Sewage sludge in microwave oven: A sustainable synthetic approach toward carbon dots for fluorescent sensing of para-Nitrophenol

Million tons of sewage sludge produced yearly creates a severe pollution problem to environment, and thus needs either to be properly disposed of, or recovered. Here, we demonstrate a value-added utilization of sewage sludge by converting its contained organics into nanosized carbon dots (CDs) with microwave irradiation. This synthetic method, using waste resources as precursors and avoiding the requirement of hazardous reagents and complex procedures, has the great advantage of low cost, environmental friendliness, and easy scalability. The resultant CDs exhibit excellent fluorescence properties with a large quantum yield (QY) of up to 21.7%, higher than the most values of waste-derived CDs. It is found that CDs can serve as a sensitive and selective sensor to detect para-Nitrophenol (p-NP), a toxic pollutant, through fluorescence quenching, giving a linear detection range of 0.2-20 μM and a detection limit of as low as 0.069 μM. Systematic investigations suggest that the inner filter effect (IFE) is the dominant sensing mechanism. Moreover, the practical applications of CDs for p-NP assay in real water samples achieve good results.