Triphenylmethyl-based carbon biradical nanoparticles for magnetic resonance imaging

Abstract

In recent years, luminescent carbon radicals have emerged as a promising functionalized material, with a primary focus on harnessing the potential of mono-radicals. The greater spin density exhibited by a single molecule in biradicals undoubtedly provides greater research value for its magnetic properties. Conventional metallic contrast agents represent the predominant choice in magnetic resonance imaging (MRI) of clinical applications. However, the accumulation of metal ions within the body poses potential safety risks. Hence, the exploration of innovative metal-free organic magnetic nanomaterials offers a safer alternative. This work introduces an innovative approach, presenting the first metal-free carbon-radical-based MRI contrast agent by encapsulating luminescent triphenylmethyl biradicals into nanoparticles (TTM-PhTTM NPs). These biradical NPs exhibit high water solubility, low cytotoxicity, and stability in highly reductive environments. Notably, TTM-PhTTM NPs maintained a strong electron paramagnetic resonance (EPR) signal even after exposure to ascorbic acid for 24 h, and cell viability remained above 80 % even at concentrations up to 1400 μg/mL. Moreover, TTM-PhTTM NPs demonstrated a T₁ longitudinal relaxation rate of 0.35 mM⁻¹s⁻¹, one of the highest recorded for metal-free organic magnetic nanomaterials. Following a 6-h incubation period, significant enhancements in imaging contrast were observed, with T₁ signal intensities increasing as the concentration of TTM-PhTTM NPs increased. This study paves the way for the utilization of stable, biocompatible carbon-based radicals as MRI contrast agents, underscoring their potential for safe and effective biomedical imaging applications and providing a solid foundation for further development of carbon radical-based imaging technologies.