Langmuir 2003, 19, 7829-7831
7829
Letters Controlling Electroosmotic Flow in Microchannels with pH-Responsive Polyelectrolyte Multilayers Zhijie Sui and Joseph B. Schlenoff* Department of Chemistry and Biochemistry, and Center for Materials Research and Technology (MARTECH), The Florida State University, Tallahassee, Florida 32306-4390 Received April 22, 2003. In Final Form: July 14, 2003 A polyelectrolyte multilayer coating comprising acrylic acid or imidazole units afforded precise control over the rate and direction of electroosmotic flow within fused silica capillaries. Morphological changes accompanying the pH-induced switching of surface charge were minimized by diluting the pH-responsive polyelectrolyte with permanently charged polymer. Electroosmotic stability over many runs was demonstrated.
Electroosmotic flow1-3 (EOF) is commonly employed to steer liquid through microfluidic channels.4-7 Since surface charge is critical in determining the direction and velocity of EOF, different substrates, having different charge densities, provide a range of flow rates. Polymeric substrates are problematic, owing to the nonuniformity and poor reproducibility of surface charge.8,9 A variety of strategies for controlling surface charge have been employed, including covalent,10 noncovalent,11 and dynamic coatings.12-14 Recently, coatings comprising polyelectrolyte multilayers (PEMUs) have shown exceptional promise for modifying microfluidic systems, including those for capillary zone electrophoresis15-17 and microscale “plumbing” (lab-on-a-chip).18,19 The coatings, which are prepared in a straightforward manner by alternate flushing with positive and negative polyelectrolytes,15-19 are robust and (1) Jorgenson, J. W.; Lukacs, K. D. Anal. Chem. 1981, 53, 12981302. (2) Hayes, M. A. Anal. Chem. 1999, 71, 3793-3798. (3) Herr, A. E.; Molho, J. I.; Santiago, J. G.; Mungal, M. G.; Kenny, T. W. Anal. Chem. 2000, 72, 1053-1057. (4) Pittman, J. L.; Henry, C. S.; Gilman, S. D. Anal. Chem. 2003, 75, 361-370. (5) Bharadwaj, R.; Santiago, J. G.; Mohammadi, B. Electrophoresis 2002, 23, 2729-2744. (6) Dutta, P.; Beskok, A.; Warburton, T. C. J. Microelectromech. Syst. 2002, 11, 36-44. (7) Liu, Y.; Wipf, D. O.; Henry, C. S. Analyst 2001, 126, 1248-1251. (8) Branham, M. M.; MacCrehan, W. A.; Locascio, L. E. J. Capillary Electrophor. Microchip Technol. 2000, 6, 43. (9) Locascio, L. E.; Perso, C.; Lee, C. S. J. Chromatogr., A 1999, 857, 275-284. (10) Culbertson, C. T.; Ramsey, R. S.; Ramsey, J. M. Anal. Chem. 2000, 72, 2285-2291. (11) Wang, Y.; Dubin, P. L. Anal. Chem. 1999, 71, 3463-3468. (12) Chiem, N.; Harrison, D. J. Anal. Chem. 1997, 69, 373-378. (13) Wang, S.-C.; Perso, C. E.; Morris, M. D. Anal. Chem. 2000, 72, 1704-1706. (14) Lurie, I. S.; Panicker, S.; Hays, P. A.; Garcia, A. D.; Geer, B. L. J. Chromatogr., A 1999, 984, 109-120. (15) Katayama, H.; Ishihama, Y.; Asakawa, N. Anal. Chem. 1998, 70, 5272-5277. (16) Katayama, H.; Ishihama, Y.; Asakawa, N. Anal. Chem. 1998, 70, 2254-2260. (17) Graul, T. W.; Schlenoff, J. B. Anal. Chem. 1999, 71, 4007-4013. (18) Barker, S. L. R.; Tarlov, M. J.; Canavan, H.; Hickman, J. J.; Locascio, L. E. Anal. Chem. 2000, 72, 4899-4903. (19) Liu, Y.; Fanguy, J. C.; Bledsoe, J. M.; Henry, C. S. Anal. Chem. 2000, 72, 5939-5944. (20) Decher, G. Science 1997, 277, 1232-1237.
