A chiral carbazole based sensor for sequential “on-off-on” fluorescence detection of Fe3+ and tryptophan/histidine

Abstract Quantitative analysis of the L-tryptophan content in commercial products by chemosensor has not been reported yet. In this study, the chiral carbazole based sensor (CCS), bearing a chiral “urea-carboxylic acid” binding site was prepared. CCS was proved to be effective in the consecutive detection of Fe3+, L-tryptophan (Trp), and L-histidine (His) by naked eyes. CCS exhibits different color changes and fluorescent ‘turn-off’ response toward Fe3+ and Fe2+. The discrimination of these two oxidation states of iron by CCS is important to understand the iron-related biological reactions. The limit of detection for Fe3+ by CCS is 84 nM in a high water-content solution (99 % water). CCS sensor can act as a reversible fluorescence quenching sensor for Fe3+. The results of Job’s plot analysis and mass spectra reveal that Fe3+ binds to CCS with a 1:1 stoichiometry. The CCS-Fe3+ complex can sense tryptophan, or histidine by a dose-dependent turn-on fluorescent response in a high water-content solution (99 % water). The mass spectra suggest the sensing of L-histidine or L-tryptophan by CCS-Fe3+ was performed with a CCS-Fe3+: amino acid binding ratio of 1:1. In comparison with other chemosensors reported in the literature, CCS can detect the L-Trp and L-His with a low limit of detection (0.31 μM L-Trp, 7.64 μM L-His) in a high water content environment. Furthermore, the CCS - Fe3+ complex based chemosensor was successfully applied to quantify the L-tryptophan content in commercial sleep-improving capsules containing various ingredients. The measured L-tryptophan contents in two samples are close to the announced values.