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Electric Field Mediated Transport in Nanometer Diameter Channels Paula J. Kemery,† Jack K. Steehler,‡ and Paul W. Bohn*,† Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois, 600 South Mathews, Urbana, Illinois 61801, and Department of Chemistry, Roanoke College, Salem, Virginia 24153 Received February 5, 1998 Nanoporous polycarbonate (PCTE) nuclear-track-etched membranes were used to effect electric field modulation of the mass transport of cationic, anionic, and neutral species in aqueous buffer. The permeability response to electric fields depended on the molecular charge and electrolyte concentration. Solute and solvent fluxes through nanopores under an electrical potential result from a balance of diffusion, electroosmosis, and ion migration. The Debye length, κ-1, associated with the electrical double layer within the pores relative to the pore diameter, 2a, plays a critical role in determining electrokinetic transport behavior. The channel walls adsorb anions preferentially to produce a largely immobile negative charge density, leaving a large and mobile cation population to mediate transport in the channel. By adjusting the supporting electrolyte concentration, κa can be tuned such that the electrical double layer is either small in relation to the pore (κa g 1) or more diffuse and spanning the pore (κa < 1). Electroosmotic transport, mediated primarily by buffer cations, dominates when κa < 1. In this case the pores are essentially permselective, and anion electromigration is virtually eliminated. When κa g 1, the sign of the applied potential can be used to select for anion vs cation/neutral molecule transport.
Introduction Increasingly, the behavior of very small ensembles of molecules is coming under scrutiny, making it imperative that tools be developed for manipulating microscopic quantities of materials with the same facility now available for macroscopic samples. Central to the problem is the ability to move molecules over nanometer dimensions with high precision, selectivity, and temporal control; a capability which, when realized, will enable advances on both fundamental and technological fronts. Active control over transport of specific molecules at nanometer dimensions would comprise an enabling technology by permitting (a) new approaches to molecular separations that augment passive analyte-stationary phase interactions, (b) the study of reactions in which one or more reagents are available in extremely limited quantities, and (c) coupling of powerful methods of molecular identification, e.g., mass spectrometry, to spatially and temporally resolved molecular sampling methods; thereby permitting inherently small samples to be manipulated spatially. Nuclear-track-etched, e.g., Nuclepore, membranes have attracted much attention, particularly for separation applications in size-based filtration.1-5 The effects of electric fields, both applied to and produced in these membranes, have generated much interest, ranging from understanding electrokinetic flow in cylindrical channels,6-11 to the observation and explanation of periodic * To whom correspondence should be addressed. † University of Illinois. ‡ Roanoke College. (1) Hernandez, A.; Martinez-Villa, F.; Ibanez, J. A.; Arribas, J. I.; Tejerina, A. F. Sep. Sci. Technol. 1986, 21, 665. (2) Wakeman, R. J.; Tarleton, E. S. Trans. AlChE 1991, 69, 386. (3) Nystrom, M.; Lindstrom, M.; Matthiasson, E. Colloids Surf. 1989, 36, 297. (4) Hope, M. J.; Bally, M. B.; Webb, G.; Cullis, P. R. Biochim. Biophys. Acta 1985, 812, 55. (5) Henry, J. D.; Lawler, L. F.; Kuo, C. H. A. AlChE J. 1977, 23, 851. (6) Ibanez, J. A.; Forte, J.; Hernandez, A.; Tejerina, F. J. Membr. Sci. 1988, 36, 45.
