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Article Cite This: Acc. Chem. Res. 2017, 50, 2756-2766
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Molecular Recognition in the Colloidal World Elizabeth Elacqua,*,†,‡ Xiaolong Zheng,† Cicely Shillingford,† Mingzhu Liu,† and Marcus Weck*,† †
Molecular Design Institute and Department of Chemistry, New York University, New York, New York 10003-6688, United States Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802-1503, United States
‡
CONSPECTUS: Colloidal self-assembly is a bottom-up technique to fabricate functional nanomaterials, with paramount interest stemming from programmable assembly of smaller building blocks into dynamic crystalline domains and photonic materials. Multiple established colloidal platforms feature diverse shapes and bonding interactions, while achieving specific orientations along with short- and long-range order. A major impediment to their universal use as building blocks for predesigned architectures is the inability to precisely dictate and control particle functionalization and concomitant reversible self-assembly. Progress in colloidal self-assembly necessitates the development of strategies that endow bonding specificity and directionality within assemblies. Methodologies that emulate molecular and polymeric three-dimensional (3D) architectures feature elements of covalent bonding, while high-fidelity molecular recognition events have been installed to realize responsive reconfigurable assemblies. The emergence of anisotropic ‘colloidal molecules’, coupled with the ability to site-specifically decorate particle surfaces with supramolecular recognition motifs, has facilitated the formation of superstructures via directional interactions and shape recognition. In this Account, we describe supramolecular assembly routes to drive colloidal particles into precisely assembled architectures or crystalline lattices via directional noncovalent molecular interactions. The design principles are based upon the fabrication of colloidal particles bearing surface-exposed functional groups that can undergo programmable conjugation to install recognition motifs with high fidelity. Modular and versatile by design, our strategy allows for the introduction and integration of molecular recognition principles into the colloidal world. We define noncovalent molecular interactions as site-specific forces that are predictable (i.e., feature selective and controllable complementary bonding partners) and can engage in tunable high-fidelity interactions. Examples include metal coordination and host−guest interactions as well as hydrogen bonding and DNA hybridization. On the colloidal scale, these interactions can be used to drive the reversible formation of open structures. Key to the design is the ability to covalently conjugate supramolecular motifs onto the particle surface and/or noncovalently associate with small molecules that can mediate and direct assembly. Efforts exploiting the binding strength inherent to DNA hybridization for the preparation of reversible open-packed structures are then detailed. We describe strategies that led to the introduction of dual-responsive DNA-mediated orthogonal assembly as well as colloidal clusters that afford distinct DNA-ligated close-packed lattices. Further focus is placed on two essential and related efforts: the engineering of complex superstructures that undergo phase transitions and colloidal crystals featuring a high density of functional anchors that aid in crystallization. The design principles discussed in this Account highlight the synergy stemming from coupling well-established noncovalent interactions common on the molecular and polymeric length scales with colloidal platforms to engineer reconfigurable functional architectures by design. Directional strategies and methods such as those illustrated herein feature molecular control and dynamic assembly that afford both open-packed 1D and 2D lattices and are amenable to 3D colloidal frameworks. Multiple methods to direct colloidal assembly have been reported, yet few are capable of crystallizing 2D and 3D architectures of interest for optical data storage, electronics, and photonics. Indeed, early implications are that [supra]molecular control over colloidal assembly can fabricate rationally structured designer materials from simple fundamental building blocks.
