Benzoindole-based bifunctional ratiometric turn-on sensor with an ICT effect for trapping of H⁺ and Al³⁺ in dual-channel cell imaging and samples

Abnormal changes in H⁺ and Al³⁺ concentrations in living cells can alter neurological diseases. A small-molecule sensor combined with a fluorescence imaging technique holds great promise for monitoring changes in proton and metal-ion concentration. In this work, a bifunctional ratiometric naked-eye fluorescence sensor (BIBC) was developed for turn-on detection of H⁺ and Al³⁺ in H₂O/EtOH (v/v = 1:1) mixtures. BIBC exhibits a pK_a value of 4.58 within a linear pH variation from 4.1 to 4.7 (R² = 0.9939). Moreover, the fluorescence intensity ratio (I_{566 nm}/I_{524 nm}) shows a good linear relationship (R² = 0.9965) within an Al³⁺ concentration range of 7.0-10.0 μM. The detection limit (DL) for the sensor was calculated to be 1.58 μM. The practical application of BIBC for Al³⁺ detection in real samples was further discussed, and satisfactory results were obtained. Furthermore, the sensor was applied to real-time visualization of changes in H⁺ and Al³⁺ concentration in living cells, with great photostability and low cytotoxicity observed. Fluorescence images of H⁺ and Al³⁺ were collected by using a fluorescence microscope in a dual-channel configuration, wherein they were labeled green and yellow, respectively.