Carbon quantum dots (CQDs) were synthesized via a hydrothermal method, in which extracellular polymeric substance (EPS) from anaerobic ammonium oxidation (anammox) granular sludge was used as a carbon precursor, while citric acid and ethylenediamine were applied as auxiliary carbon source and passivation agent, respectively. The synthesized CQDs, with orderly spherical shape and mean size of 7.15 nm, emitted blue fluorescent light under UV radiation of 365 nm. The CQDs had a high fluorescence yield (40.84%), with good water solubility and excellent spectroscopic properties. In addition, the CQDs exhibited selective, sensitive and distinctive fluorescence quenching behaviors for Cr(VI) and Mn(VII) in a PBS buffer solution (NaH₂PO₄–Na₂HPO₄) of pH 7, with a detection limit of 5.8 nM for Cr(VI) and 2.3 nM for Mn(VII). Owing to the nitrogen components from the EPS of anammox granules, the CQDs were well nitrogen-doped, promoting electron-transfer and leading to reduction between the CQDs and Mn(VII)/Cr(VI). These results indicate that CQD-based chemical sensing is a simple and efficient means for the fluorescence detection of Mn(VII) and Cr(VI).

A fast responsive and highly selective mercury(II) sensor was developed using the fluorescence quenching mechanism of 2,9-dimethyl-1,10-phenanthroline (2,9-DMP) towards mercury(II). Furthermore, two kinds of metal–organic frameworks (MOFs) of 2,9-DMP–Hg(II) have been synthesized and characterized by X-ray diffraction. The probe 2,9-DMP exhibited a fast and highly selective response to mercury(II) ions in aqueous solution, showing a good linear relationship over the concentration range of 0.050–2.0 μM for Hg(II) with R² = 0.9994. The established procedure was used to monitor the concentration of mercury(II) in drinking water and Zhujiang River water samples with satisfactory recoveries of 85.42–101.5% and 81.72–96.09%, respectively. Furthermore, the fluorescence quenching mechanism of 2,9-DMP by mercury(II) was investigated and clarified by the data of single crystal X-ray diffraction of MOFs and the Job's plot analyses of 2,9-DMP towards Hg(II).