Zn-assisted modification of the chemical structure of N-doped carbon dots and their enhanced quantum yield and photostability.

This article presents the Zn-assisted synthesis of N-doped carbon dots (N-CDs) with an enhanced quantum yield (QY) and photostability. There have been intensive studies to improve or tune the optical properties of carbon dots (CDs) to meet the demand for luminescent materials in various fields, including energy conversion, photocatalysis, bioimaging, and phototherapy. For these applications, the photostability of the CDs is also a critical factor, but the related studies are relatively less common. The Zn-assisted N-CDs (denoted as Zn:N-CDs) obtained by the addition of Zn(OAc)2 to the precursors during the synthesis of N-CDs not only exhibited an enhanced quantum yield but also improved photostability compared to those of N-CDs. A comprehensive study of the chemical composition of Zn:N-CD and N-CD using X-ray photoelectron spectroscopy indicated a correlation between their chemical structure and photostability. Zn(OAc)2, which acts as a catalytic reagent, induced the modification of chemical structures at the edges of carbogenic sp2 domains, without being doped in N-CD, and the heteroatom-carbon bonds in Zn:N-CD seemed to be more resistant to light compared to those in N-CDs. The increased QY and photostability of Zn:N-CDs make them more suitable as an optical probe and they could be used in fingerprint identification. With Zn:N-CDs, the microstructure of fingerprints was confirmed clearly for a long duration effectively.

Effect of heteroatoms on the optical properties and enzymatic activity of N-doped carbon dots.

Carbon dots (CDs) are attractive nanomaterials because of their facile synthesis, biocompatibility, superior physicochemical properties, and low cost of their precursors. Recent advances in CDs have particularly relied on the modulation of their properties by heteroatom doping (e.g., nitrogen). Although nitrogen-doped CDs (N-CDs) have attracted considerable attention owing to their different properties compared to those of the original CDs, the effects of the heteroatom content and types of bonding on the properties of N-doped CDs remain underexplored. In this work, we prepared N-CDs with controlled nitrogen contents, and fully examined their optical properties, enzymatic activity, and toxicity. We demonstrate that (i) the type of carbon-heteroatom bonding (i.e., carbon-oxygen and carbon-nitrogen bonds) can be altered by changing the ratio of carbon to heteroatom sources, and (ii) both the heteroatom content and the heteroatom-bonding character significantly influence the properties of the doped CDs. Notably, N-CDs exhibited higher quantum yields and peroxidase-like activities than the non-doped CDs. Furthermore, the negatively charged N-CDs exhibited negligible cytotoxicity. Such comprehensive investigations on the physicochemical properties of N-CDs are expected to guide the design of N-CDs for targeted applications.