Self-Assembly of Polymer Brush-Functionalized Inorganic

Perspective pubs.acs.org/JPCL

Self-Assembly of Polymer Brush-Functionalized Inorganic Nanoparticles: From Hairy Balls to Smart Molecular Mimics Matthew G. Moffitt* Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, British Columbia V8W 3V6, Canada ABSTRACT: New opportunities for complex and controllable self-organization of inorganic nanoparticles (e.g., quantum dots, plasmonic or magnetic nanoparticles) are provided when the stabilizing organic layer at the nanoparticle surface consists of a polymer brush of densely grafted chains. We highlight recent advances in the synthesis and self-assembly of these unique building blocks, termed polymer brush-functionalized nanoparticles (PBNPs). We show how the field has progressed from PBNPs with isotropic, single-component brushes showing limited self-assembly behavior to PBNPs with anisotropic brushes showing directional interactions and complex self-organization. We further discuss how PBNPs with isotropic multicomponent brushes, either mixed brushes or grafted block copolymers, can also exhibit the complex self-organization of molecular amphiphiles via rearrangements within the brush enabled by conformational flexibility. Through numerous examples, we show how established principles of polymer science and surface engineering encapsulated in these composite colloidal building blocks open up vast possibilities for functional self-assembled nanomaterials.

T

he spontaneous organization of molecular building blocks into complex structures, driven by a delicate balance of noncovalent attractive and repulsive intermolecular interactions, is the pathway to an incredible range function in living systemsfrom the self-cleaning power of the lotus leaf to the amazing complexities of the human brain. Inspired by these organizational processes in nature, chemists have recognized intriguing opportunities for a new generation of functional materials via the self-assembly of synthetic building blocks.1−6 As a result, the last 30 years or so have seen a proliferation of examples in which spontaneous ordering and structural complexity from an increasing range of molecules and particles has been demonstrated, including liquid crystals,7 artificial opals,8 self-assembled monolayers,9 and block copolymers.10,11

and semiconductor quantum dots (e.g., CdSe, CdS, PbSe, core/ shell CdSe/ZnS) with stable, size-tunable fluorescence at high quantum yield.15 Although the colloidal synthesis of various inorganic nanoparticles has been well-established, their controlled assembly into one-, two-, and three-dimensional (3D) superstructures remains an ongoing challenge,16,17 en route to applications ranging from medical therapeutics18,19 and diagnostics19−22 to photonics,23,24 photovoltaics,25,26 and computing.27−29 In general, this challenge can be met by functionalizing nanoparticle surfaces with a layer of organic ligands that modify interactions between neighboring nanoparticles or between nanoparticles and the surrounding medium. Unique opportunities for nanoparticle self-assembly are provided when the organic layer consists of a polymer brush of densely grafted chains anchored at one end to the nanoparticle surface and extending outward into the surrounding medium.30−32 The resulting “hairy” nanoparticles can be regarded as hybrid building blocks combining the optical, electronic, or magnetic properties of the core inorganic nanoparticle with the mechanical strength, flexibility, processability, and dielectric properties of the grafted polymer chains. In addition to enhancing material properties for specific nanocomposite applications,24,33,34 surface-grafted polymer brushes can also broaden the capabilities for nanoparticle self-assembly in a number of general ways: (1) by generating strong thermodynamic driving forces for self-assembly due to strongly favorable phase separation between grafted and ungrafted dissimilar polymeric components or between co-grafted dissimilar polymeric components, (2) by increasing the range of potential

Colloidal inorganic nanoparticles are particularly intriguing candidates as building blocks for self-assembly, with interesting optical, electronic, or magnetic properties associated with surface and quantum effects arising from their small size. Colloidal inorganic nanoparticles are particularly intriguing candidates as building blocks for self-assembly, with interesting optical, electronic, or magnetic properties associated with surface and quantum effects arising from their small size. These include gold (Au) and silver (Ag) nanoparticles with strong surface plasmon resonances in the visible range,12,13 metal (e.g., Fe, Co) and metal oxide (e.g., Fe2O3) magnetic nanoparticles,14 © 2013 American Chemical Society

Received: August 23, 2013 Accepted: October 14, 2013 Published: October 14, 2013 3654

dx.doi.org/10.1021/jz401814s | J. Phys. Chem. Lett. 2013, 4, 3654−3666

The Journal of Physical Chemistry Letters

Perspective

equilibrium structures due to the conformational flexibility of grafted chains, or (3) by allowing kinetic control of stable, nonequilibrium structures due to the relatively slow chain dynamics of overlapping polymer brush layers. The purpose of this Perspective is to describe and contextualize, from the author’s viewpoint, representative work on the self-assembly of polymer brush-functionalized inorganic nanoparticles (PBNPs). We will begin the discussion where the field began in the early 1990s,35−37 with PBNPs functionalized with isotropic, single-component brushes (type I PBNPs). Due to the generally isotropic and repulsive nature of particle−particle interactions for type I PBNPs, their self-assembly tends to form periodic nanoparticle arrays (Figure 1).38 However, we will also

Figure 2. Schematics showing type II PBNPs with single-component (A,C) or multicomponent (B,D) anisotropic brushes and either Janus (A,B) or patchy (C,D) surface patterning of chain tethers.

Figure 1. Periodic array formation of type I PBNPs. (A) Schematic showing type I PBNPs (isotropic, single-component polymer brush) and array formation via repulsive interactions at increased nanoparticle volume fraction. (B) Two-dimensional array of gold particles after treating a concentrated solution of PS(190)-b-P2VP(190) gold-loaded micelles with anhydrous hydrazine. Adapted from ref 53.

discuss how blending type I PBNPs with various additive components, including compatible and incompatible homopolymers and selectively compatible block copolymers, can produce a wide range of nonperiodic coassemblies with multiscale organization. Next, we will describe the more recent evolution to PBNPs exhibiting anisotropic interparticle interactions (attractive and repulsive) analogous to those of molecular amphiphiles such as block copolymers. The resulting “smart” self-assembly properties are evidenced by the morphological range and structural complexity of the resulting nanoparticle/ polymer assemblies.30,31,39 In this discussion, we distinguish between the two types of anisotropic PBNPs; type II PBNPs (Figure 2) are functionalized with anisotropic brushes, including both single-component (Figure 2A,C) and multicomponent brushes (Figure 2B,D), in which the distribution of chain tethers defines an anisotropic patterning of distinct chemical regions on the nanoparticle surface, either Janus (“two-faced”, Figure 2A,B) or patchy (Figure 2C,D). In such PBNPs, the anisotropy of the particle−particle interactions is encoded directly into the surface chemistry, such that type II PBNPs are inherently anisotropic. In contrast, type III PBNPs (Figure 3) are functionalized with isotropic, multicomponent brushes; in

