self-assembly of polymer brush-functionalized inorganic … · 2018-04-18 · latexes40,41 and...

Self-Assembly of Polymer Brush-Functionalized InorganicNanoparticles: From Hairy Balls to Smart Molecular MimicsMatthew 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 ofinorganic nanoparticles (e.g., quantum dots, plasmonic or magnetic nanoparticles) areprovided when the stabilizing organic layer at the nanoparticle surface consists of apolymer brush of densely grafted chains. We highlight recent advances in the synthesisand self-assembly of these unique building blocks, termed polymer brush-functionalizednanoparticles (PBNPs). We show how the field has progressed from PBNPs withisotropic, single-component brushes showing limited self-assembly behavior to PBNPswith anisotropic brushes showing directional interactions and complex self-organization.We further discuss how PBNPs with isotropic multicomponent brushes, either mixedbrushes or grafted block copolymers, can also exhibit the complex self-organization ofmolecular amphiphiles via rearrangements within the brush enabled by conformationalflexibility. Through numerous examples, we show how established principles of polymerscience and surface engineering encapsulated in these composite colloidal building blocksopen up vast possibilities for functional self-assembled nanomaterials.

The spontaneous organization of molecular building blocksinto complex structures, driven by a delicate balance of

noncovalent attractive and repulsive intermolecular interac-tions, is the pathway to an incredible range function in livingsystemsfrom the self-cleaning power of the lotus leaf to theamazing complexities of the human brain. Inspired by theseorganizational processes in nature, chemists have recognizedintriguing opportunities for a new generation of functionalmaterials via the self-assembly of synthetic building blocks.1−6

As a result, the last 30 years or so have seen a proliferation ofexamples in which spontaneous ordering and structuralcomplexity from an increasing range of molecules and particleshas been demonstrated, including liquid crystals,7 artificialopals,8 self-assembled monolayers,9 and block copolymers.10,11

Colloidal inorganic nanoparticles are particularly intriguingcandidates as building blocks for self-assembly, with interestingoptical, electronic, or magnetic properties associated withsurface and quantum effects arising from their small size. Theseinclude gold (Au) and silver (Ag) nanoparticles with strongsurface plasmon resonances in the visible range,12,13 metal (e.g.,Fe, Co) and metal oxide (e.g., Fe2O3) magnetic nanoparticles,14

and semiconductor quantum dots (e.g., CdSe, CdS, PbSe, core/shell CdSe/ZnS) with stable, size-tunable fluorescence at highquantum yield.15 Although the colloidal synthesis of variousinorganic nanoparticles has been well-established, their con-trolled assembly into one-, two-, and three-dimensional (3D)superstructures remains an ongoing challenge,16,17 en route toapplications ranging from medical therapeutics18,19 anddiagnostics19−22 to photonics,23,24 photovoltaics,25,26 and com-puting.27−29 In general, this challenge can be met by fun-ctionalizing nanoparticle surfaces with a layer of organic ligandsthat modify interactions between neighboring nanoparticles orbetween nanoparticles and the surrounding medium.Unique opportunities for nanoparticle self-assembly are

provided when the organic layer consists of a polymer brushof densely grafted chains anchored at one end to the nano-particle surface and extending outward into the surroundingmedium.30−32 The resulting “hairy” nanoparticles can beregarded as hybrid building blocks combining the optical,electronic, or magnetic properties of the core inorganic nano-particle with the mechanical strength, flexibility, processability,and dielectric properties of the grafted polymer chains. Inaddition to enhancing material properties for specific nano-composite applications,24,33,34 surface-grafted polymer brushescan also broaden the capabilities for nanoparticle self-assemblyin a number of general ways: (1) by generating strong ther-modynamic driving forces for self-assembly due to stronglyfavorable phase separation between grafted and ungrafted dis-similar polymeric components or between co-grafted dissimilarpolymeric components, (2) by increasing the range of potential

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

Colloidal inorganic nanoparticlesare particularly intriguing

candidates as building blocks forself-assembly, with interestingoptical, electronic, or magnetic

properties associated with surfaceand quantum effects arising from

their small size.

Perspective

pubs.acs.org/JPCL

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

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equilibrium structures due to the conformational flexibility ofgrafted chains, or (3) by allowing kinetic control of stable,nonequilibrium structures due to the relatively slow chaindynamics of overlapping polymer brush layers.The purpose of this Perspective is to describe and contex-

tualize, from the author’s viewpoint, representative work on theself-assembly of polymer brush-functionalized inorganic nano-particles (PBNPs). We will begin the discussion where the fieldbegan in the early 1990s,35−37 with PBNPs functionalized withisotropic, single-component brushes (type I PBNPs). Due tothe generally isotropic and repulsive nature of particle−particleinteractions for type I PBNPs, their self-assembly tends to formperiodic nanoparticle arrays (Figure 1).38 However, we will also

discuss how blending type I PBNPs with various additivecomponents, including compatible and incompatible homopol-ymers and selectively compatible block copolymers, can pro-duce a wide range of nonperiodic coassemblies with multiscaleorganization. Next, we will describe the more recent evolutionto PBNPs exhibiting anisotropic interparticle interactions (at-tractive and repulsive) analogous to those of molecularamphiphiles such as block copolymers. The resulting “smart”self-assembly properties are evidenced by the morphologicalrange and structural complexity of the resulting nanoparticle/polymer assemblies.30,31,39 In this discussion, we distinguishbetween the two types of anisotropic PBNPs; type II PBNPs(Figure 2) are functionalized with anisotropic brushes, includ-ing both single-component (Figure 2A,C) and multicomponentbrushes (Figure 2B,D), in which the distribution of chaintethers defines an anisotropic patterning of distinct chemicalregions on the nanoparticle surface, either Janus (“two-faced”,Figure 2A,B) or patchy (Figure 2C,D). In such PBNPs, theanisotropy of the particle−particle interactions is encodeddirectly into the surface chemistry, such that type II PBNPs areinherently anisotropic. In contrast, type III PBNPs (Figure 3)are functionalized with isotropic, multicomponent brushes; in

