Precisely Shaped, Uniformly Formed Gold Nanocubes with Ultrahigh

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Precisely Shaped, Uniformly Formed Gold Nanocubes with Ultrahigh Reproducibility in Single-Particle Scattering and Surface-Enhanced Raman Scattering Jeong-Eun Park, Yeonhee Lee, and Jwa-Min Nam Nano Lett., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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Precisely Shaped, Uniformly Formed Gold Nanocubes

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with Ultrahigh Reproducibility in Single-Particle

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Scattering and Surface-Enhanced Raman Scattering

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Jeong-Eun Park , Yeonhee Lee and Jwa-Min Nam*

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Department of Chemistry, Seoul National University, Seoul 08826, South Korea

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†

†

*Correspondence should be addressed to J.-M.N ([email protected]).

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Abstract

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Synthesizing plasmonic nanostructures in an ultra-precise manner is of paramount importance

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because the nanometer-scale structural details can significantly affect their plasmonic

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properties. Au nanocubes (AuNCs) have been a highly promising, heavily studied

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nanostructure with high potential in various fields, but an ultra-precise synthesis from 10 to

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100 nm in size over a large number of AuNCs has not been well established. Precisely

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structured AuNC-based studies for a highly reproducible, quantitative plasmonic signal

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generation [e.g., quantitative surface-enhanced Raman scattering (SERS)] are needed for

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reliable use and exploration in the beneficial properties of AuNCs. Here, we developed a

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strategy for AuNC synthesis with the desired size and shape, ranging from 17 to 78 nm

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particularly with highly controlled corner sharpness, by precisely controlling the growth rate

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of different facets and AuNC-specific flocculation which enabled ultrahigh yields (~98-99%). 1 ACS Paragon Plus Environment

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Importantly, the precisely shaped AuNCs can scatter light in a spectrally reproducible manner,

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and the SERS enhancement factors (EFs) for the AuNC dimers are very narrowly distributed

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(the EFs of 72-nm sharp-cornered cube dimers have a distribution within one order of

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magnitude). Our results pave the paths to ultrahigh yield synthesis of metal nanocubes with a

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precise size and shape and offer single-particle-level spectral controllability and reproducibility

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over a large number of particles.

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Keywords: Gold nanocube, localized surface plasmon resonance, Rayleigh scattering, surfaceenhanced Raman scattering

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Table of Contents

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The localized surface plasmon resonance (LSPR) is a unique feature of plasmonic metal

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nanostructures, and this enables numerous applications including sensing,1,2 imaging,3

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therapeutics,4 nonlinear optics,5 and catalysis6-8. Since LSPR is primarily affected by the size

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and shape of plasmonic nanostructures, plentiful studies have been conducted on creation of

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new structures and precise structural controlling strategies. Among many plasmonic metal

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nanostructures, metallic nanocubes (NCs) have been heavily studied and utilized because of 2 ACS Paragon Plus Environment

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their strong and tunable LSPR properties and availability of various synthetic strategies to form

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metal NCs.9-16 The edges and corners of metallic NCs generate significantly enhanced localized

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electromagnetic field on individual nanoparticles or between coupled nanoparticles, which can

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be employed in diverse areas including fundamental optics, plasmon-enhanced spectroscopy,

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and sensing applications.9-13,15 Furthermore, owing to its flat surfaces, it is often used as a

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building block for assembled structures such as face-to-face assembled cube dimers or as a

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template for further synthesis of more complicated structures.14,15 Thus, synthesizing

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plasmonic NCs with high reproducibility and controllability is a fundamental building block

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and of great importance in the field of materials science, chemistry, plasmonics and

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nanotechnology. Among plasmonic NCs, although silver NCs (AgNCs) have been utilized

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more heavily because of their facile synthetic method and strong plasmonic properties, AuNCs

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are more beneficial in that AuNPs offer good chemical stability, capabilities in various surface

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modifications, conjugation chemistries, biocompatibility and tunable plasmonic properties.4,17

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However, the synthesis of structurally controlled AuNCs in an ultrahigh yield is challenging,

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and the use of AuNCs has been limited due to the lack of reliable synthetic strategies with

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ultrahigh precision and yield. Although some of the previously reported papers claimed high

