Fabrication and Characterization of a Dual Submicrometer-Sized

Anal. Chem. 2009, 81, 7496–7500

Fabrication and Characterization of a Dual Submicrometer-Sized Electrode Chenxi Yang and Peng Sun* Department of Chemistry, Box 70695, East Tennessee State University, Johnson City, Tennessee 37614 The fabrication of polishable integrated dual submicrometer-sized electrodes has been reported. The electrode assembly consists of two closely spaced submicrometersized platinum disks. It has been accomplished by sealing and pulling platinum (Pt) wires in a θ glass pipet. Monitored by a microscope, the electrodes are polished on a 50 nm diamond lapping tape. Scanning electron microscopy (SEM) and cyclic voltammetry (CV) show that the electrodes are well sealed and are disks. The two submicrometer-sized Pt disk electrodes can work either independently or in the generation-collection model. The shielding effect, feedback effect, and generation-collection efficiency have been discussed. The introduction of micrometer-sized electrodes led to significant advances in studies of fast heterogeneous and homogeneous reactions, measurements in various microenvironments, and highresolution electrochemical imaging.1 To study the kinetics of complicated electrochemical reactions or detection of multiple species, dual micrometer-sized metal electrodes2 or pipet electrodes3 have been used. An integrated dual micrometer-sized electrode is attractive, because the electrode can be easily miniaturized and exhibits reproducible feedback behavior or generation-collection behavior. To perform generation-collection experiments, an integrated dual microelectrode should meet the following condition: the product that is generated at one electrode can diffuse to an adjacent electrode and be detected providing the potential setting is selected appropriately. Besides this condition, species formed at the collector electrode can diffuse to the generator electrode when a feedback model is used. To perform multiple species detection, one hope that the overlapping of the diffusion layers (shielding effect) should be negligible when both electrodes are set at the same potential. Until now, several methods have been used to prepare an integrated dual micrometer* To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Whightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15. (b) Forster, R. J. Chem. Soc. Rev. 1994, 289. (c) Amatore, C. In Physical Electrochemistry: Principles, Methods, and Applications; Rubinstein, I., Ed.; Marcel Dekker: New York, 1995. (2) (a) Wei, C.; Bard, A. J.; Nagy, G.; Toth, K. Anal. Chem. 1995, 67, 1346. (b) Zhao, G.; Giolando, D. M.; Kirchhoff, J. R. Anal. Chem. 1995, 67, 1491. (c) Niwa, O. Electroanalysis 1995, 7, 606. (d) Zhong, M.; Zhou, J.; Lunte, S. M. Anal. Chem. 1996, 68, 203. (e) Matysik, F.-M. Electrochim. Acta 1997, 42, 3113. (f) Yasukawa, T.; Kaya, T.; Matsue, T. Anal. Chem. 1999, 71, 4637. (g) Gao, N.; Lin, X.; Jia, W.; Zhang, X.; Jin, W. Talanta 2007, 73, 589. (3) (a) Chen, Y.; Gao, Z.; Li, F.; Ge, L.; Zhang, M.; Zhan, D.; Shao, Y. Anal. Chem. 2003, 75, 6593. (b) Liu, B.; Shao, Y. H.; Mirkin, M. V. Anal. Chem. 2000, 72, 510–519. (c) Shao, Y. H.; Liu, B.; Mirkin, M. V. J. Am. Chem. Soc. 1998, 120, 12700.

