Ratiometric red-emission fluorescence detection of Al³⁺ in pure aqueous solution and live cells by a fluorescent peptidyl probe using aggregation-induced emission

The development of a fluorescence method for the selective ratiometric detection of Al³⁺ ions in pure aqueous solutions and live cells is still a significant challenge. In the present study, we synthesized a new type of fluorescent probe using an Al³⁺-triggered self-assembly based on the dipeptide receptor and an aggregation-induced emission fluorophore. The fluorescent probe (1) bearing cyanostilbene with excitation by visible light detected Al³⁺ ions sensitively in pure aqueous buffered solution by ratiometric red-emission at 600 nm. 1 provided a highly selective ratiometric detection of Al³⁺ among 16 metal ions in aqueous solution. 1 exhibited sensitive ratiometric response to Al³⁺ in aqueous buffered solutions at pH ranging from 5 to 7.4. The detection limit (145 nM, R² = 0.999) for Al³⁺ ions in pure aqueous solution was much lower than the maximum allowable level of Al³⁺ in drinking water demanded by the Environmental Protection Agency (EPA). The probe provided an efficient approach to detect low concentrations of Al³⁺ in ground water, tap water, and live cells by ratiometric red-emissions at 600 nm. The binding study using dynamic light scattering, NMR, IR, and TEM revealed that the complex between 1 and Al³⁺ self-assembled to form nanoparticles, resulting in the enhancement of the emission at 600 nm and a concomitant decrease in the emission at 535 nm.