Pyrene-imidazole conjugate as a fluorescent sensor for the sequential detection of iron(III) and histidine in aqueous solution.

We developed PIM, a pyrene-based fluorescence sensor bearing an imidazole moiety and a carbonyl group as the binding sites for Fe3+ ions. The pyrene-based control compounds 1 and 2 were synthesized to demonstrate the structure-activity relationships. Compound 1, which contained a thiazoline moiety and a carbonyl group, displayed high selectivity for Cu2+ ions. This property indicated that heterocycles play an important role in the metal ion selectivity modulation. Compound 2, which lacked a carbonyl group, did not display metal ion selectivity. This characteristic demonstrated that introducing an additional recognition unit (cooperative recognition strategy) should be an effective way to improve metal ion selectivity. Furthermore, the PIM-Fe3+ ensemble can serve as a fluorescent sensor for histidine (His) detection via the removal of Fe3+ from the ensemble by His and the release of PIM. The sequential detection of Fe3+ and His exhibited on-off-on phenomenon, and the Fe3+ and His detection limits were 0.11 and 3.06 μM, respectively. These results will help in the further enhancement or modulation of metal ion selectivity in the development of fluorescent sensor systems. Moreover, the organic-metal ensemble provides an effective platform for detecting amino acids through the displacement strategy.