this case, the distribution of chain tethers defines an isotropicsurface chemistry that does not directly encode anisotropicinteractions. However, due to the chemical incompatibility andflexibility of the polymer components, microphase separation orconformational changes can induce anisotropic interactions andamphiphilic self-assembly if the global free energy is loweredoverall.30,31,39 Therefore, type III PBNPs are contextuallyanisotropic.Although important work on the synthesis and self-assembly

of colloidal polymer brushes has been carried out on polymeric

Figure 1. Periodic array formation of type I PBNPs. (A) Schematicshowing type I PBNPs (isotropic, single-component polymer brush)and array formation via repulsive interactions at increased nanoparticlevolume fraction. (B) Two-dimensional array of gold particles aftertreating a concentrated solution of PS(190)-b-P2VP(190) gold-loadedmicelles with anhydrous hydrazine. Adapted from ref 53.

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

The Journal of Physical Chemistry Letters Perspective

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latexes40,41 and block copolymer micelles with polymericcores,42−44 for the purpose of this Perspective, we will focuson the synthesis and self-assembly of PBNPs in which the coresare inorganic, especially metals, semiconductors, and metaloxides. In terms of the polymer brush materials, we will focuson PBNPs with shells consisting of synthetic macromolecules;however, we point out that inorganic nanoparticles decoratedwith DNA chains offer immense potential for organizedfunctional superstructures via specific interactions betweenbiopolymer brushes, and important work in this area has alsobeen pursued.45,46 Due to the relative importance of conforma-tional flexibility to self-assembly,31 especially in the context oftype III PBNPs, grafted particles in which the end-to-endlength of the chains is significantly greater than the diameter ofthe inorganic core (R0 ≫ dc), so-called “star-like” colloids, willbe particularly emphasized throughout this discussion. As well,it is important to note that although the vast majority of relatedliterature studies concern spherical polymer-grafted nano-particles, in which anisotropic interactions are induced bychemical, rather than shape, anisotropy, a few key examples ofnonspherical nanoparticles with both chemical and shapeanisotropy will also be discussed.47−50

PBNPs with Isotropic Single-Component Brushes (Type I). Thesynthesis of PBNPs with isotropic, single-component brushlayers of various polymers on a wide range of inorganic nano-particles has been well-documented and is generally accom-plished using one of three strategies.32 As shown in subsequentsections, type II and type III PBNPs can be synthesized usingvariations of one or more of these approaches.30 (1) In theblock copolymer micelle template approach, a diblock copol-ymer first forms reverse micelles in an organic solvent (orspherical nanodomains in the solid state), in which the core-forming blocks selectively complex metal ions or other inor-ganic nanoparticle precursors. The inorganic nanoparticles arethen synthesized within the micelle cores via a reduction ornanoprecipitation reaction, with the metal-complexing blocksforming a condensed layer at the nanoparticle surface, cova-lently attached to the external brush layer of corona-formingblocks. For example, Eisenberg’s group37 and ours51 have syn-thesized CdS quantum dots coated with a brush layer ofpolystyrene (PS) chains (graft density, σ = ∼1 chains/nm2)using a template of polystyrene-b-poly(acrylic acid) (PS-b-PAA)-based reverse micelles. PS brush-functionalized goldnanoparticles have been prepared using a similar approach byMoller and co-workers from reverse micelles of polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP) or polystyrene-b-poly-(ethylene oxide) (PS-b-PEO).52−54 (2) In the grafting-toapproach, presynthesized polymer chains with terminal func-tional groups are grafted onto preformed nanoparticles thoughligand-exchange or used as initial ligands during nanoparticlegrowth. In typical examples, Emrick and co-workers applied aligand-exchange approach to displace pyridine with thiol-terminated PEO on the surface of CdSe/ZnS core−shellquantum dots,55 whereas the Lennox group used thiol-terminated PS or PEO chains as ligands during the growth ofgold nanoparticles to generate PBNPs.34,56 (3) Finally, in thegrafting-from approach, polymer chains are polymerized frominitiators grafted onto the surface of preformed nanoparticles.For example, Emrick and co-workers have applied nitroxide-mediated living radical polymerization to generate either PS orpolystyrene-r-poly(methyl methacrylate) (PS-r-PMMA) brushlayers on CdSe quantum dots;57 surface-initiated polymer-izations of PMMA on gold58 and PS and poly(3-vinyl pyridine)

(P3VP) on magnetite59 have also been demonstrated by othergroups.Although the focus of this review is the controlled assembly

of PBNPs into condensed superstructures, another current andimportant challenge in material science, and an additionalmotivation behind functionalizing inorganic nanoparticleswith polymer brushes, is not to assemble nanoparticles butto keep them apart, specifically, to allow inorganic nanoparticlesto be dispersed at low volume fraction in a homopolymermatrix.32,34,56,60,61 For a host of potential applications, the ideaof imparting a strong, flexible, and processable polymer materialwith the unique properties of individual nanoparticles (e.g.,fluorescence, surface plasmon resonance, third-order non-linearities, superparamagnetism) is extremely appealing, andin these cases, the goal becomes achieving a solid dispersion ofwell-separated nanoparticles without aggregation or phase sepa-ration between the chemically dissimilar inorganic and poly-meric components. PBNPs offer an intriguing solution toachieving such thermodynamically stable nanoparticle/polymercomposites, with the hairy polymer layers providing compat-ibility with the surrounding homopolymer (usually of identicalcomposition to the brush chains).32 As in liquid colloidal dis-persions, repulsive steric forces between particles require thatthe brushes are “wet” with the surrounding solvent (i.e., thehomopolymer chains); otherwise, particle aggregation viaautophobic phase separation between PBNPs and homopol-ymer is expected.32,34,56,60,61 Along with enthalpically neutralmixing (χ = 0 between matrix and brush chains), brush wettingrequires a sufficiently low entropic penalty for intercolation ofmatrix chains into the sterically crowded environment of thebrush. Due to this entropic factor, PBNP dispersion is generallypromoted by brush grafting densities that are not too high andbrush chains that are relatively long compared to the matrixchains. An example of the sensitivity of particle dispersion tobrush structure comes from Lennox and co-workers, whoshowed that PS-functionalized gold nanoparticles with longPS chains (degree of polymerization, N = 125: PS125-Au)and lower graft density (σ = 0.94 chains/nm2) remainedwell-dispersed in a matrix of relatively long PS760 chains, evenwith 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 PS20chains but phase separated from matrixes of PS440 and PS760chains.56

