Light-guiding capillaries: a robust optofluidic platform for nanoparticle tracking analysis.

Understanding nanoparticle interactions at the nanoscale is essential for numerous scientific fields and applications, with waveguide-based approaches offering a promising solution. In this work, we present a fiber-integrated platform for nanoparticle tracking analysis based on a fiber-type capillary structure (hereinafter referred to as "capillary") that achieves high performance comparable to more complex microstructured fibers. By combining measurements with optical simulations, we validate light propagation and loss behavior in water-filled capillaries, showing excellent agreement with theoretical predictions. The capillary-based approach provides structural simplicity, robustness, and compatibility with fiber systems, while enabling diffraction-limited imaging and accurate determination of hydrodynamic diameters for mono- and polydisperse nanoparticle ensembles. The demonstrated platform may serve as a future foundation for the characterization of nanoscale species in confined geometries and could be relevant for applications in nanoscience, bioanalytics, and environmental analysis.