Aggregation-Induced Emission Active Benzidine-Pyridoxal Derived Scaffold for Detecting Fe3+ and pH.

Present work introduces an aggregation-induced emission (AIE) active Schiff base 4,4'-((1E,1'E)-([1,1'-biphenyl]-4,4'-diylbis(azaneylylidene))bis(methaneylylidene))bis(5-(hydroxymethyl)-2-methylpyridin-3-ol) (BNPY). Schiff base BNPY was synthesized by reacting benzidine with pyridoxal. The non-fluorescent BNPY in freely soluble DMSO medium showed a significant fluorescence enhancement at 563 nm (λex = 400 nm) upon increasing the water fraction (fw) in DMSO above 60% due to the restriction of intramolecular rotation upon the aggregation of BNPY. The AIE active BNPY was employed for the detection of metal ions in DMSO:H2O (fw = 70%). Upon the addition of Fe3+, the fluorescence emission of BNPY at 563 nm was quenched due to the chelation-enhanced fluorescence quenching (CHEQ). The Job's plot experiment supported the formation of a complex between BNPY and Fe3+ in 1:2 binding ratio. With an estimated detection limit of 5.6 × 10-7 M, BNPY was employed to detect and quantify Fe3+ ion in real water samples with satisfactory recovery percentages. Moreover, the pH studies of BNPY aggregates revealed three different fluorescence windows: non-fluorescent in acidic pH 2.02 to 3.16, yellow fluorescent between pH 3.60 to 9.33, and green fluorescent in basic pH 9.96 to 12.86.

Improving Copper(II) Sensitivity by Combined use of AIEE Active and Inactive Schiff Bases.

An aggregation-induced emission enhancement (AIEE) active Schiff base PNN was synthesized by condensing benzidine with 2-hydroxynaphthaldehyde. The green-fluorescent PNN (λem = 510 nm) in DMF turned to yellow-fluorescent PNN (λem = 557 nm) upon increasing the fractions of HEPES buffer (10 mM, pH 7.4) above 40%. The DLS study supports the self-aggregation of PNN that restricts the intramolecular rotation and activates the excited-state intramolecular proton transfer (ESIPT) process. The fluorescence emission of AIEE active PNN was quenched by Cu2+ with an estimated detection limit of 2.1 µM. Interestingly, the detection limit of PNN towards Cu2+ was improved in the presence of an AIEE inactive Schiff base PBPM obtained by reacting 1,4-diaminobenzene with pyridine-4-carbaldehyde. The mixed PNN-PBPM showed a detection limit of 0.49 µM. The practical utility of PNN-PBPM was validated by quantifying Cu2+ ions in real environmental water samples and green tea.

'AIE + ESIPT' Active 2-hydroxy-naphthalene Hydrazone for the Fluorescence Turn-on Sensing of Al3.

The aggregation-induced emission (AIE) behaviour of an easy-to-prepare Schiff base 2-hydroxy-naphthalene hydrazone (L) was explored in mixed DMSO/HEPES medium by selecting DMSO as a good solvent, whereas HEPES buffer (H2O, 10 mM, pH 7.4) as a poor solvent. The weakly fluorescent L in pure DMSO showed a fluorescence enhancement at 532 nm upon increasing the fraction of HEPES above 70% because of the self-aggregation of L and excited state intramolecular proton transfer (ESIPT) process. The AIE luminogen (AIEgen) L was applied for the sensing of metal ions in HEPES buffer (5% DMSO, 10 mM, pH 7.4). Among the fourteen different metal ions (Cu2+, Co2+, Ni2+, Mn2+, Mg2+, Fe3+, Fe2+, Zn2+, Cd2+, Hg2+, Pb2+, Al3+, Cr3+), AIEgen L showed a selective fluorescence enhancement at 435 nm in the presence of Al3+ without disturbing the fluorescence intensity at 532 nm due to the chelation-enhanced fluorescence effect (CHEF). The detection limit of 20 nM was estimated by performing the fluorescence titration of AIEgen L with Al3+. The reversibility of the Al3+ selective AIEgen L was demonstrated by adding a strong chelating agent EDTA. Finally, the practical utility of AIEgen L was validated by quantifying Al3+ in river and tap water samples.