Whispering gallery mode single nanoparticle detection and sizing: the validity of the dipole approximation.

Interactions between whispering gallery modes (WGMs) and small nanoparticles are commonly modeled by treating the particle as a point dipole scatterer. This approach is assumed to be accurate as long as the nanoparticle radius, a, is small compared to the WGM wavelength λ. In this Letter, however, we show that the large field gradients associated with the evanescent decay of a WGM causes the dipole theory to significantly underestimate the interaction strength and, hence, the induced WGM resonance shift, even for particles as small as a∼λ/10.

Label-free detection of single protein using a nanoplasmonic-photonic hybrid microcavity.

Recently we reported the detection and sizing of the smallest RNA virus MS2 with a mass of 6 ag from the resonance frequency shift of a whispering gallery mode-nanoshell hybrid resonator (WGM-h) upon adsorption on the nanoshell and anticipated that single protein above 0.4 ag should be detectable but with considerably smaller signals. Here, we report the detection of single thyroid cancer marker (Thyroglobulin, Tg) and bovine serum albumin (BSA) proteins with masses of only 1 ag and 0.

Whispering Gallery Mode Carousel--a photonic mechanism for enhanced nanoparticle detection in biosensing.

Individual nanoparticles in aqueous solution are observed to be attracted to and orbit within the evanescent sensing ring of a Whispering Gallery Mode micro-sensor with only microwatts of driving power. This Carousel trap, caused by attractive optical gradient forces, interfacial interactions, and the circulating momentum flux, considerably enhances the rate of transport to the sensing region, thereby overcoming limitations posed by diffusion on such small area detectors. Resonance frequency fluctuations, caused by the radial Brownian motion of the nanoparticle, reveal the radial trapping potential and the nanoparticle size.

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

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.

MicroParticle photophysics illuminates viral bio-sensing.

The authors present an approach for specific and rapid unlabeled detection of a virus by using a microsphere-based whispering gallery mode sensor that transduces the interaction of a whole virus with an anchored antibody. They show theoretically that this sensor can detect a single virion below the mass of HIV. A micro-fluidic device is presented that enables the discrimination between viruses of similar size and shape.

Detection of protein orientation on the silica microsphere surface using transverse electric/transverse magnetic whispering gallery modes.

The state of adsorbed protein molecules can be examined by comparing the shifts in a narrow line resonance wavelength of transverse electric (TE) and transverse magnetic (TM) whispering gallery modes (WGM) when the molecules adsorb onto a transparent microsphere that houses WGM. In adsorption of bovine serum albumin (BSA) onto an aminopropyl-modified silica microsphere, the TM/TE shift ratio indicated highly anisotropic polarizability of BSA in the direction normal to the surface, most likely ascribed to anchoring the heart-shaped protein molecule by one of its tips. The polarization-dependent resonance shift was confirmed when the surrounding refractive index was uniformly changed by adding salt, which would simulate adsorption of large objects.

Nanolayer characterization through wavelength multiplexing of a microsphere resonator.

We optically characterize nanolayer (<150 nm) formation in situ on a silica microsphere in an aqueous environment by simultaneously following the shifts of whispering-gallery modes at two wavelengths. This approach was inspired by layer perturbation theory, which indicates that these two measurements can be used to determine independently both the thickness and the optical dielectric constant. The theory is verified for extreme cases and used to characterize a biophysically relevant hydrogel nanolayer with an extremely small excess refractive index of 0.