Current Progress and Future Perspectives on Metal- and Nonmetal-Doped Carbon Dots for Fluorescence, CT, and MRI Bioimaging

Abstract The demand for innovative biocompatible contrast agents suitable for multimodal imaging is rising, driven by the increasing incidence of cancer and chronic disease, the growing use of combined imaging workflows, and the safety limitations of conventional iodinated- and gadolinium-based agents. In this regard, carbon dots (CDs) have received a lot of attention in the past decade due to their unique characteristics, such as production and modification simplicity, low cost, biocompatibility, high colloidal stability, and tunable optical properties. In this Review, we summarize recent advances in biomedical applications of metal- and nonmetal-doped carbon dots (CDs) as contrast agents in fluorescence imaging, computed tomography (CT), magnetic resonance imaging (MRI), and potential theranostic applications. Particular emphasis is placed on studies published between 2015 and 2026, representing the most recent developments in these rapidly evolving fields. This Review also compares key imaging parameters, including X-ray attenuation efficiency, longitudinal and transverse relaxivity (r1/r2), and fluorescence quantum yield, with the available data summarized in comparative tables. In addition, the current challenges associated with doped CDs, together with emerging opportunities and future research directions, are discussed. We anticipate that this Review will provide valuable guidance for the rational design and development of next-generation multimodal CDs for bioimaging and photothermal applications.