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INTRODUCTION Akin to nature’s biomaterials comprising design elements that ‘code’ for architecturally exquisite molecular and macromolecular frameworks via supramolecular interactions, synthetic assembly processes can be designed with molecular building blocks that assume predesigned hierarchical and threedimensional (3D) architectures.1−4 Nevertheless, substantial challenges remain to controllably translate the concepts of directional self-assembly over multiple length scales.5 Fabrica© 2017 American Chemical Society
tion of colloidal materials wherein the composition and surface features remain constant and uniform across the nano- and microscale is not facile, and strategies to functionalize particles selectively and/or site-specifically are not ordinarily transferrable across different length scales. Generally speaking, selfassembly strategies are among the most potent and rational Received: July 25, 2017 Published: October 6, 2017 2756
DOI: 10.1021/acs.accounts.7b00370 Acc. Chem. Res. 2017, 50, 2756−2766
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Figure 1. Examples of directional self-assembly on the nanoscale: (a) pH-responsive rotaxane-based molecular muscles featuring host−guest complexation;9 (b) MOF-5 sustained by metal cordination;8 (c) perylene bisimide foldamers engaging in solvent-driven π−π stacking;11 (d) supramolecular polymer exhibiting orthogonal hydrogen-bonding, metal-coordination, and host−guest interactions;7 (e) DNA-origami-directed Au nanoparticle assembly.12 Adapted with permission from refs 11, 7, and 12. Copyright 2015 and 2016 American Chemical Society and 2016 Nature Publishing Group, respectively.
competes with the assembly forces.13 Thus, colloidal behavior can be considered thermodynamically equivalent to that of atoms and molecules. Complex geometries observed on the molecular and macromolecular scales can be translated to colloidal systems by mimicking structures, properties, and assembly dynamics.14 Colloidal self-assembly involves various repulsive, attractive, and/or external forces that act on the particles during sequential phase changes leading to crystal formation.13 These forces are directional and/or nondirectional and generate colloidal structures with short- or long-range order. Interactions defining the self-assembly of two colloidal particles have contrasting effects relative to atomic or molecular entities; we limit our discussion to a qualitative description of major assembly forces. All interparticle, intermolecular, or interatomic forces have a distance dependence that designates the range of order that can be induced on each species.15 Colloidal particles predominantly experience attractive interactions due to van der Waals forces, which are estimated by the Hamaker model to decrease by r−2 (where r is the interparticle distance) and act over a larger range compared with the van der Waals forces between molecules (F ∝ r−6).16 Van der Waals attractions between colloidal particles are more pronounced relative to those between molecules because the total force increases with particle size and is a summation of attractions between interparticle pairs. Colloids fabricated from polymers or metal oxides will aggregate when suspended in liquid media, particularly in nonpolar media.13 Aggregation is prevented by long-range Coulombic forces generated through electrostatic stabilization of particles suspended in polar media. Coulombic forces on micron-sized objects can span up to hundreds of nanometers depending on the surface charge, size, and ion concentration and are exploited for the assembly of chemically asymmetric
modes to orchestrate well-defined ensembles on both molecular and nanoscales, with their architectures being intrinsically dictated via coded information (e.g., shape, surface properties, charge, etc.) that culminates in the organization of discrete patterns and designer functions; however, the translation of molecular design principles to the colloidal scale is in its infancy.6 Central to a given assembly is balancing covalent interactions with the manipulation of nondirectional and directional noncovalent forces. Molecular self-assembly involves the exploitation of covalent and noncovalent interactions for short-range associations. Programmable molecular architectures feature additive and orthogonal directional forces1,7 such as dynamic covalent bonds, metal coordination,8 hydrogen bonding,3,7 host−guest complexation,9,10 π−π stacking,11 and nucleic acid hybridization,12 engendering versatile materials such as metal−organic frameworks (MOFs),8 rotaxane-based molecular machines,9 foldamers,11 supramolecular polymers,7 and nanoassemblies by design (Figure 1).12 These materials are based on precise and controllable assemblies ranging from the molecular (angstrom) scale to the nanoscale. Given the wealth of molecular self-assembly motifs that are controllable, programmable, and importantly, directional and dynamic,1 successful integration and exploitation of supramolecular interactions to construct programmable assemblies should allow for colloidal architectures by design.
