.; Vinatier, P.; Sojic, N. Anal. Chem. 2004, 76, 357.

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Figure 1. (A) SEM image of a part of the nanotip electrode array. The inset shows a single nanotip at higher magnification (white bar: 1 µm). (B) Principle of the surface patterning with an etched optical fiber bundle electrode (radius: a, electrode/substrate distance: d).

To use such “stamp” electrodes in an electrochemical lithographic process, first their behavior in the SECM configuration must be known, for example, to place the electrode in the vicinity of the surface to be patterned. For this purpose, numerical simulations are performed and compared to experimental approach curves to understand the influence of the different electrode geometries and of the nanotips. Finally, such new SECM probes are used to pattern a non-conductive surface (Figure 1B). By controlling the time scale of the applied potential pulses, the thickness of the diffusion layer generated at each nanotip is modulated. Diffusional decoupling between single electrode nanotips is achieved by pulsing the array and finally, the electrode is used to pattern a polytetrafluoroethylene (PTFE) surface. EXPERIMENTAL SECTION The nanotip array was prepared by a wet-etch procedure of a coherent optical fiber bundle, which is extremely well-reproducible. Silica imaging fiber bundles with a diameter of 300 µm comprising 6000 individually cladded 3-4 µm diameter optical fibers (Sumitomo Electric Industries, IGN-035/06) were used. The etching of the fiber array was accomplished through the combination of several methods reported in the literature and resulted in the formation of conical tips whose shape, length, and angle were dependent on the experimental conditions.45-50 The polished side was left for 90 min in a buffer of hydrofluoric acid solution (HF) consisting of 40% aqueous NH4F and 48% HF in deionized water in proportions 5/1/1, and was then rinsed with deionized water. (Caution! HF etching solutions are extremely corrosive and safety procedures must be followed accordingly). Selective etching was achieved because of the different etching rates between the high-refractive index GeO2 gradient doped silica core and the low-refractive index fluorine doped cladding. The etched fiber was then sputter-coated with a 100 nm thick gold film to serve as an electrode (Emitech K550X). The diameter of the base and the height of each single nanotip element are 3 and 4 µm, respectively (Figure 1A). The SECM experiments were performed with a CH910B SECM station, with the translation stages mounted on top of 2 (47) Pantano, P.; Walt, D. R. Chem. Mater. 1996, 8, 2832. (48) Chovin, A.; Garrigue, P.; Manek-Ho ¨nninger, I.; Sojic, N. Nano Lett. 2004, 4, 1965. (49) Deiss, F.; Sojic, N.; White, D. J.; Stoddart, P. R. Anal. Bioanal. Chem. 2010, 396, 53. (50) White, D. J.; Mazzolini, A. P.; Stoddart, P. R. Photon. Nanostruct. Fund. Appl. 2008, 6, 167.

