Determination of ion pairing on capping structures of gold nanoparticles by phase extraction.

As nanoparticles with different capping structures in solution phases have found widespread applications of wide interest, understanding how the capping structure change influences their presence in phases or solutions is important for gaining full control over both the intended nanoactivity and the unintended nanotoxicity. This report describes a simple and effective phase extraction method for analyzing the degree of ion pairing in the capping molecular structure of nanoparticles. Gold nanoparticles of a few nanometers diameter with a mixed monolayer capping structure consisting of both hydrophobic and hydrophilic and reactive groups were studied as a model system, and a quantitative model was derived based on chemical equilibria in a two-phase system, and used to assess the experimental data for phase extraction by cationic species.

Oligonucleotide flexibility dictates crystal quality in DNA-programmable nanoparticle superlattices.

The evolution of crystallite size and microstrain in DNA-mediated nanoparticle superlattices is dictated by annealing temperature and the flexibility of the interparticle bonds. This work addresses a major challenge in synthesizing optical metamaterials based upon noble metal nanoparticles by enabling the crystallization of large nanoparticles (100 nm diameter) at high volume fractions (34% metal).

Counting the number of magnesium ions bound to the surface-immobilized thymine oligonucleotides that comprise spherical nucleic acids.

Label-free studies carried out under aqueous phase conditions quantify the number of Mg(2+) ions binding to surface-immobilized T40 sequences, the subsequent reordering of DNA on the surface, and the consequences of Mg(2+) binding for DNA-DNA interactions. Second harmonic generation measurements indicate that, within error, 18-20 Mg(2+) ions are bound to the T40 strand at saturation and that the metal-DNA interaction is associated with a near 30% length contraction of the strand. Structural reordering, evaluated using vibrational sum frequency generation, atomic force microscopy, and dynamic light scattering, is attributed to increased charge screening as the Mg(2+) ions bind to the negatively charged DNA, reducing repulsive Coulomb forces between nucleotides and allowing the DNA single strands to collapse or coil upon themselves.

Anisotropic nanoparticles as shape-directing catalysts for the chemical etching of silicon.

Anisotropic Au nanoparticles have been used to create a library of complex features on silicon surfaces. The technique provides control over feature size, shape, and depth. Moreover, a detailed study of the etching rate as a function of the nanoparticle surface facet interfaced with the silicon substrate suggested that the etching is highly dependent upon the facet surface energy.

A general approach to DNA-programmable atom equivalents.

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.

Hollow spherical nucleic acids for intracellular gene regulation based upon biocompatible silica shells.

Cellular transfection of nucleic acids is necessary for regulating gene expression through antisense or RNAi pathways. The development of spherical nucleic acids (SNAs, originally gold nanoparticles functionalized with synthetic oligonucleotides) has resulted in a powerful set of constructs that are able to efficiently transfect cells and regulate gene expression without the use of auxiliary cationic cocarriers. 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.

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.

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.

Establishing the design rules for DNA-mediated programmable colloidal crystallization.

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.

Conjugating Methotrexate to magnetite (Fe(3)O(4)) nanoparticles via trichloro-s-triazine.

Monodisperse Fe(3)O(4) nanoparticles (NPs) originally synthesized with a hydrophobic oleylamine capping ligand were made water soluble and conjugated to the anticancer drug Methotrexate (MTX) using a new chemistry based on the readily available linker trichloro-s-triazine (TsT). This new linker is much more versatile than those that currently exist for attaching biomolecules to magnetic NPs. The MTX-conjugated NPs were found to be stable under physiological conditions for over 72 hours and MTX was shown to maintain its anticancer activity after conjugation to the NP surface.

Plasmonically controlled nucleic acid dehybridization with gold nanoprisms.

Remote release: Triangular gold nanoprisms convert 1064 nm laser irradiation into heat selectively to allow the dehybridization of oligonucleotide conjugated to their surface (see scheme). These conjugates show unprecedented morphological stability under hours of irradiation. Released nucleic acids are unharmed by this process and can be repeatedly dehybridized and sequestered under spatiotemporal control.