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

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% H 2 O‐MeOH mixture. Formation of a larger agglomerated hetero structure of LH was confirmed by FESEM image. This probe was found to detect Al 3+ 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 Al 3+ ion and cysteine got enhanced in surfactant environment, having very low detection limit of 2.75 nM (for Al 3+ ) and 2.65 nM (for cysteine). Upon binding to Al 3 ⁺, 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' )(NO 3 ) 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.