Patterned Polymeric Multilayered Assemblies through Hydrogen

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Patterned Polymeric Multilayered Assemblies through Hydrogen Bonding and Metal Coordination Victor Piñoń III†,‡ and Marcus Weck*,† †

Molecular Design Institute and Department of Chemistry, New York University, 100 Washington Square East, New York, New York 10003, United States ‡ School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Dr., Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Patterned polymeric multilayered assemblies were formed using a combination of metal coordination and hydrogen bonding interactions. We proved that the hydrogen bonding interaction between diamidopyridine and thymine can be employed for polymeric multilayer assemblies. We then combined this strategy along with a second supramolecular interaction, metal coordination. These interactions proved to be orthogonal to one another on the surface, making each discrete region individually responsive to external stimuli.

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constant between thymine and DAP is on the order of 103 M−1 in halogenated solvents.14,15 Previous work by our group has shown that the metal coordination between Pd−pincer complexes and poly(vinylpyridine) (PVP) can be used readily for the assembly of responsive and robust multilayers.16−18 Furthermore, others have demonstrated that multilayered structures can also be assembled using other metal−ligand systems.19−21 We have also employed the metal coordination between Pt−pincer complexes and pyridine to form supramolecular polymers.22−25 Finally, we have demonstrated in solution that Pd−pincer-based metal coordination and the hydrogen bonding between THY and DAP are orthogonal to each other.26 We rationalize that the orthogonality of these two noncovalent interactions should hold true on surfaces providing us with a powerful tool by which external compounds can be introduced reliably to site-specific locations on surfaces. This strategy has the potential to allow for multiply responsive systems,14,27−29 often envisioned in triggered release devices or self-healing materials.6 Herein, we demonstrate the orthogonality of these two interactions on gold surfaces through the use of site-specific assembly for the formation of patterned monolayers using microcontact printing. Microcontact printing (μCP) is a technique commonly used to pattern surfaces with feature sizes ranging from the macroscale down to the nanometer scale.30,31 This technique uses an elastomeric stamp that, after being inked by a solution of a compound that can react with the surface, can transfer this

INTRODUCTION The ability to accurately replicate complex architectures in an efficient and simple manner is an important goal in a diverse set of fields ranging from optoelectronics1,2 to biomaterials3 and sensor technologies.4 By moving toward complex threedimensional designs, implications for use of these materials expand to applications in drug delivery devices5 and self-healing materials.6,7 By incorporating supramolecular interactions into the design of these materials, an added handle to tune the properties of the materials is obtained. Through the introduction of orthogonal supramolecular/noncovalent interactions, one can imagine using one type of interaction for the assembly of the architectures and a second interaction to hold a moiety that is to be released upon a trigger or to introduce a tunable functionality. Furthermore, supramolecular interactions allow for the disassembly of the entire architecture, since these interactions are reversible by nature.8 For hydrogen-bonding systems, the stimuli to induce a response or to reverse the interaction range from temperature to solvent and pH,9−12 while for metal−ligand complexes competing ligands serve this purpose.9,13 In this contribution, we report the realization of an orthogonal supramolecular functionalization strategy for surface patterning that allows for the stepwise multipatterning of surfaces which we view as a first step toward the fabrication of complex 3D architectures on surfaces. The supramolecular interactions used in this contribution are metal coordination between a palladium−pincer complex and pyridine and the hydrogen bonding between thymine (THY) and diamidopyridine (DAP) (Figure 1). It has been shown that the coordination between Pd−pincer complexes and pyridine is higher than 109 M−1 in chloroform while the association © 2012 American Chemical Society

Received: November 3, 2011 Revised: December 31, 2011 Published: January 8, 2012 3279

