Correlative cellular ptychography with functionalized nanoparticles at the Fe L-edge

TL;DR

This study demonstrates a correlative microscopy approach combining fluorescence, tomography, and ptychography to precisely localize and analyze functionalized nanoparticles within mammalian cells at nanometer resolution.

Contribution

It introduces a novel correlative imaging method using X-ray energies at the Fe L-edge to detect and distinguish nanoparticle oxidation states inside cells.

Findings

Successfully localized nanoparticles within cells in 3D

Achieved ~16.5 nm resolution in ptychographic imaging

Identified two oxidation states of nanoparticles

Abstract

Precise localization of nanoparticles within a cell is crucial to the understanding of cell-particle interactions and has broad applications in nanomedicine. Here, we report a proof-of-principle experiment for imaging individual functionalized nanoparticles within a mammalian cell by correlative microscopy. Using a chemically-fixed, HeLa cell labeled with fluorescent core-shell nanoparticles as a model system, we implemented a graphene-oxide layer as a substrate to significantly reduce background scattering. We identified cellular features of interest by fluorescence microscopy, followed by scanning transmission X-ray tomography to localize the particles in 3D, and ptychographic coherent diffractive imaging of the fine features in the region at high resolution. By tuning the X-ray energy to the Fe L-edge, we demonstrated sensitive detection of nanoparticles composed of a 22 nm magnetic Fe3O4 core encased by a 25-nm-thick fluorescent silica (SiO2) shell. These fluorescent core-shell nanoparticles act as landmarks and offer clarity in a cellular context. Our correlative microscopy results confirmed a subset of particles to be fully internalized, and high-contrast ptychographic images showed two oxidation states of individual nanoparticles with a resolution of ~16.5 nm. The ability to precisely localize individual fluorescent nanoparticles within mammalian cells will expand our understanding of the structure/function relationships for functionalized nanoparticles.