A Directional Entropic Force Approach to Assemble Anisotropic Nanoparticles into Superlattices

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102143/1/14230_ftp.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/102143/2/ange_201306009_sm_miscellaneous_information.pd

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. Includes Supplementary Information (44 pp.)

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. Specifically, the etching rate for Au nanocubes with {100}-terminated facets was ∼1.5 times higher than that for triangular nanoprisms with {111} facets. Furthermore, this work gives fundamental insight into the mechanism of metal-catalyzed chemical etching

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 Mg²⁺ ions binding to surface-immobilized T₄₀ sequences, the subsequent reordering of DNA on the surface, and the consequences of Mg²⁺ binding for DNA–DNA interactions. Second harmonic generation measurements indicate that, within error, 18–20 Mg²⁺ 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 Mg²⁺ 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. The impact of Mg²⁺ binding on DNA hybridization and duplex stability is assessed with spherical nucleic acid (SNA) gold nanoparticle conjugates in order to determine an optimal working range of Mg²⁺ concentrations for DNA–DNA interactions in the absence of NaCl. The findings are consistent with a charge titration effect in which, in the absence of NaCl, (1) hybridization does not occur at room temperature if an average of 17.5 or less Mg²⁺ ions are bound per T₄₀ strand, which is not reached until the bulk Mg²⁺ concentration approaches 0.5 mM; (2) hybridization proceeds, albeit with low duplex stability having an average <i>T</i>_m of 31(3)°C, if an average of 17.5–18.0 Mg²⁺ ions are bound; and (3) highly stable duplexes having a <i>T</i>_m of 64(2)°C form if 18.5–19.0 Mg²⁺ ions are bound, corresponding to saturation of the T₄₀ strand

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. 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 antisense approach

Cross-linked Heterogeneous Nanoparticles as Bifunctional Probe

Cross-linked Heterogeneous Nanoparticles as Bifunctional Prob