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COLLOIDAL SELF-ASSEMBLY We focus on colloidal particles with sizes ranging from 500 nm to 2 μm, whose material properties emerge when they selfassemble into clusters, superstructures, and crystals. Colloids inherently possess Brownian motion, which affects particle assembly as the thermal energy that elicits the disordered state 2757
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Figure 2. Nondirectional forces for colloidal self-assembly: (a) electrostatic assembly of oppositely charged colloids into an ionic lattice;19 (b) selfassembly of triblock Janus particles featuring hydrophobic and hydrophilic regions;20 (c) flotation-capillary-driven assembly of close-packed binary lattices;26 (d) convective assembly of colloidal monolayers driven by immersion capillary forces;31 (e) depletion forces acting on lock and key particles.33 Images adapted and reprinted with permission from refs 19,20,26,31,33. Copyright 2005 Nature Publishing Group, 2012 American Chemical Society, 2009 John Wiley and Sons, 2007 American Chemical Society, and 2010 Nature Publishing Group, respectively.
Figure 3. Design strategy consisting of engineering (a) functional-group-containing colloidal clusters; (b) patchy particles, wherein each patch can be functionalized with molecular recognition moieties; and (c) after functionalization, site-specific molecular recognition between functionalized patchy particles through host−guest-based, metal-coordination-based, or DNA-based self-assembly.
colloidal crystals at the air−water or water−oil interface (Figure 2c).25−29 Immersion capillary forces acting on particles that are partially wetted by a liquid film on a solid substrate have led to crystal formation at the evaporation front where colloids accumulate due to convective flux (Figure 2d).30,31 Finally, depletion interactions are inherently entropic and effective short-range forces that have been used to construct colloidal chains and clusters (Figure 2e).32,33 Whereas the aforementioned interactions are intrinsic, nondirectional forces that minimize thermodynamic free energy regardless of particle size, shape, surface charge, or dispersion medium and in the absence of external forces, there are examples wherein these interactions are used with anisotropic
particles (e.g., particles containing distinct hydrophobic and hydrophilic domains).13,17 Steric stabilization of particles containing a solvated poly(electrolyte) or surfactant layer also prevents aggregation.18 Varying the density of electrolytes, surfactants, or acid/base groups while adjusting the pH or ionic strength of the medium can tune repulsive interaction strengths. Thus, manipulation of electrostatic, hydrophobic, and steric stabilization is a powerful tool to assemble colloids into superstructures and ionic lattices (Figure 2a,b).19−24 Capillary forces are long-range attractive forces that can direct colloidal assembly.25 Long-range floatation capillary forces along with short-range electrostatic repulsion have been used to fabricate close-packed, open-packed, and binary 2758
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Figure 4. Metal-coordination-driven assembly and disassembly of colloidal particles: (a, b) schematics depicting pincer-based metal-coordination assembly/disassembly process; (c) bright-field micrographs illustrating linear and branched chains afforded via self-assembly.
particles to induce directional assembly into open-packed lattices. For example, Granick has reported that site-specific localization of hydrophobic and charged hydrophilic moieties on a triblock Janus particle surface achieved directional assembly into a kagome lattice.21 Given the wealth of anisotropic particles reported,33-35 the use of structurally complex building blocks can lead to more opportunities to construct superstructures and nontrivial lattices. Constraining colloids to exist in precise, metastable, and nonminimized free energy patterns requires particles with directional bonding capabilities akin to those of molecules. This Account addresses the translation of molecular recognition principles to the colloidal scale, highlighting directional interactions and systems that can engineer superstructures, low-coordination-number lattices, and hierarchical materials. Particular focus is placed on colloidal systems that not only are capable of achieving controllable assemblies through molecular recognition but also feature triggerable methods to disassemble and/or reassemble into different lattices.
facilitate bonding directionality upon functionalization with molecular recognition pairs. Whereas azide moieties are envisioned to increase the design landscape for a variety of alkynyl-substituted small molecules, dyes, and/or DNA that are suitable for azide−alkyne cycloadditions, carboxylic acids can be conjugated to functionalized molecules or polymers that can provide additional platforms for regioselective supramolecular recognition events. We rationalize that coupling of high-fidelity molecular recognition pairs with anisotropic colloidal molecules allows for the realization of directionally assembled open colloidal lattices that feature stimuli-responsive centers (e.g., triggerable through redox, temperature, pH, light) while providing an element of reconfigurability. Akin to atoms that bond covalently, patchy colloidal molecules would then be able to preferentially bind to specific complementary particles, with various symmetries being accommodated.
