Cylindrical Nanopore Electrode and Its Application to the Study of

Nov 30, 2009 - Department of Chemistry, East Tennessee State University, Box 70695, Johnson ... Mirkin's groups studied reactions in zeptoliter volume...
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Anal. Chem. 2010, 82, 276–281

Cylindrical Nanopore Electrode and Its Application to the Study of Electrochemical Reaction in Several Hundred Attoliter Volume Peng Sun* Department of Chemistry, East Tennessee State University, Box 70695, Johnson City, Tennessee 37614 A method to fabricate cylindrical nanopore electrodes is presented. The volume of the cavity formed in the cylindrical nanopore electrode can be as small as several hundred attoliters. It has been characterized by using scanning electron microscopy and electrochemical methods. Our results show that the radius of the cavity can affect the diffusion coefficient of a redox species in the cavity. The cylindrical nanopore electrode has also been used to study charge transfer across the interface between an aqueous phase of several hundred attoliters in volume and a bulk chloroform phase. Compared with the same charge-transfer reaction across the interface between a bulk aqueous phase and a bulk chloroform phase, the potential of the charge-transfer reaction has a negative shift. The effect of the phase ratio on the distribution of the supporting electrolyte in the aqueous and organic phases has been used to explain the shift. The studies of chemical reactions in ultrasmall volumes are motivated by the fact that the chemistry of life takes place in a small enclosed volume, which we call a cell. Another motivation to downsize volumes is the need to create large, dense arrays of sensors for multiple species detection.1 As introduced in ref 1e, ultrasmall volumes are volumes of 10 pL (10-11 L) or less. It is beneficial to create very small volumes so that we can perform controlled experiments to understand the effects of volume on reactions. Lipid vesicles,1a,2 prefabricated chambers,3 microinjectors,4 etc., have been used to create ultrasmall volumes. Currently, fluorescence detection is the usual method to study reactions in ultrasmall volumes.1d,e * To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Chiu, D. T.; et al. Science 1999, 283, 1892. (b) He, M.; Edgar, S.; Jeffries, G. D. M.; Lorenz, R. M.; Shelby, P.; Chiu, D. T. Anal. Chem. 2005, 77, 1539. (c) Jeffries, G. D. M.; Kuo, J. S.; Chiu, D. T. Angew. Chem., Int. Ed. 2007, 46, 1326. (d) Chiu, D. T.; Lorenz, R. M. Acc. Chem. Res. 2009, 42, 649. (e) Chiu, D. T.; Lorenz, R. M.; Jeffries, G. D. M. Anal. Chem. 2009, 81, 5111. (2) Karlsson, R.; Karlsson, A.; Ewing, A.; Dommersnes, P.; Joanny, J.-F.; Jesorka, A.; Orwar, O. Anal. Chem. 2006, 78, 5960–5968. (3) (a) Huang, B.; Wu, H.; Bhaya, D.; Grossman, A.; Granier, S.; Kobilka, B. K.; Zare, R. N. Science 2007, 315, 81–84. (b) Rondelez, Y.; Tresset, G.; Tabata, K. V.; Arata, H.; Fujita, H.; Takeuchi, S.; Noji, H. Nat. Biotechnol. 2005, 23, 361–365. (c) Fan, F.-R. F.; Bard, A. J. Science 1995, 267, 871. (d) Fan, F.-R. F.; Kwak, J.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 9669. (e) Sun, P.; Mirkin, M. V. J. Am. Chem. Soc. 2008, 130, 8241. (4) (a) Huebner, A.; Sharma, S.; Srisa-Art, M.; Hollfelder, F.; Edel, J. B.; deMello, A. J. Lab Chip 2008, 8, 1244. (b) Larforge, F. O.; Carpino, J.; Rotenberg, S. A.; Mirkin, M. V. Proc. Natl. Acad. Sci. 2007, 104, 11895.

