Rounded Monodisperse Fluorescent Nanodiamonds

Shape heterogeneity and surface sp 2 carbon in high‐pressure high‐temperature nanodiamonds (HPHT NDs) compromise nitrogen‐vacancy (NV) sensing and imaging capabilities. Rapid molten‐nitrate etching is investigated on a ∼gram preparative scale to round off HPHT NDs while removing sp 2 carbon. Subsequent centrifugal fractionation narrows size distribution, yielding more homogenized samples. Etching conditions of 567°C and 6 min maximize particle circularity at an acceptable mass yield. The rounding of ND particles is quantified by transmission electron microscopy (TEM), high‐resolution TEM with electron energy loss (EELS), and Raman spectroscopy, which confirm atomically stepped surfaces with markedly reduced sp 2 content. After separation, the rounded NDs exhibit a number‐weighted maximum at ∼35–40 nm (mean: 34 nm) and substantially reduced dispersity. Aqueous colloids retain negative ζ‐potential and are colloidally stable. After irradiation and annealing, the rounded NDs show an increased proportion of luminescent particles (66% vs. 24% for angular controls) and more than twofold greater single‐particle photoluminescence under identical excitation conditions. Additionally, the NV − charge state and NV spin longitudinal relaxation ( T 1 ) are preserved after the etching. Furthermore, the process is compatible with further scale‐up and provides shape‐controlled, low‐sp 2 , narrowly size‐dispersed NDs with enhanced optical properties, bringing improved reproducibility to bioimaging and quantum sensing.