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sized electrode with different geometry. One of the methods is based on lithographic techniques,2c and this method can produce an electrode with the band widths and interelectrode distance in the lower micrometer range. A ring-disk microelectrode has been prepared by chemical vapor deposition.2d Several groups have produced an integrated parallel dual micrometer-sized electrode by sealing micrometer-sized metal wires into a θ glass tube.2e-g The electrodes fabricated from the aforementioned methods are either unpolishable or in the micrometer size. Compared with microelectrodes, nanoelectrodes have smaller iR drops and double-layer charging effects.4,5 Additionally, nanoelectrodes have a higher mass transfer rate. Therefore, the kinetics of a fast electron transfer reaction can be studied. The nanoelectrode has also been used in high-resolution imaging, single cell study, and single molecule detections. Many methods have been used to fabricate unpolishable4 and polishable5 nanoelectrodes. However, nanoelectrodes are not “sensitive” to the product of the electrochemical reaction because the product can diffuse to a place which is out of reach of the electrode when its potential is scanned back. To electrochemically probe more complicated reactions or multiple species at the nanoscale or the product at an electrode surface, a dual submicrometer-sized electrode may be suitable. To our knowledge, the method that can be used to fabricate a polishable dual submicrometer-sized electrode has not been reported yet. Here, we report a simple method to prepare polishable glass sealed dual submicrometer-sized electrodes with a 1-2 µm interelectrode distance. The polished dual submicrometer-sized (4) (a) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 250, 1118. (b) Mirkin, M. V.; Fan, F.-R. F.; Bard, A. J. J. Electroanal. Chem. 1992, 328, 47. (c) Mirkin, M. V.; Fan, F.-R. F.; Bard, A. J. Science 1992, 257, 364. (d) Fan, F.-R. F.; Bard, A. J. Science 1995, 267, 871. (e) Slevin, C. J.; Gray, N. J.; MacPherson, J. V.; Webb, M. A.; Unwin, P. R. Electrochem. Commun. 1999, 1, 282. (f) Campbell, J. K.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 3779. (g) Watkins, J. J.; Chen, J.; White, H. S.; Abrun ˜a, H. D.; Maisonhaute, E.; Amatore, C. Anal. Chem. 2003, 75, 3962. (h) Sun, P.; Zhang, Z.; Guo, J.; Shao, Y. Anal. Chem. 2001, 73, 5346. (i) Kranz, C.; Friedbacher, G.; Mizaikoff, B.; Lugstein, A.; Smoliner, J.; Bertagnolli, E. Anal. Chem. 2001, 732, 2491. (j) Chen, S.; Kucernak, A. Electrochem. Commun. 2002, 4, 80. (k) Hoeben, F. J. M.; Meijer, F. S.; Dekker, C.; Albracht, S. P. J.; Heering, H. A.; Lemay, S. G. ACS Nano 2008, 2, 2497–2504. (l) Bard, A. J. ACS Nano 2008, 2 (12), 2437–2440. (5) (a) Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Palanker, D.; Lewis, A. Anal. Chem. 1997, 69, 1627. (b) Katemann, B. B.; Schuhmann, W. Electroanalysis 2002, 14, 22–28. (c) Katemann, B. B.; Schulte, A.; Schuhmann, W. Electroanalysis 2004, 16, 60–65. (d) Sun, P.; Mirkin, M. V. Anal. Chem. 2006, 78, 6526–6534. (e) Sun, P.; Abeyweera, P.; Carpino, J.; Laforge, F. O.; Rotenberg, S. A.; Mirkin, M. V. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 443–448. (f) Zhang, B.; Galusha, J.; Shiozawa, P. G.; Wang, G.; Bergren, A. J.; Jones, R. M.; White, R. J.; Ervin, E. N.; Cauley, C. C.; White, H. S. Anal. Chem. 2007, 79, 4778–4787. (g) Sun, P.; Mirkin, M. V. Anal. Chem. 2007, 79, 5809–5816. (h) Zhang, B.; Zhang, Y.; White, H. S. Anal. Chem. 2004, 76, 6229. 10.1021/ac901099n CCC: $40.75  2009 American Chemical Society Published on Web 07/29/2009

Figure 1. SEM of dual submicrometer-sized electrodes. The approximate radius of the Pt disks are (A) 240 and 170 nm and (B) 220 and 210 nm. The scale bars in both pictures are 1 µm.

Figure 2. Cyclic voltammogram obtained individually on each Pt disk of a dual submicrometer-sized electrode in 3 mM Ru(NH3)6Cl3 and 0.2 M KNO3 at a scan rate of 20 mV/s, and the radius of Pt disks are (A) 107 and (B) 78 nm. The cyclic voltammograms shown in the inset are obtained on the same dual submicrometer-sized electrode while concurrently scanning the potential at each Pt disk.

electrode is characterized by means of scanning electron microscopy and cyclic voltammetry. The shape of the two electrodes is a disk. The radius of the electrodes is varying from several tens to several hundred nanometers. Because the diffusion layer on a very small disk electrode is less than 10 times that of an electrode radius,5h integrated dual submicrometer electrodes with a micrometer interelectrode distance can satisfy the needs required in a generation-collection experiment. Moreover, the shielding effect for such an electrode assembly is very small; therefore, it can be used in multiple species detection. EXPERIMENTAL SECTION Chemicals. Pt wire (99.99%) of 25 µm in diameter was purchased from Goodfellow (Huntingdon, England). Borosilicate θ glass pipet (o.d. ) 1.5 mm) is from Sutter Instrument Co. (Novato, CA). KNO3 (99+%, Fisher Chemical) was used as the supporting electrolyte. Hexaammineruthenium(III) chloride (99%) was obtained from Strem Chemicals (Newburyport, MA).

