Gold Nanoparticles and Localized Surface Plasmon Resonance
Gold nanoparticles a few nanometers across exhibit localized surface plasmon resonance, where surface electrons oscillate with incident light. This makes colloidal gold appear red and underlies its uses in medicine, sensing, and catalysis.
Gold nanoparticles (colloidal gold) are gold particles typically a few to a few hundred nanometers in diameter, suspended in fluid. Their defining property is localized surface plasmon resonance (LSPR): the conduction electrons on a particle's surface oscillate collectively in resonance with incident light. Because this resonance absorbs and scatters specific wavelengths, a suspension of small (<100 nm) spherical gold nanoparticles appears wine-red rather than metallic gold, while larger particles and nanorods shift toward blue and purple. Color depends on size, shape, spacing, and the surrounding refractive index — which is exactly what makes them useful sensors. Synthesis routes set the size: the Turkevich method yields ~10-20 nm spheres, Brust-Schiffrin ~5-6 nm, and other methods reach 3-300 nm. The scattering from a single 60 nm particle can be on the order of 100,000 times stronger than the emission of a typical fluorescent dye molecule, and their extinction coefficient is high enough to be seen by eye at low concentration. Applications follow from plasmonics and surface chemistry. In medicine, gold nanorods convert near-infrared light to heat for photothermal therapy of tumors, and nanoparticles serve as drug-delivery carriers and imaging agents. In diagnostics they are the colored label in lateral flow tests and amplify signals in surface-enhanced Raman scattering. Below about 2-5 nm, gold nanoparticles also become catalytically active despite bulk gold's inertness. See Gold Catalysis: How Nanoparticles Revived a "Dead" Metal and Lateral Flow Tests: How Gold Nanoparticles Make a Visible Line.