Anal. Chem. 2009, 81, 5496–5502
Preparation and Electrochemical Response of 1-3 nm Pt Disk Electrodes Yongxin Li, David Bergman, and Bo Zhang* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700 The preparation and characterization of Pt nanoelectrodes in the range of 1 to 3 nm in radii are reported. A Pt microwire is sealed into a bilayer quartz capillary and pulled into an ultrasharp Pt nanowire sealed in a silica tip using a laser-assisted pulling process. The ultrasharp tip is then sealed into a piece of glass tubing, which is manually polished to expose the Pt. Transmission electron microscopy and steady-state voltammetry are utilized to characterize the nanoelectrodes. The results show that the minimum size of the Pt nanoelectrode is determined by the size of the Pt microwire and parameters used in the pulling process. The heterogeneous electron transfer rate constant for the oxidation of ferrocene, ferrocenemethanol, and potassium hexachloroiridate (III) are determined from steady-state voltammetry using the method of Mirkin and Bard and are found to be k° ) 7.6 ( 3.4 cm/s and r ) 0.85 ( 0.06 for ferrocene, k° ) 7.4 ( 6.9 cm/s and r ) 0.78 ( 0.16 for ferrocenemethanol, and k° ) 6.0 ( 4.2 cm/s and r ) 0.72 ( 0.15 for IrCl63-. Electrochemical measurements utilizing metal electrodes of nanometer dimensions have shown tremendous advantages such as smaller RC constants, increased mass-transport rates, and lower influences from solution resistance.1-3 Because of their small size, nanoelectrodes are becoming increasingly important for applications in high-resolution imaging such as scanning tunneling microscopy (STM)4,5 and scanning electrochemical microscopy (SECM)6-10 and single nanoparticle detection11,12 and bioanalysis.13,14 * To whom correspondence should be addressed. Phone: 206-543-1767. Fax: 206-685-8665. E-mail:
[email protected]. (1) Zoski, C. G. Electroanalysis 2002, 14, 1041–1051. (2) Arrigan, D. W. M. Analyst 2004, 129, 1157–1165. (3) Murray, R. W. Chem. Rev. 2008, 108, 2688–2720. (4) Penner, R. M.; Heben, M. J.; Lewis, N. S. Anal. Chem. 1989, 61, 1630– 1636. (5) Bach, C. E.; Nichols, R. J.; Beckmann, W.; Meyer, H.; Shulte, A.; Besenhard, J. O.; Jannakoudakis, P. D. J. Electrochem. Soc. 1993, 140, 1281–1284. (6) Mirkin, M. V.; Fan, F. R. F.; Bard, A. J. J. Electroanal. Chem. 1992, 328, 47–62. (7) Mirkin, M. V.; Richards, T. C.; Bard, A. J. J. Phys. Chem. 1993, 97, 7672– 7677. (8) Mirkin, M. V.; Bulhoes, L. O. S.; Bard, A. J. J. Am. Chem. Soc. 1993, 115, 201–204. (9) Sun, P.; Mirkin, M. V. Anal. Chem. 2006, 78, 6526–6534. (10) Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Palanker, D.; Lewis, A. Anal. Chem. 1997, 69, 1627–1634. (11) Xiao, X.; Bard, A. J. J. Am. Chem. Soc. 2007, 129, 9610–9612. (12) Fan, F. R. F.; Bard, A. J. Science 1997, 277, 1791–1793. (13) Cahill, P. S.; Walker, Q. D.; Finnegan, J. M.; Mickelson, G. E.; Travis, E. R.; Wightman, R. M. Anal. Chem. 1996, 68, 3180–3186.
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It is known that one of the main challenges in applying nanometer-scale electrodes is the reliable fabrication of geometrically well-defined nanoelectrodes, whose size approaches true molecular dimensions, e.g., 18 MΩ cm, Barnstead Nanopure Systems) and acetonitrile, respectively. Pt microwires with a diameter of 25 µm (99.95%, hard) were purchased from Alfa Aesar. Quartz capillaries (o.d. ) 1.2 mm, i.d. ) 0.40 mm) were obtained from Sutter Instrument Co. A silverfilled epoxy glue (DuPont) was used to contact the Pt wire with a tungsten wire. Alumina polishing powders with different particle sizes of 1.0, 0.3, and 0.05 µm and fine grit sandpapers with 400, 600, and 800 grits were purchased from Buehler. Instruments. A laser puller (P-2000, Sutter Instrument Company) was used for the preparation of the Pt nanoelectrodes. A Dagan Chem-Clamp voltammeter/amperometer was used as a potentiostat. The potentiostat was interfaced to a Dell computer through a PCI-6521 data acquisition board (National Instrument) via a BNC-2090 analog breakout accessory (National Instrument). Voltammetric data were recorded using in-house virtual instrumentation written in LabView (National Instrument). A onecompartment, two-electrode cell was employed with the cell and preamplifier in a home built Faraday cage. A Ag/AgCl electrode (Bioanalytical Sciences, Inc.) was used as the reference electrode. Transmission Electron Microscopy. TEM images of SiO2coated Pt nanoelectrodes were acquired on a Tecnai G2 F20 (FEI) microscope. No additional coatings were performed prior to imaging. Nanoelectrode Preparation. As shown in Figure 2, Pt nanoelectrodes below 3 nm were prepared utilizing a four-step process. The first two steps involved the preparation of an ultrasmall Pt nanowire sealed in an ultrasharp silica tip utilizing a laser-assisted pulling process reported by Mirkin et al. and Schuhmann et al.,10,18 which was then modified in our laboratory. In the last two steps, the sharp silica tip was sealed in borosilicate capillary tubing followed by polishing away the excess glass to expose the Pt nanosurface. It is critical to obtain ultrasharp Pt nanowires in order to prepare Pt electrodes below 3 nm in radii. To make an ultrasharp Pt nanowire, we developed a laser-assisted pulling process and applied it based on the reported procedures. First, a ∼10 mm long piece of a Pt microwire was prepared by electrochemically etching a 25 µm Pt wire in a 6 M NaCN solution containing 0.1 M NaOH. (Caution: This solution is extremely toxic, and care must be taken when making and handling this solution! All of the etching should be carried out in a ventilation hood.) Using an function generator (33220A, Agilent Technologies), we applied a 3.5 V, 600 Hz ac voltage between the Pt microwire and a 1 mm Pt wire counter Analytical Chemistry, Vol. 81, No. 13, July 1, 2009
