Branched Plasmonic Nanoparticles with High Symmetry - The Journal

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Branched Plasmonic Nanoparticles with High Symmetry Joshua D. Smith, Zachary J. Woessner, and Sara E. Skrabalak*

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Indiana University - Bloomington, Department of Chemistry, 800 E. Kirkwood Avenue, Bloomington, Indiana 47405, United States ABSTRACT: Branched plasmonic nanoparticles (NPs) composed of noble metals are an interesting subclass of plasmonic NPs due to their unique properties that arise from the strong electric field enhancements that occur at their tips. The plasmonic properties of branched metal NPs can be manipulated through altering their symmetry and structural features such as branch sharpness, composition, and their surroundings. In this Featured Article, the unique optical properties that arise from branched plasmonic NPs are introduced, which were revealed through pioneering studies of Au nanostars and related structures. Next, branched NPs with high symmetry are discussed as a model system to explore more fully how parameters such as NP size, shape, and composition impact their properties, enabling applications in chemical sensing and beyond. These studies provide design criteria and synthetic strategies toward new nanostructures with increasing structural and compositional complexity.

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INTRODUCTION Plasmonic metal nanoparticles (NPs) provide new platforms for security devices, biological and chemical sensors, heterogeneous catalysis, solar cells, plasmon-enhanced spectroscopies, and even theranostics.1−4 The diversity of these applications arises from the ability to readily tune the localized surface plasmon resonances (LSPRs) of NPs. LSPRs occur in plasmonic materials (i.e., those that exhibit negative real permittivity; metals or metal-like materials) that have one or more dimensions smaller than the wavelength of incident light.5 A surface plasmon resonance is produced when a material’s electron density couples with incident electromagnetic (EM) radiation to create a coherent, collective oscillation of the conduction electrons.5 By confining a resonance to dimensions smaller than the wavelength of light, the electron density oscillates locally around the NP, and, thus, the plasmon’s behavior is defined by the structural features of the NP.5 This localization means that the extinction (scattering + absorbance) spectra and electric field (EF) enhancements of plasmonic NPs can be modified for specific applications by changing their size, shape, composition, and environment.5 For example, the extinction spectrum of a spherical metal NP can be calculated by eq 1 Ä ÑÉÑ 3/2 Å ÅÅ ÑÑ 24π 2Na3εout εi(λ) ÅÅ ÑÑ E (λ ) = Å λ ln(10) ÅÅÅÅÇ (εr(λ) + χεout)2 + εi(λ)2 ÑÑÑÑÖ (1)

more expansive discussions. In general, the EM behavior of plasmonic NPs defined by simple convex shapes (e.g., spheres, cubes, octahedra, etc.) is well-understood and provides a foundation for understanding the plasmonic properties of branched NPs. Introducing complexity to NPs in the form of concavities (e.g, branched structures), architecture (e.g., core@ shell structures), and composition (e.g., alloys) enables new and interesting optoelectronic properties to be accessed, as will be illustrated with our discussion of branched plasmonic NPs herein. Branched plasmonic NPs, such as nanostars,8−10 spiky gold nanoshells,11,12 and Au dendrimers,13,14 have received much interest in spectroscopy and sensing applications due to the strong EF enhancements at their tips as depicted in Figure 1a. One of the earliest examples of branched plasmonic NPs came from the groups of Schatz and Hupp, wherein a mixture of Au NPs with one, two, or three tips was synthesized in over 90% yield.15 To understand the optical properties of these NPs, electrodynamics calculations based on the discrete dipole approximation (DDA) were undertaken for a model of a threetipped NP; they found that the LSPR was mainly determined by the in-plane dipole excitation associated with the sharp tips. Correspondingly, the LSPR spectrum was very sensitive to the length and sharpness of the tips but not so much to the thickness and overall NP size. Moreover, the calculated E-field revealed large enhancements at the tips of the NP, up to 3900 times greater than the applied background field. Notably, strong EF enhancement can lead to enhanced molecular signatures in surface-enhanced Raman spectroscopy (SERS)

in which size (a, radius), shape (χ, which equals 2 for a sphere), composition (εi and εr, the imaginary and real components of the metal dielectric function), and environment (εout, dielectric of surrounding media) all contribute to the scattering and absorption behavior. We refer readers to reviews by Hafner,3 Van Duyne,5 Fendler,6 and Garciá de Abajo7 for © XXXX American Chemical Society