orthogonal motorized-goniometer stages (Microcontrole, MGON65-U, stepper motor CMA25 driven by an ESP300). The whole stages assembly was mounted on an inverted Olympus IC71 microscope, and the goniometer stages were used to adjust the tilt of the SECM probe respectively to the microscope optical axis. The tilt was controlled from the observation of interference fringes on the probe surface with a dedicated Mirau-interferometric objective (Nikon, CF PLAN DI 20xA). Similarly, the glass slide used as the insulating substrate was set perpendicularly to the microscope optical axis with the Mirau-interferometric objective. The optical microscope setup was also used to detect optically the occurrence of the physical contact between the SECM probe and the insulating surface. The SECM experiments were conducted in a 30 µL droplet of electrolytic solution in a 2-electrode configuration, a 250 µm diameter Pt wire was used as the auxiliary and pseudoreference electrode. For the local etching of PTFE, the orthogonality of the probe and of the PTFE surface was not controlled (PTFE is not transparent). The parallelism between the PTFE surface and the x-y displacement of the probe was checked by SECM with a ferrocyanide solution. Typically, it consists of observing a constant probe current for ferrocyanide oxidation while the probe is scanned in the x-y plane in the vicinity of the substrate. The probe was then positioned in the close vicinity (1-2 µm from the SECM detected contact point) to the substrate from a SECM approach curve with ferrocyanide. This procedure was done only once for a given probe. Indeed, the electrochemical etching process was conducted in a DMF solution and, after the first exposure of the probe to the electrolytic solution, the probe looses its insulating sheath and can not produce anymore the typical negative feedback approach curve. Once patterned, the PTFE surface was rinsed with acetone, and the patterns were observed by optical microscopy. Interferometric profiles of the probes were obtained with a Microsurf 3D FOGALE optical profiler (Fogale Nanotech, France), or with a standard inverted microscope equipped with a Mirauinterferometric objective. The profiles were deduced from the reconstruction into 3D images of the interference fringe patterns taken at different focal planes of observation of the probe. Simulated approach curves by finite elements method were obtained with the FEMLAB 3.1 package. The simulations were carried out on axisymmetric electrodes, and the resulting problem was computed under 2D-axisymmetric geometry. All the electrodes have a same base radius unity (a ) 1) and a given height (or aspect ratio) H (H ) h/a where h is the electrode apogee depicted in Figure 2A). The geometry of the simulation space and boundary conditions are presented as Supporting Information in the case of the cone or the “volcano” shape. The simulation consists of the numerical solution of the diffusion equation under the appropriate boundary and electrode shape conditions. The probe current was evaluated using the weak constraint procedure. To generate SECM approach curves, a steady-state simulation was obtained for a given probesubstrate distance, L. L is the distance from the apogee of the probe to the substrate surface (L ) d/a); it is taken as L ) 20 (infinite distance) at the beginning of the simulation. This simulation procedure was repeated from a simple MATLAB routine, which iteratively decreases L down to 0.01.

Figure 2. (A) Schematic representation of the section of an etched optical fiber bundle electrode of radius a, presenting a protuberance of given shape (here ellipsoid) and height, h defined as the electrode apogee, and decorated with nanotips of height htip. (B) Theoretical variations of the normalized current (I ) i/iinf; iinf is the stationary current for infinite d) with the normalized distance (L ) d/a) for an insulating surface and different protuberance electrodes geometries (cone, ellipse, or “volcano”). Normalized electrode apogee: H ) h/a ) 0.06 (blue line, cone; green line, ellipsoid); 0.15 (blue line, cone; green line, ellipsoid; red line, “volcano”).

RESULTS AND DISCUSSION The electrodes under investigation can be viewed as objects with structures at 2 different scales (Figure 2A). The first one concerns the macrostructure of the electrode; it corresponds to the shape of the fiber bundle before it was etched. The shape and aspect of the fiber bundles is determined by the fabrication procedures (bundle cleaving and polishing). The second characteristic scale is the microstructure of the etched fiber bundle; it is the conical shape of each individual fiber nanotip formed during the etching process. These two different structures can be compared by their characteristic height along the z-axis. The microstructure characteristic height is about htip ) 4 µm and normalized by the bundle electrode radius, a (a ) 150 µm), is Htip ) htip/a ) 0.027. The characteristic dimension of the bundle (macrostructure) is the bundle height, h or its normalized value, H ) h/a, taken as the distance between the electrode base to its apogee. As shown below, for the electrodes under scrutiny, each individual nanotip is small compared to the characteristic dimension of the bundle and htip < h. In a first approach, it can then be considered that the diffusional behavior of the whole electrode is not ruled by the presence of the array of nanotips but rather by the shape and aspect of the bundle. As the shape and aspect of the fiber bundle are not known a priori, we wanted to describe, from finite element modeling, how the shape and aspect of a microelectrode affect its SECM approach curve. The approach of an insulating surface is the case that shows the largest dependence on the tip shape and geometry as such approach curve is governed by the hindrance of diffusion by both the probe and the surface. This was clearly shown in the investigation of electrodes of various geometries, such as cone,1-3 sphere,2,4-6 sphere-cap,7 band,8-10 ring or ring-disk,11,12 and slightly recessed discs.13 First, theoretical approach responses of a curved electrode surface, schematizing the fiber bundle electrode, toward an insulating surface were drawn (Figure 2B). They represent the normalized current, I ) i/iinf (iinf is the stationary current for infinite d, where d is the distance between the bundle apogee and the surface) as a function of the normalized distance (L ) d/a). The shape of the electroactive part of the fiber bundles was assumed to be either a cone, an ellipse, or a concave shape Analytical Chemistry, Vol. 82, No. 12, June 15, 2010

