Anal. Chem. 2000, 72, 4441-4446
Electrochemical Characterization of Electrodes with Submicrometer Dimensions Jodie L. Conyers, Jr. and Henry S. White*
Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
The construction and electrochemical characterization of electrodes with submicrometer dimensions (2 nm < rapp < 1000 nm) is reported. Electrodes are prepared by insulating etched Pt wires with electrophoretic paint, as recently reported by Slevin et al. (Electrochem. Commun. 1999, 1, 282). The voltammetric behavior of these electrodes was evaluated using nine different redox systems; well-defined and stable diffusion-limited responses were obtained in all but two cases. The behavior of these electrodes was investigated in aqueous ferrocenylmethyltrimethylammonium (FcTMA+) solutions in the presence and absence of excess supporting electrolyte to determine the influence of diffusion and migration on molecular transport in the nanometer spatial regime. Our findings indicate that the voltammetric behavior of these electrodes can be described using classical transport theory for rapp > 10 nm.
of nanostructured materials used in energy conversion and chemical sensing. In addition, nanometer-sized electrodes have been used for single-molecule detection9 and as scanned-probe tips for atomic force microscopy (AFM),10 scanning tunneling microscopy (STM),11 and scanning electrochemical microscopy (SECM).12 A number of researchers have reported the preparation of band-, disk-, or hemispherical-shaped electrodes with submicrometer dimensions by insulating the metal surface with coatings such as glass,6,13 nail varnish,14 wax,9,15 epoxy,16 and copolymers.17 More recently, electrodes with submicrometer dimensions have been prepared by electrodepositing insulating layers onto the metal surface.10-12,18 The procedure reported by Slevin et al.1 appears particularly attractive as it offers a relatively straightforward, inexpensive, and reproducible means of producing hemispherical-shaped electrodes with nanometer dimensions. In this report, we present the electrochemical characterization of hemispherical electrodes with nanometer dimensions prepared
In a recent report, Slevin et al.1 reported the fabrication of individual Pt electrodes having radii as small as 16 nm by coating etched Pt wires with an electrophoretic paint. This is an important result, as there has been significant and growing interest during the past 15 years in performing quantitative measurements using nanoscale electrodes.2 The small size of these electrodes provides a means to investigate biological3 and neurochemical4,5 microenvironments (i.e., single-cell electrochemistry), measure the rates of fast heterogeneous electron-transfer reactions,6,7 and probe the influence of interfacial fields and solvent structure on molecular transport.8 Quantitative measurements at individual nanoscale electrodes also provide insight into the electrochemical behavior
(7) (a) Howell, J. O.; Wightman, R. M.; Anal. Chem. 1984, 56, 524. (b) Russell, A.; Repka, K.; Dibble, T.; Ghoroghchian, J.; Smith, J. J.; Fleischmann, M.; Pitt, C. H.; Pons, S. Anal. Chem. 1986, 58, 2961. (c) Andrieux, C. P.; Hapiot, P.; Saveant, J.-M. J. Phys. Chem. 1988, 92, 5992. (d) Wipf, D. O.; Kristensen, E. W.; Deakin, M. R.; Wightman, R. M. Anal. Chem. 1988, 60, 306. (e) Bond, A. M.; Henderson, T. L. E.; Mann, D. R.; Mann, T. F.; Thormann, W.; Zoski, C. G. Anal. Chem. 1988, 60, 1878. (f) Bowyer, W. J.; Engelman, E. E.; Evans, D. H. J. Electroanal. Chem. 1989, 262, 67. (8) (a) Norton, J. D.; White, H. S.; Feldberg, S. W. J. Phys. Chem. 1990, 94, 6772. (b) Smith, C. P.; White, H. S. Anal. Chem. 1993, 65, 3343. (9) (a) Fan, F.-R.; Kwak, J.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 9669. (b) Bard, A. J.; Fan, F.