Fmoc-phenylalanine-based fluorimetric and colorimetric dual-channel probes for the detection of Fe3+, Cu2+ and F-

Organic fluorophores that can generate readable signals on interaction with various metal ions are extensively used for their detection and quantification. The inherent metal binding affinity of amino acids can be utilized in developing fluorescent metal ion sensors by connecting them with appropriate fluorophores. Such sensors reported thus far mainly used histidine and to a lesser extent, tyrosine, methionine, etc., due to the presence of side chains with metal binding propensity. Herein, we report four Fmoc-phenylalanine (Fmoc-Phe)-based metal ion sensors for the selective detection of Fe3+ and Cu2+. The general design of the probes consisted of Fmoc-Phe connected to naphthalene/pyrene units through amide or triazole linkers. The absorbance of the probes gradually enhanced with the addition of increasing amounts of Fe3+ and Cu2+ ions. The colorless probe solutions turned yellow under visible light with the addition of these metal ions allowing colorimetric detection. A regular decrease in the emission intensity was noticed on adding Fe3+ and Cu2+ ions to probe solutions suggesting a ‘turn-off’ response. The probes were highly selective towards these two metal ions with a response time of less than 60 s. The detection limit for both metal ions was in the lower micromolar levels (0.67 to 3.14 μM). The binding stoichiometry and quenching mechanism were analyzed using Job’s plot method and Stern-Volmer analysis. The probe-Fe3+ complexes were further used as a turn-on sensor to detect fluoride ions. We also demonstrated the real-time detection of Fe3+, Cu2+ and F- using the filter paper strips coated with the probes under UV light of 365 nm.