Figure 3. Schematic showing type III PBNPs before (top) and after (bottom) spontaneous anisotropy generation. (A) Mixed brush PBNPs develop surface anisotropy via microphase separation of incompatible chains. (B) Block copolymer PBNPs develop surface anisotropy via extension and compaction of chains.

this case, the distribution of chain tethers defines an isotropic surface chemistry that does not directly encode anisotropic interactions. However, due to the chemical incompatibility and flexibility of the polymer components, microphase separation or conformational changes can induce anisotropic interactions and amphiphilic self-assembly if the global free energy is lowered overall.30,31,39 Therefore, type III PBNPs are contextually anisotropic. Although important work on the synthesis and self-assembly of colloidal polymer brushes has been carried out on polymeric 3655



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latexes40,41 and block copolymer micelles with polymeric cores,42−44 for the purpose of this Perspective, we will focus on the synthesis and self-assembly of PBNPs in which the cores are inorganic, especially metals, semiconductors, and metal oxides. In terms of the polymer brush materials, we will focus on PBNPs with shells consisting of synthetic macromolecules; however, we point out that inorganic nanoparticles decorated with DNA chains offer immense potential for organized functional superstructures via specific interactions between biopolymer brushes, and important work in this area has also been pursued.45,46 Due to the relative importance of conformational flexibility to self-assembly,31 especially in the context of type III PBNPs, grafted particles in which the end-to-end length of the chains is significantly greater than the diameter of the inorganic core (R0 ≫ dc), so-called “star-like” colloids, will be particularly emphasized throughout this discussion. As well, it is important to note that although the vast majority of related literature studies concern spherical polymer-grafted nanoparticles, in which anisotropic interactions are induced by chemical, rather than shape, anisotropy, a few key examples of nonspherical nanoparticles with both chemical and shape anisotropy will also be discussed.47−50 PBNPs with Isotropic Single-Component Brushes (Type I). The synthesis of PBNPs with isotropic, single-component brush layers of various polymers on a wide range of inorganic nanoparticles has been well-documented and is generally accomplished using one of three strategies.32 As shown in subsequent sections, type II and type III PBNPs can be synthesized using variations of one or more of these approaches.30 (1) In the block copolymer micelle template approach, a diblock copolymer first forms reverse micelles in an organic solvent (or spherical nanodomains in the solid state), in which the coreforming blocks selectively complex metal ions or other inorganic nanoparticle precursors. The inorganic nanoparticles are then synthesized within the micelle cores via a reduction or nanoprecipitation reaction, with the metal-complexing blocks forming a condensed layer at the nanoparticle surface, covalently attached to the external brush layer of corona-forming blocks. For example, Eisenberg’s group37 and ours51 have synthesized CdS quantum dots coated with a brush layer of polystyrene (PS) chains (graft density, σ = ∼1 chains/nm2) using a template of polystyrene-b-poly(acrylic acid) (PS-bPAA)-based reverse micelles. PS brush-functionalized gold nanoparticles have been prepared using a similar approach by Möller and co-workers from reverse micelles of polystyrene-bpoly(2-vinyl pyridine) (PS-b-P2VP) or polystyrene-b-poly(ethylene oxide) (PS-b-PEO).52−54 (2) In the grafting-to approach, presynthesized polymer chains with terminal functional groups are grafted onto preformed nanoparticles though ligand-exchange or used as initial ligands during nanoparticle growth. In typical examples, Emrick and co-workers applied a ligand-exchange approach to displace pyridine with thiolterminated PEO on the surface of CdSe/ZnS core−shell quantum dots,55 whereas the Lennox group used thiolterminated PS or PEO chains as ligands during the growth of gold nanoparticles to generate PBNPs.34,56 (3) Finally, in the grafting-from approach, polymer chains are polymerized from initiators grafted onto the surface of preformed nanoparticles. For example, Emrick and co-workers have applied nitroxidemediated living radical polymerization to generate either PS or polystyrene-r-poly(methyl methacrylate) (PS-r-PMMA) brush layers on CdSe quantum dots;57 surface-initiated polymerizations of PMMA on gold58 and PS and poly(3-vinyl pyridine)

(P3VP) on magnetite59 have also been demonstrated by other groups. Although the focus of this review is the controlled assembly of PBNPs into condensed superstructures, another current and important challenge in material science, and an additional motivation behind functionalizing inorganic nanoparticles with polymer brushes, is not to assemble nanoparticles but to keep them apart, specifically, to allow inorganic nanoparticles to be dispersed at low volume fraction in a homopolymer matrix.32,34,56,60,61 For a host of potential applications, the idea of imparting a strong, flexible, and processable polymer material with the unique properties of individual nanoparticles (e.g., fluorescence, surface plasmon resonance, third-order nonlinearities, superparamagnetism) is extremely appealing, and in these cases, the goal becomes achieving a solid dispersion of well-separated nanoparticles without aggregation or phase separation between the chemically dissimilar inorganic and polymeric components. PBNPs offer an intriguing solution to achieving such thermodynamically stable nanoparticle/polymer composites, with the hairy polymer layers providing compatibility with the surrounding homopolymer (usually of identical composition to the brush chains).32 As in liquid colloidal dispersions, repulsive steric forces between particles require that the brushes are “wet” with the surrounding solvent (i.e., the homopolymer chains); otherwise, particle aggregation via autophobic phase separation between PBNPs and homopolymer is expected.32,34,56,60,61 Along with enthalpically neutral mixing (χ = 0 between matrix and brush chains), brush wetting requires a sufficiently low entropic penalty for intercolation of matrix chains into the sterically crowded environment of the brush. Due to this entropic factor, PBNP dispersion is generally promoted by brush grafting densities that are not too high and brush chains that are relatively long compared to the matrix chains. An example of the sensitivity of particle dispersion to brush structure comes from Lennox and co-workers, who showed that PS-functionalized gold nanoparticles with long PS chains (degree of polymerization, N = 125: PS125-Au) and lower graft density (σ = 0.94 chains/nm2) remained well-dispersed in a matrix of relatively long PS760 chains, even with annealing at 145 °C for up to 18 h; however, PS19-Au, with much shorter brush chains and higher graft density (σ = 3.45 chains/nm2), could only be dispersed in very short PS20 chains but phase separated from matrixes of PS440 and PS760 chains.56 When dispersed in a good solvent for the polymer brush (including homopolymer chains in the wet brush case of nanoparticle/polymer composites discussed above), the particle− particle interactions between type I PBNPs are generally repulsive and isotropic due to excluded volume effects associated with the interpenetrating brushes.38,62,63 The resulting steric interparticle potential functions show features in between the “hard”, short-range, steric potentials of uncharged and nongrafted particles and the “soft”, long-range, electrostatic potentials of charged particles. Similar to such hard and soft colloids, the self-assembly of dispersed type I PBNPs can be induced by an increase in particle volume fraction, leading to long-range ordering of particles (Figure 1A). In three dimensions, this disorder-to-order transition corresponds to colloidal crystal formation above a critical volume fraction of nanoparticles. Ohno et al. investigated the ordering of PMMAsilica particles (PMMA-SiO2, σ = 0.59−0.71 chains/nm2) in mixed organic solvents and found that the particle volume fraction of colloidal crystal formation (ϕf = 0.129−0.0392) 3656



