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.

We report on the one-photon photoluminescence of gold nanorods with different aspect ratios. We measured photoluminescence and scattering spectra from 82 gold nanorods using single-particle spectroscopy. We found that the emission and scattering spectra closely resemble each other independent of the nanorod aspect ratio. We assign the photoluminescence to the radiative decay of the longitudinal surface plasmon generated after fast interconversion from excited electron-hole pairs that were initially created by 532 nm excitation. The emission intensity was converted to the quantum yield and was found to approximately exponentially decrease as the energy difference between the excitation and emission wavelength increased for gold nanorods with plasmon resonances between 600 and 800 nm. We compare this plasmon emission to its molecular analogue, fluorescence.

Plasmonic clusters can support Fano resonances, where the line shape characteristics are controlled by cluster geometry. Here we show that clusters with a hemicircular central disk surrounded by a circular ring of closely spaced, coupled nanodisks yield Fano-like and non-Fano-like spectra for orthogonal incident polarization orientations. When this structure is incorporated into an uniquely broadband, liquid crystal device geometry, the entire Fano resonance spectrum can be switched on and off in a voltage-dependent manner. A reversible transition between the Fano-like and non-Fano-like spectra is induced by relatively low (∼6 V) applied voltages, resulting in a complete on/off switching of the transparency window.

Here we present in situ observations of adsorption of bovine serum albumin (BSA) on citrate-stabilized gold nanospheres. We implemented scattering correlation spectroscopy as a tool to quantify changes in the nanoparticle Brownian motion resulting from BSA adsorption onto the nanoparticle surface. Protein binding was observed as an increase in the nanoparticle hydrodynamic radius. Our results indicate the formation of a protein monolayer at similar albumin concentrations as those found in human blood. Additionally, by monitoring the frequency and intensity of individual scattering events caused by single gold nanoparticles passing the observation volume, we found that BSA did not induce colloidal aggregation, a relevant result from the toxicological viewpoint. Moreover, to elucidate the thermodynamics of the gold nanoparticle-BSA association, we measured an adsorption isotherm which was best described by an anti-cooperative binding model. The number of binding sites based on this model was consistent with a BSA monolayer in its native state. In contrast, experiments using poly-ethylene glycol capped gold nanoparticles revealed no evidence for adsorption of BSA.

Using light to exchange information offers large bandwidths and high speeds, but the miniaturization of optical components is limited by diffraction. Converting light into electron waves in metals allows one to overcome this problem. However, metals are lossy at optical frequencies and large-area fabrication of nanometer-sized structures by conventional top-down methods can be cost-prohibitive. We show electromagnetic energy transport with gold nanoparticles that were assembled into close-packed linear chains. The small interparticle distances enabled strong electromagnetic coupling causing the formation of low-loss subradiant plasmons, which facilitated energy propagation over many micrometers. Electrodynamic calculations confirmed the dark nature of the propagating mode and showed that disorder in the nanoparticle arrangement enhances energy transport, demonstrating the viability of using bottom-up nanoparticle assemblies for ultracompact opto-electronic devices.