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Microscopic Views of Gold Nanorods

Amneet Gulati, Hongwei Liao, and Jason H. Hafner*

Very small gold particles lack the element’s familiar yellow luster.  At a size scale of 10’s of nanometers, their optical properties are dominated by an effect called localized surface plasmon resonance (LSPR), which can be thought of as a resonance in the oscillation of the gold particle’s free electrons due to excitation by light.  It has long been known that the LSPR of spherical gold particles peaks in the green.  More recently, researchers have found that the LSPR effect can be tuned through the visible and near infrared by changing the particle size and shape.  Nanometer-scale gold shells, rods, cubes, boxes, stars, and many other shapes have been synthesized and studied intensely over the past decade.  Biological and biomedical applications are being pursued widely since these particles can be made to absorb and scatter in the near infrared, and since the optical properties are sensitive to the local environment. Gold nanorods are of particular interest for these purposes due to their small size. 

Building on previous measurements of the spectral extinction cross section of gold nanorods (Liao, Hafner, Chem. Mater. v17, p4636, 2005), Prof. Hafner’s group has calculated nanorod lengths and diameters from simple spectral extinction measurements on nanorod solutions through the predicted dependence of LSPR on nanorod size and shape (Gulati, Liao, Hafner, JPCB v110, p22323, 2007).  This connection is simple for spherical particles given the exact Mie theory description of their LSPR extinction.  However, for non-spherical particles only approximate analytical descriptions exist.  Gold nanorods are usually described by Rayleigh-Gans (R-G) theory, which is a modification of Mie theory for prolate spheroids.   While R-G theory provides the correct spectral form, it does not accurately predict resonant wavelengths or amplitudes.  Hafner’s group rescaled R-G theory based on measured nanorod cross sections to provide quantitative theoretical predictions. 

This new capability has been put to use to study nanorod synthesis, a highly complex growth reaction that is poorly understood.  The LSPR extinction spectra were recorded throughout a nanorod growth reaction and converted to length and diameter.  This yields growth rates and therefore microscopic chemical reaction rates, which will enable a traditional chemical kinetic analysis of gold nanorod synthesis. 

J. Phys. Chem. B, 110 (45), 22323 -22327, 2006      Click here for full paper

 GoldNanoLsm

GoldNanoRsm 

An electron micrograph of gold nanorods (false color, scale bar = 20 nm) and optical extinction spectra taken during their synthesis.