Photoswitchable hemithioindigo inspired copper ion selective sensing with excellent selectivity and versatile operational modes

Copper is an essential element in living organisms and the monitoring of copper ions in aquatic environments and biological systems is of great significance. Inspired by a family of visible light responsive photoswitchable compounds, we report here a fluorescent probe (HTI-Q) for Cu²⁺ with excellent selectivity, high sensitivity, and a limit of detection (LOD) of ca. 0.02 μM. While the central carbon-carbon double bond of HTI-Q is photoisomerizable between the Z and E configurations, it is locked upon binding with Cu²⁺ into the E configuration, resulting in a large bathochromic shift (102 nm) and ratiometric fluorescence changes. Different sensing modes were demonstrated including in mixed solvents, a two-phase sensing system containing cation exchanger, and fluorescent nanoprobes containing a reference dye. The absorbance change of HTI-Q upon 470 nm light illumination also depended on the concentration of Cu²⁺. HTI-Q-based sensors were successfully applied to determine Cu²⁺ in real water samples with excellent recovery. Further, fluorescent nanoprobes incorporating HTI-Q were successfully applied to image endolysosomal Cu²⁺ changes upon external stimulation.