1 Field-Directed Self-Assembly with Locking Nanoparticles2 Mikhail Motornov,† Sergiy Z. Malynych,‡ Deepthi S. Pippalla,† Bogdan Zdyrko,‡ Halyna Royter,†

3 Yuri Roiter,† Mathew Kahabka,† Alexander Tokarev,‡ Ihor Tokarev,† Ekaterina Zhulina,§

4 Konstantin G. Kornev,‡ Igor Luzinov,*,‡ and Sergiy Minko*,†

5†Department of Chemistry and Biomolecular Science, Clarkson University, Potsdam, New York 13699, United States

6‡School of Materials Science and Engineering, Clemson University, Clemson, South Carolina 29634, United States

7§Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russia

8 *S Supporting Information

9 ABSTRACT: A reversible locking mechanism is established10 for the generation of anisotropic nanostructures by a magnetic11 field pulse in liquid matrices via the balancing thermal energy,12 short-range attractive and long-range repulsive forces, and13 dipole−dipole interactions using a specially tailored polymer14 shell of nanoparticles. The locking mechanism is used to15 precisely regulate the dimensions of self-assembled magnetic16 nanoparticle chains and to generate and disintegrate three-17 dimensional (3D) nanostructured materials in solvents and18 polymers.

19 KEYWORDS: Nanoparticles, nanostructures, self-assembly

20 Self-assembly is an efficient and scalable method for the21 fabrication of complex nanostructures from small building22 blocks such as nanoparticles.1 This approach has evolved23 toward the directed self-assembly of specially designed building24 blocks2 to generate functional materials by using selective25 molecular interactions between particulates,3−7 constraints at26 interfaces,8,9 template methods,10,11 and external fields.12 These27 materials possess unique plasmonic,13 photonic,14 magnetic,15

28 spin memory,16 and electroconductive17 properties, to name a29 few. Here, we introduce a locking mechanism to control the30 generation of anisotropic nanostructures in liquid matrices by31 balancing Brownian motion, and short-range attractive and32 long-range repulsive forces, due to the specially tailored33 polymer brush shell of nanoparticles, as well as the forces34 acting on the particles in an external magnetic field. A reversible35 locking mechanism is used to precisely regulate the dimensions36 of self-assembled magnetic nanoparticle chains and to generate37 3D nanostructured materials in solvents and polymers.38 The field-directed assembly of colloids in suspensions18

39 implies the use of field-induced polarization and strong dipole−40 dipole interactions to guide the particle assembly into the

41 desired configurations.19,20 In well-known examples of

42 electro-21 and magnetorheological22 fluids, the field-induced

43 particle interactions overcome Brownian motion and the

44 particles form dipolar chains that coalesce with time. Particle

45 functionalization and the use of different shapes of particles in

46 combination with external fields is a promising methodology47 for the fabrication of complex anisotropic nanostructures.2

48Field-directed particle assembly has several limitations related49to the generation of complex nanostructures. First, the50structures change dynamically as long as an external field is51on. Hence, simultaneous control over the length and diameter52of 1D chains assembled from nanoparticles is a challenging53problem. Second, the structures are generated in accordance54with the field direction (along the field or in the orthogonal55direction). A change of the field direction will result in a56realignment of the structures. Third, the structures may be57destroyed if the field is removed. Traditionally, particles are58stabilized electrostatically and sterically using surfactants and59polymers to yield stable colloidal suspensions. Thus, when the60external field is turned off, the assembled structures are61disintegrated by thermal motion (when the particles are not62glued together).63These constraints lead to the applications of the field-64directed assembly, which are limited to the cases of switchable65and reconfigurable assemblies, when turning an external field66on and off is used to temporarily alter the structure and67rheology of the materials and fluids,22 or for the synthesis of682D- and 3D-colloidal crystals (which requires prolonged69exposure to permanent or oscillating fields).23 More complex70anisotropic structures, indeed, can be attained by using field-71directed self-assembly of nonspherical particles24 and at