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yields,18-20 supporting data such as large area TEM and SEM images for a large number of

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particles and extensive spectroscopic/microscopic analysis have not been provided to

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convincingly support such claims, and it is widely believed that highly reliable synthesis of

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AuNCs with varying sizes and shapes has been well established yet. The low yield could be

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partially resolved by universal gold nanoparticle seeds acquired through an iterative oxidative

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dissolution and re-growth reaction.21 However, the intricate and laborious seed-preparation

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process compromises practical synthesis of AuNCs. Further, the accurate control in structural

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features such as corner sharpness for NCs and the synthesis of small-sized AuNCs have not

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been achieved so far. The detailed structural features such as edge and corner of NCs can

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largely affect their optical signals,11,22,23 but systematic and reliable experimental realization is

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limited by the lack of a proper synthetic method. This could be particularly impactful for

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reproducible

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spectroscopies such as SERS and surface enhanced fluorescence.

and

quantitative

plasmonic

signal

generations

for

surface-enhanced

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Here, we report a synthetic strategy to form the AuNCs with ultra-precisely controlled size

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and shape, ranging from 17 to 78 nm in diameter, in an ultrahigh yield (98.3 ± 0.7%). The

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corner sharpness, edge and size of AuNCs were precisely controlled via anisotropic growth

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facilitated by the quantitative modulation of facet-selective protection. Moreover, the synthetic

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yield of a targeted cube structure was improved by centrifugation-driven depletion-induced

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flocculation. Single-particle-level scattering spectrum results show that the synthesized AuNCs

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generate strong and tunable scattering signals in a highly reproducible and quantifiable manner.

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Importantly, bulk UV−Vis spectral analysis for different batches of AuNCs prove that the

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optical signals from AuNCs are quite reproducible over a large number of particles. Average

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SERS EF values from AuNC dimers with 1.1 ± 0.3 nm gap were higher than 107. Notably, the

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EFs of sharp-cornered AuNC dimers were distributed within one-order-of magnitude. This is

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a big step forward in SERS because dimeric structures often generate SERS EF distribution

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over several orders of magnitude due to the heterogeneity in dimeric interfaces and gaps,24-26

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hindering the use of SERS probes for reliable

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and practical applications. The strategy and results in this study open up ultrahigh-yield

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synthesis of metal NCs with precisely controlled size and shape and reproducible, quantitative,

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plasmonically enhanced signals including SERS from metal nanostructures.

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By a seed-mediated method, we performed the selective surface-protection-directed growth

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of AuNCs surrounded by (100) facets (Figure 1a, left). In order to induce an anisotropic

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growth, the growth rate along [100] needs to be reduced, with the rates along [110] and [111]


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being higher. We introduced bromide ions that are favorably adsorbed on the (100) facet, in

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the presence of cetyltrimethylammonium chloride (CTAC) to make difference in accessibility

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of Au precursors to different facets and induce the growth-rate difference between (100) and

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the other facets. 27,28 The bromide concentration-controlled growth kinetics governs the corner

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Figure 1. Synthesis and refinement of size and shape-controlled AuNCs. (a) Synthesis of


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corner sharpness-controlled AuNCs with varied bromide densities followed by refinement by

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centrifugation-driven depletion-induced flocculation in surfactant micelle solutions. The

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proposed mechanism of modifying growth kinetics is based on the difference in growth rates

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of each facet due to the difference in adsorption of Br-. (b) Definition of edge length, edge

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radius, and SI. Representative TEM images (c) and SI value (d) of AuNCs obtained by

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adjusting the concentration of bromide from 0 to 200 mM at the fixed amounts of seed and

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gold precursor. (e) Bromide-concentration-dependent growth kinetics. The results were

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obtained at each maximum LSPR wavelength using a UV−Vis spectrophotometer. TEM

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images (f) and SI values (n=100) (g) of refined AuNCs with different amounts of seed and

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bromide; the insets show representative single-particle images to clearly visualize the

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sharpness difference. Numbers in the labels correspond to edge length, and R and S indicate

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round-cornered and sharp-cornered AuNCs, respectively. (h) Normalized UV−Vis spectra for

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a series of AuNC solutions. Solid and dashed lines correspond to round-cornered and sharp-

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cornered AuNCs, respectively. The scale bars in (c) and (f) indicate 20 nm and 100 nm,

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respectively.

