Nitrogen and sulfur co-doped carbon quantum dots as "on-off-on" fluorescence probes to detect Hg2+ and MnO4- and improving the photostability of Rhodamine B.

BACKGROUND: Carbon quantum dot (CQDs) are zero-dimensional carbon nanomaterials with a size of less than 10 nm CQDs are widely used in the field of ion detection by virtue of their fluorescence characteristics such as strong fluorescence intensity, good optical stability and tunable emission wavelength. Although the traditional atomic absorption method, electrochemical method and other metal ion detection methods are highly sensitive, the operation is complex, expensive and limited by the site. Therefore, we prepared the N, S-CQDs capable of detecting Hg2+ and MnO4- in water with the advantages of simple operation, low cost, and direct visual signal. RESULTS: N, S-CQDs with high-quantum yield (77.68%), uniform particle size (0.4 nm-2.6 nm) and green fluorescence were created utilizing a one-pot hydrothermal process with the precursors ASDA-Na4 and m-phenylenediamine. N, S-CQDs has good optical properties such as high fluorescence intensity, wavelength independence, up-conversion luminescence and fluorescence stability. We examined 27 common ions in water and found that the fluorescence of N, S-CQDs could be selectively quenched by Hg2+ and MnO4-, and the detection limits are 0.41 µM and 1.2 µM, respectively. The mechanism of quenching is further investigated. The fluorescence of N, S-CQDs-Hg2+ system can be restored by halogen ions (Cl-, Br-, I-), while the fluorescence of N, S-CQDs-MnO4- system can be partially restored by Fe2+. This forms an "on-off-on" mode of fluorescent probes. In addition, we also studied that trace amounts of N, S-CQDs can improve the photostability of RhB. SIGNIFICANCE: The N, S-CQDs are fluorescent probes in an "on-off-on" mode. N, S-CQDs with green fluorescence (on) can be quenched by Hg2+ and MnO4- (off). The fluorescence quenched by Hg2+ can be restored by halogen ions again, while the fluorescence quenched by MnO4- can partially be restored (on). This ion detection method can be used to visually detect the two ions in the field, with the advantages of low cost, simple operation and visual intuition.

Novel highly selective fluorescence sensing strategy for Mercury(â ¡) in water based on nitrogen-doped carbon quantum dots.

In this work, a fluorescent signal-closing probe of nitrogen-doped carbon quantum dots (NCQDs) was developed for quantitative detection of mercury ions (Hg2+). In this detection system, the NCQDs with high quantum yield (QY, 63.80 %) were synthesized via simple hydrothermal method with Methyl Glycine Diacetic acid Trisodium Salt (MGDA) and m-phenylenediamine (MPD) as carbon and nitrogen sources. The NCQDs have a typical surface structure and exceptional fluorescence stability, and their fluorescence zones are centered on excitation wavelengths of 440 nm and emission wavelengths of 510 nm. Under optimal conditions, the NCQDs have outstanding anti-interference ability to various ions and high selectivity to mercury ions. The fluorescence intensity of the detection system is weakened due to the generation of non-fluorescent groups caused by the static quenching effect. The fluorescence quenching efficiency shows a fascinating linear relationship with Hg2+ ions at 0-100 µM (y = 0.0051x-0.015, R2 = 0.9943), and the detection limit is 0.9 µM. Acute toxicity test shows that NCQDs have low toxicity and little harm to environment. The detection system can be used for the quantification of mercury ions in environmental water samples, and the recovery rate is between 99.64 % and 103.43 %, indicating that it is a simple and economical fluorescence detection method.

Preparation of nitrogen-doped carbon quantum dots from chelating agent and used as fluorescent probes for accurate detection of ClO- and Cr(â ¥).

In this study, a novel ionic sensor based on nitrogen-doped carbon quantum dots (N-CQDs) was developed for sensitive detection of hypochlorite ions (ClO-) and dichromate ions (Cr(Ⅵ)) by fluorescence spectrometry. The N-CQDs was synthesized by hydrothermal method using Methyl Glycine Diacetic acid Trisodium Salt (MGDA) and Ethylenediamine (EDA) with bright blue fluorescence, high fluorescence quantum yield, abundant surface groups and good dispersion. The N-CQDs had a remarkable emission characteristic at 450 nm under the ultraviolet light of 350 nm, and the ClO- and Cr(Ⅵ) ions could quantificationally quench the fluorescence of this emission band. The results showed that N-CQDs had broad linear detection range and the detection limits of trace ClO- and Cr(Ⅵ) ions are 5.0 µM and 2.1 µM, respectively. Subsequently, further verify the reliability of this study and the N-CQDs played an excellent role in the ion detection of actual water samples. The quenching mechanism of ClO- is proved to be dominated by static quenching, while the quenching mechanism of Cr(Ⅵ) is mainly due to inner filter effect. This study is envisioned to efficiently prepare N-CQDs with novel raw materials, provide enlightening insights for enriching the detection of various trace ions by CQDs, and open up a new way to use fluorescence characteristics for water detection.