The extremely large optical extinction coefficient of gold nanorods (Au-NRs) enables their use in a diverse array of technologies, rnging from plasmonic imaging, therapeutics and sensors, to large area coatings, filters, and optical attenuators. Development of the latter technologies has been hindered by the lack of cost-effective, large volume production. This is due in part to the low reactant concentration required for symmetry breaking in conventional seed-mediated synthesis. Direct scale up of laboratory procedures has limited viability because of excessive solvent volume, exhaustive postsynthesis purification processes, and the generation of large amounts of waste (e.g., hexadecyltrimethylammonium bromide(CTAB)). Following recent insights into the growth mechanism of Au-NRs and the role of seed development, we modify the classic seed-mediated synthesis via temporal control of seed and reactant concentration to demonstrate production of Au-NRs at more than 100-times the conventional concentration, while maintaining independent control and narrow distribution of nanoparticle dimensions, aspect ratio, and volume. Thus, gram scale synthesis of Au-NRs with prescribed aspect ratio and volume is feasible in a 100 mL reactor with 1/100th of organic waste relative to conventional approaches. Such scale-up techniques are crucial to cost-effectively meet the increased demand for large quantities of Au-NRs in emerging applications.
The thermal reshaping of gold nanorods in a polymer matrix is an important phenomenon for many potential applications. However, a fundamental understanding of the various mechanisms that govern the nanorod reshaping dynamics is still lacking. Here, we provide evidence for a phenomenological model of the gold nanorod shape transformation based on the measurements and detailed analysis of the time-resolved thermal reshaping for a variety of gold nanorods having different geometries (aspect ratio, volume, diameter) in a cross-linked epoxy matrix at application relevant temperatures (120−220 °C). Our analysis suggests that (a) the nanorod reshaping dynamics consist of two temporal regimes that are governed by different phenomena and (b) the ultimate amount of reshaping at a given temperature depends strongly on the initial particle geometry and the mechanical stiffness of its surroundings. At short times, the shape transformation is dominated by a curvature-induced surface diffusion process, in which the activation energy for diffusion depends on curvature. At long times, however, the surrounding environment plays a key role in slowing the diffusion and stabilizing the nanorod shape. We show that the long-time behavior can be well described using a modified surface diffusion model that takes into account the slowing of atomic diffusivity as a result of external forces arising from mechanical constraints. The ability to tune both the final shape and the reshaping dynamics in nanocomposites opens up new possibilities in tailoring the optical properties of these materials.
Polymer-grafted nanoparticles (PGNs) are widely used as additives or as single-component assemblies in numerous technologies, spanning from composites and coatings to membranes, optical elements, and printable electronics. How the design modularity of PGNs relates to their dispersibility and morphology in thermal or poor solvents, however, is not well established. Herein, we provide experimental volume fraction (ϕ)−temperature (T) coexistence curves (ϕ CE and T CE ) for polystyrene-grafted gold nanoparticles (PS-AuNPs) in solvents of various qualities (cyclohexane, cyclopentane, and ethyl acetate) using a combination of UV−vis spectroscopy and small-angle X-ray scattering. These systems exhibit upper critical solution temperatures, with coexistence curves that are broader and shifted to lower temperatures relative to their polymer analogs, consistent with prior reports of the impact of macromolecular branching. The coexistence curves span over 6 orders of magnitude in concentration, exhibit solvent-rich and solvent-poor arms that are ϕ PGN -dependent, display a reduction in critical temperature (T C *) as graft molecular weight decreases, exhibit an overall narrowing of the solvent-poor arm at higher graft molecular weights, and reveal a minimal influence of core nanoparticle volume (900−4200 nm 3 ). Additionally, the structure within the solvent swollen PS-AuNPs aggregates is disordered or face-centered cubic and depends on the aggregation temperature relative to T C * than on solvent characteristics. Such quantification of PGN phase behavior in thermal or poor solvents is used to demonstrate thermal fraction for purification and will inform future synthesis methods, self and directed assembly techniques, film and fiber processing, and formulation of inks, adhesives, thermosets, and coatings.
Coated gold nanorods (GNRs) are attractive as chemical sensors because their plasmon resonance energy is strongly dependent on the value of the dielectric constant in the local environment. For thin coatings (<≈20 nm), the plasmon resonance is sensitive to both the coating and the surrounding medium, while for thicker coatings the plasmons are effectively screened from their surroundings. We use finite element modeling to develop a semi-empirical effective medium approximation for the dielectric constant surrounding GNRs 30-50 nm in length with coating thicknesses of 0.5-200 nm. We demonstrate that this approximation can be used to correctly interpret shifts in plasmon resonance energy when the dielectric constant of the surroundings changes with temperature. We compare plasmon resonances of gold nanorods embedded in an epoxy matrix when coated with polyethylene glycol or silica of various thicknesses during thermal cycling. The derived expression for the effective medium dielectric of a coated rod will help device engineers optimize the sensitivity and robustness of coated GNR plasmonic sensors.
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