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

TLDR

Gold nanorods are prized for their strong, tunable plasmonic fields and biocompatibility, building on the ability of noble metal nanoparticles to confine photons and excite surface plasmon oscillations. This review surveys gold nanorods’ radiative and nonradiative properties, outlines seed‑mediated synthesis and assembly strategies, and examines their emerging biological and biomedical applications. Photothermal behavior arises from rapid dephasing of absorbed light and energy transfer to the lattice, while the review details radiative/nonradiative characteristics, seed‑mediated growth, and various assembly methods. Confining photons to nanorod dimensions amplifies electromagnetic fields, boosting absorption, scattering, and sub‑diffraction imaging resolution, and recent studies demonstrate promising biomedical uses.

Abstract

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 nanoparticle's dimensions leads to a large increase in its electromagnetic field and consequently great enhancement of all the nanoparticle's radiative properties, such as absorption and scattering. Moreover, by confining the photon's wavelength to the nanoparticle's 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.