Single virus detection from the reactive shift of a whispering-gallery mode

Abstract

We report the label-free, real-time optical detection of Influenza A virus particles. Binding of single virions is observed from discrete changes in the resonance frequency/wavelength of a whispering-gallery mode excited in a microspherical cavity. We find that the magnitude of the discrete wavelength-shift signal can be sufficiently enhanced by reducing the microsphere size. A reactive sensing mechanism with inverse dependence on mode volume is confirmed in experiments with virus-sized polystyrene nanoparticles. By comparing the electromagnetic theory for this reactive effect with experiments, the size and mass ( approximately 5.2 x 10(-16) g) of a bound virion are determined directly from the optimal resonance shift.

Fig. 1.

Excitation of an equatorial WGM of a microsphere by evanescent coupling to a guided wave in a tapered optical fiber. Resonance positions are detected as dips in the transmitted light T at particular laser wavelengths.

Fig. 2.

Nanoparticle wavelength shift. (A) Dip-trace for a = 250 nm PS particles interacting with a microsphere with R ≈ 45 mm. (B) A 15 second zoom-in. (C) Wavelength step statistics.

Fig. 3.

Maximum step height vs. microsphere curvature for polystyrene particles with radius a = 250 nm.

Fig. 4.

Shift signal for InfA. The data were acquired for a microsphere with R = 39 mm, and a DFB laser having a nominal wavelength of 763 nm.