Solutions were prepared from deionized water (Milli-Q, Millipore Co.). Electrode. Pretreatment of the Pt wire and glass pipet is required. Pt wire is treated as follows: 2 cm long Pt wires are immersed in acetone solution to remove grease, then the wires are rinsed with deionized water and dried in an oven. To remove impurities on the inner wall of a borosilicate θ glass pipet, it is soaked in hot 1:3 H2O2 (30%)/H2SO4 (Caution! This solution is a very strong oxidizing agent and very dangerous to handle in the laboratory. Protective equipment including gloves, goggles, and face shields should be used at all times) for at least 1 h and then thoroughly rinsed with deionized water. The glass tube is then dried in an oven at 100 °C for 3 h and is placed in a desiccator overnight. After the pretreatment, the borosilicate θ glass pipet is pulled by using a P-2000 laser puller (Sutter Instrument Co.) until achieving an hourglass shape. The hourglass is in such a shape that the thickness of the neck is approximately 60% of its original Analytical Chemistry, Vol. 81, No. 17, September 1, 2009

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thickness. The pulling parameters are heat, 650; filament, 5; velocity, 10; delay, 190; pull, 0. Then, two 2 cm long Pt wires are inserted into each barrel of the θ glass pipet in such a way that the Pt wires are located in the middle section of the capillaries. The glass pipet is positioned inside the laser heating chamber so that the laser beam is focused on the center. Both ends of the pipet are connected to flexible silicone tubes and via a Y-connector to a vacuum pump. The glass capillary is vacuumed. Then, the puller bar of the laser puller is fixed in position. A program (heat, 450; filament, 5; velocity, 10; delay, 180; pull, 0) is used to pull the glass capillary for a period of 6 s. The glass capillary is removed from the puller and is inspected under a microscope to determine whether the wire is completely sealed in the glass tube and to make sure there are no trapped air bubbles. If it is not well sealed, this program is repeated until the desired seal is achieved. After a well sealed glass capillary is ready, another program (heat, 450; filament, 2; velocity, 18; delay, 180; pull, 0) is used to break the capillary into two halves. Velocity and delay have a big effect on the shape and size of the electrode. Generally, the higher the velocity and the smaller the delay, the smaller is the size of the electrode. The electrical connection to the unsealed end of the wire is made with silver powder (99.9%, Strem Chemicals) to a Cu wire (0.25 mm diameter). Then a small amount of Torr Seal Epoxy (Varian Associates, Palo Alto, CA) is packed into the open end of the glass capillary. This seals the barrels and provides strain relief for the contact wire. The electrode is then polished by using the method mentioned in ref 5d. Briefly, a manipulator is used to move the pipet vertically toward the slowly rotating disk of the micropipet beveller (model BV-10, Sutter Instrument Co.) under the monitor of a video microscope. The polishing lasts 10-20 s once the pipet touches the disk. After polishing, there is some dust attached on the electrode surface produced from scratching between the pipet and the disk; thus, a step to remove the dust is necessary. This can be done by polishing the electrode in a 50 nm Al2O3 slurry by hand. Instrument and Procedure. A scanning electron microscope (SEM, Hitachi S-430) is used to observe the surface of the polished dual submicrometer-sized electrode. Electrochemical experiments are carried out using an Epsilon with a low current module (BSAi, West Lafayette, IN), employing a two-electrode system. A 0.25 mm in diameter Ag/AgCl wire is inserted into a glass pipet containing 100 mM NaCl to serve as a reference electrode in all experiments. RESULTS AND DISCUSSION Fabrication of an Integrated Dual Submicrometer-Sized Electrode. The application of a dual submicrometer-sized electrode in the study of the kinetics of an electrochemical reaction requires knowledge of the electrode geometry to interpret the electrochemical response correctly. Since the voltammetric response of a dual submicrometer-sized electrode does not provide sufficient information about the geometry of the electrodes, we must use SEM to check the electrode surface. Parts A and B of Figure 1 show the surface image of two different dual submicrometer-sized Pt electrodes. From the images, one can see that each Pt electrode is disk shaped. The Pt wires are well sealed in 7498

