Nitrogen-Doped Carbon Quantum Dots: an Electrochemical Insight

Carbon quantum dots (CQDs) are considered to be a next-generation green multifunctional nanomaterial because of their advantages in terms of high chemical stability, broad excitation, low toxicity, flexible surficial functionalization, and good solubility. In comparison to fluorescent semiconductor quantum dots (QDs), which are usually composed of heavy metals as the essential elements to achieve high performance fluorescence, CQDs are advantageous both in their green synthesis and good biocompatibility for biomedical applications. So far, various methods have been explored to prepare the CQDs, which may be mainly grouped into two classifications: top-down synthetic route, such as laser ablation, electrochemical exfoliation, and arc discharge, and bottom-up synthetic method, just like ultrasonic-assisted route, microwave pyrolysis, and hydrothermal means. Doping with heteroatoms (e.g., oxygen, nitrogen, or sulfur) is one way of tuning the electronic and optical properties in CQDs. In particular, nitrogen doping is considered to be the most effective strategy at present because the small atomic size and rich valence electrons of nitrogen atom can bond with the carbon atoms. Moreover, nitrogen-doped CQDs can effectively adjust their electronic characteristics, surface and local chemical reactivity. In nitrogen-doped CQDs (N-CQDs), some of the carbon atoms at the core/edges of the CQDs honeycomb matrix are replaced by nitrogen, or N-containing groups reside at edges. The most common types of nitrogen atoms found in N-CQDs are graphitic, pyridinic, pyrrolic, and amino centers. In recently reported N-doped CQDs, pyridinic, pyrrolic, and amino N atoms were mainly located at edge sites, with pyridinic and pyrrolic N centers being more abundant. Recent studies have shown that N-CQDs display reversibly switchable "on-off" fluorescence via redox reactions and photoluminescence spectro-electrochemistry determinations provided direct in-situ evidence of the dependence of the N-CQDs luminescence and their redox state3b. By contrast, the electrochemical behavior of N-CQDs is considered quite complex and a detailed interpretation of their oxidation/reduction processes is less investigated. Herein, citric acid (CA) and urea were used as the precursors to synthetize luminescent nitrogen-doped CQDs by facile hydrothermal method, in which urea acted both as base and N-doping source. The resulted materials were characterized by UV-vis and FT-IR spectroscopies, X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS). Cyclic Voltammetry (CV) combined with UV-vis and XPS spectroctospies has been employed to give an interpretation of N-CQDs oxidation/reduction processes.