Received: February 22, 2019 Revised: April 20, 2019

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DOI: 10.1021/acs.jpcc.9b01703 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

specific synthetic conditions. Regardless, this size dependence is similar to reports of NPs with convex shapes and arises because of the increased distance between regions of opposite charges (i.e., the plasmon length), in turn, reducing the restoring force for the oscillation.24 Using finite-difference time-domain (FDTD) calculations, Hafner and Nordlander showed that the plasmons of such nanostars arise from hybridization of the core and tip plasmon resonances, providing a model for the physical origin of plasmons in structurally complex NPs.25 Their analysis also showed that the core serves as an antenna, increasing both the excitation cross section and near-field enhancement at the tips.25 The ability to tune the LSPR wavelengths and the strong EF enhancement at the tips of Au nanostars makes them ideal substrates in SERS applications.25,26 The former can ensure good overlap with specific laser wavelengths, while the latter can provide a large enhancement factor (i.e., the magnitude of increase in the Raman scattering cross-section), scaling as E4, where E is the electromagnetic field.26 For example, simulations by Liz-Marzán and co-workers revealed that molecules adsorbed preferentially to the sharp tips of Au nanostars should experience 10-fold SERS signal enhancements when compared to molecules adsorbed preferentially to the convex cores. 20 Recent work by the Lu group demonstrated that highly symmetric (Ih symmetry) Au nanostars provide fourfold greater SERS enhancement and higher reproducibility than asymmetrically branched Au NPs.9 Along with SERS applications, branched plasmonic NPs are interesting platforms for drug delivery and as contrast enhancement agents and photothermal therapeutics; these potential applications are enabled by the ability to tune the LSPR of such structures into the near-infrared region, which corresponds with where light has its maximum depth of penetration in tissue (i.e., the therapeutic window).27−30 Notably, precise control over the structural features of branched plasmonic NPs is central to their use in these applications, where branch parameters such as their length and width as well as the sharpness of the tips, the number of branches, and the angles between branches are important.24,31−34 As one demonstration of how the LSPR depends on the finer structural features of branched NPs, Fan and coworkers prepared Au tetrapods with different branch lengths and varying tip sharpness by reducing Au salts with 4-(2hydroxyethyl)-1-piperazine propanesulfonic acid in an aqueous solution. The synthetic control of the branching allowed the LSPR position of the Au tetrapods to be tuned between 650 and 785 nm.35 These experimental findings were coupled with FDTD simulations of model Au tetrapods (Figure 1b−d) and revealed that the LSPR peak shifts to longer wavelengths by increasing the branch length (L) (Figure 1b), decreasing the apex angle (θ) (Figure 1b), or decreasing the angle between branches (α) (Figure 1c).31 The red-shift in the LSPR peak from increasing L is expected given that the plasmon length increases.24 These angle dependences arise as they modify the dipole interaction.35,36 Given the sensitivity of the LSPR position and profile to the structural features of branched NPs, much effort has been dedicated to controlling the structural features of branched NPs.21,37−41 Although substantial improvements in NP uniformity have been achieved in many cases, fine-tuning structural features such as apex angle and branch length has remained difficult.42 These variations in structure can give rise to highly variable optical properties when comparing individual particles and

Figure 1. Schematic and simulations displaying the plasmonic behavior of branched NPs. (a) Schematic illustrating the hot spots at which the EF enhancement is greatest about various NPs and (b− d) finite-different time-domain simulations demonstrating the impact of (b) branch length, apex angle, and (c) angle between branches. (d) The gold tetrapod model used in simulations that displays L as the branch length, θ as the apex angle, and α as the angle between branches. (a) and (b−d) reproduced with permission from refs 12 and 31, respectively.

and other plasmon-enhanced spectroscopies, as reported by many groups.16,17 A particularly promising and well-studied class of branched NPs is Au nanostars. These nanostructures consist of Au cores from which many branches with very sharp tips and with random orientations emanate.18,19 There are several different synthetic procedures to achieve Au nanostars, with most based on seeded methods.20−25 During seed-mediated growth of Au nanostars, Au salts are reduced in the presence of seeds and capping agents.21 This process results in deposition of Au atoms at poorly defined sites on the seeds, resulting in randomly oriented branching.21 Given this mechanism, the plasmon resonance of Au nanostars is traditionally tuned by adjusting the ratio between metal precursors and the number of seeds. For example, decreasing the amount of seeds in a solution (while keeping all other parameters constant) usually results in the size of the NP to increase and, in turn, for the LSPR to red-shift. This increase in size can correspond to both an increase in core dimensions (conformal deposition) as well as branch lengths (anisotropic growth) depending on the B


