In vivo tumor ultrasound-switchable fluorescence imaging via intravenous injections of size-controlled thermosensitive nanoparticles

Near-infrared fluorescence imaging has emerged as a noninvasive, inexpensive, and ionizing-radiation-free monitoring tool for assessing tumor growth and treatment efficacy. In particular, ultrasound switchable fluorescence (USF) imaging has been explored with improved imaging sensitivity and spatial resolution in centimeter-deep tissues. This study achieved size control of polymer-based and indocyanine green (ICG) encapsulated USF contrast agents, capable of accumulating at the tumor after intravenous injections. These nanoprobes varied in size from 58 nm to 321 nm. The bioimaging profiles demonstrated that the proposed nanoparticles can efficiently eliminate the background light from normal tissue and show a tumor-specific fluorescence enhancement in the BxPC-3 tumor-bearing mice models possibly via the enhanced permeability and retention effect. In vivo tumor USF imaging further proved that these nanoprobes can effectively be switched 'ON' with enhanced fluorescence in response to a focused ultrasound stimulation in the tumor microenvironment, contributing to the high-resolution USF images. Therefore, our findings suggest that ICG-encapsulated nanoparticles are good candidates for USF imaging of tumors in living animals, indicating their great potential in optical tumor imaging in deep tissue.