pubs.acs.org/Langmuir © 2010 American Chemical Society
Electromagnetic Micropores: Fabrication and Operation Joseph R. Basore,† Nickolay V. Lavrik,‡ and Lane A. Baker*,† †
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States, and ‡Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Received October 3, 2010. Revised Manuscript Received October 26, 2010 We describe the fabrication and characterization of electromagnetic micropores. These devices consist of a micropore encompassed by a microelectromagnetic trap. Fabrication of the device involves multiple photolithographic steps, combined with deep reactive ion etching and subsequent insulation steps. When immersed in an electrolyte solution, application of a constant potential across the micropore results in an ionic current. Energizing the electromagnetic trap surrounding the micropore produces regions of high magnetic field gradients in the vicinity of the micropore that can direct motion of a ferrofluid onto or off of the micropore. This results in dynamic gating of the ion current through the micropore structure. In this report, we detail fabrication and characterize the electrical and ionic properties of the prepared electromagnetic micropores.
Introduction We have recently described the application of electromagnetic micropores for controlling ion transport through a silicon membrane.1 In these devices, a micropore fabricated on a silicon wafer is encompassed by a single-turn microcoil. A local electromagnetic field is produced when an electric current is passed through the microcoil. In the presence of an additional external magnetic field, these local electromagnetic fields create regions of significant field gradient that can be used to actuate magnetic materials in the vicinity of the micropore. Addition of a hydrophobic ferrofluid over the micropore forms a gate that can be actuated to control transport of ions through the micropore. In this report, we detail the fabrication protocol and functional aspects of the electromagnetic micropore. Microelectromagnetic traps of various geometries are used in a wide range of applications to enhance separation and detection techniques.2-5 Such traps have previously served to position magnetic particles in a specific location,6-9 separate materials from a complex mixture,10-13 or manipulate atoms or small molecules in vacuum.14,15 For the device described here, incorporation of a micropore within the microelectromagnetic trap results in a focused magnetic response that can be exploited to *To whom correspondence should be addressed. E-mail: lanbaker@ indiana.edu. Telephone: 812-856-1873. Fax: 812-855-8300.
(1) Basore, J. R.; Lavrik, N. V.; Baker, L. A. Adv. Mater. 2010, 22, 2759–2763. (2) Gijs, M. A. M. Microfluid. Nanofluid. 2004, 1, 22–40. (3) Gijs, M. A. M.; Lacharme, F. D. R.; Lehmann, U. Chem. Rev. 2009, 110, 1518–1563. (4) Pamme, N. Lab Chip 2005, 6, 24–38. (5) Ramadan, V. Q. S.; Poenar, D.; Yu, C. Int. J. Nanosci. 2005, 4, 489–499. (6) Lee, C. S.; Lee, H.; Westervelt, R. M. Appl. Phys. Lett. 2001, 79, 3308–3310. (7) Ramadan, Q.; Poenar, D. P.; Yu, C. Microfluid. Nanofluid. 2008, 6, 53–62. (8) Smistrup, K.; Hansen, O.; Bruus, H.; Hansen, M. F. J. Magn. Magn. Mater. 2005, 293, 597–604. (9) Smistrup, K.; Tang, P. T.; Hansen, O.; Hansen, M. F. J. Magn. Magn. Mater. 2006, 300, 418–426. (10) Lee, H.; Liu, Y.; Ham, D.; Westervelt, R. M. Lab Chip 2007, 7, 331–337. (11) Lee, H.; Purdon, A. M.; Westervelt, R. M. IEEE Trans. Magn. 2004, 40, 2991–2993. (12) Ramadan, Q.; Samper, V.; Poenar, D.; Yu, C. J. Magn. Magn. Mater. 2004, 281, 150–172. (13) Lee, H.; Purdon, A. M.; Chu, V.; Westervelt, R. M. Nano Lett. 2004, 4, 995– 998. (14) Johnson, K. S.; Drndic, M.; Thywissen, J. H.; Zabow, G.; Westervelt, R. M.; Prentiss, M. Phys. Rev. Lett. 1998, 81, 1137–1141. (15) Drndic, M.; Lee, C. S.; Westervelt, R. M. Phys. Rev. B 2001, 63, 085321.
