Thiourea modification of fluorescent nanodiamonds towards enhanced quantum sensing

We functionalized fluorescent nanodiamonds of various sizes using a thiourea-based thiolation reaction to tailor their surface chemistry for biological and quantum technological applications. Spectroscopic analyses revealed that this reaction generates a complex mixture of sulfur- and nitrogen-containing groups, arising from the reaction of thiourea with surface functional groups and from oxidative cyclization. Since the charge stability of negatively charged nitrogen-vacancy (NV⁻) centers is strongly influenced by the near-surface electronic structure, surface modifications that enhance this stability—while preserving colloidal dispersibility and enabling further functionalization—are essential for quantum sensing applications. We show that the surface chemistry produced through the reaction of nanodiamonds with thiourea increases electron availability and favors the stabilization of the NV⁻ charge state. These results highlight the potential of thiourea-derived surface modification as an effective route to improve the quantum performance of nanodiamonds.