“…Given the high flexibility in the design of FRET probes and the wide choice of fluorophore available, FRET has been widely utilized for the design of ratiometric probes, such as for pH [29] , Fe 3+ [30] , Cu 2+ [31,32] , Hg 2+ [33] , H2S and sulfide/sulfite [34,35] . Notably, so far only a few FRET-based probes for the detection of Al 3+ have been reported [36][37][38] .…”