Rounded Monodisperse Fluorescent Nanodiamonds

Shape heterogeneity and surface sp² 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 sp² 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 sp² 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 (T₁) are preserved after the etching. Furthermore, the process is compatible with further scale-up and provides shape-controlled, low-sp², narrowly size-dispersed NDs with enhanced optical properties, bringing improved reproducibility to bioimaging and quantum sensing.