Measuring the Optical Absorption Cross Sections of Au−Ag Nanocages and Au Nanorods by Photoacoustic Imaging

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Abstract
This paper presents a method for measuring the optical absorption cross sections (σa) of Au−Ag nanocages and Au nanorods. The method is based on photoacoustic (PA) imaging, where the detected signal is directly proportional to the absorption coefficient (μa) of the nanostructure. For each type of nanostructure, we first obtained μa from the PA signal by benchmarking against a linear calibration curve (PA signal versus μa) derived from a set of methylene blue solutions with different concentrations. We then calculated σa by dividing the μa by the corresponding concentration of the Au nanostructure. Additionally, we obtained the extinction cross section (σe, sum of absorption and scattering) from the extinction spectrum recorded using a conventional UV−vis−NIR spectrometer. From the measurements of σa and σe, we were able to easily derive both the absorption and scattering cross sections for each type of gold nanostructure. The ratios of absorption to extinction obtained from experimental and theoretical approaches agreed well, demonstrating the potential use of this method in determining the optical absorption and scattering properties of gold nanostructures and other types of nanomaterials.