Anal. Chem. 2007, 79, 5809-5816
Scanning Electrochemical Microscopy with Slightly Recessed Nanotips Peng Sun and Michael V. Mirkin*
Department of Chemistry and Biochemistry, Queens CollegesCUNY, Flushing, New York 11367
Slightly recessed nanoelectrodes were prepared by controlled etching of nanometer-sized, flat Pt electrodes. By using high-frequency (e.g., 2 MHz) ac voltage, the layer of Pt as thin as J 3 nm was removed to produce a cylindrical cavity inside the insulating glass sheath. The etched electrodes were characterized by combination of voltammetry and scanning electrochemical microscopy (SECM) to determine the radius and the effective depth of the recess. The theory was developed for current versus distance curves obtained with a recessed tip approaching either a conductive or an insulating substrate. Good agreement between the theoretical and experimental approach curves indicated that recessed nanotips are suitable for quantitative feedback mode SECM experiments. An electrode with the conductive metal surface recessed into the insulator is a major headache for an electrochemist doing kinetics experiments. The results obtained with such an electrode may be completely misleading.1 Nevertheless, recessed electrodes (and conceptually similar pore-based or microcavity-based electrodes) were found to be useful for various applications from voltammetry2,3 to sensors4 and liquid/liquid electrochemistry5 to studies of molecular transport in nanopores6 and single-molecule experiments.7 Several groups have fabricated and characterized arrays of recessed microelectrodes.8-11 The theory was developed for diffusion currents to deeply recessed microdisk electrodes8 * Corresponding author. E-mail:
[email protected]. (1) (a) Baranski, A. S. J. Electroanal. Chem. 1991, 307, 287. (b) Oldham, K. B. Anal. Chem. 1992, 64, 646. (2) (a) Zhang, B.; Zhang, Y.; White, H. S. Anal. Chem. 2004, 76, 6229. (b) Zhang, B.; Zhang, Y.; White, H. S. Anal. Chem. 2006, 78, 477. (c) Zhang, Y.; Zhang, B.; White, H. S. J. Phys. Chem. B 2006, 110, 1768. (3) Lemay, S. G.; van den Broek, D. M.; Storm, A. J.; Krapf, D.; Smeets, R. M. M.; Heering, H. A.; Dekker, C. Anal. Chem. 2005, 77, 1911. (4) Sundfors, F.; Bereczki, R.; Bobacka, J.; To´th, K.; Ivaska, A.; Gyurcsa´nyi, R. E. Electroanalysis 2006, 18, 1372. (5) Cunnane, V. J.; Schiffrin, D. J.; Williams, D. E. Electrochim. Acta 1995, 40, 2943. (6) Wang, G.; Bohaty, A. K.; Zharov, I.; White, H. S. J. Am. Chem. Soc. 2006, 128, 13553. (7) (a) Fan, F.-R. F.; Bard, A. J. Science 1995, 267, 871. (b) Fan, F.-R. F.; Kwak, J.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 9669. (c) Bard, A. J.; Fan, F.-R. F. Acc. Chem. Res. 1996, 29, 572. (8) Bond, A. M.; Luscombe, D.; Oldham, K. B.; Zoski, C. G. J. Electroanal. Chem. 1988, 249, 1. (9) Morita, K.; Shimizu, Y. Anal. Chem. 1989, 61, 159. (10) Brumlik, C. J.; Martin, C. R.; Tokuda, K. Anal. Chem. 1992, 64, 1201. (11) (a) Ito, T.; Audi, A. A.; Dible, G. P. Anal. Chem. 2006, 78, 7048. (b) Lanyon, Y. H.; De Marzi, G.; Watson, Y. E.; Quinn, A. J.; Gleeson, J. P.; Redmond, G.; Arrigan, D. W. M. Anal. Chem. 2007, 79, 3048. 10.1021/ac070771m CCC: $37.00 Published on Web 06/22/2007
© 2007 American Chemical Society
