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.

Nanoparticles can be combined with nucleic acids to programme the formation of three-dimensional colloidal crystals where the particles' size, shape, composition and position can be independently controlled. However, the diversity of the types of material that can be used is limited by the lack of a general method for preparing the basic DNA-functionalized building blocks needed to bond nanoparticles of different chemical compositions into lattices in a controllable manner. Here we show that by coating nanoparticles protected with aliphatic ligands with an azide-bearing amphiphilic polymer, followed by the coupling of DNA to the polymer using strain-promoted azide-alkyne cycloaddition (also known as copper-free azide-alkyne click chemistry), nanoparticles bearing a high-density shell of nucleic acids can be created regardless of nanoparticle composition. This method provides a route to a virtually endless class of programmable atom equivalents for DNA-based colloidal crystallization.

Due to their potential for creating 'designer materials,' the 3D assembly of nanoparticle building blocks into macroscopic structures with well-defi ned order and symmetry remains one of the most important challenges in materials science. [1][2][3][4][5] Furthermore, superlattices consisting of noble-metal nanoparticles have emerged as a new platform for the bottom-up design of plasmonic metamaterials. [6][7][8] The allure of optical metamaterials is that they provide a means for altering the temporal and spatial propagation of electromagnetic fi elds, resulting in materials that exhibit many properties that do not exist in nature. [9][10][11][12][13] With the vast array of nanostructures now synthetically realizable, computational methods play a crucial role in identifying the assemblies that exhibit the most exciting properties. [ 14 ] Once target assemblies are identifi ed, the synthesis of nanometer-scale structures for use at optical and IR wavelengths must be taken into account. Many of the current methods to fabricate metamaterials in the optical range use serial lithographic-based approaches. [ 6 ] The challenge of controlled assembly into well-defi ned architectures has kept bottom-up methods that rely on the self-organization of colloidal metal nanoparticles from being fully explored for metamaterial applications. [ 8 ] DNA-mediated assembly of nanoparticles has the potential to help overcome this challenge. The predictability and programmability of DNA makes it a powerful tool for the rational assembly of plasmonic nanoparticles with tunable nearest-neighbor distances and symmetries. [ 1,[15][16][17][18] Herein, we combine theory and experiment to study a new class of plasmonic superlattices-fi rst using electrodynamics simulations to identify that superlattices of spherical silver nanoparticles (Ag NPs) have the potential to exhibit emergent metamaterial properties, including epsilon-near-zero (ENZ) behavior, [ 13 ] and a region with an 'optically metallic' response.Optically metallic materials are DC insulators that refl ect in the visible spectrum. This behavior can be described as the opposite of the common touch-screen material, indium tin oxide (ITO), which is transparent in the visible spectrum, but conducts electricity. We then synthesize the fi rst examples of silver nanoparticle superlattices using DNA-mediated assembly and characterize their optical properties with both ensemble measurements and measurements of individual superlattices using spectroscopy. Furthermore, we expand beyond monometallic nanoparticle superlattices to create novel binary superlattices of gold and silver nanoparticle building blocks and observe a Fano-like interference between the two components, leading to a signifi cant dampening of the plasmonic response.ENZ materials [ 19 ] are a new class of metamaterials that allow for the tunneling of light through a barrier and present the opportunity for arbitrary phase manipulation of light. [ 13,[20][21][22] With the global progression of optical fi bers and the potential ...

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.anisotropic | tunable | small angle X-ray scattering | depletion interaction T he ability to form ensembles of inorganic nanoparticles with a high degree of control has become one of the main areas of focus in nanoscience research (1). This interest stems from the fact that nanocrystal superlattices often exhibit electronic (2), optical (3), and magnetic (4) properties that are distinct from both the corresponding individual particles and the bulk solid as a result of the interactions between the excitons, surface plasmons, or magnetic moments of the assembled particles (5). Superlattices composed of spherical building blocks have been extensively studied, and researchers now have the ability to synthesize a wide variety of structures (6). However, as new techniques are developed to synthesize high-quality anisotropic nanoparticles with new physical properties that cannot be obtained with spheres alone, researchers are increasingly interested in the rich assembly behavior of particles with reduced symmetry (7-9). Indeed, periodic arrays of these anisotropic building blocks have been shown to possess unique collective properties with applications in various fields including plasmonics (10) and photonics (11). However, to take full advantage of these collective properties, it is necessary to understand the relationship between the architectural parameters of the ensemble and the emergent physical properties. For this purpose, it is crucial to be able to "engineer" the various interactions that exist between nanoparticle building blocks to produce a desired structure (12, 13). The assembly of nanocrystals into ordered arrays can be induced via the manipulation of interparticle interactions including van der Waals (14), electrostatic (15), entropic (16-21), and through highly specific biological interactions (22-24). Herein, we report the assembly of colloidal triangular gold nanoprisms protected by a cetyltrimethylammonium bromide (CTAB) bilayer in a solution of CTAB micelles into highly ordered 1D crystals with unexpected structural f...

