The size of a probe bead reported by holographic particle characterization depends on the proportion of the surface area covered by bound target molecules and so can be used as an assay for molecular binding.
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-Hückel 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 potential measurements and outside of one standard deviation. However, non-linear Gouy-Chapman theory when applied to our measured data arrives at a charge that overlaps zeta potential measurements. Our method is non-destructive, enabling the same particle to be passed on for further characterization.
Holographic particle characterization yields the diameter of individual colloidal spheres with nanometer precision and can resolve probe beads growing as molecules bind to their surfaces. We demonstrate label-free holographic assays for antibodies and for antigenic proteins from pathogenic viruses, including SARS-CoV-2 and H1N1.
Holographic molecular binding assays detect target molecules binding to the surfaces of specifically functionalized probe beads by measuring the associated increase in bead diameter with holographic video microscopy. Holograms of individual colloidal beads are analyzed by fitting to analytic predictions of the Lorenz-Mie theory of light scattering, yielding measurements of bead diameter with the nanometer precision required to detect binding events. Holographic binding assays share the specificity and robustness of industry-standard bead-based assays. Direct holographic readout eliminates the processing time, expense and uncertainty associated with fluorescent labeling. The underlying technology for holographic particle characterization also has a host of other applications in biopharmaceuticals, semiconductor processing and fundamental research.
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