Measuring Nanoscale Torques with Cylindrical-Polarization-based Interferometric Scattering Microscopy

TL;DR

The paper introduces cypiSCAT, a novel interferometric microscopy technique that enables high-precision, rapid measurement of nanoscale rotational dynamics and torques in single molecules, overcoming previous detection limitations.

Contribution

It presents a new method for directly observing and quantifying nanoscale torques and rotational dynamics with high temporal and angular resolution.

Findings

Achieved sub-degree angular precision in rotational tracking.

Measured optically induced torques as small as ~1 pN nm.

Enabled rapid detection of rotational events in biomolecular systems.

Abstract

The ability to observe rotational dynamics and measure underlying torques is a crucial component in understanding the function and mechanics of nanoscale systems. Yet, direct observation of rotational dynamics at the single-molecule level in liquids remains challenging due to the trade-off between optical detectability and hydrodynamic responsiveness. Labels that are bright enough for rapid readout typically introduce excessive drag, while minimally perturbing probes are difficult to detect at high speed. This limits access to fast rotational dynamics required for direct torque measurements. Here, we introduce cylindrical-polarization-based interferometric scattering microscopy (cypiSCAT), a method encoding the orientation of anisotropic scatterers directly into a single interferometric point spread function, while intrinsically suppressing the isotropic background. We achieve rotational tracking of low-drag orientation labels based on DNA origami-attached gold nanorods with sub-degree angular precision and microsecond temporal resolution, allowing quantitative characterization of nanoscale rotational dynamics. This capability provides direct access to torque metrology at the single-molecule level, here demonstrated through the extraction of optically induced torques as small as ~1 pN nm. Relying on elastic scattering, cypiSCAT combines ultrafast temporal resolution with long observation times, making it well-suited for capturing rapid and rare rotational events and reaction steps in nanoscale biomolecular systems.