DESIGN PRINCIPLES Our initial exploits to design materials imbued with bonding specificity and molecular recognition handles led to anisotropic molecular-geometry-mimicking colloids.36 Well-defined colloidal particles of various shapes (spherical, linear, triangular, tetrahedral, trigonal-bipyramidal, octahedral, etc.; Figure 3) were fabricated from colloidal clusters and subsequently encapsulated to afford patchy colloids.36 The resulting particles are ‘colloidal molecules’ with valence, wherein the geometry mimics that of hybridized atomic orbitals. The patches present as distinct sites for molecular recognition (Figure 3) based upon metal coordination, host−guest complexation, hydrogen bonding, and DNA hybridization. Thus, assemblies are constructed that feature sp, sp2, sp3 (and so on) central valency. We envisioned that colloidal molecules featuring pendant functionalities such as amidine, sulfonate, carboxylic acid, and/ or azide moieties studded along the protruding patches can
Metal-Coordination-Based Self-Assembly
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MOLECULAR-RECOGNITION-BASED ASSEMBLY OF COLLOIDS
Metal-coordination-based recognition has been utilized in both molecular and polymer chemistry to generate a wealth of structures through high-specificity and high-fidelity associations.1,7 We envisioned that metal coordination could facilitate the assembly of colloidal patchy particles into open lattices while providing a strategic opportunity to disassemble the particles via the introduction of competitive ligands (Figure 4). We demonstrated this strategy by engineering poly(styrene)based patchy particles that are functionalized with triblock copolymers (tbcs).37 The colloidal particles contain −COOH moieties on the patches, while the tbcs feature side chains with hydroxyl groups and coumarin dyes as well as metalcoordination motifs. Patch−polymer conjugation is realized via esterification, and assembly is visualized through fluorescence. Particles featuring SCS-PdII-pincer and pyridine motifs on the exterior patches are achieved. The assembly 2759
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Figure 5. Host−guest-based reversible complexation of colloidal particles featuring (a) CB[8]-mediated assembly of azobenzene and viologen,40 (b) β-CD-mediated assembly with azobenzene,42 (c) CB[8]-mediated assembly of azobenzene- and viologen-functionalized anisotropic colloids,43 and (d) CB[7]-mediated self-assembly of sulfonated patchy particles with DPV.44 Adapted with permission from refs 40 and 42. Copyright 2014 John Wiley and Sons and 2015 The Royal Society of Chemistry, respectively.