-R. Acc. Chem. Res. 1996, 29, 572. (10) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2000, 72, 276. (11) Bach, C. E.; Nichols, R. J.; Beckmann, W.; Meyer, H.; Schulte, A.; Besenhard, J. O.; Jannakoudakis, P. D. J. Electrochem. Soc. 1993, 140, 1281. (12) (a) Wipf, D. O.; Bard, A. J. J. Electrochem. Soc. 1991, 138, 469. (b) Mirkin, M. V.; Richards, T. C.; Bard, A. J. J. Phys. Chem. 1993, 97, 7672. (c) Bath, B. D.; Lee, R. D.; White, H. S. Anal. Chem. 1998, 70, 1047. (13) (a) Penner, R. M.; Heben, M. J.; Lewis, N. S. Anal. Chem. 1989, 61, 1630. (b) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 250, 1118. (c) Pendley, B. D.; Abruna, H. D. Anal. Chem. 1990, 62, 782. (d) Wong, D. K. Y.; Xu, L. Y. F. Anal. Chem. 1995, 67, 4086. (14) (a) Green, M. P.; Hanson, K. J.; Scherson, D. A.; Xing, X.; Richter, M.; Ross, P. N.; Carr, R.; Lindau, I. J. Chem. Phys. 1989, 93, 2181. (b) Vitus, C. M.; Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 7559. (15) (a) Wiechers, J.; Twomey, T.; Kolb, D. M.; Behm, R. J. J. Electroanal. Chem. 1988, 248, 451. (b) Nagahara, L. A.; Thundat, T.; Lindsay, S. M. Rev. Sci. Instrum. 1989, 60, 3128. (16) Gewirth, A. A.; Craston, D. H.; Bard, A. J. J. Electroanal. Chem. 1989, 261, 477. (b) Kozminski, K. D.; Gutman, D. A.; Davila, V.; Sulzer, D.; Ewing, A. G. Anal. Chem. 1996, 68, 3123. (17) (a) Heben, M. J.; Dovek, M. M.; Lewis, N. S.; Penner, R. M.; Quate, C. F. J. Microsc. 1988, 152, 651. (b) Clark, R. A.; Ewing. A. G. Anal. Chem. 1998, 70, 1119. (18) (a) Strein, T. G.; Ewing, A. G. Anal. Chem. 1992, 64, 1368. (b) Schulte, A.; Chow, R. H. Anal. Chem. 1996, 68, 3054. (c) Xin, Q.; Wightman, R. M. Anal. Chem. 1998, 70, 1677.
* Corresponding author: (e-mail)
[email protected]; (phone) 801/ 585-6256; (fax) 801/585-3207. (1) Slevin, C. J.; Gray, N. J.; Macpherson, J. V.; Webb, M. A.; Unwin, P. R. Electrochem. Commun. 1999, 1, 282. (2) Morris, R. B.; Franta, D. J.; White, H. S. J. Phys. Chem. 1987, 91, 3559. (3) (a) Dayton, M. A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980, 52, 946. (b) Ciolkowski, E. L.; Maness, K. M.; Cahill, P. S.; Wightman, R. M.; Evans, D. H.; Fosset, B. Amatore, C. Anal. Chem. 1994, 66, 3611. (c) Pihel, K.; Schroeder, T. J.; Wightman, R. M. Anal. Chem. 1994, 66, 4532. (d) Cahill, P. S.; Wightman, R. M. Anal. Chem. 1995, 67, 2599. (e) Cahill, P. S.; Walker, Q. D.; Finnegan, J. M.; Mickelson, G. E.; Travis, E. R.; Wightman, R. M. Anal. Chem. 1996, 68, 3180. (4) (a) Chien, J. B.; Wallingford, R. A.; Ewing, A. G. J. Neurochem. 1990, 54, 633. (b) Lau, Y. Y.; Chien, J. B.; Wong, D. K. Y.; Ewing, A. G. Electroanalysis 1991, 3, 87. (5) (a) Adams, R. M. Anal. Chem. 1976, 48, 1126A. (b) Giros, B.; Jaber, M. Jones, S. R.; Wightman, R. M. Nature 1996, 379, 606. (c) Zerby, S. E.; Ewing, A. G. Brain Res. 1996, 712, 1. (6) Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Palanker, D.; Lewis, A. Anal. Chem. 1997, 69, 1627. 10.1021/ac000399+ CCC: $19.00 Published on Web 08/10/2000
© 2000 American Chemical Society
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using technology adapted from the report by Slevin et al.1 By controlling the amount of electrophoretic insulation deposited onto etched Pt microwires, we have been able to reproducibly fabricate electrodes with effective radii between 2 and 1000 nm. We have evaluated the voltammetric behavior of these electrodes using nine redox systems and observe well-defined and highly stable diffusion-limited responses in all but two cases. Preliminary investigations of the voltammetric behavior of these electrodes in the presence and absence of supporting electrolyte have also been conducted to examine the influence of diffusion and migration on the electrochemical behavior of nanometer-sized electrodes. Our results indicate that nearly ideal behavior can be obtained with electrodes with radii as small as 10 nm. Electrodes with smaller radii exhibit a consistent