D. Keng

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. 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 (≈5.2 × 10−16 g) of a bound virion are determined directly from the optimal resonance shift.

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. Since the attractive forces draw particles to the highest evanescent intensity at the surface, binding steps are found to be uniform.

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 0.11 ag (66 kDa), respectively. However, the wavelength shifts are enhanced beyond those anticipated in our earlier work by 240% for Tg and 1500% for BSA. This surprising sensitivity is traced to a short-range reactive field near the surface of our Au nanoshell receptor due to intrinsic random bumps of protein size, leading to an unanticipated increase in sensitivity to single protein, which grows larger as the protein diminishes in size. As a consequence of the largest signal-to-noise ratio in our BSA experiments (S/N ≈ 13), we conservatively estimated a new protein limit of detection for our WGM-h of 5 kDa.

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.

Molecular weight dependence of a whispering gallery mode biosensor

We report on molecular weight dependence measurements for an optical resonance biosensor. A dielectric microparticle is evanescently coupled with an optical fiber for the resonance stimulation, and a shift of the resonance wavelength is measured to monitor protein monolayer formation on the microparticle surface. Wavelength shifts for proteins over two orders of magnitude in molecular weight are measured. We show that the shift is proportional to molecular weight to the one-third power. Our result demonstrates that the optical resonance biosensor provides protein size information upon detection. This molecular weight dependency differentiates optical resonance sensing from electrical detection using field-effect transistors.

Resonance fluctuations of a whispering gallery mode biosensor by particles undergoing Brownian motion

Nanoparticles suspended in the vicinity of a whispering gallery mode (WGM) biosensor are detected from fluctuations in the driving light-guide transmission. These fluctuations are described by Brownian particles perturbing the resonance wavelength in reaction to being polarized by the WGM’s evanescent field. Comparison between the autocorrelation of the measured fluctuations and theory provides a first order approximation for the nanoparticle size and lays the basis for future studies of interfacial dynamics. With this advance, the WGM biosensor goes beyond low-frequency measurements of adsorption and desorption and into a world which has been dominated by fluorescence correlation spectroscopy, but without labels.

Whispering gallery micro-global positioning system for nanoparticle sizing in real time

We have devised a simple means for determining the size of a nanoparticle in one binding event (i.e., real time) by utilizing two polar modes of a slightly eccentric Whispering Gallery Mode (WGM) spheroidal resonator. The ratio of shifts of these modes locates the absolute latitude angle at which a nano-particle binds. From this location, the size of the nanoparticle is calculated using the reactive sensing principle. Although our latitude-only micro-global positioning scheme is applied to nanoparticle sizing using slightly eccentric spheroids in aqueous solution, this approach can be applied to WGM micro-resonators having a variety of shapes.

Nanolayer characterization through wavelength multiplexing a microsphere resonator

Hydrogel nanolayer (<150 nm) formation on a silica microsphere is optically characterized while forming in situ by simultaneously following the shifts of whispering gallery modes at two wavelengths.

Label-free single cancer marker protein detection using a nanoplasmonic-photonic hybrid whispering gallery mode biosensor

The reactive sensing principle applied to hybrid plasmonic whispering gallery mode biosensor has recently demonstrated detection of individual protein cancer markers. The rough surface of a gold nanoparticle affixed to the resonator surface acts like a nanoscopic antenna, significantly boosts the local electric field within the cavity mode. Adsorption of a target protein onto this nanoscopic antenna results in an enhanced response of the resonator system to the binding event. We have demonstrated detection of individual protein molecules (66 kDa) with good signal-to-noise (S/N > 10), and project that detection of proteins as small as 5 kDa are possible.

Whispering gallery mode coulometry of the nanoparticle-microcavity interaction in aqueous solution

Charge influences the binding of virus and other nano-particles to microcavity bio-sensors, although surprisingly there have been no reports of the determination of either cavity charge density σw or nanoparticle charge qp using these sensors. In this letter, we experimentally demonstrate an approach for the determination of both. We use an opto-mechanical Whispering Gallery Mode (WGM) Carousel trap to extract the electrostatic interaction energy versus separation s between the cavity surface and a nanoparticle from WGM frequency fluctuations induced by the orbiting particle. Next, we fit this interaction energy to linearized wall-colloid theory (Debye-Huckel theory) for a particle whose charge is known and determine σw. With this microcavity charge density in hand, a larger particle having unknown charge and orbiting the same microcavity has its charge measured from its associated electrostatic interaction energy. This charge is found to be smaller by 10% when compared to results from independent zeta po...