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Electrochemistry can also be used to study reactions in small volumes. Generally, these studies can be classified into two categories: electrochemical studies of reactions in isolated small volumes and electrochemical studies of reactions in a small volume dispersed in another bulk phase. In the first category, Ewing et al. have electrochemically studied reactions in picoliter volume.5a Copper et al. have developed a method that could routinely conduct assays in subnanoliter volume.5b On the basis of the positive feedback that occurred in a thin layer cell, Bard and Mirkin’s groups studied reactions in zeptoliter volumes.3c-e Systems in the second category include biological cells in an aqueous environment, microemulsion, etc. Bard et al. and Yu et al. have studied local electrochemical reactions by inserting a needlelike small electrode into a micrometer-sized droplet.6 Several groups have studied reactions at the organic droplets/ aqueous phase interface by attaching millimeter- or micrometersized droplets to an electrode surface. Scholz et al.7 developed a method to study the Gibbs energies of the transfer of ions across three phase junctions formed at aqueous/metal surface/organic droplet of several microliters in volume. On the basis of their method, the Gibbs energies of the transfer of ions across a liquid/ liquid interface, which has a very narrow potential window, could be studied.7b Shao and Girault used a similar method to study the effect of the volume ratio on the charge transfer across a liquid/liquid interface.8 Aoki et al. studied the mechanism of charge transfer at the three phase boundaries.9 Compton’s group studied the reaction at the interface between micrometer-sized random droplet arrays and an aqueous phase.10 Using a laser trapping technique, Nakatani et al. have studied the kinetics of electron transfer across the interface between organic droplets (5) (a) Clark, R. A.; Hietpas, P. B.; Ewing, A. G. Anal. Chem. 1997, 69, 259. (b) Bratten, C. D.; Cobbold, P. H.; Copper, J. M. Anal. Chem. 1997, 69, 253. (6) (a) Zhan, W.; Bard, A. J. Anal. Chem. 2006, 78, 726–733. (b) Yum, K.; Cho, H. N.; Hu, J.; Yu, M. ACS Nano 2007, 1, 440–448. (7) (a) Scholz, F.; Komorsky-Lovric, Sˇ.; Lovric, M. Electrochem. Commun. 2000, 2, 112–118. (b) Gulaboski, R.; Mirceski, V.; Scholz, F. Electrochem. Commun. 2002, 4, 277–283. (c) Scholz, F.; Gulaboski, R. ChemPhysChem 2005, 6, 16–28. (8) (a) Ulmeanu, S.; Lee, H.; Fermin, D.; Girault, H.; Shao, Y. Electrochem. Commun. 2001, 3, 219. (b) Zhang, M.; Sun, P.; Chen, Y.; Li, F.; Gao, Z.; Shao, Y. Anal. Chem. 2003, 75, 4341–4345. (9) (a) Tasakorn, P.; Chen, J.; Aoki, K. J. Electroanal. Chem. 2002, 533, 119– 126. (b) Aoki, K.; Tasakorn, P.; Chen, J. J. Electroanal. Chem. 2003, 542, 51–60. (10) (a) Qiu, F.; Ball, J.; Marken, F.; Compton, R.; Fisher, A. Electroanalysis 2000, 12, 1012. (b) Wadhawan, J.; Evans, R.; Banks, C.; Wilkins, S.; France, R.; Oldham, N.; Fairbanks, A.; Wood, B.; Walton, D.; Schro1der, U.; Compton, R. J. Phys. Chem. B 2002, 106, 9619. 10.1021/ac9019335  2010 American Chemical Society Published on Web 11/30/2009

