We introduce a new technique to measure tracer dynamics, which is sensitive to single metal nanoparticles down to a radius of 2.5 nm with a time resolution of a few microseconds. It is based on a fluctuation analysis of a heterodyne photothermal scattering signal emanating from the hot halo around the laser-heated tracer. A generalized Stokes-Einstein relation for "hot brownian motion" is developed and verified. Exploiting the excellent photostability of gold nanoparticles, the developed method promises broad applications especially in the field of quantitative biomedical screening.

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