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decreased with increasing height of the polymer brush, which was experimentally varied by changing both the grafting density and chain length.38,62 Confocal laser scanning microscopy (CLSM) of the resulting 3D colloidal crystals demonstrated a strong correlation between grafted chain length and the spacing between SiO2 cores, as well as an influence on the array structure, with mixtures of face-centered cubic (fcc) and hexagonal close-packed (hcc) lattices, showing increased fcc fractions with increasing PMMA chain length.62 Importantly for potential materials applications, Goel et al. have shown that such 3D ordering can occur for highly grafted PBNPs even in the absence of dispersing solvents or homopolymer diluents.63 Using smallangle X-ray scattering (SAXS), they measured a weak but discernible higher-order scattering peak from poly(n-butyl acrylate)grafted SiO2 nanoparticles (PBA-SiO2, σ ≈ 0.8 chains/nm2) in the melt state, which persisted after repeated solvent and thermal annealing, suggesting fcc arrays of thermodynamic origin. The same repulsive particle−particle interactions described above, but modulated by particle−substrate interactions and capillary forces between particles during solvent evaporation, can also be used to generate two-dimensional (2D) ordered arrays of PBNPs in thin polymeric films.53,64−66 Möller and coworkers dip-coated various substrates, including glass, mica, and carbon-coated transmission electron microscope (TEM) grids with toluene dispersions of PS190-Au PBNPs, fabricated from gold-loaded reverse micelles. Applying a controlled withdrawal speed (∼10 mm/min), a well-ordered 2D hexagonal array of ∼10 nm gold nanoparticles with ∼35 nm interparticle spacing extending over an area of several square centimeters was obtained (Figure 1B).53 The Ritcey group has similarly applied drop-coating chloroform dispersions of PS-Au (∼10 nm gold cores) with variable grafted chain lengths (N = 36 − 635) onto glass or carbon-coated TEM grids, generating monolayers consisting of short-range-ordered 2D hexagonal arrays of gold nanoparticles.64 Furthermore, the authors demonstrated the influence of a self-assembled structure on the monolayer optical properties, showing an increased red shift in the gold surface plasmon resonance with decreased polymer chain length, attributed to closer proximy and thus stronger dipolar coupling between nanoparticles within the 2D arrays.64,65 Finally, Ohno et al. have produced highly ordered hexagonal 2D arrays of gold nanoparticles with variable interparticle spacings by spreading PMMA-Au with different chain lengths (N = 120−620, σ ≈ 0.8 chains/nm2) from benzene onto the air−water interface of a Langmuir trough, followed by compression and Langmuir− Blodgett (LB) transfer to carbon-coated TEM grids.66

The isotropic and repulsive interactions between type I PBNPs encode their self-assembly into ordered arrays with individual inorganic nanoparticles at each lattice point, as shown above for both 3D and 2D cases. However, it is possible to generate a range of other, nonperiodic superstructures from such grafted nanoparticles by adding coassembling, often polymeric, components that template or direct the self-assembly process (Figure 4). The role of these added components during self-assembly is generally to develop phases (Figure 4A−C) or interfaces (Figure 4C,D) to which the PBNPs are selectively attracted via their polymeric brushes, thus directing their spatial distribution. For example, in our group, we have shown that the interfacial self-assembly of PS-b-PEO copolymers at the air− water interface directs the organization of PS-CdS nanoparticles into hierarchically structured wires, cables, and rings with mesoscale external dimensions and internal nanoscale dispersions of quantum dots (Figure 4A).67,68 The resulting stable kinetic structures could be LB transferred to glass, TEM grids, or microcontact-printed substrates69 for further organization and were explained by synergistic self-assembly of the two components, driven by PEO-regulated dewetting of PS from the water surface. In other work, we have directed the self-assembly of PS-CdS into isotropic islands or directional stripes via spincoating-induced phase separation from a PMMA homopolymer on bare70 or patterned71 glass substrates (Figure 4B). In the colloid state, we have produced kinetically tunable mesoscale spherical aggregates with complex internal structure via dropwise water addition to PS-CdS dispersions with codissolved PSb-PAA stabilizing chains.72 Following a hierarchical selfassembly strategy, the ∼100−200 nm spheres from the latter study were further self-assembled into weakly ordered closepacked arrays, which combined the photoluminescence of the nanoscale quantum dots with a photonic stopband associated with the mesoscale periodicity generated through a series of sequential self-assembly steps.73 Various other studies have also explored the use of additive components to direct the self-assembly of type I PBNPs. In particular, the microphase separation of block copolymers in the solid state and concomitant selective localization of polymergrafted nanoparticles and nanorods into one of the microdomains, or at the polymer/polymer interface, is an intriguing strategy for forming functional nanocomposites with complex hierarchical organization. For example, Kramer and co-workers have shown that the grafting density of PS-Au provides a handle on steering the specific localization of the PBNPs within the lamellar domains of microphase separated PS-b-P2VP, with higher grafting density (σ > 1.6 chains/nm2) directing nanoparticle localization to the center of the PS phase and lower grafting density (σ < 1.3 chains/nm2) directing their localization to the PS/P2VP interface (Figure 4C).74 The same authors have also shown that by stabilizing the PS/P2VP interface, PS-Au with sufficiently low grafting density can trigger a change from lamellar to bicontinuous block copolymer morphologies above a critical particle volume fraction.75 The latter study demonstrates an interesting case of synergistic selfassembly, in which co-organization strongly influences the spatial distribution of both the PBNPs and the “directing” component. A different example of directed PBNP self-assembly is the spontaneous migration of PEO-CdSe/ZnS quantum dots dispersed in a PMMA film to cracks within an adjoining silica layer (Figure 4D).55 In this case, the localization of PBNPs provides an appealing strategy for self-healing multilayer structures and is driven by a combination of surface energy lowering

