Anal. Chem. 2001, 73, 5346-5351
Fabrication of Nanometer-Sized Electrodes and Tips for Scanning Electrochemical Microscopy Peng Sun, Zhiquan Zhang, Jidong Guo, and Yuanhua Shao*
Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences, Changchun, Jilin 130022, China
A novel method for fabrication of nanometer-sized electrodes and tips suitable for scanning electrochemical microscopy (SECM) is reported. A fine etched Pt wire is coated with polyimide, which was produced by polymerization on the Pt surface initiated by heat. This method can prepare electrodes with effective radii varying from a few to hundreds of nanometers. Scanning electron microscopy, cyclic voltammetry, and SECM were used to characterize these electrodes. Well-defined steady-state voltammograms could be obtained in aqueous or in 1,2dichloroethane solutions. This method produced the nanoelectrodes with exposed Pt on the apex, and they can also be employed as the nanotips for SECM investigations. Different sizes of Pt nanotips made by this method were employed to evaluate the kinetics of the redox reaction of Ru(NH3)63+ on the surface of a large Pt electrode by SECM, and the standard rate constant K0 of this system was calculated from the best fit of the SECM approach curve. This result is similar to the values obtained by analysis of the obtained voltammetric data. During the last two decades, there has been growing interesting in using of nanoelectrodes in electrochemical investigations.1,2 The advantages arise from the fact that these nanoelectrodes can reduce iR drop and double-layer charging effects.3,4 Additionally, the rate of mass transport at these electrodes is so high that fast heterogeneous electron-transfer kinetic measurements would be carried out by steady-state measurements rather than by transient techniques, which provided more reliable information and simpli* To whom the correspondence should be addressed. E-mail: yhshao@ ns.ciac.jl.cn. (1) (a) Penner, R. M.; Heben, M. J.; Lewis, N. S. Anal. Chem. 1989, 61, 16301636. (b) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 250, 1118-1121. (2) (a) Morris, R. B.; Franta, D. J.; White, H. S. J. Phys. Chem. 1987, 91, 35593564. (b) Norton, J. D.; White, H. S.; Feldberg, S. W. J. Phys. Chem. 1990, 94, 6772-6780. (c) Conyers, J. L.; White, H. S. Anal. Chem. 2000, 72, 4441-4446. (3) (a) Heinze, J. Angew. Chem., Int. Ed. Engl. 1993, 32, 1268-1288. (b) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15. (c) Fleischmann, M.; Pons, S.; Rolison, D. R.; Schmidt, P. P. Ultromicroelectrodes; Datatech Systems, Inc: Morganton, NC, 1987; Chapter 3. (4) (a) Howell, J. O.; Wightman, R. M. Anal. Chem. 1984, 56, 524-529. (b) Bond, A. M.; Fleischmann, M.; Robinson, J. J. Electroanal. Chem. 1984, 168, 299-312.
