Carbon dots with tailor-made chelating ligands for specific metal ions recognition: Target synthesis and prediction of metal ions selectivity

Carbon dots (CDs) have been widely applied in metal ion recognition and detection, which makes them promising in the fields of environmental monitoring and biomedical science. However, CDs usually contain multiple functional groups that have different affinities for metal ions, so the relationship between CDs and recognized metal ions is ambiguous, which limits the prediction of recognized metal ions. Herein, CDs with different chelating ligands were target synthesized via free radical polymerization in combination of hydrothermal carbonization technology, the precursors directional converted into CDs. The tailor-made CDs with ethylenediamine tetraacetic acid (EDTA, EfCDs) and hydroxyethylidene diphosphonic acid (HEDP, HfCDs) chelating ligands were synthesized with high synthetic yields of 86.7% and 72.5%, respectively. The coordination constant (K) and the number of chelation site (n) of EfCDs for Fe³⁺ were (3.92 ± 0.03) × 10⁴ M⁻¹ and 3.62 ± 0.18, while the values of K and n of HfCDs for Cu²⁺ were (5.95 ± 0.04) × 10⁵ M⁻¹ and 1. EfCDs and HfCDs can serve as fluorescence probes for specific recognition of Fe³⁺ and Cu²⁺, respectively. Based on the metal-chelating affinity of EDTA and HEDP for Fe³⁺ and Cu²⁺ (lgK_EDTA-Fe = 25.1, lgK_HEDP-Cu = 12.5), EfCDs and HfCDs were predicted to specifically recognize Fe³⁺ and Cu²⁺. We established the exponential type functions between fluorescence quenching degree (1-F/F₀) and coordination constant (lgK) of EDTA or HEDP to facilitate the prediction of specific recognition of metal ions. This study proposes a new strategy for heavy metal ions specific recognition using CDs with tailor-made chelating ligands.