The isotropic and repulsive interactions between type I PBNPs encode their self-assembly into ordered arrays. However, it is possible to generate a range of other, nonperiodic superstructures from such grafted nanoparticles by adding coassembling, often polymeric, components that template or direct the self-assembly process. 3657



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Figure 4. Examples of directed self-assembly of Ttype I PBNPs with various coassembling polymer components. (A) Schematic of coassembly of PS-CdS and PS-b-PEO at the air−water interface and representative AFM and TEM images of the resulting hierarchical assemblies. Adapted from ref 67. (B) Schematic showing PS-CdS patterning by spin-casting PS-CdS/PMMA blends from toluene solutions followed by selective etching of the PMMA domains; representative fluorescence image of patterned features from the PS-CdS/PMMA (w/w) = 10/90 blend spin-cast at 3000 rpm. Adapted from ref 70. (C) Cross-sectional TEM images of a PS-b-P2VP block copolymer containing PS-Au, whose surfaces are covered with areal densities of PS chains (Mn = 3.4 kg/mol) of 1.64 (left) and 0.83 (right) chains/nm2. Adapted from ref 74. (D) Fluorescence microscope image of a crack in a SiOx layer on PMMA containing PEO-CdSe/ZnS PBNPs. Reprinted with permission from ref 55. Copyright 2006, Nature Publishing Group, U.K.

autophobic phase separation between brush and homopolymer chains in the limit of high grafting densities referred to earlier.56 In this case, self-assembly is explained by microphase separation between the ungrafted inorganic surfaces and the polymer phase, enabled at low grafting densities by the relatively low entropic penalty for grafted chains to distort as particles come into contact. In this way, these anisotropic interactions between PBNPs are amphiphilic in nature, arising from a balance of attractive and repulsive forces between spatially and chemically distinct regions on each particle. In a Perspective published earlier this year, Kumar and coauthors proposed a grafting threshold of σN1/2 ≈ 2 for good particle dispersion, below which anisotropic self-assembly predominates;61 we point out that for polymer chain lengths ranging from N = 100 to 1000, this threshold corresponds to grafting densities from 0.06 to 0.2 chains/nm2, which is well below graft densities for all other type I PBNP cases described above (typically σ ≈ 1 chain/nm2). Although this behavior of type 1 PBNPs is still poorly understood and the range of associated assemblies is as yet limited, the work of Akcora underlines the potential to generate complex structural hierarchies from particles that mimic the self-assembly behavior of molecular amphiphiles. We now turn to recent efforts to specifically engineer PBNPs for amphiphilic self-assembly via a range of strategies that offer fine chemical and spatial

(PEO has a lower surface energy than PMMA) and the entropic exclusion of polymer-grafted nanoparticles from the PMMA matrix. The discussion thus far has highlighted the repulsive and isotropic forces (in the absence of directing components) between type I PBNPs dispersed in good solvent environments. However, we now consider two exceptional examples of selfassembly arising from anisotropic and attractive interactions between hairy nanoparticles functionalized with isotropic, single-component brushes. In the first example from the Pyun lab, anisotropic dipolar attractive interactions between ferromagnetic cobalt nanoparticle cores of PS-Co PBNPs overcame the isotropic steric repulsion of the surrounding PS brushes in organic solvents (tetrahydrofuran (THF), methylene chloride, toluene), leading to self-assembly into long, highly anisotropic nanowires.76 The PS-stabilized nanowires could then be oxidized by bubbling the colloidal dispersions with oxygen to form electroactive cobalt oxide nanowires of potential interest as nanoscale electrodes for energy storage applications.77 The second example is the recent discovery by Akcora et al. of anisotropic self-assembly of sparsely grafted PS-SiO2 particles in a PS homopolymer matrix to form strings, sheets, and platelet-like objects.78 These results, qualitatively supported by Monte Carlo simulations, are fundamentally different from the 3658



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anisotropic brushes, chemical anisotropy is the result of partial functionalization of the nanoparticle surface with the polymer chains, with the brush distributed nonuniformly over the surface in either a Janus (Figure 2A) or patchy (Figure 2C) manner; here, the chemical contrast driving self-assembly is between polymer-grafted and ungrafted regions (generally passivated with low-molecular-weight ligands) on the nanoparticle surfaces. In cases of anisotropic binary brushes, on the other hand, the two chain types are segregated into distinct grafted regions over the surface in either a Janus (Figure 2B) or patchy (Figure 2D) arrangement, and self-assembly is driven by chemical contrast between the incompatible polymer regions. The inherent shape and surface energy anisotropies of nonspherical nanoparticles provide excellent opportunities for producing selectively grafted PBNPs with strongly directional interparticle interactions. Kumacheva and co-workers have produced facinating “pom-pom” building blocks consisting of gold nanorods with hydrophilic cetyl trimethylammonium bromide (CTAB) ligands passivating the longitudinal sides of the nanorods and brushes of ∼13 PS chains grafted to each end (Figure 5A).47 The synthesis of the anisotropic PBNPs applied a grafting-to approach, while taking advantage of the preferential binding affinity of CTAB to the (100) gold faces of the longitudinal faces followed by grafting the unpassivated (111) ends with thiol-terminated PS brush “crowns”. The chemical

manipulation of nanoparticle surfaces, enabling exquisite control of particle interactions and assembled structures. PBNPs with Anisotropic Brushes (Type II). Theoretical studies by Glotzer and others have shown that by designing nanoparticle surfaces with distinct and spatially separated chemical regions (either patchy or Janus particles), colloidal building blocks can be produced with the essential chemical anisotropy of molecular amphiphiles, enabling their spontaneous selfassembly into complex three-dimensional ensembles in the manner of surfactants, phospholipids, and block copolymers.79−81 Moreover, if one or more of the distinct chemical regions is a grafted polymer brush, then the block copolymer analogy becomes particularly apt because the resulting colloids will possess the combined features of chemical anisotropy and conformational flexibility that are germane to the diverse and complex self-assembly characteristics of those macromolecular amphiphiles.10,11,31 Producing PBNPs with anisotropically grafted surfaces requires special consideration for breaking mirror symmetry in the case of a Janus brush structure or breaking spherical symmetry in the case of a patchy brush structure. In this section, we consider the various synthetic strategies for symmetry breaking applied to PBNPs, as well as the opportunities for self-assembly arising from the resulting directional interparticle interactions (Figure 5). In cases of PBNPs functionalized with single-component

