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Description:

Detection of single particles or molecules represents a key milestone in the development of biosensing technologies. Recently developed optical sensors based on quasi-bound states in the continuum (qBICs) have primarily focused on detecting global refractive index changes, aiming to simultaneously enhance both refractive index sensitivity and quality (Q) factors. However, sensors capable of resolving local refractive index perturbations, such as the binding of a nanometer-sized molecule on a surface, remain elusive and have not yet been demonstrated in BIC metasurfaces due to the limited Q factors and relatively large mode volumes. Here, we demonstrate low-contrast BIC metasurfaces enabling sensing with virus-sized single-nanoparticle resolution. The qBIC resonance operating at the critical coupling condition exhibits an experimental Q factor of approximately 4.5 × 10^4 in heavy water. Strong interactions between the localized electric field and polystyrene nanoparticles with a diameter of 100 nm allow the experimental observation of step-like resonance wavelength shifts, serving as signatures of individual particle binding events. Furthermore, binding-induced modifications to the qBIC resonance alter the optical confinement and asymmetry factor, inducing changes not only in the resonance wavelength but also in the linewidth and amplitude with single-particle sensitivity. Combined with position-insensitive response and free-space accessible features, low-contrast BIC metasurfaces provide a user-friendly platform for next-generation single-molecule sensing integrated with microfluidic systems.

Rights:

• Creative Commons BY Attribution 4.0 International
  

Keyword: metasurfaces, BIC, silicon, single-molecule, biosensors

Date published: 2026-08-04

Publisher: Wiley

Journal:

• Small Methods (ISSN: 23669608) vol. 10 issue. 17 e70931

Funding:

• Ministry of Education, Culture, Sports, Science and Technology JPMXP1224NM5259
• Ministry of Education, Culture, Sports, Science and Technology JPMXP1225NM5235
• Iketani Science and Technology Foundation 0361252‐A

Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.1002/smtd.70931

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Updated at: 2026-09-11 11:14:33 +0900

Published on MDR: 2026-09-11 14:26:05 +0900