Fluorescence tunable thiophene-bis(benzimidazole)-based probes for a cascade trace detection of Hg2+ and lysine: A molecular switch mimic

Abstract

Six thiophene-bis(benzimidazole)-based molecular probes with tunable fluorescence emission were synthesized for selective Hg2+ detection. Probe 1 contains a thiophene-bis(benzimidazole) unit without an appendage, while probes 2-4 have multiple appendages on benzimidazole rings of the core and probe 6/7 has two biphenyl appendages on the thiophene ring. Fluorescence emissions of all these probes were selectively quenched upon Hg2+ addition, but probe 7 provided the most sensitive detection of Hg2+, giving the highest association constant (Ka (7) = 2.38 × 106 M−1) and the lowest observed and calculated limit of detections (LODs) (5 nM and 0.047 pM, respectively). Thus, the probe 7-Hg2+ complex was further tested for detection of a biomolecule as a next step. Among the various biomolecules tested (including amino acids), L-lysine (Lys) uniquely showed a full restoration of the fluorescence intensity of [probe 7+Hg2+ (1:2)]-complex to the level of the free probe, with an LOD of 5 nM. Thus, a cascade detection of Hg2+ and Lys was attained by probe 7via selective quenching and recovering of the fluorescence intensity, which enables us to build an “ON−OFF−ON” switch and logic gate at a molecular level. The current system was fully studied by various analytical methods such as UV–vis, fluorescence, and 1H NMR spectroscopies, IR, FE-SEM images, and DFT calculations.