of micrometer radius and the aqueous phase.11 They found that electron-transfer rate significantly depends on the size of the droplet.11b,c By using a nanopipette, Shao and Mirkin’s groups have accomplished the study of charge transfer across a liquid/ liquid interface in which the volume of one phase is much smaller than that of another phase.12 Theories have been developed for reactions in systems with big or small volume ratio (Vw/Vo).13 Droplet volumes used in the aforementioned studies were bigger than 1 pL. The question of what may happen when the size of the droplet is smaller than 1 pL is still open. To our knowledge, the electrochemical study of reactions in an attoliter volume aqueous phase dispersed in another phase has never been reported. Here we introduce a method that can be used to study reactions in an attoliter volume. The fabrication of a cavity of an attoliter in volume is similar to the method used by White et al.14 Briefly, a nanoelectrode is etched so that a cavity whose volume is in the attoliter range is formed. We have characterized the cavity by using scanning electron microscopy (SEM) and electrochemical methods. Charge transfer across the interface between an aqueous solution trapped in the cavity and a bulk organic phase has been studied. Our results show that the size of the droplet can affect the electrochemical reaction in the droplet. EXPERIMENTAL SECTION Chemicals. NaCl and KNO3 (99+%, Fisher Chemical) were used as supporting electrolytes. Ru(NH3)6Cl3 was purchased from Strem Chemicals (Newburyport, MA). Tetrabutylammonium perchlorate (TBAP) was obtained from Aldrich (Milwaukee, WI). Chloroform is freshly distilled. Aqueous solutions were prepared with deionized water (Milli-Q, Millipore Co.). Cylindrical Nanopore Electrodes. A puller-made nanoelectrode15 is polished in such a way that the angle between the electrode and the rotating disk of a micropipet beveller (Sutter instrument Co, Novato, CA) is 3°. Therefore, the polished electrode is wedge-shaped (see Figure 2B,C). By using eq 1, the radius of the polished electrode can be obtained from the steadystate diffusion-limiting current of a cyclic voltammogram at the electrode. Then, the electrode is dried and hanged in a 50 mL vial, in which 0.5 mL of octadecyltrimethoxysilane (90%, Aldrich) is present at the bottom. The covered vial is then put into an oven at 90 °C overnight to silanize the glass wall of the electrode.16 After silanization, the electrode is rinsed several times by acetone and dried at 80 °C for 2-3 h. It is then etched with an alternating (11) (a) Nakatani, K.; Uchino, M.; Suzuki, S.; Negishi, T.; Osakai, T. Anal. Sci. 2009, 25, 183. (b) Terui, N.; Nakatani, K.; Kitamura, N. J. Electroanal. Chem 2000, 494, 41. (c) Nakatani, K.; Chikama, K.; Kim, H.; Kitamura, N. Chem. Phys. Lett. 1995, 237, 133. (12) Shao, Y.; Mirkin, M. V. J. Am. Chem. Soc. 1997, 119, 8103. Sun, P.; Zhang, Z.; Gao, Z.; Shao, Y. Angew. Chem., Int. Ed. 2002, 41, 3445. (13) Kakiuchi, T. Anal. Chem. 1996, 68, 3658. (14) (a) Zhang, B.; Zhang, Y.; White, H. S. Anal. Chem. 2004, 76, 6229. (b) 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, 778–4787. (15) (a) Katemann, B. B.; Schuhmann, W. Electroanalysis 2002, 14, 22–28. (b) Katemann, B. B.; Schulte, A.; Schuhmann, W. Electroanalysis 2004, 16, 60–65. (c) Sun, P.; Mirkin, M. V. Anal. Chem. 2006, 78, 6526–6534. (d) Sun, P.; Mirkin, M. V. Anal. Chem. 2007, 79, 5809–5816. (16) (a) Hozumi, A.; Ushiyama, K.; Sugimura, H.; Takai, O. Langmuir 1999, 15, 7600. (b) Hozumi, A.; Sugimura, H.; Siroyama, H.; Yokogawa, Y.; Kameyama, T.; Takai, O. J. Vac. Sci. Technol., A 2001, 19, 1812. (c) Hayashi, K.; Saito, N.; Sugimura, H.; Takai, O.; Nakagiri, N. Langmuir 2002, 18, 7469.

current of 5 V amplitude and 60 Hz frequency in a solution containing 60% (by volume) distilled water, 30% 5 M CaCl2, and 10% HCl. After etching, the outer glass surface remains hydrophobic, while the inner surface of the etched cavity is hydrophilic. Thus, one could fill the hydrophilic cavity with an aqueous solution without forming a film of water on the hydrophobic outer surface of the probe. Electrochemical Characterization of Cylindrical Nanopore Electrodes. A 20 mL vial is fully filled with a 2 mM Ru(NH3)6Cl3 and 0.2 M KNO3 aqueous solution, which is saturated by a 5 mM TBAP/chloroform solution. Then, a small hole is made on the cover of the vial so that the etched electrode can be inserted into the solution. The hole is then sealed by using Teflon tape. Then, the electrode and vial are inverted and sonicated for 2 min so that air bubbles in the cavity could be removed. The cover of the vial is removed, and the electrode is fixed on a micromanipulator (note that the etched electrode is always immersed in the aqueous solution during the whole process). Electrochemical measurements have been done in the solution. Formation of an Aqueous Droplet of an Attoliter in Volume in Chloroform. To form an attoliter aqueous droplet in chloroform, the aforementioned process is continued by dropping a 5 mM TBAP/chloroform solution saturated by a 2 mM Ru(NH3)6Cl3 and 0.2 M KNO3 aqueous solution into the vial. The organic solution should not touch the nanopore electrode during this process. Then, the manipulator is used to move the electrode vertically into the organic solution. The aqueous solution in the cavity is thus sealed by the organic phase. Instrument and Procedure. A scanning electron microscope (Hitachi S-430) is used to observe the surface of a nanopore electrode. Voltammetries are performed in two-electrode mode using an Epsilon potentiostat with a low current module (BSAi, West Lafayette, IN). For experiments in the aqueous phase alone, a 0.25-mm-diameter Ag/AgCl wire is inserted into a glass pipet containing 100 mM NaCl to serve as the reference electrode. For experiments involving an organic phase, a 0.25-mm-diameter Ag/ AgClO4 wire is immersed in the organic phase to serve as the reference electrode. All experiments have been done at room temperature. RESULTS AND DISCUSSION Fabrication and Characterization of Cylindrical Nanopore Electrodes. A SEM image of a well-polished planar nanometersized disk electrode was shown in our previous manuscript.15c The etching reaction may potentially affect the glass wall because it is violet and oxygen or hydrogen can be released.17 Thus, it is necessary to check if the glass orifice is still a disk after etching. Figure 1 shows the SEM image of an etched cylindrical nanopore electrode. The orifice is a uniform disk. No cracks can be observed around the orifice. This means that only platinum was removed in the etching reaction. Another concern is whether the etched nanometer-sized hole is cylindrical. Because the shape of the hole is determined by the shape of the Pt wire, which was removed by etching, a cylindrical hole should be obtained if the shape of the Pt wire is (17) Nam, A. J.; Teren, A.; Lusby, T. A.; Melmed, A. J. J. Vac. Sci. Technol. B 1995, 13, 1556–1559.

