Biowaste-Derived Nitrogen-Doped Carbon Dots for Real-Time Detection of Fe³⁺ and DFT Mechanistic Study

Nitrogen-doped carbon dots (N-CDs) are emerging as materials for advancing technologies in fields such as environmental analysis, biosensing, and fluorescent probes due to their outstanding fluorescence and excellent biofriendliness. The synthesis of N-CDs involved a facile hydrothermal treatment of agricultural waste bagasse and urea, which acted as the carbon and nitrogen source, respectively. The as-synthesized dots were thoroughly characterized to elucidate the structure and properties, demonstrating that the N-CDs possessed well-defined particle sizes, rich surface functional groups, and promising luminescent characteristics. Specifically, the measured quantum yield was 27.8%. N-CDs exhibit an immediate fluorescence quenching response toward Fe³⁺, and based on this property, a novel sensing platform for real-time Fe³⁺ detection was constructed, achieving a limit of detection (LOD) of 0.17 μM. Mechanistic investigations, including fluorescence lifetime decay analysis, temperature-dependent Stern-Volmer studies, and DFT simulations, collectively reveal that the quenching process is primarily governed by dynamic electron transfer from the excited state of N-CDs to the 3d orbital of Fe³⁺. This work not only presents a sustainable and effective fluorescent sensor for Fe³⁺ detection but also provides deeper insight into the photophysical interactions between heteroatom-doped carbon dots and metal ions.