A high-performance biosensor based on quasi-bound states in the continuum for the detection of bacterial species in water

We propose a high-performance quasi-bound state in the continuum (quasi-BIC) optical biosensor for the label-free detection of bacterial species in aqueous media. The sensor exploits the redshift of a sharp resonance peak in response to variations in the effective refractive index. Six clinically relevant bacterial species (P. aeruginosa, B. anthracis, E. faecalis, E. coli, S. haemolyticus, and S. aureus) were analyzed across volume fractions from 0% to 100%, corresponding to an index range of 1.3330–1.4160 RIU. The biosensor was rigorously simulated using the Fourier Modal Method (FMM), demonstrating high sensitivity (up to 297.60 nm/RIU), an ultra-narrow FWHM of 0.4 nm, and high quality factors reaching 2451. Additionally, the design achieved a maximum detection accuracy of 2.5 nm⁻¹, a limit of detection as low as 3.36 × 10⁻⁴ RIU, and excellent figures of merit: FoM up to 739.13 RIU⁻¹, dip FoM up to 1.488 × 10⁶ nm/RIU⁻¹, and CSF up to 738.91 RIU⁻¹. Comparative analysis with recent literature confirms that the proposed biosensor outperforms existing designs across all major metrics. Its high resolution, broad index sensitivity range, and robustness make it a promising platform for real-time, label-free bacterial sensing in biomedical and environmental applications.