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glass and do not recess into the insulation layer. The radius of the Pt disks for each dual submicrometer-sized electrode is similar (240 and 170 nm for Figure 1A, 220 nm and 210 nm for Figure 1B). This means that the Pt wires are evenly pulled by using the pulling parameter in the separation process. The geometry of the insulation layer can affect the current of the electrode. To make theoretical treatment possible, the surface of a dual submicrometer-sized electrode should be symmetrical along the two Pt disks. This is one of the main problems encountered in the fabrication of dual submicrometer-sized electrodes. The problem can be solved when a dual submicrometer-sized electrode is obtained by making a θ glass pipet into an hourglass shape prior to loading the Pt wires (see Figure 1A). If the Pt wires are loaded in advance, the shape of the glass insulation layer is not symmetrical (see Figure 1B). Cyclic Voltammetry of an Integrated Dual SubmicrometerSized Electrode. Parts A and B of Figure 2 are obtained when only one electrode is connected to the clamp of the working electrode. Both of them show a small charging current. This also indicates that these electrodes are well sealed. Assuming the geometry of both submicrometer-sized electrodes to be disk shaped, the radius of the electrochemical active surface (r) determined from eq 1 is 107 and 78 nm. iss ) 4nFDCr

(1)

where iss is the limiting current, n is the number of electrons transferred per molecule, F is the Faraday’s constant, and D and C are the diffusion coefficient and bulk concentration of the electroactive species, respectively. Using the described fabrication procedure, we can easily produce a dual submicrometer-sized electrode in which the radius of both electrodes is below 50 nm (result not shown). The cyclic voltammograms (CVs) in the inset of Figure 2 are obtained when the potential of the two electrodes is scanned concurrently. Compared with the CVs in Figure 2, one can find that the limiting current of the same electrode is almost identical. This means that the two submicrometer-sized Pt electrodes can independently work without significantly affecting another Pt electrode. The shielding effect in the dual submicrometer-sized electrode is very small. The generation-collection behavior is one of the most important characteristics of a dual submicrometer-sized electrode. In Figure 3, cyclic voltammetry is performed with one electrode (generator) while the potential of another electrode is kept at 0.05 V (collector). The electrochemical reaction at the two electrodes is shown in eq 2a Ru(NH3)63+ + e f Ru(NH3)62+ (generator electrode) (2a) Ru(NH3)62+ - e f Ru(NH3)63+ (collector electrode) (2b) Since the potential of the collector is more positive than E1/2, the reaction at the collector electrode is diffusion controlled. It is clearly shown in Figure 3 that well-defined steady-state CVs can be obtained at both electrodes. There is a small hysteresis in

Figure 3. Voltammograms obtained on a dual nanometer-sized electrode in a generation-collection experiment in 3 mM Ru(NH3)6Cl3 and 0.2 M KNO3. (A) Current response of the generator and (B) current response of the collector. Scan rate 20 mV/s. The potential of the collector is kept at 0.05 V. The radius of the Pt disks of the generator and collector are 107 and 78 nm, respectively. The upper inset shows the collection efficiency of this electrode.

Figure 4. Double step chronoamperomatric generation-collection experiment of a dual submicrometer-sized electrode. The number beside the arrow shows the peak position (for clarity, only curves around the peak area have been shown). (A) Current response of the 325 nm in radius generator in cell 1, (B) current response of the 325 nm in radius generator in cell 2, (C) current response of the 468 nm in radius collector in cell 2, and (D) current response of the 468 nm in radius collector in cell 1. Experimental condition for the generator: initial potential 100 mV, first step potential 100 mV, second step potential -300 mV, step width 200 ms, sample interval 100 µs. The potential of the collector is always held at 100 mV. The inset shows the cyclic voltammograms of the generator and collector, and the potential of the collector is held at 100 mV. Electrochemical cell 1, Ag/AgCl/ 3 mM Ru(NH3)6Cl3 and 0.2 M KNO3/Pt; electrochemical cell 2, Ag/AgCl/0.2 M KNO3/Pt.