Langmuir 2010, 26(24), 19239–19244
control transport through the pore. Several groups have developed synthetic platforms to control the transport of ions or small molecules through micro- and nanoscale structures, including colloidal assemblies,16-21 single-pore devices,22-26 and multipore membranes.27-34 Transport through these structures relies on a number of factors, including the size of the structures and the incorporation of specific chemical functionalities, and has been applied to fundamental studies of transport,23-26,35-37 construction of fluidic diodes,38-40 and the development of platforms for separation41-44 and detection.27,41,45-47 (16) Cichelli, J.; Zharov, I. J. Am. Chem. Soc. 2006, 128, 8130–8131. (17) Newton, M. R.; Bohaty, A. K.; White, H. S.; Zharov, I. J. Am. Chem. Soc. 2005, 127, 7268–7269. (18) Schepelina, O.; Zharov, I. Langmuir 2006, 22, 10523–10527. (19) Schepelina, O.; Zharov, I. Langmuir 2007, 23, 12704–12709. (20) Schepelina, O.; Zharov, I. Langmuir 2008, 24, 14188–14194. (21) Smith, J. J.; Zharov, I. Langmuir 2008, 24, 2650–2654. (22) Kawano, R.; Schibel, A. E. P.; Cauley, C.; White, H. S. Langmuir 2009, 25, 1233–1237. (23) White, H. S.; Bund, A. Langmuir 2008, 24, 12062–12067. (24) White, H. S.; Bund, A. Langmuir 2008, 24, 2212–2218. (25) White, R. J.; Ervin, E. N.; Yang, T.; Chen, X.; Daniel, S.; Cremer, P. S.; White, H. S. J. Am. Chem. Soc. 2007, 129, 11766–11775. (26) White, R. J.; White, H. S. Anal. Chem. 2007, 79, 6334–6340. (27) Choi, Y.; Baker, L. A.; Hillebrenner, H.; Martin, C. R. Phys. Chem. Chem. Phys. 2006, 8, 4976–4988. (28) Lakshmi, B. B.; Martin, C. R. Nature 1997, 388, 758–760. (29) Lee, S.; Zhang, Y.; White, H. S.; Harrell, C. C.; Martin, C. R. Anal. Chem. 2004, 76, 6108–6115. (30) Lee, S. B.; Martin, C. R. Anal. Chem. 2001, 73, 768–775. (31) Lee, S. B.; Mitchell, D. T.; Trofin, L.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. Science 2002, 296, 2198–2200. (32) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700–702. (33) Sexton, L. T.; Horne, L. P.; Martin, C. R. Mol. Biosyst. 2007, 3, 667–685. (34) Steinle, E. D.; Mitchell, D. T.; Wirtz, M.; Lee, S. B.; Young, V. Y.; Martin, C. R. Anal. Chem. 2002, 74, 2416–2422. (35) Ervin, E. N.; White, R. J.; White, H. S. Anal. Chem. 2009, 81, 533–537. (36) Zhang, B.; Zhang, Y.; White, H. S. Anal. Chem. 2006, 78, 477–483. (37) Zhang, Y.; Zhang, B.; White, H. S. J. Phys. Chem. B 2006, 110, 1768–1774. (38) Chang, C.; Tran, V. H.; Wang, J.; Fuh, Y.-K.; Lin, L. Nano Lett. 2010, 10, 726–731. (39) Vlassiouk, I.; Smirnov, S.; Siwy, Z. ACS Nano 2008, 2, 1589–1602. (40) Yusko, E. C.; An, R.; Mayer, M. Biophys. J. 2009, 96, 646a–646a. (41) Baker, L. A.; Choi, Y.; Martin, C. R. Curr. Nanosci. 2006, 2, 243–255. (42) Feng, X.; Huang, R. Y. M. Ind. Eng. Chem. Res. 1997, 36, 1048–1066. (43) Rodriguez, M. A.; Armstrong, D. W. J. Chromatogr., B 2004, 800, 7–25. (44) Noble, R. D.; Stern, S. A. Membrane Separations Technology: Principles and Applications; Elsevier Science: 1995; Vol. 2, p 738. (45) Dekker, C. Nat. Nanotechnol. 2007, 2, 209–215. (46) Howorka, S.; Cheley, S.; Bayley, H. Nat. Biotechnol. 2001, 19, 636–639. (47) Lagerqvist, J.; Zwolak, M.; Di Ventra, M. Nano Lett. 2006, 6, 779–782.