Received: May 10, 2012Revised: June 13, 2012

Letter

pubs.acs.org/NanoLett

© XXXX American Chemical Society A dx.doi.org/10.1021/nl301780x | Nano Lett. XXXX, XXX, XXX−XXX

amc00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.1.5.i2 HF01:3738 | 2.0 alpha 39) 2012/05/23 16:28:00 | PROD-JCAVA | rq_1583663 | 6/21/2012 14:04:01 | 7

72 interfaces25 when the spatial constraints change the mechanism73 of particle assembly. There are also methods reported for the74 scalable field-directed formation of stable (after removal of a75 magnetic field) chains of magnetic nanoparticles based on the76 permanent locking of the structures via covalent and non-77 covalent bonding.26 However, these methods have limitations78 for the fabrication of bulk materials and scaling up. There are as79 well limitations in control over the nanowire length and80 thickness, energy of interactions, 3D alignment, and reversi-81 bility of self-assembly.82 Here, we introduce a method for field-directed self-assembly83 using specially designed spherical core−shell magnetic particles84 with locking properties of the polymeric shell. Specifically, we85 synthesized responsive nanoparticles that are capable of86 reorganization in an external magnetic field. The magnetic87 field turns on interactions between the particles. The88 interaction remains unchanged even after the removal of the89 external magnetic field due to the specially tailored polymer90 shell of the nanoparticles. This mechanism is termed here the91 “locking mechanism.” The “locking particles” can be unlocked92 by applying external stimuli, such as changes in temperature or93 in pH. We also demonstrated that a mixture of two populations94 of nanoparticles with different surface functionalizations that95 provide control over intermolecular forces can be used to96 regulate the length of the self-assembled 1D chains. The chains97 can be used as the second generation building blocks for the98 field-assisted fabrication of 3D-nanostructured materials.99 In the examples presented here, 30 and 100 nm particles,100 made of γ-iron oxide, were enveloped by a thin silica shell and101 decorated by tethered polymer chains of poly(2-vinylpyridine-102 b-ethylene oxide) (P2VP-b-PEO) block copolymer, which103 provided colloidal stability to the particles and imbued them104 with the mechanism of reversible locking. The spherical105 magnetic nanoparticles encapsulated into a silica shell with a106 mean diameter of 50 and 100 nm (core plus shell) were107 synthesized using two different methods described in refs 27108 and 28, respectively. Although similar results were obtained for109 both samples of the particles, in this article we report the results110 obtained for particles with the 30 nm core. For synthesis of the111 locking particles, the silica shell was functionalized with (3-112 bromopropyl)trimethoxysilane, and then P2VP-b-PEO (Mn =113 3000 g/mol for P2VP and Mn = 9000 g/mol for PEO) was114 grafted via a quaternization reaction in nitromethane at 60 °C.115 The thickness of the grafted polymer shell was 7 ± 1.5 nm (as116 measured using scanning electron microscopy (SEM) and117 atomic force microscopy (AFM)). Details regarding particle118 synthesis, functionalization, and characterization are available in119 Supporting Information.120 The water-soluble nonionic PEO layer constitutes the outer121 shell of the nanoparticle. The PEO shell has a low interfacial122 energy in an aqueous environment. The PEO shell contributes123 a steric repulsive force when interacting with other particles.124 P2VP chains are not soluble in water at pH > 4 due to a125 hydrophobic backbone. P2VP macromolecules, however, can126 be protonated and dissolved in an acidic aqueous solution. In127 general, P2VP demonstrates the properties of a weak128 polyelectrolyte. The polymer is soluble at pH < 4 due to the129 protonated charged pyridine nitrogen when electrostatic130 repulsion overcomes the short-range hydrophobic attraction131 in the backbone of the polymer chain. Conformation of the132 P2VP chain is sensitive to the pH and ionic strength of aqueous133 solutions.29

134In an external magnetic field, the kinetically stable particle135 f1suspension (Figure 1a) forms discontinuous strings (Figure

