Quantum dots (QDs) are nanoparticles with size-dependent optical and electronic properties that have been proposed for various applications, such as energy-efficient displays and lighting, photovoltaic devices, and biological markers. [1][2][3][4][5][6] Compared with other fluorescent (FL) materials (conventional dyes, polymers, or propeins), QDs offer many promising advantages, such as bright fluorescence, high photostability, and resistance to metabolic degradation in bioapplications. [7][8][9] However, most high-performance QDs are limited by toxicity from their metal elements, such as cadmium. [10,11] Extensive efforts have thus been made on the development of non-or low-toxic FL nanomaterials as alternatives to the semiconductor-based QDs. Carbon-based dots (CDs), including carbon nanoparticles of less than 10 nm in size, [12] which are known as carbon quantum dots (CQDs) and graphene nanosheets of less than 100 nm in size, [13] which are known as graphene quantum dots (GQDs), are particularly encouraging owing to their outstanding optical properties, low toxicity, good biocompatibility, and robust chemical inertness. [6,12] Various methods have been demonstrated in preparation of FL CDs, such as electrochemical oxidation processes, [14,15] chemical oxidation methods, [6,12,[16][17][18] hydrothermal cutting strategies, [19,20] and carbonizing organics routes. [21][22][23][24] Nevertheless, most of the developed methods are unsatisfactory owing to expensive equipment required, low yields, or complicated procedures. In particular, most obtained CDs have a relatively low FL quantum yield (FLQY, usually less than 50 %) in comparison to the conventional semiconductor QDs. Most recently, doped CDs were proposed for highly FL dots. [25][26][27] For example, ZnS-doped CDs with the passivation of oligomeric poly(ethylenelycol) diamine (PEG1500N) molecules show a 78 % FLQY after a gel column fractionation. [26] However, the preparation of highly FL ZnS-doped CDs is complicated. Moreover, the poor chemical inertness of the ZnS would be a severe limitation to broad applications of the CDs. Thus, there is a great need to develop a facile, low-cost, and high-yield method for the preparation of CDs with strong FL emission. Herein, citric acid (CA) and l-cysteine were used to produce nitrogen and sulfur co-doped CDs (N,S-CDs) through a one-step hydrothermal treatment. The CA serves as the carbon source, while the l-cysteine provides nitrogen and sulfur. Compared with other reported CDs, the as-prepared N,S-CDs exhibit very high FLQY (73 %) and excitationindependent emission, resulting from the synergy effect of the doped nitrogen and sulfur atoms.The obtained N,S-CD solution exhibits a long-term homogeneous phase without any noticeable precipitation at room temperature. The transmission electron microscopy (TEM) image (Figure 1 a,g) shows that the size of the asprepared N,S-CDs is distributed in the range from 5 to 9 nm, with an average size of 7 nm. High-resolution TEM (HRTEM) images (Figure 1 b) reveal the high crystallin...