<p>The integration of functional components into flexible photonic environments is a critical area of research in integrated photonics and is essential for high-precision sensing. This work presents a novel concept of interfacing square-core hollow-core waveguides with commercially available optical fibers using 3D nanoprinting, and demonstrates its practical relevance through a nanoscience-based characterization technique. In detail, this innovative concept results in a monolithic, fully fiber-integrated device with key advantages such as alignment-free operation, high-purity fundamental mode excitation, full polarization control, and a unique handling flexibility. For the first time, the application potential of a fiber-interfaced waveguide in nanoscale analysis is demonstrated by performing nanoparticle-tracking-analysis experiments. These experiments involve the tracking and analysis of individual gold nanospheres diffusing in the hollow core waveguide, enabled by nearly aberration-free imaging, extended observation times, and homogeneous light-line illumination. The study comprehensively covers design strategy, experimental implementation, key principles, optical characterization, and practical applications. The fiber-interfaced hollow-core waveguide concept offers significant potential for applications in bioanalytics, environmental sciences, quantum technologies, optical manipulation, and life sciences. It also paves the way for the development of novel all-fiber devices that exploit enhanced light-matter interaction in a monolithic form suitable for flexible and remote applications.</p>