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Figure 3. (A) Theoretical influence of the electrode geometry (blue line, cone; gren line, ellipsoid; red line, “volcano”) and the bunch apogee H ) h/a on the normalized current for L ) 0 (contact between the electrode and the surface). (B) Influence of the global geometry (shape and aspect) on the normalized current for L ) 0 at H ) 0.15.

that we call a “volcano”, emerging from an insulating sheath (whose RG was taken as 1.1, RG ) ratio of insulating sheath to bundle radii). The curves represented in Figure 2B show that the electrode shape has no significant influence on the approach curves; only the bundle height, h/a (restricted to h/a ) 0.06 and 0.15 in Figure 2B), has an effect for L < 1. As expected, the maximum effect is observed when the electrode contacts the insulating surface (intercepts). The intercepts of the approach curves in Figure 2B are reported in Figure 3A as a function of the bundle heights for three electrode geometries. They show that the bundle height (0.03 < h/a < 0.3) has a large effect on the intercept, in agreement with Figure 2B. Conversely, for a given bundle height h/a, the electrode shape has no significant influence on the normalized electrode current when the electrode contacts the surface; there is only a slight effect for h/a > 0.15. The mean normalized height of the electrode, 〈H〉 given by eq 1, has been introduced to better characterize the shape of a given electrode. 〈H〉 )

2 a3

∫

a

0

h(r)r dr

(1)

Indeed, 〈H〉 is shape-dependent and represents the sharpness of the electrode and therefore the couple of H and 〈H〉 fully characterizes a given electrode of an unknown shape. For example, a cone electrode with a given H has a mean height of 〈H〉 ) H/3 while an ellipsoid one with the same H has a mean height of 〈H〉 ) 2H/3 meaning that the cone is sharper than the ellipsoid. 5172

