Water-Soluble Bimodal Magnetic-Fluorescent Radical Dendrimers as Potential MRI-FI Imaging Probes

Dual or multimodal imaging probes have become potent tools for enhancing detection sensitivity and accuracy in disease diagnosis. In this context, we present a bimodal imaging dendrimer-based structure that integrates magnetic and fluorescent imaging probes for potential applications in magnetic resonance imaging and fluorescence imaging. It stands out as one of the rare examples where bimodal imaging probes use organic radicals as the magnetic source, despite their tendency to entirely quench fluorophore fluorescence. Opting for organic radicals over metal-based contrast agents like gadolinium (Gd³⁺)-chelates is crucial to mitigate associated toxicity concerns. We utilized an amino-terminated polyamide dendrimer containing a 1,8-naphthalimide (Naft) fluorescent group, amino acid derivatives as linkers to enhance water solubility, and TEMPO organic radicals as terminal groups. The same dendrimer structure, featuring an equivalent number of branches but lacking the fluorophore group, was also functionalized with amino acid and terminal radicals to serve as a reference. Remarkably, we achieved a fully water-soluble dendrimer-based structure exhibiting both magnetic and fluorescent properties simultaneously. The fluorescence of the Naft group in the final structure is somewhat quenched by the organic radicals, likely due to photoinduced electron transfer with the nitroxyl radical acting as an electron acceptor, which has been supported by density functional theory calculations. Molecular dynamics simulations are employed to investigate how the dendrimers' structure influences the electron paramagnetic resonance characteristics, relaxivity, and fluorescence. In summary, despite the influence of the radicals-fluorophore interactions on fluorescence, this bimodal dendrimer demonstrates significant fluorescent properties and effective r₁ relaxivity of 1.3 mM^-1 s^-1. These properties have proven effective in staining the live mesenchymal stem cells without affecting the cell nucleus.