Svetlana Neretina

Gold Nanorods: From Synthesis and Properties to Biological and Biomedical Applications

Noble metal nanoparticles are capable of confining resonant photons in such a manner as to induce coherent surface plasmon oscillation of their conduction band electrons, a phenomenon leading to two important properties. Firstly, the confinement of the photon to the nanoparticles dimensions leads to a large increase in its electromagnetic field and consequently great enhancement of all the nanoparticles radiative properties, such as absorption and scattering. Moreover, by confining the photons wavelength to the nanoparticles small dimensions, there exists enhanced imaging resolving powers, which extend well below the diffraction limit, a property of considerable importance in potential device applications. Secondly, the strongly absorbed light by the nanoparticles is followed by a rapid dephasing of the coherent electron motion in tandem with an equally rapid energy transfer to the lattice, a process integral to the technologically relevant photothermal properties of plasmonic nanoparticles. Of all the possible nanoparticle shapes, gold nanorods are especially intriguing as they offer strong plasmonic fields while exhibiting excellent tunability and biocompatibility. We begin this review of gold nanorods by summarizing their radiative and nonradiative properties. Their various synthetic methods are then outlined with an emphasis on the seed-mediated chemical growth. In particular, we describe nanorod spontaneous self-assembly, chemically driven assembly, and polymer-based alignment. The final section details current studies aimed at applications in the biological and biomedical fields.

Altering the dewetting characteristics of ultrathin gold and silver films using a sacrificial antimony layer.

Solid state dewetting of ultrathin films is the most straightforward means of fabricating substrate-supported noble metal nanostructures. This assembly process is, however, quite inflexible, yielding either densely packed smaller structures or widely spaced larger structures. Here, we demonstrate the utility of introducing a sacrificial antimony layer between the substrate and noble metal overlayer. We observe an agglomeration process which is radically altered by the concurrent sublimation of antimony. In stark contrast with conventional dewetting, where the thickness of the deposited metal film determines the characteristic length scales of the assembly process, it is the thickness of the sacrificial antimony layer which dictates both the nanoparticle size and interparticle spacing. The result is a far more flexible self-assembly process where the nanoparticle size and areal density can be varied widely. Demonstrations show nanoparticle areal densities which are varied over four orders of magnitude assembled from the identical gold layer thickness, where the accompanying changes to nanostructure size see a systematic shift in the wavelength of the localized surface plasmon resonance. As a pliable self-assembly process, it offers the opportunity to tailor the properties of an ensemble of nanostructures to meet the needs of specific applications.

Plasmon Field Effects on the Nonradiative Relaxation of Hot Electrons in an Electronically Quantized System: CdTe-Au Core-Shell Nanowires

The intense electromagnetic fields of plasmonic nanoparticles, resulting from the excitation of their localized surface plasmon oscillations, are known to enhance radiative processes. Their effect on the nonradiative electronic processes, however, is not as well-documented. Here, we report on the enhancement of the nonradiative electronic relaxation rates in CdTe nanowires upon the addition of a thin gold nanoshell, especially at excitation energies overlapping with those of the surface plasmon oscillations. Some possible mechanisms by which localized surface plasmon fields can enhance nonradiative relaxation processes of any quantized electronic excitations are proposed.

Kinetically Controlled Nucleation of Silver on Surfactant-Free Gold Seeds

We report on the heterogeneous nucleation of Ag on Au seeds using a surfactant-free synthesis where nanoparticle aggregation is nullified through the immobilization of bare Au seeds on the surface of a substrate. Requiring only silver nitrate, ascorbic acid, and Au seeds, the synthesis is facile and, from a mechanistic standpoint, far less convoluted than conventional protocols. The results reveal that, even in the absence of surfactants, highly anisotropic growth modes are achieved which result in a lone Ag structure emanating from a single (100) Au facet. Consistent with surfactant-based protocols is the ability to vary the product of the reaction by varying the reaction rate. It allows for kinetic control which is able to direct the reaction toward either a bimetallic heterodimer or a core-shell configuration. The observed growth modes cannot be explained in terms of those proposed for surfactant-based growth modes where surfactants, surface diffusion, and/or collision patterns are used to rationalize the reaction product. We, instead, propose a growth mode reliant on the formation of a space charge region around each seed consisting of a double layer of ions, where the integrity of the layer is dependent upon the facets expressed by the seed, the rate at which the reduced ions are being deposited, and the pH of the solution. Our work reveals the rich nature of surfactant-free heteroepitaxial growth modes as well as the utility of the substrate-based platform in defining growth pathways.

Mechanistic study of substrate-based galvanic replacement reactions

AbstractThe sacrificial templates used in galvanic replacement reactions dictate the properties of the hollow metal nanostructures formed. Here, we demonstrate that substrate-based Au-Ag nanoshells with radically altered properties are obtained by merely coating silver templates with an ultrathin layer of gold prior to their insertion into the reaction vessel. The so-formed nanoshells exhibit much smoother surfaces, a higher degree of crystallinity and are far more robust. Dealloying the nanoshells results in the first demonstration of substrate-based nanocages. Such cages exhibit a well-defined pattern of geometric openings in directions corresponding to the {111}-facets of the starting template material. The ability to engineer the cage geometry through adjustments to the orientational relationship between the crystal structure of the starting template and that of underlying substrate is demonstrated. Together these discoveries provide the framework to advance our understanding of the mechanisms governing substratebased galvanic replacement reactions.