1361b), which was monitored in situ in suspensions using dark137field optical microscopy. The particles are locked in the strings138because the field-induced forces bring particles into close139contact when the locking mechanism comes into effect. This140locking mechanism is not directly related to magnetism, but is141caused by the interaction between the specially designed142polymeric shells of the particles. When the magnetic field was143removed, the strings remained intact (Figure 1c, Supporting144Information, Video 1) due to the particle locking. This behavior145of the particle suspension was compared with the reference146particles that did not possess any locking shell. The particles147with grafted PEO chains were used in the reference experiment.148PEO-decorated particles were prepared by grafting 3-amino149propyl terminated PEO (Mn = 1500 g/mol) to the surface of150the core−shell magnetic particles, which were functionalized151with 3-glycidoxypropyl trimethoxy silane. The PEO-decorated152particles formed stable colloids due to steric repulsion between153the PEO grafted layers. The strings, which were formed when154the magnetic field was applied to the reference sample, were155destroyed by thermal motion and the particles returned to the156original state of the stable colloidal suspension after removal of157the magnetic field (Supporting Information, Video 2).158The mechanism of locking is demonstrated first using an159example of the interaction of a plane substrate (Si-wafer) and160an AFM tip. The surfaces of both materials were functionalized161by grafting a P2VP-b-PEO copolymer with molecular weights162of 3 and 9 kg/mol for the P2VP and PEO blocks, respectively;163the architecture of the grafted layer is schematically shown in164 f2Figure 2a,b. The hydrophobic P2VP surface is screened by the165swollen hydrophilic PEO brush (Figure 2b). In the approach166mode, when the AFM tip approaches the Si-wafer, the brushes167exert a repulsive force at pH 5.5 (Figure 2d and e). A further168movement of the tip toward the wafer brings the two surfaces169into contact when they are locked due to hydrophobic170interactions (Figure 2g and h). The locking effect manifests171itself as a nonzero pool-off force in the withdrawal mode

Figure 1. Locking of magnetic nanoparticle chains (in situ dark fieldoptical microscopy images). (a) Kinetically stable suspension of thebrush-decorated nanoparticles. (b) Discontinuous chains are formedfrom these particles in the 18 mT magnetic field at pH4. (c) Theformed particle chains remain intact (locked) after the field removal incontrast to the reference particles (without the locking brush shell).(d) The locked strings are disassembled by changes in pH (at pH 2)or temperature.

Nano Letters Letter

dx.doi.org/10.1021/nl301780x | Nano Lett. XXXX, XXX, XXX−XXXB

172 (Figure 2g). Thus, in an aqueous environment at 5.5 > pH >173 3.5 two solid materials decorated with the grafted copolymer174 can be locked to each other due to hydrophobic interactions175 between the P2VP blocks if the external force overcomes the176 steric repulsion (Figure 2d) of the PEO blocks and the tip-Si-177 wafer interaction is governed by hydrophobic forces (Figure178 2g). Thus, we observe the locking effect in the range 5.5 > pH179 > 3.5.180 At pH < 3, the P2VP chains are charged and swollen. In181 acidic solutions, the two surfaces are repulsive due to the182 combination of steric and electrostatic repulsions (Figure 2c183 and f). There is no locking effect in this pH range.184 The diagram in Figure 2i is a summary of the pH-dependent185 balance between the energy (EA) needed to bring the polymer-186 decorated AFM tip and Si-wafer into the contact and the187 energy (ER) needed to retract the AFM tip once it is in contact188 with the Si-wafer. The EA and ER values were obtained by the189 integration of the repulsive and attractive parts of the force190 distance curves (Figure 2c, d, f, and g), respectively. This191 diagram helps to predict and explain the behavior of colloids192 decorated with the P2VP-b-PEO grafted copolymer. In the

193range of pH values between pH 3.5 and pH 5.5, EA is high194enough to stabilize colloids, whereas ER is sufficient to lock195particles via hydrophobic interactions between the P2VP blocks196as soon as the particles are in intimate contact. (The theoretical197estimates for EA and ER are presented in the Supporting198Information.) Thus, if the external force overcomes the199repulsions in this pH range, the colloids will be locked into200aggregates.201The graphs in Figure 2j show the dynamic light scattering202hydrodynamic radii of silica particles (nonmagnetic particles)203that are decorated with the P2VP-b-PEO copolymer in204comparison with the reference colloidal particles: the bare205silica particles and the silica particles decorated with grafted206PEO and P2VP polymers. The dynamic light scattering (DLS)207data were used to distinguish between colloidal dispersions of208nonaggregated particles and aggregated particles. No aggrega-209tion was observed for the silica and PEO-coated particles. The210P2VP-coated particles aggregate strongly when P2VP turns into211the hydrophobic state at pH > 5.5. For the P2VP-b-PEO212decorated particles, aggregation is detected at pH > 5.5.213Thermal energy overcomes repulsion at pH > 5.5 (Figure 2j),

