Anticounterfeiting Feature of a Writable and Self-Erasable Ni(II)-Metallogel Pad via Fluorescent "Turn-On" Detection of Cyanide.

A Schiff base 5-(2-hydroxy-3-methoxybenzylidieneamino)-1- H -imidazole-4-carboxamide ( HL ) comprising multibinding sites has been synthesized with the aim of fabricating a supramolecular gel. The gelator HL was characterized by FT-IR, 1 H & 13 C NMR, and ESI-MS techniques and also formed a [Ni(L) 2 ] complex. The gelation property of HL was investigated with various metal ions, wherein Ni(II) selectively forms a mechanically and thermally stable supramolecular metallogel ( MG ) in the presence of a triethylamine base in DMF-MeOH media. Characterization of MG was accomplished with different spectro-analytical techniques such as FT-IR, ESI-MS, powder-XRD, SEM, rheological investigations, UV/vis, and fluorescence. The gelator HL displays moderate emission upon addition of Ni 2+ and gives "turn-off" fluorescence output by forming the complex [Ni(L) 2 ] ( MG ) due to the chelation-enhanced quenching of fluorescence (CHEQ). Job plot and ESI-MS data suggested a 2:1 stoichiometry between HL and Ni(II) in MG . Further, MG exhibited highly selective and ultrasensitive " turn-on " fluorescence signaling with CN - in the background presence of several cations and anions. The limit of detection (LoD) of MG was determined to be 6.9 × 10 -9 M for CN - using the fluorescence technique. Notably, MG behaves as a fluorescent writable pad material explicitly with CN - under 365 nm UV light but not under ordinary light and the fluorescent text is self-erased after 15 min. Hence, MG can be used as a metallogel pad in the presence of CN - to communicate secret messages. Overall, the present work explores the fabrication of a thermo- and mechanostable Ni(II)-metallogel ( MG ), which selectively and ultrasensitively detects CN - both in the solution phase and in the gel form, wherein MG behaves as a writable and self-erasable pad with anticounterfeiting features for practical applications.