When dispersed in a good solvent for the polymer brush(including homopolymer chains in the wet brush case of nano-particle/polymer composites discussed above), the particle−particle interactions between type I PBNPs are generally repul-sive and isotropic due to excluded volume effects associatedwith the interpenetrating brushes.38,62,63 The resulting stericinterparticle potential functions show features in between the“hard”, short-range, steric potentials of uncharged and non-grafted particles and the “soft”, long-range, electrostaticpotentials of charged particles. Similar to such hard and softcolloids, the self-assembly of dispersed type I PBNPs can beinduced by an increase in particle volume fraction, leading tolong-range ordering of particles (Figure 1A). In threedimensions, this disorder-to-order transition corresponds tocolloidal crystal formation above a critical volume fraction ofnanoparticles. Ohno et al. investigated the ordering of PMMA-silica particles (PMMA-SiO2, σ = 0.59−0.71 chains/nm2) inmixed organic solvents and found that the particle volumefraction of colloidal crystal formation (ϕf = 0.129−0.0392)



decreased with increasing height of the polymer brush, whichwas experimentally varied by changing both the grafting densityand chain length.38,62 Confocal laser scanning microscopy(CLSM) of the resulting 3D colloidal crystals demonstrated astrong correlation between grafted chain length and the spacingbetween SiO2 cores, as well as an influence on the array struc-ture, with mixtures of face-centered cubic (fcc) and hexagonalclose-packed (hcc) lattices, showing increased fcc fractions withincreasing PMMA chain length.62 Importantly for potentialmaterials applications, Goel et al. have shown that such 3D or-dering can occur for highly grafted PBNPs even in the absenceof dispersing solvents or homopolymer diluents.63 Using small-angle X-ray scattering (SAXS), they measured a weak but dis-cernible 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 ther-mal annealing, suggesting fcc arrays of thermodynamic origin.The same repulsive particle−particle interactions described

above, but modulated by particle−substrate interactions andcapillary forces between particles during solvent evaporation,can also be used to generate two-dimensional (2D) orderedarrays of PBNPs in thin polymeric films.53,64−66 Moller and co-workers dip-coated various substrates, including glass, mica, andcarbon-coated transmission electron microscope (TEM) gridswith toluene dispersions of PS190-Au PBNPs, fabricated fromgold-loaded reverse micelles. Applying a controlled withdrawalspeed (∼10 mm/min), a well-ordered 2D hexagonal array of∼10 nm gold nanoparticles with ∼35 nm interparticle spacingextending over an area of several square centimeters wasobtained (Figure 1B).53 The Ritcey group has similarly applieddrop-coating chloroform dispersions of PS-Au (∼10 nm goldcores) with variable grafted chain lengths (N = 36 − 635) ontoglass or carbon-coated TEM grids, generating monolayersconsisting of short-range-ordered 2D hexagonal arrays of goldnanoparticles.64 Furthermore, the authors demonstrated theinfluence of a self-assembled structure on the monolayer opticalproperties, showing an increased red shift in the gold surfaceplasmon resonance with decreased polymer chain length,attributed to closer proximy and thus stronger dipolar couplingbetween nanoparticles within the 2D arrays.64,65 Finally, Ohnoet al. have produced highly ordered hexagonal 2D arrays of goldnanoparticles with variable interparticle spacings by spreadingPMMA-Au with different chain lengths (N = 120−620, σ ≈0.8 chains/nm2) from benzene onto the air−water interface of aLangmuir trough, followed by compression and Langmuir−Blodgett (LB) transfer to carbon-coated TEM grids.66

The isotropic and repulsive interactions between type IPBNPs encode their self-assembly into ordered arrays withindividual inorganic nanoparticles at each lattice point, asshown above for both 3D and 2D cases. However, it is possibleto generate a range of other, nonperiodic superstructures fromsuch grafted nanoparticles by adding coassembling, often poly-meric, components that template or direct the self-assemblyprocess (Figure 4). The role of these added components duringself-assembly is generally to develop phases (Figure 4A−C) orinterfaces (Figure 4C,D) to which the PBNPs are selectivelyattracted via their polymeric brushes, thus directing their spatialdistribution. For example, in our group, we have shown that theinterfacial self-assembly of PS-b-PEO copolymers at the air−water interface directs the organization of PS-CdS nanoparticlesinto hierarchically structured wires, cables, and rings with meso-scale external dimensions and internal nanoscale dispersions ofquantum dots (Figure 4A).67,68 The resulting stable kineticstructures could be LB transferred to glass, TEM grids, ormicrocontact-printed substrates69 for further organization andwere explained by synergistic self-assembly of the two com-ponents, driven by PEO-regulated dewetting of PS from thewater surface. In other work, we have directed the self-assemblyof PS-CdS into isotropic islands or directional stripes via spin-coating-induced phase separation from a PMMA homopolymeron bare70 or patterned71 glass substrates (Figure 4B). In thecolloid state, we have produced kinetically tunable mesoscalespherical aggregates with complex internal structure via drop-wise water addition to PS-CdS dispersions with codissolved PS-b-PAA stabilizing chains.72 Following a hierarchical self-assembly strategy, the ∼100−200 nm spheres from the latterstudy were further self-assembled into weakly ordered close-packed arrays, which combined the photoluminescence of thenanoscale quantum dots with a photonic stopband associatedwith the mesoscale periodicity generated through a series ofsequential self-assembly steps.73

