Anisotropic Nanoparticles and Anisotropic Surface Chemistry - The

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Anisotropic Nanoparticles and Anisotropic Surface Chemistry Nathan D. Burrows, Ariane M. Vartanian, Nardine S. Abadeer, Elissa M. Grzincic, Lisa M. Jacob, Wayne Lin, Ji Li, Jordan M. Dennison, Joshua G. Hinman, and Catherine J. Murphy* Department of Chemistry, University of Illinois at UrbanaChampaign, 600 South Matthews Avenue, Urbana, Illinois 61801, United States ABSTRACT: Anisotropic nanoparticles are powerful building blocks for materials engineering. Unusual properties emerge with added anisotropyoften to an extraordinary degreeenabling countless new applications. For bottom-up assembly, anisotropy is crucial for programmability; isotropic particles lack directional interactions and can self-assemble only by basic packing rules. Anisotropic particles have long fascinated scientists, and their properties and assembly behavior have been the subjects of many theoretical studies over the years. However, only recently has experiment caught up with theory. We have begun to witness tremendous diversity in the synthesis of nanoparticles with controlled anisotropy. In this Perspective, we highlight the synthetic achievements that have galvanized the field, presenting a comprehensive discussion of the mechanisms and products of both seed-mediated and alternative growth methods. We also address recent breakthroughs and challenges in regiospecific functionalization, which is the next frontier in exploiting nanoparticle anisotropy.

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improved properties in applications such as electronic circuitry,21 optical filters and films,22−24 and sensors based on surface-enhanced Raman scattering (SERS).25,26 Nanoparticles can also be assembled in polymer films27 with alignment control through mechanical deformation28,29 or geometric confinement within block copolymers30 as well as inorganic templates.31 Further dynamic manipulation of nanoparticle assemblies and their alignment is possible through composites with liquid crystals.32,33 Composite films can be produced that imbue chirality into the assembly and therefore the optical properties.34

olloidal nanoparticles can be prepared in an everincreasing number of shapes and sizes, giving rise to a host of varied physicochemical properties.1−7 Anisotropic nanoparticles possess asymmetric axes, and this break in symmetry is the origin of remarkable physical properties in metallic, semiconducting, and polymeric materials. These particles also provide interesting targets for regiospecific functionalization. Together, these features make anisotropic nanoparticles attractive subjects for fundamental and application-driven research. Gold nanorods are a classic example of nanoparticles whose shape anisotropy results in unique optical properties (Figure 1A).8 The isotropic counterpart to the gold nanorod, the gold nanosphere, possesses a single localized surface plasmon resonance (LSPR), but the two different axes of rods (the longitudinal and transverse) give rise to two distinct plasmon resonances. The location of the longitudinal plasmon band is influenced by the aspect ratio (length/width) of the nanorod, making it possible to tune the LSPR.9 Discovery of shape-dependent properties of noble metal and semiconductor nanoparticles has prompted the development of a myriad of exotic particle shapes,1 such as stars, flowers, wires, triangles, and plates, for a host of applications including sensing, imaging, and photothermal therapy.10−15 Anisotropic particles have been explored as near field transducers for heat-assisted magnetic recording for data storage.16 Electronic properties can also emerge from structural anisotropy, as observed with carbon nanotubes, which have garnered much attention in the past 20 years for their utility in electronics and sensing.17,18 Furthermore, the assembly of nanoparticles can result in unique mesoscale phenomena that show emergent properties beyond the sum of the material’s disparate parts19 and can even effect how cells respond to nanoparticles.20 The patterned deposition of nanoparticles on various substrates can lead to © XXXX American Chemical Society

Anisotropic nanoparticles possess asymmetric axes, and this break in symmetry is the origin of remarkable physical properties in metallic, semiconducting, and polymeric materials. Anisotropy is not limited to shape but can also arise from the chemical composition and surface chemistry of the particle. The most well-known chemically anisotropic colloid is the Janus particle, a sphere whose hemispheres feature distinctly different chemical compositions.37 Such diverse chemistry can afford Janus particles amphiphilicity and novel optical and magnetic properties, making them useful as emulsion-stabilizing surfactants,38 catalysts,39,40 and as building blocks for selfReceived: October 3, 2015 Accepted: January 28, 2016

