Hitting two birds with one stone: The synthesis of carbon quantum dots for optical detection of Fe³⁺ and ZnO microsphere optical coating through Zn-triethanolamine bridging polydentate ligand

The development of highly selective and sensitive fluorescence-based nanoprobes for monitoring Fe³⁺ concentration is important in environmental analyses. Herein, unique metal complex precursors involving triethanolamine (TEA) and zinc acetate Zn(OAc)₂ were designed to synthesize carbon quantum dots (CDs) via hydrothermal carbonization approach. The prepared water-soluble Zn-N codoped CDs are ∼2.9 nm in size and exhibit excitation-dependent blue emission and good photostability. Zn²⁺ ions, coordinated via Zn-TEA complexes, are incorporated into the carbon matrix and function as anchors to bridge and stabilize sp² domains. The CDs serve as an ideal platform for sensitive detection of Fe³⁺ in aqueous media through fluorescence quenching, with a detection limit of 3.48 μM, which is below the water quality standard for Fe³⁺ (0.3 ppm, ∼5.36 μM) in drinking water suggested by the U.S. Environmental Protection Agency (EPA). Interference studies reveal that the presence of 17 other competing ions does not alter the sensing of Fe³⁺, indicating the high selectivity of the CDs toward Fe³⁺, while the fluorescence quenching mechanism is attributed to the formation of non-fluorescent complexes between Fe³⁺ and surface functional groups of the CDs, leading to nonradiative recombination. Simultaneously, Zn high concentration favors the preparation of ZnO microsphere pigment composed of nanocrystals through TEA ligand modified bridge effect.