Figure 2. Conditions for the locking effect. (a and b) Schematics of a P2VP-b-PEO brush at ionized (a) and nonionized states (b). (c−h) Thesurfaces of a brush-coated Si-wafer and an AFM tip (simplified geometry of a spherical particle) are repulsive at pH 2 (c) and 5.5 (d) in theapproaching mode due to steric plus electrostatic interactions (e) and repulsive at pH 2 (f) and attractive at pH 5.5 (g) in the withdrawal mode dueto the hydrophobic locking effect (h). (i) Dependences of the energy of approaching, EA (black squares), and retraction, ER (red circles), on pHexhibit three regions of colloidal stability of the brush-decorated nanoparticles: stable colloids, kinetically stable colloids (conditions for the lockingeffect), and unstable colloids. (j) The above data are in accord with the results of light scattering measurements for the P2VP-b-PEO-decoratedparticles vs the reference silica, PEO- and P2VP-coated particles.

Nano Letters Letter

dx.doi.org/10.1021/nl301780x | Nano Lett. XXXX, XXX, XXX−XXXC

214 and the colloids aggregate. Thus, colloidal dispersions of the215 P2VP-b-PEO decorated particles are stable in a broad pH range216 below pH 5.5 without an external magnetic field, and we could217 expect the locking effect at pH < 5.5 according to the218 mechanism depicted in Figure 2.219 This concept of locking has been applied to suspensions of220 magnetic particles when an external magnetic field was used to221 activate the locking mechanism. The dispersion was kinetically222 stable at room temperature at pH < 5.5 (Figure 2i). In the223 magnetic field, the dipole−dipole interaction overcame the224 steric repulsion, and the generated strings of magnetic particles225 were locked and stabilized by hydrophobic forces (Figure 1b,c).226 However, changes in pH from 5 to 2 led to disassembly of the227 strings and formation of a stable particle suspension (Figure228 1d) due to ionization of P2VP chains and strong electrostatic229 repulsion between the particles. Consequently, within the pH230 range of 5.5 > pH > 3.5 when EA is high enough to stabilize the231 colloids and ER is sufficient to lock the particles, the external232 magnetic field was used to overcome the EA barrier and233 reversibly lock the particles. A decrease in pH below pH 5.5234 results in unlocking the particles due to strong electrostatic and235 steric repulsions.236 It is evident that different kinds of particle−particle237 interactions could be explored for the locking mechanism. In238 another example, we used the P2VP-b-PEO decorated239 nanoparticles that were further modified with 2−4 nm Prussian240 Blue (PB) clusters as described in the Supporting Information.241 The PB clusters were deposited and adhered to the P2VP242 chains in the particle shell. The PB clusters interact strongly243 with P2VP blocks30 so that the PB clusters can be used to lock244 the particles aggregates as soon as they are brought in close245 contact by the external magnetic field. In other words, we246 replaced the hydrophobic locking mechanism in aqueous247 suspensions by locking due to the P2VP-PB interactions248 between particles in polymer melts. This replacement enables

