Photoluminescence quenching of thermally treated waste-derived carbon dots for selective metal ion sensing.

In the present study, carbon-dots (CDs) were derived from the thermal oxidation of an agricultural waste, bitter tea residue, to obtain different sp/sp ratios and electronic structures for metal sensing. The CDs obtained from calcination at 700 °C exhibited the highest photoluminescence (PL) quantum yield (QY) of 11.8% among all the samples treated at different temperatures. These CDs had a high degree of graphitization, which resulted in a strong π-π* electron transition, and hence in a high QY. The strong photoluminescence of the CDs could be used to sense the metal ions Ag, Sr, Fe, Fe, Co, Ni, Cu, and Sn by monitoring their PL intensity at an excitation wavelength of 320 nm. The metals inhibited the PL intensity in the order Ag > Fe, Fe, Ni > Sr, Co, Cu, Sn, which demonstrated that the CDs exhibited high metal ion detection capability and selectivity. The detection of Fe using CDs was performed in the range of 10-100 ppm with a LOD (limit of detection) value of 0.380 ppm. Theoretical calculations demonstrated that Ag, Sr, and Sn induced charge transfer excitation and that Fe and Ni induced d-d transitions via complexation with the sp clusters. The charge transfer excitation and d-d transitions hindered the π-π* transition of the sp clusters, leading to a quenching effect. On the other hand, Li, Na, and K ions did not alter the π-π* transition of the sp clusters, resulting in a negligible quenching effect. In summary, the oxidation level and electronic structure of CDs derived from bitter tea residue could be tailored, and the CDs were shown to be a facile, sustainable, and eco-friendly material for metal sensing.