  Optically accessible spin-active nanomaterials are promising as quantum
nanosensors for probing biological samples. However, achieving bioimaging-level
brightness and high-quality spin properties for these materials is challenging
and hinders their application in quantum biosensing. Here, we demonstrate
ultrabright fluorescent nanodiamonds (NDs) containing 0.6-1.3-ppm
nitrogen-vacancy (NV) centers by spin-environment engineering via enriching
spin-less 12C-carbon isotopes and reducing substitutional nitrogen spin
impurities. The NDs, readily introduced into cultured cells, exhibited
substantially narrow optically detected magnetic resonance (ODMR) spectra,
requiring 16-times less microwave excitation power to give an ODMR depth
comparable to that of conventional type-Ib NDs. They show average
spin-relaxation times of T1 = 0.68 ms and T_2 = 1.6 us (1.6 ms and 2.7 us
maximum) that were 5- and 11-fold longer than those of type-Ib, respectively.
The bulk-like NV spin properties and bright fluorescence demonstrated in this
study significantly improve the sensitivity of ND-based quantum sensors for
biological applications.
