Benzimidazole–Piperazine–Coumarin/Cucurbit[7]uril Supramolecular Photoinduced Electron Transfer Fluorochromes for Detection of Carnosol by Stimuli-Responsive Dye Displacement and pK_a Tuning

Abstract

A new fluorescent dye (4PBZC) comprising coumarin (C), piperazine (P), and benzimidazole (BZ) was designed, prepared, and complexed to cucurbit[7]­uril (CB7) to detect carnosol (CAR), an anti-breast cancer drug, in sub-nanomolar concentrations utilizing the supramolecular indicator displacement assay strategy, the CB7-assisted pK a shift, and the CB7-retarded photoinduced electron transfer process. The host–guest complexation was confirmed by UV–visible absorption, fluorescence, and 1H NMR spectroscopy, which established the binding of 4PBZC to CB7. CB7 preferentially binds the indicator dye (4PBZC) via the protonated BZ residue compared to the neutral BZ one, demonstrated by a higher binding constant of the complex in its di-protonated form, which led to an increase in the pK a of the BZ moiety by ca. 3.0 units after the addition of CB7. In aqueous solution (pH 6), switching the emission signals between 4PBZH+C/CB7 (ON state) and 4PBZC (OFF state) was achieved by displacement of the protonated dye from the cavity of CB7 by the CAR analyte. An efficient sensor was obtained for the sensitive detection of CAR in aqueous solution with a low detection limit of 0.148 ng/mL (0.45 nM) and a linear range from 20 to 627 ng/mL.

Separation of mercuric ions using 2-thienylbenzimidazole/cucurbit[7]uril/iron-oxide nanoparticles by pH control

“…The CB7-induced p K a shifts are unsurprising and agree with previous reports on other benzimidazole derivatives 14 , 24 – 27 . To ensure a controlled release and retention of the guest from the host cavity, the binding affinity of the protonated and neutral forms of TBI to CB7 must be different.…”

“…S1 in the Supporting Information). Previous reports on other benzimidazole derivatives demonstrated similar host-induced shifts on account for the shielding of the protons of the benzimidazole guest molecules when encapsulated in the electron-donating cavities of CB7 (e.g., fuberidazole, where the sulfur atom is replaced by oxygen) 14 , 24 – 27 . Two pH values, pH 2 and 7, were selected for the binding experiments (Fig.…”