Tracking Brownian motion in three dimensions and characterization of individual nanoparticles using a fiber-based high-finesse microcavity

TL;DR

This paper presents a fiber-based microcavity method for three-dimensional tracking of unlabeled nanoparticles with high spatial and temporal resolution, enabling detailed nanomaterial and biomolecular motion analysis.

Contribution

It introduces a novel, label-free approach using a high-finesse microcavity to track nanoparticles in 3D with nanometer precision and microsecond temporal resolution.

Findings

Achieved 8 nm spatial resolution in 300 μs

Quantitatively determined nanoparticle properties like polarizability and hydrodynamic radius

Enabled high-bandwidth analysis of biomolecular motion

Abstract

The dynamics of nanosystems in solution contain a wealth of information with relevance for diverse fields ranging from materials science to biology and biomedical applications. When nanosystems are marked with fluorophores or strong scatterers, it is possible to track their position and reveal internal motion with high spatial and temporal resolution. However, markers can be toxic, expensive, or change the object's intrinsic properties. Here, we simultaneously measure dispersive frequency shifts of three transverse modes of a high-finesse microcavity to obtain the three-dimensional path of unlabeled SiO$_2$ nanospheres with $300$$\mathrm{\mu}$s temporal and down to $8$nm spatial resolution. This allows us to quantitatively determine properties such as the polarizability, hydrodynamic radius, and effective refractive index. The fiber-based cavity is integrated in a direct-laser-written microfluidic device that enables the precise control of the fluid with ultra-small sample volumes. Our approach enables quantitative nanomaterial characterization and the analysis of biomolecular motion at high bandwidth.