to host one or more guest compounds) can dictate a homoternary or heteroternary complexation effect.39 Integration of photoresponsive guests, such as azobenzene, affords the opportunity to fine-tune or reconfigure colloidal assemblies. Studies utilizing molecular containers (e.g., cucurbit[n]urils (CB[n]) and cyclodextrins) have been reported on colloidal matter in aqueous solutions. Scherman and co-workers described hybrid raspberry-like colloids (HRCs) that are light-responsive and based upon the heteroternary complexation of CB[8] with azobenzene-studded silica particles and viologen-conjugated nanoparticles.40 UV irradiation led to the triggered release of viologen−CB complexes from the core via isomerization of the azobenzene, while exposure to visible light led to reassembly (Figure 5a). Work with hollow mesoporous raspberry-like colloids (HMRCs) demonstrated their use as an on-demand cargo release system. This system assembled azobenzene-functionalized HMRCs with viologen-functionalized iron oxide particles in the presence of CB[8].41 UV irradiation allowed for the photocontrolled release of viologen− iron oxide particles from the surface. Additional photoresponsive colloidal Janus particles have been engineered through the complexation of β-cyclodextrin (β-CD) and azobenzene (Figure 5b). The Janus particles had β-CD grafted onto one end and demonstrated light-controlled assembly/ disassembly in the presence of an azobenzene-functionalized polymer.42 We have studied CB[n]-related assembly and disassembly of anisotropic colloids in aqueous environments (Figure 5c,d). Using sulfonated anisotropic particles, we demonstrated that CB[7] could be noncovalently grafted onto the patches of dimers and form small daisy-chain-like architectures in the presence of diphenyl viologen (DPV) (Figure 5d). The resulting assemblies were sustained through CB[7]−DPV−
was initiated by the addition of AgBF4, which frees a coordination site for the pyridine-functionalized particles, thus allowing for the formation of linear and branched colloidal chains (Figure 4c). Disassembly was triggered by the addition of PPh3, which is a stronger ligand for the Pd pincer species. Whereas reassembly is not possible in this system, metalcoordination-based assembly and disassembly provides a unique method to assemble colloidal particles within organic media. Hydrogen-Bond-Mediated Self-Assembly
Although prominent in small-molecule and polymeric assemblies, hydrogen bonding in colloids remains in its infancy. Recent reports detail the assembly behavior of stearyl alcoholcoated silica particles that feature covalently-grafted selfcomplementary benzene-1,3,5-tricarboxamide (BTA) moieties.38 The BTA units were protected by photolabile onitrobenzyl groups, thus providing a stimulus to trigger assembly of the spheres into colloidal clusters. Upon UVtriggered deprotection, the particles engage in short-range hydrogen bonding. The newly formed clusters also disassemble at higher temperatures, where the hydrogen-bonding strength is lower. In contrast to metal coordination, hydrogen bonding is fully reversible and can fine-tune colloidal assemblies, leading to potential thermally reconfigurable materials given the proper colloidal platform. Hydrogen bonding is a robust platform for colloidal assembly because of its ability to facilitate assembly in either organic or aqueous media, pending the desired recognition sequence. Host−Guest-Based Complexation of Colloids
The inherent selectivity and switchability/stimuli-responsive nature of host−guest assemblies39 render them ideal targets for colloidal assembly. Their selectivity based on size (the capacity 2760
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Figure 6. (a) (left) Schematic of colloidal cluster formation from crystal templating and (right) colored SEM micrograph of the icosahedral cluster (yellow) surrounded by face-bound particles with dodecahedral symmetry (blue) and vertex-bound particles (orange).50 (b) (left) SEM micrographs and confocal micrographs of clusters and patchy particles and (right) supracolloidal AB4 formed by mixing monovalent and tetravalent particles.36 Adapted with permission from refs 50 and 36. Copyright 2015 American Chemical Society and 2012 Nature Publishing Group, respectively.
Figure 7. Directional bonding of DNA-functionalized patchy particles: colloidal molecules assembled from (a) monofunctional patchy particles36 and (b) bifunctional patchy particles.53 Adapted with permission from refs 36 and 53. Copyright 2012 Nature Publishing Group and 2016 American Chemical Society, respectively.
CB[7] bridges,44 with redox-responsive disassembly in the presence of sodium dithionite and reassembly after air oxidation being observed. We further used CB[n]-mediated self-assembly strategies, combined with covalent chemistry, to assemble anisotropic colloids.43 Heterotelechelic diblock copolymers bearing pendant azobenzene and viologen side chains were conjugated to carboxylated patch surfaces and assembled in the presence of CB[8]. The designed heteroternary complexation led to the fabrication of chains
and clusters that underwent light-triggered disassembly/ reassembly (Figure 5c). DNA-Mediated Self-Assembly
The rapid development of DNA nanotechnology has led to the realization of various architectures, including 2D/3D crystals, nanotubes, other periodic and aperiodic structures, and DNA origami.45 Further integration of DNA into hybrid structures has led to the emergence of DNA-coated colloids, owing to pioneering work by Mirkin and Alivisatos on the nanoscale.46,47 Recent reviews cover DNA-directed assembly on length scales 2761
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Figure 8. Crystallization of DNA-coated colloids: (a) colloidal crystals using DNA-coated spheres with different size ratios;61 (b) colloidal isomorphs of the MgCu2 Laves phase;64 (c) double diamond lattice.65 Reprinted with permission from refs 61, 64, and 65. Copyright 2015, 2017, and 2017, respectively, Nature Publishing Group.