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Figure 1. Components utilized for multilayer assembly. Metal coordination interaction of Pd−bispincer and PVP and hydrogen bonding interactions of poly(DAP) and poly(THY). followed by a hexanes rinse and drying under vacuum to afford 0.23 g (0.70 mmol, 70% yield) of a white solid. 1H NMR (400 MHz; CDCl3): δ 7.90 (d, J = 8.1 Hz, 2H, Ar H), 7.71 (t, J = 8.1 Hz, 1H, Ar H), 7.52 (s, 2H, −NH), 5.81 (m, 1H, −CHCH2), 4.97 (m, 2H, −CHCH2), 2.4 (m, 4H, −CH2CO, 2.04 (m, 2H, −CH2), 1.7−1.3 (br, 13H, −CH2 and −CH3). 13C NMR (400 MHz: CDCl3) δ 172.2, 171.6, 149.6, 141.0, 139.3, 114.3, 109.5, 109.5, 38.0, 33.9, 31.0, 29.4, 29.3, 29.2, 29.0, 25.5, 9.5. HRMS (m/z): [M + H]+ calcd for C19H29N3O2, 332.23; found 332.23. Anal. calcd for C19H29N3O2: C, 68.85; H, 8.82; N, 12.68. Found: C, 68.15; H, 8.69; N, 12.33. Synthesis of S-(11-Oxo-11-((6-propionamidopyridin-2-yl)amino)undecyl)ethanethioate (2). In a round-bottom flask containing toluene (30 mL), 500 mg of 1 (1.6 mmol), thiolacetic acid (0.25 g, 3.2 mmol), and AIBN were combined, and Ar was bubbled through the reaction. The reactants were stirred for 2 h under an argon atmosphere. The solution was then washed with water and NaHCO3. After drying the organic phase with MgSO4, the solvent was removed in vacuo. The product was further purified via precipitation into hexanes to afford 460 mg (1.1 mmol, 71% yield) of a white solid. 1 H NMR (400 MHz; CDCl3): δ 7.90 (d, J = 8.0 Hz, 2H, Ar H), 7.69 (t, J = 8.1 Hz, 1H, Ar H), 7.53 (m, 2H, −NH), 2.86 (t, J = 7.3 Hz, 2H, −CH2CH2S−), 2.44−2.35 (m, 4H, −CH2CO), 2.32 (d, J = 0.7 Hz, 3H, CH3CO), 1.71 (t, J = 7.3 Hz, 2H, −CH2), 1.60−1.54 (m, 2H, −CH2), 1.36−1.23 (br, 15H, −CH2). 13C NMR (400 MHz: CDCl3) δ 196.1, 172.0, 171.5, 149.4, 140.9, 109.3, 109.3, 37.9, 30.9. 30.7. 29.5,29.3, 29.3, 29.3, 29.2, 29.0, 28.8, 25.3, 9.4. HRMS (m/z): [M + H]+ calcd for C21H33N3O3S, 408.22; found 408.22. Anal. calcd for C21H33N3O3S: C, 61.88; H, 8.16; N, 10.31. Found: C, 61.71; H, 8.17; N, 10.30. Synthesis of 11-Mercapto-N-(6-propionamidopyridin-2-yl)undecanamide (3). A solution of 2 (105 mg, 0.26 mmol) in 1 mL of MeOH was added to a freshly made solution of sodium methoxide (0.15 g of Na in 15 mL of MeOH). The solution was degassed for 90 min and allowed to stir overnight. The reaction was then quenched slowly with NH4Cl. Water was added, and the solution was extracted into CHCl3. The organic phase was washed with water and then dried with MgSO4. Removal of the solvent in vacuo afforded 63 mg (0.17 mmol, 65% yield) of a white solid. 1H NMR (400 MHz; CDCl3): δ 7.81 (d, J = 8.1 Hz, 2H, Ar H), 7.62 (m, 1H, Ar H), 7.4 (m, 2H, −NH), 2.59 (t, J = 7.35 Hz, 2H, SH), 2.43 (q, J = 7.30 Hz, 2H, −CH2SH), 2.31 (m, 4H, −CH2CO), 1.62 (br, 4H, −CH2), 1.50 (m, 2H, −CH2), 1.20 (br, 15H, CH2 and CH3). 13C NMR (400 MHz; CDCl3): δ 172.1. 171.6, 171.6, 149.6, 141.0, 141.0, 109.5, 39.4, 38.0, 34.2, 31.0, 29.6, 29.3, 29.2, 28.6, 28.5. 25.5, 24.8, 9.5. HRMS (m/z): [M + H]+ calcd for C19H31N3O2S, 366.21; found 366.22. Anal. calcd for C19H31N3O2S: C, 62.43; H, 8.55; N, 11.50. Found: C, 62.17; H, 8.59; N, 11.11. Preparation of PDMS Stamps. Sylgard 184 elastomer and curing agent (Dow Corning Corp.; purchased from Ellsworth Adhesives) were mixed in a 10:1 ratio and poured over a silicon master prepared using SU-8 resist (MicroChem) exposed through custom photomasks (printed by CAD/Art Services, Inc.). The PDMS was cured in a

