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Switching Surface Chemistry with Supramolecular Machines† Bruce C. Bunker,*,‡ Dale L. Huber,‡ James G. Kushmerick,‡ Timothy Dunbar,‡ Michael Kelly,‡ Carolyn Matzke,‡ Jianguo Cao,§ Jan O. Jeppesen,§ Julie Perkins,§ Amar H. Flood,§ and J. Fraser Stoddart*,§ Sandia National Laboratories, Albuquerque, New Mexico 87185, Department of Chemistry and Biochemistry and the California NanoSystems Institute, UniVersity of California, Los Angeles, California 90095 ReceiVed June 1, 2006. In Final Form: August 6, 2006 Tethered supramolecular machines represent a new class of active self-assembled monolayers in which molecular configurations can be reversibly programmed using electrochemical stimuli. We are using these machines to address the chemistry of substrate surfaces for integrated microfluidic systems. Interactions between the tethered tetracationic cyclophane host cyclobis(paraquat-p-phenylene) and dissolved π-electron-rich guest molecules, such as tetrathiafulvalene, have been reversibly switched by oxidative electrochemistry. The results demonstrate that surface-bound supramolecular machines can be programmed to adsorb or release appropriately designed solution species for manipulating surface chemistry.
Introduction Devices containing microfabricated fluidic channels are under development for a wide range of applications1,2 including proteomics and DNA analysis, biomedical implants, and microanalytical systems for Homeland Security. As devices get smaller, interactions between fluids and interfaces become more important. Researchers are developing materials whose interfacial characteristics can be made to change in response to external stimuli such as heat, light, or electrical impulses for functions ranging from the hydrophilic-hydrophobic pumping of liquids, programmable chromatography, and the selective adsorption and release of specific analytes such as proteins.3-8 Here we report the use of a new class of active SAMs containing supramolecular machines that can be electrochemically activated to move or exchange guest molecules relative to a substratefluid interface. Examples of such machines include mechanically interlocked molecules9-11 such as catenanes and rotaxanes, as well as their precursor complexes including precatenanes and pseudorotaxanes.12 The actuator in these machines is the †
Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * To whom correspondence should be addressed. E-mail: stoddart@ chem.ucla.edu (J.F.S.);
[email protected] (B.C.B.). ‡ Sandia National Laboratories. § University of California, Los Angeles. (1) Auroux, P. A.; Iossifidis, D.; Reyes, D. R.; Manz, A. Anal. Chem. 2002, 74, 2637-2652. (2) de Mello, A. Lab Chip 2002, 2, 48N-54N. (3) Burns, M. A.; Mastrangelo, C. H.; Sammarco, T. S.; Man, F. P.; Webster, J. R.; Johnson, B. N.; Foerster, B.; Jones, D.; Fields, Y.; Kaiser, A. R.; Burke, D. T. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 5556-5561. (4) Walker, G. M.; Beebe, D. J. Lab Chip 2002, 2, 131-134. (5) Chaudhury, M. K.; Whitesides, G. M. Science (USA) 1992, 256, 15391541. (6) Huber, D. L.; Manginell, R. P.; Samara, M. A.; Kim, B. I.; Bunker, B. C. Science (USA) 2003, 301, 352-354. (7) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science (USA) 2000, 288, 16241626. (8) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abbott, N. L. Science (USA) 1999, 283, 57-60. (9) Ballardini, R.; Balzani, V.; Credi, A.; Gandolfi, M. T.; Venturi, M. Acc. Chem. Res. 2001, 34, 445-455. (10) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3349-3391. (11) Gomez-Lopez, M.; Preece, J. A.; Stoddart, J. F. Nanotechnology (UK) 1996, 7, 183-192.
