Biodegradable germanium nanoparticles as contrast agents for near-infrared-II photoacoustic imaging.

Photoacoustic (PA) imaging using contrast agents with strong near-infrared-II (NIR-II, 1000-1700 nm) absorption enables deep penetration into biological tissue. Besides, biocompatibility and biodegradability are essential for clinical translation. Herein, we developed biocompatible and biodegradable germanium nanoparticles (GeNPs) with high photothermal stability as well as strong and broad absorption for NIR-II PA imaging. We first demonstrate the excellent biocompatibility of the GeNPs through experiments, including the zebrafish embryo survival rates, nude mouse body weight curves, and histological images of the major organs. Then, comprehensive PA imaging demonstrations are presented to showcase the versatile imaging capabilities and excellent biodegradability, including PA imaging which can bypass blood absorption, dual-wavelength PA imaging which can clearly distinguish the injected GeNPs from the background blood vessels, and PA imaging with deep penetration, time-lapse PA imaging of a mouse ear for observing biodegradation, time-lapse PA imaging of the major organs of a mouse model for observing the biodistribution after intravenous injection, and notably dual-modality fluorescence and PA imaging of osteosarcoma tumors. The biodegradation of GeNPs is observed not only in the normal tissue but also in the tumor, making the GeNPs a promising candidate for clinical NIR-II PA imaging applications.