Linear self-assembly and grafting of gold nanorods into arrayed micrometer-long nanowires on a silicon wafer via a combined top-down/bottom-up approach

Nanoparticle Size Influences the Self-Assembly of Gold Nanorods Using Flexible Streptavidin-Biotin Linkages

The self-assembly of colloidal nanocrystals remains of robust interest due to its potential in creating hierarchical nanomaterials that have advanced function. For gold nanocrystals, junctions between nanoparticles yield large enhancements in local electric fields under resonant illumination, which is suitable for surface-enhanced spectroscopies for molecular sensors. Gold nanorods can provide such plasmonic fields at near-infrared wavelengths of light for longitudinal excitation. Through the use of careful concentration and stoichiometric control, a method is reported herein for selective biotinylation of the ends of gold nanorods for simple, consistent, and high-yielding self-assembly upon addition of the biotin-binding protein streptavidin. This method was applied to four different sized nanorods of similar aspect ratio and analyzed through UV-vis spectroscopy for qualitative confirmation of self-assembly and transmission electron microscopy to determine the degree of self-assembly in end-linked nanorods. The yield of end-linked assemblies approaches 90% for the largest nanorods and approaches 0% for the smallest nanorods. The number of nanorods linked in one chain also increases with an increased nanoparticle size. The results support the notion that the lower ligand density at the ends of the larger nanorods yields preferential substitution reactions at those ends and hence preferential end-to-end assembly, while the smallest nanorods have a relatively uniform ligand density across their surfaces, leading to spatially random substitution reactions.

Exosome-like Nanoparticles: A New Type of Nanocarrier

Nanoparticles are one of the most commonly used systems for imaging or therapeutic drug delivery. Exosomes are nanovesicular carriers that transport cargo for intercellular communication. These nanovesicles are linked to the pathology of some major diseases, in some cases with a central role in their progression. The use of these carriers to transport therapeutic drugs is a recent and promising approach to treat diseases such as cancer and Alzheimer disease. The physiological production of these structures is limited impairing its collection and subsequent purification. These drawbacks inspired the search for mimetic alternatives. The collection of exosome-like nanoparticles from plants can be a good alternative, since they are easier to extract and do not have the drawbacks of those produced in animal cells. Both natural and synthetic exosome-like nanoparticles, produced from serial extrusion of cells or by bottom up synthesis, are currently some of the most promising, biocompatible, high efficiency systems for drug delivery.

Salt Mediated Self-Assembly of Poly(ethylene glycol)-Functionalized Gold Nanorods

Although challenging, assembling and orienting non-spherical nanomaterials into two- and three-dimensional (2D and 3D) ordered arrays can facilitate versatile collective properties by virtue of their shape-dependent properties that cannot be realized with their spherical counterparts. Here, we report on the self-assembly of gold nanorods (AuNRs) into 2D films at the vapor/liquid interface facilitated by grafting them with poly(ethylene glycol) (PEG). Using surface sensitive synchrotron grazing incidence small angle X-ray scattering (GISAXS) and specular X-ray reflectivity (XRR), we show that PEG-AuNRs in aqueous suspensions migrate to the vapor/liquid interface in the presence of salt, forming a uniform monolayer with planar-to-surface orientation. Furthermore, the 2D assembled PEG functionalized AuNRs exhibit short range order into rectangular symmetry with side-by-side and tail-to-tail nearest-neighbor packing. The effect of PEG chain length and salt concentration on the 2D assembly are also reported.