REPORTS component of the Bloch electric field, as in E,=2a 28. The simulation uses a thick perfectly Layer 30. M. Koshiba, Y Tsuji, 5. Sasaki, IEEE Microwave Wireless boundary region that overlaps 10 periods of the Components Lett. 11, 152(2001). iv, P. Yeh, Optical Waves in Crystals: Propaga that used in(30) and can 31. Supported in part by NSF's Material tion and Control of Laser Radiation( wiley, New York, contour formed by u is similar to the ray surface e of Naval Research, Multidisciplinary Universit in(29), though it now represents group velocities of tion and whose position depends on time. esearch Initiative program (grant no. N00014-01-1 29. M. Born, E. Wolf, Principles of Optics: Electromagn 27.S. G Johnson, J D Joannopoulos, Opt. Express 8, 173 Light(Cambridge Univ. Press, New York, ed. 7, 1999) 18 October 2002: accepted 5 December 2002 A Reversibly Switching Surface whitin in the range of chemical stability of the Joerg Lahann, Samir Mitragotri, 2 Thanh-Nga Tran SAM(18). In other words, conventional SAMs are too dense to allow conformational Hiroki Kaido, Jagannathan Sundaram,2 Insung S. Choi, ansitions and consequently do not allow for Saskia Hoffer, Gabor A. Somorjai, Robert Langer't witching. To explore SAMs as a model sys- tem for switching, we must establish suffi We report the design of surfaces that exhibit dynamic changes in interfacial cient spatial freedom for each molecule. Once properties, such as wettability, in response to an electrical potential. The change a low-density SAM is created, preferential in wetting behavior was caused by surface-confined, single-layered molecules exposure of either hydrophilic or hydropho- undergoing conformational transitions between a hydrophilic and a moderately bic moieties of the SAM to the surrounding hydrophobic state. Reversible conformational transitions were confirmed at a medium could be exploited for the switching ao molecular level with the use of sum-frequency generation spectroscopy and at a macroscopic level with the use of contact angle measurements. This type of 1 6-Mercapto )hexadecanoic acid (MHA) S surface design enables amplification of molecular-level conformational tran- was chosen as a model molecule because it(1)? itions to macroscopic changes in surface properties without altering the chem- self-assembles on Au(1 11) into a monolayer al identity of the surface. Such reversibly switching surfaces may open pre and (ii) has a hydrophobic chain capped by a viously unknown opportunities in interfacial engineering. hydrophilic carboxylate group, thus poten tially facilitating changes in the overall sur- g Interfacial properties, such as wetting behav- using an active stimulus, such as an electrical face properties. To create a monolayer with ior, are defined by the molecular-level struc- potential, to trigger specific conformational sufficient spacing between the individual ture of the surface(1). Diverse modification transitions(e.g, switching from an all-trans MHA molecules, we used a strategy that g procedures have been used to permanently to a partially gauche oriented conformation; exploits synthesis and self-assembly of a alter wettability(2-4). Control of wettability see Fig. 1). Amplification of conformational MHA derivative with a globular end group, has been recently demonstrated by elegant transitions to macroscopically measurable which results in a SAM that is densely methods including light-induced (5-6) and changes requires synergistic molecular rori- packed with respect to the space-filling end electrochemical surface modifications entations of ordered molecules. In principle, groups but shows low-density packing with p (-10) These systems require chemical reac- this is attainable with a single-molecular lay tions in order to control wettability er, such as a self-assembled monolayer quent cleavage of the space-filling end g We demonstrate an alternative (SAM) of alkanethiolates on gold(15). How- groups establishes a low-density SAM of for dynamically controlling interfa ever, the dense molecular packing in SAMs MHA. The spatial dimensions of the precur- a erties that uses conformational m and the strong interactions between the met sor molecule to be used were adapted to (switching) of surface-confined molecules. ylene groups restrict dynamic molecular mo- match the optimum alkanethiolate density for O Polymers have been shown to undergo con- tions to the outermost atoms(16, 17). All in conformational rearrangements formational reorientations when changed situ evidence so far indicates that applied The equilibrium low-energy conformational from one solvent to another (In) or from one electrical potentials have no effect on long- state of each of the sparsely packed MHA temperature to another (12, 13) because of ohase transitions between a well solvated and a poorly solvated state(14). In contrast, our approach maintains the systems environment unaltered (including solvent, electrolyte con- tent, pH, temperature, and pressure)while Institute of Technology(MIT ) 45 Carleton Street, idge, MA 02139, USA. Department of Chemi- lh Hydrophilic Barbara, CA 93106, USA. Department nistry, University of Califomi group terial Science Division, Lawrence Berkeley Laboratory. *Present address: Depart ydrophoble Gold Electrode ute of Science and Technology, Daejeon 305- ealized representation of the transition between straight(hydrophilic) and bent(hydro- = not shown). The precursor tTo whom correspondence should be addressed. E- MHAE, characterized by a nd group and a thiol head group, was synthesized from maiL: ranger@mit.edu MHA by introducing the(2-chlorop phenylmethyl ester group www.sciencemagorgSciEnceVol29917JanUary2003 371component of the Bloch electric field, as in Ekn
G eknGei(k  G)