ink to a surface through the portions of the stamp that come into contact with the surface, thus replicating the pattern designed on the stamp.32 In the study presented herein, we patterned a gold surface with a thiol-containing diamidopyridine (DAP-SH) via PDMS stamp-based microcontact printing followed by backfilling the negative pattern with 4-mercaptopyridine (MP). Multilayered assemblies can then be created using a layer-by-layer33 methodology. Finally, both hydrogen bonding and metal coordination were combined into a single, multilayered assembly in which the discrete domains of each functionality were retained. Each domain was proven to be individually addressable by external stimuli.

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EXPERIMENTAL SECTION

Reagents and Materials. All reagents were purchased from Sigma-Aldrich, Alfa Aesar, and Acros and used without further purification unless otherwise noted. Poly(4-vinylpyridine) (PVP), Mw = 160 000 and Tg = 142.0 °C, was purchased from Sigma-Aldrich and used without further purification. NMR spectra were recorded on a Bruker AV-400 (1H: 400.1 MHz; 13C: 100.6 MHz) spectrometer. Chemical shifts are reported in ppm and referenced to the corresponding residual nuclei in deuterated solvents. High-resolution mass spectrometry was performed using an Agilent 6224 TOF LC/ MS. Gel-permeation chromatography (GPC) analyses were carried out using a Shimadzu pump coupled to a Shimadzu UV detector with tetrahydrofuran (THF) as the eluant and a flow rate of 1 mL/min on an American Polymer Standards column set (100, 1000, 100 000 Å, linear mixed bed). All GPCs were calibrated using poly(styrene) standards and carried out at 25 °C. Mw, Mn, and PDI represent the weight-average molecular weight, number-average molecular weight, and polydispersity index, respectively. AFM measurements were carried out using an Asylum Research MFP-3D or a Veeco Dimension 3100 using Olympus AC240TS cantilevers operating in tapping mode under ambient conditions. Step height analyses were performed employing Gwyddion software, using averaged line analysis and at least 100 lines for each image. Step height analyses were performed on at least three separate areas of each substrate. Gold-coated slides (1 in. × 3 in. × 0.40 in.; 50 Å Cr, 1000 Å Au) were purchased from Evaporated Metal Films (EMF) (Ithaca, NY) and were cleaned using a Harrick plasma cleaner prior to use. Poly(DAP) (20-mer), poly(THY) (20mer), and the palladium bispincer were synthesized according to literature procedures.26,34,35 Synthesis of N-(6-Propionamidopyridin-2-yl)undec-10-enamide (1). To a round-bottom flask containing N-(6-aminopyridin2-yl)propionamide (165 mg, 1.00 mmol) in dichloromethane (20 mL), undec-10-enoyl chloride (202 mg, 1.00 mmol) was added along with triethylamine (0.14 mL, 1.00 mmol), and the reactants were stirred overnight. The reaction mixture was then washed using NaHCO3 followed by water and dried using MgSO4. After removal of the solvent in vacuo, a yellow solid was obtained which was further purified by column chromatography (3:2 hexanes:ethyl acetate) 3280

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Scheme 1. Synthesis of DAP-SH 3

Figure 2. AFM micrographs of (a) patterned monolayer of DAP-SH/dodecylthiol and (b) three bilayers of poly(DAP)/poly(THY).

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vacuum oven at (120 °C, 15 in. Hg) for 2 h. The PDMS stamps were then removed from the silicon master, cut to the desired size, and stored using Scotch tape covering the patterned side until ready for use. Monolayer Formation. The PDMS stamps were “inked” using a 10 mM solution of DAP-SH in absolute EtOH for 5 min before being dried under a stream of N2. The “inked” stamps were then placed into contact with the plasma cleaned Au slides for 5 min under a 50 g weight to apply pressure and ensure sufficient contact of the stamp to the surface. The stamps were then carefully removed, and the Au substrates were rinsed with copious amounts of absolute EtOH and dried under a stream of N2. The patterned substrates were then placed in a solution of either dodecylthiol or 4-mercaptopyridine (10 mmol) in absolute EtOH overnight, removed, rinsed copiously, and dried under a stream of N2. Multilayer Buildup. The multilayered assembly of the hydrogenbonding poly(THY)/poly(DAP) system were conducted in solutions of each polymer in 3% isopropyl alcohol in CHCl3. The deposition solvent for PVP was CHCl3 and DMF for the Pd−bispincer complex. Deposition concentrations were varied, as shown in Figure 3, and rinsings were performed using the deposition solvent. Multilayers were assembled using the layer-by-layer technique in which the substrate was submerged in alternating baths of complementary compounds while rinsing between each deposition step. In the case of the metal coordination assembly, the substrates were submerged for 5 min to allow for complete coordination, whereas in the case of hydrogen bonding, 30 min was required at the highest concentration studied and increased up to 90 min for the lowest concentration.

