Eugene R. Zubarev

Functional Gold Nanorods: Synthesis, Self‐Assembly, and Sensing Applications

Gold nanorods have received much attention due to their unique optical and electronic properties which are dependent on their shape, size, and aspect ratio. This article covers in detail the synthesis, functionalization, self-assembly, and sensing applications of gold nanorods. The synthesis of three major types of rods is discussed: single-crystalline and pentahedrally-twinned rods, which are synthesized by wet chemistry methods, and polycrystalline rods, which are synthesized by templated deposition. Functionalization of these rods is usually necessary for their applications, but can often be problematic due to their surfactant coating. Thus, general strategies are provided for the covalent and noncovalent functionalization of gold nanorods. The review will then examine the significant progress that has been made in controllable assembly of nanorods into various arrangements. This assembly can have a large effect on measurable properties of rods, making it particularly applicable towards sensing of a variety of analytes. Other types of sensing not dependent on nanorod assembly, such as refractive-index based sensing, are also discussed.

Gold nanorods 3D-supercrystals as surface enhanced Raman scattering spectroscopy substrates for the rapid detection of scrambled prions

Highly organized supercrystals of Au nanorods with plasmonic antennae enhancement of electrical field have made possible fast direct detection of prions in complex biological media such as serum and blood. The nearly perfect three-dimensional organization of nanorods render these systems excellent surface enhanced Raman scattering spectroscopy substrates with uniform electric field enhancement, leading to reproducibly high enhancement factor in the desirable spectral range.

Semiconductor nanohelices templated by supramolecular ribbons

Nanohelices of cadmium sulfide (CdS) have been made by the mineralization of supramolecular organic ribbons. A schematic representation of how coiled CdS helices (yellow) can be templated from a twisted ribbon (blue) by growth along one face of the ribbon is shown top right.

Therapeutic platforms based on gold nanoparticles and their covalent conjugates with drug molecules.

This review will first look at the various covalent strategies that have been developed to attach drugs to gold nanoparticles as well as the strengths and limitations of such strategies. After examining general strategies for the synthesis of gold nanoparticles and their subsequent covalent functionalization, this review will focus on nanoparticle conjugates for gene therapy, antibacterial, and anticancer applications including the use of gold nanoparticles with intrinsically therapeutic properties. The effects of targeting and cellular uptake of gold nanoparticles will also be discussed.

Purification of High Aspect Ratio Gold Nanorods: Complete Removal of Platelets

Because physical and chemical properties of nanostructures strongly depend on their shape, it is of great importance to find either synthetic or separation techniques that can produce objects of a particular shape in a pure state. This manuscript describes a solution to a long standing problem of separating 2D platelets from 1D nanorods. The key aspect of our approach relies on the partial dissolution of faceted platelets with Au(III)/CTAB complex that transforms them into smaller nanodisks. Because of the reduction in size, the 2D structures become fully soluble in water and can be separated from the nanorods that undergo slow precipitation. In addition, the isolated nanodisks can be converted back into initial faceted platelets upon treatment with Au(I)/ascorbic acid mixture. As a result of these simple procedures, a seemingly inseparable mixture of rods, platelets, and spheres is converted into nearly pure individual components.

Propagation Lengths and Group Velocities of Plasmons in Chemically Synthesized Gold and Silver Nanowires

Recent advances in chemical synthesis have made it possible to produce gold and silver nanowires that are free of large-scale crystalline defects and surface roughness. Surface plasmons can propagate along the wires, allowing them to serve as optical waveguides with cross sections much smaller than the optical wavelength. Gold nanowires provide improved chemical stability as compared to silver nanowires, but at the cost of higher losses for the propagating plasmons. In order to characterize this trade-off, we measured the propagation length and group velocity of plasmons in both gold and silver nanowires. Propagation lengths are measured by fluorescence imaging of the plasmonic near fields. Group velocities are deduced from the spacing of fringes in the spectrum of coherent light transmitted by the wires. In contrast to previous work, we interpret these fringes as arising from a far-field interference effect. The measured propagation characteristics agree with numerical simulations, indicating that propagation in these wires is dominated by the material properties of the metals, with additional losses due to scattering from roughness or grain boundaries providing at most a minor contribution. The propagation lengths and group velocities can also be described by a simple analytical model that considers only the lowest-order waveguide mode in a solid metal cylinder, showing that this single mode dominates in real nanowires. Comparison between experiments and theory indicates that widely used tabulated values for dielectric functions provide a good description of plasmons in gold nanowires but significantly overestimate plasmon losses in silver nanowires.