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DOI: 10.1021/acs.jpclett.5b02205 J. Phys. Chem. Lett. 2016, 7, 632−641

The Journal of Physical Chemistry Letters

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Figure 1. A Venn diagram of morphological and chemical anisotropy with examples: (A) gold nanorods, (B) gold nanoparticles grown site selectively on cadmium selenide nanoplatelets (adapted from Naskar et al),35 and (C) polymer Janus nanoparticles featuring poly(L-lactide) and poly(styrene-co-aminoethyl methacrylate) faces (adapted from Urban et al).36

assembly.41 Figure 1C shows an example of a Janus particle, in which one hemisphere is composed of poly(L-lactide) (PLLA) and the other is composed of poly(styrene-co-aminoethyl methacrylate) (PS-co-AEMA), allowing for chemical reactions to selectively occur on one face of the particle.36 A given nanoparticle can exhibit both shape and chemical anisotropy (Figure 1B) by employing regiospecific surface functionalization. For example, anisotropic semiconductor nanoparticles with site-selective attachment of gold nanoparticles on the ends have been developed for catalyst and sensing applications.35 Figure 2 illustrates the most common and general ideas on how to control nanoparticle shape; namely through templating,43 vapor−liquid−solid extrusion,44 preferential absorption of ligands and minimization of surface energy,45 defect/ dislocation driven growth,46 and coalescence and oriented attachment.47 This Perspective highlights insights into the mechanisms and kinetics of anisotropic nanoparticle growth with an emphasis on seed-mediated growth, introduces alternative methods to seed-mediated growth, and addresses exciting developments and challenges in regiospecific surface chemistry. Anisotropic Seed-Mediated Growth. The seed-mediated growth of anisotropic nanoparticles is a logical extension of the principles of LaMer for the synthesis of monodisperse colloids. The well-known LaMer diagram48 (Figure 3A) illustrates the ideal temporal separation of nucleation (Aii) from growth (Aiii) and describes the preparation of seed nanoparticles. It starts with the generation of solute monomers (Ai) building in concentration through a variety of mechanisms, including: the decomposition of inert compounds, hydrolysis in organic or aqueous media, a redox reaction, precipitation by poor solvents, and the direct reaction of ions or chelates.48−50 At some point, the concentration of monomers (C) surpasses its solubility level (Cs) and becomes supersaturated. During this prenucleation stage (Ai), no self-initiated, stable nucleation takes place. In this supersaturated state, clusters of monomers are continually forming and dissolving; however, thermodynamically, as seen through an application of the well-known Kelvin equation, there is a energetic barrier to the formation of a new solid phase due to a high surface area to volume ratio.51 A nucleation stage (Aii) occurs when the monomer concentration surpasses a selfinitiated nucleation concentration (Cmin * ), where clusters of a sufficient size are able to form, overcoming that energetic barrier, which then favors continued nanoparticle growth over

Figure 2. General ideas on how to control nanoparticle shape. (A) Examples of the multitude of shapes achievable for noble metal nanoparticles. Adapted from ref 42 with permission from The Royal Society of Chemistry, copyright 2009. (B) Assembled cowpea chlorotic mottle virus virion templates with mineralized cores of (NH4)10H2W12O42, unstained (left) and negatively stained (right). Adapted from ref 43 with permission from Wiley, copyright 1999. (C) Silicon nanowires grown by vapor−liquid−solid extrusion in 1 h at 600 °C in a 20% disilane and 80% He solution at a pressure of 5 × 10−4 Torr. Adapted from ref 44 with permission from Macmillan Publishers Ltd. (Nature), copyright 2006. (D) Cartoon demonstrating the preferential binding of the surfactant to the side Au{100} and Au{110} faces, over the {111} faces at the ends, results in blocking the nanorod growth at the sides and promotion of nanorod growth at the ends. Adapted from ref 45 with permission from Elsevier, copyright 2011. (E) Branched nanotree structures grown by screw dislocation driven growth. Adapted from ref 46 with permission, copyright 2013. (F) Single crystal of anatase hydrothermally grown by oriented attachment. Adapted from ref 47 with permission from Elsevier, copyright 1999.

