A hyperspectral imaging method was developed that allowed the identification of heterogeneous plasmon response from 50 nm diameter gold colloidal particles on a conducting substrate in a transparent three-electrode spectroelectrochemical cell under non-Faradaic conditions. At cathodic potentials, we identified three distinct behaviors from different nanoparticles within the same sample: irreversible chemical reactions, reversible chemical reactions, and reversible charge density tuning. The irreversible reactions in particular would be difficult to discern in alternate methodologies. Additional heterogeneity was observed when single nanoparticles demonstrating reversible charge density tuning in the cathodic regime were measured dynamically in anodic potential ranges. Some nanoparticles that showed charge density tuning in the cathodic range also showed signs of an additional chemical tuning mechanism in the anodic range. The expected changes in nanoparticle free-electron density were modeled using a charge density-modified Drude dielectric function and Mie theory, a commonly used model in colloidal spectroelectrochemistry. Inconsistencies between experimental results and predictions of this common physical model were identified and highlighted. The broad range of responses on even a simple sample highlights the rich experimental and theoretical playgrounds that hyperspectral single-particle electrochemistry opens.

Redox electrochemistry was used to reversibly tune the optical properties of plasmonic core-shell nanoparticles and dimers.

Photoluminescent Au nanoparticles are appealing for biosensing and bioimaging applications because of their non-photobleaching and non-photoblinking emission. The mechanism of one-photon photoluminescence from plasmonic nanostructures is still heavily debated though. Here, we report on the one-photon photoluminescence of strongly coupled 50 nm Au nanosphere dimers, the simplest plasmonic molecule.We observe emission from coupled plasmonic modes as revealed by single-particle photoluminescence spectra in comparison to correlated dark-field scattering spectroscopy.The photoluminescence quantum yield of the dimers is found to be surprisingly similar to the constituent monomers, suggesting that the increased local electric field of the dimer plays a minor role, in contradiction to several proposed mechanisms. Aided by electromagnetic simulations of scattering and absorption spectra, we conclude that our data are instead consistent with a multistep mechanism that involves the emission due to radiative decay of surface plasmons generated from excited electronÀhole pairs following interband absorption.

Nanoparticle and thin film surface plasmons are highly sensitive to electrochemically induced dielectric changes. We exploited this sensitivity to detect reversible electrochemical potential-driven anion adsorption by developing single-particle plasmon voltammetry (spPV) using plasmonic nanoparticles. spPV was used to detect sulfate electroadsorption to individual Au nanoparticles. By comparing both semiconducting and metallic thin film substrates with Au nanoparticle monomers and dimers, we demonstrated that using Au film substrates improved the signal in detecting sulfate electroadsorption and desorption through adsorbate modulated thin film conductance. Using single-particle surface plasmon spectroscopic techniques, we constructed spPV to sense sulfate, acetate, and perchlorate adsorption on coupled Au nanoparticles. spPV extends dynamic spectroelectrochemical sensing to the single-nanoparticle level using both individual plasmon resonance modes and total scattering intensity fluctuations.

A spectroelectrochemical flow cell is used to probe the localized surface plasmon resonance (LSPR) of the same single gold nanorods (AuNRs) in sodium fluoride, sodium chloride, and sodium bromide electrolytes using dark-field scattering microscopy. The changes in resonance energy, line width (full-width at half-maximum, fwhm), and peak intensity of a Lorentzian fit to single AuNR scattering spectra as the rods are charged are compared to determine the role of anion adsorption. We demonstrate that at positive potentials up to +0.25 V relative to a Pt quasi-reference electrode, the induced changes in the LSPR are independent of halide anion. At more positive potentials (+0.3 to +0.35 V) bromide and chloride ions damp the AuNR LSPR, observed as an increase in the line width. At the most positive potential investigated in all three electrolyte solutions (+0.35 V), the AuNR scattering intensity decreases irreversibly in bromide electrolyte, indicating dissolution. The kinetics of the bromide-mediated dissolution can be controlled by the electrolyte concentration and show that the change in resonance energy due to dissolution increases with each cycle from negative to positive potential.