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Nitrogen doping can effectly adjust the compositions and structures of carbon dots and hence enhance their fluorescence. In this work, we report a fast and low-cost route for synthesis of nitrogen-doped carbon dots (N-CDs) by microwave pyrolysis of citric acid and ammonium within 7 min. The as-prepared N-CDs contain plentiful oxygen and nitrogen functional groups, and dispaly intense fluorescence with high quantum yield of ca. 44.3% and have an average size of 1.8 nm. The obtained N-CDs exhibit highly stable against photobleaching, ionic strengths, and can be used for selective and sensitive detection of Fe(III). It is postulated that the Fe 3+ -mediated fluorescence quenching is attributed to the charge transfer between N-CDs and Fe 3+ . In particular, the emission peaks from blue to red region can be tuned by interparticle distance of N-CDs, simply by increasing the concentration of N-CDs in aqueous solution, which indicates its potential applications as a promising optical image probe in multicolor cellular imaging. [ABSTRACT FROM AUTHOR]

Abstract: A dual‐emission ratiometric fluorescent sensing film for metal ion detection is designed. This dual‐emission film is successfully prepared from chitosan, graphitic carbon nitride (g‐C3N4), and gold nanoclusters (Au NCs). Here, it is shown that the g‐C3N4 not only serves as the fluorescence emission source, but also enhances the mechanical and thermal stability of the film. Meanwhile, the Au NCs are adsorbed on the surface of chitosan film by the electrostatic interaction. The as‐prepared dual‐emission film can selectively detect Cu2+, leading to the quench of red fluorescence of Au NCs, whereas the blue fluorescence from g‐C3N4 persists. The ratio of the two fluorescence intensities depends on the Cu2+ concentration and the fluorescence color changes from orange red to yellow, cyan, and finally to blue with increasing Cu2+ concentration. Thus, the as‐prepared dual‐emission film can be worked as ratiometric sensing paper for Cu2+ detection. Furthermore, the film shows high sensitivity and selectivity, with low limit of detection (LOD) (10 ppb). It is observed that this novel gold‐cluster‐based dual‐emission ratiometric fluorescent sensing paper is an easy and convenient way for detecting metal ions. It is believed that this research work have created another avenue for the detection of metal ions in the environment. [ABSTRACT FROM AUTHOR]

A series of dual-emission fluorescent probes was prepared from copper nanoclusters (Cu NCs) and carbon dots (CDs). They show two emission peaks (blue at 469 nm and red at 622 nm) when photoexcited at 365 nm. Upon exposure to sulfide, the Cu NCs will be deteriorated because they react with sulfide to form CuS. This results in the quenching of the red fluorescence of the Cu NCs, while the blue fluorescence of the CDs remains constant. Thus, the color of the nanocomposite changes from red to blue. The ratio of the fluorescences at the two wavelengths decreases linearly in the 2–10 ppb (26–128 nM) sulfide concentration range, and the limit of detection is 0.33 ppb (4.3 nM). The nanocomposite also was placed in an agar gel and then incorporated into a paper strip for fluorometric monitoring of gaseous hydrogen sulfide. Graphical abstract Schematic presentation of the synthesis of Cu NCs (copper nanoclusters)-CDs (carbon dots) dual-emission nano-assembly, Cu NCs-CDs-agar fluorescent film and their application for the detection of sulfide and H2S. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Visual and dual-emission-fluorometric sensor paper containing copper nanoclusters (CuNCs) and carbon dots (CDs) reported. • Sensing elements are uniformly and homogenously distributed over large area. • Linear relationship observed between fluorescence intensity ratio (red/blue) and humidity. • System shows promise for sensing of humidity and ethanol (which differ in color-change-trends). Abstract A novel visual and fluorometric-sensor paper is prepared for the monitoring of atmospheric humidity and ethanol. The paper contains a combination of copper nanoclusters (CuNCs) and carbon dots (CDs), immobilized on a filter paper. This makes it a dual-emission system. The visual sensor shows bright fluorescence and has a diameter of ∼8 cm with uniform and homogeneous surface. The detection mechanism of the sensing paper is based on the fluorescence intensity variation of CuNCs in different environments. Desirable linear relationship is observed between fluorescence intensity ratio (red/blue) and humidity. Visible color transformation of this sensor paper is observed in the humidity range ∼40-80%, implying utility for easy and facile atmospheric humidity determination. Furthermore, due to the water and ethanol have different impacts on the spectrum, this paper-based fluorescent probe shows excellent response to ethanol. Therefore dual-emitting sensing paper, when suitably calibrated, shows promise for visual and fluorometric sensing platforms aimed at humidity or ethanol detection. [ABSTRACT FROM AUTHOR]