Quantum Metrology for Biological Systems using the Nitrogen-Vacancy Center in Nanodiamonds

The Nitrogen-Vacancy (NV) center in fluorescent nanodiamonds (FNDs) has emerged as an excellent candidate for nanoscale sensing of temperature, pH, and paramagnetic species concentration, potentially enhancing our understanding of complex biological processes. Their use in aqueous environments, however, has been challenging. Here we present a series of proof-of-principle experiments to illustrate potential applications of FNDs in sensing biologically relevant species. Through T1 relaxometry, we achieve highly reproducible nanomolar sensitivity to the paramagnetic ion, gadolinium (Gd3+) using optically-trapped FNDs. This behavior is characterized by three distinct phases, modeled through a nanoscale Langmuir adsorption framework combined with spin coherence dynamics. Additionally, we leverage T1 relaxometry to demonstrate the pH sensitivity of FNDs, made possible through surface functionalization. Finally, using optically detected magnetic resonance (ODMR), we demonstrate intracellular temperature sensing, with a particular focus on the role of mitochondria in establishing the temperature of the intracellular environment.