Robert J. Macfarlane

Nanoparticle Superlattice Engineering with DNA

Design rules allow the synthesis of nanoparticle-DNA superlattices in nine different lattices. A current limitation in nanoparticle superlattice engineering is that the identities of the particles being assembled often determine the structures that can be synthesized. Therefore, specific crystallographic symmetries or lattice parameters can only be achieved using specific nanoparticles as building blocks (and vice versa). We present six design rules that can be used to deliberately prepare nine distinct colloidal crystal structures, with control over lattice parameters on the 25- to 150-nanometer length scale. These design rules outline a strategy to independently adjust each of the relevant crystallographic parameters, including particle size (5 to 60 nanometers), periodicity, and interparticle distance. As such, this work represents an advance in synthesizing tailorable macroscale architectures comprising nanoscale materials in a predictable fashion.

DNA-nanoparticle superlattices formed from anisotropic building blocks

Directional bonding interactions in solid-state atomic lattices dictate the unique symmetries of atomic crystals, resulting in a diverse and complex assortment of three-dimensional structures that exhibit a wide variety of material properties. Methods to create analogous nanoparticle superlattices are beginning to be realized, but the concept of anisotropy is still largely underdeveloped in most particle assembly schemes. Some examples provide interesting methods to take advantage of anisotropic effects, but most are able to make only small clusters or lattices that are limited in crystallinity and especially in lattice parameter programmability. Anisotropic nanoparticles can be used to impart directional bonding interactions on the nanoscale, both through face-selective functionalization of the particle with recognition elements to introduce the concept of valency, and through anisotropic interactions resulting from particle shape. In this work, we examine the concept of inherent shape-directed crystallization in the context of DNA-mediated nanoparticle assembly. Importantly, we show how the anisotropy of these particles can be used to synthesize one-, two- and three-dimensional structures that cannot be made through the assembly of spherical particles.

Establishing the design rules for DNA-mediated programmable colloidal crystallization

DNA-programmable colloidal crystals are assembled with 5–80 nm nanoparticles, and the lattice parameters of the resulting crystals vary from 25 to 225 nm. A predictable and mathematically definable relationship between particle size and DNA length dictates the assembly and crystallization processes, creating a set of design rules for DNA-based nanoscale assembly.

Nanoparticle Shape Anisotropy Dictates the Collective Behavior of Surface-Bound Ligands

We report on the modification of the properties of surface-confined ligands in nanoparticle systems through the introduction of shape anisotropy. Specifically, triangular gold nanoprisms, densely functionalized with oligonucleotide ligands, hybridize to complementary particles with an affinity that is several million times higher than that of spherical nanoparticle conjugates functionalized with the same amount of DNA. In addition, they exhibit association rates that are 2 orders of magnitude greater than those of their spherical counterparts. This phenomenon stems from the ability of the flat, extended facets of nonspherical nanoparticles to (1) support more numerous ligand interactions through greater surface contact with complementary particles, (2) increase the effective local concentration of terminal DNA nucleotides that mediate hybridization, and (3) relieve the conformational stresses imposed on nanoparticle-bound ligands participating in interactions between curved surfaces. Finally, these same trends are observed for the pH-mediated association of nanoparticles functionalized with carboxylate ligands, demonstrating the generality of these findings.

Assembly and organization processes in DNA-directed colloidal crystallization

We present an analysis of the key steps involved in the DNA-directed assembly of nanoparticles into crystallites and polycrystalline aggregates. Additionally, the rate of crystal growth as a function of increased DNA linker length, solution temperature, and self-complementary versus non-self-complementary DNA linker strands (1- versus 2-component systems) has been studied. The data show that the crystals grow via a 3-step process: an initial “random binding” phase resulting in disordered DNA-AuNP aggregates, followed by localized reorganization and subsequent growth of crystalline domain size, where the resulting crystals are well-ordered at all subsequent stages of growth.

Modeling the crystallization of spherical nucleic acid nanoparticle conjugates with molecular dynamics simulations.

We use molecular dynamics simulations to study the crystallization of spherical nucleic-acid (SNA) gold nanoparticle conjugates, guided by sequence-specific DNA hybridization events. Binary mixtures of SNA gold nanoparticle conjugates (inorganic core diameter in the 8-15 nm range) are shown to assemble into BCC, CsCl, AlB(2), and Cr(3)Si crystalline structures, depending upon particle stoichiometry, number of immobilized strands of DNA per particle, DNA sequence length, and hydrodynamic size ratio of the conjugates involved in crystallization. These data have been used to construct phase diagrams that are in excellent agreement with experimental data from wet-laboratory studies.

Controlling the Lattice Parameters of Gold Nanoparticle FCC Crystals with Duplex DNA Linkers

DNA-functionalized gold nanoparticles can be used to induce the formation and control the unit cell parameters of highly ordered face-centered cubic crystal lattices. Nanoparticle spacing increases linearly with longer DNA interconnect length, yielding maximum unit cell parameters of 77 nm and 0.52% inorganic-filled space for the DNA constructs studied. In general, we show that longer DNA connections result in a decrease in the overall crystallinity and order of the lattice due to greater conformational flexibility.