Various other studies have also explored the use of additivecomponents to direct the self-assembly of type I PBNPs. In par-ticular, the microphase separation of block copolymers in thesolid state and concomitant selective localization of polymer-grafted nanoparticles and nanorods into one of the micro-domains, or at the polymer/polymer interface, is an intriguingstrategy for forming functional nanocomposites with complexhierarchical organization. For example, Kramer and co-workershave shown that the grafting density of PS-Au provides ahandle on steering the specific localization of the PBNPs withinthe lamellar domains of microphase separated PS-b-P2VP, withhigher grafting density (σ > 1.6 chains/nm2) directing nano-particle localization to the center of the PS phase and lowergrafting density (σ < 1.3 chains/nm2) directing their local-ization to the PS/P2VP interface (Figure 4C).74 The sameauthors have also shown that by stabilizing the PS/P2VP inter-face, PS-Au with sufficiently low grafting density can trigger achange from lamellar to bicontinuous block copolymer mor-phologies above a critical particle volume fraction.75 The latterstudy demonstrates an interesting case of synergistic self-assembly, in which co-organization strongly influences thespatial distribution of both the PBNPs and the “directing” com-ponent. A different example of directed PBNP self-assembly isthe spontaneous migration of PEO-CdSe/ZnS quantum dotsdispersed in a PMMA film to cracks within an adjoining silicalayer (Figure 4D).55 In this case, the localization of PBNPs pro-vides an appealing strategy for self-healing multilayer structuresand is driven by a combination of surface energy lowering

The isotropic and repulsiveinteractions between type I

PBNPs encode their self-assemblyinto ordered arrays. However, it ispossible to generate a range ofother, nonperiodic superstruc-tures from such grafted nano-

particles by adding coassembling,often polymeric, componentsthat template or direct the

self-assembly process.



(PEO has a lower surface energy than PMMA) and theentropic exclusion of polymer-grafted nanoparticles from thePMMA 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 self-assembly arising from anisotropic and attractive interactionsbetween hairy nanoparticles functionalized with isotropic,single-component brushes. In the first example from thePyun lab, anisotropic dipolar attractive interactions between fer-romagnetic cobalt nanoparticle cores of PS-Co PBNPsovercame the isotropic steric repulsion of the surrounding PSbrushes in organic solvents (tetrahydrofuran (THF), methylenechloride, toluene), leading to self-assembly into long, highlyanisotropic nanowires.76 The PS-stabilized nanowires couldthen be oxidized by bubbling the colloidal dispersions withoxygen to form electroactive cobalt oxide nanowires ofpotential interest as nanoscale electrodes for energy storageapplications.77

The second example is the recent discovery by Akcora et al.of anisotropic self-assembly of sparsely grafted PS-SiO2 particlesin a PS homopolymer matrix to form strings, sheets, andplatelet-like objects.78 These results, qualitatively supported byMonte Carlo simulations, are fundamentally different from the

autophobic phase separation between brush and homopolymerchains in the limit of high grafting densities referred to earlier.56

In this case, self-assembly is explained by microphase separationbetween the ungrafted inorganic surfaces and the polymerphase, enabled at low grafting densities by the relatively lowentropic penalty for grafted chains to distort as particles comeinto contact. In this way, these anisotropic interactions betweenPBNPs are amphiphilic in nature, arising from a balance ofattractive and repulsive forces between spatially and chemicallydistinct regions on each particle. In a Perspective publishedearlier this year, Kumar and coauthors proposed a graftingthreshold of σN1/2 ≈ 2 for good particle dispersion, belowwhich anisotropic self-assembly predominates;61 we point outthat for polymer chain lengths ranging from N = 100 to 1000,this threshold corresponds to grafting densities from 0.06 to0.2 chains/nm2, which is well below graft densities for all othertype I PBNP cases described above (typically σ ≈ 1 chain/nm2).Although this behavior of type 1 PBNPs is still poorly understoodand the range of associated assemblies is as yet limited, the workof Akcora underlines the potential to generate complex struc-tural hierarchies from particles that mimic the self-assemblybehavior of molecular amphiphiles. We now turn to recent effortsto specifically engineer PBNPs for amphiphilic self-assemblyvia a range of strategies that offer fine chemical and spatial

Figure 4. Examples of directed self-assembly of Ttype I PBNPs with various coassembling polymer components. (A) Schematic of coassembly ofPS-CdS and PS-b-PEO at the air−water interface and representative AFM and TEM images of the resulting hierarchical assemblies. Adapted fromref 67. (B) Schematic showing PS-CdS patterning by spin-casting PS-CdS/PMMA blends from toluene solutions followed by selective etching of thePMMA 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 arealdensities 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 acrack in a SiOx layer on PMMA containing PEO-CdSe/ZnS PBNPs. Reprinted with permission from ref 55. Copyright 2006, Nature PublishingGroup, U.K.



manipulation of nanoparticle surfaces, enabling exquisite controlof particle interactions and assembled structures.PBNPs with Anisotropic Brushes (Type II). Theoretical studies

by Glotzer and others have shown that by designing nano-particle surfaces with distinct and spatially separated chemicalregions (either patchy or Janus particles), colloidal buildingblocks can be produced with the essential chemical anisotropyof molecular amphiphiles, enabling their spontaneous self-assembly into complex three-dimensional ensembles in themanner of surfactants, phospholipids, and block copoly-mers.79−81 Moreover, if one or more of the distinct chemicalregions is a grafted polymer brush, then the block copolymeranalogy becomes particularly apt because the resulting colloidswill possess the combined features of chemical anisotropy andconformational flexibility that are germane to the diverse andcomplex self-assembly characteristics of those macromolecularamphiphiles.10,11,31

