J. Eur. Opt. Society-Rapid Publ. 22, 53( 2026) 517
nucleic acids( NAs), antibodies, and extracellular vesicles( EVs) such as exosomes [ 12 – 14 ]. For EVs and exosomes in particular, microtoroid WGM resonators have enabled real-time, label-free single-exosome detection in complex media [ 13 ], complementing other optical platforms such as interferometric imaging and nanoparticle-tracking-based methods [ 15 – 17 ]. Collectively, these characteristics established WGM-mode microresonators as an attractive platform for single-nanoparticle detection and characterization.
Exosomes are of particular interest for diagnostics because their molecular cargo reflects the physiological and pathological state of their parent cells, making them promising biomarkers for a range of diseases, including cancer [ 18, 19 ]. This property supports early-stage disease detection through non-invasive or minimally invasive liquid biopsies, as exosomes can be isolated from readily accessible body fluids such as blood, saliva, and urine [ 20, 21 ]. Tumorderived exosomes often carry altered protein and nucleicacid signatures compared with exosomes from healthy cells, which can manifest as subtle shifts in their effective refractive index( ERI); such shifts are, in principle, detectable with WGM-based biosensors and may enable cancer diagnosis [ 22 – 24 ]. Nevertheless, direct phenotyping and robust health-state classification remain challenging due to the small size of exosomes( typically 30 – 150 nm) and their low refractive-index contrast relative to the surrounding medium [ 25 – 27 ].
Recent optical studies have begun to quantify the ERI of extracellular vesicles( EVs) and to correlate subtle ERI variations with differences in biochemical composition [ 17, 25, 26 ]. In parallel, a range of hybrid and coated WGM architectures, including plasmonic-enhanced resonators, high-index coatings, and polymer microlaser platforms, have been proposed to further localize the optical field at the sensor surface and to push detection limits toward the single-molecule regime [ 28 – 30 ]. Metal-halide perovskites have emerged as particularly attractive high-index, excitonic materials, enabling high-Q f WGM lasing and strong exciton – photon coupling in CsPbX 3 and FAPbX 3 microcavities [ 31 – 35 ]. However, to the best of our knowledge, perovskite nanocoatings have not yet been leveraged to engineer WGM microresonators specifically for exosome sensing, nor has ERI-based classification of individual exosomes been explored using such hybrid structures.
The goal of this work is to discriminate the health state( cancerous versus healthy) of single exosomes using coated WGM microresonators. Realizing this capability would enable an early-stage, non-invasive, label-free biosensing approach to cancer diagnostics based on liquid biopsy. Achieving robust single-exosome classification requires further improvements in the sensing limit of optical microresonators. A particularly effective strategy is to introduce a carefully engineered nanocoating with a refractive index higher than that of the underlying resonator. We have previously shown that such high-index nanocoatings can enhance sensing performance by pulling the supported modes toward the coating region, increasing Q f and | E |, and simultaneously reducing the effective mode volume V mode [ 36 – 38 ]. Results show that, depending on the coating material, | E | can be amplified by up to a factor of 2.25 relative to the uncoated microresonator, V mode can be reduced by 21 %– 77 %, and Q f can increase from approximately 5 10 4( uncoated) up to 8 10 7 for the case of Si 3 N 4 nanocoating [ 36 ].
In this work, microdisks with tailored nanocoatings are modeled and simulated using the finite-element method( FEM)-based simulation platform Comsol Multiphysics. Full-wave, driven-mode simulations are performed, in which WGMs are excited in the microresonator via fiber optic coupling. Although computationally demanding, this modeling approach allows a comprehensive investigation of the impact of the nanocoating on the microresonator characteristics and enables precise calculation of the resonance wavelength shifts induced by the presence of healthy and cancerous exosomes. A 30 lm silica microdisk in an aqueous medium is considered, to which a 100 nm-thick nanocoating of either a polymer or a perovskite material is added. The sensing performance of the uncoated microresonator, the polymer-coated microresonator and the perovskite-coated microresonator is evaluated in terms of Q f, mode volume, coupling efficiency and electric field strength at the sensing location.
Additionally, we introduce and implement a novel approach for determining the effective refractive index( ERI) of exosomes via numerical simulations. This method yields a dispersive ERI for healthy and cancerous exosomes, as well as for exosomes whose ERI lies in the borderline region between the two health states. These exosomes are then placed in the near field of the modeled microresonators and the resulting resonance wavelength shifts are computed.
Our results show that selecting a suitable perovskite material with a bandgap around the WGM resonance wavelengths of the microresonator enables strong photonic – excitonic interaction between the perovskite coating and the resonator, leading to a substantial enhancement of the quality factor. These improvements, in turn, enable the detection of very small variations in the exosomes’ ERI and thereby the determination of the health state of individual exosomes.
The rest of the paper is organized as follows. In Section 2, the nanocoating concept is discussed in detail and the choice of nanocoating materials is explained. Section 3 introduces exosomes and our approach to determine their ERI in different health states; the modeling details and the resulting dispersive ERIs are presented there. The microresonator modeling and simulations are described in Section 4, while the impact of the nanocoating on the microresonator performance is analyzed in Section 5. InSec- tion 6, single exosomes are positioned next to the microresonators and the corresponding resonance wavelength shifts are evaluated. Finally, Section 7 summarizes the main conclusions and provides an outlook on future work.
2 The nanocoating
The rationale for introducing a high-refractive-index nanocoating is to directly engineer the three figures of merit that determine the resonance shift of whispering-gallery –