RESULTS AND DISCUSSION The key building block for the hydrogen bonding based functionalization of surfaces is the thiol-containing diaminopyridine 3 (DAP-SH). The synthesis of 3 was carried out as described in Scheme 1. N-(6-Aminopyridin-2-yl)propionamide was coupled to undec-10-enoyl chloride in the presence of a base to afford 1. A radical reaction with thiolacetic acid afforded the protected thiol, 2, which could then be deprotected quantitatively using sodium methoxide. Functionalization of gold surfaces using 3 was carried out in a cleanroom. Thiol 3 was dissolved in absolute EtOH and subsequently used as “ink” on PDMS stamps containing a pattern of 20 μm stripes. After 5 min, a stream of N2 was used to dry any residual solvent on the stamp. The inked stamps were then used to transfer 3 to plasma-cleaned gold slides using a 50 g weight to apply pressure and to provide uniform monolayers. Using AFM, we investigated the kinetics of the monolayer formation by examining layer thickness and surface roughness to quantify the monolayer coverage upon the surface. A well-defined smooth patterned surface was desired. It was determined that a contact time of 5 min produced dense monolayers of 3. Longer contact times did not improve the quality of the monolayers; however, shorter contact times investigated (30 s, 1 min, 2 min, and 3 min) produced incomplete monolayer formation, as well-defined edges were 3281

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not evident throughout the areas stamped. Immediately after stamping, the substrates were rinsed copiously with ethanol to remove any physisorbed species. The patterned substrates were then placed in a 10 mmol solution of either MP or dodecylthiol in ethanol overnight to ensure complete coverage of the negative pattern, affording well-defined mixed monolayers. We investigated first whether the hydrogen bonding between diamidopyridine and thymine is sufficiently strong enough to allow for the formation of polymer-based multilayer structures on patterned surfaces. For this study, we immersed gold substrates expressing a patterned monolayer (20 μm stripes) of DAP-SH and dodecylthiol in a 0.5 mmol solution of poly(THY) in dioxane/CHCl3 (15/85). We have shown previously that this solvent mixture is an appropriate cosolvent system for the DAP/THY assembly in solution. Using a solution of 3% isopropyl alcohol (IPA) in CHCl3, we were able to achieve polymer solubility and the multilayered assemblies were conducted under these conditions. Studies were performed on the hydrogen bonding moieties to study the effect of deposition concentration on the thicknesses of the layers deposited. Solutions of poly(THY) and poly(DAP) were prepared at concentrations of 0.5, 0.25, and 0.1 mmol in 3% isopropyl alcohol in chloroform. Patterned substrates were placed in a solution of poly(THY) or poly(DAP), depending on the nature of the exposed group on the gold surface, at the desired concentration. Every 30 min, a sample was removed from the solution, rinsed, and examined using AFM to determine layer thickness. The time after which no increase in layer thickness was discernible was used as the deposition time for each concentration. Each kinetic study was carried out in triplicate. The highest deposition concentration required the least amount of exposure time (30 min); as the concentration decreased, more time was required to reach full monolayer coverage: 60 min for 0.25 mmol and 90 min for 0.1 mmol. The layer thickness as a function of deposition solution concentration is shown in Figure 3. Concentration plays a key role in the thicknesses of the layers deposited which is in agreement with other studies in the literature.18,36 By plotting the change in height (from the mixed monolayer) versus the bilayer number, the slope of the line fitting the data allows for the determination of the average value for the thickness of each deposition step. Bilayer thicknesses depending on solution concentrations were found to be 3.69 nm (0.5 mmol), 2.30 nm (0.25 mmol), and 0.60 nm (0.1 mmol). These findings parallel the concentration effects on multilayer buildup of systems using other types of noncovalent interactions such as metal coordination.18 Upon exposure of our multilayers to a DMF rinse, the hydrogen-bonding interactions holding the multilayers together were disrupted within 20 s, and only the patterned monolayer remained (see Supporting Information). This gives us an easy handle on responsive and controllable hydrogen bonding based multilayer assembly in a layer-by-layer fashion. We also followed the hydrogen bonding multilayer formation using IR and UV−vis spectroscopies (Supporting Information). As described previously for our metal coordination multilayers,17,18 we were able to show the stepwise increase in bilayer formation using UV−vis spectroscopy. While the combination of IR and UV−vis spectroscopies as well as our patternformation/AFM studies cannot prove the formation of multilayers through hydrogen bonding, the total absence of any multilayer formation without the deposition of 3 and the