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Figure 2. Finite-difference time-domain simulation comparing the plasmonic behavior of branched NPs with different symmetries. The far-field scattering spectra of (a) Au D4h bowtie and Oh octopod with unpolarized light being introduced along the C4 and C2 axes of the bowtie and C4, C3, and C2 axes of the octopod, (b) the bowtie with the incident light polarized along the transverse (black), longitudinal (red), face diagonal (dashed) axes, (c, d) the near-field enhancement maps of an octopod with light introduced along the C4 axis, and (e−g) a bowtie with light introduced along the C2 axis with the light polarized in directions indicated by the white arrows. Models on the right indicate the introduction of light at the origin for different rotational axes. The scale for near-field enhancement maps in (e, f) are equivalent. Adapted from reference (56). Copyright 2016 American Chemical Society.

may lead to batch-to-batch variations.9,21,29 Moreover, branched Au NPs often suffer from structural instability due to the high surface mobility of Au atoms.43,44 Recently, Bals and co-workers studied the restructuring of Au nanostars into Au spheres by fast in situ electron tomography.45 They reported significant restructuring after only 30 s of nanostar heating at 200 °C.45 Others have reported short shelf life when branched NPs are stored in solution as well as restructuring after illumination, likely due to the high temperatures reached during photothermal heating.46,47 Such instability can be problematic, as many of the applications of these structures require that the LSPR overlap with specific wavelengths of light (e.g., SERS, photothermal, and photocatalytic applications). Being inspired by the opportunities that arise from branched plasmonic NPs, this Feature Article highlights our group’s research involving the synthesis of branched metal NPs with high symmetry and the study of their symmetry- and composition-dependent optical properties.

occurs if the charges are separated by a mirror plane),53 and (iv) the near-field enhancement is largest when the charge accumulation occurs at the least number of vertices.54,55 Recently, our group used the FDTD method to investigate how the symmetry of branched NPs influenced both LSPR position and near-field enhancement.56 Models of branched Au NPs with D3h, C2v, Td, D4h, and Oh symmetries were analyzed to elucidate the polarization-dependent LSPR responses and EF enhancements as a function of NP symmetry.56 Shown in Figure 2 are FDTD simulations that compare the plasmonic responses of eight-branched Au octopods (with Oh symmetry) and eight-branched Au bowties (with D4h symmetry). Both models were investigated by introducing light sources that propagated along their various rotational k-Cn axes, as defined by their symmetry; the light was either polarized in the direction indicated by an arrow in the figure or unpolarized, depending on the objective of the study.56 Shown in Figure 2a are the far-field scattering spectra of the eight-branched NPs with Oh symmetry when unpolarized light is introduced down the k-C2, k-C3, and k-C4 axes.56 Only one LSPR feature is evident, and its position is largely unaffected by the propagation direction. These spectra are consistent with the high symmetry of the structure. For comparison, shown in Figure 2a are also the far-field scattering spectra of the eightbranched bowtie NPs with D4h symmetry when unpolarized light is introduced down the k-C2 and k-C4. Now, two major LSPR features are evident for k-C2, consistent with a reduction in symmetry. Similar to Au nanorods,51 transverse and longitudinal LSPR bands can be observed when the light is introduced down the k-C2 axis of the bowtie (Figure 2a,b). Interestingly, the transverse or longitudinal band can be turned on or off by simply changing the polarization and direction of incident light (Figure 2b) and not reducing the symmetry.56 Given this, a more universal rule can be established in which the number of LSPR resonances, as well as their position, is

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DISCUSSION Plasmonic Properties of Branched Nanoparticles with High Symmetry. As the study of Au nanostars and related structures has shown, both increasing the size of the NPs and tip sharpness result in red-shifts in the LSPR. Finer control over both the near- and far-field properties of branched NPs can be envisioned through symmetry control, as the study of convex plasmonic NPs has revealed symmetry-specific design rules.48−52 For example, the study of shape-controlled Ag NPs has led to the following guidelines: (i) the number of LSPR peaks is determined by the number of different dipole resonance modes available, with lower symmetry structures displaying more modes;34,50 (ii) the position of the LSPR is determined by the plasmon length (i.e., the distance between the charge separations);24 (iii) the intensity of the LSPR peak will be large when the effective dipole moment is large (which C