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sharpness of AuNCs. At low bromide densities, the number of adsorbed bromide ions is

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insufficient to block the (100) facet completely. Hence, the relative growth-rate difference

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between [100] and [111]/[110] is less significant, producing round-cornered AuNCs. When a

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sufficient amount of facet-directing agents is provided, effective and preferential binding to the

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(100) facet decreases the reduction rate, while the (111)/(110) facet are less affected by bromide

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ions, maximizing the growth-rate difference between (100) and the other two facets to be sharp-

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cornered AuNCs.

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We then employed the shape-selective refinement strategy to maximize the synthetic

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yields for AuNCs (Figure 1a, right). The method is based on centrifugation-driven depletion6 ACS Paragon Plus Environment

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induced flocculation, which consists of the selective flocculation of AuNCs using surfactant

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micelles and reversible redispersion of the sediment. The flocculation has been applied to select

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nanorods or nanobipyramids among mixtures of nanoparticles and typically takes more than

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10 h to make nanoparticles settle down.29-31 Since the attractive force between two particles is

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proportional to the flat surface areas facing each other, precisely synthesized AuNCs with flat

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surfaces are advantageous in flocculation compared to those with curved surfaces, such as rods,

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spheres and bipyramids. In this study, we dramatically shortened the time by adopting a brief

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centrifugation step, and the efficiency for flocculation here was much higher than the

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previously reported cases with non-cube structures. During the centrifugation, centrifugal force

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pushes particles close to each other, giving extra driving force to flocculation. Therefore,

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effective flocculation can occur with a relatively low depletion potential in a very short time

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(97-98%.

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As a proof of concept, first, we controlled the corner sharpness of AuNCs in a similar size

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(Figure 1c, Figure S1 and Figure S2). To demonstrate the effects of the bromide amount, the

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bromide concentration was varied from 0 to 200 mM, while the amounts of CTAC, seed,

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precursor and ascorbic acid were kept constant (see Methods for details). To quantitatively

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characterize the structural features of each AuNC based on the transmission electron

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microscopy (TEM) images, we coined a term, sharpness index (SI) based on the edge length

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defined as the distance between the two closest faces of AuNCs and the edge radius defined as

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the radius of the circle that perfectly matches the corner curvature (Figure 1b). As shown in

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Figure 1d, the corner sharpness initially increased and then decreased with increasing bromide

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concentration. This could be explained by the growth rate changes through adsorbed bromides

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on each facet, as illustrated in Figure 1a. In the case of excessive bromide concentration,

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bromide ions can be additionally adsorbed on (111)/(110) surfaces as well as (100), decreasing

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the overall growth rate and rate difference between facets. Further, we explored the growth


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kinetics during shape evolution by UV−Vis spectroscopy with three selected bromide

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concentrations (1, 40 and 200 mM). After adding Au precursors, we started to monitor the

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changes in extinction intensity with 10-s intervals at each maximum LSPR wavelength of the

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fully-grown structures (Figure 1e and Figure S3). The slowest increase of extinction for 200

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mM NaBr suggests that increasing the bromide concentration slows the reduction, supporting

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the proposed mechanism. More experimental studies on the possibility of reduction rate change

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due to ligand exchange of Au-halide complexes, the effect of other halide ions, and the

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advantage of using CTAC and Br- are separately shown in the supporting information (Figure

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S4-6).

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To substantiate the above principles, we controlled the size and corner sharpness

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simultaneously by changing the amounts of seed and bromide. The seed amount was adjusted

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by controlling the volume of seed solution (300, 30, 9, 6, and 2 𝜇𝜇L). We also varied the bromide

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sharp-cornered AuNCs, we applied a lower density of bromide to produce the round-cornered

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AuNCs. The Br- densities adsorbed on the round-cornered AuNC and the sharp-cornered

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AuNC, obtained from ion chromatography, confirm a higher number of Br- on the sharp-

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cornered AuNC surface (Table S2). After synthesis, the AuNCs were dispersed in a mixture

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of benzyldimethyldodecylammonium chloride (BDAC) and cetyltrimethylammonium bromide

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(CTAB) solution and centrifuged for 5−10 min at 500−1000 g. Since flocculation force has a

