Interstitial diffuse radiance spectroscopy of gold nanocages and nanorods in bulk muscle tissues

Abstract

Radiance spectroscopy was applied to the interstitial detection of localized inclusions containing Au nanocages or nanorods with various concentrations embedded in porcine muscle phantoms. The radiance was quantified using a perturbation approach, which enabled the separation of contributions from the porcine phantom and the localized inclusion, with the inclusion serving as a perturbation probe of photon distributions in the turbid medium. Positioning the inclusion at various places in the phantom allowed for tracking of photons that originated from a light source, passed through the inclusion's location, and reached a detector. The inclusions with high extinction coefficients were able to absorb nearly all photons in the range of 650-900 nm, leading to a spectrally flat radiance signal. This signal could be converted to the relative density of photons incident on the inclusion. Finally, the experimentally measured quantities were expressed via the relative perturbation and arranged into the classical Beer-Lambert law that allowed one to extract the extinction coefficients of various types of Au nanoparticles in both the transmission and back reflection geometries. It was shown that the spatial variation of perturbation could be described as 1/r dependence, where r is the distance between the inclusion and the detector. Due to a larger absorption cross section, Au nanocages produced greater perturbations than Au nanorods of equal particle concentration, indicating a better suitability of Au nanocages as contrast agents for optical measurements in turbid media. Individual measurements from different inclusions were combined into detectability maps.

Keywords: Beer–Lambert law; diffuse radiance spectroscopy; gold nanocages; gold nanorods; perturbation; porcine muscles; turbid media.

Figure 1

A schematic of the experimental setup for diffuse radiance spectroscopy. Notes: Left panel: side view of a schematic of the experimental set-up for diffuse radiance spectroscopy. Right panel: top view of the porcine phantom with representative positions marked for the light source, the detector and the inclusion of Au NPs (gold nanoparticles).

Figure 2

Extinction coefficients of various types of NPs extracted from the conventional spectroscopy measurements using the Beer–Lambert law. Abbreviations: NP, nanoparticles; NC, nanocages; NR, nanorods.

Figure 3

Representative spectro-angular maps for inclusions in the porcine phantom: (A) NC-12.5 at 0° and 5 mm distance from the detector, (B) NC-100 at −45° and 3 mm distance from the detector. Abbreviation: NC, nanocage.

Figure 4

A sequence of data analysis steps in radiance measurements for an inclusion made from various NPs positioned at −45° and 3 mm distance from the detector: (A) radiance extinction ratio, RER, (B) relative perturbation, (C) relative transmitted radiance with (relative) incident radiance, (D) extracted extinction coefficient. Abbreviations: AU, arbitrary units; NC, nanocage; NR, nanorod; RER, radiation extinction ratio; NP, nanoparticle.

Figure 5

The extracted extinction coefficient from diffuse radiance measurements for the inclusion positioned at (A) 0° and 5 mm distance from the detector, (B) −90° and 3 mm distance from the detector, (C) −135° and 3 mm distance from the detector. Abbreviations: NC, nanocage; NR, nanorod.

Figure 6

Dependence of the relative perturbation (∆I/I₀) at plasmon resonance on concentration of Au NC in the inclusion’s volume for selected detector–inclusion separations (r) for the inclusion positioned at (A) −45° and (B) −90° angle. Abbreviations: NP, nanoparticle; NC, nanocage.

Figure 7

Combined perturbation or detectability maps for (A) NC-100, (B) NC-12.5, (C) NR-10 in porcine muscle phantoms (at the corresponding plasmon resonance). Abbreviations: NC, nanocage; NR, nanorod.

Figure 8

Spectro-angular map of the phantom containing five inclusions of Au NRs (gold nanorods) positioned at 0°, 45°, −90°, 135°, and −160° (all 5 mm away from the detector as seen in the inset).