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The Journal of Physical Chemistry C determined by the particle symmetry and orientation with respect to incoming light.56 Along with this rule, the scattering efficiency and near-field enhancement of the branched NPs are greatest when the dipolar modes are on opposite sides of a mirror plane and the core is in a plane perpendicular to the direction of the incident light, as evident in scattering efficiency of longitudinal and transverse bands in Figure 2b and the improved near-field enhancement for the transverse (Figure 2g) and longitudinal modes (Figure 2e).56 Overall, Figure 2 demonstrates that the number and intensity of the LSPR characteristics are dependent on the polarization of light, the orientation of the particle, and the contribution from the core plasmon. Similar analyses of branched Au NPs with different symmetries revealed the following additional rules. First, increasing the number of dipolar modes does not require a reduction in symmetry, and a reduction in symmetry does not necessarily lead to in an increase in the number of modes.56 This rule became evident when comparing three-branched Ttripods (C2v), three-branched Y-tripods (D3h), and fivebranched pentapods (D3h), where the position of the LSPR peak shifted, but the total number of LSPR peaks remained constant for the three different structures.56 Along with the farfield properties of branched NPs, the EF enhancement is greatest when the charge separation results in the largest effective dipole as observed in convex shapes (Figure 2c−g);54 however, the largest EF enhancement does not require that charge accumulate on the fewest tips.56 This is readily observed in Figure 2e−g, in which all three cases result in charge accumulation at the same number of tips but with drastically different enhancement factors.56 Furthermore, Au hexapods with six branches and Oh symmetry displayed equivalent EF enhancements when the light was polarized in a manner to have charge accumulation on two and four tips.56 This result further suggests that overall effective dipole and not the number of tips govern the strength of EF enhancement observed for branched NPs.56 In an analysis of the impact of symmetry on the plasmonic properties of branched NPs, similarities to convex NPs were identified, including the orientation-dependent plasmonic properties of a NP, the ability to turn on specific LSPR bands, and the increased EF enhancement that occurs at the greatest effective dipole.56 However, branched NPs have an amazing ability to concentrate EFs at their tips, where the magnitude and distribution of this enhancement depends on NP symmetry as well as branch size, tip sharpness, and particle orientation relative to the incident light. Resultantly, as the understanding of the plasmonic properties of branched NPs has grown, so has the understanding of the synthesis of branched metal NPs. Toward the objective of controlling structural features of branched metal NPs, syntheses that start with shape-controlled NPs have opened pathways toward branched NPs with high symmetry.9,38,57−59 For example, shown in Figure 3 are examples of branched Au/Pd NPs with different, well-defined symmetries; these NPs were synthesized by seed-mediated coreduction (SMCR) wherein Au and Pd precursors were coreduced in the presence of shape-controlled seeds, with branches proceeding from the seed vertices under kinetically controlled growth conditions (Figure 4a).57 Plasmonic Properties of Branched Au/Pd Nanoparticles with High Symmetry. When considering eq 1 and the discussion of the previous section, the contribution of NP size and shape to the plasmonic properties of branched

Figure 3. Different symmetries synthetically available, where the SMCR product retains the symmetry of its seed after coreduction. Different symmetries available are (a, f) Oh, (b, g) D4h, (c, h) Oh, (d, i) Td, and (e, j) D3h. Models of the final NPs are shown in (a−e), with the SMCR products shown in (f−j). Adapted from reference (57). Copyright 2013 American Chemical Society.

Figure 4. Synthetic route to alloyed Au−Pd octopods via seedmediated coreduction (a) and typical composition of a Au−Pd octopods. TEM image of an alloyed Au−Pd octopod (b) with STEMEDS elemental mapping for Pd (c), shown in red, Au (d), shown in yellow, and the overlaid image (e). Reprinted from reference (85). Copyright 2012 American Chemical Society.

