Carbon dots as multi-modal contrast agents: opportunities and open challenges for in vivo bioimaging

Carbon dots (CDs), a class of carbonaceous and fluorescent nanomaterials with ultra-small dimensions (below 10 nm), have become popular nanoparticles in several fields, such as nanomedicine, catalysis, sensing or electronics. Due to their chemical versatility, luminescence, surface functional groups, biocompatibility and water solubility, they have been extensively studied in biomedical field. In this review, we focused on CDs used as nano contrast agents (nCAs) for fluorescence, magnetic resonance or computed tomography imaging in small animal models. We described the advantages of using CDs, highlighting some relevant examples, as well as the current issues that slow down their effective use in biomedicine, such as the rational synthetic design, scant purification and poor characterization. Lastly, we gathered some perspectives to develop carbon dots with more reliable properties and accomplish cutting-edge applications.