Dual channel selective fluorescent detection of Al3+ and PPi in mixed aqueous media: DFT studies and cell imaging applications

Abstract

A new, easily synthesizable chemosensor, DFC-EN-p-Ph-NO₂, derived by the Schiff base condensation between 2,6-diformyl-p-cresol and N-(4-nitrophenyl)ethylenediamine, with potential N₄O donor atoms was found to act as a dual channel (colori- and fluorimetric) sensor towards Al³⁺ and PPi emitting at 486 nm (blue region) and 534 nm (green region), respectively in MeOH–H₂O (8 : 2, v/v) at pH 7.2 (10 mM HEPES buffer), μ = 0.05 M (LiCl), temperature 25 °C. The binding stoichiometries and formation constants of the sensor towards both Al³⁺ and PPi were determined by the combined UV-Vis and fluorescence titrations and Job's method, and confirmed by MS (m/z) studies. The corresponding detection limits as calculated by the 3σ method are: 7.55 μM and 3.34 μM. The most interesting part of this study is that on addition of 230 μM PPi to an ensemble of DFC-EN-p-Ph-NO₂−Al³⁺ (20 μM Ligand and 380 μM Al³⁺) the fluorescence is totally quenched but on further addition of PPi a new emission peak appears at 534 nm. All biologically relevant metal ions and toxic heavy metals did not interfere with the detection of Al³⁺ ion. Its bio-compatibility with respect to its good solubility in mixed organo-aqueous media (MeOH–H₂O) and cell permeability with no or negligible cytotoxicity provide good opportunities towards in vitro cell imaging studies of these ions. In particular, the fluorescent detection of PPi was not interfered by the presence of 400 μM of ATP or Pi although most reported PPi sensors that work in aqueous solution displayed cross-sensitivities toward ATP or Pi. The obvious excellent sensing capability of DFC-EN-p-Ph-NO₂ towards PPi and Al³⁺ was further scrutinized in HCT116 cell lines without much cytotoxicity. The modes of 1 : 1 binding of DFC-EN-p-Ph-NO₂ towards Al³⁺ and PPi were delineated by DFT calculations.