Article pubs.acs.org/JPCC
Large-Scale Plasmonic Pyramidal Supercrystals via Templated SelfAssembly of Monodisperse Gold Nanospheres Christoph Hanske,◆,† Guillermo González-Rubio,◆,†,‡ Cyrille Hamon,† Pilar Formentín,§ Evgeny Modin,∥,⊥,# Andrey Chuvilin,#,▽ Andrés Guerrero-Martínez,‡ Lluis F. Marsal,§ and Luis M. Liz-Marzán*,†,▽,○ †
BioNanoPlasmonics Laboratory, CIC biomaGUNE, Paseo de Miramón 182, 20014 Donostia - San Sebastián, Spain Departamento de Química Física I, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain § Department of Electronic Engineering, Universitat Rovira i Virgili, Av. Països Catalans, 26, 43007 Tarragona, Spain ∥ National Research Centre “Kurchatov Institute”, Kurchatov Sq. 1, 123182 Moscow, Russia ⊥ Electron Microscopy and Image Processing Interdisciplinary Laboratory, Far Eastern Federal University, Sukhanova 8, 690000 Vladivostok, Russia # Electron Microscopy Laboratory, CIC nanoGUNE, Tolosa Hiribidea, 76, 20019 Donostia-San Sebastian, Spain ▽ Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain ○ Biomedical Research Networking Center in Bioengineering, Biomaterials, and Nanomedicine, CIBER-BBN, 20014 Donostia - San Sebastián, Spain ‡
S Supporting Information *
ABSTRACT: Three-dimensional supercrystals of plasmonic nanoparticles are a novel class of materials with exciting applications in technologies such as light harvesting or metamaterials. However, their realization relies on extraordinarily regular colloidal building blocks and accurate self-assembly methods. We present here a simple and up-scalable protocol for the synthesis of smooth gold nanospheres with high monodispersity in size and sphericity. The synthesis involves rapid growth up to the desired size and subsequent removal of surface roughness via an efficient etching step, so that nanospheres with diameters ranging between 10 and 110 nm can be obtained in large quantities. Upon functionalization with thiolated polyethylene glycol and low surfactant concentration, Au nanospheres were employed as building blocks to produce uniform arrays of micron-sized 3D pyramidal supercrystals over large areas, by means of a template-assisted approach. Focused ion beam cutting and SEM characterization revealed a facecentered cubic lattice within individual pyramidal supercrystals.
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INTRODUCTION More than 150 years after the first scientific report on the synthesis of plasmonic colloids,1 gold nanoparticles (AuNPs) are still the spearhead of nanotechnology, offering a unique combination of tunable optical properties (localized surface plasmon resonances, LSPRs), high stability, and controllable electrochemistry.2,3 Covering a wide range of topics in natural and health sciences, AuNPs are used, for example, in energy harvesting, catalysis, artificial photosynthesis, metamaterials, and ultradetection of contaminants and toxins as well as in the treatment of cancer.4−8 An essential feature behind the impact of AuNPs is the tunability of their optical properties. This can be controlled either by tailoring the shape and size of the nanoparticles or by changing their environment, for example, through assembly into larger superstructures.9−11 Because of plasmon coupling, densely packed AuNP arrangements often © XXXX American Chemical Society
exhibit a completely different optical response than their constituent building blocks.12−16 However, the collective coupling modes that give rise to this fascinating phenomenon are highly sensitive to structural irregularities.17,18 Consequently, a central challenge lies in the construction of hierarchical arrays of supercrystals with well-defined 3D order. Such assemblies of AuNPs, which hold great potential for applications in optical metamaterials, as signal enhancers for ultradetection spectroscopy, or for subwavelength light Special Issue: ISSPIC XVIII: International Symposium on Small Particles and Inorganic Clusters 2016 Received: December 2, 2016 Revised: January 3, 2017 Published: January 6, 2017 A
DOI: 10.1021/acs.jpcc.6b12161 J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