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Figure 1. SEM image of the surface of a cylindrical nanopore electrode.

cylindrical. Although it is hard to ensure that all of the Pt wire stretched in a pulling is cylindrical, it is possible to make part of the stretched Pt wire cylindrical by using appropriate pulling parameters. In a pulling program, four parameters (heat, velocity, filament, and delay) can be adjusted. Velocity and delay are two important parameters that mainly affect the shape of the Pt wire. Our experience showed that more cylindrical Pt wire can be obtained in a pulling when the velocity ranges from 20 to 50 and the delay ranges from 165 to 195 (these numbers have no units, but they can be recognized by laser puller P-2000). An optical image can be used to roughly check the shape of a substance in the submicrometer dimension. For example, the thickness of the Pt wire (black line in Figure 2) between a1 and a2 in Figure 2A is constant. This means that the shape of the Pt wire is cylindrical because the intersection of the Pt wire of a puller-made nanoelectrode is a disk.15a,c The shape of the Pt wire can also be electrochemically evaluated because its radius can be obtained from eq 1: iss ) 4nFDCr

(1)

where iss is the limiting current, F is Faraday’s constant, D is the diffusion coefficient [for Ru(NH3)63+ in 0.2 M KNO3, D is 6.1 × 10-6 cm2/s18]. Curve 1 in the inset of Figure 2 is obtained when the electrode is polished to a1 shown in Figure 2A. Then, a layer is removed from the nanoelectrode by polishing. The radius of the Pt disk is checked once again by using cyclic voltammetry (CV). We repeat the procedures until the electrode is polished to point a2. Thus, a series of cyclic voltammograms can be obtained (curves 1-4 in the inset in (18) Pyot, M.; Bard, A. J. Electrochim. Acta 1997, 42, 3077.

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Figure 2). Although there is a trend that the limiting current increases with each polishing, the difference is quite small. This means that the radii of the Pt wires between a1 and a2 are identical. Therefore, the Pt wire between a1 and a2 is cylindrical, and its diameter is around 287 nm. From the aforementioned results, we can conclude that the Pt wire in a region is cylindrical if the optical image shows that the thickness of the Pt wire in the region is constant. This conclusion can help us to find an appropriate nanoelectrode to fabricate a nanopore electrode. The depth of a nanopore electrode can be measured by using an optical microscope because the nanopore electrode we used is deeply etched (>0.5 µm). For example, the depth of the nanopore electrode in Figure 3 is 5.1 µm. Voltammetry of Cylindrical Nanopore Electrodes. Parts A and B of Figure 4 were obtained at two different nanopore electrodes. The two electrodes have the same ratio of radius to depth, L, which is 30. The radii of the cylindrical nanopores were electrochemically measured as 65 and 867 nm, respectively. When the electrode is small, steady-state cyclic voltammograms of the reduction of Ru(NH3)63+ can be obtained at slow and fast scan rates (see curves 1and 2 in Figure 4A). When the electrode is large, a steady-state cyclic voltammogram of the reduction of Ru(NH3)63+ can be observed at a slow scan rate (see curve 1 in Figure 4B); however, two peaks corresponding to the reduction of Ru(NH3)63+ and the oxidation of Ru(NH3)62+ can be observed at a fast scan rate (see curve 2 in Figure 4B). This means that the shape of the cyclic voltammograms at a cylindrical nanopore electrode is related to the radius of the nanopore, L, and the scan rate. Bond et al. gave the following expression for the limiting current of a steady-state cyclic voltammogram at a recessed microelectrode:19

