Fabrication of Carbon-Based Quantum Dots via a "Bottom-Up" Approach: Topology, Chirality, and Free Radical Processes in "Building Blocks".

The discovery of carbon-based quantum dots (CQDs) has allowed opportunities for fluorescence bioimaging, tumor diagnosis and treatment, and photo-/electro-catalysis. Nevertheless, in the existing reviews related to the "bottom-up" approaches, attention is mainly paid to the applications of CQDs but not the formation mechanism of CQDs, which mainly derived from the high complexities during the synthesis of CQDs. Among the various synthetic methods, using small molecules as "building blocks", the development of a "bottom-up" approach has promoted the structural design, modulation of the photoluminescence properties, and control of the interfacial properties of CQDs. On the other hand, many works have demonstrated the "building blocks"-dependent properties of CQDs. In this review, from one of the most important variables, the relationships among intrinsic properties of "building blocks" and photoluminescence properties of CQDs are summarized. The topology, chirality, and free radical process are selected as descriptors for the intrinsic properties of "building blocks". This review focuses on the induction and summary of recent research results from the "bottom-up" process. Moreover, several empirical rules pertaining thereto are also proposed.

Synergistic Effect of Oxygen- and Nitrogen-Containing Groups in Graphene Quantum Dots: Red Emitted Dual-Mode Magnetic Resonance Imaging Contrast Agents with High Relaxivity.

Contrast agents (CAs) in magnetic resonance imaging generally involve the dissociative Gd3+. Because of the limited ligancy of Gd3+, the balance between Gd3+ coordination stability (reducing the concentration of dissociative Gd3+) and increases in the number of coordination water molecules (enhancing the relaxivity) becomes crucial. Herein, the key factor of the synergistic effect between the O- and N-containing groups of graphene quantum dots for the structural design of CAs with both high relaxivity and low toxicity was obtained. The nitrogen-doped graphene quantum dots (NGQDs) with an O/N ratio of 0.4 were selected to construct high-relaxivity magnetic resonance imaging (MRI)-fluorescence dual-mode CAs. The coordination stability of Gd3+ can be increased through the synergetic coordination of O- and N-containing groups. The synergetic coordination of O- and N-containing groups can result in the short residency time of the water ligand and achieve high relaxivity. The resulting CAs (called NGQDs-Gd) exhibit a high relaxivity of 32.04 mM-1 s-1 at 114 µT. Meanwhile, the NGQDs-Gd also emit red fluorescence (614 nm), which can enable the MRI-fluorescence dual-mode imaging as the CAs. Moreover, the bio-toxicity and tumor-targeting behavior of NGQDs-Gd were also evaluated, and NGQDs-Gd show potential in MRI-fluorescence imaging in vivo.