π-electron-deficient tetracationic cyclophane cyclobis(paraquatp-phenylene) (CBPQT4+, Scheme 1). In its tetracationic state, the central cavity can attract and be filled by π-electron-rich donor molecules. Electrochemical oxidation of the donor guest or reduction of the cyclophane disrupts the attractive interactions, allowing the guest to be expelled or released, respectively. In the pseudorotaxanes studied here (Scheme 1), reversible complexation involves donor molecules that are present in solution. In bistable molecular machines, electrochemical actuation induces the cyclophane to move between two nonequivalent recognition sites in the same molecule. Depending on how machine components are interconnected, either piston-like (in the [2]rotaxanes,13) or wheel-like motions (the [2]catenanes) are available for moving functional groups relative to the substratesolution interface.14,15 Recently, electrochemically active monolayers that switch between linear and bent-chain states have been reported16 that represent a third class of switchable “molecular machines”. Synthesis and Characterization of Pseudorotaxanes For deploying molecular machines at interfaces, monolayer structures are required that allow machine components to move without steric interference and allow the electromechanical switching of [π-π] stacking interactions to be freely expressed. To explore these interfacial parameters, we have produced SAMs in which CBPQT4+ is tethered to gold surfaces and investigated how these monolayers interact with π-donor guest molecules in acetonitrile solutions. One class of monolayers was produced by first depositing a SAM of 11-mercaptoundecanoic acid onto a gold surface from methanol solutions. Exposing the substrate to acetonitrile solutions containing the PF6- salt of the cyclophane resulted in the formation of a self-assembled “salt” layer in which the +4 charge on the (12) Odell, B.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1547-1550. (13) Bissell, R. A.; Cordova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994, 369, 133-137. (14) Tseng, H. R.; Wu, D. M.; Fang, N. X. L.; Zhang, X.; Stoddart, J. F. ChemPhysChem 2004, 5, 111-116. (15) Bryce, M. R.; Cooke, G.; Duclairoir, F. M. A.; John, P.; Perepichka, D. F.; Polwart, N.; Rotello, V. M.; Stoddart, J. F.; Tseng, H. R. J. Mater. Chem. 2003, 13, 2111-2117. (16) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science (USA) 2003, 299, 371-374.
10.1021/la0615793 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/24/2006
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Scheme 1. (Left) Pseudorotaxane Components, Including the Gold-Tethered, π-Electron Deficient Host (CBPQT4+, Blue) and Three Solution-Based π-Electron-Rich Guests Tetrathiafulvalene (TTF, green), 4,4′-di(diethylene glycol)Tetrathiafulvalene (TTF-thread), and 1,5-Di(diethylene glycol)naphthalene (DNP-thread). (Right) Exposure of the Cyclophane Host to the Guests (a) Leads to the Formation of a Supramolecular Complex. The TTF Guest Can Be Expelled or Released by (b) Reduction of the Host or (c) Oxidation of the Guest, Respectively.
cyclophane was compensated by four tethered carboxylate anions. A second class of monolayers was produced by functionalizing the cyclophane with a disulfide-terminated side chain that could be adsorbed onto gold like an alkane thiol. SAMs of the tethered cyclophane (14+) were produced by immersing gold surfaces into 1 mM solutions of 14+ in acetonitrile. Samples were periodically removed from solution and examined by ellipsometry to determine the average film thickness and atomic force microscopy (AFM) to determine film morphology. AFM images (see Supporting Information) show that monolayers of 14+ form slowly (50% coverage after 1 day, with complete coverage requiring 4 days). Ellipsometry and AFM measurements on the completed monolayers indicate a film thickness of 1.5-1.7 nm. The measured film thickness is shorter than the 2.1 nm expected if both the tether and the long axis of CBPQT4+ were oriented perpendicular to the surface. The two preparations of the surface-bound CBPQT4+ were exposed to solutions containing one of three different guest species (Scheme 1) known to form stable complexes displaying large association constant (Ka) values with CBPQT4+ in solution: tetrathiafulvalene (TTF, Ka ) 10 000 M-1),17 TTF functionalized with two diethylene glycol chains (TTF-thread, Ka ) 500 000 M-1),17 and 1,5-dioxynaphthalene similarly functionalized with two diethylene glycol chains (DNP-thread, Ka ) 25 000 M-1).18 The remainder of this communication describes the properties of the SAMs as these guest species enter and leave the central CBPQT4+ cavity.
is reduced from its native +4 oxidation state to the +2 oxidation state at a peak potential of -0.74 V (relative to a Ag/AgPF6 reference electrode). On the basis of solution-phase cyclic voltammetry, facile species desorption from the cyclophane is expected at potentials that are more negative than this first bielectronic reduction process. A second bielectronic reduction process at a peak potential of -1.26 V generates the electrically neutral cyclophane. In solution, oxidation of either TTF or the TTF-thread has also been shown to promote desorption due to electrostatic repulsion between the cationic guest and the
Electrochemical Switching of Tethered Pseudorotaxanes Cyclic voltammetry experiments were performed in acetonitrile on SAMs of 14+ to determine the molecular density of active molecules in the film and to establish the redox potentials at which film switching is expected to occur. Cyclic voltammograms of surface-bound 14+ (Figure 1) mirror those obtained for CBPQT4+ in acetonitrile solutions,19 indicating that its redox properties are not sensitive to the tethering to gold. The cyclophane (17) Balzani, V.; Credi, A.; Mattersteig, G.; Matthews, O. A.; Raymo, F. M.; Stoddart, J. F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Org. Chem. 2000, 65, 1924-1936. (18) Castro, R.; Nixon, K. R.; Evanseck, J. D.; Kaifer, A. E. J. Org. Chem. 1996, 61, 7298-7303. (19) Ashton, P. R.; Ballardini, R.; Balzani, V.; Boyd, S. E.; Credi, A.; Gandolfi, M. T.; Gomez-Lopez, M.; Iqbal, S.; Philp, D.; Preece, J. A.; Prodi, L.; Ricketts, H. G.; Stoddart, J. F.; Tolley, M. S.; Venturi, M.; White, A. J. P.; Williams, D. J. Chem.-Eur. J. 1997, 3, 152-170.