discrepancy between experimental and theoretical predictions. EXPERIMENTAL SECTION Chemicals and Reagents. Ferrocenylmethyltrimethylammonium hexafluorophoshate was prepared via the metathesis of the corresponding iodide salt (Strem, 99%) with ammonium hexafluorophosphate (Strem, 99.5%). The yellow crystals were collected via vacuum filtration and recrystallized from water. Iron(II) tris(phenanthroline) sulfate, [Fe(phen)3]SO4, was prepared from ammonium iron(II) sulfate hexahydrate (Aldrich, 99.997%) and 1,10-phenanthroline (Aldrich, 99+%). Iron(II) tris(bipyridyl) perchlorate, [Fe(bpy)3](ClO4)2, was prepared from iron(II) sulfate heptahydrate (Aldrich, 99+%) and 2,2′-dipyridyl (Aldrich, 99+%). Solutions of ferrocene monocarboxylate, Fc(COO-), and 1,1′ferrocene dicarboxylate, Fc(COO-)2, were prepared by titrating the corresponding acids (Strem, 97 and 96%, respectively) with an equimolar amount of base using 0.1 M NaOH (Aldrich, 97+%). Potassium ferrocyanide trihydrate (99%), potassium hexachloroiridate(III) (99%), sodium sulfate (99%), sodium perchlorate (99+%), potassium chloride (99+%), and 1,1′-ferrocenedimethanol (98%) were all purchased from Aldrich and used as received. Hexaamineruthenium(III) chloride (99%) was purchased from Strem and used as received. All solutions were prepared using water obtained from a Barnstead “E-pure” water purification system. Electrode Construction. Inlaid Pt disk electrodes were constructed from 25-µm-diameter Pt wires (Alfa-Aesar, 99.999%) sealed in glass. The melted end of the glass was polished to a flat surface using sandpaper of successively finer grit to expose a Pt disk. The electrodes were then polished in alumina (0.3-0.01µm diameter) slurry and rinsed with distilled water. Nanometer-sized electrodes were constructed using a procedure adapted from Slevin et al.,1 originally employed by Bach et al.11 for the preparation of coated tips used in STM studies. Electrical contact was made between a 3-cm length of 25-µmdiameter Pt wire and a 15-cm length of 250-µm-diameter W wire (FHC, 99.95%) using a Ag conductor composition (W. K. Robson, 5504N). A 20-cm portion of glass tubing (Corning, 2-mm o.d., 1-mm i.d.) was carefully heated in a butane flame and pulled gently while rotating. After cooling in air for several seconds, the glass tubing was carefully separated in the middle, yielding two ∼12cm lengths of glass tubing with capillaries (∼250-mm o.d.) at one end. The Pt/W wire ensemble was then inserted into the large end of the glass tubing until ∼5 mm of Pt wire extended from the end of capillary. The W wire was secured to the other end of 4442
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the glass tubing using 5-min epoxy (Devcon). The Pt wire was then sealed in glass by resistive heating. The lower ∼5 mm of the glass capillary was positioned in the center of a Nichrome coil (Goodfellow, Ni80Cr20, 0.75-mm diameter) using a vertical translation stage. An ac voltage of ∼6 V was applied to the Nichrome coil for ∼20 s to melt the capillary around the Pt wire, yielding a “bead” of glass at the end. Inspection of the glass/ metal junction with an optical microscope suggests that the lower ∼1 mm of glass is completely melted around the Pt wire. The ∼5 mm of Pt wire extruding from the glass capillary was electrochemically etched in 10 M aqueous NaNO2 employing a Pt coil as a counter electrode. Using a vertical translation stage, ∼1 mm of the Pt wire was carefully immersed in the NaNO2 solution and positioned in the center of the Pt coil to ensure uniform etching. An ac voltage of ∼1.6 V was applied for ∼2 min to ensure complete etching of the wire. During electrochemical etching, bubbles could be observed at the metal/liquid interface for ∼90 s. Optical micrographs indicate that a blunt tip is formed if the electrochemical cell is disconnected once