NPs become fully evident, but the impact of composition has yet to be discussed.5 Multimetallic plasmonic NPs have received much interest due to the combination of two or more elements leading to multifunctional NPs.60−62 For example, Christopher, Norlander, and Halas prepared antenna-reactor complexes in which core@shell Ag@SiO2 structures were used to support Pt catalysts, where the plasmonic Ag core introduced hot carriers in the Pt, which led to efficient CO oxidation.63 Moreover, mixing two metals in an alloy allows the dielectric function of the material to be tuned as a function of composition, and in turn, the plasmonic response.5,61 Although much research has gone into understanding plasmonic properties of bimetallic NPs as well as heterostructures,50,61,62 extending this insight into stellated NPs is in its infancy. The branched Au/Pd NPs with high symmetry achieved by our group (Figure 3) are a fantastic platform for studying how composition influences the plasmonic properties of branched NPs.57 Of the stellated Au/Pd NPs synthesized to date, eightbranched octopods with Oh symmetry have been most wellstudied. Scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDS) D


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correspond with a lower energy proximal mode (i.e., field intensity penetrating mainly into the higher refractive index Si3N4 substrate) and a higher energy distal mode (i.e., field intensity penetrating mainly into the lower refractive index vacuum).71 Strong near-field enhancements were observed despite the tips being enriched with the poor plasmonic metal Pd.71 Excitingly, the presence of distal and proximal modes of different energies indicates that these branched NPs are capable of sensing two different dielectric environments (see eq 1), opening LSPR sensing possibilities despite the poor plasmonic tips. The LSPR sensing capabilities of the Au/Pd octopods were also revealed during the collection of single-particle light scattering spectra of individual NPs. Shown in Figure 6a are the results obtained from an octopod with a tip-to-tip distance of 112 nm, with a proximal mode from penetration into the glass substrate (i.e., LSPR character occurring from tips being in proximity to the glass substrate) and a distal mode from greater penetration into air (i.e., LSPR character occurring from tips being distant from the glass substrate).72 A larger octopod with a tip-to-tip distance of 262 nm shows only the distal mode at longer wavelength as the LSPR restoring force is reduced with increasing size (note that the proximal mode is out of the detector range). FDTD simulations are in good agreement with simulations in Figure 2, where charge localizes at the tips of the octopod, but the anisotropic environment increases the number of plasmon peaks (Figure 6a−c).72 Interestingly, bimetallic branched NPs appear to follow the structural rules previously discussed that determine the LSPR response, where the symmetry of the bimetallic NP dictates the number of LSPR features present. Figure 6d displays simulated scattering spectra of Au octopods, and Figure 6e shows the dark-field spectra of Au/Pd octopods.72 Increasing the branch length and tip sharpness resulted in a red-shifted LSPR position, which is in good agreement with previously discussed rules of branched NP.72 However, an analysis of nearly 100 individual particles revealed that this structural analysis is too

reveal a complex Au−Pd distribution, wherein the nanostructures consist of a monometallic core that is consistent with the initial seed composition (typically Au or Pd) and Au−Pd alloy branches, wherein Pd is enriched at their tips (Figure 4c− e).64,65 Such a system is interesting, because it integrates traditionally good (e.g., Au, Cu, and Ag) and poor (e.g., Pd and Pt) plasmonic materials into one structure, opening the possibility for plasmon-enhanced catalysis, among other applications.62,66−68 Still, the addition of a poor plasmonic metal could be damaging to the quality of the plasmonic response and limit utility.69−71 The desire to better understand the plasmonic properties of these branched NPs led to a collaboration with Ringe and coworkers, who investigated the influence of Pd on the near-field response of the Au/Pd octopods with nanometer-resolution STEM electron energy loss spectroscopy (EELS).71 As can be seen in Figure 5, two plasmonic modes were deconvoluted that

Figure 5. Electron energy loss spectroscopy maps of a typical Au−Pd octopod. Schematic for the proximal mode of Au−Pd octopod and EELS probability at −30°, 0°, and 30° left to right (a) and for the distal mode of Au−Pd octopod with EELS probability at −30°, 0°, and 30° left to right (a). Reprinted from reference (71). Copyright 2015 Science Reports.

Figure 6. Investigation into the influence of tip sharpness and tip distance on the plasmonic response of bimetallic octopods. Spectral assignments (a) of plasmon modes for alloyed Au−Pd octopods of different sizes (112 nm face diagonal and 262 nm face diagonal). Solid red and black lines indicate experimental dark-field scattering for the 112 and 262 nm Au−Pd octopods, respectively, with the dashed-red line indicating the scattering spectrum for a 112 nm Au−Pd octopod via FDTD numerical simulation. FDTD simulated electric-field enhancement for a 112 nm Au−Pd octopod at both the distal mode maximum wavelength (b) and the proximal mode maximum wavelength (c). FDTD numerical simulations of the scattering spectra for Au octopods (d) of different face diagonal and tip widths. Dark-field scattering for alloyed Au−Pd octopods (e) of different sizes. Reprinted from reference (72). Copyright 2015 American Chemical Society. E


