A primary challenge in cellular nanotechnology is the assessment of how nanoparticles interact with living cells. Key to this is the imaging of nanoparticles within cells, which potentially allows for elucidation of particle uptake, processing and potential exocytosis pathways. Unfortunately, imaging nanoparticles is extremely difficult without resorting to non-quantitative techniques such as transmission electron microscopy, or techniques that may themselves alter nanoparticle uptake, such as surface modification to attach fluorescent tags. Here, we present an x-ray microfluorescence-based technique that allows extra- and intra-cellular imaging and quantitation of cobalt lattice-doped ZnO in human immune cells.
This study used two x-ray fluorescence detectors at the Australian Synchrotron: a low resolution scanning detector (resolution ~3µm) and a high resolution mapping detector (resolution ~150nm). At low resolution, we elementally mapped a field of approximately 300 fixed cells over several hours. Whilst endogenous zinc levels are generally too high to provide certainty in locating ZnO nanoparticles within cells, we were able to accurately locate nanoparticle-exposed cells using cobalt (which has very low abundance in cells) that was lattice-doped into the nanoparticles. We were then able to detect and quantify cells containing co-localised zinc and cobalt across the field of cells. We found two distinct populations, which contained either a low or a very high quantity of nanoparticles. Selecting a cell from the highly-exposed population, we imaged the cell at high resolution and mapped clusters of nanoparticles associated with the cell membrane. To investigate the intracellular location of these agglomerates, we then performed progressive ion beam ablation cycles, followed by further elemental mapping, which showed that the particle agglomerates were present within the cell body. This dual-label technique also provided information concerning cellular processing and fate, as these intracellular agglomerates were enriched with cobalt – strongly suggesting intracellular dissolution of the nanoparticles.