Topotactic Interconversion of Nanoparticle Superlattices

Sticking with DNA One strategy for creating superlattices from nanoparticles is to coat the particles with DNA strands that have sticky ends that can be exploited to control the assembly of the lattice. This method can create binary lattices, but now Macfarlane et al. (p. 1222, published online 22 August) have succeeded in inserting a third type of nanoparticle into a predetermined site by tuning the strength of the relative DNA binding interactions. DNA-coated nanoparticles can reversibly incorporate a third nanoparticle. The directed assembly of nanoparticle building blocks is a promising method for generating sophisticated three-dimensional materials by design. In this work, we have used DNA linkers to synthesize nanoparticle superlattices that have greater complexity than simple binary systems using the process of topotactic intercalation—the insertion of a third nanoparticle component at predetermined sites within a preformed binary lattice. Five distinct crystals were synthesized with this methodology, three of which have no equivalent in atomic or molecular crystals, demonstrating a general approach for assembling highly ordered ternary nanoparticle superlattices whose structures can be predicted before their synthesis. Additionally, the intercalation process was demonstrated to be completely reversible; the inserted nanoparticles could be expelled into solution by raising the temperature, and the ternary superlattice could be recovered by cooling.

Nucleic Acid‐Modified Nanostructures as Programmable Atom Equivalents: Forging a New “Table of Elements”

The establishment of the Periodic Table of the Elements almost 150 years ago was the first step towards transforming how scientists organized and understood the elemental building blocks of matter. Before its introduction, elements were viewed as separate and independent entities, each with their own unique set of properties. By arranging elements based upon their characteristics, the Periodic Table enabled scientists to understand their behavior as members of collective sets. Their properties could be discussed in the context of logical trends, and these trends could be used to predict the properties of as-of-yet undiscovered elements and yet-to-be synthesized molecules, bulk materials, and extended lattice structures. Indeed, for decades the Periodic Table has served as a guide for the synthesis of new structures and given us a framework to understand important scientific advances. Today, the field of nanoscience and nanotechnology offers scientists new ways to think about materials synthesis. Nanoscience is an interdisciplinary field focused on the synthesis, manipulation, characterization, and application of structures with at least one dimension on the 1 to 100 nm length scale. In this size regime, materials possess properties that are significantly different than their macroscopic analogues, and these properties are highly dependent on the nanostructure s composition, size, shape, and local environment. In 2000, with the introduction of the National Nanotechnology Initiative (NNI), nanoscience research and development were prioritized in the United States, and since then US scientists and other researchers around the globe have devised myriad methodologies to generate nanoparticles of many compositions (e.g., metallic, semiconducting, insulating, carbon-based, polymeric) in high yield in the solid, solution, and gas phases, as well as on surfaces. Several methods also have been introduced to tune the size and shape of nanoparticles with nanometer precision, and to couple materials of different compositions together to create hybrid structures (e.g., alloys, core–shell structures). These new nanoparticles have a variety of interesting chemical and physical properties, which have been applied in a range of fields from catalysis to biomedicine to energy. This explosion in research aimed at discovering, understanding, and refining nanoparticle syntheses to realize highly sophisticated nanoscale architectures can be likened to the early rush in chemistry to discover new elements. A key area of nanotechnology research deals with the assembly of these building blocks into more complex structures, just as the discovery of different elements led to the synthesis of many new materials. Although analogues consisting of small clusters of nanoparticles have also been developed, we will focus herein primarily on extended networks. In many cases, these assemblies have been shown to exhibit novel and extremely useful emergent properties that are a direct result of the arrangement of the individual nanostructures within the assembly. As a result of these promising but nascent discoveries with nanoparticle-based constructs, there has been intense interest in devising strategies that can be used to organize nanoparticles of all types into well-defined hierarchical arrays, in which the spacing and symmetry between the particles are precisely controlled. Indeed, one of the main challenges currently facing nanoscience researchers is the [*] R. J. Macfarlane, M. N. O’Brien, Dr. S. H. Petrosko, Prof. C. A. Mirkin Department of Chemistry and International Institute for Nanotechnology, Northwestern University 2145 Sheridan Road, Evanston, IL 60208 (USA) E-mail: chadnano@northwestern.edu

Assembly of reconfigurable one-dimensional colloidal superlattices due to a synergy of fundamental nanoscale forces

We report that triangular gold nanoprisms in the presence of attractive depletion forces and repulsive electrostatic forces assemble into equilibrium one-dimensional lamellar crystals in solution with interparticle spacings greater than four times the thickness of the nanoprisms. Experimental and theoretical studies reveal that the anomalously large d spacings of the lamellar superlattices are due to a balance between depletion and electrostatic interactions, both of which arise from the surfactant cetyltrimethylammonium bromide. The effects of surfactant concentration, temperature, ionic strength of the solution, and prism edge length on the lattice parameters have been investigated and provide a variety of tools for in situ modulation of these colloidal superstructures. Additionally, we demonstrate a purification procedure based on our observations that can be used to efficiently separate triangular nanoprisms from spherical nanoparticles formed concomitantly during their synthesis.