Carbon dots: materials, synthesis, properties and approaches to long-wavelength and multicolor emission.

Herein, we summarize recent research developments and progress of carbon dots (CDs), which have been attracting considerable attention as a new type of photoluminescent material. Raw materials, from single carbonaceous compounds to colorful natural substances, for the synthesis of CDs are discussed. A range of diverse synthetic methodologies to achieve better photoluminescence performance and more advanced functions are summarized, and these are basically divided into two classes: top-down and bottom-up.

Synthesis of nitrogen-doping carbon dots with different photoluminescence properties by controlling the surface states.

Surface states of carbon dots (CDs) are critical to the photoemission properties of CDs. By carefully adjusting the reaction conditions in a hydrothermal synthesis route, we have prepared a series of CDs with excitation-dependent emission (EDE) and excitation-independent emission (EIE) properties by controlling the content of nitrogen elements, confirming that the characteristic optical properties of CDs originate from their energy levels. It has been found that surface-passivation of the as-prepared CDs by nitrogen doping can improve the emission efficiency and be beneficial to EIE features due to the single electron transition resulting from the single functional groups.

Germanium-doped carbon dots as a new type of fluorescent probe for visualizing the dynamic invasions of mercury(II) ions into cancer cells.

Carbon dots doped with germanium (GeCDs) were firstly prepared by a new simple 15 min carbonation synthesis route, exhibiting excitation-independent photoluminescence (PL), which could avoid autofluorescence in bioimaging applications. The as-prepared GeCDs have low cell toxicity, good biocompatibility, high intracellular delivery efficiency, stability and could be applied for detection of mercury(II) ions with excellent selectivity in complicated medium. It is to be noted that the as-prepared GeCDs used as a new type of probe for visualization of dynamic invasions of mercury(II) ions into Hep-2 cells display greatly different properties from most of the previously reported CDs which are regularly responsive to iron ions.