ranging from nanostructures to colloidal particles48 along with progress in simulation and experimental research.49 Herein we focus on experimental progress of DNA-directed assembly of micron-sized particles. Crocker and co-workers synthesized DNA-coated colloidal clusters with polyhedral symmetries.50 They developed a crystal templating approach wherein binary crystals were first formed from a majority ‘host’ and a minority ‘impurity’ species, followed by selective dissociation of all particle bonds associated with the host. By variation of the size ratio of the host and impurity spheres and the timing of ligation, different cluster symmetries could be obtained. The authors further demonstrated that clusters with uniformly covered singlestranded DNA (ssDNA) strands could display directional binding to spheres of complementary DNA strands (Figure 6a).50 We developed a method to engineer colloidal particles with chemically distinct patches and symmetries that mimic hybridized atomic orbitals (Figure 6b).36 DNA was siteselectively attached to the patch surface to enable directional binding. The anisotropic particles assembled into colloidal molecules, with the particle symmetry or patch size dictating possible architectures (Figure 7a).36 Accordingly, linear, trigonal, and tetrahedral colloidal molecules were obtained by mixing monovalent particles with divalent, trivalent, or tetravalent particles, respectively. Alternating copolymers or homopolymers akin to covalent block polymers were also achieved from complementary divalent particles. Colloidal isomers of a nonlinear AB2-type assembly that mimics cis and trans conformations of ethylene formed when divalent particles with larger patches were assembled in aqueous media with monovalent particles.36 By this approach, extended kagome, honeycomb, and diamond lattices can be envisioned using
particles with divalent, trivalent, and tetravalent symmetries.51 Covalent capture of the assembled structures (e.g., by photocross-linking of cinnamate moieties) can be envisioned as a method to harvest the assembled architectures.52 We also designed bifunctional patchy particles wherein two orthogonal functional groups were spatially located.53 Two types of DNA with distinct terminal sequences and melting temperatures were site-specifically conjugated to the patches and the matrix, employing orthogonal coupling strategies.53 The regioselective molecular recognition on both the patches and matrix enabled stepwise assembly of more complex architectures, wherein the different DNA melting temperatures allowed for a reversible and stepwise temperature-gated assembly (Figure 7b).53
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MOLECULAR-RECOGNITION-DRIVEN COLLOIDAL CRYSTALLIZATION AND RECONFIGURABLITY
Crystallization of DNA-Coated Colloids
In 2008, Gang54 and Mirkin55 first crystallized DNA-coated nanoparticles. Since then, various lattices including facecentered cubic (FCC), CsCl, Cr3Si, AlB2, hexagonal closepacked, Cs6C60, NaCl, simple cubic, and diamond were obtained from DNA-coated particles on the nanoscale.56 Crystallization of micron-sized particles, however, remains a challenge57 because of kinetic traps resulting from surface inhomogeneity and low areal grafting density. Strategies to overcome this include insertion of polymeric linkers (e.g., Pluronic F108 or poly(styrene)-b-poly(ethylene oxide) copolymers)58,59 between the particle surface and DNA to increase the overall length and flexibility as well as integration of mobile DNA linkers onto the particle surface, thus allowing the particles to roll around each other and rearrange.60 2762
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Accounts of Chemical Research We have developed a synthetic strategy to optimize the surface DNA areal density and smoothness.61 In particular, chlorine-functionalized spherical particles were fabricated from a sol−gel/free radical polymerization, and the chlorine groups were subsequently converted to azides. The azides then underwent click-type reactions with cyclooctyne-terminated DNA. Our method increased the DNA surface-grafting density to approximately 115 000 ssDNA per particle, which is well in excess of the required 10 000 strands needed to diffuse and anneal.62 The particles were reconfigurable, and crystals with a range of lattices (e.g., FCC, CsCl, AlB2, and Cs6C60) were fabricated by varying the DNA strands or particle size ratio (Figure 8a). The crystallization kinetics were sensitive to the quenching depth. Shallow quenches (i.e., smaller temperature changes) achieved larger crystals through nucleation and growth mechanisms, whereas