Near‐Bulk Conductivity of Gold Nanowires as Nanoscale Interconnects and the Role of Atomically Smooth Interface

The electronics industry has consistently decreased the dimensions of structural components and they are now well into the nanoscale range. Naturally, a significant portion of the chip is composed of interconnects. Besides, the engineering problems associated with short wavelength lithography to achieve smaller components, the performance of increasing number of interconnections has become one of the biggest limiting factors in device performance. [1,2] The power loss, signal degradation, interconnection delays, and other performance limitations related to interconnects should be minimized. The importance of such a task can be seen from the perspective of power dissipation by computation elements. The energy dissipation density in electronic chips approaches that in nuclear reactors. [3,4] Therefore, the decrease of the resistivity of metal interconnects is one of the key challenges in the design of nanoscale electronic circuits. Intense studies on methods of preparation of interconnects by advanced lithographic techniques including e-beam lithography lead to the preparation of Au, Pd, Pt, and Cu nanowires (NWs) with resistivities much greater than the bulk metal value (see Supporting Information). [5‐11] The reason for the drastic increase in resistivity for NWs is that charge carriers experience grain boundaries reflections and surface scattering. The smaller the diameter of the conductor, the greater this effect becomes. Even NWs with diameters much above the nanometer scale can exhibit resistivities as high as two orders of magnitude greater that the bulk. [12]

Starfruit-Shaped Gold Nanorods and Nanowires: Synthesis and SERS Characterization

Recently, branched and star-shaped gold nanoparticles have received significant attention for their unique optical and electronic properties, but most examples of such nanoparticles have a zero-dimensional shape with varying numbers of branches coming from a quasi-spherical core. This report details the first examples of higher-order penta-branched gold particles including rod-, wire-, and platelike particles which contain a uniquely periodic starfruitlike morphology. These nanoparticles are synthesized in the presence of silver ions by a seed-mediated approach based on utilizing highly purified pentahedrally twinned gold nanorods and nanowires as seed particles. The extent of the growth can be varied, leading to shifts in the plasmon resonances of the particles. In addition, the application of the starfruit rods for surface-enhanced Raman spectroscopy (SERS) is demonstrated.

Scaffolding of polymers by supramolecular nanoribbons

metallic structures with a height of 100 lm have been microfabricated and characterized. Photoactive nanocomposites have been formed using nanoparticles with covalently attached photoreducing dye molecules. These nanocomposites are active to both photon and electron-beam stimuli and yield continuous metallic structures. We believe that optical or electron-beam induced growth of nanoparticles provides a versatile new approach to the patterning of metal from the micrometer to the nanometer length scales in 2D and 3D. This approach may have applications in the fabrication of new types of structures, interconnects, and components in electronic, optical, and electromechanical devices.

One-Dimensional Coupling of Gold Nanoparticle Plasmons in Self-Assembled Ring Superstructures

Plasmon coupling in ordered metal nanoparticle assemblies leads to tunable collective surface plasmon resonances that strongly depend on the interparticle distance. Here we report on the surface plasmon scattering of polystyrene-functionalized 40 nm gold nanoparticles self-assembled into close-packed rings. Using single particle dark-field scattering spectroscopy, we observed strong near-field coupling between neighboring nanoparticles, which results in red-shifted multipolar plasmon modes highly polarized along the ring circumference. Correlated optical spectroscopy and scanning electron microscopy of individual rings with different diameters revealed that the plasmon coupling is independent of ring curvature and mostly insensitive to the local nanoparticle arrangement. Our results further suggest that a one-dimensional gold nanoparticle assembly yields long-range collective plasmonic properties similar to those of metallic nanowires.