Published on Web 11/18/2010
DOI: 10.1021/la103977e
19239
Article
Basore et al.
Microelectromagnetic traps provide a method to tune the local magnetic field at the microscale, precisely controlling magnetic fields and magnetic field gradients of various magnitudes, shapes, and sizes, which can be used further to manipulate magnetic material. The properties of local electromagnetic fields are dictated by a number of factors, which include trap shape, current carried, and distance from the trap.5,7,8,12,48-52 The combination of electromagnetic traps and micropores into a single electromagnetic micropore structure can be used to directly manipulate magnetic material and to indirectly manipulate nonmagnetic species such as ions and small molecules in solution. In this report, magnetic fields produced by single-coil gold microelectromagnetic traps have been calculated with finite element simulations. Methods to move magnetic material into and out of the center of the coil, based on the magnetic fields produced, have been explored. Reversible gating of ion transport through the electromagnetic micropore was controlled by reversal of electronic current through the electromagnetic trap in the presence of an external magnetic field.
Experimental Section Electromagnetic Micropore Design and Fabrication. Initially, microelectromagnetic traps were fabricated on 100 mm double-side polished pyrex wafers (thickness ∼ 700 μm) with minor modifications to previously reported procedures (a detailed procedure of this process is included in the Supporting Information).11,53 Computer-aided-design (CAD) software was used to design nine chips (22 mm 22 mm each) on a single 100 mm wafer. Each chip contained six single-coil microelectromagnetic traps (outer diameters: 20-100 μm) that were connected to larger (3 mm 3 mm) contact pads for externally addressing the coils. The trap wire widths ranged from 4 to 20 μm, depending on the outer diameter (OD) of the coil (e.g., 100 μmOD coil, 20 μm wire width; 20 μmOD coil, 4 μm wire width). This initial protocol was transferred from pyrex to silicon and then further modified to incorporate a single pore (5-25 μm in diameter) in the center of each single-coil microelectromagnetic trap to form the electromagnetic micropore. For these devices, p-type CZ (100) oriented single-side polished silicon wafers (100 mm) with a nominal thickness of 500 μm were chosen. Silicon oxide was thermally grown (600 nm) onto both sides of the wafers. The fabrication sequence of the electromagnetic micropores employed four photolithographic patterning steps combined with reactive ion etching and gold deposition. Briefly, a positive tone photoresist (SPR 220-7.0, MicroChem Corp) was patterned to form a mask for etching wells in the back side of the wafer. Reactive ion etching (RIE) was then used to remove the silicon oxide in the well areas, and an anisotropic deep reactive ion etch (DRIE/Bosch) was performed (System 100 Plasma etcher, Oxford Instruments) to create 300 μm deep wells (Figure 1a). Front-side pore wells were created with a similar approach, except 15 nm of chromium (Cr) was first deposited on the front side of the wafer. This Cr layer served as an extra protective layer to ensure the front side was not damaged in the etching process prior to coil deposition. Six pore wells (5-25 μm in diameter) were created by photolithographic patterning of SPR 220-7.0, followed by a Cr etch. Once the patterned Cr layer was formed, 100 μm deep pore (48) Ramadan, Q.; Samper, V.; Poenar, D.; Yu, C. Biomed. Microdevices 2006, 8, 151–158. (49) Ramadan, Q.; Samper, V.; Poenar, D. P.; Yu, C. Biosens. Bioelectron. 2006, 21, 1693–1702. (50) Ramadan, Q.; Samper, V. D.; Poenar, P. D.; Yu, C. J. Microelectromech. Syst. 2006, 15, 624–638. (51) Ramadan, Q.; Yu, C.; Samper, V.; Poenar, D. P. Appl. Phys. Lett. 2006, 88, 032501. (52) Smistrup, K.; Bu, M.; Wolff, A.; Bruus, H.; Hansen, M. F. Microfluid. Nanofluid. 2008, 4, 565–573. (53) Dubus, S.; Gravel, J.-F.; Le Drogoff, B.; Nobert, P.; Veres, T.; Boudreau, D. Anal. Chem. 2006, 78, 4457–4464.