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Then, for a given value of H ) h/a ) 0.15, we have varied the shape of the bundle electrode and compared the normalized currents at the electrode/insulating surface contact. Figure 3B represents the current at the electrode/surface contact for h/a ) 0.15 as a function of the mean height 〈H〉. All the electrodes modeled present a cylindrical symmetry and have the same electroactive radius, a, but different sharpnesses (and therefore different 〈H〉). For example, an electrode having a conical shape with the same height H but a smaller mean height 〈H〉 may be built by embedding a cone of height H of smaller aperture (or base) over an electroactive disk of radius a (the flat part of the schematic electroactive parts presented in Figure 3B). Figure 3B shows that for all the bundle shapes considered with the same apogee, H ) 0.15, the normalized electrode current IL)0,H)0.15 ) 0.402 ± 2%. For H ) 0.06, the normalized current at the contact IL)0,H)0.06 ) 0.275 ± 0.8%. It means that, to first order, the bundle shape has no significant influence on the normalized current with an insulating surface. For a curved electrode of unknown shape, its approach curve toward an insulating surface is then not significantly dominated by its aspect ratio, 〈H〉, but rather by the height of the electrode protuberance, H. A curved electrode of unknown shape but known protuberance height, H, in contact with an insulating substrate behaves as a disk electrode (as indicated by the simulated point at 〈H〉 ) 0 in Figure 3B, Idisk,L)0.15 ) 0.412) held at a distance L ) H from an insulating substrate. Indeed, whatever the electrode shape, the current is related to the concentration field of the generated species, which is dictated by the diffusion volume available for edge or radial escape of the electrogenerated species from the electroactive part. The volume occupied by the electrode protuberance is then negligible in regard to this diffusion volume, which is dominated by the electrode protuberance height H and the electrode insulating sheath (the electrode RG value). The influence of the volume occupied by the electrode protuberance has a second order effect, and the normalized current (Figure 3B) decreases, almost linearly, with the electrode aspect ratio, 〈H〉. We have inspected the effect of the presence of nanotips (Figure 1A) on the electrode response. A single conical nanotip of height Htip and base 0.35Htip was added on top of an ellipsoidal electrode of height H-Htip. This composite probe of height H and 〈H〉 ) 0.55 behaves as an ellipsoid probe of height H. The decoration of a probe with multiple nanotips would require a time-consuming 3D simulation. Instead, we generated 2Daxisymmetric shapes formed by rotating a half-ellipse of height H - Htip on top of which are added 1 central nanotip plus 4 or 9 triangles of height Htip, half-base 0.35Htip and intertip distance 0.2 (see Supporting Information, Figure SI2). The rotation around the z-axis of this 2D shape generates an ellipsoid composite electrode decorated with a central nanotip and 4 to 9 nanocrowns. As shown in Figure 3B, the approach curves of these composite probes of total height H and different 〈H〉 are very similar to that of a curved electrode of height H. This suggests that a composite electrode made of a network of small nanotips (of height Htip) on top of a curved electrode (of protuberance H > Htip) should behave similarly to a curved electrode of height H + Htip, the actual height of the composite electrode.

We have drawn the experimental approach curve toward an insulating surface of a high-density array of 6000 electrode nanotips obtained by etching of an optical fiber bundle followed by sputter-coating with a thin gold layer (thickness ∼100 nm). As a result, such electrode behaves as a macroelectrode in cyclic voltammetry (Supporting Information, Figure SI3). Indeed, for scan rates slower than 3 V/s, the electrode response to a potential ramp is diffusive, the peak current is proportional to the squareroot of the scan rate, indicating that for characteristic times longer than 10 ms the nanotips are not individual nanoelectrodes but present interpenetrating diffusion layers. This experimental configuration is likely easier to handle for SECM experiments. First, such incomplete insulation step is more reproducible and easier to achieve while a more careful insulation may bury some nanotips. Second, the approach of an insulating surface with a network of independent nanotips is expected to be similar to the approach curve of a single nanotip probe. This means that each individual nanotip detects the substrate presence by a tip current change for nanotip-substrate separation distances d < 4 times the nanoprobe radius, which then imposes electrode-substrate separation distances d < 2 µm. Even if approaching a 150 µm radius (nanotip array) electrode at L ) d/a ) 2/150 ) 0.013 is feasible, it requires a careful parallelism of the electrode and substrate planes. Moreover, as discussed later, with fiber bundle electrodes that present a curvature larger than the nanotip height, only a small fraction of the nanotip array can be held in this farfield region where it can sense the substrate. As the overall electrode current is the arithmetic sum of each individual nanoprobe current, the contact between the electrode array and the substrate may not be detected by a significant probe current variation. Owing to its dimension the fiber bundle electrode could act as a macroelectrode at the time scale of the SECM (tip approaching rate 10 s). It was, however, shown, for band microelectrodes, that it is still possible to carry out SECM experiments with macroelectrodes since the electrode response is governed by a pseudo-steady state enforced by convection, either from the slow movement of the probe51,52 or from natural convection.8,53-55 For the fiber bundle electrode, at slow scan rate, 10 mV/s, the cyclic voltammogram presents a mixed mass-transfer control as a sigmoidal shape appears on the voltammogram with a limiting current similar to the steady-current observed at large electrode-substrate separations during SECM experiments (ilim ) 0.43 µA for the electrode considered in Supporting Information, Figure SI3). If this steady-state current is controlled by mass-transfer limitation, one expects, for a oneelectron exchange: iT,inf ) 4ε1FC0Da

