  Viscoelasticity of the cytoplasm plays a critical role in cell morphology and
division. In parallel, local temperature is coupled to viscoelasticity and
influences cellular bioenergetics. Probing the interdependence of intracellular
temperature and viscoelasticity provides an exciting opportunity for the study
of metabolism and disease progression. Here, we present a dual-mode quantum
sensor, capable of performing simultaneous nanoscale thermometry and rheometry
in a dynamic cellular environment. Our technique uses nitrogen-vacancy centres
in nanodiamond, combining sub-diffraction resolution single-particle tracking
in a fluidic environment with optically detected magnetic resonance
spectroscopy. We demonstrate nanoscale sensing of temperature-dependent
viscoelasticity in complex media. We then use our sensor to investigate the
interplay between intracellular forces and cytoplasmic rheology in live cells,
revealing details of active trafficking and nanoscale viscoelasticity.