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The Journal of Physical Chemistry C simple; composition and structure are highly intertwined. In fact, heavy loading of Pd at the tips can lead to a red-shift even if the loading leads to wider tips.72 These single-particle studies reveal important properties of the Au/Pd octopods, including their potential utility for LSPR sensing applications. In practice, such applications typically rely on ensemble measurements, with one objective being to obtain high refractive index sensitivity (RIS). RIS is defined as the change in LSPR position (Δλ) per change in refractive index (Δn) (eq 2):5 RIS =

Δλ Δn

(2)

The ability to tune the LSPR of Au and Ag NPs through size, shape, and compositional control has led to the advancement of such NPs as platforms for LSPR sensors.73−76 As the LSPR maximum of a NP occurs when εi is relatively small and when εr = −χεout (eq 1), the material’s dielectric function plays a crucial role in determining the RIS.5,77 Depending on the spectral dispersion of the real part of the dielectric function, (i.e., the slope), the refractive index sensitivity can be predicted.77 This idea of spectral dispersion is illustrated when comparing Au and Ag nanoparticles, in which Ag particles have higher RISs than Au nanoparticles of similar size and shape, despite Ag having a more blue-shifted LSPR.74,78,79 Recently, interest in Pd as an alternative platform for LSPR sensing has emerged.77,80,81 This interest is, in part, motivated by the possibility for new functionality; however, Pd also has very low spectral dispersion of εr despite having a high εi.77 In effect, this situation means that, even though the LSPR of Pd NPs is expected to be broad, such NPs should have high RIS.73 This situation leads to an interesting balancing act where, on one hand, high RIS (through Pd εr) can be achieved, but, on the other hand, increased line-broadening (through Pd εi) of the LSPR peak occurs. This increase in line broadening can decrease the figure of merit (FOM) of LSPR sensors, which is defined as the RIS over the full width at half-maximum (fwhm) of the LSPR (eq 3). FOM =

RIS fwhm

Figure 7. Analysis of how the composition of the branch and core influence the refractive index sensitivity of branched NPs. STEM and STEM-EDX elemental mapping of octopodal structures consisting of a cubic Pd core, Au−Pd alloyed branches (a), cubic Pd core, Au branches (b), cubic Au core, Au−Pd alloyed branches (c), and Au core, Au overgrowth (d). Experimental refractive index sensitivity (e) for all Au octopods (red), Pd core, Au tips (red dash), Au core with 4.2% Pd tip (black dot), 5.8% Pd tip (black dash), 6.4% Pd tip (black), small Pd core with 4% Pd tip (blue dash), and large Pd core with 1.2% Pd tip (blue dot). FDTD simulated RIS (f) of all Au octopods (red), Au-core with a 4% Pd tip (black dot) and Au-core with 8% Pd tip (black), 4% Pd core, 4% Pd tip (blue dash), and 32% Pd core, 4% Pd tip (blue). Reprinted from reference (83). Copyright 2016 Royal Society of Chemistry.

nanostars typically have RIS values on the order of 350 nm RIS −1 . 84 This finding was corroborated with FDTD simulations (Figure 7f), where all structures containing Pd exhibited higher RIS than the all-Au counterpart.83 Other work in our group highlighted the importance of NP size toward achieving NPs with high RIS.85 Au/Pd octopods of different sizes and tip sharpness were synthesized by SMCR on Au octahedra.85 With the mole ratio of Au/Pd kept constant at a 10:1 Au-to-Pd precursor ratio, the size was varied by modifying the concentration of the metal precursors relative to the number of seeds in solution (Figure 8a−h).85 Notably, an increase in octopod size corresponded with a red shift in the LSPR (Figure 8i). This increase in NP size was also accompanied by line broadening, which can be attributed to greater polydispersity in the samples with larger NPs as well as increased radiative damping. RIS measurements were conducted for each sample using standard methods.85 As the aspect ratio (face−face diagonal over tip-width) decreases, the RIS increases, with sample f having the highest RIS (Figure 8j,k).85 This work shows that Au−Pd octopod size can be increased to red-shift the LSPR into a region that is ideal for high RIS. Although this case study has focused on the plasmonic properties of Au/Pd octopods, advances in SMCR have allowed for a variety of multimetallic branched NPs to be accessed, including branched NPs with D3h, D4h, and Td symmetries, among others.57,86,87 Also, similar to the work shown in Figure 7, branched NPs with similar symmetries but different cores are available by simply changing the