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electrode for a short period of time. Depending on the etching time, we easily controlled the size of the Pt microwire from as small as 5-25 µm. A smaller voltage (e.g., ∼2.5 V) is needed to reduce the etching rate in order to obtain microwires smaller than 5 µm. The Pt microwire was then inserted into a 1 cm long piece of a silica capillary with an inner diameter of ∼80 µm and an outer diameter of ∼350 µm with the help of a stereo microscope (Bausch & Lomb). The small silica capillary/Pt ensemble was then inserted into a 7.5 cm long piece of a silica capillary tubing (i.d. ) 0.40 mm, o.d. ) 1.2 mm) as shown in Figure 2. The Pt microwire was then sealed in silica using a laser-assisted micropipet puller (P-2000). A homemade aluminum clamp was utilized to secure the moving puller bars in place.18 The puller bars are the two holders for mounting the quartz capillary to be pulled. An in-house vacuum was connected to the silica tubing during the sealing process. To obtain a complete seal between the microcapillary and Pt without melting the metal, we employed the following heating parameters: heat ) 760 and filament ) 4. Other parameters were unimportant because no pulling was allowed with the puller bars clamped. This program was applied for three cycles, each cycle consisting of a 45 s heating period followed by a 20 s cooling period. The clamp was then removed, and the sealing was visually checked under an optical microscope to ensure that no gap was present between the Pt and SiO2. In the second step, the Pt microwire was pulled into two Pt nanowire tips sealed in silica using the following program: heat ) 820, filament ) 1, velocity ) 80, delay ) 150, and pull ) 225. The Pt nanowire tip was visualized using the transmission mode of a 400X magnification microscope (Olympus, BX51). In the last two steps, the Pt nanowire tip was exposed to make the nanoelectrode by a special sealing and polishing process. The pulled Pt nanowire tip was inserted into a 6 cm long piece of 2 mm o.d. borosilicate glass tubing. The entire tip portion was then sealed into borosilicate under in-house vacuum using a hydrogen flame. A 150 µm tungsten wire was used to make electrical contact with the Pt microwire using conductive silver paste. To expose the Pt nanowire tip, we ground the sealed end using regular sandpaper and an alumina suspension on a polishing cloth. The whole process was monitored by using an optical microscope and a homemade ultrasensitive continuity tester to ensure the polishing stops when the very end of the Pt is exposed.17 RESULTS AND DISCUSSION Electrochemical Etching to Obtain Pt Microwires. Electrochemical etching is utilized to prepare Pt microwires smaller than 25 µm. The setup is similar to the one used to fabricate ultrasharp Pt tips for scanning tunneling microscopy (SEM). Figure SI1A of the Supporting Information shows such an electrochemical cell containing a 25 µm Pt wire and a Pt macrowire counter electrode. The size of the Pt microwire is controlled by varying the time duration of the etching. Depending on the final dimension of the Pt microwire and the length of the Pt immersed in the etchant, we find the etching can usually take from 30 to 90 s to finish. The etched Pt microwires are uniform in diameter and smooth at the surface. Panels B-D of Figure SI1of the Supporting Information show optical micrographs of three Pt microwires having diameters of 20, 12, and 6 µm, respectively. 5498
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Sealing Silica-Coated Ultrasharp Pt in Borosilicate Glass and Exposing Pt Nanosurfaces. Because of the conical geometry of the pulled Pt nanowire, it is critical to expose the metal right at the tip apex to obtain an electrode with its minimum size.9,10,18 Special polishing processes have been developed to obtain Pt nanoelectrodes as small as ∼4 nm in radii.9 However, the extremely small size of the ultrasharp silica tip (∼200 nm) makes it extremely hard to expose the Pt by direct polishing on a surface without breaking the tip at larger dimensions. To better control the polishing process, we sealed the silica-coated ultrasharp Pt nanowires in a 2 mm borosilicate glass capillary and polished it using fine sandpapers and aluminum oxide powers. The final process in exposing the Pt nanosurfaces is carried out using a high-sensitivity continuity tester as described elsewhere.17 The polishing rate can be well-controlled in this fashion, which results in >90% reproducibility in obtaining Pt electrodes