Figure 5. Various examples of self-assembly of type II PBNPs. (A) Self-assembly of polymer-tethered gold nanorods and SEM images of assemblies in various selective solvent mixtures. Reprinted with permission from ref 47. Copyright 2007, Nature Publishing Group, U.K. (B) Schematic showing eccentric AuNP@polymer and TEM images of ecc-[AuNP@polymer] before and after incubation with basic NaCl solution. Adapted from ref 83. (C) Schematic representation of self-assembly of CdSe/CdS with bound hydrophilic polymer chains and TEM images of the structures formed from different polymer/nanoparticle ratios. Adapted with permission from refs 82 and 94. Copyright 2009, John Wiley & Sons, Ltd., or related companies. All rights reserved. (D) Schematic illustration of synthesis of polymer-functionalized Janus AuNPs combining “solid-state grafting-to” and “graftingfrom” methods. Reprinted from ref 86. TEM micrographs of J-AuNPs and S-AuNPs in dioxane after 120 and 180 min incubation times, respectively. Reprinted from ref 87. 3659



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functionalized silica. Dynamic light scattering (DLS) and cryoTEM showed that the resulting Janus PBNPs underwent reversible amphiphilic clustering at low pH values due to aggregation of the relatively hydrophobic uncharged PAA faces with cluster stabilization by the hydrophilic chains of the PSSNa or PDMAEMA brushes. On the other hand, at intermediate pH values, the PDMAEMA Janus PBNPs exhibited macroscopic precipitation due to uncontrolled aggregation of electric dipoles from negatively charged PAA and positively charged PDMAEMA faces. Another elegant surface masking approach using a solid substrate was applied by the Li group, but this time to produce Janus PBNPs with bicompartment brushes consisting of hydrophobic and hydrophilic chains grafted to opposite faces of ∼6 nm gold nanoparticles (Figure 5D).85−87 First, the nanoparticles were immobilized on a substrate of single-crystal thiol (SH)-terminated PEO via bonding of multiple exposed thiol groups to each gold nanoparticle.85 Then, grafting-from polymerization was used to decorate the exposed gold faces with brushes of PMMA chains.86 Finally, the PEO substrate was dissolved to release amphiphilic PBNPs with both hydrophobic (PMMA) and hydrophilic (PEO) brush faces. Due to low electron densities of both PMMA and PEO chains compared to the that of the gold, it was impossible to directly observed the asymmetric brush structure by TEM; however, by using an identical procedure to obtain PBNPs with PAA (instead of PMMA) and PEO faces, followed by selective decoration of the PAA chains with platinum (Pt) nanoparticles, the Janus brush structure was confirmed from TEM images showing Pt nanoparticles to occupy only part of the gold nanoparticle surface. The same authors subsequently investigated the solution selfassembly of the Janus PBNPs with PMMA and PEO faces, noting reversible clustering to form branched wormlike aggregates in dioxane (a selective solvent for PMMA) by a proposed mechanism of amphiphilic self-assembly.87 The importance of the Janus structure to the observed self-assembly behavior in dioxane was also demonstrated using a control sample of gold nanoparticles grafted with an isotropic mixed brush of PEO and PMMA chains. In contrast to the aggregation behavior of the Janus particles (J-AuNPs), the corresponding symmetric mixed brush PBNPs (S-AuNPs) did not aggregate but instead remained well dispersed in dioxane.87 PBNPs with Isotropic Multicomponent Brushes (Type III). Even as interest in amphiphilic nanoparticle self-assembly continues to increase, examples of Janus PBNPs functionalized with bicompartment brushes such as those described in refs 86 and 87 are extremely rare due in part to the stated challenges of anisotropic functionalization and characterization of compartmentalized brushes. In addition, a growing number of literature examples suggest that Janus or other anisotropic functionalization is not a requirement for amphiphilic self-assembly of PBNPs; these studies provide compelling evidence for highly directional and attractive interactions between nanoparticles decorated with isotropic polymer brushes containing two or more incompatible polymeric components.50,88−91 Their interesting self-assembly properties, evidenced by a wealth of structurally complex assemblies along with the relative simplicity of isotropic (compared to anisotropic) functionalization, make this class of PBNPs particularly intriguing. The amphiphilic self-assembly of type III PBNPs can be explained by the spontaneous development of particle anisotropies via microphase separation of incompatible brush polymers (Figure 3A) or via stretching and compacting of polymer chains

anisotropy of the resulting pom-pom nanoparticles was analogous to an ABA triblock copolymer with a hydrophilic center block flanked by two hydrophobic end blocks. Self-assembly was triggered by water addition to dispersions of the nanorods in various polar organic solvents, which induced directional aggregation of the PS brushes to form a range of assemblies (e.g., rings, nanochains, and bundles) depending on the specific solvent mixture. Significant blue shifts (up to 200 nm) in the longitudinal surface plasmon peak with increasing water content, attributed to increased distances and decreased electrodynamic coupling between nanorods, were also described. In follow-up work, the association modes of the nanorod pompom assemblies were further controlled by variation of the molecular weight of grafted chains, allowing determination of a phase diagram for self-assembly of the metal−polymer colloids.48 Furthermore, by monitoring the statistics and kinetics of growth of self-assembled chains of nanorods, the authors noted intriguing analogies, including molecular weight and branching control, with step-growth polymerization of molecular monomers.49 These results nicely highlight the potential for applying amphiphilic principles to tuning both the structure and ensemble properties (e.g., optical, electronic, magnetic) of self-assembled nanomaterials. For producing spherical PBNPs with anisotropic brushes, spherical symmetry can be broken through surface segregation processes via migration of ligands on the nanoparticle surface.82,83 For example, Chen et al. applied block-copolymermediated competitive binding and surface phase segregation of hydrophobic and hydrophilic ligands to produce eccentrically encapsulated gold nanoparticles (Figure 5B).83 The resulting Janus PBNPs possessed both a block-copolymer-encapsulated surface surrounded by a PAA polymer brush and an “exposed” surface passivated with nonpolymeric hydrophilic ligands. Saltinduced aggregation of the resulting Janus PBNPs in water was observed with a predominance of dimer formation. In other work, Förster and co-workers demonstrated amphiphilic selfassembly of spherical CdSe/CdS core/shell nanoparticles decorated with PEO brushes via relatively weak attachment with terminal amine groups (Figure 5C).82 In this case, it was proposed that surface reorganization led to PEO ligand depletion in between adjacent nanoparticles, effectively forming patchy or Janus brushes (depending on the nanoparticle environment), with exposed hydrophobic regions driving anisotropic attractive interactions. The PBNPs were modeled as amphiphiles with rigid cores constituting the hydrophobic part and the packing parameter increasing as the density of PEO chains decreased. Using such packing arguments based on molecular amphiphiles, observed morphology changes of selfassembled structures from single particles, to strings, to monolayer vesicles with a decreasing polymer/nanoparticle ratio were explained. Another strategy for producing anisotropic PBNPs begins with binding nanoparticles to a solid−liquid interface, which breaks their mirror symmetry and opens routes to Janus brushes on the nanoparticle surfaces.84−87 In one such study, Hatton and co-workers used ∼700 nm positively charged surface-treated silica particles as colloidal substrates for electrostatic binding of ∼10 nm PAA-coated magnetite nanoparticles.84 The exposed PAA surfaces were then selectively functionalized with either polystyrene sodium sulfonate (PSSNa) or polydimethylamino ethylmethacrylate (PDMAEMA) chains using the grafting-to method, followed by nanoparticle release via pH-triggered charge reversal of the 3660