(3)

High FOM values are preferred because subtle changes in LSPR position are easier to detect; this feature also enables multiplexing in LSPR sensor design.5,82 Because of their chemical stability, Au NPs are typically the preferred platform for LSPR sensing; however, strategies for combining the narrow line-width of Au nanostructures with the high RIS of Pd nanostructures are emerging.5,77,80−82 Work by Rodal-Cedeira and co-workers showed that core@ shell Au@Pd nanorods had higher RIS than all-Au nanorods, and the Au@Pd nanorods provided narrower line-widths than all-Pd nanorods.81 In related work, our group studied the impact of composition on the RIS of eight-branched NPs with Oh symmetry.83 Specifically, octopods with cubic Pd cores and Au−Pd alloyed branches (Figure 7a), cubic Pd cores and Au branches (Figure 7b), cubic Au cores and Au−Pd alloyed branches (Figure 7c), and Au cores and Au branches (Figure 7d) were synthesized, and their RISs were measured. All structures containing Pd provided higher RIS than the all-Au structure (Figure 7e), with highest Pd content on Au cores yielding the highest RIS.83 For example, the octopods with Au−Pd alloyed branches (6.4% atomic Pd on exterior) and cubic Au cores gave an RIS of 583 nm RIU−1, while Au F


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Figure 8. Investigation into how the size of branched bimetallic NPs influences their refractivity sensitivity. SEM images of alloyed Au−Pd octopods of different sizes (a−g) with a model of octopod structure (h); inset scale bars indicate 50 nm. Individual normalized extinction spectra (i) of samples A−G. Refractive index sensitivity of samples A−G (j) and spectral shift of the LSPR of sample F (k) in different water/DMSO mixtures with the inset illustrating the linear relationship between SPR shift and refractive index. Reprinted from reference (85). Copyright 2012 American Chemical Society.

composition or symmetry of the initial seeds.57 Furthermore, nanostructures of different compositions (e.g., Au/Ag/Pd, Pd/ Cu, and Pt/Cu) and convex shapes are readily available through SMCR.88−91 These synthetic advances may enable the discovery of new properties, including plasmonic properties.

general, this rule is consistent with the idea that increasing the size of a NP results in an increase in the plasmon length (i.e., the distance along which the osculation occurs).24 Furthermore, a change in the orientation with respect to injection axis of light can also change the overall plasmon length.56

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CONCLUSIONS Branched plasmonic NPs are exciting platforms with potential to revolutionize a variety of applications due to tunable far-field response and strong near-field enhancements. By examining these recent studies, general criteria that dictate the optoelectronic response of branched plasmonic NPs with high symmetry can be established. (1) The confinement of the charge separation to tips of branched NPs leads to strong EF enhancements that are localized at their tips.16,20 This rule is evident from the study of Au nanostars in which molecules adsorbed preferentially to the branch tips are anticipated to have 10-fold SERS enhancements compared to molecules adsorbed to the nonbranched core.20 (2) For structures of constant composition, increasing both the overall size and branch length of a branched NP will cause a red shift in the plasmon resonance.35,91 In

(3) Increasing the tip sharpness causes the LSPR to shift to lower energy as well as increase the EF enhancement at the tip.35,55 FDTD simulations in Figure 1b support this rule.35 (4) When considering the symmetry of a branched NP, increasing the number of dipolar modes does not require a reduction in symmetry, and a reduction in symmetry does not necessarily lead to in an increase in the number of modes.56 This rule is evident when comparing threebranched T-tripods, Y-tripods, and five-branched pentapods display the same number of LSPR peaks.56 (5) Similar to Au nanorods,38 plasmonic modes can be selectively turned on or off by changing the polarization of light or by changing the particles’ orientation with respect to the incident light.56 This idea is readily observable in Figure 2b, in which the longitudinal and G


