Manganese-Doped Carbon Dots with Redshifted Orange Emission for Enhanced Fluorescence and Magnetic Resonance Imaging

Metal-doped carbon dots (CDs) exhibited promising application in fluorescence and magnetic resonance (MR) imaging, but developing manganese-doped CDs (Mn-CDs) with long wavelength emission and enhanced MR performance is a challenge. Herein, using a one-step solvothermal method, Mn-CDs with redshifted orange emission and enhanced longitudinal relaxation were synthesized for fluorescence and MR imaging. The results indicated that the prepared Mn-CDs had a uniform size distribution, and the average size was 5 nm. Moreover, Mn-CDs possessed a stronger fluorescence performance than Mn-free CDs, and simultaneously, the emission wavelength can redshift from 542 nm (green emission) to 578 nm (orange emission), owing to the increasing N-doping. Because of the movement limit of Mn²⁺, Mn-CDs exhibited high T₁-weighted MR imaging performance with a longitudinal relaxation rate of 12.69 mM^-1·s^-1. The in vivo experiments demonstrated their excellent fluorescence and MR imaging with safety and reliability. Therefore, the prepared Mn-CDs with orange emission can be an excellent candidate as a dual-modal nanoprobe for fluorescence and MR imaging.