249the locking mechanism in a nonaqueous environment, for250example, in polymer melts. In our experiments, the PB clusters251were used to lock the particle strings via P2VP-PB-P2VP252bridging in PEO polymer melts of various molecular weights in253the presence of a magnetic field. For a higher molecular weight254PEO, that is, PEO 1500 g/mol, the sample was locked at 60 °C255(above the melting point of PEO) to enable free movement of256the particles. The obtained strings were stable up to 75 °C in257PEO 200 g/mol and up to 120 °C in PEO 1500 g/mol after the258magnetic field was removed. At higher temperatures we259observed a fragmentation of the strings with complete260disassembly and aggregation at 160 °C in PEO 1500 g/mol261as shown in the in situ (in melt) experiments using dark field262 f3optical microscopy (Figure 3).263Thus, the locking mechanism can be used to preserve264anisotropic nanostructures in solutions and polymeric matrices265via exposure of the material to a pulse of a magnetic field. That266brings a substantial technical advantage in terms of energy267saving for the fabrication of anisotropic composites. An268anisotropic composite with common magnetic particles could269be prepared by hardening the composite in a magnetic field270until the solid or cross-linked polymer stabilizes the assembled271particles. For structures with the locking mechanism, the272magnetic field could be switched off as soon as the structures273are formed. The locking mechanism will preserve the structures274while the process of hardening is conducted.275Physical properties, including conductivity and optical and276mechanical properties of composite nanostructured materials,277depend on the dimensions of nanostructures. The locking effect278brings a new opportunity for control over the dimensions of279nanostructures, as discussed below.280The length of the particle chains depends on the balance of281thermal energy and the energy of dipole−dipole interactions in282the magnetic field. In a strong magnetic field, magnetic chains283are very long; however, they disintegrate as soon as the field is

Figure 3. Locking effect in the polymer melt (in situ dark field optical microscopy images). (a) Suspension of magnetic particles in PEG at 60 °C atno external magnetic field. (b) The same suspension generates particle chains in the 0.1 T magnetic field. (c) These particle chains are locked viaPrussian Blue (PB) nanoclusters and remain intact after removal of the magnetic field. (d−f) The particle chains are stable at 120 °C (d), whiledisintegration begins at 140 °C (e), and the structures are transformed into aggregates at 160 °C (f).

Nano Letters Letter

dx.doi.org/10.1021/nl301780x | Nano Lett. XXXX, XXX, XXX−XXXD

284 off. The situation is quite different for the locking particles. As285 soon as the magnetic field is off, the length of chains is affected286 by the balance of the energy of the particle repulsion, the287 energy of locking, the energy of dipole−dipole interactions288 between the particles in the chain (at the moment when the289 magnetic field was on), and the strength of the magnetic field290 (when it was on). The polydispersity of the particles by size291 manifests itself in the dependence of the chain length on the292 strength of the magnetic field (see the Supporting Information293 for a more detailed discussion). If the locking is not activated294 for some fraction of the nanoparticles (because the balance

295between the forces is particle size dependent), then the particle296chain is broken in that unlocked site after the magnetic field is297 f4turned off (Figure 4a−e). Thus, a fraction of “defective298particles” (by size and quality of coatings) creates defects in the299particle chains (locking is not activated at the given strength of300the magnetic field). In locations of those defects, the particle301chains are “cut” by thermal energy.302Control over the particle chain length could be attained using303specially incorporated defects via a combination of two304populations of magnetic nanoparticles. One population of the305nanoparticles has a shell with the locking effect, whereas

Figure 4. Behavior of locking particles and their mixtures with antilocking particles in external magnetic fields (in situ dark field optical microscopyimages). (a−e) The length of wires is adjusted in the magnetic field: 2.2 mT (a), 3.1 mT (b), 5.0 mT (c), and 10.0 mT (d) showing an increasingeffect of the magnetic field (e). (f−j) The length of the wires can be regulated by adding antilocking particles in different weight fractions: 10% (f),25% (g), 50% (h), and 75% (i) showing a decreasing effect of the fraction of antilocking particles (j). (k−m) The wires generated at 18.0 mT (k) arelocked, remain intact after removal of the field (l), and form a 3D network upon application of the orthogonal field (m). (n and o) The redirection ofthe field results in the elastic stretching of the network (n), which relaxes after reverting of the field direction (o). Dark field microscopy images are150 × 150 μm.