reproducible.23-25 Effective resistance to protein adsorption yields exceptional efficiency in the capillary zone electrophoretic separation of proteins that normally adsorb irreversibly to bare silica.17 Similar efficacy is expected on polymeric substrates,22 since the substrate properties are effectively masked after a few layers. The mechanisms of formation and charge distribution of PEMUs, including surface charge, have been probed by electrokinetic17,23 and radiochemical techniques.24,25 Excess polymer, shown to be excluded to the surface, is controlled by the last layer added in the buildup sequence in a very reproducible manner.17 Highly reproducible switching in EOF direction17 provides a valuable dimension in microfluidic flow control. However, the ability to determine rate as well as the direction of EOF would be an additional benefit, and adding another polymer layer to effect the latter is somewhat cumbersome. We describe here multilayers comprising weak acid/base moieties that permit both. Fused silica capillaries of diameter 50 µm were coated with multilayers made from poly(styrene sulfonate), PSS, as a permanently charged negative polyelectrolyte, and a mixture of poly(diallyldimethylammonium chloride), PDADMA, and a random copolymer of diallyldimethylammonium and acrylic acid, PDADMA-co-PAA (0.64/0.36 mole ratio of respective monomer units, rendering the copolymer net positive at all pHs) (Chart 1). Coatings were prepared in buffers at low pH with the last layer being the copolymer (positive) having the carboxylic acid groups in the protonated (neutral) state. On exposure to sufficiently high pH, carboxylic groups are ionized, overwhelming the surface positive excess polymer charge and therefore switching the surface charge to negative.26 The function of the permanent positive charges (quaternary ammoniums) is essentially to keep the multilayer stitched together at all pH values. Too many counterion(21) Decher, G.; Schlenoff, J. B. Multilayer Thin Films. Sequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, 2003. (22) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78-86. (23) Caruso, F.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 2011-2016. (24) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626-7634. (25) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592598.
10.1021/la034682q CCC: $25.00 © 2003 American Chemical Society Published on Web 08/19/2003
7830
Langmuir, Vol. 19, No. 19, 2003
Letters
Chart 1. Structures of Polyelectrolytes
compensated units within a multilayer cause hyperswelling and decomposition.27,28 For example, we have used this same copolymer, which is commercially available (Merquat 280, Calgon Inc.) as an ingredient for personal care products, to build pH-releasable multilayers.29 Therefore, the carboxylates were “diluted” by adding pure homopolyDADMAC. Since the concentration of PAA in the pure Merquat polymer is 36 mol %, the final concentration of carboxylates in the multilayer, expressed as a molar ratio or mole percent of total PAA + PDADMA units, is given by
PEMU mol % PAA )
[Merquat] × 36 [Merquat] + [PDADMA]
An appropriate balance between sufficient copolymer to induce switching, yet avoid film disruption, was about 7 mol % PAA, or a mixture of 0.20 mM copolymer and 0.80 mM PDADMAC. We verified, using Fourier transform infrared spectra of comparison samples on silicon wafers, that the multilayer carboxylate composition approximately reflects the solution composition from which the films are deposited.30 The EOF of a capillary coated with an 11-layer PEMU (copolymer on top), where the positive layers were composed of 20% copolymer, exhibited very reproducible switching of flow direction with alternating between buffers of “high” (7.30) and “low” (2.78) pH. The fact that such flow reversal was observed for positive layers containing as little as 7.2 mol % PAA was initially surprising. However, it is likely that PAA within the bulk of the multilayer participates, after migration, in the surface charge switching: efficient ion pairing tends to drive excess charge to the surface,24 where there are fewer excluded volume constraints.25 When internal PAA is ionized, it is probable that the charge is extruded to the surface. Evidence for the motion of polyelectrolytes within multilayers has been acquired via atomic force microscopic measurements of PEMU surface smoothing.31 pH-induced phase separation of PAA-containing PEMUs, yielding porous films, has also been observed.32 A key variable in pH-sensitive coatings was the total thickness of the PEMU. While it is desirable to minimize (26) EOF was determined with a Beckman Coulter P/ACE MDQ capillary electrophoresis system with 254 nm UV detection operating at 15 kV and 25 °C. Fused silica capillary, 50 µm (i.d.) × 31.2 cm (effective length 21 cm), was employed. PEMUs were made by alternate flushing with polymer solutions and water rinse. The buffer was citric (pH 2.787.30). Acetone was used as a neutral electroosmotic flow marker. We use the convention that the normal direction, from anode to cathode, is positive. (27) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301310. (28) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 37363740. (29) Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2001, 123, 5368-5369. (30) Sui, Z.; Schlenoff, J. B. To be published. (31) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001 17, 7725-7727. (32) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017-5023.
