Research / 2021 / Article / Fig 7

Review Article

Afterglow Carbon Dots: From Fundamentals to Applications

Figure 7

(a) Schematic representation of the synthesis of self-protective afterglow CDs. In this strategy, a mixture of molecules or molecules and polymers was heated at high temperatures for carbonization synthesis of afterglow CDs. The unique structural characteristic of the resulting CDs is the presence of a layer of highly cross-linked polymer-like chains on the surface of the graphitized core. The surface layer can serve as a protective matrix to stabilize excited triplet species of CDs due to the existence of covalent and hydrogen bonding between the polymer-like chains. (b) TEM image of self-protective afterglow CDs after being dispersed in ethanol. Note that the CDs were prepared by the carbonization of PVA and ethylene diamine under hydrothermal conditions (200°C, 8 h). Inset: sheet-like CD aggregates. (c) Schematic illustration of a stabilization effect of the excited triplet states by the surface polymer-like chains (adapted and copyright permission [91] (b, c), Royal Society of Chemistry). (d) Photographs of self-protective afterglow CDs at different delay times after the removal of UV excitation. Note that PCDi-1, PCDi-2, and PCDi-3 were prepared with the use of mixtures of PAA and EDA, PAA and EAM, and PAA and ethylene glycol as precursors, respectively (adapted and copyright permission [45], Wiley-VCH Verlag GmbH & Co. KGaA). (e) Schematic representation of the preparation of self-protective afterglow CDs via a microwave-assisted heating method. (f) Photographs of self-protective afterglow CDs under daylight, UV excitation on, and UV excitation off. (g) Phosphorescence profiles of the CD powder as a function of excitation wavelengths and the excitation profile at the phosphorescence emission of 535 nm. (h) The decay curve of the phosphorescence of CDs at 535 nm (adapted and copyright permission [30] (e–h), Wiley-VCH Verlag GmbH & Co. KGaA). (i) Photographs of self-protective afterglow CDs under daylight, UV excitation on, and UV excitation off at different delay times. Note that the afterglow CDs were prepared by microwave irradiation of a mixture of L-aspartic acid (AA) in the presence of ammonia (adapted and copyright permission [92], MDPI).