bubbling ceases. Conversely, a sharp tip results when the ac voltage is maintained for ∼30 s beyond cessation of bubbling at the metal/solution interface. Once etching is complete, the exposed wire is thoroughly rinsed with triply distilled water to remove any residual NaNO2. SEM images of these etched wires are consistent with those reported by Slevin et al.1 Insulation of the etched Pt wire was accomplished using an anodic electrophoretic paint (PPG, ZQ84-3225). The paint consists of poly(acrylic acid) (PAAH) with an excess of base added in order to deprotonate the acid groups, yielding a water-soluble species (PAA-). The exposed portion of the Pt wire was completely immersed in a dilute aqueous paint solution (50:1) and positioned in the center of a Pt coil (∼1-cm diameter). A dc potential of +2.1 V was applied between the Pt wire and the Pt coil to initiate the oxidation of water.
2H2O h O2 + 4H+ + 4e-
(1)
The local change in the pH at the electrode surface shifts the equilibrium to the protonated species, PAAH,
PAA- + H+ h PAAH
(2)
which is water insoluble and adsorbs onto the Pt surface. Nanometer-sized electrodes were obtained by applying multiple coatings of PAAH. For the first three coats of insulation, electrodes were immersed in the paint solution for ∼1 min while under potential control. This procedure yields electrodes with radii between 100 and 1000 nm. To obtain a further reduction in electrode size, the above procedure was repeated in 10-s intervals. After each electrophoretic deposition, the electrode was removed from solution, inverted for ∼1 min to allow excess solution to drain away from the tip, and cured in an oven for 5 min at 200 °C to harden the insulation layer. Bach et al. have speculated that the curing stage results in shrinkage of the layer, causing it to retract away from the sharp tip.11 To achieve electrodes with radii less than 100 nm required three to six additional applications of the electrophoretic paint. We have found it difficult to precisely control the radius of the electrode based on the number of coatings and
Figure 1. Voltammetric response of Pt electrodes electrophoretically insulated with (A) three, (B, C) six, and (D-F) nine coatings of PAAH. The voltammograms were recorded in a 2.0 mM FcTMA+ solution with 0.1 M KCl as supporting electrolyte. Electrode radii, rapp, indicated on the figure were calculated using eq 3. Scan rate, 10 mV/s.
other coating parameters. However, greater than 90% of the etched Pt wires that were insulated displayed well-defined i-V characteristics. Electrochemical Apparatus. A Cypress model EI-400 bipotentiostat interfaced to a computer through an AT-MIO16E-10 data acquisition board (National Instruments) was controlled via a BNC-2090 analog breakout accessory (National Instruments). Voltammetric data were recorded using in-house virtual instrumentation written in LabVIEW (National Instruments). The highsensitivity preamplifier (100 pA/V to 10 nA/V) of the bipotentiostat was used in all experiments. The electrochemical cell consisted of a one-compartment, two-electrode configuration employing Ag/ AgCl (3 M NaCl) as the reference/counter electrode. The nanometer-sized electrodes were introduced into the solution via a vertical translation stage. Only the tip of the electrode was immersed into the solution to prevent leaching of solution through the glass/metal junction. The electrochemical cell and the preamplifier were both housed in a Faraday cage. RESULTS AND DISCUSSION Characterization of Electrode Size. Following the approximation of Slevin et al.,1 we assume that the exposed tip has a hemispherical shape. The apparent radius of a hemispherical microelectrode, rapp, can be determined by measuring the steadystate diffusion-limited current, iD, in a solution containing a known concentration of a redox-active molecule.