dissolution. Both nucleation and growth occur during this stage; however, this stage is dominated by rapid nucleation. Nucleation continues until the monomer concentration drops back below (Cmin * ), signaling the end of this stage (Aii) and the beginning of the growth stage (Aiii), where monomer is consumed through addition to previously nucleated particles until the monomer concentration reaches saturation (Cs).48,49 Successful preparation of monodisperse particles in the LaMer model requires temporal separation of nucleation and growth. 633



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seed-mediated synthesis has led to the synthesis of low aspect ratio (2−5) single-crystalline nanorods utilizing smaller, CTABstabilized seed nanoparticles (1.5 nm) with varying trace amounts of silver nitrate to control the aspect ratio.1,62,63 Additional empirical developments in synthesis include a “seedless” (in situ seed generation) synthesis,64,65 adjustment of gold and ascorbic acid ratios for gram-scale synthesis,66 millifluidic synthesis,67 identification of a problematic trace iodide impurity in stock reagents and its synthetic effects,68−70 and addition of salicylate compounds,57,71 Gemini surfactants,72 and alternative reducing agents.73 Most anisotropic nanoparticles produced by seed-mediated growth are kinetically controlled products. Thermodynamic arguments for morphological control are typically invoked through preferential adsorption of surfactant molecules on different crystal facets of seed crystals. However, recent electrochemical experiments on the adsorption of CTAB on specific crystallographic planes of flat gold demonstrate little thermodynamic preference between crystal faces, despite different levels of bromide adsorption.74,75 It is possible that sufficient differences in the electronic state of each facet exists and promote preferential surfactant cation adsorption.74,75 Overall, the extensive body of work on anisotropic seedmediated growth illustrates that there are many subtle avenues of influence over the growth kinetics that produce dramatic effects in product morphology and yield. The seed-mediated growth of anisotropic nanoparticles is not limited to gold; the subtle manipulation of kinetically controlled growth is observed in other materials as well. A synthesis involving the heating of a polyol with a metal salt precursor and a polymeric capping agent has been developed to generate anisotropic metal colloids. The Xia group used this method to produce bimetallic nanowires of silver on platinum seeds using ethylene glycol, silver nitrate, and poly(vinylpyrrolidone).76−78 Further work with this polyol synthesis has revealed numerous variables in manipulating the anisotropic growth, including: the metal salt and capping agent ratio,77 temperature,77 seed concentration,77 seed size distribution,77 seed crystal structure,77,78 seed material,77,78 capping agent and its molecular weight,78 counterions and their concentrations,79−81 and iron and oxygen impurities.79 Similar polyol-based routes to anisotropic particles have been observed for platinum,82−84 palladium,85−87 gold,85,88 rhodium,89 and numerous bimetallic systems.76,87,90 Seed-mediated growth has also been combined with coreduction techniques to create bimetallic nanoparticles. Notable work by the Skrabalak group with gold and palladium resulting in two structurally distinct but kinetically related productsoctopods and concave core@shell nanocrystals illustrate the importance of careful kinetic control.91 Continued research on such coreduction techniques has shown the importance of the ratio of metal precursors,92,93 growth solution pH,92,94 capping agent concentration,92 seed morphology,95,96 and counterions94 in controlling the morphology of branched bimetallic nanocrystals. Most interesting in this system is that despite dramatic changes in morphology as a function of pH,92,94 little effect on the metal deposition rate is observed.94 Anisotropic and heterogeneous semiconductor nanocrystals compose yet another important class of nanomaterials aided by seed-mediated growth techniques. As with gold, the first seedmediated semiconductor structures were epitaxially grown, isotropic core@shell structures.97 Advances in manipulating the

Figure 3. (A) LaMer model for monodisperse nanoparticle formation employed in seed nanoparticle formation (Cs, solubility level; C*min, * , maximum minimum monomer concentration for nucleation; Cmax monomer concentration for nucleation; Ai, prenucleation stage; Aii, seed nucleation stage; Aiii, seed growth stage). (B) Extension of the LaMer model to seed mediated growth (C*seeds, minimum monomer concentration for nucleation on a seed; Bi, prenucleation stage; Bii, nucleation stage on a seed nanoparticle; Biii, nanoparticle growth stage).

