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 sp²/sp³ 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⁺, Sr²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, and Sn²⁺ by monitoring their PL intensity at an excitation wavelength of 320 nm. The metals inhibited the PL intensity in the order Ag⁺ > Fe²⁺, Fe³⁺, Ni²⁺ > Sr²⁺, Co²⁺, Cu²⁺, Sn²⁺, which demonstrated that the CDs exhibited high metal ion detection capability and selectivity. The detection of Fe³⁺ 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⁺, Sr²⁺, and Sn²⁺ induced charge transfer excitation and that Fe²⁺ and Ni²⁺ induced d-d transitions via complexation with the sp² clusters. The charge transfer excitation and d-d transitions hindered the π-π* transition of the sp² clusters, leading to a quenching effect. On the other hand, Li⁺, Na⁺, and K⁺ ions did not alter the π-π* transition of the sp² 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.