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fied data analysis.5 The nanoelectrodes also show promising applications in the study of microenvironmental and biological specimens.6,7 Scanning electrochemical microscopy (SECM) is one of the major developments in electrochemistry in the past decade.8 It combines the advantages of ultramicroelectrodes (UMEs) and scanning probe microscopes (SPMs). Many electrochemical techniques can be implemented in situ with additional high spatial resolution with SECM. It has been proved to be a powerful tool in investigation of fast heterogeneous kinetics on electrode surfaces, microfabrication, and imaging of local electrochemical reactivity. The attainable information by SECM strongly depends on the type and size of the tips used, which is normally an UME or a micro-ion-selective electrode (MISE). To obtain nanometer resolution, and completely eliminate the iR effect in many lowconductivity solvents, fabrication of nanometer tips is desirable and the trend of development.5 Crooks et al.9 recently reported that single carbon nanotubes could be used for electrochemical studies. The problem is that it is not easy to handle this tiny creature. Nanopipets have also been used to evaluate the fast kinetics of facilitated potassium ion transfer across a water/1,2dichloroethane (DCE) interface by dibenzo-18-crown-6 (DB18C6), and it can be easily handled and fabricated.10a Nevertheless, it is confined only to the study of charge transfer across liquid/liquid interfaces. Pt has been used widely in electrochemical investigations. However, at present only a few attempts to make Pt electrodes with the radii in nanometer ranges have been reported.1b,2,10b-12 Fabrication of a nanometer Pt electrode usually involves two major steps. First, a Pt wire is electrochemically etched to an ultrafine (5) (a) Oldham, K. B.; Myland, J. C.; Zoski, C. G.; Bond, A. M. J. Electroanal. Chem. 1989, 270, 79-101. (b) Mirkin, M. V.; Fan, F.-R. F.; Bard, A. J. J. Electroanal. Chem. 1992, 328, 47-62. (6) (a) Pihel, K.; Schroeder, T. J.; Wightman, R. M. Anal. Chem. 1994, 66, 4532-4537. (b) Cahill, P. S.; Walker, Q. D.; Finnegan, J. M.; Mickelson, G. E.; Travis, E. R.; Wightman, R. M. Anal. Chem. 1996, 68, 3180-3186. (c) Giros, B.; Jaber, M.; Jones, S. R.; Wightman, R. M. Nature 1996, 379, 606-611. (7) (a) Chien, J. B.; Wallingford, R. A.; Ewing, A. G. J. Neurochem. 1990, 54, 633-637. (b) Lau, Y. Y.; Chien, J. B.; Wong, D. K. Y.; Ewing, A. G. Electroanalysis 1991, 3, 87-95. (8) (a) Lee, C.; Miller, C. J.; Bard, A. J. Anal. Chem. 1991, 63, 78-83. (b) Mirkin, M. V.; Arca, M.; Bard, A. J. J. Phys. Chem. 1993, 97, 10790-10795. (9) Campbell, J. K.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 37793783. (10) (a) Shao, Y.; Mirkin, M. V. J. Am. Chem. Soc. 1998, 119, 8103-8104. (b) Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Plalanker, D.; Lewis, A. Anal. Chem. 1997, 69, 1627-1634. 10.1021/ac010474w CCC: $20.00
© 2001 American Chemical Society Published on Web 09/22/2001
point. This is easy to archive by using several environmentally benign etching solutions.13 Second, the etched ultrafine Pt wire is coated with an insulating material, except at the apex of the wire. Although a number of coating materials have been employed, such as Apiezon wax,14 varnish,15 molten paraffin,16 glass,1,10b,12 poly(R-methylstyrene),1a silica coatings,17 electrochemically polymerization of a layer of phenol,18 and electrophoretic paint,2c,11 these insulating procedures have other problems. Either the timeconsuming process19 or the easily cracking of this material14a prevents these methods from wide use. Some of these materials cannot be used in organic solvents. Recently, Conyers and White2c fabricated submicroelectrodes following the method proposed by Unwin et al.11a to cover the etched Pt wires by electrophoretic paint. They demonstrated that well-defined cyclic voltammograms could be obtained. Various sizes of Pt nanoelectrodes can also be made using a laser puller. By this way, the