Multimodal functionalization of nanodiamonds for advanced biomedical and optoelectronic applications

Fluorescent nanodiamonds (FNDs) emit light through a specific type of photoluminescence in which light emission is very rapid and is quenched almost immediately upon removal of the light source. To enhance the fluorescent properties of FNDs, a sustainable synthesis and functionalization strategy was developed by combined physicochemical treatments. The primary nanodiamonds (NDs) were obtained using a hydrothermal method and subsequently functionalized using ascorbic acid (AA), theraphthal (TP), urea (UR), and 30 % hydrogen peroxide (H 2 O 2 ). The surface activation was enhanced by applying direct ultrasound treatment for 15 and 20 h, along with ozone (O 3 ) exposure for 0.75 and 1.5 h, followed by ultrasound for 5 h. Prior to the final functionalization step, the NDs suspensions were subjected to a cleaning process by centrifugation and filtration with 200 nm membranes. After functionalization, the samples were duly characterized by FT-IR, UV-Vis, and photoluminescence (PL) spectroscopy, ζ-potential analysis and electron microscopy, i.e., HR-TEM and SEM. From the detailed analyses, incorporation of oxidative and nitrogen-containing functional groups on the NDs was confirmed. Which was further corroborated by the appearance of a faint blue to lime-green photoluminescence (∼500–520 nm), which is attributed to nitrogen-related defects near the surface (with N–V–N as the most consistent interpretation under our conditions). It is deduced that, the combined treatment with O 3 and ultrasound effectively favored the formation of the emissive centers and significantly reduced the duration of the experimental process. FNDs developed by this method have been shown to be extremely stable and resistant to photo-bleaching, making them highly useful for long-term bioimaging applications, diagnostics and drug release systems. • Post-synthetic ultrasound and O3 treatments enabled sustainable surface activation of NDs. • Functionalization with TP, AA, UR, and H2O2 enhanced surface reactivity and photoluminescence response. • Green photoluminescence observed in NDs after ultrasound and sequential ozone–ultrasound. • O3–ultrasound effectively induced luminescence and reduced processing time by over 60 %. • Applicable to other carbon nanostructures for optical and surface modification.