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positive correlation with the overlaid surface area between nanoparticles as well as the micelle

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concentration, the surfactant concentration required for flocculation decreases as AuNCs

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become larger (see Methods for details). Based on the results obtained with varied

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flocculation conditions, we determined the optimal surfactant concentration for each NC size

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(Figure S7 and Note S1). Figure 1f shows different sizes of refined AuNCs, and the insets

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show representative images for clear visualization of representative structures; The numbers in

concentration to control the corner sharpness at a fixed seed amount. Compared to synthesis of


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the labels indicate the edge length, and R and S correspond to round-cornered and sharp-

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cornered NCs, respectively. With decreasing the seed volume, the edge length increased from

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17 to 78 nm (Figure S8 and Table S1). Larger-sized cubes can be also synthesized by this

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method (data not shown). Although synthesizing AuNCs smaller than 25 nm by a solution-

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based method was challenging and has not been achieved before because the self-diffusion

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distance of gold atoms is higher than those of other metals such as platinum,18 we successfully

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synthesized AuNCs with an edge length of 17−18 nm using our method. To our knowledge,

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this is the smallest size reported so far for synthetically controlled AuNCs. While the SI values

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of four large, sharp-cornered AuNCs are similar, that of 17S was lower than others (Figure

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1g). It is because their edge radius was similar to 32S, which can be mainly attributable to the

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surface energy that prohibits smaller edge radius. Normalized UV−Vis spectra for a series of

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AuNC solutions show a gradual red-shift as the corner sharpens and the edge length increases,

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owing to the retardation effect (Figure 1h).32

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After the refinement, the yield was improved to >97-99% for all the AuNCs, measured with

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the images with a large number of particles over a large area, except the smallest two NCs and

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32S (n > 400, Figure 2 and Figure S9 for larger scanning electron microscopy (SEM)

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54R, 97.2% for 53S, 99.0% for 78R, and 98.2% for 72S). Narrower spectral linewidth and

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removal of shoulder peak at ~800 nm after refinement imply highly monodisperse AuNCs

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(Figure S10). It was more difficult to determine an appropriate surfactant condition to make

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small particles, 17S and 18R AuNCs, flocculated owing to their low surface area resulting in

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low flocculation potential. 32S AuNCs have flocculation potentials similar to those of their

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byproducts, providing relatively lower yields (~90%). Long-term stability of AuNC solution

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stored in an ambient condition was confirmed by the extinction spectra acquired with AuNC

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samples stored for ~16 months (Figure S11).

images; yields are 98.0% for 37R, 90.0% for 32S, 99.5% for 41R, 98.4% for 41S, 98.0% for


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Next, we obtained the light scattering images from individual particles using dark-field (DF)

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microscopy. The scattering properties of 18R, 17S, 37R, and 32S AuNCs could not be clearly

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measured owing to the low signal intensity caused by their small volumes. The DF micrographs

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of six different larger AuNCs exhibit a highly uniform scattering intensity for each sample, and

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the scattering colors were identical (green), except for 78R and 72S AuNCs, which showed

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yellow color of scattering (Figure 3a). Significantly, the highly consistent spectra from 25

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different individual AuNCs for each case are reminiscent of the utterly narrow size and shape

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distribution of the synthesized AuNCs and indicate a unique and reproducible spectral feature

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from each particle (Figure 3b). The representative spectrum of each sample is marked with a

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dotted box in Figure 3c. As the corner becomes sharper between AuNCs of similar sizes and

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Figure 2. Precisely shaped AuNCs in a high yield. Low-magnification SEM images showing

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AuNCs in a high yield after refinement. The samples were prepared by drop casting AuNCs

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on a silicon wafer. The image was taken at the boundaries where the particles were densely

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packed. The scale bars indicate 1 𝜇𝜇m.