Substrate-based galvanic replacement reactions carried out on heteroepitaxially formed silver templates

AbstractGalvanic replacement reactions have been widely used to transform solution dispersed silver template structures into intricate nanoshell geometries. Here, we report on the use of these same reactions to form hollow substrate-supported Au-Ag nanoshells from silver templates having a heteroepitaxial relationship with the underlying single crystal substrate. The structures obtained exhibit a nanohut geometry, show highly tunable plasmonic properties and are formed as periodic arrays using a lithography-free technique. When removed from the substrate the inverted nanohuts appear as nanobowls with a notch in the rim. The study lays the groundwork for wafer-based devices utilizing nanoshells located at site-specific locations.

Manipulating the size distribution of supported gold nanostructures

Gold nanostructures, with a wide size distribution, are confined between a metal foil and the oxide substrate upon which they were formed. When heated the surface energy gradient between the oxide and foil results in a net migration of gold atoms from the nanostructure to the foil. With time, the nanostructures show a size reduction and a narrowed size distribution. The narrowing results from the formation of foil contact points with only the largest nanostructures, a characteristic which leaves small nanostructures intact while consuming larger ones. Also demonstrated is the size reduction of arrayed gold structures to nanoscale dimensions.

The Dependence of the Plasmon Field Induced Nonradiative Electronic Relaxation Mechanisms on the Gold Shell Thickness in Vertically Aligned CdTe−Au Core−Shell Nanorods

The dependence of the plasmon field enhancement of the nonradiative relaxation rate of the band gap electrons in vertically aligned CdTe-Au core-shell nanorods on the plasmonic gold nanoshell thickness is examined. Increasing the thickness of the gold nanoshell from 15 to 26 nm is found to change the decay curve from being nonexponential and anisotropic to one that is fully exponential and isotropic (i.e., independent of the nanorod orientation with respect to the exciting light polarization direction). Analysis of the kinetics of the possible electronic relaxation enhancement mechanisms is carried out, and DDA simulated properties of the induced plasmonic field of the thin and thick gold nanoshells are determined. On the basis of the conclusions of these treatments and the experimental results, it is concluded that by increasing the nanoshell thickness the relaxation processes evolve from multiple enhancement mechanisms, dominated by highly anisotropic Auger processes, to mechanism(s) involving first-order excited electron ejection process(es). The former is shown to give rise to nonexponential anisotropic decays in the dipolar plasmon field of the thin nanoshell, while the latter exhibits an exponential isotropic decay in the unpolarized plasmonic field of the thick nanoshell.

A Wulff in a Cage: The Confinement of Substrate-Based Structures in Plasmonic Nanoshells, Nanocages, and Nanoframes Using Galvanic Replacement

Galvanic replacement reactions carried out on solid core-shell structures typically yield a noble metal nanorattle geometry in which a mobile core is contained within a hollowed shell. Here, we adapt this colloidal synthesis to substrate-based structures to obtain a fundamentally altered product in which an immobilized core is separated from the shell by a well-defined gap, an architecture unobtainable using colloidal techniques and that offers unique advantages in terms of generating plasmonic near-field effects within the confines of a single structure. In the devised route, Wulff-shaped templates of Au, Pt, or Pd, formed through the dewetting of ultrathin films, are first transformed into core-shell structures through the reduction of Ag(+) ions onto their surface and then further transformed through the galvanic replacement of Ag with Au. Through suitable adjustments to the shell geometry, the epitaxial relationship with the substrate, and the extent to which the shell is replaced, it is possible to generate an entire family of nanostructures in which a Wulff-shaped core is confined within a nanoshell, nanocage, or nanoframe, where, in all cases, bonds formed between the structure and the substrate preclude motion. With the potential to tune the gap width, the geometry of the confining structure, and the composition of the core, shell, and substrate, these structures could find application as catalytic nanoreactors able to drive both single-step and cascade reactions or as plasmon-based sensing elements for biological and chemical detection.

Eutectic Combinations as a Pathway to the Formation of Substrate‐Based Au‐Ge Heterodimers and Hollowed Au Nanocrescents with Tunable Optical Properties

Pairs of immiscible elements with deep eutectics are used to synthesize periodic arrays of heterodimers and hollowed metal nanocrescents. In the devised route, substrate-immobilized Au or Ag nanostructures act as heterogeneous nucleation sites for Ge adatoms. At elevated temperatures the adatoms collect in sufficient quantities to transform each site into a AuGe liquid alloy which, upon cooling, phase separates into elemental components sharing a common interface. The so-formed Au-Ge and Ag-Ge heterodimers exhibit a complex morphology characterized by a noble metal nanocrescent which partially encapsulates one end of the Ge domain. Through the use of a selective etch the Ge component is removed, leaving behind a periodic array of hollow noble metal nanocrescents on the surface of the substrate. Optical characterization of both the heterodimers and nanocrescents indicates that the presence of Ge gives rise to a relative blue-shift in the localized surface plasmon peak, a result that is in stark contrast to the red-shifts typically observed when plasmonic nanostructures are in contact with a dielectric medium. Simulations are used to both rationalize the observed shift and show the potential for deriving unexpected behaviors when semishell-like noble metal structures are in contact with high permittivity dielectric mediums.