deeper quenches favored smaller crystals. The deepest quench (i.e., the largest temperature change) afforded a dense amorphous aggregate, followed by slow crystallization. The synthetic strategy could be extended to different materials, including a variety of polymers and inorganic spheres. Combinations of these materials achieved true binary crystals.63 This method was used by Pine and coworkers to combine tetrahedral clusters with spherical particles containing complementary DNA strands, thus allowing for the crystallization of colloidal isomorphs of the MgCu2 Laves phase (a diamond lattice that interpenetrates a pyrochlore lattice; Figure 8b).64 Crocker and co-workers observed the double diamond lattice (two interpenetrating diamond lattices) using different-sized microspheres with complementary DNA strands on the surface (Figure 8c).65
Figure 9. Programmed phase transition with DNA strand displacement:67 (a) measured pair interactions (symbols) agree quantitatively with model calculations (curves); (b−d) confocal images of (b) CsCl binary crystals in coexistence with a fluid, (c) a homogeneous fluid phase of all three species, and (d) CsCl crystals in coexistence with a fluid. Reprinted with permission from ref 67. Copyright 2015 American Association for the Advancement of Science.
of these molecular concepts on the colloidal scale has led to the formation of ‘colloidal molecules’ and open-packed colloidal frameworks, each of which differs in the strength of featured noncovalent binding elements as well as the capacity to controllably assemble/disassemble colloidal frameworks. In addition to achieving molecular bonding directionality, major strides to fabricate larger organizational (crystalline) domains have been aided by the realization that synthetic strategies common to molecular designs facilitate high grafting densities upon colloidal particles, a prerequisite to achieving colloidal crystals. Our introduction of molecular click chemistry to the colloidal world has led to the formation of multiple higher-order crystals and frameworks based upon different spherical building blocks as well as clusters that achieve binary lattices. Further studies of colloidal phase transitions have demonstrated additional methods to fine-tune colloidal architectures. Indeed, multiple elements of molecular synthesis and assembly have realized complex architectures along the micron scale with colloidal particles. Efforts to combine these concepts, although not accomplished yet, will lead to multiple designer frameworks and robust materials. Current approaches have separately utilized both engineering- and chemistry-based insights to design approaches toward novel architectures. Common to both is the breakdown of the architectures into simpler building blocks; however, construction of the most complex frameworks will necessitate synergistic approaches that feature elements of both molecular recognition and molecular synthesis combined with engineering approaches to afford colloidal architectures by design. The realization of hierarchical lattices such as the diamond lattice, which is predicted to contain a 3D photonic band gap, can be broken down into building blocks with fourfold coordination that pack into a tetrahedral lattice. The general notion of taking concepts from the molecular world and translating them to the colloidal scale is not limited to binding interactions. A rational design could also lead to structures with chirality, self-
Programmable Colloidal Phase Transitions
The assembly of spherical colloidal particles produces crystals with specific static structures and limited ability to reconfigure the structures. Tunable and dynamic colloidal assemblies can be achieved by taking advantage of DNA nanotechnology.45 By applying a toe-hold exchange or strand displacement hybridization reaction to DNA-coated colloids,66 Manoharan and coworkers were able to tune particle interactions and control temperature-dependent phase behavior.67 They described the phase behavior of DNA-coated colloids from zero-, one-, and two-displacement reactions, including wider gas−solid coexistence, re-entrant melting, and reversible transitions between distinct crystal phases. The reversible phase transitions between compositionally distinct phases were observed by combining a zero-displacement reaction with a two-displacement reaction, wherein CsCl crystals form at relatively low and high temperatures between different composites yet melt at intermediate temperatures (Figure 9). Unique crystal structures and phase transitions that may be inaccessible through direct nucleation from the fluid are achieved.