Seed-mediated growth involves not only the temporal separation of nucleation and growth, but quite frequently their spatial separation into different reaction vessels. This allows for the careful manipulation of growth kinetics and the morphology and crystal habit of the material produced during growth (Figure 3B). This spatial separation also allows for the material precipitated during growth to be of a different elemental composition from the seed material. Similar to the LaMer diagram, monomers are generated by various methods; however, ideally monomer concentration remains below the level required for self-induced nucleation (Bi). The addition of seed nanoparticles (Bii) allows for the nucleation of the second material on the seed nanoparticles only, which requires a lower solution concentration than self-induced nucleation. In practice, however, the monomer concentration can be high enough for self-induced in situ generation of new seeds, contributing to increased polydispersity of the product nanoparticles. When the seeds and growth material are the same, distinguishing between intentionally added seeds and in situ seeds becomes impossible. However, careful control and manipulation of the growth conditions can avoid in situ seeds with seemingly small changes leading to anisotropic growth of the material onto the intentionally added seed nanoparticles (Biii).

Tiny variations in the growth conditions are important in determining anisotropy. One of the first examples of seed-mediated growth of colloids was done by Zsigmondy for spherical gold particles.49,52 More recently, anisotropic gold colloids have been produced via seedmediated growth,53 illustrating that tiny variations in the growth conditions are important in determining anisotropy in this type of growth. The earliest protocol for the seed-mediated growth of gold nanorods employed 3.5 nm citrate-stabilized gold nanoparticles as seeds added sequentially to three growth solutions, each consisting of HAuCl4, cetyltrimethylammonium bromide (CTAB), and ascorbic acid.54−56 The aspect ratio of the rods obtained can be controlled by varying the amount and size of seeds added, organic additives and cosurfactants,57,58 and the halide counterion.54,55,59−61 The morphological yield of the penta-twinned nanorods of high aspect ratio (>10) can also be improved by adjusting the pH.56 Further development of this 634



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morphology of semiconductor nanocrystals98 using seedmediated growth have produced heterogeneous anisotropic nanoparticles of rod99−101 and tetrapod100,101 morphologies. Morphological control in these semiconductor systems has been achieved by controlling the seed’s crystal phase100,102 or morphology,103 leading to very different final nanoparticle morphologies. This work has even been extended to the synthesis of heterogeneous anisotropic structures consisting of both semiconductors and metals.35,104,105

similar precursors, surfactants, and solvents, which are all mixed into the same reaction vessel, and all of the contents are heated. The elevated temperature leads to the formation of monomers, which leads to nucleation and nanoparticle growth. Nanoparticle synthesis using the noninjection method has the advantage of scalability over hot-injection, but the less-rapid onset of nucleation can increase the polydispersity.114 Like in the hot-injection method, shape anisotropy can be achieved by choosing the surfactants such that the crystal faces grow at different rates. Park et al. used the noninjection method to made anisotropic CdSe nanoparticles with tetrapod and matchstick shapes.114,115 The preparation of anisotropic oxide nanoparticles is difficult using sol-gel type syntheses. Sol-gel methods usually rely on the hydrolysis of precursor compounds into monomers that coalesce via condensation reactions to form particles. Because the syntheses are usually conducted at room temperature, the resultant nanoparticles are typically amorphous116 and the adsorption of monomers and particles is not directed but occurs isotropically, as in the Stöber method to synthesize silica nanospheres.117 High temperature syntheses, such as hydrothermal and solvothermal methods, result in more crystalline nanoparticles, for which kinetic shape control (such as that used for seed-mediated growth) can lead to anisotropic nanoparticles.118,119 Templated synthesis methods can also be used to prepare anisotropic nanoparticles. Hard templates can be rigid nanostructures that the desired anisotropic nanoparticles are grown in, such as the porous membranes described by Martin in which conductive nanocylinders were grown.120 Anisotropic nanoparticles can also serve as hard templates on which other materials can be grown: Obare et al. demonstrated the growth of polystyrene and silica shells on long gold nanorods,121 and Abadeer et al. successfully grew silica shells on short gold nanorods.51 The hard template acts to direct the shape, even though the materials being synthesized are isotropic. Hard particle templates can be selectively dissolved away, yielding hollow nanoparticles of the shell material.121 In soft template methods, microemulsions and micelles act as the templates. The use of soft templates often takes advantage of differences in the solubility of precursor and monomer compounds between the templating droplets and the continuous phase. Although soft templates are often spherical, Kujik et al. used a related technique to synthesize silica nanorods.122 Cryogenic transmission electron microscopy (Cryo-TEM) images showing silica nanorod growth over time are shown in Figure 4. To

