Tailoring Nanodiamonds for High-Contrast EPR Imaging: Size, Surface Properties, and Spectroscopic Performance

Electron paramagnetic resonance (EPR) spectroscopy is a tool that provides sensitive detection of uncoupled electron spins for a variety of applications. This technique enables the specific detection and quantification of radical species while also being able of generating high-contrast, background-free images. However, the EPR labeling and imaging techniques encounter limitations mainly due to the instability of organic radicals from organic probes, which can influence the reliability and scope of the experiment. In that context, the use of nanodiamonds (NDs) in EPR may be a promising route for understanding their unique properties and potential biomedical applications. The ability to perform EPR imaging in combination with the stable intrinsic properties of paramagnetic centers within these particles raises the possibility of extending nanodiamond-based imaging capabilities. Herein, we present a preliminary demonstration of a practical spectroscopy and imaging application using nanosized diamond particles (<18 nm) for electron paramagnetic resonance imaging (EPRI). The discretization of two different nanodiamond production sources among the most studied NDs (HPHT or detonation) allows further characterization of their physicochemical properties. In addition, we have investigated variations in the physicochemical properties of nanodiamonds, including size effects and surface treatments. Finally, we provide experimental evidence of the conditions required for optimal spectroscopic and imaging resolution (R < 1 mm) as well as achievable EPR sensitivity.