Abstract
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.
Highlights
Light forces interacting with mechanical systems provide a unique tool for studying small biological objects [1]
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
Summary
Light forces interacting with mechanical systems provide a unique tool for studying small biological objects [1]. Measured binding rates of bioparticles in aqueous solution using a toroidal WGM bio-sensor [3] at ultra-low concentrations appear to be about one hundred times higher than calculated based on diffusive and convective transport theory [4]. At low ionic strength an addition electrostatic force repels the nanoparticle from the surface, contributing to a radial trap. The maximum fluctuation enables the size and mass of the trapped nanoparticle to be determined without binding, suggesting that the WGM Carousel mechanism can be used for size/mass spectrometry in solution. At a considerably high ionic strength the electrostatic field is screened to a much shorter depth, and the particle is drawn closer to the surface where it is caught by a van der Waal interaction and binds. Resonance shifts due to these binding events are found to be steps having uniform heights
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