and for the lower recess depth under both steady-state and chronoamperometric conditions12 assuming the cylindrical pore geometry. In a series of reports, White and co-workers recently described the preparation and characterization of nanopore electrodes.2 They were fabricated by etching submicrometer-sized metal electrodes to produce a pore inside the glass insulator. The produced pores were deep, i.e., the recess depth (l) was significantly larger than the orifice radius (a), and conically shaped. The modification of the glass surface allowed such useful applications as electrostatic13 and photochemical6 control of molecular transport and measurements of the in-plane ionic conductivity of the ∼1 nm thick aqueous layer.14 The essential difference between the probes described here and those reported in ref 2 is a much smaller recess depth. We start with a disk-type, flat, polished nanoelectrode fabricated by pulling a metal wire into a glass capillary (Figure 1A).15 Presently, the radius of the smallest electrode that can be produced in this way and polished is ∼5 nm. The next step is to etch away a thin layer of metal to produce a nanocavity (Figure 1B). By using highfrequency etching current, one can control the thickness of the etched layer, which can be smaller than the orifice radius, so that the radius of the recessed conductive disk is similar to a. We found recently that a quasi-cylindrical nanocavity produced in this way can be filled with solution and immersed in the pool of dry mercury to produce an electrochemical thin-layer cell that can contain only a few redox molecules.16 To carry out quantitative voltammetric experiments in such a thin-layer cell, one must know the a and l values. Here we develop the methodology for characterization of recessed nanoelectrodes. To use a nanoelectrode for kinetic measurements, one has to determine its size and shape. It was shown previously that the geometry of conical and spherical cap nanoelectrodes17 as well as nanopipet-based probes18 and disk-type polished nanoelectrodes15 can be determined by combination of steady-state voltammetry and scanning electrochemical microscopy (SECM) mea(12) Bartlett, P. N.; Taylor, S. L. J. Electroanal. Chem. 1998, 453, 49. (13) Wang, G.; Zhang, B.; Wayment, J. R.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 2006, 128, 7679. (14) White, R. J.; Zhang, B.; Daniel, S.; Tang, J. M.; Ervin, E. N.; Cremer, P. S.; White, H. S. Langmuir 2006, 22, 10777. (15) Sun, P.; Mirkin, M. V. Anal. Chem. 2006, 78, 6526. (16) Sun, P.; Mirkin, M. V. Manuscript in preparation. (17) (a) Mirkin, M. V.; Fan, F.-R. F.; Bard, A. J. J. Electroanal. Chem. 1992, 328, 47. (b) Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Palanker, D.; Lewis, A. Anal. Chem. 1997, 69, 1627. (18) (a) Shao, Y.; Mirkin, M. V. J. Phys. Chem. B 1998, 102, 9915. (b) Amemiya, S.; Bard, A. J. Anal. Chem. 2000, 72, 4940.
Analytical Chemistry, Vol. 79, No. 15, August 1, 2007 5809
feedback mode with a one-step heterogeneous electron-transfer reaction occurring at the recessed, disk-shaped tip (because of the system axial symmetry, only a half of the domain shown in Figure 2 was needed for our simulations). The electrodes are immersed in solution containing redox mediator (e.g., either reducible species, O, or oxidizable species, R). The tip is held at a potential at which the reduction (or the oxidation) of the solution species is diffusion-limited. In the case of a conductive substrate, the reaction at its surface is also diffusion-controlled, while no mediator regeneration occurs at the insulating substrate. The steady-state diffusion equation in cylindrical coordinates is
∂2c 1 ∂c ∂2c + )0 + ∂r2 r ∂r ∂z2
Figure 1. Schematic representation of the fabrication of a recessed nanoelectrode and its use in the feedback mode SECM experiment. (A) A polished disk-type nanoelectrode is prepared by heat-pulling of a Pt microwire into glass capillary. (B) A nanocavity inside the glass sheath is produced by etching away a thin layer of Pt. (C) Oxidized form of the mediator is reduced at the etched tip and regenerated at the conductive substrate.