iss ) 4nFDbC br

π 4L + π

(2)

where Cb is the bulk concentration of the electroactive species and Db is the diffusion coefficient of the electroactive species in bulk solution. This equation is based on the assumption that the diffusion coefficients inside and outside the cavity are the same. In a nanometer-sized cavity, the diffusion of a solute resulting from a “random walk” process can be interferred with because the presence of the wall of the cavity will frequently change the direction of motion of the solute. If the diffusion coefficient in the cavity is different from the diffusion coefficient in the bulk phase, the following equation can be derived (see the Supporting Information) to describe the limiting current:

iss ) 4nFDbDcC br

π 4LDb + πDc

(3)

where Dc is the apparent diffusion coefficient of the electroactive species in the nanometer-sized cavity. Figure 5 shows plots of the limiting current over the concentration at different sized nanopore electrodes. On the basis of eq 3 and the values of the (19) Bond, A. M.; Luscombe, D.; Oldham, K. B.; Zoski, C. G. J. Electroanal. Chem. 1988, 249, 1–14.

Figure 2. Optical microscopic images of a nanoelectrode. The black line is the Pt wire. (A) Unpolished puller-made nanoelectrode. (B) Electrode shown in part A polished to position a1. (C) Electrode shown in part A polished to position a2. The scale bar is 7 µm. Cyclic voltammograms shown in the inset are obtained at the electrode between a1 and a2.

Figure 3. Optical microscopy image of a cylindrical nanohole electrode. The scale bar is 7 µm.

slopes of curves 1-4 in Figure 5, the apparent diffusion coefficient of Ru(NH3)63+ in each electrode can be obtained and they are (5.9 ± 0.8) × 10-6 cm2/s (for electrode 1), (5.7 ± 1.2) × 10-6 cm2/s (for electrode 2), (5.1 ± 0.5) × 10-6 cm2/s (for electrode 3), and (4.5 ± 1.0) × 10-6 cm2/s (for electrode 4). This means that the apparent diffusion coefficient in a big cavity is similar to that in a bulk solution. However, it decreases as the radius of the nanopore becomes smaller. Comparison between an Electrochemical Reaction in the Bulk Phase and in Several Hundred Attoliters. When a nanopore electrode is moved from an aqueous solution into an organic solution, our experience showed that a nanopore electrode fabricated from a vertically polished nanoelectrode can trap an aqueous solution whose volume is much bigger than the volume of the cavity. However, a nanopore electrode fabricated from a wedge-shaped electrode (see Figure 2B) can trap an aqueous solution whose volume is almost the same as the volume of the

Figure 4. Cyclic voltammograms obtained at nanopore electrodes: (A) nanopore electrode whose radius is 65 nm and etched depth is 2.0 µm; (B) nanopore electrode whose radius is 867 nm and etched depth is 26 µm. The scan rate is 50 mV/s for curve 1 and 200 mV/s for curve 2. Before etching, the limiting currents are 30 pA (for electrode A) and 403 pA (for electrode B).

cavity. Because the angle between the axis and plane of a wedgedshaped electrode is around 87°, the orifice of the nanopore electrode fabricated from a wedged-shaped nanoelectrode is still a disk. Analytical Chemistry, Vol. 82, No. 1, January 1, 2010

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Figure 5. Limiting currents for different nanopore electrodes at different concentrations. The line and function beside the line show the best fitting of the limiting current versus concentration for each electrode. For all electrodes, the ratio of depth to electrode radius is 30. Electrode radius: (1) 867 nm; (2) 613 nm; (3) 261 nm; (4) 65 nm. The scan rate is 50 mV/s.