There are many subtle avenues of influence over the growth kinetics that produce dramatic effects in product morphology and yield. Alternative Synthetic Techniques for Anisotropic Nanoparticles. Besides seed-mediated growth, a number of other techniques have been developed for the shape-controlled synthesis of metallic and semiconducting nanoparticles. In the case of gold nanorods, various methods, including photochemistry,106 electrochemistry107 and sonochemistry,108 have been applied to reduce the Au3+ ions to Au0, replacing molecular reducing agents and seeds. As in the seed-mediated growth of gold nanorods, CTAB directed the anisotropic growth and Ag+ controlled the aspect ratio in these examples.106,107 Single crystalline gold nanobelts have also been successfully synthesized in the presence of α-D-glucose and under ultrasound irradiation to form reducing radicals.108 The hot-injection method has been particularly important for synthesizing anisotropic semiconductor nanocrystals. This method has been popular since Murray and co-workers demonstrated the utility of the hot-injection method for the synthesis of cadmium chalcogenide quantum dots in the early 1990s.109 To make nanoparticles using the hot-injection method, precursors are injected into a hot solution of surfactants in a solvent. The elevated temperature causes the precursors to react or decompose to form the atomic or molecular monomers that form the nanoparticles. High initial monomer concentrations induce the nucleation of seed particles, followed by a phase during which the leftover monomers add onto the seeds, leading to the growth of nanoparticles.110 By choosing surfactants and cosurfactants judiciously, Peng et al. demonstrated hot injection methods could be used to grow anisotropic nanoparticles.98 As mentioned earlier, the mechanism of anisotropic nanoparticle growth is typically governed by kinetics; anisotropic shapes are not the most thermodynamically favorable products.110,111 Yin and Alivisatos have suggested that the anisotropic growth of CdSe nanorods could be due to the stabilizing effects of organic ligands binding selectively to certain crystal faces, causing slower growth relative to other faces.110 Peng suggested that anisotropic growth may also be attributed to the chemical potential of the monomers: the higher their chemical potential, the faster the growth will be of high energy faces relative to low energy faces.112 The dependence of particle morphology on the concentration of the monomers lends support to this idea.112,113 However, explaining the initial symmetry-breaking events that lead to anisotropic growth remains a challenge.111 Closely related to the hot-injection method is the noninjection, or heat-up method.114 The noninjection method uses

Figure 4. Cryo-TEM images of silica nanorods after (A) 30, (B) 60, and (C) 120 min of growth. Adapted from ref 122 with permission, copyright 2011.

make these rods, the authors first prepared a polyvinylpyrrolidone- and sodium citrate-stabilized microemulsion of water in pentanol. When tetraethyl orthosilicate was added, the hydrolyzed form partitioned to the aqueous phase, so the condensation reactions to form silica occurred at the surface of the water droplets. Because the condensation reactions took 635



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rods.126−129 These chemically anisotropic rods thus have “activated” ends that can direct further chemistry or interparticle interactions.

