  Identifying, visualising and ultimately tracking dynamically moving
non-fluorescent nanoparticles in the presence of non-specific scattering is a
long-standing challenge across the nano- and life-sciences. In this work we
demonstrate that our recently developed ultrafast holographic transient (UHT)
microscope is ideally suited for meeting this challenge. We show that UHT
microscopy allows reliably distinguishing off-resonant, dielectric, from
resonant, metallic, nanoparticles, based on the phototransient signal: a
pre-requisite for single-particle tracking in scattering environments. We then
demonstrate the capability of UHT microscopy to holographically localize in 3D
single particles over large volumes of view. Ultimately, we combine the two
concepts to simultaneously track several tens of freely diffusing gold
nanoparticles, within a 110x110x110 $\mu$m volume of view at an integration
time of 10 ms per frame, while simultaneously recording their phototransient
signals. The combined experimental concepts outlined and validated in this work
lay the foundation for background-free 3D single-particle tracking applications
or spectroscopy in scattering environments and are immediately applicable to
systems as diverse as live cells and tissues or supported heterogeneous
catalysts.