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Figure 3. Single-particle scattering analysis on AuNCs and UV-Vis spectra of different

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batches of AuNC solutions. Dark-field microscope images (a) and 25 single-particle

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scattering spectra (b) of individual AuNCs [the particles in (a) do not correspond to the particles

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in (b)]. (c) The corresponding representative spectra of each sample marked with a dotted box

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in (b). Analyzed maximum peak position (d), scattering intensity (e), and spectral linewidth (f)

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of each AuNC obtained from the analysis of (b). (g) The solution extinction spectra from nine

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different batches of 53S AuNC solutions. The exposure times for acquisition of scattering

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images in (a) are 200 ms for 41R and 41S AuNCs, 120 ms for 54R and 53S AuNCs, and 80 ms

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for 78R and 72S AuNCs. The images in (a) were additionally adjusted to visualize the

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scattering signal while ensuring differences among each other. We acquired scattering spectra

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of 41R and 41S AuNCs with an exposure time of 20 s and those of 54R, 53S, 78R, and 72S

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AuNCs with an exposure time of 3 s in (b). The 20-s cases were normalized to the 3-s case

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under the assumption that the signals increase linearly with exposure time. The scale bars in (a)

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indicate 2 μm.


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the size increases, the average peak wavelength was red-shifted, which is mainly attributable

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to the retardation effect,32 along with the increase in scattering intensity (Figure 3d and 3e).

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The 72S AuNCs showed a slightly lower scattering intensity compared to that of 78R AuNCs

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owing to the increased radiative decay for the larger NCs, as the light scattering scales with the

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square of the particle volume in general. Notably, although the maximum peak position of 78R

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was 562 nm as in the 53S case, an increased contribution from wavelengths greater than 600

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nm results in a distinct color difference in the micrograph, as shown in Figure 3a. Analyzed

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spectral linewidths also manifest the uniform scattering property (Figure 3f). When reference

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AuNCs synthesized according to the previously reported method18 were compared to our

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AuNCs of a similar scattering intensity, our AuNCs showed the linewidths with clearly smaller

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deviations (Figure S12), implying the high homogeneity in particle structures synthesized with

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our method. Importantly, we also discovered the extraordinary reproducibility of the AuNCs

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by synthesizing 9 different batches of 53S AuNC solutions and comparing their ensemble

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extinction spectra in solution (Figure 3g). The spectral information including the maximum

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peak position and linewidth suggest negligible batch-to-batch variations in the synthesized

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AuNCs, proving the reproducibility in structure and spectrum are very high in bulk scales.

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Formation of dimeric or multimeric structures is a typical way to create plasmonic couplings

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between particles to strongly enhance the signals from plasmonic nanostructures.4,33,34 Among

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those structures, cube dimers have gained attentions as a plasmonically coupled nanostructure

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with flat gap, showing relatively uniform electric field inside the gap.35-39 However,

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experimentally obtaining reproducible and reliable signals from these structures with a largely

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enhanced plasmonic signal (typically, many orders-of-magnitude enhancements) is highly

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challenging. To investigate our dimeric AuNC structures with ultra-small nanogap (~1 nm) on

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near-field enhancement, we observed the 1,4- benzenedithiol (BDT) SERS signal from face–


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face oriented AuNC dimers with two differently cornered AuNC structures at single-particle

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level (Figure 4a and 4d). BDT was used as linker molecules to assemble the NCs and to form

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a nanogap between NCs.

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Figure 4. Single-particle SERS from AuNC dimers with face–face orientation. (a)

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Schematic illustration of face–face oriented AuNC dimers (78R and 72S AuNC dimers) with

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1,4-benzenedithiol. (b) Cross-sectional images of simulated near-field enhancement for 78R

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and 72S dimers, respectively. Solid line depicts the boundary of AuNCs and dotted boxes

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represent the flat interfacial area for each dimer. (c) 3D spatial plot images of simulated electric

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field enhancements in (b). (d) Representative Raman scattering spectra from individual AuNC

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dimers. (e-f) Single-particle SERS enhancement factor distributions for the Raman peak at

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1069 cm-1 [log scale (e) and linear scale (f) obtained in the boxed regions in (e)] from the

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measured Raman signals for 78R NC and 72S NC dimers, respectively (n = 22 for each case). 14 ACS Paragon Plus Environment

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Assembled NCs were loaded to SiO2-supported TEM grid and mapped with TEM. Then, TEM

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images were correlated with photomultiplier tube images to distinguish face–face dimers from

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monomers and other types of assemblies including multimers and face–edge dimers in Raman

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measurement setup (Figure S13). From the analysis of 22 different dimers for each case, the

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average gap size was obtained as 1.1 ± 0.3 nm (Figure S14). The Raman signal polar plot from

