Photoacoustic Vaporization of Endoskeletal Droplets Loaded with Zinc Naphthalocyanine

Vaporizable endoskeletal droplets are solid hydrocarbons in liquid fluorocarbon droplets in which melting of the hydrocarbon phase leads to the vaporization of the fluorocarbon phase. In prior work, vaporization of the endoskeletal droplets was achieved thermally by heating the surrounding aqueous medium. In this work, we introduce a near-infrared (NIR) optically absorbing naphthalocyanine dye (zinc 2,11,20,29-tetra-tert-butyl-2,3-naphthalocynanine) into the solid hydrocarbon (eicosane, n-C₂₀H₄₂) core of liquid fluorocarbon (C₅F₁₂) drops suspended in an aqueous medium. Droplets with a uniform diameter of 11.7 ± 0.7 μm were formed using a flow-focusing microfluidic device. The solid hydrocarbon formed a crumpled spherical structure within the liquid fluorocarbon droplet. The photoactivation behavior of these dye-containing endoskeletal droplets was investigated using NIR laser irradiation. When exposed to a pulsed laser of 720 nm wavelength, the dye-containing droplets vaporized at an average laser fluence of 65 mJ/cm², whereas blank droplets without the dye did not vaporize at any fluence up to 100 mJ/cm². Furthermore, dye-loaded droplets with a smaller, polydisperse size distribution were prepared using a simple shaking method and studied in a flow phantom for their photoacoustic signal and ultrasound contrast imaging. These results demonstrate that dye-containing endoskeletal droplets can be made to vaporize by externally applied optical energy. Such droplets may be useful for a variety of photoacoustic applications for sensing, imaging, and therapy.