Solvent-Tuned Fluorometric Detection of Al3+ Ion and Cysteine by a Benzimidazole-Linked AIE Active Sensor in Solution and Solid Matrix

Abstract

The probe 6-(quinolin-2-yl)-5,6-dihydrobenzo[4,5]imidazo[1,2-c]quinazoline (LH) was synthesised and found to exhibit weak fluorescence in pure methanol but exhibited typical AIE phenomenon in 90% H₂O-MeOH mixture. Formation of a larger agglomerated hetero structure of LH was confirmed by FESEM image. This probe was found to detect Al³⁺ ion and cysteine in three aqueous solvents, viz., (i) MeOH/HEPES buffer (8:2, pH = 7.34, rt), (ii) water and (iii) aqueous CTAB medium. The sensitivity as well as selectivity of Al³⁺ ion and cysteine got enhanced in surfactant environment, having very low detection limit of 2.75 nM (for Al³⁺) and 2.65 nM (for cysteine). Upon binding to Al³⁺, cyclic bidentate LH was converted to tridentate L′, having an open structure. Cysteine converted LH to fluorescent L–L via formation of L^•, which was detected using EPR spectroscopy at 77 K. Molecular ion peaks due to cystine and L–L were found in ESI(-) mass spectrometry. Larger aggregates of LH formed through AIE activity in the presence of these two analytes were observed in FESEM and DLS experiments. Outcome of DFT/TDDFT calculation performed on LH, [Al(L')(NO₃)₃], and L–L supported experimental findings. On-site detection in solution and solid-state matrix, as well as reversible fluorometric outcomes, were formulated to construct a logic gate circuit.