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EXPERIMENTAL DETAILS Chemicals. Hexadecyltrimethylammonium chloride (CTAC, 25% w/w, 756 mM), hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, ≥ 99.9%), L-ascorbic acid (≥99%), sodium hypochlorite solution (available chlorine 10−15%), sodium borohydride (NaBH4, 99%), tetramethylammonium hydroxide (TMAH), trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOTS, 97%), and poly(ethylene glycol) methyl ether thiol (PEG-6K-SH, av. Mn = 6000 g/mol) were purchased from Aldrich. All chemicals were used without further purification, and Milli-Q water (resistivity 18.2 MΩ·cm at 25 °C) was used in all experiments. Synthesis of AuNSs. Gold Seeds. A reported CTAC/ NaBH4 procedure was used with minor modifications:36 50 μL of a 0.05 M HAuCl4 solution was added to 5 mL of a 0.1 M CTAC solution; 200 μL of a freshly prepared 0.02 M NaBH4 (7.6 mg/10 mL) solution was then injected under vigorous stirring. After 3 min the mixture was diluted 10 times in CTAC 100 mM. 10 nm Spheres. 900 μL of the seed solution and 40 μL of 0.1 M ascorbic acid were added to 10 mL of 25 mM CTAC solution. Then, 50 μL of a 0.05 M HAuCl4 solution was injected under vigorous stirring. The mixture was left undisturbed at room temperature for at least 10 min. The resulting gold nanospheres presented an LSPR band centered at 520 nm. Seeded Growth. A selected amount of 10 nm AuNS suspension and 40 μL of 0.1 M ascorbic acid were added to 10 mL of a 25 mM CTAC solution. Listed in Table 1 are the
management, have been elusive mainly due to the high demands on the building block quality.19−22 Besides local colloidal interactions, geometry-dependent long-range forces play a decisive role in the formation of supercrystals through a variety of assembly pathways.23−25 Therefore, procedures that yield uniform AuNPs in large quantities are a prerequisite for novel functional materials based on plasmonic supercrystals. During the past 20 years, colloid chemistry has evolved in such a manner that we currently have access to a large library of methods for the synthesis of monodisperse gold nanoparticles of different morphologies that can be utilized as building blocks for mesostructured materials, including nanorods, nanotriangles, nanostars, nanocubes, and various platonic shapes.26−29 Even though large AuNPs (>75 nm) display stronger plasmonic near-fields and optical scattering, most of the reported methods for the synthesis of AuNPs with the basic spherical geometry are still limited to small diameters in the range of 3−30 nm. For instance, the Turkevich method has been extensively used for the synthesis of spherical gold nanocrystals in water, whereas the use of oleylamine in organic solvents such as toluene, both for reducing the gold salt precursor and as a stabilizing agent, results in monodisperse spheres.30,31 In the context of colloidal assembly, the term monodispersity comprises control over not only the size distribution in a population of particles but also the regularity of their shape. In the case of gold nanospheres (AuNSs), this means a high degree of sphericity and minimum roughness. However, for larger AuNSs, the structure of gold nanocrystals, which inherently exhibit facets of different surface energies, combined with the need for adsorbing ligands granting colloidal stability, hinders the direct growth of smooth and truly spherical nanoparticles. Although several protocols have been reported for the direct synthesis of spheres with uniform size, based on citrate or cetyltrimethylammonium bromide (CTAB), the resulting particles generally display a significant degree of roughness.32,33 Xia and coworkers recently reported a protocol using the surfactant cetyltrimethylammonium chloride (CTAC), which comprises successive growth steps and yields monodisperse, smooth gold nanospheres in the size range from 5 to 150 nm.34 On the contrary, Ruan et al. described a synthesis of monodisperse spheres with diameters ranging between 20 and 220 nm, in which gold polyhedra of different sizes were first grown in the presence of CTAC and subsequently etched into a rounded shape by oxidation with Au(III) in the presence of CTAB.35 We found that these methods require time-consuming centrifugation steps between the various reaction stages, and in addition the use of a syringe pump eventually complicates the potential upscaling to prepare large quantities. Herein, we describe an economic, scalable synthesis of smooth AuNSs with uniform size in the range of 10−110 nm based on seeded growth and highly efficient oxidative etching. Such AuNSs were used to build 3D pyramidal supercrystals by self-assembly within elastomeric templates. The obtained arrays of micron-sized pyramidal AuNS arrangements exhibit exceptional regularity over macroscopic surface areas. In contrast with previous work on pyramid-shaped assemblies,19 we achieved a clean separation between neighboring pyramids and ordered packing of the constituent AuNSs, which form high-quality supercrystals with a face-centered cubic (fcc) lattice.
