Abstract
A strong intrinsic signal is advantageous over labeling for optical detection of nanoparticles. Intense scattering and absorption by the surface plasmon resonance, which exceeds molecular cross sections, provides a direct method for visualizing noble metal nanoparticles. While two-photon luminescence in gold nanoparticles yields a strong signal, one-photon luminescence is generally regarded to be much weaker and has seldom been employed for optical nanoparticle detection. In this article we investigated one-photon luminescence of gold nanospheres and nanorods using single particle spectroscopy with excitation at 514 and 633 nm. We characterized the polarization dependence, determined the quantum yield, and present a mechanism describing one-photon luminescence. Our results suggest fast interconversion between surface plasmons and hot electron-hole pairs and show that the luminescence occurs via emission by a surface plasmon. Using the information obtained from the single particle studies, we were able to successfully employ one-photon luminescence for correlation Spectroscopy measurements and to correctly interpret auto- and cross-correlation functions, which were used to determine the hydrodynamic sizes of several gold nanoparticle samples and to extract rotational dynamics of nanorods. Because of the difference in size dependence for one-photon luminescence compared to scattering, luminescence correlation spectroscopy of metal nanoparticles is advantageous as it is not as strongly affected by the presence of larger nanoparticles or aggregates. This was verified by measuring luminescence as well as scattering correlation traces for a mixture of nanoparticles containing 98% 57 nm and 2% 96 nm gold nanospheres.