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the BDT with varying incident laser polarizations reveals that the maximum Raman

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enhancement occurs at the longitudinal mode of dimers (Figure S15). The assembled dimers

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generated highly symmetrical polar plot results, suggesting that the dimers were formed

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impeccably and symmetrically. The averaged EF of 78R dimer is ~8.0 × 107 to be five times

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higher than that of 72S being ~1.6 × 107 with linearly polarized laser along with the longitudinal

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dimer axis. These values are believed to be strong enough for single-molecule detection

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experiments (Figure 4e and 4f).40 This trend is consistent with the electromagnetic-field

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simulation results, which shows higher maximum field enhancement for round-cornered

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AuNCs than sharp-cornered AuNCs (Figure 4b and 4c).

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Importantly, we found that a very narrow distribution of SERS EFs was acquired with both

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78R and 72S cube dimers; The SERS EFs for the entire dimers that we analyzed are ranged

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from 1.6 × 107 to 3.1 × 108 for 78R cube dimers and from 5.4 × 106 to 4.9 × 107 for 72S cube

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dimers (Figure 4e and 4f). It is remarkable that the SERS EFs are distributed within one order

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of magnitude for the 72S cube dimer case, and this is the narrowest distribution of SERS EFs

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with dimeric structures ever reported. Given that uniformly formed Ag nanosphere arrays41

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generated widely distributed SERS EF values between 2.8 × 104 and 4.1 × 1010 and smaller

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SERS EF values from face-to-face AgNC dimers25 are broadly distributed from 98-99 %

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yields for a targeted AuNC structure. The ultrahigh structural precision for size and sharpness

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and high synthetic yield enable the fine-tuning of plasmonic optical responses from AuNCs,

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allowing single-particle-level scattering signal reproducibility over a large number of particles.

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Dark-field light scattering images and spectra were obtained with a highly reproducible manner

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from particle to particle, and, further, the bulk solution UV-Vis spectra from different batches

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of AuNC solution samples were perfectly matched with each other. Remarkably, SERS

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analysis for the individual AuNC dimers shows that a very narrow distribution of large SERS

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EF values can be obtained; all of the EFs for sharp-cornered AuNC dimers are distributed

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within one order of magnitude (1.6 × 107 to 3.1 × 108 for 78R cube dimers and 5.4 × 106 to 4.9

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× 107 for 72S cube dimers). Rigorous analysis results of single-particle-level optical signals for


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Nano Letters

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AuNCs and AuNC dimers suggest that the synthesized AuNCs open avenues for highly reliable

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surface-enhanced spectroscopic applications including SERS with high sensitivity and

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quantification capability in a controllable and predictable manner, which have been major

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hurdles for the wide applications of nanostructure-based surface-enhanced spectroscopies. We

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expect our work to lead to the development of practically useful and scalable synthetic methods

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for metal building blocks to form more complex, sophisticated structures with varying

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functionalities and quantitative plasmonics with metal nanocubes.

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AUTHOR INFORMATION

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Corresponding Author

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*E-mail: [email protected]

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Author contributions

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J.-E.P., Y.L. and J.-M.N. designed the experiments. J.-E.P., Y.L. carried out experiments under

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guidance of J.-M.N. Y.L. performed computational simulations under guidance of J.-M.N. J.-

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E.P., Y.L. and J.-M.N. analyzed the data and wrote the paper. †These authors equally

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contributed.

18 19

Additional information

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The Supporting Information is available free of charge on the ACS Publications website at

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DOI: XXX. *Correspondence and requests for materials should be addressed to J.-M.N

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([email protected]).

23


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Notes

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The authors declare no competing financial interests.

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Acknowledgements

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This work was supported by BioNano Health-Guard Research Center funded by the Ministry

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of Science and ICT(MSIT) of Korea as Global Frontier Project (Grant number H-

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GUARD_2013M3A6B2078947). This research was also supported by Basic Science Research

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Program through the National Research Foundation of Korea(NRF) funded by the Ministry of

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Science and ICT(MSIT) (Grant No. 2016R1A2A1A05005430) and the National Research

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Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. NRF-

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2017R1A5A1015365).

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Precisely Shaped, Uniformly Formed Gold Nanocubes with Ultrahigh

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