Integrated sensor with a whispering-gallery mode and surface plasmonic resonance for the enhanced detection of viruses

Surface plasmonic resonance (SPR) is integrated into a whispering-gallery mode (WGM) optical microsensor to augment sensitivity in this study. The performance of such WGM silica ring sensors of 20 µm in size with an Ag or Au metal core was evaluated for detection of small respiratory viruses such as COVID-19 via the finite-element modeling. Compared with pure WGM sensors, the integration with SPR enhances sensitivity by 3–5 times and facilitates combination with the polymerase chain reaction method to achieve fast, accurate, and specific virus detection. The presence of a single respiratory virus of 70 to 100 nm in diameter in air environment could shift the sensor resonance wavelength by 36 to 64 pm in the case of an Au metal core or by 34 to 63 pm of an Ag metal core. With use of a general-purpose optical analyzer of 10 pm resolution, a single airborne virus of 20 nm in size is detectable using the proposed hybrid sensor. This corresponds to about 0.005 vol. %. For viruses in aqueous solution, the detection limit rises to about 0.2 vol. %. A fundamental enhancement factor based on relative electric energy ratio is introduced and defined to analyze and quantify sensitivity enhancement for the first time, to the best of our knowledge.