Ultrasensitive Detection of PD‐L1‐Expressing Extracellular Vesicles via Reflection‐Mediated Ellipsometry: From Nanoscale Optical Modeling to Immunotherapy Response Prediction

Abstract Programmed death‐ligand 1 (PD‐L1) expression in extracellular vesicles (EVs) has emerged as a promising biomarker for predicting responses to immune checkpoint inhibitors (ICIs), but current detection methods lack the sensitivity needed for clinical translation. Here, a reflection‐mode solution‐immersed silicon (R _p ‐SIS) ellipsometry sensor is adapted, originally developed for thin‐film measurements, for the ultra‐sensitive detection of PD‐L1–expressing EVs. Computational and experimental validation demonstrated that the selective use of p‐polarized light, combined with a fixed polarizer configuration, enables highly sensitive detection of nanoscale surface changes. Applied to cell‐line–derived EVs, the sensor achieved a limit of detection of 3.8 × 10 ¹ particles mL ⁻¹ —exceptionally high sensitivity for label‐free EV detection—with also high reproducibility and specificity. In clinical samples from patients receiving ICI therapy, PD‐L1⁺ EV levels measured by the R _p ‐SIS sensor distinguished responders from non‐responders and correlated with progression‐free survival, outperforming conventional ELISA and tissue‐based PD‐L1 staining. Additionally, it is demonstrated that direct detection of intact EVs yields stronger signals than lysate‐based detection due to amplified optical responses from vesicle‐scale binding. The platform's versatility is further demonstrated by detecting alternative tumor‐derived EV biomarkers and employing diverse capture ligands. These findings establish the R _p ‐SIS sensor as a clinically actionable platform for high‐sensitivity, EV‐based liquid biopsy.