Figure 1. Cyclic voltammograms of (a) 14+ attached to a gold electrode surface (8 cm2 in area) and (b) a 1 mM solution of TTF (MeCN) in contact with a bare gold electrode.
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tetracationic host. Cyclic voltammetry experiments (Figure 1b) performed in solution show that TTF is oxidized to the +1 state at + 0.035 V and to the +2 state at +0.45 V. Both oxidation processes are reversible and can be performed in aqueous media (falling within the electrochemical stability window for water). The total current passed in the first reduction wave in Figure 1a leads to a calculated surface coverage for 14+ on gold of 1.25 nm2/cyclophane, consistent with a densely packed monolayer in which all cyclophane molecules are oriented parallel with the surface. (Coverages calculated based on the known dimensions of 14+ are 1.1, 0.5, and 0.4 nm2/cyclophane for a complete monolayer of molecules with the two longest axes parallel to the surface, with the second longest axis perpendicular to the surface, and with the longest axis perpendicular to the surface, respectively.) The inferred cyclophane orientation is expected to offer the most open access to the cyclophane’s central cavity to species dissolved in solution.
Capture and Release of Guests from Tethered Cyclophane Hosts While electrochemical measurements indicate that the components of our pseudorotaxane should be switchable when tethered to a surface, they do not provide direct evidence that the cyclophane host captures and releases guests under the same conditions seen in solution. As the standard 1H NMR and UV-visible spectroscopy methods used in solution are not sensitive enough20 to detect activity in a single monolayer, we have used polarized reflection-absorption infrared spectroscopy (RAIRS) to characterize capture and release processes. For the RAIRS experiments, large (2 cm × 4 cm) gold-coated substrates were immersed in solution (MeCN, 0.1 M tetrabutylammonium hexafluorophosphate) and employed as working electrodes in an electrochemical cell. After electrolysis at specified voltages, substrates were removed, rinsed in acetonitrile, and examined as dry samples using RAIRS. RAIRS on gold is sensitive enough to detect vibrational modes within thin organic monolayers even at sub-monolayer coverages. The RAIRS spectra show clear evidence for reversible capture and release of guest molecules. However, the conditions required for adsorption and desorption are substantially different from those reported for cyclophanes in solution. On adsorption, the most dramatic spectral changes are associated with interactions involving the TTF-thread (Figure 2). New intense peaks appear at 1260 and 800 cm-1 corresponding to vibrational modes associated with the five-membered rings of TTF. The observed ring breathing mode intensities of the host (1640 cm-1) and TTF thread (1260 cm-1) are consistent with a cyclophane occupancy of over 80%. (This calculation requires the assumption that the rings have the same orientation relative to the surface, a situation that is to be expected in a system of π-stacked aromatics.) In a 1 mM solution of normal TTF (no thread), similar peaks appear, but with intensities that are 20-25% of those seen with TTFthread indicating partial cyclophane occupancy (expected on the basis of the lower association constant reported in solution). Spectral changes suggestive of adsorption are also seen on exposing the surface-bound cyclophanes to the naphthalenethread DNP (not shown). On the basis of solution association constants, DNP adsorption should be comparable to that seen for TTF. However, the extent of DNP adsorption was difficult to make quantitative. RAIRS spectra are sensitive to molecular orientation relative to the substrate. Most observed spectral intensity changes could be rationalized on the basis of changes (20) Bradshaw, A. M.; Schweizer, E. AdV. Spectrosc. 1988, 16, 413-483.
Figure 2. RAIRS spectra for CBPQT4+ adsorbed to a -COOHterminated SAM, (a) in the native state, (b) after exposure to a 1 mM solution of the TTF thread (arrows indicate new bands associated with threading), and (c) after dethreading of the TTF thread with an applied voltage of +1 V (vs Ag/Ag+ reference electrode).