iD ) gnFDC*rapp
(3)
In eq 3, D and C* are the diffusion coefficient and bulk concentration of the electroactive species, respectively, n is the number of electrons transferred per molecule, F is Faraday’s constant, and the geometric factor, g, is a constant indicative of the shape of the electrode. For a hemispherical-shaped electrode, the geometric factor is 2π. Values of rapp are apparent values
because they are based on an assumed geometry and an assumed mode of transport. Other nonideal geometries (e.g., lagoon electrodes19) may mimic the response of a true hemisphericalshaped microelectrode. In addition, the use of continuum-based transport expressions for describing the voltammetric response of ultrasmall electrodes is questionable.2,8 However, we believe that eq 3 is useful in providing a rough estimate of the exposed surface area of the electrodes. Ferrocenylmethyltrimethylammonium (FcTMA+) was employed as the electroactive species for characterization of electrode radii. The diffusivity of FcTMA+ was measured from the voltammetric limiting current at calibrated inlaid disk electrodes (nominal radius, 12.5 µm). The radii of the inlaid disks, a, were first determined using a 4.0 mM ferrocyanide (Fe(CN)64-) solution containing 0.1 M KCl. The diffusivity of Fe(CN)64- in these solution conditions has been previously reported20 to be (6.50 ( 0.02) × 10-6 cm2/s. The steady-state limiting currents, iD, of five inlaid-disk Pt microelectrodes were recorded 10 times each in the Fe(CN)64- solution, polishing the electrode with 0.01-µm-diameter alumina after each measurement. The radii of the electrodes were determined from eq 3 (replacing 2π by 4 and rapp by a) using the averaged value of iD. These electrodes were then used to determine the diffusivity of 2.0 mM FcTMA+ in solutions containing 0.1 M KCl. This value, (7.47 ( 0.09) × 10-6 cm2/s, has been used to determine the apparent radii of the PAAH-coated Pt wires. Figure 1 shows the voltammetric response of six PAAH-coated electrodes in a 2.0 mM FcTMA+ solution. After three coatings of insulation, electrodes exhibit apparent radii, rapp, of less than 1 µm (Figure 1A). The voltammetric response is sigmoidal in shape, a characteristic of well-behaved microelectrodes. With additional coating of electrophoretic paint, the apparent radii of the electrode decreases, eventually approaching nanometer dimensions. After (19) Baranski, A. S. J. Electroanal. Chem. 1991, 307, 287. (20) von Stackelberg, M.; Pilgram, M.; Toome, V. Z. Elektrochem. 1953, 57, 342.
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six coatings (Figure 1B,C), electrode radii are measured between 50 and 200 nm. Electrodes coated nine times have radii almost always below 50 nm (Figure 1D-F). Figure 1F shows the voltammetric response of an electrode with rapp ) 1.7 nm. Although the voltammetric response is well-defined and stable in KCl solutions, the behavior of electrodes with radii less than 10 nm in low ionic strength solutions displays nonideal behavior (vide infra). In addition, we speculate that, when such small electrodes are used, the current due to steady-state transport of FcTMA+ through the insulating PAAH layer, i.e., membrane diffusion, may also be comparable to that arising from radial diffusion of the molecule in solution to the exposed tip. Thus, while we report the data at the smallest electrodes, the shape of the tip and coating for these electrodes is questionable. On occasion, electrodes become completely insulated after continual coating (>6 total) with the electrophoretic paint. This is usually observed with electrodes in the 20-30-nm range that are further insulated, resulting in a response that does not exhibit any measurable current for FcTMA+ oxidation. However, when overinsulated electrodes are held at negative potentials (-0.3 V vs Ag/AgCl) for 1-3 min, the electrophoretic paint is slowly removed, exposing a portion of the Pt wire. The dissolution of the PAAH layer at negative potentials limits the application of these electrodes to positive potentials. This point is discussed further below. The stability of these electrodes has been examined by monitoring their steady-state currents over extended periods of time. Voltammetric data were periodically recorded for several electrodes for a period of up to 2 weeks in order to determine whether the insulation layer cracks or deteriorates with time. Electrodes were stored in air between voltammetric experiments.
