Determining the health state of a single exosome via nanocoated silica microresonator

We present a numerical study of a nanocoated whispering-gallery-mode (WGM) silica microdisk as a label-free platform for single-exosome detection and refractive-index–based health-state classification. The device is modeled as a fiber-coupled silica microdisk in water, functionalized with a thin nanocoating of either polystyrene (PS) or the metal-halide perovskite CsPbI_3. Using full-wave driven-mode simulations in Comsol Multiphysics, we show that nanocoatings reshape the WGM field distribution and improve sensing-relevant figures of merit by enhancing surface-field confinement while preserving high-Q_f operation. Beyond the field-pulling mechanism provided by polymer coatings, we demonstrate a distinct sensitivity enhancement enabled by perovskites: spectral alignment of the WGM with an excitonic resonance in CsPbI_3 supports a hybrid excitonic--photonic mode that concentrates optical energy at the sensing interface and increases the transduction of minute effective-refractive-index (ERI) variations into measurable resonance shifts. To connect the exosome composition to the optical response, we introduce a physics-based workflow to estimate dispersive ERIs of individual exosomes from their protein and nucleic-acid content using a Barer-type relation and a core--shell geometry, and we map these ERIs to resonance-wavelength shifts for single exosomes at the sensing position. The resulting resonance signatures provide separable responses for healthy-like, borderline, and cancer-like exosomes, indicating that the proposed excitonically engineered WGM microresonator can not only detect single exosomes but also classify their health state, supporting a route toward non-invasive liquid-biopsy diagnostics.