19240 DOI: 10.1021/la103977e
Figure 1. Fabrication process of the microelectromagnetic pore. Etch 300 μm deep backside well (a); etch front side pore well (b); deposition of 10 nm titanium and 200 nm of gold and subsequent liftoff resulting in a trap seed layer (c); electrodeposition of 3-4 μm gold (d); final etch to produce continuous pore (e). wells were etched with the same protocol used to etch the back side wells (Figure 1b). The Cr layer was subsequently removed and the front side was coated with a double-layer photoresist (lift-off resist LOR-1A overcoated by SPR 995-2.0, MicroChem Corp), which then defined the microcoil and contact pad areas. Titanium (15 nm) and gold (200 nm) were deposited onto the surface by electron-beam evaporation and subsequently patterned by lift-off, resulting in a metallic seed layer for deposition of the microelectromagnetic trap (Figure 1c). In a final photolithographic patterning step, the coil and pad areas were redefined. In this step, the patterned photoresist (SPR 220-7.0) acted as a mold to allow electrodeposition of 3-4 μm of Au (Figure 1d) onto the seed layer. The thickness of the Au coils was determined with a contact profilometer (Dektak, Veeco Inc.). Next, the pore wells were accessed from the back side with a DRIE Bosch process (Figure 1e), and the Au coils were electrically insulated with 200 nm aluminum oxide using atomic layer deposition (FlexAL deposition tool, Oxford Instruments), followed by plasma enhanced chemical vapor deposition (System 100 plasma deposition tool, Oxford Instruments) of 1 μm of silicon oxide. The processed wafer was then scribed and cleaved, resulting in nine chips, each containing six microelectromagnetic traps with central micropores. To decrease nonspecific adsorption, chips were chemically modified in a solution of silanol-PEG (mPEG-silane MW 5000, Laysan Bio), ethanol, and acetic acid (1:18:1 by weight, respectively) for 1 h, followed by curing at 100 °C for 1 h. Experimental Protocols. A polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) insulation layer was spin-coated (520 μm) onto the coils fabricated on pyrex and cured at 65 °C. Electromagnetic micropores on silicon wafers contained an insulation layer of alumina and silicon oxide, as described above. PDMS was applied to all areas of these devices except the electromagnetic trap being examined. This PDMS layer acted both to isolate a single electromagnetic micropore and to minimize possible damage to the 1.2 μm insulating layer. The PDMS-coated chips were mounted in a two-compartment test cell (Supporting Information Figure S1), and in most experiments a rare earth magnet was placed directly below the test cell to bathe the device in an external magnetic field. For measurements of ion current through the micropore, each compartment contained a silver/silver chloride (Ag/AgCl) electrode and a solution of potassium chloride containing 0.01% Dowfax 21A surfactant. This test cell allowed application of a potential difference (Keithley 6487 picoammeter) Langmuir 2010, 26(24), 19239–19244
Basore et al.
Article
across the silicon pore with the Ag/AgCl electrodes while the microelectromagnetic trap could be energized via contact pads and a secondary power source. The coils were energized either with a power supply connected to an H-bridge or to a function generator (model D5354, Stanford Research System) connected to a buffering amplifier with unity gain. In a typical experiment, magnetic particles (MyOne magnetic particles, Dynalbead (diameter 1.05 ( 0.1 μm) or ferrofluid FFL 001, Ferrotec (viscosity 6 cP @ 27 °C)) were placed in the top compartment of the test cell and moved into and out of the center of the coil by reversal of current through the coil. When measuring ion currents, a 0.5 V potential difference was applied across the pore and the ion current was measured. Optical micrographs of the traps and magnetic particle manipulation were recorded with a CCD camera.