Reaction in Several Hundred Attoliters. Cell 1 is used to study reactions in a small volume: Ag/AgClO4/10 mL of chloroform solution containing 5 mM TBAP/2 mM Ru(NH3)63+ and 0.2 M KNO3 aqueous solution in a small cavity/Pt cell 1. The cyclic voltammogram shown in Figure 6B is obtained from cell 1 and has two peaks. These two peaks are from the reduction of Ru(NH3)63+ or the oxidation of Ru(NH3)62+. One can find that the electrode current is almost the same as its background current before and after the peak appears (see curves A and B in Figure 6) and is stable at different scan rates. This means that all electroactive species (Ru(NH3)63+ or Ru(NH3)62+) in the nanopore electrode are involved in the reaction, and there is no leakage of the electroactive species. From the area covered by the reduction peak, one can calculate that 7.7 × 10-19 mol of Ru(NH3)63+ has been reduced. Because the concentration

of Ru(NH3)63+ is 0.002 M, this means the volume of the droplet is 3.8 × 10-16 L. Because the depth of the cavity is 2.8 µm (measured with an optical microscope) and the radius of the nanopore electrode is 182 nm (measured electrochemically before etching), the volume of the cavity, which is 2.9 × 10-16 L, almost matches the electrochemically measured volume. The peak height is proportional to the scan rate (see the inset in Figure 6). This means that the cyclic voltammograms have thin layer cell properties. The mechanism of the reduction of Ru(NH3)63+ in cell 1 is the reduction of Ru(NH3)63+, followed by ion compensation at the liquid/liquid interface. The ion compensation can be done by anion transfer from the aqueous phase to the organic phase or cation transfer from the chloroform phase to the aqueous phase. Considering the Gibbs transfer energy of cation (TBA+) or anion (ClO4-), the ion compensation should be ClO4transfer from the aqueous phase to the chloroform phase. Thus, the mechanism can be expressed by eq 4: Ru(NH3)63+(aq) + e- + ClO4-(aq)/Ru(NH3)62+(aq) + ClO4-(chloroform)

(4)

Because the cyclic voltammogram has thin layer cell properties, the peak potential of the reaction can be expressed by the following equation (see the Supporting Information):

0

Ep ) E +

0 ∆w0φClO 4

( )

w″ cClO 4 RT ln 0* + F c ClO4-

(5)

0* - is the bulk concentration of ClO4 where cClO in the organic 4 w  phase, cClO4- is the equilibrium surface concentration of ClO4in the aqueous phase, E0′ is the standard potential for the 0 - is the standard potenreduction of Ru(NH3)63+, and ∆wOφClO 4 tial of the transfer of a ClO4 ion between water and the organic phase.

Figure 6. Red, purple, and green curves obtained in cell 1 at different scan rates. Scan rates: 50 mV/s (red); 100 mV/s (purple); 150 mV/s (green). The black curve is obtained when there is no Ru(NH3)6Cl3 in the aqueous phase in cell 1. Scan rate: 50 mV/s. The inset shows the peak current of green, brown, and red curves over their scan rates. 280

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Reaction in the Bulk Phase. Cell 2 is used to study reactions in a bulk phase: Ag/AgClO4/10 mL of chloroform solution containing 5 mM TBAP/10 mL of 2 mM Ru(NH3)63+ and 0.2 M KNO3 aqueous solution/Pt cell 2. Cyclic voltammograms obtained at a 2-µm-radius Pt electrode in cell 2 are sigmoid-shaped (figure not shown). Its E1/2, which is -0.15 V, can be expressed by the following equation (see the Supporting Information):





0 - + E1/2 ) E0 + ∆w0φClO 4

( )

w* cClO 4 RT ln 0* F cClO4-

(6)

w* - is the bulk concentration of ClO4 where cClO in the aqueous 4 phase. w″ - is Comparing eq 5 with eq 6, one can conclude that cClO 4 w* almost 20 times smaller than cClO4- provided that E1/2 is -0.15V and Ep is -0.226V. This means that ClO4- in the aqueous phase could be squeezed out if the volume of the aqueous phase is very small. This conclusion matches Kakuchi’s theory, which said that an inorganic electrolyte in the aqueous phase can cause a marked “salting out” of lipophilic ions into the organic phase when the volume ratio of the aqueous to organic phase is very small.13

CONCLUSION A nanopore electrode has been fabricated. The nanopore electrode has been verified to be a useful tool to study charge transfer across the interface between the aqueous and organic phases in which the volume ratio of the aqueous phase to the organic phase is very small. We found that the distribution of lipophilic ion in the aqueous phase could be affected when the volume ratio of the aqueous to organic phase is very small. ACKNOWLEDGMENT 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; also we appreciate that Dr. John A. Hyatt and Chu-Ngi Ho proofed the manuscript. SUPPORTING INFORMATION AVAILABLE Derivation of eqs 3, 5, and 6. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review August 26, 2009. Accepted November 6, 2009. AC9019335

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