Depth and extent of gold nanorod photothermal conversion in tissue-mimicking phantoms

Abstract

Gold nanorods (AuNRs) efficiently absorb pulsed near-infrared (NIR) light. If the fluence is sufficiently high, then the absorption of pulsed light results in photothermal conversion to spherical morphology and a decrease in NIR absorption. In aqueous media, photothermal conversion also produces localized microbubble formation, which has the potential to kill nearby cells. Our objective was to study the potential of AuNRs to elicit cell killing effects at depth within a tissue-like phantom material. The approach was to measure photothermal conversion in phantoms with embedded inclusions representative of breast tumors. Phantoms were prepared with a homogeneous mixture of 1% Intralipid™ and 1% agarose to simulate tissue optical properties. Gold nanorod-loaded spherical inclusions 6 mm in diameter were prepared at an optical density of 0.67 at 800 nm. Inclusions were cast into the phantom material at a depth of 0, 5, 10, or 15 mm. Phantoms were then exposed to pulsed laser (800 nm, 5 ns pulse duration) for a range of fluence (17-100 mJ/cm²) and pulse count (10-1000). Each phantom was then cut longitudinally and imaged with a NIR camera. The images were analyzed for changes in contrast representative of photothermal conversion. Preliminary results indicated that photothermal conversion occurred only in spherical AuNR-loaded inclusions at or within 10 mm of the phantom surface. Based on these results, we concluded that within ANSI limits of laser exposure photothermal therapy with AuNR-based agents will be limited to lesions at or near the surface and lesions accessible with needlebased light delivery.