Results and Discussion Finite Element Simulations and Magnetic Particle Manipulation. Development of an electromagnetic micropore capable of reversibly gating ion transport with magnetic material relies on the location and magnitude of the maximum magnetic field produced by a microelectromagnet. Magnetic material will migrate to regions of high field; thus, the location of the micropore with respect to the electromagnetic trap is an important consideration. Single-coil microelectromagnetic traps have been shown to produce maximum magnetic fields in the center of the coil that are capable of manipulating magnetic particles.6,11,13-15,53,54 A single-coil design is also favorable for the electromagnetic micropore, as the pore structure can easily be incorporated in the overall trap design and the magnetic response focused at the micropore. Finite element simulations provide valuable information for the design of the electromagnetic micropore. To this point, magnetic fields produced by a 100 μmOD and a 20 μmOD coil were modeled with the COMSOL multiphysics magnetostatics application. In these simulations, a current of 300 mA is introduced through a 4 μm thick single-coil gold trap (100 or 20 μm diameter), and the magnetic field generated is determined. Magnetic fields produced by the 100 μm coil were largest near the surface of the microwire, until approximately 20 μm above the coil, where fields become strongest in the center of the coil (see the Supporting Information). A similar calculation was performed for the 20 μm diameter coil, except the resulting fields and field gradients occur over different spatial distributions (see the Supporting Information). These simulations demonstrate that fields and gradients produced are dependent on coil size and can be tuned by varying the coil size, currents through the coil, and thickness of insulting layers. Of key importance is the realization that producing a maximum magnetic field at the center of the coil (i.e., at the micropore) requires that the coil reside below the opening of the micropore by a specific distance. This is essential to maximize the magnetic response generated at the micropore by the electromagnetic trap. To validate finite element simulations prior to fabrication of the electromagnetic micropore, single-coil traps were fabricated on a pyrex chip and insulated with various thicknesses of PDMS. Magnetic manipulation experiments were then performed by mounting the chip in a two-compartment test cell containing a solution of magnetic particles in the top compartment and energizing a single coil with a 300 mA current (see the Supporting Information, performed without the use of the external magnet shown). Results from the largest (100 μm) and smallest (20 μm) coils are shown in Figure 2 and confirm the simulations. An insulating PDMS layer 5 μm thick resulted in particle trapping at the surface of the microwire (54) Drndic, M.; Johnson, K. S.; Thywissen, J. H.; Prentiss, M.; Westervelt, R. M. Appl. Phys. Lett. 1998, 72, 2906–2908.
Langmuir 2010, 26(24), 19239–19244
Figure 2. Optical micrographs of particle trapping experiments. In all experiments, the coils were on pyrex (no pores) and energized with 300 mA of current. The left column represents the initial coil images and the right column represents the coil images after being energized for 5 min: (a,b) 100 μm trap with a 5 μm PDMS insulating layer; (c,d) 100 μm trap with a 20 μm PDMS insulating layer; (e,f) 20 μm trap with a 5 μm PDMS insulating layer.
(Figure 2a,b), while the use of a 20 μm thick PDMS layer trapped particles at the center of the 100 μm coil (Figure 2c,d). The smallest coil trapped particles at the center of the coil with a 5 μm insulating layer. Development of the electromagnetic micropore requires not only trapping particles at the center of microcoil but also the subsequent removal of particles. This was accomplished with a uniform external magnetic field. Previous studies have shown the interaction of an external magnetic field in concert with the electromagnetic trapping field is dependent on the direction of current traveling through the microelectromagnet.55-57 Here, finite element simulations of a 100 μm coil were performed in the presence of a 2.5 mT uniform magnetic field (Figure 3a,b), oriented perpendicular to the surface of the device. A value of 2.5 mT was chosen to simulate the magnetic field produced 1 cm away from a rare earth magnet. In the simulations shown, the magnetic field (mT) produced 20 μm above the coil is depicted in false color. Under the configuration modeled, current traveling clockwise (Figure 3a) produces a focused magnetic field in the center of the microcoil. Current traveling counterclockwise (Figure 3b) produces a focused magnetic field at the periphery of the microcoil. The calculated magnetic fields across the center of the two coils, at successive heights above the coils, were (55) Rida, A.; Fernandez, V.; Gijs, M. A. M. Appl. Phys. Lett. 2003, 83, 2396– 2398. (56) Beyzavi, A.; Nguyen, N.-T. J. Phys. D: Appl. Phys. 2009, 42, 015004. (57) Melikhov, Y.; Lee, S. J.; Jiles, D. C.; Schmidt, D. H.; Porter, M. D.; Shinar, R. J. Appl. Phys. 2003, 93, 8438–8440.
DOI: 10.1021/la103977e
19241
Article
Basore et al.