Figure 1. Electroosmotic flow vs number of runs for a fused silica capillary coated with (squares) an 11-layer PEMU of one PEI layer (the first), five 20% PDADMA-co-PAA/80% PDADMA layers, and five PSS layers; and (circles) a 10-layer PEMU of five PSS and five QPVI layers (PSS on top). The open triangle is the EOF for a bare capillary at pH 7.30. Even runs were in pH 7.30 buffer; odd runs at pH 2.78. Conditions: citric buffer at 10 mM total ionic strength; 15 kV applied voltage; 31.2 cm capillary. Positive EOF indicates the surface charge is negative.
the number of layers deposited, a minimum number of layers is also required to mask the bare silica. Microscopic investigations (atomic force microscopy) of 21-layer PEMUs on a Si wafer revealed some evidence of pH-induced cratering in the surface morphology. PEMUs made with 11 layers showed no such topological nonidealities and exhibited higher EOF on pH switching. This difference was attributed to the loss of material from the surface of 21-layer PEMUs. Loss of thickness from multilayers on silicon wafers is evidence for not only polymer charge extrusion but also charge expulsion. These multilayers were less stable when the last layer was negative as-deposited (PSS).30 Similar behavior was observed using a weak base polyelectrolyte, such as a poly(vinyl imidazole) (PVI), that was 60% methylated to impart some permanent positive charges (quaternized PVI or QPVI), and PSS as the multilayering partner. In this case, the QPVI was deposited from pH 7.3 citric buffer, which is above the pKa (∼6) of the imidazole repeat units,33 allowing extra positive charge to appear within the PEMU as the pH is lowered. As seen in Figure 1, EOF switching of approximately the same magnitude as with the PAA copolymer is observed. Note that “dilution” of PVI units was not required, due to the enhanced stability of the QPVI-PSS ion pair compared to the PDADMA-PSS ion pair. Fine control of EOF is possible by selection of intermediate pH for buffers. Figure 2 depicts EOF as a function of pH for bare fused silica and the PAA and QPVI PEMU coatings. The latter two reverse directions, but the former simply offers a range of EOF on the positive (“normal” flow) side. The pH of zero flow, corresponding to the pH of zero net surface charge, was initially presumed to occur at a point near the pKa of the PAA or PVI within the multilayer.34 Although the switching buffers (Figure 1) were chosen to bracket a pH of 5 (i.e., approximately the pKa of solution polycarboxylic acid or polyimidazole), the pH of zero charge may not coincide with the solution pKa. (33) Overberger, C. G.; Kawakami, Y. J. Polym. Sci. 1978, 16, 12371248. (34) The pH of zero flow cannot be identified exactly, since the neutral marker elutes at very long times close to this point. An EOF lower than 0.06 × 10-4 cm2 V-1 s-1, corresponding to an elution time greater than 120 min, was assigned zero flow rate in our experiments. A couple of error bars in Figure 2 illustrate the uncertainty.
Letters
Figure 2. EOF as a function of citric buffer pH (total ionic strength, 10 mM) for the PDADMA-co-PAA & PDADMA/PSS (squares) and QPVI/PSS (circles) PEMUs from Figure 1. The dashed line represents EOF vs pH for a bare capillary.
Langmuir, Vol. 19, No. 19, 2003 7831
polyelectrolyte units, as described32,35,36 and rationalized36,37 previously. Additionally, it is not known what fraction of weak acid/base must be ionized in order to neutralize the existing surface charge. PDADMA by itself has been employed as a positive coating for silica capillaries in capillary zone electrophoresis.11,17 In the present work, a single coating of PDADMA-co-PAA also afforded pH-controlled switching of EOF, but this single layer yielded irreproducible and unstable flows, due to contributions from the underlying silica silanol, which were insufficiently masked. Slow surface rearrangements may also impact negatively. Run-to-run reproducibility was excellent and is illustrated by Figure 3, which shows sequential runs following pH switching. After a short time, the EOF is very stable. This contrasts strongly with the notorious instability of EOF in bare capillaries in response to wide swings in pH.38 Other advantages of the present coating include the possibility to separate acidic and basic proteins on one column, either by choosing pH values that minimize irreversible adsorption of all components or by switching the pH to release proteins that may have adsorbed to the surface. Finally, choosing a pH where the EOF is truly zero (hard to accomplish in silica capillaries) allows separation based solely on electrophoretic mobility and may allow isoelectric focusing. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research and to Husam Jumaa for synthesizing the QPVI.
Figure 3. EOF vs number of runs with switching pH for the 11-layer PDADMA-co-PAA & PDADMA/PSS PEMU from Figure 1 with pH controlled by a phosphate buffer series (total ionic strength, 10 mM). Consecutive runs at pH 2.78, diamonds and triangles; runs at pH 7.30, squares.
The pKa of weak acid/base units within the PEMU is modified by their interaction with oppositely charged
Supporting Information Available: Description of experimental materials and methods. This material is available free of charge via the Internet at http://pubs.acs.org. LA034682Q (35) Xie, A. F.; Granick, S. Macromolecules 2002, 35, 1805-1813. (36) Kabanov, V. A.; Zezin, A. B. Sov. Sci. Rev., Sect. B 1982, 4, 207. (37) Rmaile, H. H.; Schlenoff, J. B. Langmuir 2002, 18, 8263-8265. (38) Lambert, W. J.; Middleton, D. L. Anal. Chem. 1990, 62, 15851587.
