pH-enhanced red-emitting carbon dots for discrimination of Cu(II), Cr(III), and Fe(III) in acidic environments

The excellent luminescent properties of red-emitting carbon dots (RCDs) lead to their extensive utilization in the colorimetric sensing of heavy metal ions (HMIs). However, the luminescence of RCDs is typically quenched within extremely acidic environments. This represents a significant challenge to pursuing practical applications and must be addressed. Herein, N, S co-doped RCDs (NS-RCDs) were customized for H⁺ and HMIs sensing under various acidic environments. NS-RCDs exhibited visual pH sensing, with a pKa value of 4.99. Furthermore, visual detection of different metal ions was achieved by precisely controlling the pH of the NS-RCDs solution. At pH 6.0, Fe³⁺ and Cr³⁺ enhanced the red luminescence, while Cu²⁺ led to fluorescence quenching, with detection limits of 0.20, 0.90, and 0.12 μM, respectively. Notably, the established NS-RCDs assay enables the quantitative analysis of the three HMIs in actual water samples from rivers, tap water, and lakes. Additionally, at pH 3.0, only Fe³⁺ caused luminescence quenching, achieving visual Fe³⁺ detection with exclusive selectivity and a detection limit of 0.75 μM, with quantitative accuracy confirmed by ICP-MS. Finally, we investigated the optical sensing mechanisms of metal ions. This work demonstrates pH-tunable NS-RCDs for the exclusive detection of Fe³⁺ under extreme acidity (pH 3.0) and multiplex discrimination, validated in real water samples.