Figure 3. Effect of a uniform magnetic field on particle movement. Finite element simulations of the magnetic field produced 20 μm above a 100 μm coil simulated in a 2.5 mT external magnetic field (a,b). The color scale represents the magnetic field in mT. Optical micrographs of magnetic particle manipulation with coils on pyrex (c,d) with 20 μm PDMS insulating layer. The current through the coils in these simulations and experiments is 300 mA. Current is traveling clockwise in (b,d) and counterclockwise in (a,c).
also calculated (see the Supporting Information ). These simulations show that, with the addition of an external uniform magnetic field, a trap either at the center or at the periphery of the microcoil, as determined by the direction of the current flow, is created. Additionally, the summation of the magnetic field produces a greater overall magnetic field gradient when the field is focused at the center of the coil, as opposed to the reverse case where the trap resides at the periphery of the coil and the field is effectively defocused. Figure 3c shows an optical micrograph of a 100 μm coil (20 μm PDMS insulating layer), in which a rare earth magnet is placed directly below the two-compartment test cell, after magnetic particles have been trapped at the center of the coil with a 300 mA clockwise current. Figure 3d shows the same coil in a uniform field in which the direction of the current was reversed (counterclockwise). Experimentally, reversal of the current moves particles into and out of the center of the coil, supporting the simulations depicted in Figure 3a,b. These simulations and experiments provide significant evidence that the coil design is sufficient to develop a reversible magnetic gate. Electromagnetic Pore Fabrication and Functional Tests. Findings described above suggest that placing a micropore in the center of the microcoil, in combination with an insulting layer of appropriate thickness, is an appropriate design for the electromagnetic micropore. Fabrication of a micropore within the center of a single-coil trap relies on four photolithographic patterning steps combined with RIE and DRIE. The electromagnetic micropore is fabricated on silicon instead of pyrex because welldeveloped DRIE processes for creating a micropore via anisotropic etch steps exist.58 In the initial steps of the process, large back side wells and front side pore wells are etched to depths such that the two wells nearly converge. The coils are then created through e-beam evaporation and subsequent electrodeposition of (58) Franssila, S.; Kiiham€aki, J.; Karttunen, J. Microsyst. Technol. 2000, 6, 141– 144.
19242 DOI: 10.1021/la103977e
Figure 4. Optical micrographs of the final device and four different electromagnetic micropores. Silicon chip containing six gold microelectromagnetic micropores connected to contact pads (a). 100 μm diameter trap, 5 μm pore (b); 100 μm diameter trap, 10 μm pore (c); 50 μm diameter trap, 5 μm pore (d); 20 μm diameter trap, 5 μm pore (e).
gold. Lastly, the back side wells are etched to meet the front side pore wells, forming a continuous micropore. An insulting layer 1.2 μm in thickness is then deposited over the coils, and the device is functionalized with PEG. Final devices consisted of six singlecoil traps connected to contact pads (Figure 4a). Coils ranged in size from 20 to 100 μm, and each contains a centered micropore, 5-10 μm in diameter (Figure 5b-e). A critical aspect of the fabrication process is the addition of back side wells. These wells provide a straightforward approach to complete a through-wafer pore in the final steps of fabrication. Likelihood of etching the micrometer-sized front side pores completely through the chip is significantly increased by decreasing the depth of the anisotropic etches. The large back side wells decrease the aspect ratio required of the anisotropic etch. In addition, decreasing the length of the narrowest regions of the pore also serves to lower the resistance of the pore in subsequent measurements of ion current. Placement of a pore in the center of the coil allows magnetic material to be trapped over the micropore, which physically changes the state of the pore from an on-state to an off-state. The off-state is a result of an increase in resistance to the movement of ions through Langmuir 2010, 26(24), 19239–19244
Basore et al.
Figure 5. Gating of a 5 μm electromagnetic pore with a ferrofluid droplet. Optical micrographs of ferrofluid droplet with microelectromagnetic micropore (a,b). Images were taken during square wave gating experiments and show the corresponding off-state (a) and on-state (b) of the pore. Ion current versus time plot (c); the blue trace shows gating where a 0.01 Hz triangle wave (800 mA Ipp) runs through the 100 μm coil, and the red trace represents gating where a 0.01 Hz square wave (800 mA Ipp) runs through the 100 μm coil. The waveforms are shown in the upper right corner, and a 1 M KCl solution was used.