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self-assembly for the mixed brush PBNPs in dioxane (ref 87) suggests that the entropic penalty of chains wrapping around the core (Figure 3A) was not sufficiently balanced by favorable free-energy terms, in contrast to similar PBNPs in water (ref 50). Although both PBNPs are functionalized with a mixed brush of PMMA and PEO chains, the weight ratio of PEO to PMMA is not given in ref 87, and the graft densities and core sizes are somewhat different in the two studies so that only a qualitative comparison of these systems can be made. However, we note that despite possible small structural differences in the PBNP building blocks, the difference in solvent should play a much stronger role in the self-assembly of these two systems. Compared to PMMA/PEO-Au in dioxane,87 the aqueous system studied by Song et al.50 contributed a greater solubility contrast between blocks, in addition to the contribution of the hydrophobic effect in water, leading to significantly stronger driving forces for self-assembly. By comparing these two studies, therefore, it can be concluded that a compartmentalized binary brush structure can help promote nanoparticle selfassembly in nonaqueous selective solvents,87 where the entropic and enthalpic attractive forces between particles are relatively weak; however, in aqueous systems, where favorable free-energy terms for particle aggregation are large compared to the entropic penalty of surface chain reorganization, a much simpler isotropic binary brush structure can be sufficient to drive nanoparticle self-assembly into complex hierarchical structures, as evidenced by ref 50 and the following examples. Amphiphilic self-assembly of PBNPs functionalized by mixed binary brushes in aqueous media was also demonstrated by Zubarev and co-workers, who used a grafting-to strategy to attach V-shaped ligands, each containing one PEO arm and one PS arm, to the surfaces of gold and silver nanoparticles (Figure 6B).88 This method provides functionalization with an exact 50:50 mix of hydrophilic and hydrophobic chains, although it does not allow the compositional control of attaching the two chain types as separate grafts, either sequentially or simultaneously. The mixed brush PS/PEO-Au PBNPs were self-assembled by dispersion in either THF or dimethylformamide (DMF), followed by dropwise water addition to 75% (v/v) water, and finally dialysis against water to remove residual THF or DMF. The resulting assemblies were strongly reminiscent of amphiphilic block copolymer micelle morphologies, including rods, long wormlike aggregates, and vesicles, depending on the initial solvent and nanoparticle concentration, but with arrays of gold or silver nanoparticles residing at the PS core/PEO corona interfaces. On the basis of the assembly geometry in all of these structures, it was proposed that as the water concentration increases, the initially isotropic brushes reorganize via chain wrapping to form a Janus surface topology (Figure 3A, bottom right) with subsequent aggregation of the hydrophobic PS faces; following similar microphase separation principles as amphiphilic diblock copolymers in solution, the morphologies of such PBNP assemblies represent the lowest free-energy balance of interfacial tension and entropy loss due to stretching of hydrophobic and hydrophilic chains. In our group, we have extended the block copolymer paradigm for nanoparticle self-assembly to the solution properties of ionic block copolymers, by generating CdS nanoparticles functionalized by dense mixed brushes of hydrophobic PS and hydrophilic and partially ionized PMAA chains (Figure 6C).89 We applied a similar block copolymer micelle template approach to that used in earlier work to produce PS-CdS,51 except

(Figure 3B), in both cases leading to directional attractive interactions in selective solvents and aggregation into thermodynamically favored structures. This feature is a direct result of the chemical incompatibility of the polymeric components and their conformational flexibility, along with the relatively small size of the core compared to the chain lengths, which allows chains to wrap around or stretch past nanoparticle cores to generate patterned surface topologies of segment-enriched regions. Despite their isotropically grafted chains, therefore, these mechanisms of surface reorganization allow type III PBNPs to behave as true colloidal analogues of block copolymers, with spatially separated surface regions that are both chemically incompatible and conformationally flexible.

The amphiphilic self-assembly of type III PBNPs can be explained by the spontaneous development of particle anisotropies via microphase separation of incompatible brush polymers or via stretching and compacting of polymer chains. In one category of type III PBNPs, nanoparticle surfaces are decorated with binary brushes with intimately mixed graft junctions of two different chain types (“mixed brushes”, Figures 3A and 6).50,88,89 For example,50 Song et al. applied sequential grafting-to/grafting-from reactions to gold nanoparticle surfaces in which a mixture of hydrophilic PEO chains and ATRP initiator groups was first attached via Au−S bonds in a statistical ligand-exchange step, followed by growth of hydrophobic chains, either PMMA or copolymers containing various ratios of methyl methacrylate (MMA) and 4-vinyl pyridine (4VP) repeat units (designated PMMAVP), from the grafted initiator groups (Figure 6A). When dispersed in water, the resulting PBNPs with mixed binary brushes self-assembled to form vesicles with walls consisting of a densely packed monolayer of gold nanoparticles. This result suggests that the initially isotropic PBNP surfaces reorganize by chain wrapping to generate a patchy topology (Figure 3A, bottom left) followed by aggregation of the hydrophobic patches (PMMA or PMMAVP) to form the vesicle walls. It was also shown, in the PMMAVP case, that the assemblies underwent spontaneous dissociation into nanoparticles when the pH was tuned from 7.0 to 5.0 due to positive charging of 4VP in the vesicle walls, opening up possibilities for pH-triggered drug delivery. Finally, the authors demonstrated that gold nanorods functionalized with mixed PMMA/PEO brushes also self-assembled into monolayer vesicles in water; of interest for photothermal therapies, these vesicles could be disrupted by irradiation with near-infrared light due to localized heating associated with excitation of the nanorod longitudinal surface plasmon resonance. It is instructive to compare the mixed brush PMMA/PEO-Au nanoparticle building blocks in ref 50, which self-assembled into vesicles in water, with the compositionally similar mixed brush PMMA/PEO-Au control sample in ref 87 (vida supra) that remained individually dispersed in dioxane (while the analogous Janus brush structure self-assembled into wormlike aggregates in the same solvent). The absence of spontaneous 3661



