Anisotropic Nanostructures

As materials are reduced in size from the bulk to the nanoscale, they begin to exhibit new and unusual chemical and physical properties.

Noble metal nanostructures are of particular interest because their chemical and physical properties can be controlled not only by varying material composition, but also by tuning size and shape. Anisotropic nanoparticles – non-spherical structures (e.g. prisms, rods, cubes) with unique shape-dependent properties and functionalities – can be utilized in a number of important applications ranging from catalysis to sensing to optics.

In the Mirkin group, one of our primary goals is to develop synthetic strategies for creating a toolbox of structurally uniform anisotropic noble metal nanoparticles in high-yield with fine control over their shape and size. Our work focuses on two particular types of synthetic methodologies: seed-mediated and photo-mediated synthesis. After synthesis, anisotropic nanoparticles can be coated with a dense shell of oligonucleotides to form “programmable atom equivalents”, where directional DNA “bonds” program the arrangement of nanoparticle “atoms” into one-, two-, and three-dimensional colloidal crystals with new emergent properties.

Seed-Mediated Synthesis

Seed-mediated syntheses utilize nanoparticle reactants, or seeds, as templates for the heterogeneous nucleation of anisotropic nanoparticle products. In this way, the processes of seed nucleation and subsequent nanoparticle growth can be separated to allow for better control of each step. Initial research in our group focused on the reaction conditions required to control shape, with a specific emphasis on halides and silver (via underpotential deposition) as shape-directing agents. This work led to a number of novel morphologies, including concave cubes, which are composed of twenty-four high-index facets, {110}-faceted bipyramids, and octahedra with hollow features, and a set of design rules for how to control shape.

Figure 1

High quality seeds can be used interchangeably to generate eight different shapes. Each panel represents a different shape synthesized and is arranged counterclockwise from top left as three-dimensional graphic rendering of the shape; TEM image (scale bars are 100 nm); high-magnification SEM image of crystallized nanoparticles (scale bars are 500 nm) with FFT pattern inset. Moving clockwise from the top left, the shapes described are cubes, concave rhombic dodecahedra, octahedra, tetrahexahedra, truncated ditetragonal prisms, cuboctahedra, concave cubes, and rhombic dodecahedra.

Subsequent research has focused on the mechanisms behind seed-mediated synthesis and how a deeper understanding can enable unprecedented control over the structural uniformity and yield of anisotropic nanoparticles.8 In particular, while the field has predominantly focused on how to transform an ill-defined initial state (<5 nm seeds) into a well-defined end state through manipulation of reaction conditions, we have instead developed chemistry to control the uniformity of the seeds. Recently, we reported how iterative reductive growth and subsequent oxidative dissolution can be used for the stepwise refinement of gold nanoparticle seeds used for anisotropic particle synthesis.

This novel capability allowed us to systematically study how the size dispersity, shape variation, and crystalline structure of the seed influence anisotropic nanoparticle products and enables the synthesis of eight classes of single crystalline nanostructures from the same batch of seeds, each consisting of a different shape, where the shape and size uniformity exceeds that of all previously reported syntheses. Subsequently, we extended this work to two-dimensional particles, where a nonuniform mixture of triangular, truncated triangular, and hexagonal plates can be oxidized in a self-limiting, tip-selective reaction that converts each of these products into similarly sized circular disks, resulting in considerable particle homogenization and narrower plasmon resonances. As part of this work, we developed software to algorithmically analyze electron microscopy images that allows for the rapid quantification of size and shape distribution for different nanoparticle populations.

Figure 2

Overview of the process for quantitative analysis of nanoparticle structure from electron microscopy (EM) images, including EM methods and computational processing. In particular, the most appropriate EM data collection requires dilute sample preparation and acquisition of images at diverse grid locations. Computationally, the program processes raw EM images and extracts angular distance data, d(θ), from individual particles. It then performs a shape fit and repeats the process iteratively for multiple images to produce population statistics.