r . 25. A. Yariv, P. Yeh, Optical Waves in Crystals: Propaga￾tion and Control of Laser Radiation (Wiley, New York, 1984), chap. 6. 26. The contour formed by u is similar to the ray surface in (29), though it now represents group velocities of different frequencies. 27. S. G. Johnson, J. D. Joannopoulos, Opt. Express 8, 173 (2001). 28. The simulation uses a thick perfectly matched layer boundary region that overlaps 10 periods of the photonic crystal similar to that used in (30) and can absorb the Bloch waves away from a band edge. The moving charge is implemented as a point-like current density that is oriented toward the direction of mo￾tion and whose position depends on time. 29. M. Born, E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light (Cambridge Univ. Press, New York, ed. 7, 1999). 30. M. Koshiba, Y. Tsuji, S. Sasaki, IEEE Microwave Wireless Components Lett. 11, 152(2001). 31. Supported in part by NSF’s Materials Research Sci￾ence and Engineering Center program (grant no. DMR-9400334) and the Department of Defense, Of- fice of Naval Research, Multidisciplinary University Research Initiative program (grant no. N00014-01-1- 0803). 18 October 2002; accepted 5 December 2002 A Reversibly Switching Surface Joerg Lahann,1 Samir Mitragotri,2 Thanh-Nga Tran,1 Hiroki Kaido,1 Jagannathan Sundaram,2 Insung S. Choi,1 * Saskia Hoffer,3 Gabor A. Somorjai,3 Robert Langer1 † We report the design of surfaces that exhibit dynamic changes in interfacial properties, such as wettability, in response to an electrical potential. The change in wetting behavior was caused by surface-confined, single-layered molecules undergoing conformational transitions between a hydrophilic and a moderately hydrophobic state. Reversible conformational transitions were confirmed at a molecular level with the use of sum-frequency generation spectroscopy and at a macroscopic level with the use of contact angle measurements. This type of surface design enables amplification of molecular-level conformational tran￾sitions to macroscopic changes in surface properties without altering the chem￾ical identity of the surface. Such reversibly switching surfaces may open pre￾viously unknown opportunities in interfacial engineering. Interfacial properties, such as wetting behav￾ior, are defined by the molecular-level struc￾ture of the surface (1). Diverse modification procedures have been used to permanently alter wettability (2–4). Control of wettability has been recently demonstrated by elegant methods including light-induced (5–6) and electrochemical surface modifications (7–10). These systems require chemical reac￾tions in order to control wettability. We demonstrate an alternative approach for dynamically controlling interfacial prop￾erties that uses conformational transitions (switching) of surface-confined molecules. Polymers have been shown to undergo con￾formational reorientations when changed from one solvent to another (11) or from one temperature to another (12, 13) because of phase transitions between a well solvated and a poorly solvated state (14). In contrast, our approach maintains the system’s environment unaltered (including solvent, electrolyte con￾tent, pH, temperature, and pressure) while using an active stimulus, such as an electrical potential, to trigger specific conformational transitions (e.g., switching from an all-trans to a partially gauche oriented conformation; see Fig. 1). Amplification of conformational transitions to macroscopically measurable changes requires synergistic molecular reori￾entations of ordered molecules. In principle, this is attainable with a single-molecular lay￾er, such as a self-assembled monolayer (SAM) of alkanethiolates on gold (15). How￾ever, the dense molecular packing in SAMs and the strong interactions between the meth￾ylene groups restrict dynamic molecular mo￾tions to the outermost atoms (16, 17). All in situ evidence so far indicates that applied electrical potentials have no effect on long￾chain alkanethiolate monolayers on gold within the range of chemical stability of the SAM (18). In other words, conventional SAMs are too dense to allow conformational transitions and consequently do not allow for switching. To explore SAMs as a model sys￾tem for switching, we must establish suffi￾cient spatial freedom for each molecule. Once a low-density SAM is created, preferential exposure of either hydrophilic or hydropho￾bic moieties of the SAM to the surrounding medium could be exploited for the switching of macroscopic surface properties. (16-Mercapto)hexadecanoic acid (MHA) was chosen as a model molecule because it (i) self-assembles on Au(111) into a monolayer and (ii) has a hydrophobic chain capped by a hydrophilic carboxylate group, thus poten￾tially facilitating changes in the overall sur￾face properties. To create a monolayer with sufficient spacing between the individual MHA molecules, we used a strategy that exploits synthesis and self-assembly of a MHA derivative with a globular end group, which results in a SAM that is densely packed with respect to the space-filling end groups but shows low-density packing with respect to the hydrophobic chains. Subse￾quent cleavage of the space-filling end groups establishes a low-density SAM of MHA. The spatial dimensions of the precur￾sor molecule to be used were adapted to match the optimum alkanethiolate density for conformational rearrangements. The equilibrium low-energy conformational state of each of the sparsely packed MHA 1 Department of Chemical Engineering, Massachusetts Institute of Technology (MIT ), 45 Carleton Street, Cambridge, MA 02139, USA. 2 Department of Chemi￾cal Engineering, University of California at Santa Bar￾bara, Santa Barbara, CA 93106, USA. 3 Department of Chemistry, University of California at Berkeley, Ma￾terial Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA. *Present address: Department of Chemistry and School of Molecular Science (BK21), Korean Advanced Institute of Science and Technology, Daejeon 305- 701, Korea. †To whom correspondence should be addressed. E￾mail: rlanger@mit.edu Fig. 1. Idealized representation of the transition between straight (hydrophilic) and bent (hydro￾phobic) molecular conformations (ions and solvent molecules are not shown). The precursor molecule MHAE, characterized by a bulky end group and a thiol head group, was synthesized from MHA by introducing the (2-chlorophenyl)diphenylmethyl ester group. R EPORTS www.sciencemag.org SCIENCE VOL 299 17 JANUARY 2003 371 on June 8, 2007 www.sciencemag.org Downloaded from