Fiber-Optic Interference Microscopy for Label-Free Monitoring of Cancer Cell Secretion at a Single-Molecule Level

Understanding cell secretion at the single-molecule level in a label-free manner is crucial for elucidating numerous biological processes relevant to health and disease. However, most current optical label-free approaches rely on strong photon confinement to enhance the signal contrast, and the associated photothermal effect limits their applications in continuous monitoring of molecular dynamics. Here, we propose fiber-optic interference microscopy (FOIM), which achieves label-free, high-contrast visualization of single molecules by monitoring the interference patterns on a microfiber. We demonstrated that the observed interference patterns originate from the in-plane scattering of natural defects on the microfiber. We found that spatially resolved interference patterns can reveal localized phase changes induced by single molecules, providing a significant sensitivity enhancement over conventional spectral interrogation methods. After the optical fiber surface is functionalized with capture antibodies, FOIM exhibits high target affinity and strong resistance to interfering molecules. Subsequently, the highly specific detection of cancer cell secretions in complex extracellular environments was demonstrated. Our work provides a powerful tool for label-free monitoring of molecular dynamics, paving the way for next-generation biomolecular analysis and early diagnostic strategies.