Values of iD in the FcTMA+ solution varied less than ∼5%, suggesting that the electrophoretic paint is very stable. Electrochemical Behavior in Low Ionic Strength Solutions. In the absence of convective stirring, migration and diffusion of the electroactive species to the electrode surface govern the magnitude of the steady-state limiting current, iD. By adding an excess of supporting electrolyte to the solution such that COR , CSE, where COR and CSE are the concentrations of the redox couple and the supporting electrolyte, respectively, contributions to the current arising from migration are minimized.21 In solutions where the concentration of electrolyte is low (i.e., when COR/CSE > 1), the steady-state limiting current contains a significant migrational contribution. Amatore et al.22 has shown that the relative contribution of migrational transport to the total current, iT, at a hemispherical electrode is dependent upon the magnitude and sign of the charge, z, of the electroactive species, as well as the number of electrons transferred (n) in the reaction. For electrochemical reactions where z is identical (sign and magnitude) to n (i.e., z ) n), iT, is given by
iT/iD ) 1 + |n|
(4)
When z * n,
iT/iD ) 1 ( z{1 + (1 + |z|)(1 - z/n) × ln(1 - 1/[(1 + |z|)(1 - z/n)])} (5) where the sign is positive for n < z and negative for n > z. In eq 5, n is negative for oxidations and positive for reductions. Equations 4 and 5 are derived by assuming local electroneutrality
Figure 2. Voltammetric response (A-C) of electrophoretically insulated Pt electrodes in a 1.0 mM FcTMA+ solution with (i) and without (ii) supporting electrolyte (0.1 M KCl). Electrode radii were determined using eq 3 from the text. Scan rate, 10 mV/s. (D) Dependence of current diminishment, % d, on electrode radius, rapp. Current diminishments, % d, were estimated from the ratio of the steady-state limiting current in the absence of supporting electrolyte, iT, to that with electrolyte present, iD. The horizontal dashed line corresponds to the predicted % d using eq 5. 4444
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Figure 3. Voltammetric response of electrophoretically insulated Pt microwires in (A) 20.2 mM Fe(CN)63- + 0.2 M KCl, (B) 10.3 mM Ru(NH3)3+ + 0.1 M KCl, (C) 10.1 mM Fe(phen)32+ + 0.1 M Na2SO4, and (D) 3.2 mM Fe(bpy)32+ + 0.1 M NaClO4. The initial scan direction is indicated in (A) and (B) and the direction of increasing oxidationreduction cycles is indicated by c. Scan rate, 10 mV/s.
and that molecular transport of each species is governed by the Nernst-Planck equation. Equations 4 and 5 are applicable when COR/CSE . 1. For the FcTMA+/2+ redox system (n ) -1 and z ) +1), eq 5 predicts that a diminishment in the current of 15.1% (relative to the diffusion-limited value) should be observed when COR . CSE. Figure 2A-C shows the effects of migration for electrodes having apparent radii ranging from 7 nm to 1.71 µm. In each case, the magnitude of the limiting current decreases significantly in the absence of supporting electrolyte, as predicted by eq 5. The dependence of the current diminishment, % d, on electrode radius, rapp, is shown in Figure 2D. The experimentally observed vales of % d agree well with the theoretical value of 15.1% (dashed line in Figure 2D) for rapp > 10 nm. Below this value, the observed diminishment is significantly less than that predicted using eq 5. For example, only a 9.4% diminishment in the current is observed at the 7-nm-radius electrode when no supporting electrolyte is present (Figure 2C). A similar nonideal dependence on ionic strength at nanometer-scale electrodes was reported by Menon and Martin.23 In the current case, we speculate that the nonideal % d for rapp < 10 nm is either due to the partial coating of the exposed tip by the PAAH layer or to diffuse double-layer effects. Current theoretical predictions based on electrostatic arguments suggest that the i/V response should deviate from that predicted by eq 5 when the electrode size approaches the thickness of the diffuse double layer, as occurs in electrochemistry when a characteristic dimension of the electrode is reduced below ∼10 nm.8b Investigations are currently underway to fully understand (21) Montenegro, I. M.; Queiros, M. A.; Daschbach, J. L. Microelectrodes: Theory and Applications; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991. (22) Amatore, C.; Fosset, B.; Bartlett, J.; Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1988, 256, 255. (23) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928
Figure 4. Voltammetric response of electrophoretically insulated Pt microwires in (A) 9.9 mM IrCl63- + 0.1 M KCl, (B) 1.2 mM 1,1′ferrocenedimethanol, (C) 1.6 mM ferrocenemonocarboxylic acid, and 1.5 mM 1,1′-ferrocenedicarboxylic acid. Scan rate, 10 mV/s.