place only on one side of the water droplet, the growth of the silica nanorods was anisotropic. Oriented attachment is another method for growing anisotropic particles, in which small nanocrystals serve as building blocks that self-assemble into complicated structures. Due to the high surface-to-volume ratio and high surface energy of the nanocrystals, the assembly of the crystals is favored. Because the facets’ energies tend to be different, often due to preferential binding of ligands, the assembly can be oriented and anisotropic architectures can be achieved. Banfield and coworkers demonstrated the successful synthesis of a 1D structure from the attachment of TiO2 nanocrystals along the (001) direction, where the facets have the highest surface energy.47 Similar methods have also been applied to synthesize gold nanowires assembled from nanocrystals with a diameter of around 2 nm.123 On the basis of a detailed electron-microscopy study, the nanocrystals were found to align along the (111) direction, followed by a side-smoothing diffusion process to achieve nanowires up to 1 μm long. Regiospecif ic Surface Chemistry. Nanoparticles with shape anisotropy can be further modified to create chemical anisotropy. Many anisotropic growth mechanisms leave multiple exposed crystal facets on a single nanoparticle. As in the oriented self-assembly mentioned above, the reactivities of the exposed facets can be exploited to direct regioselective chemistry on the surface of the particle. A basic example of regiospecific chemistry on anisotropic nanoparticles was shown by Banin and co-workers through the deposition of metallic gold onto the tips of CdSe nanorods and tetrapods.104 In a series of groundbreaking experiments, gold salt was reduced onto CdSe nanoparticles with a clear preference for the tips rather than the sides. This phenomenon was further expounded upon by Kudera et al., who achieved control of PbSe growth on one or both tips of CdS and CdSe nanorods by a simple injection of PbSe precursors.103 In both cases, the differing crystal facets allowed for preferential deposition of metal atoms on the ends, rather than the sides. Transmission electron microscopy (TEM) imaging was used to provide clear confirmation of metal overgrowth, whereas optical absorbance experiments provided ensemble measurements that showed a distinct difference in optical properties of materials with and without deposited metal. Although deposition of metallic salts provides an inorganic route to postsynthetic modification, it is also desirable to assign organic functionality to certain locations of an anisotropic particle. Much research effort has been dedicated to controlling the placement of functional ligands on gold nanorods, in particular, because CTAB-stabilized gold nanorods have an inherent, sterically driven advantage in anisotropic functionalization. Because there is a higher radius of curvature at the nanorods’ ends, the arrangement of surfactant molecules is expected to be less dense. Whereas CTAB is tightly packed at the rods’ longitudinal facets, the end faces are more chemically accessible. The rods’ ends are indeed more reactive to cyanide dissolution, presumably because of the paucity of CTAB there.124 Interestingly, this anisotropic effect seems to be unique to CTAB-like surfactants: rods passivated by a bilayer of phospholipids appear to have a fairly uniform, densely packed bilayer, even at the highly curved ends, perhaps a function of the lipid’s geometry.125 CTAB’s presumed fluxionality at the ends appears to be a fortuitous blessing because many scientists have taken advantage of the increased accessibility to adsorb functional thiols regioselectively to the tips of gold nano-

Chemically anisotropic rods thus have “activated” ends that can direct further chemistry or interparticle interactions. Self-assembly behavior, usually visualized by TEM and ultraviolet−visible (UV−vis) absorption spectral changes, is often the primary evidence for successful anisotropic functionalization. One such example is in our group, where a biotin-disulfide molecule was synthesized and placed on the ends of gold nanorods.130 Because of the tight CTAB packing along the sides of the rod, the disulfide was hypothesized to adsorb only onto the tips. Further incubation with the protein streptavidin, which can bind multiple biotin molecules, led to favorable end-to-end assembly of the rods. In a similar fashion, Zhen et al. showed that the tips of gold rods could be functionalized with a thrombin-binding aptamer, and the protein thrombin could be used to self-assemble the rods end-to-end.131 There are many such examples of “endactivated” rods, but it is much more difficult to exclusively activate the sides of the rods, which are typically coated with CTAB, which is functionally less interesting. Xu et al. cleverly solved this issue by “deactivating” the ends with nonreactive ligands. They first treated the CTAB rods with small amounts of a thiolated nontarget DNA strand, which attached to the ends and blocked them from further reaction. Then, they added active DNA thiols, which were forced to adsorb onto the sides of the rods, presumably penetrating the CTAB bilayer. After incubating the rods with complementary DNA-functionalized gold nanospheres, the authors observed by TEM that the spheres attached almost exclusively to the sides.132 Their study illuminates the importance of the ratio of thiol to nanorods; at low concentrations of thiol, the ligand attaches primarily to the ends, but high concentrations preclude any type of regioselective functionalization. In an experiment that exploits the enhanced reactivity of loose regions of the CTAB bilayer and exposed parts of the metal surface, Wang and co-workers developed methods to selectively deposit metals such as platinum and palladium onto the ends or sides of gold nanorods (Figure 5).133 To achieve this, the authors first coated either the ends or sides of the rods with silica. Silica coating of the rods’ ends was realized through preferential adsorption of tetraethyl orthosilicate (TEOS) on the ends, whereas side coating involved an initial end-blocking step with thiolated PEG, which forced TEOS to hydrolyze along the rods’ sides. Following this, anisotropic overgrowth of metals was performed by simply reducing metal salt on the exposed nanorod surfaces. Successful metal overgrowth was shown by TEM, as silica and gold atoms under an electron beam will have clear contrast differences; furthermore, the optical properties of the gold nanorods were changed as the plasmon was perturbed by the presence of deposited metals, which was shown in the optical absorbance spectra. In a large collection of work, Kumacheva and colleagues have shown that even bulky, hydrophobic polymers can be affixed preferentially to gold nanorod ends. By using a thiol-terminated polystyrene to target the ends, the authors created “triblock” nanorods, the plasmonic analogue of amphiphilic block 636



