Effects of Thermal Oxidation and Proton Irradiation on Optically Detected Magnetic Resonance Sensitivity in Sub-100 nm Nanodiamonds

In recent decades, nanodiamonds (NDs) haveemerged as innovative nanotools for weak magnetic fields andsmall temperature variation sensing, especially in biologicalsystems. At the basis of the use of NDs as quantum sensors arenitrogen-vacancy center lattice defects, whose electronic structuresare influenced by the surrounding environment and can be probedby the optically detected magnetic resonance technique. Ideally,limiting the NDs’ size as much as possible is important to ensurehigher biocompatibility and provide higher spatial resolution.However, size reduction typically worsens the NDs’ sensingproperties. This study endeavors to obtain sub-100 nm NDssuitable to be used as quantum sensors. Thermal processing andsurface oxidations were performed to purify NDs and control theirsurface chemistry and size. Ion irradiation techniques were also employed to increase the concentration of the nitrogen-vacancycenters. The impact of these processes was explored in terms of surface chemistry (diffuse reflectance infrared Fourier transformspectroscopy), structural and optical properties (Raman and photoluminescence spectroscopy), dimension variation (atomic forcemicroscopy measurements), and optically detected magnetic resonance temperature sensitivity. Our results demonstrate how surfaceoptimization and defect density enhancement can reduce the detrimental impact of size reduction, opening to the possibility ofminimally invasive high-performance sensing of physical quantities in biological environments with nanoscale spatial resolution.