the micropore, which is recorded as a decrease in transpore ion current. The micropore is returned to an on-state by moving the magnetic field maxima to the periphery of the coil, which results in movement of the magnetic material trapped over the pore. Reversibility is an important characteristic of this device that allows switching to control the transport of nonmagnetic material over long periods of time and is important in efforts to develop the device further for applications in sensing and separations. To test reversibility, an electromagnetic micropore is mounted between two compartments (see the Supporting Information) and a 0.01 Hz alternating current square wave (Ipp = 800 mA) is run through a 100 μm coil containing a 5 μm pore at the center. A 100 μm coil is used in all experiments described further; smaller coils were also tested and produced results similar to those of the 100 μm coil. A hydrophobic ferrofluid droplet (∼100-200 μm in diameter) is trapped at the electromagnetic micropore (Figure 5b). The ferrofluid droplet is used as a model gate because the hydrophobic nature of this fluid produces a large difference in resistance between the two states of the pore. We have previously shown that the difference in the measured ion current between the pore’s two states is approximately 1 order of magnitude.1 Here, unlike the manipulation of magnetic microparticles described earlier, the ferrofluid remains a single droplet upon movement into and out of the center of the coil. This allows movement of the droplet into the center of a 100 μm coil with only a 1.2 μm insulation layer. In this situation, the droplet is attracted to the entire interior surface of the coil and pulled over the whole electromagnetic micropore (Figure 5a). The droplet remains Langmuir 2010, 26(24), 19239–19244
Article
intact even after being pulled to the periphery of the coil (Figure 5b). Ferrofluid droplets smaller than the coil diameters were investigated but did not provide sufficient pore blockage to function as optimal gating elements. The measured ion current response in 1 M KCl is shown in Figure 5c (red trace). Here, the ion current, sampled every 1 s, shows an apparent immediate response to movement of the droplet over the micropore. The ion current of the on-state is approximately 1 order of magnitude higher than that of the off-state and is completely restored, indicating the reversibility of the electromagnetic pore. The blue trace (Figure 5c) shows the effect of a 0.01 Hz alternating current with a triangular waveform (Ipp=800 mA) traveling through the coil. In this case, the droplet moves onto and off of the coil much slower in comparison to the square wave experiments (see Supporting Information movies). Application of a triangle wave reveals the minimum coil current required to actuate movement of the ferrofluid. This is an important parameter to determine, as lower currents result in lower power consumption and reduced heating of the device. Though the present experimental setup does not allow for simultaneous recording of the coil current and ion current, a slight coil current leak was observed, which provided a means to correlate the coil current and the ion current. A control experiment employing a triangle wave (no droplet) showed a periodic ion current that can be correlated to the 0.01 Hz wave (see Supporting Information Figure S4). Assuming the high points on the control trace represent the highest coil currents (400 and -400 mA), specific coil currents can be assigned for each point in the plot. The on-states of the triangle wave experiment (Supporting Information Figure S4, blue trace) also showed these high points, which were used to correlate the coil current. Here, a clockwise coil current (represented by negative numbers in Supporting Information Figure S4) of 80-160 mA resulted in movement of the magnetic droplet into the center of the coil. For clockwise currents between 0 and 80 mA, the droplet moved off of the center of the coil. To confirm the coil currents described above, additional trapping experiments were performed where the coil currents were controlled manually. These tests showed a similar current-dependent response for movement of the ferrofluid (not shown), supporting the assignment of coil currents shown in Supporting Information Figure S4. In addition, the initial droplet placement was found to influence the current-dependent movement of the droplet. For example, a droplet initially placed at the coil periphery showed the trend described above, but a droplet initially placed in the center of the coil resulted in the opposite trend (80-160 mA counterclockwise current to move the droplet to the periphery of the trap and 0-80 mA counterclockwise current to move the droplet back to the center of the trap). These results show an important consideration for the electromagnetic micropore. Namely, the magnetic fields are not solely responsible for the manipulation of magnetic material. Nonlinear trends observed for the dependence of movement of the ferrofluid droplet into and out of the microcoil with respect to current most likely occur due to solvation energies of the nonpolar ferrofluid droplet, as well as surface tension of the droplet. To further explore the influence of the material being gated, trapping experiments were also performed with magnetic microparticles. These particles did not significantly change the ion current (on/off state) of a 5-10 μm pore, but did provide a means of determining if the trends observed with ferrofluid were due to the magnetic field produced by the coil or the hydrophobic nature of the droplet. A 5 μm electromagnetic micropore was found to move particles into the center of the coil with a minimum clockwise current of 100 mA and to move particles out of the center to the periphery of the coil with a minimum counterclockwise DOI: 10.1021/la103977e
19243
Article
Basore et al.
