  Optical bio-markers have been used extensively for intracellular imaging with
high spatial and temporal resolution. Extending the modality of these probes is
a key driver in cell biology. In recent years, the NV centre in nanodiamond has
emerged as a promising candidate for bio-imaging and bio-sensing with low
cytotoxicity and stable photoluminescence. Here we study the
electrophysiological effects of this quantum probe in primary cortical neurons.
Multi-electrode array (MEA) recordings across five replicate studies showed no
statistically significant difference in 25 network parameters when nanodiamonds
are added at varying concentrations over various time periods 12-36 hr. The
physiological validation motivates the second part of the study which
demonstrates how the quantum properties of these biomarkers can be used to
report intracellular information beyond their location and movement. Using the
optically detected magnetic resonance from the nitrogen vacancy defects within
the nanodiamonds we demonstrate enhanced signal-to-noise imaging and
temperature mapping from thousands of nanodiamond probes simultaneously. This
work establishes nanodiamonds as viable multi-functional intraneuronal sensors
with nanoscale resolution, that may ultimately be used to detect magnetic and
electrical activity at the membrane level in excitable cellular systems.
