Sub-20 nm Hard X-ray Spectromicroscopy Reveals Nanoscale Redox Heterogeneity in Ultrafine Particulates

Spatially resolving chemical state variations within complex hierarchical materials is essential for understanding their reactivity. Yet, achieving sub-50 nm resolution for hard X-ray spectromicroscopy remains a long-standing challenge. Here, we demonstrate high-sensitivity, sub-20 nm X-ray absorption near-edge structure spectromicroscopy or nano-XANES enabled by high resolution imaging using multilayer Laue lenses. This approach achieved sub-20 nm chemical speciation mapping and a chemical state detection threshold of <18,000 atoms, a regime where current techniques are limited by insufficient sensitivity. We showcase the power of this methodology through the chemical speciation of ultrafine arsenopyrite particles, which are widespread environmental contaminants. Our results reveal previously inaccessible insights into the oxidation pathways of submicrometer particles, including the discovery of a labile As(III) corona that creates an inversion of the expected oxidation gradient. These findings establish nano-XANES as a high-fidelity probe for mapping localized redox gradients, surface phase transformations, and heterogeneous charge transfer at (buried) interfaces and nanoscale boundaries.