Perspective

Figure 6. Various examples of self-assembly of type III PBNPs with mixed polymer brushes. (A) Schematic of amphiphilic nanoparticle self-assembly and TEM (left) and SEM (right) images of monolayer vesicles assembled from gold nanocrystals with mixed PEO and PMMA brushes. Reprinted from ref 50. (B) Schematic representation of the amphiphilicity-driven self-assembly of Au-(PS-PEO)n PBNPs with V-shaped ligands and TEM image of assemblies prepared from an aqueous solution of Au-(PS-PEO)n after dialysis of a THF/H2O solution against DI water. Reprinted from ref 88. (C) Schematics and accompanying TEM images illustrating self-assembly of CdS nanoparticles with mixed brushes of hydrophobic (PS) and partially charged (PMAA) chains above the critical water concentration (cwc). Images include segmented wormlike assemblies (with image of the microtomed section showing internal distribution of CdS nanoparticles) formed without added NaCl and bilayer vesicles and spheres formed with different degrees of charge screening (different quantities of added NaCl). Reprinted from ref 89.

with a PMMA-b-PAA-b-PS triblock copolymer as the starting material.92,93 The copolymer was dissolved in organic solvent followed by addition of cadium acetate, which neutralized the PAA blocks and generated micelles with poly(cadium acrylate) (PACd) cores and a mixed brush corona of PMMA and PS chains (intimate mixing of PS and PMMA within the corona was confirmed by 1H-NOESY).92 Exposure of the micelles to hydrogen sulfide (H2S), followed by covalent cross-linking of the PAA core layer, and finally hydrolysis of PMMA coronal chains to PMAA generated the amphiphilic mixed brush PBNPs, PS/PMAA-CdS.89 Because a portion of the MAA groups will be negatively charged in water, the resulting PBNPs were shown to possess many of the features of self-assembling ionic block copolymers in aqueous environments, characterized by superstrong segregation between nonionic and ionic chains, conformational flexibility of both chemical regions, and saltand pH-tunable morphologies. Self-assembly of PS/PMAACdS was initiated by water addition to PBNPs dispersed in THF, which triggered chain reorganization from an isotropic mixed brush into a Janus topology. Above a critical water content, Janus-organized PS/PMAA-CdS self-assembled by aggregation of PS faces and either solubilization or microprecipitation of PMAA faces, depending on the pH and ionic

strength. Electrostatic repulsion between partially charged PMAA chains forced nonequilibrium pathways to variable kinetic structures with internal lamellar organization of nanoparticles, including segmented wormlike assemblies. Decreasing electrostatic interactions through salt or acid addition was shown to promote tunable equilibrium self-assembly into either supermicelles or bilayer vesicles of nanoparticles, depending on the pH and ionic strength. Finally, we consider a second category of type III PBNPs recently pioneered by the Nie group, in which the hydrophobic and hydrophilic polymer components are covalently linked blocks of a copolymer tethered at one end to the nanoparticle surface (Figures 3B and 7).90,91 In this case, spontaneous anisotropy generation and directional interactions do not require chains to wrap around the nanoparticle core; rather, stretching and compacting of tethered hydrophobic blocks along a single axis give rise to a linear conformation reminiscent of an ABA triblock copolymer consisting of a middle hydrophobic section (with the nanoparticle at its center) and hydrophilic end sections (Figure 3B, bottom). The Nie group used a ligand-exchange approach to graft PEO-b-PS or poly(2(2-methoxyethoxy)ethyl methacrylate)-b-polystyrene (PMEO2MA-b-PS) copolymers onto 14 nm gold nanoparticles via a terminal SH group on 3662



Perspective

Figure 7. (A) Amphiphilic self-assembly of type III PBNPs with tethered block copolymer chains into monolayer vesicles or tubules and representative TEM images. Reprinted from ref 90. (B) Mice treated with and without plasmonic PBNP vesicles under laser irradiation and tumor growth curves of different groups of mice after treatment. Reprinted from ref 91.

separation of the polymer components requires chains to wrap around the inorganic core. This underlines the different mechanisms of self-assembly in these two cases but also highlights the relative importance of particle deformability to anisotropic interactions in different type III PBNPs. Furthermore, the optical and photothermal properties and resulting bioimaging and therapeutic applications of assemblies of self-assembled gold nanoparticles were also investigated by the Nie group.91 On the basis of the effects of near-field plasmon coupling between nanoparticles, the ensemble absorbance spectra could be tuned by varying the sizes of the gold cores and PS chains, which changed both the distances between gold cores and the morphologies of the assemblies. For example, assemblies of relatively large gold cores and long PS blocks yielded two plasmonic peaks in the visible and NIR spectral range, respectively, attributed to plasmon hybridization associated with the hollow gold shell structure of the vesicle morphology. In proof-of-concept bioimaging studies, multiphoton-absorption-induced luminescence (MAIL) imaging with 800 nm excitation was applied to cancer cells loaded with various PBNP assemblies; the contrast intensity was shown to increase with increasing PS block length due to stronger plasmonic coupling associated with higher aggregation numbers of the resulting assembled colloids. Finally, vesicles of gold nanoparticles were injected into 4T1 tumors of mice

the PS blocks to form block copolymer brush layers with a grafting density of ∼0.1 chains/nm2.90 Self-assembly was triggered by film rehydration (used commonly for forming phospholipid or polymeric vesicles), forming vesicles or tubules with walls composed of a monolayer of hexagonally packed gold nanoparticles (Figure 7A). In a recent follow-up study,91 the same authors studied the effect of the relative sizes of the hydrophobic PS blocks and the gold nanoparticle cores on the self-assembled morphologies by initiating self-assembly closer to equilibrium using the method of dropwise water addition to THF dispersions of copolymerfunctionalized nanoparticle building blocks. From the resulting phase diagram, it was shown that as the PS chain length decreases relative to the gold nanoparticle diameter, the morphologies change from vesicles, to small clusters, to unimer micelles (no self-assembly). These transitions were explained based upon changes in the deformability of the nanoparticle surfaces. If the flexible PS chains are too short relative to the rigid core size, then the PS chains cannot stretch and compact sufficiently to generate the anisotropic conformation, and no self-assembly occurs. Upon the basis of the phase diagram in ref 91, the boundary between clusters and unimer micelles indicates that self-assembly in this system requires R0/dc > ∼0.4. This requirement is less stringent that that proposed by Zubarov for mixed brush systems (R0/dc > π/2 = 1.6) in which phase 3663