the collection response. The hysteresis may come from capacitive coupling, since the two electrodes are closely spaced. The collection efficiency, which is defined here as the ratio of the limiting generator current to the limiting collector current, is 0.075 (see the inset in Figure 3). The collection efficiency is stable, and this means its generation-collection behavior is reproducible. The feedback effect can be observed when the electrodes operate in the generation-collection mode. The feedback factor FF is given in eq 3.

FF ) 1 -

0 iss * iss

(3)

where t0ss is the generator current without feedback (Figure 2A) and iss* is the generator current with feedback (Figure 3A). FF for this electrode was determined to be 0.04, which is a very small value. Considering the fact that this kind of electrode is mainly used in multiple species detection and generationcollection experiments, it can satisfy our needs even though the feedback factor is not high. Interelectrode Distance Detection. Interelectrode distance can affect the collection efficiency and the feedback factor. Moreover, this parameter is also important when a theoretical simulation is needed. An optical microscope can be used to detect the interelectrode distance when the distance is larger than 1 µm. Since the distance that the product molecules have moved from Analytical Chemistry, Vol. 81, No. 17, September 1, 2009

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an electrode in a certain time can be estimated from eq 4, this means the interelectrode distance can also be detected electrochemically. d ) √2Dt

(4)

where D is the diffusion coefficient and t is the time lapse. Figure 4 shows the current response of a chronoamperomatric generation-collection experiment. Initially, the potential of both the generator and collector is held at 0.1 V, and no Ru(NH3)62+ can be generated at the generator. The potential of the generator is then jumped to -0.3 V where the reduction of Ru(NH3)63+ occurs suddenly at a diffusion controlled rate; therefore, a peak of the reduction of Ru(NH3)63+ appears (see Figure 4A). Since the potential of the collector is still at 0.1 V, Ru(NH3)62+, which is produced at the generator, can be detected at the collector and, therefore, a collection peak can be observed (see Figure 4D). Since the two electrodes are closely spaced, the capacitive coupling between the two electrodes is big in a transit time (as mentioned by a reviewer). The charging current is proportional to the electrode area; therefore, the peak of the collector is bigger than the generator. This can be verified by the result of the same experiment in supporting electrolyte alone. As shown in parts B and C of Figure 4, the capacitive coupling current is big but the peak position is similar. This means that a 3 ms time lapse between the peak position in parts A and D of Figure 4 is from the transportation of the electroactive species. On the basis of eq 4, one can find that the interelectrode distance is around 2 µm provided that the diffusion coefficient for Ru(NH3)62+ is 6.5 × 10-6 cm2/s.5d To get rid of highfrequency noise, the filter cutoff frequency was chosen to be

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100. This means that noise whose frequency is below that value will be retained in the signal. Thus, oscillation is present in Figure 4. However, the peak position can still be recognized. CONCLUSIONS A simple method to fabricate dual submicrometer-sized disk electrodes has been presented. Its generation-collection behavior as well as the shielding and feedback properties and interelectrode distance have been characterized. Since the size of both electrodes is very small, the charging current on both electrodes is also small. This advantage means that the potential of both electrodes can be scanned at a fast rate. Therefore, it can be used to detect the lifetime of short-lived electrogenerated radicals. Well-defined electrochemical behaviors at both electrodes mean that they can be employed to perform quantitative multiple species detection at the nanoscale. Another advantage is that the electrodes are polishable, and thus, reproducible results can be obtained. ACKNOWLEDGMENT The support of this work by the New Faculty Start Up grant and Student-Faculty Collaborative grant from East Tennessee State University is gratefully acknowledged. The authors thank Dr. Michael S. Zavada for obtaining the SEM images, and also we appreciate Dr. Jeff Wardeska and Betty Riccio for proofing the manuscript. NOTE ADDED AFTER ASAP PUBLICATION This manuscript originally posted ASAP on July 29, 2009. A correction was made to eq 3, and the corrected manuscript posted ASAP on July 31, 2009. Received for review May 19, 2009. Accepted July 15, 2009. AC901099N