It seems that the fluorescence enhancement was derived from the more widespread formation of the π-conjugation system in PSI upon metal binding, and the fluorescence was sensitive to specific metal ions. The selectivity for Al3+ with 1 μM PSI was plotted as a bar graph in Figure 2b. Only Al3+ resulted in a pronounced fluorescence enhancement relative to the control ions. When the PSI sensor was titrated with Al3+, the fluorescence
intensity increased up to 13 equiv and then no changes were observed up to 27 equiv (Figure 3)