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SUMMARY/PROSPECTUS The engineering of colloidal architectures by design is a multifaceted process that enlists design enlightenment from molecular species. In this Account, we have discussed supramolecular techniques that led to the formation of nontrivial colloidal molecules, clusters, chains, and crystals through molecular recognition. The ability to simplify desired architectures into small molecular-geometry-mimicking building blocks has led to multiple approaches to control colloidal self-assembly based on metal coordination, hydrogen bonding, host−guest complexation, and DNA hybridization. Realization 2763
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Accounts of Chemical Research replication, and/or shape-memory features, where the emphasis should be on the precise control of colloidal molecules that feature distinct geometries, multiple different recognition motifs, and an intrinsic shape reconfigurability mechanism.
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orthogonal functionalization methods to synthesize polymers, organized assemblies, biomaterials, and nanostructures.
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ACKNOWLEDGMENTS We gratefully acknowledge the support and enthusiasm of current and former group members, collaborators, and colleagues. The work described herein was primarily supported by the MRSEC Program of the National Science Foundation (DMR-1420073). C.S. acknowledges support from a National Science Foundation Graduate Research Fellowship (DGE1342536). This work was supported in part by the Department of Energy under Grant Award No. DE-SC0007991 (ML).
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Elizabeth Elacqua: 0000-0002-1239-9560 Xiaolong Zheng: 0000-0002-7749-6265 Cicely Shillingford: 0000-0001-6443-9715 Marcus Weck: 0000-0002-6486-4268
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Author Contributions
REFERENCES
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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest. Biographies Elizabeth Elacqua obtained her Ph.D. in 2012 from the University of Iowa with Leonard R. MacGillivray. She then worked as a postdoctoral fellow with Marcus Weck at NYU, where her research was focused toward the directional self-assembly of polymeric and colloidal materials. In 2017 she joined the faculty at The Pennsylvania State University as an Assistant Professor in Chemistry. Her research interests are in supramolecular, organic, and polymer science, in particular focusing on responsive and directional self-assembly using both dynamic covalent and noncovalent chemistry. Xiaolong Zheng obtained his Ph.D. in 2017 from NYU and is currently a postdoctoral research associate with Marcus Weck, where his research focuses on directed self-assembly and crystallization of colloidal particles. He received his B.S. and M.S. in chemistry from Wuhan University in China, studying organic synthetic methodology and fluorescent probe synthesis. Cicely Shillingford received her B.S. in Biochemistry from the University of Waterloo and is currently pursuing her Ph.D. with Marcus Weck at NYU. She studies the manipulation of capillary forces and DNA-mediated interactions for directional assembly of colloidal superstructures. Her previous work includes loop modeling of engineered immunoglobulin-like domains with Elizabeth Meiering (Waterloo) and engineering of bioinspired materials with Joanna Aizenberg (Harvard). Mingzhu Liu holds a B.E. from the University of Science and Technology of China and is currently pursuing his Ph.D. supervised by Marcus Weck at NYU. His research focuses on directing the selfassembly of colloidal particles through tunable interactions such as DNA hybridization. His undergraduate research, guided by Professor Hangxun Xu, focused on developing nanomaterials and polymers. Marcus Weck obtained his Ph.D. in 1998 from Caltech with Robert H. Grubbs. After a postdoctoral stay at Harvard University with George M. Whitesides, he joined the faculty at Georgia Tech. In 2007 he moved to NYU, where he is a Professor in the Chemistry Department and the Associate Director of the Molecular Design Institute. His research interests are in organic and polymer chemistry as well as materials science. The main foci of his group are supported catalysis and the introduction of complexity through the use of 2764
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