The assembly of DNA-programmable colloidal crystals is presented, where the sizes of nanoparticles used vary from 5 to 80 nm 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 is demonstrated to dictate the assembly and crystallization processes, creating a set of design rules for DNA-based nanoscale assembly. ** We acknowledge George Schatz for helpful discussions regarding the theoretical calculations of DNA flexibility and relative DNA concentrations in aggregates. C. A. M. acknowledges the NSF-NSEC and the AFOSR for grant support. He also is grateful for a NIH Director's Pioneer Award and an NSSEF Fellowship from the DoD. R. J. M. acknowledges Northwestern University for a Ryan Fellowship. M. R. J.

Linking hydrophilic macromolecules, especially biomolecules, to magnetic nanoparticles is a vital step for producing water-based ferrofluids for biomedical applications. Magnetic nanoparticles present in these fluids can be used as highly sensitive labels as they exhibit a magnetic signal that far exceeds that from any of the biomolecules. This, plus their capability of being manipulated under a magnetic field, provides a controllable means of magnetically tagging biomolecules or cells, leading to highly efficient bio-separation, drug delivery, biosensing, magnetic fluid hyperthermia, and magnetic resonance imaging contrast enhancement. 1 Functionalization of magnetic nanoparticles with biomolecules can be achieved through an organic linker that is designed to couple two different kinds of molecules. Such organic linkers have been well developed to couple macromolecules/biomolecules with organic dyes so that the molecules can become optically active and be detected. 2 However, using the similar coupling chemistry to link macromolelcules/biomolecules to monodisperse magnetic nanoparticles has been limited to date, 3 and the linkers used for this purpose, such as succinimidyl-4-(N-maleimidomethyl) cyclohexane-l-carboxylate (SMCC) and sulfo-SMCC, are often not economically available.

Cellular transfection of nucleic acids is necessary for regulating gene expression through anti-sense or RNAi pathways. The development of spherical nucleic acids (SNAs, originally gold nanoparticles functionalized with synthetic oligonucleotides) has resulted in a powerful set of construct that are able to efficiently transfect cells and regulate gene expression without the use of auxiliary cationic co-carriers. The gold core in such structures is primarily used as a template to arrange the nucleic acids into a densely packed and highly oriented form. In this work, we have developed methodology for coating the gold particle with a shell of silica, modifying the silica with a layer of oligonucleotides, and subsequently oxidatively dissolving the gold core with I2. The resulting hollow silica-based SNAs exhibit cooperative binding behavior with respect to complementary oligonucleotides and cellular uptake properties comparable to their gold-core SNA counterparts. Importantly, they exhibit no cytotoxicity and have been used to effectively silence the eGFP gene in mouse endothelial cells through an anti-sense approach.

Learning how to assemble inorganic nanoparticles into ordered lattices may prove to be important for applications, such as, electronics, photonics, and catalysis. [1] Indeed, theoretical studies have shown that certain types of crystalline arrays of nanoparticles could potentially be used to generate photonic band-gap materials, negative index materials, and metamaterials at visible and infrared length scales. [2,3] The vast majority of work in this area has focused on the assembly of spherical particles. However, anisotropic nanoparticles, which display rich assembly behavior owing to their reduced symmetry, and have unique physical properties that can be engineered by controlling interparticle spacing and orientation, may provide access to even more interesting materials. [4][5][6][7][8] Moreover, they require design rules for predicting the way such nanoparticles can assemble and the types of structures that may be realized. The rapidly expanding library of available anisotropic nanoparticle building blocks provides exciting new opportunities to study colloidal assembly as a function of particle shape. [9][10][11] Herein, we introduce a directional entropic force approach (DEFA) for controlling the assembly of anisotropic nanoparticles into crystalline lattices. The method relies on surfactant micelle-induced depletion interactions [12] to assemble anisotropic gold nanoparticles into reconfigurable, nonclose-packed (open) superlattices in solution. The anisotropic nanoparticles align along their flat facets to maximize entropy, and therefore minimize the free energy of the system, leading to assemblies with long-range order. [13,14] Importantly, our experimental work complements recent theoretical work that proposes directional entropic forces between nanoparticle facets as a viable means for thermodynamically assembling nanoparticle superlattices. [14][15][16][17] The experimental work herein uses depletants to create strong attractive forces that can drive assembly of reversible superlattices with tunable spacing in solution. These directional entropic forces are analogous to the directional bonding between atoms in molecules. [14,15,18] The resulting crystalline superlattices are therefore shape-dependent. We show that the electrostatic and depletion interactions combine to determine the lattice spacing, and can be tuned independently with surfactant concentration and ionic strength to reconfigure the lattice constant.The DEFA presented herein complements several assembly strategies that have been developed to create superlattices of anisotropic nanostructures including evaporationinduced [19,20] and sedimentation-based methods, [8] as well as ones based upon the manipulation of electrostatic, [21,22] entropic, [13,[23][24][25][26][27][28] block copolymer, [29,30] and biological recognition interactions. [31] However, unlike the majority of these methods, our approach yields reversible crystals with tunable lattice constants in situ.Depletion interactions are purely entropic in nature and arise when non-ads...