Facile preparation and the stepwise formation mechanistic investigation of gram-scale nitrogen-doped graphene quantum dots

Although nitrogen-doped graphene quantum dots (N-doped GQDs) have been widely studied for their unique properties and promising applications, large scale synthesis of N-doped GQDs has been rarely reported. Besides, the formation mechanism was always explained simply. Herein we report a facile one-step gram-scale approach to prepare N-doped GQDs by polymerization of nitrilotriacetic acid (NTA) under high temperature and pressure in a nitrogen environment. A mechanism, which is rooted in the chemical properties of NTA and consists of dehydration and nucleophilic addition, has been proposed to explain the formation of N-doped GQDs step by step. Utilizing this method, the change in the structure, functional groups and FL properties can be explained intuitively and clearly. Because of the significant difference in ethanol solubility between the raw material and product, N-doped GQD powder with a purity higher than 99% can be easily obtained by a dissolution-centrifugation-drying-dissolution-freeze drying process without a tedious dialysis process. 1.38 grams of N-doped GQDs could be prepared from 3 grams of NTA. The 46% synthetic yield is higher than that obtained using a large majority of methods reported so far. With the introduction of an N atom, the N-doped GQDs showed bright blue fluorescence (FL) under 365 nm ultraviolet (UV) light, exhibiting a quite high quantum yield of 45.8%. A short preparation time, the low cost of the raw materials, a simple preparation process, high synthetic yield, high quantum yield and a facile purification method make this approach promising in the industrial production of N-doped GQDs. Besides, the N-doped GQDs were used as handwriting FL ink in the form of aqueous and oil-like ink, and used in a fountain pen and ball pen respectively. The words written with FL ink showed strong contrast under daylight and UV light, and no obvious change can be observed after long-term high temperature treatment, indicating that the N-doped GQDs have obvious practicability and good stability as an FL ink.