fabricated nanoelectrodes could be employed as SECM tips.10b Another point needing to be emphasized is that only a few methods mentioned previously can fabricate nanoelectrodes that are suitable for SECM studies because the nanometer-sized Pt wire must be on the apex of the electrode for a tip of SECM.5b,10b,11a,c In this paper, we describe a simple, inexpensive method to produce Pt nanometer-sized electrodes by coating etched Pt wire with polyimide produced by the polymerizing reaction. The effective radii of the electrodes fabricated this way vary from a few to hundreds of nanometers. The nanoelectrodes can be used in aqueous or organic solvents and have a wide potential window in both solutions. This method overcomes the defects in some of the previous methods.2c,11,14-18 Scanning electron microscopy (SEM) images and SECM experimental results indicated that the fabricated nanoelectrodes can also be used as the SECM tips. EXPERIMENTAL SECTION Chemicals and Apparatus. Hexaammineruthenium(III) chloride ((Ru(NH3)63+, 98%, Aldrich), 7,7,8,8-tetracyanoquinodimethane (TCNQ, 98%, ACROS), and potassium ferricyanide (K3Fe(CN)6, A. R. Beijing Chemicals, Co., China) were used. 1,2,4,5-Benzenetetracarboxylic anhydride, 4,4′-diaminodiphenyl ether, 1,2-dichloroethane (DCE), N,N-dimethylacetamide, potassium chloride, cadmium chloride, and hydrochloric acid were supplied by Shanghai Chemicals Co., and they are all analytical grade or better. 1,2,4,5-Benzenetetracarboxylic anhydride and 4,4′-diaminodiphenyl ether were sublimated at 260 and 160 °C in a vacuum, (11) (a) Slevin, C. J.; Gray, N. J.; Macpherson, J. V.; Webb, M. A.; Unwin, P. R. Electrochem. Commun. 1999, 1, 282-288. (b) Bach, C. E.; Nichols, R. J.; Beckmann, W.; Meyer, H. J. Electrochem. Soc. 1993, 40, 1281-1284. (c) Zu, Y.; Ding, Z.; Zhou, J.; Lee, Y.; Bard, A. J. Anal. Chem. 2001, 73, 21532156. (12) Lee, Y.H.; Tsao, G. T.; Wankant, P. C. Ind. Eng. Chem. Fundam. 1978, 17, 59-81. (13) Nam, A. J.; Teren, A.; Lusby, T. A.; Melmed, A. J. J. Vac. Sci. Technol. B 1995, 13, 1556-1559. (14) (a) Nagahara, L. A.; Tundat, T.; Lindsay, S. M. Rev. Sci. Instrum. 1989, 60, 3128-3130. (b) Wiechers, J.; Twomey, T.; Kolb, D. M.; Behm, R. J. J. Electroanal. Chem. 1988, 248, 451-457. (15) (a) Gewirth, A. A.; Craston, D. H.; Bard, A. J. J. Electroanal. Chem. 1989, 261, 477-482. (b) Vitus, C. M.; Chang, S. C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 7559-7563. (16) Zhang, B.; Wang, E. Electrochim. Acta 1994, 39, 103-107. (17) Zhao, G.; Giolando, D. M.; Kirchhoff, J. R. Anal. Chem. 1995, 67, 25922598. (18) Potje-Kamloth, K.; Janata, J.; Jossowicz, M. Ber. Bunsen-Ges. Phys. Chem. 1990, 93, 1480-1485. (19) Schulte, A.; Chow, R. H. Anal. Chem. 1996, 68, 3054-3058.
respectively. N,N-Dimethylacetamide was newly distilled, and some molecular sieves were added in it to ensure that it was very dry. Etching solution contained 50% (by volume) saturated CaCl2, 25% HCl, and 25% deionized water. Coating solution was prepared by dissolution of 0.5000 g of 1,2,4,5-benzenetetracarboxylic anhydride and 0.4590 g of 4,4′-diaminodiphenyl ether in 7 mL of N,Ndimethylacetamide; this solution was mixed thoroughly before use. The aqueous solution used to characterize the fabricated nanoelectrodes was 1 × 10-2 mol dm-3 Ru(NH3)63+ with 0.2 mol dm-3 KCl as supporting electrolyte, and the DCE solution contained 1 × 10-3 mol dm-3 TCNQ without supporting electrolyte. All aqueous solutions were prepared using deionized water (Millipore Corp.). A CHI 900 setup (CH Instrument) was employed to perform cyclic voltammetry and SECM experiments. An optical microscope (BX-60, Olympus) and the JXA-840 scanning electronic microscope (JEOL) were used to check the shapes and insulating coverage of the electrodes. A 0.2-cm-diameter Pt disk electrode acted as the SECM substrate. A KCl-saturated Ag/AgCl electrode and a 0.125-cm-diamter Pt wire were used as reference and counter electrodes, respectively. Fabrication of Nanoelectrodes. A micropipet with a ∼30µm (i.d) sharp tip was pulled from a