Background-free four-wave mixing microscopy of small gold nanoparticles inside a multi-cellular organ

Abstract

The ability to detect small metallic nanoparticles by optical microscopy inside environmentally relevant species may have a wide impact for ecotoxicology studies. Here, we demonstrate four-wave mixing microscopy on individual small gold nanoparticles inside the hepatopancreas of Oniscus Asellus, a terrestrial isopod, which ingests metals found in the soil. After the exposure to food containing 10 nm radius gold nanoparticles, hepatopancreas tubules were collected, and nanoparticles were imaged by four-wave mixing microscopy with high contrast, locating them with sub-cellular resolution in the volume, despite the significant light scattering from these multi-cellular organs. Notably, the ultrafast dynamics of the four-wave-mixing non-linearity of gold nanoparticles resonantly excited and probed at their localized surface plasmon allows them to be distinguished from other metal deposits in the hepatopancreas, which manifest as a long-lived photothermal contrast. Our findings bring unexpected insight into the location of gold nanoparticles in relation to the cell types forming the hepatopancreas. Considering its simplicity, volumetric imaging capabilities, specificity, and compatibility with living cell studies, four-wave mixing microscopy holds great potential to investigate the fate of metal nanoparticles inside biological systems.