Figure 3. Thickness of layer vs bilayer number at concentrations of 0.5 mmol (red), 0.25 mmol (blue), and 0.1 mmol (green) of poly(THY)/ poly(DAP).

complete removal of patterns via a simple DMF rinse that is know to disrupt hydrogen bonds strongly suggests that the polymer multilayers are formed via hydrogen bonding. Next, we investigated whether we can combine hydrogen bonding with metal coordination on a single patterned surface to create a dually responsive system. Previously, we have shown that the Pd−bispincer/PVP interaction can be used to assemble multilayered structures of controllable thicknesses.17,18 Because of the solubility of the Pd−bispincer complex in DMF, which weakens significantly the hydrogen-bonding interaction between thymine and diaminopyridine, we carried out the metal coordination first in DMF followed by hydrogen bonding under the conditions described above. After preparing mixed monolayers of 4-MP and DAP-SH on gold substrates (Figure 4a), we immersed them for 5 min iteratively into a 1 mmol solution of the bispincer in DMF and then into a 0.05 mmol solution of PVP in chloroform, rinsing copiously between depositions. AFM was used to monitor the multilayer growth after each assembly step, using step height analysis to ensure full assembly was achieved. For the concentrations used, each bilayer was roughly 3.6 nm thick. After the formation of three metal coordination based bilayers, the thickness obtained was 10.83 ±1.0 nm (Figure 4b). We then started to fill in the negative pattern using hydrogen bonding. The substrate containing the metal coordination patterned multilayered structure was immersed in a 0.5 mmol solution of poly(THY) (3% IPA in CHCl3) for 30 min. After rinsing and drying under a stream of N2, the poly(THY)containing substrates were characterized by AFM, measuring a layer thickness of 3.43 nm before proceeding to the next deposition step which included immersing the substrates into a poly(DAP) solution for 30 min, increasing the hydrogen bonding-based multilayer thickness an additional 3.13 nm. This process was repeated until three bilayers had been assembled. Upon completion of this assembly, the thickness of the hydrogen bonding based multilayers was found to be 13.49 ± 3282

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Figure 4. AFM micrographs and average line scans of (a) patterned monolayer of DAP-SH and 4-mercaptopyridine (20 μm stripes), (b) 3 bilayers of bispincer/PVP, (c) 3 bilayers of bispincer/PVP alongside 3 bilayers of p(THY)/p(DAP), and (d) after exposure to PPh3 for 2 min followed by a DMF rinse for 30 s. Averaged line traces for the entire scans are included below each micrograph.

resulting substrate using AFM. The AFM characterization confirmed the complete removal of the bispincer/PVP multilayers, giving a step height of 13.36 nm, which is very close to the calculated thickness of 13.49 nm using step height analysis of the multilayer buildup, supporting the claim that the hydrogen bonding based multilayers were left untouched. Next, we reversed the removal strategy by subjecting a second substrate to a DMF wash. Complete removal of the poly(THY)/poly(DAP) was achieved within minutes as confirmed by AFM. In this case, the metal coordination based mutilayers remained with a thickness of 10.66 nm that is

1.7 nm (Figure 4c), which is in agreement with the average thickness for three bilayers as shown in Figure 3. One of the main advantages of the use of supramolecular chemistry for materials fabrication is the reversibility of the noncovalent interactions.9,37,38 Therefore, after demonstrating that mixed patterned surfaces using two orthogonal noncovalent interactions can be fabricated, we investigated whether selective patterns of discrete areas can be removed. For these studies, we subjected a hydrogen bonding/metal coordination containing multilayer substrate to a solution of PPh3 in CHCl3 (5 mmol) for 5 min followed by the characterization of the 3283

