Axial Profiling of Interferometric Scattering Enables an Accurate Determination of Nanoparticle Size

TL;DR

This paper introduces an advanced interferometric scattering technique that accurately measures nanoparticle sizes beyond the Rayleigh limit by analyzing axial contrast variations and employing a vectorial PSF model.

Contribution

It presents a novel axial profiling method for iSCAT microscopy that overcomes previous size limitations and provides nanometer precision in nanoparticle sizing.

Findings

Accurately measures spherical dielectric nanoparticle sizes.

Works beyond the Rayleigh scattering limit.

Correlates fluorescence signal with nanoparticle size.

Abstract

Interferometric scattering (iSCAT) microscopy has undergone significant development in recent years. It is a promising technique for imaging and tracking nanoscopic label-free objects with nanometer localization precision. The current iSCAT-based photometry technique allows quantitative estimation for the size of a nanoparticle by measuring iSCAT contrast and has been successfully applied to nano-objects smaller than the Rayleigh scattering limit. Here we provide an alternative method that overcomes such size limitations. We take into account the axial variation of iSCAT contrast and utilize a vectorial point spread function model to uncover the position of a scattering dipole and, consequently, the size of the scatterer, which is not limited to the Rayleigh limit. We found that our technique accurately measures the size of spherical dielectric nanoparticles in a purely optical and non-contact way. We also tested fluorescent nanodiamonds (fND) and obtained a reasonable estimate for the size of fND particles. Together with fluorescence measurement from fND, we observed a correlation between the fluorescent signal and the size of fND. Our results showed that the axial pattern of iSCAT contrast provides sufficient information for the size of spherical particles. Our method enables us to measure the size of nanoparticles from tens of nanometers and beyond the Rayleigh limit with nanometer precision, making a versatile all-optical nanometric technique.