Water-soluble, fluorescent carbon dots (CDs) have been successfully synthesized from waste paper. The as-prepared CDs, with an amorphous structure and small particle size, exhibit good photostability, high photoluminescence quantum yields (PLQYs) and fairly low toxicity, which indicate potential applications in the field of biolabelling.Due to its important role and increasing consumption in civilization processes, especially in writing and painting, large quantities of paper is manufactured each year, leading to increased amounts of waste paper. To date, waste paper is mainly recycled for paper manufacturing. However, its potential to be applied in material science has been ignored. As is known to all, the main components of paper are cellulose, hemicelluloses and lignin, which means that paper could act as a new source of carbonaceous materials, such as CDs.Ever since their discovery from the purification of crude nanotubes in 2004, 1 CDs have been attracting more and more attention due to their chemical inertness, excitation wavelength dependent photoluminescence (PL) behavior, optical stability, biocompatibility and low toxicity. 2-4 So far, a variety of methods have been developed to prepare fluorescent CDs, such as arc-discharge, laser ablation, 5 electrochemical oxidation, 6 and microwave heating, 7,8 etc. However, these approaches often suffer from the involvement of complex post-treatment processes, expensive carbon sources and experimental equipment, which severely restricts the practical applications of CDs. Not long ago, a simple and effective synthetic route was reported for the preparation of fluorescent CDs from waste paper ash. 9However, this method involved open flame, uncontrollable reaction conditions and products. Recently, there have been several interesting demonstrations of routes to prepare CDs using renewable carbon sources by green hydrothermal methods, which exhibited potential applications in various fields, such as biological nanomaterials, energy nanomaterials and other functionalized materials. 10-13 The hydrothermal method is a rising technique for the synthesis of reactants, due to the easy control of the reaction reactants, and low energy consumption. 14 In this work, fluorescent CDs were prepared by a hydrothermal method using waste paper as the carbon source. It was proved that the as-prepared CDs exhibit fairly high PLQYs, good water solubility, good particle size distribution and low cytotoxicity. It is worth mentioning that the reaction conditions and products can be controlled by this method, compared with previous reports. 9 A definite mechanism of the hydrothermal synthesis of carbon dots has not been explained, and the plausible reasons for this is that the mechanism is the carbonization of the major constituents of waste paper such as cellulose, hemicellulose and lignin. This work would pave the way for the recycling of waste paper and support the potential applications of CDs in bio-imaging.Briefly, waste paper was shredded and dispersed in deionized (DI) water, and so...