Producing PBNPs with anisotropically grafted surfaces re-quires special consideration for breaking mirror symmetry inthe case of a Janus brush structure or breaking spherical sym-metry in the case of a patchy brush structure. In this section, weconsider the various synthetic strategies for symmetry breakingapplied to PBNPs, as well as the opportunities for self-assemblyarising from the resulting directional interparticle interactions(Figure 5). In cases of PBNPs functionalized with single-component

anisotropic brushes, chemical anisotropy is the result of partialfunctionalization of the nanoparticle surface with the polymerchains, with the brush distributed nonuniformly over thesurface in either a Janus (Figure 2A) or patchy (Figure 2C)manner; here, the chemical contrast driving self-assembly isbetween polymer-grafted and ungrafted regions (generallypassivated with low-molecular-weight ligands) on the nano-particle surfaces. In cases of anisotropic binary brushes, on theother hand, the two chain types are segregated into distinctgrafted regions over the surface in either a Janus (Figure 2B) orpatchy (Figure 2D) arrangement, and self-assembly is driven bychemical contrast between the incompatible polymer regions.The inherent shape and surface energy anisotropies of non-

spherical nanoparticles provide excellent opportunities forproducing selectively grafted PBNPs with strongly directionalinterparticle interactions. Kumacheva and co-workers have pro-duced facinating “pom-pom” building blocks consisting of goldnanorods with hydrophilic cetyl trimethylammonium bromide(CTAB) ligands passivating the longitudinal sides of the nano-rods and brushes of ∼13 PS chains grafted to each end (Figure5A).47 The synthesis of the anisotropic PBNPs applied agrafting-to approach, while taking advantage of the preferentialbinding affinity of CTAB to the (100) gold faces of the lon-gitudinal faces followed by grafting the unpassivated (111)ends with thiol-terminated PS brush “crowns”. The chemical

Figure 5. Various examples of self-assembly of type II PBNPs. (A) Self-assembly of polymer-tethered gold nanorods and SEM images of assembliesin various selective solvent mixtures. Reprinted with permission from ref 47. Copyright 2007, Nature Publishing Group, U.K. (B) Schematic showingeccentric 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 fromdifferent 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 “grafting-from” 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.



anisotropy of the resulting pom-pom nanoparticles was anal-ogous to an ABA triblock copolymer with a hydrophilic centerblock flanked by two hydrophobic end blocks. Self-assemblywas triggered by water addition to dispersions of the nanorodsin various polar organic solvents, which induced directionalaggregation of the PS brushes to form a range of assemblies(e.g., rings, nanochains, and bundles) depending on the specificsolvent mixture. Significant blue shifts (up to 200 nm) in thelongitudinal surface plasmon peak with increasing water con-tent, attributed to increased distances and decreased electro-dynamic coupling between nanorods, were also described. Infollow-up work, the association modes of the nanorod pom-pom assemblies were further controlled by variation of themolecular weight of grafted chains, allowing determination of aphase diagram for self-assembly of the metal−polymercolloids.48 Furthermore, by monitoring the statistics andkinetics of growth of self-assembled chains of nanorods, theauthors noted intriguing analogies, including molecular weightand branching control, with step-growth polymerization ofmolecular monomers.49 These results nicely highlight thepotential for applying amphiphilic principles to tuning both thestructure 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 segregationprocesses via migration of ligands on the nanoparticlesurface.82,83 For example, Chen et al. applied block-copolymer-mediated competitive binding and surface phase segregation ofhydrophobic and hydrophilic ligands to produce eccentricallyencapsulated gold nanoparticles (Figure 5B).83 The resultingJanus PBNPs possessed both a block-copolymer-encapsulatedsurface surrounded by a PAA polymer brush and an “exposed”surface passivated with nonpolymeric hydrophilic ligands. Salt-induced aggregation of the resulting Janus PBNPs in water wasobserved with a predominance of dimer formation. In otherwork, Forster and co-workers demonstrated amphiphilic self-assembly of spherical CdSe/CdS core/shell nanoparticlesdecorated with PEO brushes via relatively weak attachmentwith terminal amine groups (Figure 5C).82 In this case, it wasproposed that surface reorganization led to PEO ligand de-pletion in between adjacent nanoparticles, effectively formingpatchy or Janus brushes (depending on the nanoparticleenvironment), with exposed hydrophobic regions driving aniso-tropic attractive interactions. The PBNPs were modeled asamphiphiles with rigid cores constituting the hydrophobic partand the packing parameter increasing as the density of PEOchains decreased. Using such packing arguments based onmolecular amphiphiles, observed morphology changes of self-assembled structures from single particles, to strings, tomonolayer vesicles with a decreasing polymer/nanoparticleratio were explained.Another strategy for producing anisotropic PBNPs begins

with binding nanoparticles to a solid−liquid interface, whichbreaks their mirror symmetry and opens routes to Janusbrushes on the nanoparticle surfaces.84−87 In one such study,Hatton and co-workers used ∼700 nm positively chargedsurface-treated silica particles as colloidal substrates forelectrostatic binding of ∼10 nm PAA-coated magnetite nano-particles.84 The exposed PAA surfaces were then selectivelyfunctionalized with either polystyrene sodium sulfonate(PSSNa) or polydimethylamino ethylmethacrylate (PDMAE-MA) chains using the grafting-to method, followed bynanoparticle release via pH-triggered charge reversal of the