(2)

where F is the Faraday, C0 and D are respectively the concentration and diffusion coefficient of the redox probe (for ferrocya(51) Combellas, C.; Fermigier, M.; Fuchs, A.; Kanoufi, F. Anal. Chem. 2005, 77, 7966. (52) Nkuku, C. A.; LeSuer, R. J. J. Phys. Chem. B 2007, 111, 13271. (53) Bento, M. F.; Thouin, L.; Amatore, C. J. Electroanal. Chem. 1998, 446, 91. (54) Amatore, C.; Combellas, C.; Kanoufi, F.; Sella, C.; Thie´bault, A.; Thouin, L. Chem.sEur. J. 2000, 6, 820. (55) Amatore, C.; Pebay, C.; Thouin, L.; Wang, A. Electrochem. Commun. 2009, 11, 1269.

Figure 4. Experimental variations of the normalized current (I ) i/iinf; iinf is the stationary current for infinite d) with the normalized distance (L ) d/a) for two electrode shapes (two experiments for each) in water with 0.1 M KCl and 3 × 10-3 M potassium ferrocyanide; electrode biased at 0.5 V/Ag/AgCl. The two bottom curves (electrode 1, black diamonds, red triangles) and the two top curves (electrode 2, blue diamonds, magenta squares) are fitted with the theoretical curves for respectively H ) h/a ) 0.06 and 0.15. Insets: 3D image of electrode 1 (see Supporting Information, Figure SI4 for electrode 2) by interferometric microscopy (rugosity is subtracted); deduced H is 0.073 and 0.13, respectively, for electrodes 1 and 2.

nide, D ) 6 × 10-6 cm2/s) and ε1 is a geometric correction factor taking into account the shape of the protuberance and the insulating sheath thickness (RG value) of the electrode of radius a. From the simulation performed with the different shapes considered, ε1 ) 1.35 ± 0.05. As proposed by Amatore et al,56 the comparison of the experimental steady state current value with the diffusive peak current of the redox probe, at a given scan rate v, allows a precise estimate of the electrode radius a. The diffusive peak current is given by

DFv RT

ip ) 0.446Fπa2ε2C0

(3)

where ε2 is another geometric factor that corrects the real electrode surface area from that of an ideal disk electrode. With the most voluminous ellipsoidal electrode of protuberance H, ε2 is lower than 1.07 when H is smaller than 0.15 (see Supporting Information). From the comparison of the diffusive peak, ip, and steady, iinf, experimental currents values and their theoretical expressions (2) and (3) one obtains

a)

( ) ip iinf

4ε1 0.446πε exp 2

DRT Fv

(4)

which gives, for v < 1 V/s, a value of a ) 145 ± 7 µm, in agreement with the optical fiber bundle radius. Therefore, at the time scale of the SECM approach curve experiment, the fiber bundle electrode behaves as a microelectrode and is not affected by convection, in agreement with the description of a convective layer larger than few 100 µm.55 The experimental approach curves for two probes have been compared, in Figure 4, to the theoretical approach curve for protuberant electrodes with different shapes (two sets of experi(56) Amatore, C.; Azzabi, M.; Calas, P.; Jutand, A.; Lefrou, C.; Rollin, Y. J. Electroanal. Chem. 1990, 288, 45.