Table 1. Average Diameter and Corresponding Standard Deviation, Volume of 10 nm Seed Solution, and Details of Oxidative Etching for AuNS of 28, 52, 71, and 110 nm volume of 10 nm NSs (μL)
diameter (nm)
SD (nm)
28 52
2 2
125 25
71
2
10
110
5
2.5
oxidative etching 10 μL of NaClO 10 μL of NaClO sol. and 2.5 of μL of 0.05 M HAuCl4 10 μL of NaClO sol. and 3.75 μL of 0.05 M HAuCl4 10 μL of NaClO sol. and 6.25 μL of 0.05 M HAuCl4
time of etching at 30 °C (min) 10 45 90 180
amounts of 10 nm seeds required for the growth of AuNS with different particle sizes. Subsequently, 50 μL of a 0.05 M HAuCl4 solution was added under vigorous stirring. The mixture was left undisturbed at room temperature for at least 1 h. The growth times varied depending on the amount of seeds, with longer times being required to grow larger particles. The zeta potential of such faceted AuNPs was recorded to be 38 ± 2 mV. Oxidative Etching. To 10 mL of grown nanoparticles was injected 10 μL of a dilute sodium hypochlorite solution (1 to 1.5 wt % of available chlorine) under rapid stirring. After 5 min, different amounts of a 0.05 M HAuCl4 solution were added under stirring (details are provided in Table 1 for the different particle sizes). The mixture was left undisturbed at 30 °C until oxidation was completed, as indicated by a constant absorbance at 400 nm. The zeta potential of the AuNSs was determined as 36 ± 2 mV. B
DOI: 10.1021/acs.jpcc.6b12161 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Surface Functionalization. For the fabrication of colloidal supercrystals, two batches of nanoparticles with average diameters of 52 nm (LSPR at 532 nm) and 105 nm (LSPR at 568 nm) were selected. These dispersions were centrifuged, concentrated, and redispersed in 1 mM CTAC solution. For ligand exchange, the particles were mixed with thiolated PEG6K dissolved in 1 mM CTAC solution to obtain a final Au0 concentration of 5 mM (according to the absorbance at 400 nm)37 and a PEG-6K-SH content of 1 mg/mL. For this, typically, 10 mg of PEG-6K-SH dissolved in 5 mL of a 1 mM CTAC solution was added to 5 mL of a 10 mM AuNP dispersion (@ 1 mM CTAC). After stirring overnight the particles were washed by centrifugation, and the PEGylation step was repeated at a lower CTAC concentration of 0.5 mM. Following the removal of residual PEG-SH, the particles were concentrated by centrifugation to set the final Au concentration to 750 mM and the CTAC concentration to 50 μM. Hereby, the pellets were redispersed in 100 μM CTAC solution to prevent aggregation, and the final concentrations were reached by dilution after the last centrifugation step. Template Fabrication. Topographically structured Si templates of inverted pyramids were produced by a direct laser writing lithography technique and chemical etching in 8% solution of tetramethylammonium hydroxide (TMAH) for 7 to 9 min at a temperature of 80 °C.19 The templates were washed with acetone, ethanol, and water before activating their surfaces in a plasma etcher (Diener PICO, 0.4 mbar O2, 200 W, 10 min) and conducting a vapor phase silanization with a small droplet of trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOTS) in a sealed vessel (24 h under Ar at ambient pressure and temperature, followed by annealing at 120 °C for 1 h). The passivated masters were replicated by molding with polydimethylsiloxane (PDMS, Dow Corning, Sylgard 184) using 5:1 and 10:1 mixtures of prepolymer and curing agent. To achieve an accurate structure replication, PDMS was degassed during the first hour after casting onto the Si master, gelled overnight at room temperature, and then cured for 3 h at 70 °C. The entire procedure including plasma activation (2 min) and FOTS passivation was repeated to obtain soft PDMS replicates resembling the topography of the original Si masters. Templated Self-Assembly. All supercrystals were prepared on borosilicate glass substrates cut into pieces of 6 × 3 mm2. The employed microscopy coverslips (Menzel-Gläser, Nr. 1.5) were precleaned with a diluted Hellmanex III solution, sonicated in isopropanol, and rinsed with water. After hydrophilization in O2 plasma (20 min), the substrates were used within 2 h. To assemble the PEGylated AuNPs, PDMS molds resembling inverted pyramids were placed onto a leveled table before a 1 μL droplet of concentrated particle dispersion was applied to the stamp and covered with a pretreated substrate. The liquid was then allowed to evaporate completely (usually
