Surface-reduced and hydroxymethylated nanodiamonds via mechanochemical NaBH4/I2 reduction for stable dispersion

Abstract Detonation nanodiamonds (DNDs) integrate the advantages of bulk diamond (e.g., exceptional mechanical and optical properties) with nanoscale functionality, yet their practical applications are severely limited by high surface energy-induced spontaneous aggregation and surface chemical heterogeneity. Here, we report a mechanochemical approach that simultaneously achieves DND deaggregation and surface hydroxylation by co-milling with sodium borohydride (NaBH4) and auxiliary reducing agents (I2, H2SO4, or AlCl3) to form distinct reduction systems. Among these, the NaBH4/I2 system demonstrates the highest reduction efficiency. The resulting hydroxymethylated DNDs (DND-CH2-OH) exhibit an average primary particle size of 24.6 nm and a maximum absolute zeta potential of 59.1 mV, enabling stable and uniform dispersion/re-dispersion in diverse solvents (water, ethylene glycol, and PAO2) without re-aggregation after freeze-drying. Notably, this NaBH4/I2 mechanochemical product shows negligible cytotoxicity toward HepG2 and LO2 cells and no acute adverse effects in mice, confirming its green and nontoxic nature. This DND-CH2-OH serves as an ideal starting material for further functionalization and holds promise for applications in drug delivery, bioimaging, and beyond.