functionalized silica. Dynamic light scattering (DLS) and cryo-TEM showed that the resulting Janus PBNPs underwentreversible amphiphilic clustering at low pH values due toaggregation of the relatively hydrophobic uncharged PAA faceswith cluster stabilization by the hydrophilic chains of thePSSNa or PDMAEMA brushes. On the other hand, at inter-mediate pH values, the PDMAEMA Janus PBNPs exhibitedmacroscopic precipitation due to uncontrolled aggregation ofelectric dipoles from negatively charged PAA and positivelycharged PDMAEMA faces.Another elegant surface masking approach using a solid

substrate was applied by the Li group, but this time to produceJanus PBNPs with bicompartment brushes consisting ofhydrophobic and hydrophilic chains grafted to opposite facesof ∼6 nm gold nanoparticles (Figure 5D).85−87 First, thenanoparticles were immobilized on a substrate of single-crystalthiol (SH)-terminated PEO via bonding of multiple exposedthiol groups to each gold nanoparticle.85 Then, grafting-frompolymerization was used to decorate the exposed gold faceswith brushes of PMMA chains.86 Finally, the PEO substrate wasdissolved to release amphiphilic PBNPs with both hydrophobic(PMMA) and hydrophilic (PEO) brush faces. Due to lowelectron densities of both PMMA and PEO chains compared tothe that of the gold, it was impossible to directly observed theasymmetric brush structure by TEM; however, by using anidentical procedure to obtain PBNPs with PAA (instead ofPMMA) and PEO faces, followed by selective decoration of thePAA chains with platinum (Pt) nanoparticles, the Janus brushstructure was confirmed from TEM images showing Pt nano-particles to occupy only part of the gold nanoparticle surface.The same authors subsequently investigated the solution self-assembly of the Janus PBNPs with PMMA and PEO faces,noting reversible clustering to form branched wormlikeaggregates in dioxane (a selective solvent for PMMA) by a pro-posed mechanism of amphiphilic self-assembly.87 The im-portance of the Janus structure to the observed self-assemblybehavior in dioxane was also demonstrated using a controlsample of gold nanoparticles grafted with an isotropic mixedbrush of PEO and PMMA chains. In contrast to the aggregationbehavior of the Janus particles (J-AuNPs), the correspondingsymmetric mixed brush PBNPs (S-AuNPs) did not aggregatebut instead remained well dispersed in dioxane.87

PBNPs with Isotropic Multicomponent Brushes (Type III). Evenas interest in amphiphilic nanoparticle self-assembly continuesto increase, examples of Janus PBNPs functionalized withbicompartment brushes such as those described in refs 86 and87 are extremely rare due in part to the stated challenges ofanisotropic functionalization and characterization of compart-mentalized brushes. In addition, a growing number of literatureexamples suggest that Janus or other anisotropic functionaliza-tion is not a requirement for amphiphilic self-assembly ofPBNPs; these studies provide compelling evidence for highlydirectional and attractive interactions between nanoparticlesdecorated with isotropic polymer brushes containing two ormore incompatible polymeric components.50,88−91 Theirinteresting self-assembly properties, evidenced by a wealth ofstructurally complex assemblies along with the relativesimplicity of isotropic (compared to anisotropic) functionaliza-tion, make this class of PBNPs particularly intriguing.The amphiphilic self-assembly of type III PBNPs can be

explained by the spontaneous development of particle aniso-tropies via microphase separation of incompatible brush polymers(Figure 3A) or via stretching and compacting of polymer chains



(Figure 3B), in both cases leading to directional attractive interac-tions in selective solvents and aggregation into thermodynamicallyfavored structures. This feature is a direct result of the chemicalincompatibility of the polymeric components and their conforma-tional flexibility, along with the relatively small size of the corecompared to the chain lengths, which allows chains to wraparound or stretch past nanoparticle cores to generate patternedsurface topologies of segment-enriched regions. Despite theirisotropically grafted chains, therefore, these mechanisms of surfacereorganization allow type III PBNPs to behave as true colloidalanalogues of block copolymers, with spatially separated surfaceregions that are both chemically incompatible and conformation-ally flexible.

In one category of type III PBNPs, nanoparticle surfaces aredecorated with binary brushes with intimately mixed graftjunctions of two different chain types (“mixed brushes”, Figures3A and 6).50,88,89 For example,50 Song et al. applied sequentialgrafting-to/grafting-from reactions to gold nanoparticle surfacesin which a mixture of hydrophilic PEO chains and ATRPinitiator groups was first attached via Au−S bonds in a sta-tistical ligand-exchange step, followed by growth of hydro-phobic chains, either PMMA or copolymers containing variousratios of methyl methacrylate (MMA) and 4-vinyl pyridine(4VP) repeat units (designated PMMAVP), from the graftedinitiator groups (Figure 6A). When dispersed in water, theresulting PBNPs with mixed binary brushes self-assembled toform vesicles with walls consisting of a densely packed mono-layer of gold nanoparticles. This result suggests that the initiallyisotropic PBNP surfaces reorganize by chain wrapping togenerate a patchy topology (Figure 3A, bottom left) followedby aggregation of the hydrophobic patches (PMMA orPMMAVP) to form the vesicle walls. It was also shown, inthe PMMAVP case, that the assemblies underwent spontaneousdissociation into nanoparticles when the pH was tuned from7.0 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 functionalizedwith mixed PMMA/PEO brushes also self-assembled intomonolayer vesicles in water; of interest for photothermaltherapies, these vesicles could be disrupted by irradiation withnear-infrared light due to localized heating associated withexcitation of the nanorod longitudinal surface plasmonresonance.It is instructive to compare the mixed brush PMMA/PEO-Au

nanoparticle building blocks in ref 50, which self-assembled intovesicles in water, with the compositionally similar mixed brushPMMA/PEO-Au control sample in ref 87 (vida supra) thatremained individually dispersed in dioxane (while theanalogous Janus brush structure self-assembled into wormlikeaggregates in the same solvent). The absence of spontaneous