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sensing. The use of anisotropic noble metal nanoparticles in SERS is preferred because upon light irradiation, electromagnetic fields at tips in anisotropic particles are orders of magnitude higher than in isotropic nanoparticles. It would be particularly noteworthy to be able to use end-to-end dimers of plasmonic nanorods with nanoscopic space between them for SERS applications. This type of assembly would generate an even more enhanced electric field, specifically at the intersection between the tips. However, molecules of interest would need to have access to that space to induce a highly enhanced Raman signal. A few researchers have reported the preparation of nanoparticle dimers,140,141 but dimers of anisotropic nanoparticles are difficult to prepare in high yields. More often, extended lengths of self-assembled nanorod chains (larger than dimers) are prepared and result in variation in the strength of the electromagnetic field and SERS effect.142 With better knowledge and control of anisotropic surface coatings and anisotropic nanoparticle dimers, researchers will be able to demonstrate even greater potential in SERS applications.

Figure 5. Routes of metal overgrowth. (A) Au nanorod capped with a CTAB bilayer. (B) Nanorod coated with silica at the ends. (C) Nanorod overgrown with a metal on the side surface. (D) Au nanorod bonded with mPEG-SH at the ends. (E) Nanorod coated with silica on the side surface. (F) Nanorod overgrown with a metal at the ends. Reproduced from ref 133 with permission from Wiley, copyright 2013.

polymers.134 Their amphiphilic nature afforded the rods interesting self-assembly properties and in various solvent mixtures they formed chains, rings, bundles, or spheres. Tuning the molecular weight of the polystyrene altered the final morphology of the supramolecular assemblies. In subsequent studies, similar amphiphilic rods were treated as monomers that could be “polymerized” in a step-growth-like mechanism into supramolecular plasmonic chains.135,136 The rates and degrees of polymerization, and the bond angles and lengths between monomers, were characterized and controlled in an elegant extension of the analogy. In one example of chain length control, patchy Au/Fe3O4 nanospheres with polystyrene and polyethylene glycol domains served as colloidal chain stoppers, as the “reactive” polystyrene patch attached to the growing chain, and the exposed polyethylene glycol halted further hydrophobically driven polymerization.137 Clearly, amphiphilic nanorods show promise as a way to achieve fine control in bottom-up nanoassemblies. Theoretical studies have aimed to generalize the design rules of these polymer-tethered rods for further insight.138,139

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE 1306596) for funding. REFERENCES

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Amphiphilic nanorods show promise as a way to achieve fine control in bottom-up nanoassemblies. Future Directions. Nanoparticles of noble metals, semiconductors, and soft materials have become ubiquitous in research studies, and their applications range across fields such as cancer therapy, disease detection, and energy generation. The advent of facile, reproducible, and scalable synthesis techniques for anisotropic nanoparticles has extended these potential applications even further. By using different reactivities of crystal facets and packing of ligand layers, the field has gained another degree of control for the self-assembly and application of nanoparticles. However, much more work needs to be done in order to fully characterize and exploit this mechanism of control. One key to better understanding such coatings is the ability to identify and quantify the ligands on a nanoparticle surface as a function of position, a challenge due to the small size of nanoparticles and polydispersity often present in as-synthesized nanoparticle dispersions. Surface enhanced Raman spectroscopy (SERS) is a technique that has garnered much interest for molecular 637



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