in the orientation of the cyclophane (from slightly tilted to planar) rather than the appearance of new vibrational modes. (The sixmembered rings in naphthalene derivatives have IR bands that are difficult to discriminate from those in the tethered cyclophanes.) We believe that the change in cyclophane tilt angle is induced by the DNP guest. (In the absence of the guest, adjacent cyclophanes within the monolayer can partially satisfy their desire for π-stacking by tilting to maximize interactions with adjacent cyclophanes.) Contact angle measurements (see below) confirm that significant levels of DNP are captured by the cyclophane host film. No spectral changes were seen in the case of naphthalene (no thread), as expected on the basis of the low association constant reported in solution. In solution, desorption from the cyclophane can be induced via either electrochemical reduction of the host or (for TTF derivatives) oxidation of the guest. However, the RAIRS spectra show that desorption from tethered cyclophanes is substantially different than that reported in solution. First, the electrochemical reduction of the tethered cyclophane does not appear to promote desorption. Even when the cyclophane is completely reduced to the neutral state (at voltages exceeding -1.3 V), changes in RAIRS spectra associated with adsorption of TTF, TTF-thread, and DNP are not reversed. Reversible desorption can be achieved by oxidizing TTF guests (Figure 2c). However, the voltage required to induce switching (+ 1.0 V) is even higher than the + 0.4 V required to oxidize TTF to the +2 state. It appears that, in addition to the electrostatic repulsion between the dicationic form of the TTF-thread and the tetracationic cyclophane, added repulsions associated with an applied voltage are required for detachment. At +1.0 V, RAIRS peak intensities suggest that at least 90% of the TTF-thread is released. The adsorption and desorption process has been observed for four cycles without apparent loss of the surface-bound cyclophane or surface reversibility.
Switching of Surface Chemistry Now that reversible electrochemical switching of supramolecular machines has been demonstrated, it is important to consider how such switching might be used to program the chemistry of a surface. The capture and release of guest molecules can
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potentially be used to change nonspecific surface interactions such as wetting, surface charge, or hydration forces or to program surfaces to selectively capture and release specific species such as a single protein from a complex mixture (e.g., by functionalizing TTF with an antibody). As one example, it has been reported that active transport of droplets in microchannels can be driven by a contact angle differential of less than 10°.21 Although the guests investigated in this study were not selected to promote specific changes in surface chemistry, contact angle measurements performed on gold-bound cyclophanes using both Neumann’s modification of the Wilhemy plate method22 and sessile drop instrumentation indicate that substantial changes in interfacial interactions can be achieved using supramolecular machines. The advancing (50°) and receding (16°) contact angles for both the -COOH-bound CBPQT4+ and the tethered 14+ are most dramatically increased by incorporation of the DNP-thread (advancing contact angle ) 100°; receding contact angle ) 54°), presumably due to screening of the N+ sites in the cyclophane. If reversible, such changes would be sufficient to achieve fluid transport. The reversible TTF-thread guest exhibits more modest contact angle changes (advancing ) 67°; receding ) 27°). However, the DNP-thread results suggest that, with appropriate functionalization of the TTF, programmable wetting-driven transport of fluids in microchannels should be possible.
Conclusions We have demonstrated that supramolecular pseudorotaxanes can be bound to gold surfaces and that electrochemical actuation can be made to release captured electron-rich aromatic guests from surface-bound, electron-poor CBPQT4+ hosts. When used in conjunction with microelectrodes or conductive scanning probe tips, electrochemical actuation of SAMS containing these supramolecular machines should facilitate the creation of programmable patterns of functional groups that can be reversibly (21) Sammarco, T. S.; Burns, M. A. Aiche J. 1999, 45, 350-366. (22) Adamson, A. Physical Chemistry of Surfaces, 3rd ed.; John Wiley and Sons: New York, 1976.
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switched and controlled with a high degree of lateral resolution. However, the results on the guests studied here indicate that interactions between the guest and surface-bound cyclophanes differ from those reported in solution and must be carefully tuned to achieve reversible switching. If the interactions are too weak (as for TTF and naphthalene), negligible capture occurs, while if they are too strong (as for the DNP-thread), guest capture cannot be reversed. For the host-guest complexes studied, it was found that guest release required an electrostatic repulsion component provided by oxidation of the guest (such as TTF), while the tuning of π-π interactions by reduction of the cyclophane host was ineffective in promoting desorption at the voltages studied. In all cases, guests were considerably more difficult to remove from a surface-bound host than from an equivalent solvent-borne host. While we have no firm explanation for this tendency, the distances from the surface are so small that it could be something as mundane as van der Waals attraction to the gold surface. Attachment of host molecules to surfaces with widely varying Hamaker constants and careful measurement of the voltage required to dethread guest molecules would be an excellent way to prove or refute this proposed explanation. In any case, functionalization of completely reversible systems (e.g., the TTF-thread) to express specific molecules at a surface will have to be done with care to ensure that the incorporation of desired surface properties does not compromise switching. Acknowledgment. Funding was provided by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, United States Department of Energy, and by Sandia’s Laboratory Directed Research and Development program. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. Supporting Information Available: AFM images of the formation of monolayers of 14+. This material is available free of charge via the Internet at http://pubs.acs.org. LA0615793