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Ghosh and co-workers showed that these types of materials can be prepared as branched NPs and sustain LSPRs; for example, shown in Figure 9e is an example of Sn:CdO NPs prepared by tuning precursor reactivity and choice of capping agent.96 Advances in the synthesis of these types of materials could enable an analysis of their plasmonic properties as a function of branching symmetry and provide a greater test to these guidelines, which are outlined for branched metal NPs with high symmetry.95,97

transverse LSPR peaks are selectively excited by the polarization of light.56 (6) The far-field scattering intensity is greatly influenced by the core lying within the same plane as the charge accumulation.56 It is important to note that this rule applies as long as the plane of interest is aligned perpendicular to the incident light.56 This idea is readily observed in the LSPR peak intensity in Figure 2b and the near-field enhancement maps in Figure 2e,f.56 (7) The EF enhancement is the greatest when an NP is orientated in a manner that yields the greatest effective dipole.56 We expect this rule given previous results observed for NPs with simple shapes;54 however, the greatest near-field enhancement does not necessarily occur when the charge is accumulated on the least number of tips.56 (8) Similar to what has been observed in bimetallic plasmonic NPs,62,92 the plasmon resonance of multimetallic branched NPs can be tuned by changing the composition of the branch as well as the composition of the core.83 For example, the plasmon resonance of Au/ Pd octopods can shift between 646 and 759 nm depending on the composition of the core, branches, and volume of the core.83 (9) The incorporation of a poor plasmonic metal into a branched NP is not necessarily detrimental to the LSPR response. This rule is based on the results in Figure 4 and the RIS measurement of Au/Pd octopods, in which near-field enhancements were preserved and the LSPRs were highly sensitive to changes in the environment.71,83 It is important to note that these rules were developed by analyzing results from NPs based on noble metals. Although these guidelines are expected to be general and applicable to a variety of systems, more research into different systems is required. For example, many sensors based on the RIS of NPs require that the NPs be deposited as a film.93 In the case of designer NPs, ordered arrays may provide the most consistent responses, and our group demonstrated that assemblies of Au/ Pd octopods can be prepared by standard drop-casting methods if the samples are sufficiently monodispersed (Figure 9a−d).94 Still, greater understanding of NP-NP coupling in such systems is required as well as how these NPs interact with substrates. Also of interest are new plasmonic compositions, for example, heavily doped metal oxides.95 Recent work by

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sara E. Skrabalak: 0000-0002-1873-100X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies

Joshua (Josh) Smith is a Ph.D. candidate in the Skrabalak research group at Indiana University, Bloomington. He received his American Chemical Society accredited BS degree (2016) in chemistry from Ball State University while playing football for the Cardinals. During his time at Ball State University, Josh conducted research with Dr. Tykhon Zubkov on developing new photocatalytic materials for oxidation of organic matter. His current research is focused on developing new synthetic pathways to highly symmetric branched plasmonic nanostructures with interesting optical properties.

Figure 9. Future directions into the assembly of branched NPs and synthesis of branched plasmonic semiconductors. (a) SEM image of alloyed Au−Pd octopods in large assemblies, with highlighted regions corresponding to (b) (100), (c) (110), and (d) (111) terminations. TEM image of branched Sn:CoO plasmonic nanoparticles. (a−d) Reprinted from reference (94), and (e) reprinted with permission from reference (96). (a−d) Copyright 2014 American Chemical Society, and (e) Copyright 2017 John Wiley & Sons, Inc. H


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Zachary Woessner received his BS degree from Westminster College in New Wilmington, Pennsylvania, in 2017. He is currently working towards a Ph.D. in chemistry at Indiana University, Bloomington, in the Skrabalak Group. His current scientific interests are plasmonics, LSPR sensors, and nanoparticle assembly.

Sara Skrabalak received her B.A. in chemistry from Washington University in St. Louis in 2002, where she conducted research with Professor William E. Buhro. She then moved to the University of Illinois at Urbana−Champaign, where she completed her Ph.D. in chemistry in 2006 under the tutelage of Professor Kenneth S. Suslick. After conducting postdoctoral research at the University of Washington−Seattle with Professors Younan Xia and Xingde Li, she began on the faculty at Indiana University−Bloomington in 2008. She is currently the James H. Rudy Professor at Indiana University. She is a recipient of both NSF CAREER and DOE Early Career Awards. She is a 2012 Research Corporation Cottrell Scholar, a 2013 Sloan Research Fellow, a 2014 Camille Dreyfus Teacher-Scholar, and 2017 Guggenheim and Fulbright Fellows. In 2014, she received the ACS Award in Pure Chemistry and in 2017 was the recipient of the Frontiers in Research Excellence & Discovery Award from Research Corporation. Her group is developing new synthetic methods to solid materials with defined shapes and architecture and then studying the properties of the materials as they are applied to applications in energy science, chemical sensing, and secured electronics.

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ACKNOWLEDGMENTS This work was supported by National Science Foundation (Award No. CHE-1602476) and Research Corporation for Science Advancement (Frontiers in Research Excellence & Discovery Grant).

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