the observed behavior in this spatial regime. The data in Figure 2D indicate that the voltammetric behavior of the PAAH-coated electrodes can be described using classical transport theory for rapp > 10 nm. Voltammetric Response Using Other Redox Couples. Slevin et al.1 reported the voltammetric response for electrodes as small as 16 nm in radius in solutions containing 20 mM Fe(CN)64- + 0.2 M KCl and employed these electrodes to study the kinetics of Fe(CN)64- oxidation. However, they only presented the initial positive-going scan of the voltammetric wave. As shown in Figure 3A, we find that while the initial scan yields a near-ideal response, the reverse and subsequent scans are quite poorly defined, with the current decreasing and becoming drawn out with each scan (the direction of increasing scans is indicated by the arrow labeled c). Pharr and Griffiths have shown that the electroactive species has a propensity to adsorb onto the electrode surface from KCl solutions as a polymeric hexacyanoferrate species, thus inhibiting electron transfer leading to a gradual degradation of the voltammetric response.24 The poor voltammetric behavior observed in Figure 3A is most likely associated with adsorption of the electroactive species. Figure 3B shows the reduction of ruthenium(III) hexaamine, Ru(NH3)63+, at an electrophoretically insulated Pt electrode. It is well established that Ru(NH3)63+/2+ is a nonadsorbing redox couple with a very high rate of electron transfer.13b Therefore, a steady-state limiting current should easily be achieved at a nanometer-sized electrode. However, as can be seen in Figure 3B, continual cycling suggests that the electrode size increases over time. A steady-state response is never reached. More detailed measurements indicate that the PAAH layer dissolves at potentials negative of -0.2 V vs Ag/AgCl. The dissolution of the insulation layer determines the negative potential limit of investigations in which the electrodes can be employed. The voltammetric response of the electrophoretically coated electrodes in solutions containing other redox couples with (24) Pharr, C. M.; Griffiths, P. R. Anal. Chem. 1997, 69, 4673.
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standard potentials between 0 and 0.8 V vs Ag/AgCl is nearly ideal. Panels C and C of Figure 3 show the oxidations of Fe(phen)32+ and Fe(bpy)32+, respectively. Figure 4shows the oxidation of IrCl63-, 1,1′-ferrocenedimethanol (Fc(CH2OH)2), ferrocenemonocarboxylate, and 1,1′-ferrocenedicarboxylate. The voltammetric response for (Fc(COO-)2) typically displays a larger hysteresis than that of the other redox couples for reasons that are not understood. However, the i-V response for each of these species is highly reproducible and stable. Detailed investigations of migrational effects in low ionic strength solutions for each of these redox couples will be discussed in a future report. CONCLUSIONS Electrodes of nanometer dimensions can be readily constructed by insulating etched Pt wires with electrophoretic paint. However, care must be taken in choosing an electrochemical system due to problems associated with adsorption of the redox species (e.g., Fe(CN6)4-) and dissolution of the insulation layer. These elec-
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trodes yield reproducible i-V responses over an extended period of time in aqueous solutions. The voltammetric behavior of nanometer-size electrodes, in the presence and absence of supporting electrolyte in FcTMA+ solutions, can be described using classical transport theory for rapp > 10 nm. The deviation between predicted and observed limiting currents for rapp < 10 nm is currently under study. ACKNOWLEDGMENT The authors gratefully acknowledge Dr. Julie V. Macpherson, University of Warwick, for her assistance in the initial stages of this work, and Chett Boxley, University of Utah, for his contributions to the electrode fabrication procedure. This work has been supported by the Office of Naval Research. Received for review April 6, 2000. Accepted July 6, 2000. AC000399+
