Pithecellobium dulce Leaf-Derived Carbon Dots for 4-Nitrophenol and Cr(VI) Detection

Luminescent carbon dots (CDs) synthesized from Pithecellobium dulce (P. Dulce) leaves, in a simple, single-step carbonization procedure, were used as optical nanosensors. TEM revealed the crystalline nature of the CDs with the average dimension of 20 nm with a quantum yield of 24%. In addition to carbon, the X-ray photoelectron spectroscopy shows the presence of oxygen and nitrogen. The FTIR spectra and Zeta potential were used for additional characterization of the nanoprobes. Among the contaminants and heavy metals, the proposed nanoprobes were found to be selective towards 4-nitrophenol (4-NP) and Cr(VI), respectively. The emission response of CDs towards 4-NP solution not only reveals the high sensitivity of the CDs (Limit of detection (LOD) of 14 nM) but also demonstrates a color change (light to dark yellow) that is attributed to spontaneous deprotonation detectable with the naked eye. The selectivity of CDs towards Cr(VI) (LOD 0.9 nM) was also tested in the presence of other metals. The quenching mechanism has been attributed to the inner filter effect for both analytes. The observed low detection limits in river and tap water opens up the possible applicability of the proposed nanoprobes as optical sensors in environmental pollution monitoring.