J. Eur. Opt. Society-Rapid Publ. 2026, 22, 53 Ó The Author( s), published by EDP Sciences, 2026 https:// doi. org / 10.1051 / jeos / 2026046 Available online at: https:// jeos. edpsciences. org
EOSAM 2025 Guest editors: Omar El Gawhary, Stefan Witte, Ignacio Moreno
Journal of the European Optical Society-Rapid Publications
RESEARCH ARTICLE
Determining the health state of a single exosome via nano-coated silica microresonator
Maíra Martins Garcia, Brahim Bessif, Daniel Erni, and Mandana Jalali * General and Theoretical Electrical Engineering( ATE), University of Duisburg-Essen( UDE) and Center for Nanointegration Duisburg-Essen( CENIDE), D-47048 Duisburg, Germany Received 30 January 2026 / Accepted 10 May 2026
Abstract. 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.
Keywords: Whispering gallery mode( WGM) microresonator, Nanocoating, Polystyrene, Perovskite, Exosome health state, Excitonic-photonic coupling.
1 Introduction
Optical biosensors that enable direct, real-time, non-invasive and label-free detection of bioelements are highly desired in biological studies. These sensors require ultra-sensitive mechanisms to detect single nanoscale particles and, when the goal is specificity, should also detect and distinguish even small variations in the particle’ s composition. In this regard, optical microresonators, sustaining whispering gallery modes( WGMs) have been established as ultrasensitive sensors and are the appropriate candidates for such task.
The first demonstrations of using WGM microresonators for biosensing emerged in the early 2000s, such as the 2002 report by Vollmer et al. on protein detection using a resonant microcavity and the 2003 study by Arnold et al. discussing the frequency shifts induced by protein adsorption in microspheres sustaining WGMs [ 1, 2 ]. Since then,
* Corresponding author: mandana. jalali @ uni-due. de interest in using WGM-based biosensors has increased substantially, and major advances in both sensing mechanisms and experimental techniques have been made [ 3 – 6 ].
Therefore, WGM biosensing has developed into a central platform for label-free biological and chemical sensing, owing to their ultra-high quality factors( Q f), small mode volumes( V mode), and strong electric field intensity at the sensing location | E |. A wide variety of ultra-high Q f WGM microresonator designs have already been fabricated, with various geometries including microspheres, microdisks, microrings, microtoroids, microbubbles, microbottles, and microcapillaries, presenting ultra-high Q f values in the range of 10 4 – 10 10 [ 3 – 5, 7, 8 ].
Comprehensive reviews report applications of WGM microresonators ranging from bulk refractometry and protein binding to single virus and nanoparticle detection, as well as recent progress towards integrated and optofluidic implementations for clinical diagnostics [ 3, 6, 7, 9 – 11 ]. More specifically, for single-molecule and single-particle detection, WGM biosensors have also been applied to sense
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