Quantum dots as handles for optical manipulation

TL;DR

This paper demonstrates the use of individual colloidal quantum dots as optical handles for manipulation and force measurement, enabling simplified two-photon excitation and precise calibration in single-molecule assays.

Contribution

It introduces a method for using quantum dots as handles for optical manipulation and force calibration, with an Allan variance analysis for optimal measurement timing.

Findings

Optimal measurement time is around 0.3 seconds.

Quantum dots enable two-photon excitation without additional lasers.

Smallest tracer for tethered particle assay reported.

Abstract

Individual colloidal quantum dots can be optically trapped and manipulated by a single infrared laser beam operated at low laser powers. If the absorption spectrum and the emission wavelength of the trapping laser are appropriately chosen, the trapping laser light can act as a source for two-photon excitation of the trapped quantum dot. This eliminates the need for an additional excitation laser in experiments where individual quantum dots are used both as force transducers and for visualization of the system. To use quantum dots as handles for quantitative optical force transduction, it is crucial to perform a precise force calibration. Here, we present an Allan variance analysis of individual optically trapped quantum dots and show that the optimal measurement time for experiments involving individual quantum dots is on the order of 0.3 seconds. Due to their small size and strong illumination, quantum dots are optimal for single molecule assays where, optimally, the presence of the tracer particle should not dominate the dynamics of the system. As an example, we investigated the thermal fluctuations of a DNA tether using an individual colloidal quantum dot as marker, this being the smallest tracer for tethered particle method reported.