flux oscillations.12 Surface charge, a manifestation of the enhanced surface area-to-volume ratio (As/V), is a critically important property influencing all these phenomena.13,14 It determines the magnitude of the surface potential and the applicability of the Debye-Huckel approximation, and ultimately, it provides an experimental handle to adjust the microscopic processes that determine transport in the channel. While much effort has gone into understanding the role of surface potential, relatively little has gone into trying to control it. In this regard the recent discovery by Martin et al. that ion permselective membranes can be obtained using electroless Au-coated channels in nucleartrack-etched membranes is notable.15 The well-defined properties of nuclear-track-etched membranes make them a nearly ideal system to study controlled molecular transport; it is possible to study transport under a wide range of conditions in structures with a preselected pore areal density and a nearly monodisperse distribution of channel diameters. It is particularly instructive to explore how the relative sizes of the double layer and the channel diameter affect transport. The relevant parameter to consider is the product of the channel radius, a, and the inverse Debye length, κ. When κa e 1, the electrical double layer extends across the entire channel. In aqueous systems κ-1 is very large, being limited by autoprotolysis of water, meaning that achieving channels where the electrical double layer (7) Brendler, E.; Ratkje, S. K.; Hertz, H. G. Electrochim. Acta 1996, 41, 169. (8) Wan, Q.-H. Anal. Chem. 1997, 69, 361. (9) Anderson, J. L.; Koh, W.-H. J. Colloid Interface Sci. 1977, 59, 149. (10) Rice, C. L.; Whitehead, R. J. Phys. Chem. 1965, 69, 4017. (11) Westermann-Clark, G. B.; Anderson, J. L. J. Electrochem. Soc. 1983, 130, 839. (12) Meares, P.; Page, K. R. Phil. Trans. R. Soc. London 1972, 272, 1. (13) Fair, J. C.; Osterle, J. F. J. Chem. Phys. 1971, 54, 3307-3316. (14) Martinez, L.; Gigosos, M. A.; Hernandez, A.; Tejerina, F. J. Membr. Sci. 1987, 35, 1. (15) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700.
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can be made to span the channel requires diameters of several hundred nanometers or less. Thus, nanometer pores occupy a special place in the study of electrokinetic aqueous transport in confined geometries. Anionic surface functional groups and/or the adsorption of anions from solution produce a low mobility anionic surface charge on the pore wall,16-20 thereby determining that the mobile ions filling the channel are primarily cations. Thus, under conditions where the double layer extends throughout the pore, electroosmotic transport mediated by buffer cations is primarily responsible for moving species across the membrane. Alternatively, under conditions in which κa > 1, the double layer is collapsed near the channel wall, and electromigration is possible. By simultaneously manipulating buffer conditions, pore properties, and surface chemistry, it should be possible to effect molecular transport with widely varying character. The work described herein lays out the basic phenomenology of electrokinetic transport in nanometer-diameter channels in PCTE, establishes the ranges of important parameters, and illuminates those conditions likely to be fruitful for investigation of controlled transport. Experimental Section Materials. Deionized water, F ) 18.2 MΩ cm, was obtained from a Milli-Q UV plus system (Millipore). The fluorescent probes 1,1′,3,3,3′,3′-hexamethylindocarbocyanine, DiIC1(3)+ (Molecular Probes), disodium fluorescein, Fl2- (Matheson, Coleman and Bell), and d-tryptophan, Trp (Sigma), were used as received. Nuclepore PCTE membranes were purchased from Costar and stored in a 0% humidity environment prior to use. Membranes were 2.5 cm in diameter and 6 µm thick with a highly monodisperse distribution of 6 × 108/cm2 ( 1.2 × 108/cm2 pores with average diameter 15 nm. PCTE membranes added 200-400 Ω to the total resistance of the permeation cell. Phosphate buffer solution (PBS) was prepared from mono- and dibasic potassium phosphate at pH 6.8 ( 0.2, ionic strength 10 mM e µ e 1 M, and resistivity 140 Ω cm e F e 370 Ω cm. Permeability Measurements. Fluxes of fluorescent probes through PCTE membranes were monitored in situ in a Spex fluorometer in the 90° geometry. Instrument control and data collection were achieved with a personal computer running LabView (National Instruments) software written specifically for this instrument. The fluorescent probes used were the zwitterionic Trp, the cation DiIC1(3)+, and the dianion Fl2-. Excitation (emission) maxima for the probes are 289 (366), 539 (553), and 488 (510) nm, respectively. Limits of detection via fluorescence in our apparatus were determined to be 3 nM for Trp, 2 nM for DiIC1(3)+, and 0.2 nM for Fl2-. A PCTE membrane was mounted to a custom-made stainless steel ring holder and positioned between two cells of 2.5 mL each. The initial concentration of fluorescent probe on the source side ranged from 5 × 10-6 to 1 × 10-4 M and was set such that the change in probe