Fe-doped carbon dots as dual-mode T₁/T₂ nano-contrast agents for ultra-high field MRI

Carbon dots (CDs) doped with iron are valuable nano-contrast agents (nCAs) for magnetic resonance imaging (MRI) owing to their optimal compatibility, stability and interesting relaxivity, even at ultra-high fields (UHF-MRI, ≥7 T). Moreover, they represent suitable alternatives to Gd-based contrast agents frequently associated with moderate toxicity. Notably, UHF-MRI offers enhanced spatial resolution and improved signal-to-noise ratios, challenging some limitations encountered for standard MRI of vascular tissues, brain or applications demanding high sensitivity. The design of nCAs for UHF-MRI is not straightforward due to their tendency to show predominant T₂-character at high fields, hence leading to image darkening and resolution loss. For this reason, we propose novel Fe-doped CDs (Fe@CDs) working as dual mode T₁/T₂ nCAs for UHF-MRI; as a grounding concept, our synthesis was supported by stringent purification, a practice extremely overlooked in the CD field. Pure Fe@CDs (Fe content 3.7%) were isolated from fluorescent molecular-like subproducts and uncoordinated metals by size exclusion chromatography (SEC-HPLC); the particles showed good dimensional homogeneity (AFM = 5.0 ± 2.2 nm; TEM = 4.0 ± 1.3 nm), with blue centered and typical wavelength-dependent fluorescence. The structure of Fe@CDs was enriched in carboxylic moieties, enabling stable coordination of iron (in a mixed +2/+3 oxidation state), and optimal in vitro safety was observed on A549 cell line at the highest concentration investigated (500 mg mL^-1). Fe@CDs presented low r₂/r₁ ratios at high fields, behaving as T₁/T₂ dual-mode probes (r₂/r₁@7 T = 2.78 and r₂/r₁@11.7 T = 3.23); notably, the magnetic properties remained almost unaltered for 8 days in water and cellular medium. Our results provide a combined and wide approach for synthesizing artifact-free Fe-doped CDs with potential in advanced UHF-MRI techniques.