Selective detection of mitochondrial Cu²⁺ in living cells by a near-infrared iridium(III) complex

The widespread use of copper (Cu) has raised concerns about environmental pollution and adverse effects on human health, highlighting the need to develop copper detection methods. Developing near-infrared (NIR) luminescent probes for imaging subcellular Cu²⁺ is still a challenge. In this work, we have developed a luminescence probe based on a NIR iridium(III) complex, which rapidly detects Cu²⁺ by combining salicylaldehyde and amine groups through a simple Schiff base reaction on the N^N ligand. The probe exhibits strong luminescence with a quantum yield of 0.66 and is able to detect Cu²⁺ with a limit of detection (LOD) of 0.31 μM, without interference from other metal ions. Mechanistic studies showed that the probe coordinated Cu²⁺ ions with a molar ratio of 1:1 and binding constant as low as 4.02 × 10^-2 μM, and operated through photoinduced electron transfer (PeT) for luminescence quenching. Importantly, the photostability experiments confirmed the desirable photostability of the probe in aqueous solution and in cellulo compared with a commercial organic dye. Furthermore, cellular imaging experiments demonstrated its capability for the visualization of Cu²⁺ in the mitochondria of living cells, which paves the way for the study of the subcellular distribution of Cu²⁺ and related toxicity analysis.