Gold nanoparticles have received much attention due to their potential diagnostic and therapeutic applications. Gold
nanoparticles are attractive in many biomedical applications because of their biocompatibility, easily modifiable
surfaces for targeting, lack of heavy metal toxicity, wide range of sizes (35-100 nm), tunable plasmonic resonance
peak, encapsulated site-specific drug delivery, and strong optical absorption in the near-infrared regime. Specifically,
due to their strong optical absorption, gold nanoparticles have been used as a contrast agent for molecular photoacoustic
(PA) imaging of tumor. The plasmonic resonance peak of the gold nanocages (AuNCs) was tuned to the near-infrared
region, and the ratio of the absorption cross-section to the extinction cross-section was approximately ~70%, as
measured by PA sensing. We used PEGylated gold nanocages (PEG-AuNCs) as a passive targeting contrast agent on
melanomas. After 6-h intravenous injection of PEG-AuNCs, PA amplitude was increased by ~14 %. These results
strongly suggest PA imaging paired with AuNCs is a promising diagnostic tool for early cancer detection.
This study demonstrates a method for measuring the optical absorption cross-sections (σa) of Au-Ag nanocages and Au
nanorods using photoacoustic (PA) sensing. PA signals are 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 vs. 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 cross-sections) 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. This method can potentially
provide the optical absorption and scattering properties of gold nanostructures and other types of nanomaterials.
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