Label-Free Detection of Respiratory Syncytial Virus in Clinical Nasopharyngeal Swab Samples Using a Silicon-Nitride Slot Microring Resonator

We report a silicon-nitride slot-waveguide microring resonator functionalized with monoclonal antibodies against the respiratory-syncytial-virus (RSV) fusion (F) protein for label-free, real-time detection of RSV. The 100 μm-radius slotted ring exhibits a loaded quality factor of 3.5 × 104 and a bulk sensitivity of 275 nm/RIU at 1310 nm. Langmuir fitting of recombinant F-protein titrations (0.05–3 nM) gives an equilibrium dissociation constant KD = 5.4 × 10-2nM and Δλmax = 7.3 nm (R² = 0.997), yielding a limit-of-detection of 31 pM and a linear dynamic range spanning three orders of magnitude. When challenged with nasopharyngeal swab extracts from RSV-positive patients, the sensor produces wavelength shifts of 0.35–1.45 nm that correlate quantitatively with viral load over > 10³-fold concentration range and show 100% concordance with RT-qPCR on three clinical specimens. The wash-free assay requires low volumes of samples and no labeling, and it is fully compatible with disposable CMOS-compatible photonic chips, underscoring its potential for near-patient RSV surveillance during seasonal outbreaks.