Photothermal Single Particle Microscopy

TL;DR

This paper reveals that photothermal microscopy detects signals from single molecules and nanoparticles through a nanolensing effect, challenging the previous assumption of an extinction measurement, and provides a foundation for future quantitative applications.

Contribution

The study demonstrates that the photothermal signal arises from nanolensing rather than extinction, supported by combined microscopy and Mie scattering calculations, clarifying the physical detection mechanism.

Findings

Photothermal signal is due to nanolensing, not extinction.

Combined microscopy and Mie calculations explain the detection mechanism.

Results enable quantitative single molecule absorption microscopy.

Abstract

Photothermal microscopy has recently complemented single molecule fluorescence microscopy by the detection of individual nano-objects in absorption. Photothermal techniques gain their superior sensitivity by exploiting a heat induced refractive index change around the absorbing nano-object. Numerous new applications to nanoparticles, nanorods and even single molecules have been reported all refering to the fact that photothermal microscopy is an extinction measurement on a heat induced refractive index profile. Here, we show that the actual physical mechanism generating a photothermal signal from a single molecule/particle is fundamentally different from the assumed extinction measurement. Combining photothermal microscopy, light scattering microscopy as well as accurate Mie scattering calculations to single gold nanoparticles, we reveal that the detection mechanism is quantitatively explained by a nanolensing effect of the long range refractive index profile. Our results lay the foundation for future developments and quantitative applications of single molecule absorption microscopy.