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mental values for each probe). They show a good agreement and are fitted by the model for a protuberant electrode with a height of h/a ) 0.06 and 0.15. The shape of the SECM probes has been determined by interferometric microscopy, and the total protuberance height of the two probes (respectively 0.073 and 0.13) is in good agreement with the values obtained from the fit. Last, the ability of such nanotip to locally modify a surface was tested. The issue is to find if there are experimental conditions in which the nanotips are not coupled, although they are addressed electrically as a whole. In that case, it should be possible to obtain a modified surface with as many modified domains as individual nanotips. A PTFE surface was chosen for that test since the local reduction of PTFE surfaces is well-documented.54,57 It involves strong reducers generated at a cathode, such as the 2,2′-dipyridyl radical anion. An array of 3 µm center to center spaced nanotips was contacted with the PTFE surface and submitted for 10 s to a succession of sequences consisting of (i) a pulse at -2.2 V/Ag/ AgCl for a time τ and (ii) the application of 0 V/Ag/AgCl for 5 τ, with τ ) 0.05, 0.1, and 1 s. During the first part, (i), of the sequence, the reducer is generated and allowed to diffuse toward the PTFE surface where its surface transformation takes place. During the longer second part, (ii), of the pulse, the system is allowed to relax diffusionally, by the forward oxidation of the reducer created during step (i), before a new cycle is started. The images of the PTFE surface at the end of the process are given in Figure 5 for the three values of τ. For the two shortest pulses, the PTFE surface is reduced locally and small dark ∼2-2.5 µm domains of carbonized PTFE are observed (Figures 5 A, B). This means that the nanotips are not diffusionaly coupled and function as isolated nanotips without the need to insulate them with an electrophoretic paint.46,58,59 It is noteworthy that the electrophoretic paint is not stable at the applied potential and in such an organic solvent. It would dissolve under these experimental conditions and have no electrical insulating properties. For the longest pulse, the surface is reduced everywhere, and then the nanotips are coupled and the image of the whole bundle is imprinted on the PTFE. To diffusionally isolate the nanotips, it is necessary to restrain the overlapping of the diffusion layer that develops over each nanotip. This is achieved by adapting the time duration of the pulse, τ, to the internanotip distance, dt, and the shorter the pulse duration, the smaller the diffusion layer. In the case of PTFE reduction, it is difficult to relate the diffusion layer expansion, δ, and τ for mechanistic and experimental reasons. The mechanism is intricate since the reducer (i) is chemically unstable in the presence of water, (ii) reacts with oxygen in a redox catalysis type process (the water and oxygen contents are not controlled), and (iii) also reacts with PTFE in a catalytic process. The experimental drawbacks are related to the difficulties to control precisely the tip/surface distance since the tip is manually positioned onto the surface and is slightly tilted to increase the contacting surface area. (57) Combellas, C.; Kanoufi, F.; Mazouzi, D. J. Phys. Chem. B 2004, 108, 19260. (58) Szunerits, S.; Tam, J. M.; Thouin, L.; Amatore, C.; Walt, D. R. Anal. Chem. 2003, 75, 4382. (59) Szunerits, S.; Garrigue, P.; Bruneel, J.-L.; Servant, L.; Sojic, N. Electroanalysis 2003, 15, 548.

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Figure 5. Electrochemical patterning by reduction of a PTFE surface with a contacting optical fiber bundle electrode in DMF with 10-2 M 2,2′-dipyridyl and 0.1 M NBu4BF4. The electrode is submitted for 10 s to a succession of sequences consisting of (i) a pulse at -2.2 V/Ag/AgCl for a time τ and (ii) the application of 0 V/Ag/AgCl for 5 τ. The images are taken at the end of the process for τ ) (A) 0.05, (B) 0.1, and (C) 1 s. Scale bar: on the left, 100 µm; on the right, 10 µm.

