Optical biomarkers 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 nitrogen-vacancy (NV) center in nanodiamond has emerged as a promising candidate for bioimaging and biosensing with low cytotoxicity and stable photoluminescence. Here we study the electrophysiological effects of this quantum probe in primary cortical neurons. Multielectrode array 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 h. 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 multifunctional intraneuronal sensors with nanoscale resolution, which may ultimately be used to detect magnetic and electrical activity at the membrane level in excitable cellular systems.