Manipulation of the Geometry and Modulation of the Optical Response of Surfactant-Free Gold Nanostars: A Systematic Bottom-Up Synthesis

Among plasmonic nanoparticles, surfactant-free branched gold nanoparticles have exhibited exceptional properties as a nanoplatform for a wide variety of applications ranging from surface-enhanced Raman scattering sensing and imaging applications to photothermal treatment and photoimmunotherapy for cancer treatments. The effectiveness and reliability of branched gold nanoparticles in biomedical applications strongly rely on the consistency and reproducibility of physical, chemical, optical, and therapeutic properties of nanoparticles, which are mainly governed by their morphological features. Herein, we present an optimized bottom-up synthesis that improves the reproducibility and homogeneity of the gold-branched nanoparticles with desired morphological features and optical properties. We identified that the order of reagent addition is crucial for improved homogeneity of the branched nature of nanoparticles that enable a high batch-to-batch reproducibility and reliability. In addition, a different combination of the synthesis parameters, in particular, additive halides and concentration ratios of reactive Au to Ag and Au to Au seeds, which yield branched nanoparticle of similar localized surface plasmon resonances but with distinguishable changes in the dimensions of the branches, was realized. Overall, our study introduces the design parameters for the purpose-tailored manufacturing of surfactant-free gold nanostars in a reliable manner

Spectroelectrochemistry of Halide Anion Adsorption and Dissolution of Single Gold Nanorods

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

Spectral Response of Plasmonic Gold Nanoparticles to Capacitive Charging: Morphology Effects

We report a study of the shape-dependent spectral response of the gold nanoparticle surface plasmon resonance at various electron densities to provide mechanistic insight into the role of capacitive charging, a topic of some debate. We demonstrate a morphology-dependent spectral response for gold nanoparticles due to capacitive charging using single-particle spectroscopy in an inert electrochemical environment. A decrease in plasmon energy and increase in spectral width for gold nanospheres and nanorods was observed as the electron density was tuned through a potential window of −0.3 to 0.1 V. The combined observations could not be explained by existing theories. A new quantum theory for charging based on the random phase approximation was developed. Additionally, the redox reaction of gold oxide formation was probed using single-particle plasmon voltammetry to reproduce the reduction peak from the bulk cyclic voltammetry. These results deepen our understanding of the relationship between optical and electronic properties in plasmonic nanoparticles and provide insight toward their potential applications in directed electrocatalysis