concentration during a typical experiment was 8 at low ionic strength (Table 1). The same field application in the presence of 500 mM buffer, however, produces only a moderate 2-fold enhancement of Trp permeability. The negative permeability displayed in Table 1 for Trp in 500 mM PBS indicates that positive polarity applied fields reverse Trp flux. Cationic Probe. The permeability trends in Table 1 and Figure 2 for the charged probe DiIC3(1)+ indicate behavior similar to that observed for Trp, although somewhat more pronounced, as would be expected given the cationic nature of the probe. At 5 mM PBS a 10-fold permeability enhancement is observed when a negative potential is applied, while a positive potential inhibited transport in the diffusive direction. In the presence of 500 mM PBS the same trends are observed, but the magnitudes of effects are much smaller, e.g., a 1) responds to field application by exhibiting distinctive periods of delay (cf. Figure 4), suggesting that under these conditions fluorescein interacts with the nanoporous membrane structure, requiring that equilibrium be established prior to flux control by ion migration. The most likely explanation for this interaction is that Fl2- interacts with the membrane surfaces and pore walls by participating in the electrical double layer. Consider that as Fl2- flux changes due to electric field application, the concentration of Fl2- in the channels varies relative to the bulk. Fl2- interaction would be expected to display a different behavior than that of cationic (or neutral) probes in the electrical double layer, since the anion layer is much less mobile than the associated cations. Thus, Fl2partitioning between the mobile fluid phase and the relatively immobile anionic portion of the double layer would lead to characteristic delays in establishing steadystate transport conditions after changing the magnitude or polarity of the bias potential. This effect is expected to be larger for the relatively compact double layer obtained at κa > 1, since the entire double layer is much more closely associated with the pore surface. The sign of the effect is also consistent with this interpretation. When the structure is switched from field-free to positive bias conditions (30 min in Figure 4), Fl2- transport is inhibited initially before reaching steady-state flux, while switching from positive to negative bias (40 min in Figure 4) leads to a transient period of enhanced Fl2- transport. Under conditions appropriate to establishing a compact double layer, anions in the center of the pore would be expected
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to partition between the mobile center and the relatively immobile region near the channel walls. Switching from field-free to positive bias would draw an enhanced steadystate concentration of Fl2- into the channel, and consequently, steady-state partitioning into the double layer would occur, thereby giving rise to an apparent inhibition in the Fl2- transport. Subsequent application of a negative bias would decrease the steady-state concentration of Fl2in the center of the channel. To establish the steadystate partitioning between channel center and double layer, Fl2- would be expelled from the double layer and subsequently from the channel, leading to a transient increase in the observed Fl2- concentration on the receiving side of the membrane. Again, after the steady state is established, Fl2- transport is inhibited because it is controlled by ion migration and is reverse-biased under these conditions. Conclusions By controlling the dimensions of the electrical double layer relative to the pore diameter and the magnitude and sign of applied potential, transport of charged and uncharged species over micron dimensions through nanoporous media may be spatially and temporally manipulated with electric fields. PCTE nanoporous polymer substrates provide uniform, rugged media for exploiting electrokinetic transport, because adjusting buffer concentrations can produce electrical double layer dimensions over the entire range from κa , 1 to κa . 1. Field-induced transport behavior is dominated by electrokinetic effects. The determining factor of the electrokinetic transport is the propensity of anions to become surface-associated, forming a relatively immobile layer on the channel walls. In the diffuse double layer regime (κa , 1) field-induced transport is then dominated by electroosmotic flow of buffer cations. In the opposing limit of a compact double layer, electromigration plays a more important role, although, for anionic probes, partitioning of probe into the immobile surface-associated layer produces significant transient effects. It is of significant interest to derivatize the pore surfaces to further dictate the structure of the electrical double layer and effect a greater degree of control over field-induced mass transport, and experiments targeting this capability are underway in our laboratory. Acknowledgment. This work was supported by the National Science Foundation through grant CHE 9420211 and by the Biotechnology Research and Development Corp. LA980147S