Nano Letters Letter

dx.doi.org/10.1021/nl301780x | Nano Lett. XXXX, XXX, XXX−XXXE

306 another population of the particles is decorated with an307 antilocking shellthe shell that is always repulsive to any308 particles in the suspension. The antilocking shell was made of309 grafted polyethyleneimine (PEI, Mn = 2000 g/mol). PEI is310 positively charged in aqueous solutions. Thus, the PEI-311 decorated particles are repulsive to P2VP-b-PEO decorated312 particles in a broad pH range. Mixtures of those two313 populations of the particles were used to “cut” the strings314 generated in the magnetic field. Specifically, the locked chains315 were cut into smaller fragments (particle chains) by thermal316 energy. The rupture of the strings occurred in the locations of317 antilocking particles as soon as the external field was removed318 (Supporting Information, Video 3). The generated short319 particle chains exhibit random distribution by length; however,320 the mean length can be regulated with an accuracy of321 micrometers (or 10−20 particle in the chain) by adjusting322 the fraction of antilocking particles in the mixture (Figure 4f−323 j).324 The combination of both the locking effect and the control325 over the particle length offers an additional opportunity to326 generate 3D nanostructured materials as shown in Figure 4(k−327 o). The mobility of the magnetic chains obviously depends on328 their length. By optimizing the mobility characteristics through329 the adjustment of the chain length, we found a regime in which330 a rotation of the magnetic field could be used to approach an331 entanglement of the particle chains and the formation of a 3D-332 network. For example, exposure of a suspension of locking333 nanoparticles to the 18 mT field resulted in the formation of 25334 μm long chains (Figure 4k) in which the particles were locked,335 as concluded from the lack of any sign of chain breakup after336 removal of the field (Figure 4l). The subsequent exposure of337 this aligned structure to the magnetic field applied in338 orthogonal directions led to the formation of a 3D network339 of the chains (Figure 4m). The 3D network was locked through340 the same mechanism. Experiments with a change in the341 magnetic field direction (Figure 4n) and then the reversion of342 the field (Figure 4o) revealed the recovery of the same network343 patterns. In particular, the experiments showed that the initially344 formed network (Figure 4m) deformed elastically by the345 redirected magnetic field (Figure 4n) and relaxed after the field346 direction was restored (Figure 4o). The reversible deformation347 of the field-assembled 3D network was confirmed through the348 quantitative analysis of the particle strand conformation (see349 Supporting Information).350 In summary, the generated structures demonstrate that a351 combination of locking and antilocking nanoparticles is a352 powerful tool to control the anisotropic self-assembly of353 nanostructures in external fields. Obviously, the particles could354 be prepared from metals and semiconductor materials. Self-355 assembly could be attained in magnetic or electric fields (in the356 latter case in a dielectric environment). The energy of particle−357 particle interactions in the locked state could be regulated by358 appropriate selection of the locking mechanism. All of those359 combinations could bring into existence a number of novel360 approaches for the development of nanostructured composite361 materials with advanced optical, conductive, magnetic, and362 mass transport properties.

363 ■ ASSOCIATED CONTENT364 *S Supporting Information365 Methods of the particle synthesis, force−distance measure-366 ments, magnetic field directed assembly, theoretical analysis of367 particle−particle interactions, and video files that monitor the

368field-directed self-assembly. This material is available free of369charge via the Internet at http://pubs.acs.org.

370■ AUTHOR INFORMATION

371Corresponding Author372*E-mail: sminko@clarkson.edu; luzinov@clemson.edu.

373Notes374The authors declare no competing financial interest.

375■ ACKNOWLEDGMENTS

376The research was supported by the National Science377Foundation (Grants CBET-0756461, CBET-0756457).

378■ REFERENCES(1) 379Whitesides, G. M.; Grzybowski, B. Science 2002, 295 (5564),

3802418−2421.(2) 381Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzan, L. M. ACS

382Nano 2010, 4 (7), 3591−3605.(3) 383Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294

384(5547), 1684−1688.(4) 385Ikkala, O.; ten Brinke, G. Science 2002, 295 (5564), 2407−2409.(5) 386Chen, Q.; Bae, S. C.; Granick, S. Nature 2011, 469 (7330), 381−

387384.(6) 388Nykypanchuk, D.; Maye, M. M.; van der Lelie, D.; Gang, O.