However, some approximations may be made to get more insight into the global etching process. We have supposed, as Szunerits et al,58 that each active conical nanotip can be considered as equivalent to a single isolated hemispherical electrode. Then, the reducer generated at each nanotip that diffuses within a layer of thickness δ, is supposed to expand in the substrate plane over a distance ∆ given by the aperture angle of the nanotip (θ ∼18°), ∆ ) 2δ/cos θ. This assumption is valid as long as the diffusion layers do not overlap (when the diffusion layer thickness exceeds the intertip distance) and as long as the tip-substrate distance is small compared to the intertip distance. These conditions are satisfied for short potential pulses, τ ) 0.05 or 0.1 s, since wellseparated spots are formed at the substrate. For τ ) 0.05 or 0.1 s, the modification develops over ∆ ∼ 2-2.5 µm spots, and then the reducer diffusion layer is δ ∼ (∆cos θ)/2 ) 1-1.2 µm. This value is 1 order of magnitude smaller than the distance expected for 1-D diffusive travel (Dτ)1/2, with D the reducer diffusion coefficient (∼10-5 cm2 s-1). The chemical instability of the reducer quenches its lateral propagation as would do a

“chemical lens”,34,60 and δ represents the thickness of this reaction layer. For τ ) 1 s, the reaction layers of the different nanotips overlap, this means that ∆ > 3.5 µm, that is, δ > 1.8 µm. The propagation of the redox probe is limited to a reaction layer very similar to that observed for the electrochemiluminescence system reported by Szunerits et al.58 for which a 1-2 µm diffusion layer develops in 0.1 s, while for τ > 0.4 s, the size of the diffusion layer implies that the nanotips are coupled. The similarity of the results allows us to deduce the approximate apparent chemical rate constant of the reducer that should be of the same order as that of the electrochemiluminescence system,58 that is, kapp ∼ 5.102 s-1. This result is in good agreement with the approximation kapp ∼ D/δ2, which gives kapp ∼ 103 s-1. A more rigorous finite element analysis of the time dependent process of diffusion and chemical reactions taking place at the nanotip array should be possible. However, this would not help with the patterning system tested here (PTFE patterning). Indeed, as previously shown, the PTFE SECM etching does not only depend on the diffusion-chemical stability of the reducer but involves much more complicated surface and solid diffusionreaction processes.54,57 CONCLUSION A new type of multiscaled electrochemical probe for SECM is characterized and used to pattern a non-conductive surface. Numerical simulations show that approach curves toward an insulating surface are mainly governed by the electrode protuberance. The nanotips decorating the global shape of the electrode have no significant influence in this diffusional regime. However, the thickness of the diffusion layer generated at each nanotip is modulated by controlling the time scale of the applied potential (60) He, C.; Borgwarth, K.; Ricken, C.; Ebling, D. G.; Heinze, J. Electrochim. Acta 1997, 42, 3065.

pulses. Diffusional decoupling between individual electrode nanotips is achieved by decreasing the pulse time. Such a diffusionindependent behavior is exploited to pattern a PTFE surface by local reduction. The transition between a diffusional decoupling regime and a total overlap regime is evidenced by imaging the patterned PTFE surface at different times. Well-separated spots observed at short time scale are the image of the nanotip electrode array. Indeed, the reaction layer is confined at each nanotip by the pulse duration and by the instability of the electrogenerated reducer. Since the approach SECM curves are governed by the global shape of the electrode whereas the nanotip array pattern is transferred electrochemically to the surface in a different diffusional regime, both scales show distinct and complementary features for the positioning of the probe and the subsequent patterning step. Therefore, this electrochemical approach allows to pattern surfaces in a parallelized and high-throughput manner. Moreover, since the size between adjacent nanotips may be decreased to a few hundreds of nanometers, we plan to generate electrochemically various patterns of micro/nanostructures for applications in biosensing. SUPPORTING INFORMATION AVAILABLE Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT Ms. Huayun Xuan is thanked for preliminary experiments. This work was supported by CNRS, the Re´gion Aquitaine (F.D. and N.S.) and ESPCIParisTech (C.F., C.C., and F.K.). Received for review February 12, 2010. Accepted May 8, 2010. AC100399Q

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