Three-dimensional tracking of nanoparticles by dual-color position retrieval in a double-core microstructured optical...

Elastic light scattering-based three-dimensional (3D) tracking of objects at the nanoscale level is essential for unlocking the dynamics of individual species or interactions in fields such as biology or surface chemistry. In this work, we introduce the concept of dual-color 3D tracking in a double-core microstructured optical fiber that for the first time allows for full 3D reconstruction of the trajectory of a diffusing nanoparticle in a water-filled fiber-integrated microchannel. The use of two single-mode cores provides two opposite decaying evanescent fields of different wavelengths within the microchannel, bypassing spatial domains of ambiguous correlation between the scattered intensity and position. The novelty of the fiber design is the use of two slightly different single-mode cores, preventing modal crosstalk and thus allowing for longitudinally invariant dual-color illumination across the entire field of view. To demonstrate the capabilities of the scheme, a single gold nanosphere (80 nm) diffusing in the water-filled microchannel was tracked for a large number of images (about 32 000) at a high frame rate (1.389 kHz) over a long time (23 s), with the determined hydrodynamic diameters matching expectations. The presented 3D tracking approach yields unique opportunities to unlock processes at the nanoscale level and is highly relevant for a multitude of fields, particularly within the context of understanding sophisticated interaction of diffusing species with functionalized surfaces within the context of bioanalytics, nanoscale materials science, surface chemistry or life science.