Dual-mode Cu2+ and Fe3+ sensing using Na2SO4 enhanced red-emissive carbon dots

Dual-mode detection of environmentally relevant metal ions remains a significant challenge using highly stable and sensitive red-emissive carbon dots. This work presents a one-pot synthesis of red-emitting carbon dots (CDs) using p-phenylenediamine and Na 2 SO 4 . Highly stable red photoluminescence (650 nm) was observed via the synergistic effect of Na + and SO 4 2- ions, in which the CDs revealed a photoluminescence quantum yield of 45.9% in ethanol. The CD sample prepared using optimized conditions (sample 2-CDs) served as a versatile probe for the dual-mode Cu 2+ and Fe 3+ detecting in water. Colorimetry detection was finished in solutions, by smartphone, and in filter paper. As for Cu 2+ , the detection limits reached 0.013 μM (fluorometry) and 1.65 nM (smartphone colorimetry). A portable test paper based on 2-CDs achieved a detection limit of 0.517 μM for Cu 2+ . In the case of Fe 3+ , the corresponding detection limits were 1.5 μM (fluorometry) and 76.9 nM (colorimetry). Fluorescent degradation kinetics by Cu and Fe ions were discussed in detail. The sulfate ions form a multifunctional protective layer that enhanced hydrophilicity, passivated surface defects, and resulted in the CDs with remarkable photothermal stability (retaining the initial intensity of 93% after 120 min of ultraviolet/boiling) and long-term storage stability (47.2% retention after 300 days). These results supplied a novel strategy for creating high performance red-emissive CDs, paving the way for their practical application in on-site environmental monitoring.