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(17) South, C. R.; Pinon, V. III; Weck, M. Angew. Chem., Int. Ed. 2008, 47, 1425−1428. (18) South, C. R.; Weck, M. Langmuir 2008, 24, 7506−7511. (19) Altman, M.; Shukla, A. D.; Zubkov, T.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Am. Chem. Soc. 2006, 128 (22), 7374−7382. (20) Tuccitto, N.; Ferri, V.; Cavazzini, M.; Quici, S.; Zhavnerko, G.; Licciardello, A.; Rampi, M. A. Nature Mater. 2009, 8, 41−46. (21) Veinot, J. G. C.; Yan, H.; Smith, S. M.; Cui, J.; Huang, Q.; Marks, T. J. Nano Lett. 2002, 2, 333−335. (22) Ambade, A. V.; Yang, S. K.; Weck, M. Angew. Chem., Int. Ed. 2009, 48, 2894−2898. (23) Yang, S. K.; Ambade, A. V.; Weck, M. Chem.Eur. J. 2009, 15, 6605−6611. (24) Gerhardt, W. W.; Zucchero, A. J.; South, C. R.; Bunz, U. H. F.; Weck, M. Chem.Eur. J. 2007, 13, 4467−4474. (25) Gerhardt, W. W.; Zucchero, A. J.; Wilson, J. N.; South, C. R.; Bunz, U. H. F.; Weck, M. Chem. Commun. 2006, 2141−2143. (26) Nair, K. P.; Pollino, J. M.; Weck, M. Macromolecules 2006, 39, 931−940. (27) Guo, W.; Xia, H.; Cao, L.; Xia, F.; Wang, S.; Zhang, G.; Song, Y.; Wang, Y.; Jiang, L.; Zhu, D. Adv. Funct. Mater. 2010, 20, 3561− 3567. (28) Wang, R.; Jiang, X.; Di, C.; Yin, J. Macromolecules 2010, 43, 10628−10635. (29) Xing, Z.; Wang, C.; Yan, J.; Zhang, L.; Li, L.; Zha, L. Colloid Polym. Sci. 2010, 288, 1723−1729. (30) Yan, L.; Huck, W. T. S.; Whitesides, G. M. Polym. Rev. 2004, 44, 175−206. (31) Koide, Y.; Wang, Q.; Cui, J.; Benson, D. D.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 11266−11267. (32) Qin, D.; Xia, Y.; Whitesides, G. M. Nature Protoc. 2010, 5, 491− 502. (33) Decher, G. Science 1997, 277, 1232−1237. (34) Burd, C.; Weck, M. Macromolecules 2005, 38, 7225−7230. (35) van Manen, H.-J.; Nakashima, K.; Shinkai, S.; Kooijman, H.; Spek, A. L.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Eur. J. Inorg. Chem. 2000, 2533−2540. (36) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153− 8160. (37) Rowan, S. J.; Beck, J. B. Faraday Discuss. 2005, 128, 43−53. (38) De Greef, T. F. A.; Smulders, M. M. J.; Wolffs, M.; Schenning, A. P. H. J.; Sijbesma, R. P.; Meijer, E. W. Chem. Rev. 2009, 109, 5687− 5754.

within the error range of our initial metal-coordination-based multilayer assembly of 10.83 nm. We can also remove the hydrogen bonding as well as the metal coordination pattern at the same time by exposing the substrates to a solution of PPh3 followed by a rinse with DMF (sequence independent). AFM images of this experiment can be seen in Figure 4. In summary, in this contribution, we have taken interactions found to be orthogonal to one another in solution and transferred them to surfaces. We have demonstrated that these interactions are strong enough to drive site-specific multilayer assembly with high fidelity. Each interaction type is individually addressable, giving control of assembly in three dimensions. This proof-of-concept work has many implications ranging from smart materials and drug delivery vehicles to replicative devices. Furthermore, this strategy can be expanded by varying the polymer backbone, functionality and density within.

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra for all new compounds synthesized; additional evidence of multilayer removal presented by AFM. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-0911460) for financial support of this research. REFERENCES

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