Helpful elements: A facile bottom-up method using citric acid and L-cysteine as a precursor has been developed to prepare graphene quantum dots (GQDs) co-doped with nitrogen and sulfur. A new type and high density of surface state of GQDs arises, leading to high yields (more than 70 %) and excitation-independent emission. FLQY = fluorescence quantum yield.

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...

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Novel organic-inorganic mesoporous luminescent hybrid materials were prepared by linking the binary and ternary Eu 3+ complexes to the functionalized ordered mesoporous SBA-15 with the modified 1-(2-naphthoyl)-3,3,3-trifluoroacetonate (NTA) by co-condensation of tetraethoxysilane (TEOS) in the presence of Pluronic P123 surfactant as a template. 1-(2-Naphthoyl)-3,3,3-trifluoroacetonate (NTA) grafted to the coupling agent 3-(triethoxysilyl)propyl isocyanate (TEPIC) was used as the precursor for the preparation of mesoporous materials. SBA-15 consisting of the highly luminescent ternary complex Eu(NTA) 3 bpy covalently bonded to the silica-based network, which was designated as Eu(NTA-SBA-15) 3 bpy, was obtained by introducing EuCl 3 and 2,2′-bipyridine (bpy) complex into the hybrid material via a ligand-exchange reaction. In addition, for comparison, SBA-15 doped with Eu(NTA) 3 ‚2H 2 O and Eu(NTA) 3 bpy complexes and SBA-15 covalently bonded with the binary Eu 3+ complex with NTA ligand were also synthesized, denoted as Eu(NTA) 3 /SBA-15, Eu-(NTA) 3 bpy/SBA-15, and Eu(NTA-SBA-15) 3 , respectively. The luminescence properties of these resulting materials were characterized in detail, and the results reveal that they all have high surface area, uniformity in the mesostructures, and crystallinity. The efficient intramolecular energy transfer in mesoporous material Eu(NTA-SBA-15) 3 bpy mainly occurs between the modified ligand NTA-Si and the central Eu 3+ ion. Furthermore, Eu(NTA-SBA-15) 3 bpy exhibited the characteristic emission of Eu 3+ ion under UV irradiation with higher 5 D 0 luminescence quantum efficiency than the pure Eu(NTA) 3 bpy complex and the other materials. Thermogravimetric analysis on the mesoporous material Eu(NTA-SBA-15) 3 bpy indicated that the thermal stability of the complex was improved as it was covalently bonded to the host SBA-15 matrix.