self-assembly for the mixed brush PBNPs in dioxane (ref 87)suggests that the entropic penalty of chains wrapping aroundthe core (Figure 3A) was not sufficiently balanced by favor-able free-energy terms, in contrast to similar PBNPs in water(ref 50). Although both PBNPs are functionalized with a mixedbrush of PMMA and PEO chains, the weight ratio of PEO toPMMA is not given in ref 87, and the graft densities and coresizes are somewhat different in the two studies so that only aqualitative comparison of these systems can be made. However,we note that despite possible small structural differences in thePBNP building blocks, the difference in solvent should play amuch stronger role in the self-assembly of these two systems.Compared to PMMA/PEO-Au in dioxane,87 the aqueoussystem studied by Song et al.50 contributed a greater solubilitycontrast between blocks, in addition to the contribution of thehydrophobic effect in water, leading to significantly strongerdriving forces for self-assembly. By comparing these twostudies, therefore, it can be concluded that a compartmentalizedbinary brush structure can help promote nanoparticle self-assembly in nonaqueous selective solvents,87 where theentropic and enthalpic attractive forces between particles arerelatively weak; however, in aqueous systems, where favorablefree-energy terms for particle aggregation are large compared tothe entropic penalty of surface chain reorganization, a muchsimpler isotropic binary brush structure can be sufficient todrive nanoparticle self-assembly into complex hierarchicalstructures, as evidenced by ref 50 and the following examples.Amphiphilic self-assembly of PBNPs functionalized by

mixed binary brushes in aqueous media was also demonstratedby Zubarev and co-workers, who used a grafting-to strategyto attach V-shaped ligands, each containing one PEO arm andone PS arm, to the surfaces of gold and silver nanoparticles(Figure 6B).88 This method provides functionalization with anexact 50:50 mix of hydrophilic and hydrophobic chains,although it does not allow the compositional control of at-taching the two chain types as separate grafts, either sequen-tially or simultaneously. The mixed brush PS/PEO-Au PBNPswere self-assembled by dispersion in either THF ordimethylformamide (DMF), followed by dropwise wateraddition to 75% (v/v) water, and finally dialysis against waterto remove residual THF or DMF. The resulting assemblieswere strongly reminiscent of amphiphilic block copolymermicelle morphologies, including rods, long wormlike aggre-gates, and vesicles, depending on the initial solvent and nano-particle concentration, but with arrays of gold or silver nano-particles residing at the PS core/PEO corona interfaces. On thebasis of the assembly geometry in all of these structures, it wasproposed that as the water concentration increases, the initiallyisotropic brushes reorganize via chain wrapping to form a Janussurface topology (Figure 3A, bottom right) with subsequentaggregation of the hydrophobic PS faces; following similarmicrophase separation principles as amphiphilic diblock copol-ymers in solution, the morphologies of such PBNP assembliesrepresent the lowest free-energy balance of interfacial tensionand entropy loss due to stretching of hydrophobic and hydro-philic chains.In our group, we have extended the block copolymer para-

digm for nanoparticle self-assembly to the solution propertiesof ionic block copolymers, by generating CdS nanoparticlesfunctionalized by dense mixed brushes of hydrophobic PS andhydrophilic and partially ionized PMAA chains (Figure 6C).89

We applied a similar block copolymer micelle template ap-proach to that used in earlier work to produce PS-CdS,51 except

The amphiphilic self-assembly oftype III PBNPs can be explainedby the spontaneous development

of particle anisotropies viamicrophase separation of incom-patible brush polymers or viastretching and compacting of

polymer chains.



with a PMMA-b-PAA-b-PS triblock copolymer as the startingmaterial.92,93 The copolymer was dissolved in organic solventfollowed by addition of cadium acetate, which neutralized thePAA blocks and generated micelles with poly(cadium acrylate)(PACd) cores and a mixed brush corona of PMMA and PSchains (intimate mixing of PS and PMMA within the coronawas confirmed by 1H-NOESY).92 Exposure of the micelles tohydrogen sulfide (H2S), followed by covalent cross-linking ofthe PAA core layer, and finally hydrolysis of PMMA coronalchains to PMAA generated the amphiphilic mixed brushPBNPs, PS/PMAA-CdS.89 Because a portion of the MAAgroups will be negatively charged in water, the resulting PBNPswere shown to possess many of the features of self-assemblingionic block copolymers in aqueous environments, characterizedby superstrong segregation between nonionic and ionic chains,conformational flexibility of both chemical regions, and salt-and pH-tunable morphologies. Self-assembly of PS/PMAA-CdS was initiated by water addition to PBNPs dispersed inTHF, which triggered chain reorganization from an isotropicmixed brush into a Janus topology. Above a critical water con-tent, Janus-organized PS/PMAA-CdS self-assembled byaggregation of PS faces and either solubilization or micro-precipitation of PMAA faces, depending on the pH and ionic

strength. Electrostatic repulsion between partially charged PMAAchains forced nonequilibrium pathways to variable kineticstructures with internal lamellar organization of nanoparticles,including segmented wormlike assemblies. Decreasing electro-static interactions through salt or acid addition was shown topromote tunable equilibrium self-assembly into either super-micelles or bilayer vesicles of nanoparticles, depending on thepH and ionic strength.Finally, we consider a second category of type III PBNPs

recently pioneered by the Nie group, in which the hydrophobicand hydrophilic polymer components are covalently linkedblocks of a copolymer tethered at one end to the nanoparticlesurface (Figures 3B and 7).90,91 In this case, spontaneous aniso-tropy generation and directional interactions do not requirechains to wrap around the nanoparticle core; rather, stretchingand compacting of tethered hydrophobic blocks along a singleaxis give rise to a linear conformation reminiscent of an ABAtriblock 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-exchangeapproach to graft PEO-b-PS or poly(2(2-methoxyethoxy)ethylmethacrylate)-b-polystyrene (PMEO2MA-b-PS) copolymersonto 14 nm gold nanoparticles via a terminal SH group on