Photo-Mediated Synthesis and the Concept of Plasmonic Seeds

Anisotropic nanoparticle synthesis research in the Mirkin group began with our discovery and development of a photo-mediated (also referred to as a plasmon-mediated) synthesis for silver triangular nanoprisms, which was one of the first high-yield anisotropic nanoparticle syntheses ever developed. We have shown that the triangular nanoprism morphology can be controlled photochemically by adjusting the wavelength of irradiation or chemically by varying the pH of the reaction solution. The photo-mediated synthesis also can be used to generate Au-core/Ag-shell structures by seeding the reaction with Au particles and irradiating the reaction solution at a wavelength equal to the plasmon resonance of the Au core. In this way, these particles serve as “plasmonic seeds” where light can be used to control particle growth and resulting shape. By changing the Ag precursor in the photo-mediated synthesis from Ag nanospheres to AgNO3, we have synthesized a variety of additional particle morphologies, such as right-triangular bipyramids and penta-twinned rods. Through this work, we have begun to work out the mechanisms that underlie these unusual transformations.

Figure 3

Silver nanostructures prepared by plasmon-mediated syntheses. Left column: Solutions of triangular nanoprisms prepared at different excitation wavelengths and a representative transmission electron micrograph. Center column: Scanning electron micrograph of right triangular bipyramids and solutions of bipyramids prepared by different excitation wavelengths. Right column: Penta-twinned nanorods (top) and gold-core/silver-shell bimetallic icosahedra (bottom).

Anisotropic Nanoparticle Assembly

With this incredible toolbox of anisotropic nanoparticle building blocks, we have investigated a number of different properties, ranging from plasmonics to assembly. Assembly represents a particularly interesting avenue of research, as shape can be used to introduce directional interactions between particles. We first investigated the effect of shape in the context of DNA-mediated nanoparticle crystallization. In this work, DNA is used as a surface ligand to mediate interparticle interactions through sequence specific hybridization events. The DNA forms a densely packed, conformal shell around the nanoparticle, such that the shape controls the strength, number and directionality of the “DNA bonds” for a given particle.

Our initial work showed that a given shape (e.g. triangular prisms, rods, octahedra, rhombic dodecahedra) will crystallize with a lattice symmetry that maximizes the number of face-to-face interactions between particles (and thus maximizes the number of DNA hybridization events). We have subsequently investigated how the DNA bond strength between particles changes as a function of shape and how differences in bonding strength can be exploited to separate otherwise difficult to purify mixtures. Recently, we demonstrated the selective co-crystallization of mixtures of two different anisotropic nanoparticles and investigated the effects of nanoparticle size and shape complementarity on the resultant crystal symmetry, microstrain, and effective ‘DNA bond’ length and strength.

Additionally, we have developed a Langmuir-based assay to elucidate the binding thermodynamics of nanoparticles as a function of shape and size. Using DNA-mediated nanoparticle adsorption, we have also constructed optical metasurfaces of ordered arrays of anisotropic nanoparticles, which allows for the systematic control of photonic and plasmonic coupling modes in a single device and has implications for advanced nanophotonic structures in sensing, quantum plasmonics, and tunable absorbers.

Figure 4

Nanoparticles with different shapes and sizes (left two columns) can be selectively cocrystallized together with complementary DNA sequences, seen in drawings (center column) and false-colored TEM images (right). Small-angle x-ray scattering produced a characteristic diffraction pattern for each type of crystal (second from right).

Figure 5

Optical metasurfaces with control of both photonic and plasmonic coupling can be constructed using an approach that combines top-down and bottom-up processes, wherein gold nanocubes are assembled into ordered arrays via DNA hybridization events onto a gold film decorated with DNA-binding regions defined using electron beam lithography.

Anisotropic Nanostructures Subgroup Members

Standing (L-R): Eileen Seo, Jingshan Du, Jinghan Zhu, Taegon Oh, Devleena Samanta (subgroup leader), Oliver Hayes (subgroup leader) , Melissa Puga, Zack Urbach, Lin Sun, Rohit Murthy, Peter Winegar, Haixin Lin, Pengcheng Chen

Sitting (L-R): Mike Ashley, Christine Laramy, Katherine Bujold, Lam-Kiu Fong