Carbon dots (CDs) are relatively new and one of the most propitious nanomaterials ever known to humanity, primarily consisting of a carbonized carbon core with heteroatoms in organic functional groups attached. CDs show various fascinating properties, such as tunable excitation/emission, chemical inertness, photostability, low toxicity, good biocompatibility, ease of handling, and eco-friendliness. Due to the anomalous optical and chemical properties of the CDs, they have a wide range of applications in the fields of bioimaging, biosensing, photocatalysis, optoelectronics, etc. In this Review, we intend to cover the many strides in CDs chemistry, which is an emerging paradigm, in conjunction with the most recent discoveries of CDs with near-infrared fluorescence, phosphorescence, electroluminescence, chirality, and antibacterial activity. Our main emphasis will be on the contemporary evolution in synthetic strategies, optical properties, and biomedical applications of CDs in nanomedicine and nanotheranostics.

The pure and Al-doped NiO nanorod-flowers with uniform sizes and well-defined morphologies were synthesized for the first time by a facile solvothermal reaction. As the gas sensing materials of MOS gas sensors, their sensing properties were investigated systematically. The results indicated that the 2.15 at% Al-doped NiO nanorod-flowers showed improved gas sensing properties compared to those of pure NiO nanorod-flowers. The incorporation of Al ions with NiO nanocrystals adjusts the carrier concentration, and induces the change of the oxygen deficiency and chemisorbed oxygen of NiO nanorod-flowers. Thus, the doping of Al 3+ into NiO nanorod-flowers should be a promising method for designing and fabricating the high performance gas sensor.

Carbon dots (CDs), with excellent optical property and cytocompatibility, are an ideal class of nanomaterials applied in the field of biomedicine. However, the weak response of CDs in the near‐infrared (NIR) region impedes their practical applications. Here, UV–vis–NIR full‐range responsive fluorine and nitrogen doped CDs (N‐CDs‐F) are designed and synthesized that own a favorable donor‐π‐acceptor (D‐π‐A) configuration and exhibit excellent two‐photon (λex = 1060 nm), three‐photon (λex = 1600 nm), and four‐photon (λex = 2000 nm) excitation upconversion fluorescence. D‐π‐A‐conjugated CDs prepared by solvothermal synthesis under the assistance of ammonia fluoride are reported and are endowed with larger multiphoton absorption (MPA) cross sections (3PA: 9.55 × 10−80 cm6 s2 photon−2, 4PA: 6.32 × 10−80 cm8 s3 photon−3) than conventional organic compounds. Furthermore, the N‐CDs‐F show bright deep‐red to NIR fluorescence both in vitro and in vivo, and can even stain the nucleoli of tumor cells. A plausible mechanism is proposed on the basis of the strong inter‐dot and intra‐dot hydrogen bonds through NH···F that can facilitate the expanding of conjugated sp2 domains, and thus not only result in lower highest occupied molecular orbital‐lowest unoccupied molecular orbital energy level but also larger MPA cross sections than those of undoped CDs.

Hierarchical α-Fe2O3/NiO composites with a hollow nanostructure were synthesized by a facile hydrothermal method. The structures and morphologies of the composites were investigated by different kinds of techniques, including X-ray diffraction, field-emission electron scanning microscopy, transmission electron microscopy, and energy dispersive spectroscopy. Hierarchical α-Fe2O3/NiO composites were fabricated by growing the α-Fe2O3 nanorods on the surfaces of porous NiO nanosheets with a thickness of ∼12 nm. The gas sensing properties of hierarchical α-Fe2O3/NiO composites toward toluene were investigated using a static system. The response of α-Fe2O3/NiO composites to 100 ppm toluene was ∼18.68, which was 13.18 times higher than that of pure NiO at 300 °C. The enhanced response can be attributed to heterojunction. Meanwhile, the rapid response and recovery characteristics were observed because of the porous hollow structural characteristics and catalytic actions of α-Fe2O3 and NiO.