Effect of Gadolinium Doping on the Optical and Magnetic Properties of Red-Emitting Dual-Mode Carbon Dot-Based Probes for Magnetic Resonance Imaging

Bioimaging probes based on carbon dots (CDs) can become a useful replacement for existing commercial probes, benefiting clinical diagnostics. While the development of dual-mode CD-based probes for magnetic resonance imaging (MRI), which provides the ability for photoluminescence (PL) detection at the same time, is ongoing, several challenges have to be addressed. First, most of the CD-based probes still emit at shorter wavelengths (blue/green spectral range), which is harmful to biological objects or have very low PL intensity in the biological window of tissue transparency (red/near-infrared spectral range). Second, the relaxation characteristics of the proposed CD-based probes are still similar or only slightly larger than those of commercial contrast agents. Herein, we introduce a dual-mode probe allowing for simultaneous PL detection and MRI analysis, based on CDs doped with gadolinium ions (Gd³⁺) with a PL peak in the red (640 nm), a PL quantum yield up to 24%, and high longitudinal and transverse relaxivities reaching 25.76 and 40.57 L mmol^–1 s^–1, respectively. Moreover, Gd-doped CDs show good biocompatibility, which was studied on H9c2 and 4T1 cell lines with a cell viability above 70%. The developed Gd-doped red-emissive CDs can be utilized as efficient and nontoxic dual-mode probes for PL and MRI measurements carried out simultaneously.