Optical label-free detectors, such as the venerable surface plasmon resonance (SPR) sensor, are generally favored for their ability to obtain quantitative data on intermolecular binding. However, before the recent introduction of resonant microcavities that use whispering gallery mode (WGM) recirculation, sensitivity to single binding events had not materialized. Here we describe the enhancement mechanisms responsible for the extreme sensitivity of the WGM biosensor, review its current implementations and applications, and discuss its future possibilities.

We present an optical biosensor with unprecedented sensitivity for detection of unlabeled molecules. Our device uses optical resonances in a dielectric microparticle (whispering gallery modes) as the physical transducing mechanism. The resonances are excited by evanescent coupling to an eroded optical fiber and detected as dips in the light intensity transmitted through the fiber at different wavelengths. Binding of proteins on the microparticle surface is measured from a shift in resonance wavelength. We demonstrate the sensitivity of our device by measuring adsorption of bovine serum albumin and we show its use as a biosensor by detecting streptavidin binding to biotin.

Biosensors based on the shift of whispering-gallery modes in microspheres accompanying protein adsorption are described by use of a perturbation theory. For random spatial adsorption, theory predicts that the shift should be inversely proportional to microsphere radius R and proportional to protein surface density and excess polarizability. Measurements are found to be consistent with the theory, and the correspondence enables the average surface area occupied by a single protein to be estimated. These results are consistent with crystallographic data for bovine serum albumin. The theoretical shift for adsorption of a single protein is found to be extremely sensitive to the target region, with adsorption in the most sensitive region varying as 1/R(5/2). Specific parameters for single protein or virus particle detection are predicted.

Biosensing relies on the detection of molecules and their specific interactions. It is therefore highly desirable to develop transducers exhibiting ultimate detection limits. Microcavities are an exemplary candidate technology for demonstrating such a capability in the optical domain and in a label-free fashion. Additional sensitivity gains, achievable by exploiting plasmon resonances, promise biosensing down to the single-molecule level. Here, we introduce a biosensing platform using optical microcavity-based sensors that exhibits single-molecule sensitivity and is selective to specific single binding events. Whispering gallery modes in glass microspheres are used to leverage plasmonic enhancements in gold nanorods for the specific detection of nucleic acid hybridization, down to single 8-mer oligonucleotides. Detection of single intercalating small molecules confirms the observation of single-molecule hybridization. Matched and mismatched strands are discriminated by their interaction kinetics. Our platform allows us to monitor specific molecular interactions transiently, hence mitigating the need for high binding affinity and avoiding permanent binding of target molecules to the receptors. Sensor lifetime is therefore increased, allowing interaction kinetics to be statistically analysed.

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 whisperinggallery 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 (Ϸ5.2 ؋ 10 ؊16 g) of a bound virion are determined directly from the optimal resonance shift.biosensor ͉ influenza ͉ optical resonance V irus particles are a major cause for human disease, and their early detection is of added urgency since modern day travel has enabled these disease agents to be spread through populations across the globe (1). Fast and early detection on site of an outbreak requires biosensors where ideally individual viral particles produce a quantitative signal. Here, we report the observation of discrete changes in frequency of whispering gallery modes (WGMs) as Influenza A virions bind to a microsphere cavity. A ''reactive'' perturbation of the resonant photon state is confirmed in measurements with similar-sized polystyrene (PS) particles near a wavelength of 1,310 nm: The frequency/ wavelength shift signal follows a strong dependence on cavity curvature near the predicted ϳR Ϫ5/2 scaling (2), providing a mechanism for increasing signal by limiting modal volume. By reducing the microsphere radius to just Ͻ40 m and operating at a more favorable wavelength near 760 nm where reduced water absorption is expected to enhance sensitivity (3), binding steps of individual Influenza A (InfA) virions are seen that easily exceed the experimental noise level. Analytic equations are derived that relate discrete changes in resonance wavelength to the size and mass of adsorbed virions. Although field effect techniques using nanofibers (4) and interferometric approaches based on light scattering (5) have demonstrated single virion sensing in the past, reactive WGM sensing adds new dimensions to what can be learned: The measured wavelength shift enables one to quantitatively identify the virion size and mass.Experimental Approach WGM resonances are perturbed toward longer wavelength as particles with polarizability in excess to that of water adsorb to a microsphere cavity. Individual binding events have been theorized to produce discrete steps in a time-trace of the wavelength shift signal (2). To probe for single binding events we immerse a silica microsphere in a suspension of polystyrene particles (PS) with radius a ϭ 250 nm (Fig. 1). The PS particles are diluted in PBS to final concentrations Ϸ10-50 fM. A tunable distributed feedback laser (DFB) (Ϸ1,311 nm nominal wavelength) excites WGMs by evanescent coupling from a tapered optical fiber. A WGM mode is detected as a Lorentzian-shaped trough in ...