piece of borosilicate capillary (1-mm o.d, 0.58-mm i.d) by use of a P-2000 laser puller (Sutter Instrument Co.). A 1-cm length of 20-µm Pt wire was transferred carefully inside the capillary from the back and left ∼0.5 cm of the Pt wire outside of the sharp tip. The Pt wire was secured in position by melting the sharp tip around the Pt wire using a gas flame. Electrical connection to the inside end of the Pt wire was made with silver epoxy to a copper wire. The open end of the capillary was sealed with epoxy resin. This provided strain relief for the conductive wire. Etching of a Pt wire to form an ultrafine point was accomplished using the procedure that is outlined in Figure 1a. A Pt (radius 0.125 mm) ring was covered with a film of etching solution, and this film can exist stably because of the surface tension. An ac voltage of 2 V was applied between the Pt wire and the Pt ring. The etching procedure was completed when the current dropped to zero. The image of an etched Pt wire is shown in Figure 2a. The etched Pt wire was cleaned by using 0.01 mol dm-3 NaOH, 1 mol dm-3 HNO3, and amounts of deionized water, respectively. Then, the tip was dipped in the coating solution, and heat was added to the part of the capillary that was very near to the tip (See Figure 1b). The heat can be transferred through the glass tube to the tip, so that the solution temperature around the tip is much higher than the solution far from the tip. The high temperature can induce the formation of a layer of polyimide around the tip. The polyimide is a tightly adherent and highly resistive insulator. Finally, the tip was taken out of the solution and was put in an oven at 180-220 °C for ∼1 h (See Figure 1c.). The major problem is that sometimes the electrodes are thoroughly insulated. Since the polyimide can hydrolyze in 0.01 M NaOH solution very slowly, those very small electrodes (only a few nanometers in diameter) can be obtained by dipping the electrodes in 0.01 M NaOH for a few minutes. Analytical Chemistry, Vol. 73, No. 21, November 1, 2001
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Figure 1. Schematic diagram of the fabrication of nanometer-sized electrodes. (a) The setup was used to etch the Pt wires; (b) and (c) the setups were employed to insulate the etched Pt wires.
RESULTS AND DISCUSSION Characterization of the Fabricated Nanoelectrodes by SEM and Cyclic Voltammetry. The polymerization is as follows:
The polyimide is a very good insulating material. It is hard and oil insoluble and has an expansion coefficient similar to that of Pt.20 Figure 2 shows the SEM images of an electrode before 5348 Analytical Chemistry, Vol. 73, No. 21, November 1, 2001
Figure 2. SEM images of the nanoelectrodes: (a) an etched Pt wire, (b) the side view of a nanoelectrode, and (c) the top view of a nanoelectrode.
and after insulation; its radius could be estimated from Figure 2c and is equal to ∼120 nm. The surface is smooth with no cracks. There are no other defects in the polymer/metal seal. From the top view and the side view of the insulated tip, we could assume that the exposed area is almost a disk shape. The stability of the polyimide coated on the surface of Pt nanoelectrodes in various solvents can be tested experimentally by cyclic voltammetry. Ru(NH3)63+ and TCNQ were chosen as the mediators in the aqueous and the DCE solutions, respectively. For both cases, it is profitable to perform the experiments in dim light. Figure 3a shows the voltammograms of reduction of Ru(20) Ying, L.; Xu, X. Fundmental of Adhension and Adhensive; Aviation Industrual Press Inc.: Beijing, 1988; Chapter 5.
Figure 3. Cyclic voltammograms of the mediators at Pt nanoelectrodes obtained both in aqueous and in DCE solutions. The numbers refer to the cycling numbers: (a) A radius of 46-nm nanoelectrode in 1 × 10-2 mol dm-3 Ru(NH3)63+ and 0.2 M KCl aqueous solution and its potential window (see the inset). (b) A radius of 52-nm nanoelectrode in 1 × 10-3 mol dm-3 TCNQ in the DCE phase and its potential window (see the inset).
