nanorods

Nanorods are nanoscale particles with a rod-like geometry, typically 1–100 nm in diameter and longer than they are wide, yielding aspect ratios from about 2 to well over 10. They can be metallic, oxide, or semiconductor and occur in many compositions, including gold, silver, iron oxide, zinc oxide, TiO2, CdSe/CdS, and other combinations. The elongated shape produces anisotropic properties that differ along the longitudinal axis compared with the transverse axis, affecting optical responses, charge transport, and catalytic activity.

Synthesis methods include seed-mediated growth, template-assisted deposition, hydrothermal or solvothermal routes, and self-assembly. In metal nanorods, surfactants such as CTAB enable controlled growth and aspect ratio tuning; precise control of reaction conditions yields monodisperse populations. Semiconductor nanorods are often grown by colloidal synthesis with ligands that passivate surfaces and influence emission properties. Post-synthesis ligand exchange can adjust solubility and bioconjugation.

Properties include optical anisotropy and plasmonic resonances for metal nanorods, whose longitudinal plasmon peak shifts with aspect ratio. Semiconductor nanorods show size-tunable photoluminescence due to quantum confinement. Electrical and catalytic properties are often anisotropic, with enhanced reactivity along the rod axis in some materials.

Applications span bioimaging, biosensing, photothermal therapy, and drug delivery for biomedical uses; in energy devices such as solar cells and LEDs; and in catalysis and nanoelectronics.

Characterization typically uses transmission electron microscopy and scanning electron microscopy for morphology, UV–vis spectroscopy for plasmonic or band-edge features, photoluminescence for emitters, X-ray diffraction for crystal phase, and dynamic light scattering for size distribution.

Challenges include achieving uniform size and aspect ratio, stable surface ligands, reliable surface functionalization, and, for some compositions, toxicity and environmental impact.