From solutions to living systems: a colorimetric and "turn-on" chemosensor for Co²⁺ with paper-strip and bioimaging...

5-(4-Chlorophenoxy)-3-methyl-1-phenyl-1H-pyrazole-4-carbaldehyde (PYA), a pyrazole-based fluorescent chemosensor, was synthesised and studied for the selective detection of Co²⁺ ions. With exceptional selectivity over other metal ions that are relevant to biology and the environment, the sensor displays a clear "turn-on" fluorescence response towards Co²⁺. Strong complexation is confirmed by UV-visible and fluorescence titration tests, with a low detection limit of 25 nM. A 1 : 1 binding stoichiometry is revealed by Job's plot analysis, and the sensing performance is consistent over a broad pH range. By clarifying the electronic structural alterations and charge-transfer processes upon Co²⁺ binding, density functional theory (DFT) computations corroborate the experimental findings. Additionally, effective zebrafish bioimaging studies show that PYA can be used for in vivo Co²⁺ detection. These findings demonstrate that PYA is a useful, selective, and sensitive probe for tracking Co²⁺ ions in chemical and biological systems. Most importantly, the practical use of the sensor is demonstrated by the invention of paper strip devices, which enable fast, portable, and visible detection of Co²⁺ ions without the need for complicated apparatus.