Tunable multicolour S/N co-doped carbon quantum dots synthesized from waste foam and application to detection of Cr³⁺...

In this study, by adjusting sulfuric acid concentrations, tunable multicolour S/N-carbon quantum dots (CQDs) were synthesized from waste foam as the raw material. The S/N-CQDs presented blue, blue-green, green, green-yellow and yellow emission with an emission peak shifting from 475 to 589 nm and with optimum excitation wavelengths of 385, 405, 440, 450, and 500 nm, respectively. Using transmission electron microscopy, the S/N-CQDs were seen to be spherical in morphology with a size around 6-8 nm. Fourier transform infrared spectra and X-ray photoelectron spectroscopy indicated that the surface of the S/N-CQDs was highly oxidized and sulfur doped. The fluorescence mechanism of multicolour S/N-CQDs emission was mainly related to a band gap change caused by the surface state. Blue-emitting S/N-CQDs were used as a fluorescent probe that was highly selective and sensitive to Cr³⁺ ions, with a low detection limit of 6 μM. The waste foam-derived S/N-CQDs exhibited promising potential for ion detection in real water samples due to its excellent fluorescence activity.