Figure 6. Gating of a electromagnetic pore at various frequencies (100 μm coil diameter, 10 μm pore diameter). A square wave (600 mA Ipp) runs through the coil at (a) 1 Hz, (b) 5 Hz, (c) 10 Hz, and (d) 25 Hz. Current sampled every 0.01 s. The black trace represents the control (no ferrofluid droplet), and the blue trace is the gating experiment (ferrofluid droplet). A 1 M KCl solution was used.
current of the same magnitude. This suggests that magnetic fields are likely not the direct cause of the trend observed with the ferrofluid droplet. The speed at which the electromagnetic micropore functions is also an important aspect of this device. Mechanical microvalves have been shown to function in the kHz regime within microfluidic systems but are significantly complex.59-61 An electromagnetic micropore could provide a simple alternative for mechanical microvalves, provided the rate of switching was adequate to a specific application. To investigate the maximum frequency at which the electromagnetic fields generated can control ion transport through the micropore, the current was sampled approximately every 0.01 s for at least 20 s using an electromagnetic micropore with a 100 μm coil and a 10 μm pore. Figure 6 shows the ion current response in which an alternating current square wave (Ipp = 600 mA) is passed through the coil at various rates (1-25 Hz). Here, the ion current response is only shown for a 1 s interval for visual clarity. As in the previous case, the ion current response is almost immediate after reversal of current through the coil and the measured ion current frequency correlates to the coil current frequency. The pore states in these examples are found to change by approximately 1 order of magnitude and are completely reversible over the entire measurement time. Frequencies faster than 25 Hz were tested, but did not result in a measurable ion current response because the ion current sampling rate was limited to approximately 0.01 s by the measurement device. Additionally, the droplet viscosity could result in a slow response at these faster switching rates. Data taken from the entire time duration for each frequency are shown in Figure 7, plotted as histograms of occurrence versus current. For each gating frequency (1-25 Hz), populations of the on and off state are clearly discernible. In all examples (except 25 Hz), a current threshold of 20 μA results in populations of nominally 50% for the on and off state. Two populations (threshold 20 μA)
Acknowledgment. Financial support was provided by the NSF (CHE-0847624). Portions of this research were conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy.
(59) Fu, C.; Rummler, Z.; Schomburg, W. J. Micromech. Microeng. 2003, 13, S96. (60) Oh, K. W.; Ahn, C. H. J. Micromech. Microeng. 2006, 16, R13. (61) Yanagisawa, K.; Kuwano, H.; Tago, A. Microsyst. Technol. 1995, 2, 22–25.
Supporting Information Available: Details of pyrex chip fabrication and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.
19244 DOI: 10.1021/la103977e
Figure 7. Histograms of occurrence versus ion current with various gating frequencies. Histograms of gating shown are for the entire data set. Two distinct states with clearly observable threshold ion currents are shown for all gating frequencies.
are observed at 25 Hz, but the on-state population (58.1%) is slightly larger than the off state population (41.9%). This offset is only seen with the 25 Hz gating rate and can likely be eliminated by increasing the sampling rate, a goal of future experiments. These tests show that the electromagnetic micropore is capable of functioning at speeds of at least 25 Hz in a highly reversible manner. Improvements in the instrumental setup, as well as smaller droplets and coil diameters, are presently being examined to determine an ultimate limit on gating frequency.
Conclusions We have fabricated and described the operation of a reversible electromagnetic micropore. Finite element simulations and particle manipulation experiments showed these electromagnets can move magnetic material into and out of the center of the trap, resulting in control over ion transport through the micropore. The frequency response to ion current gating was investigated, demonstrating functional current blockages at rates up to 25 Hz. These experiments show that we have extended the function of microelectromagnetic traps to produce a magnetically controlled pore with potential applications in fundamental studies of ion transport, fluidic switching, and possibly detection and separation of chemical and biochemical species.
Langmuir 2010, 26(24), 19239–19244