389Nature 2008, 451 (7178), 549−552.(7) 390Motornov, M.; Sheparovych, R.; Lupitskyy, R.; MacWilliams, E.;

391Minko, S. J. Colloid Interface Sci. 2007, 310 (2), 481−488.(8) 392Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P.

393Science 2003, 299 (5604), 226−229.(9) 394Lin, Y.; Boker, A.; He, J. B.; Sill, K.; Xiang, H. Q.; Abetz, C.; Li, X.

395F.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P.396Nature 2005, 434 (7029), 55−59.

(10) 397Maune, H. T.; Han, S. P.; Barish, R. D.; Bockrath, M.; Goddard,398W. A.; Rothemund, P. W. K.; Winfree, E. Nat. Nanotechnol. 2010, 5399(1), 61−66.

(11) 400Sacanna, S.; Irvine, W. T. M.; Chaikin, P. M.; Pine, D. J. Nature4012010, 464 (7288), 575−578.

(12) 402Bigioni, T. P.; Lin, X. M.; Nguyen, T. T.; Corwin, E. I.; Witten,403T. A.; Jaeger, H. M. Nat. Mater. 2006, 5 (4), 265−270.

(13) 404Fan, J. A.; Wu, C. H.; Bao, K.; Bao, J. M.; Bardhan, R.; Halas, N.405J.; Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F. Science4062010, 328 (5982), 1135−1138.

(14) 407Hynninen, A. P.; Thijssen, J. H. J.; Vermolen, E. C. M.; Dijkstra,408M.; Van Blaaderen, A. Nat. Mater. 2007, 6 (3), 202−205.

(15) 409Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. H. Nature 2002,410420 (6914), 395−398.

(16) 411Kroutvar, M.; Ducommun, Y.; Heiss, D.; Bichler, M.; Schuh, D.;412Abstreiter, G.; Finley, J. J. Nature 2004, 432 (7013), 81−84.

(17) 413Sun, B.; Sirringhaus, H. Nano Lett. 2005, 5 (12), 2408−2413.(18) 414Pileni, M. P. J. Phys. Chem. B 2001, 105 (17), 3358−3371.(19) 415Gast, A. P.; Zukoski, C. F. Adv. Colloid Interface Sci. 1989, 30,

416153−202.(20) 417Safran, S. A. Nat. Mater. 2003, 2 (2), 71−72.(21) 418Parthasarathy, M.; Klingenberg, D. J. Mater. Sci. Eng. R, Rep.

4191996, 17 (2), 57−103.(22) 420Carlson, J. D.; Jolly, M. R. Mechatronics 2000, 10 (4−5), 555−

421569.(23) 422Hynninen, A. P.; Dijkstra, M. Phys. Rev. Lett. 2005, 94 (13),

423138303.(24) 424Lee, S. H.; Liddell, C. M. Small 2009, 5 (17), 1957−1962.(25) 425Gong, T. Y.; Wu, D. T.; Marr, D. W. M. Langmuir 2002, 18

426(26), 10064−10067.(26) 427Yuan, J. Y.; Xu, Y. Y.; Muller, A. H. E. Chem. Soc. Rev. 2011, 40

428(2), 640−655.(27) 429Levy, L.; Sahoo, Y.; Kim, K. S.; Bergey, E. J.; Prasad, P. N. Chem.

430Mater. 2002, 14 (9), 3715−3721.

Nano Letters Letter

dx.doi.org/10.1021/nl301780x | Nano Lett. XXXX, XXX, XXX−XXXF

(28)431 Tsang, S. C.; Yu, C. H.; Gao, X.; Tam, K. J. Phys. Chem. B 2006,432 110 (34), 16914−16922.

(29)433 Minko, S.; Roiter, Y. Curr. Opin. Colloid Interface Sci. 2005, 10434 (1−2), 9−15.

(30)435 Kiriy, A.; Gorodyska, G.; Minko, S.; Tsitsilianis, C.; Jaeger, W.;436 Stamm, M. J. Am. Chem. Soc. 2003, 125 (37), 11202−11203.

Nano Letters Letter

dx.doi.org/10.1021/nl301780x | Nano Lett. XXXX, XXX, XXX−XXXG