(1)
where r and z are the coordinates in directions parallel and normal to the electrode base plane, respectively, and c(r, z) is the concentration of species O. The normalized dimensionless variables can be introduced as follows:
R ) r/a
(2a)
Z ) z/a
(2b)
C ) c(r, z)/c*
(2c)
H ) l/a
(2d)
L ) d/a
(2e)
LL ) dd/a
(2f)
RG ) rg/a
(2g)
RS ) rs/a
(2h)
surements. To use a recessed electrode as a tip for feedback mode SECM measurements, one must be able to bring it sufficiently close to the substrate so that the distance between the conductive surface of the tip and the substrate (i.e., d + l, where d is the distance between the substrate surface and the orifice of the recessed tip) was not significantly larger than one tip radius.19 Since the shortest attainable distance between the conductive tip surface and the substrate is l, only slightly recessed tips (e.g., l j a) are suitable for feedback mode experiments. In a feedback SECM experiment, a redox species is either reduced or oxidized at the tip electrode (Figure 1C). The product of this reaction diffuses to the substrate, where it may be reoxidized or rereduced. This process produces an enhancement in the faradic current at the tip electrode depending on the tip shape and the normalized separation distance. An SECM current versus distance (iT-d) curve is obtained by measuring the diffusion current to the tip while moving it slowly toward the substrate surface. From a high-quality current-distance curve one can determine a, the RG value (i.e., the ratio of the insulating sheath radius to a), and other shape parameters.15,17,18 If the tip is recessed, the feedback response should be significantly lower than that for a planar tip whose entire flat surface can be brought close to the substrate. In this article, we evaluate the radius of a polished disk-type nanoelectrode from steady-state voltammetry. After etching that electrode to produce a nanocavity, the recess depth is determined and the radius of the conductive surface is re-evaluated by combination of voltammetry and SECM feedback measurements.
where c* is the bulk concentration of O, rg is the insulator radius, rs is the simulation space limit in the radial direction, and dd is the z coordinate of the simulation limit behind the electrode surface (Figure 2). To calculate the tip current, eq 3 in dimensionless form
THEORY The steady-state diffusion problem formulated in this section applies to a schematic model in Figure 2, which defines the SECM
where ∂C(R, Z)/∂n is the normal derivative on the surface of the surrounding glass.
(19) Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001.
C(R, Z) ) 1 R ) RS, LL e Z e L and RG e R e RS, Z ) LL simulation space limits (6)
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Analytical Chemistry, Vol. 79, No. 15, August 1, 2007
∂2C 1 ∂C ∂2C + )0 + ∂R2 R ∂R ∂Z2
0 e R e RS, LL < Z < L (3)
was solved with the following boundary conditions:
C(R, -H) ) 0 ∂C(R, Z)/∂n ) 0 and
0eRe1
tip surface
(4)
R ) 1,-H < Z < 0; 1 e R e RG,Z ) 0
R ) RG,LL < Z < 0
insulation region (5)
(∂C(R, Z)/∂R)R)0 ) 0
-H e Z e L axis of symmetry (7)
C(R, L) ) 1
0 e R < RS conductive substrate surface (8a)
or
(∂C(R, Z)/∂Z)Z)L ) 0
0 e R < RS insulating substrate surface (8b)
The dimensionless tip current (IT) was obtained by integrating the flux over the disk surface and normalizing it by the steadystate current to the same recessed tip at infinite separation from the substrate (iT,∞). To obtain the iT,∞ for a specific H value, the same diffusion problem was solved numerically with L ) 100 using COMSOL Multiphysics 3.3a commercial software.20 To compare our results to simulations reported in ref 12, the normalized steadystate diffusion limiting currents (IT,∞ ) iT,∞/4nFDc*a, where D is the diffusion coefficient of the redox species) were also computed for the inlaid recessed dick geometry (RG ) RS ) 100). An excellent agreement between the two sets of simulations (within