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(4) Yu, D.; Dai, L. Self-Assembled Graphene/Carbon Nanotube Hybrid Films for Supercapacitors. J. Phys. Chem. Lett. 2010, 1, 467− 470. (5) Mondal, J.; Yethiraj, A. Driving Force for the Association of Amphiphilic Molecules. J. Phys. Chem. Lett. 2011, 2, 2391−2395. (6) Discher, B. M.; Won, Y.-Y.; Ege, D. S.; Lee, J. C.-M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Polymersomes: Tough Vesicles Made from Diblock Copolymers. Science 1999, 284, 1143−1146. (7) Fong, C.; Le, T.; Drummond, C. J. Lyotropic Liquid Crystal Engineering-Ordered Nanostructured Small Molecule Amphiphile Self-Assembly Materials by Design. Chem. Soc. Rev. 2012, 41, 1297− 1322. (8) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; van Driel, H. M. Large-Scale Synthesis of a Silicon Photonic Crystal with a Complete Three-Dimensional Bandgap near 1.5 Micrometers. Nature 2000, 405, 437−440. (9) Xia, Y.; Whitesides, G. Soft Lithography. Annu. Rev. Mater. Sci. 1998, 28, 153−184. (10) Zhang, L.; Eisenberg, A. Multiple Morphologies of “Crew-Cut” Aggregates of Polystyrene-b-poly(acrylic acid) Block Copolymers. Science 1995, 268, 1728−1731. (11) Jain, S.; Bates, F. S. On the Origins of Morphological Complexity in Block Copolymer Surfactants. Science 2003, 300, 460−464. (12) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine. Acc. Chem. Res. 2008, 41, 1578−1586. (13) Halas, N.; Lal, S.; Chang, W.; Link, S.; Nordlander, P. Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev. 2011, 111, 3913−3961. (14) Xie, J.; Liu, G.; Eden, H. S.; Ai, H.; Chen, X. Surface-Engineered Magnetic Nanoparticle Platforms for Cancer Imaging and Therapy. Acc. Chem. Res. 2011, 44, 883−892. (15) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E = Sulfur, Selenium, Tellurium) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706−8715. (16) Mao, Z.; Xu, H.; Wang, D. Molecular Mimetic Self-Assembly of Colloidal Particles. Adv. Funct. Mater. 2010, 20, 1053−1074. (17) Nie, Z.; Petukhova, A.; Kumacheva, E. Properties and Emerging Applications of Self-Assembled Structures made from Inorganic Nanoparticles. Nat. Nanotechnol. 2010, 5, 15−25. (18) Gopalakrishnan, G.; Danelon, C.; Izewska, P.; Prummer, M.; Bolinger, P. Y.; Geissbuhler, I.; Demurtas, D.; Dubochet, J.; Vogel, H. Multifunctional Lipid/Quantum Dot Hybrid Nanocontainers for Controlled Targeting of Live Cells. Angew. Chem., Int. Ed. 2006, 45, 5478−5483. (19) Jia, H.; Titmuss, S. Polymer-Functionalized Nanoparticles: From Stealth Viruses to Biocompatible Quantum Dots. Nano Med. 2009, 4, 951−966. (20) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Selective Colorimetric Detection of Polynucleotides Based on the Distance-Dependent Optical Properties of Gold Nanoparticles. Science 1997, 277, 1078−1081. (21) Lee, J. S.; Ulmann, P. A.; Han, M. S.; Mirkin, C. A. A DNA− Gold Nanoparticle-Based Colorimetric Competition Assay for the Detection of Cysteine. Nano Lett 2008, 8, 529−533. (22) Lee, J.; Hernandez, P.; Lee, J.; Govorov, A. O.; Kotov, N. A. Exciton−Plasmon Interactions in Molecular Spring Assemblies of Nanowires and Wavelength-Based Protein Detection. Nat. Mater. 2007, 6, 291−295. (23) Ozbay, E. Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions. Science 2006, 311, 189−193. (24) Schwerzel, R. E.; Spahr, K. B.; Kurmer, J. P.; Wood, V. E.; Jenkins, J. A. Nanocomposite Photonic Polymers. 1. Third-Order Nonlinear Optical Properties of Capped Cadmium Sulfide Nanocrystals

followed by exposure to 808 nm laser light, resulting in complete tumor ablation with no reoccurrence as a result of localized photothermal heating from NIR surface plasmon excitation of the nanoparticle-based therapeutic agents (Figure 7B). In this Perspective, we have attempted to highlight pertinent advances in the synthesis and self-assembly of polymer brushcoated inorganic nanoparticles. In only recent years, the field has moved from investigations of simple hairy particles showing isotropic repulsive interactions to finely engineered colloidal molecular mimics exhibiting the complex interplay of anisotropic attractive interactions and conformational flexibility. In many ways, this new field has mirrored the somewhat older history of block copolymer self-assembly, in which increased capabilities for producing compositionally and structurally complex building blocks, together with improved understanding of their interactions, has led to exquisite control and complexity of self-assembled nanoscale materials. In the case of PBNP selfassembly, the functional possibilities of such structural control is even further enhanced by the large variability in the optical, electronic, and magnetic properties of numerous potential inorganic nanoparticles, together with their tunable collective properties that can be harnessed by precise control of nanoparticle spatial arrangements within composite assemblies. Expanding the complexity and control of assembly function by combining nanoparticle surface engineering and the resulting spatial addressability with different inorganic nanoparticle types and combinations of nanoparticle types (e.g., magnetic, fluorescent, plasmonic) will lead to continued exciting developments. With an impressive toolbox for structural control now developed, the field is poised to produce a diverse array of functional materials with wide-ranging applications in biology, medicine, photonics, and computing.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. Biography Matthew Moffitt (Ph.D., McGill University; postdoctoral, University of Toronto) is currently Associate Professor of Chemistry at the University of Victoria. His research program combines “bottom-up” self-assembly processes with “top-down” methodologies, including surface patterning and microfluidics, for the generation of polymer/ nanoparticle materials with controllable structure and function. Website: http://web.uvic.ca/~mmoffitt/.

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ACKNOWLEDGMENTS The author expresses sincere gratitude for the collaborative efforts of all co-workers whose names appear in the references. This work is financially supported by the Natural Science and Engineering Research Council of Canada (NSERC), along with infrastructure support from the Canada Foundation for Innovation (CFI) and the British Columbia Knowledge Development Fund (BCKDF).

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