Figure 6. Various examples of self-assembly of type III PBNPs with mixed polymer brushes. (A) Schematic of amphiphilic nanoparticle self-assemblyand TEM (left) and SEM (right) images of monolayer vesicles assembled from gold nanocrystals with mixed PEO and PMMA brushes. Reprintedfrom ref 50. (B) Schematic representation of the amphiphilicity-driven self-assembly of Au-(PS-PEO)n PBNPs with V-shaped ligands and TEMimage of assemblies prepared from an aqueous solution of Au-(PS-PEO)n after dialysis of a THF/H2O solution against DI water. Reprinted from ref88. (C) Schematics and accompanying TEM images illustrating self-assembly of CdS nanoparticles with mixed brushes of hydrophobic (PS) andpartially charged (PMAA) chains above the critical water concentration (cwc). Images include segmented wormlike assemblies (with image of themicrotomed section showing internal distribution of CdS nanoparticles) formed without added NaCl and bilayer vesicles and spheres formed withdifferent degrees of charge screening (different quantities of added NaCl). Reprinted from ref 89.



the PS blocks to form block copolymer brush layers with agrafting density of ∼0.1 chains/nm2.90 Self-assembly was tri-ggered by film rehydration (used commonly for forming phos-pholipid or polymeric vesicles), forming vesicles or tubules withwalls composed of a monolayer of hexagonally packed goldnanoparticles (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 thegold nanoparticle cores on the self-assembled morphologies byinitiating self-assembly closer to equilibrium using the methodof dropwise water addition to THF dispersions of copolymer-functionalized nanoparticle building blocks. From the resultingphase diagram, it was shown that as the PS chain length de-creases relative to the gold nanoparticle diameter, the morphol-ogies change from vesicles, to small clusters, to unimer micelles(no self-assembly). These transitions were explained basedupon changes in the deformability of the nanoparticle surfaces.If the flexible PS chains are too short relative to the rigid coresize, then the PS chains cannot stretch and compact sufficientlyto generate the anisotropic conformation, and no self-assemblyoccurs. Upon the basis of the phase diagram in ref 91, theboundary between clusters and unimer micelles indicates thatself-assembly in this system requires R0/dc > ∼0.4. Thisrequirement is less stringent that that proposed by Zubarov formixed brush systems (R0/dc > π/2 = 1.6) in which phase

separation of the polymer components requires chains to wraparound the inorganic core. This underlines the different mecha-nisms of self-assembly in these two cases but also highlights therelative importance of particle deformability to anisotropicinteractions in different type III PBNPs.Furthermore, the optical and photothermal properties and

resulting bioimaging and therapeutic applications of assembliesof self-assembled gold nanoparticles were also investigatedby the Nie group.91 On the basis of the effects of near-fieldplasmon coupling between nanoparticles, the ensemble absor-bance spectra could be tuned by varying the sizes of the goldcores and PS chains, which changed both the distances betweengold cores and the morphologies of the assemblies. For ex-ample, assemblies of relatively large gold cores and long PSblocks yielded two plasmonic peaks in the visible and NIRspectral range, respectively, attributed to plasmon hybridizationassociated with the hollow gold shell structure of the vesiclemorphology. In proof-of-concept bioimaging studies, multi-photon-absorption-induced luminescence (MAIL) imagingwith 800 nm excitation was applied to cancer cells loadedwith various PBNP assemblies; the contrast intensity wasshown to increase with increasing PS block length due to stron-ger plasmonic coupling associated with higher aggregationnumbers of the resulting assembled colloids. Finally, vesicles ofgold nanoparticles were injected into 4T1 tumors of mice

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



followed by exposure to 808 nm laser light, resulting in com-plete tumor ablation with no reoccurrence as a result of local-ized photothermal heating from NIR surface plasmon excitationof 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 brush-coated inorganic nanoparticles. In only recent years, the fieldhas moved from investigations of simple hairy particles showingisotropic repulsive interactions to finely engineered colloidalmolecular mimics exhibiting the complex interplay of aniso-tropic attractive interactions and conformational flexibility. Inmany ways, this new field has mirrored the somewhat olderhistory of block copolymer self-assembly, in which increasedcapabilities for producing compositionally and structurally com-plex building blocks, together with improved understanding oftheir interactions, has led to exquisite control and complexity ofself-assembled nanoscale materials. In the case of PBNP self-assembly, the functional possibilities of such structural controlis even further enhanced by the large variability in the optical,electronic, and magnetic properties of numerous potential inor-ganic nanoparticles, together with their tunable collective pro-perties that can be harnessed by precise control of nanoparticlespatial arrangements within composite assemblies. Expandingthe complexity and control of assembly function by combiningnanoparticle surface engineering and the resulting spatialaddressability with different inorganic nanoparticle types andcombinations of nanoparticle types (e.g., magnetic, fluorescent,plasmonic) will lead to continued exciting developments. Withan impressive toolbox for structural control now developed, thefield is poised to produce a diverse array of functional materialswith wide-ranging applications in biology, medicine, photonics,and computing.

■ AUTHOR INFORMATIONNotesThe authors declare no competing financial interest.Biography

Matthew Moffitt (Ph.D., McGill University; postdoctoral, Universityof Toronto) is currently Associate Professor of Chemistry at theUniversity of Victoria. His research program combines “bottom-up”self-assembly processes with “top-down” methodologies, includingsurface patterning and microfluidics, for the generation of polymer/nanoparticle materials with controllable structure and function.Website: http://web.uvic.ca/~mmoffitt/.

■ ACKNOWLEDGMENTSThe author expresses sincere gratitude for the collaborativeefforts of all co-workers whose names appear in the references.This work is financially supported by the Natural Science andEngineering Research Council of Canada (NSERC), along withinfrastructure support from the Canada Foundation forInnovation (CFI) and the British Columbia KnowledgeDevelopment Fund (BCKDF).

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