(NH3)63+ on one Pt nanoelectrode in the 0.2 mol dm-3 KCl solution. Figure 3b shows the voltammograms of reduction of TCNQ obtained on another Pt nanoelectrode in pure DCE. The voltammograms in both cases were recorded after more than 30 times cycling. For the first cycling and the one after 30 times, the steady-state current is almost the same, suggesting that the insulation layer does not crack or deteriorate with time in both solutions. Moreover, the fabricated nanoelectrodes can work in a rather wide potential window for both systems (see the insets in Figure 3), and they can be also used to study many other systems. The following eq 1, where c° is the buck concentration of mediator
iT,∞ ) 4nFDc°r
(1)
species, F is the Faraday constant, n is the number of electrons transferred, D is the diffusion coefficient of the mediator, and r is the effective radius of the electrode, can be employed to quantitatively analyze the voltammetric data and evaluate the effective radii of these nanoelectrodes because the SEM images suggested that they are nanodisk electrodes. By use of this equation, and the diffusion coefficients of Ru(NH3)63+ and TCNQ, 7 × 10-6 8b and 1.5 × 10-5cm2 s-1,21 the radii of the nanoelectrodes used in parts a and b of Figure 3 can be calculated and are equal to 46 and 52 nm, respectively. (21) Cheng, Y. F.; Schiffrin, D. J. J. Chem. Soc., Farady Trans. 1994, 90, 25172523.
Figure 4. Cyclic voltammograms obtained with different sizes of Pt electrodes. System: 1 × 10-2 mol dm-3 Ru(NH3)63+ and 0.2 mol dm-3 KCl for (a-c), The effective radii calculated by use eq 1 are (a) 91, (b) 17, and (c) 1.8 nm.
Voltammetric responses of different sizes of electrodes obtained in the aqueous solution containing 1 × 10-2 mol dm-3 Ru(NH3)63+ and 0.2 mol dm-3 KCl are shown in Figure 4. All of them are well-defined steady-state voltammograms. The effective radii of these nanoelectrodes can be calculated from the diffusionlimiting steady-state currents using eq 1. All of them show a relative small charging current except for the very small electrodes. This indicates that these electrodes are not “lagooned” electrodes.10b We even can obtain a reasonable steady-state voltammogram for a radius of a 1.8-nm electrode in this solution (See Figure 4c). Figure 4 also shows that the voltammograms of the reduction of Ru(NH3)63+ become worse with decreasing sizes of the electrodes. It means that the reversibility of this process becomes worse with increase of the mass transport rate with smaller electrodes. Using the approach developed by Mirkin and Bard,22 the kinetic parameters can be obtained and are listed in Table 1. A number of voltammograms obtained at radii of 1.8-17 Analytical Chemistry, Vol. 73, No. 21, November 1, 2001
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Table 1. Kinetic Parameters for the Reduction of Ru(NH3)63+ no.
radii (nm)
∆E1/4 (mV)
∆E3/4 (mV)
κ0 (cm/s)
1 2 3 4 5 6
91 46 17 12 8 1.8
28.6 28.6 30.5 31.0 35.1 36.2
28.6 28.6 31.0 32.3 37.4 38.7
a a 2.0 5.3 4.5 4.9
a
Reversible reaction (no kinetic parameter can be extracted).
nm electrodes yield a value of the standard rate constant of κ0 ) 4.2(0.9 cm s-1. Characterization of the Fabricated Nanoelectrodes by Scanning Electrochemical Microscopy. SECM has been proved to be a reliable method to study fast heterogeneous electrontransfer kinetics.23 The resolution of SECM largely depends on the size of the tip and its geometry. To determine very fast heterogeneous electron-transfer kinetics and to obtain high resolution of the specimen, it is necessary to use a nanometer electrode. Practically, it is not easy to obtain good approach curves using a very small tip,24 for example,
