From Waste Polyethylene to Photophysical Probes: Mechanistic Investigation of Hg2+-Induced Fluorescence Quenching in Carbon Dots

Abstract

Understanding the photophysical origin of metal-ion-induced fluorescence quenching in carbon dots remains challenging because of their structural heterogeneity and complex surface chemistry. Herein, waste polyethylene-derived carbon dots (PECDs) were synthesized and employed as a model system to investigate Hg²⁺-induced fluorescence quenching through combined experimental and computational analyses. The fluorescent carbon dots were synthesized via a hydrothermal method and characterized by HR-TEM, XRD, FTIR, Raman, and XPS. This valorized material can detect mercury ions in the nanomolar range, by fluorescence quenching, with a limit of detection (LOD) of 31.1 nM (6.24 ppb). The fluorescence quenching is substantiated as a static quenching mechanism by fluorescence lifetime studies and ζ-potential analysis. Density functional theory (DFT) calculations were performed to model the PECD structure and evaluate the binding of Hg²⁺ and selected metal ions to different surface functional groups. Time-dependent DFT (TDDFT) analysis of the absorption and emission properties reproduces the experimental fluorescence trends and provides insights into the observed quenching behavior of the PECD–metal ion complexes.