Molecular engineering of ruthenium(ii) complexes with (3-polyamino)phenanthroline ligands for developing reusable optical sensors for Cu(ii) ions

Abstract

This work is aimed at the development of synthetic approaches to luminescent hybrid organic–inorganic materials based on chemosensors bearing phosphonate anchoring groups for the detection of toxic metal ions in aqueous solutions. To that end, the sensing properties of phosphonate-substituted ruthenium(II) complex with (3-polyamino)phenanthroline Ru(N₃P₃phen) toward toxic metal ions in aqueous media at physiological pH was investigated and compared to those of the known selective chemosensor Ru(N₂P₂phen) for copper(II) ions. The Ru(N₃P₃phen) complex enables us to detect copper(II) by luminescence measurements and is much more sensitive (limit of detection (LOD) is 0.02 μM) than Ru(N₂P₂phen). The main advantage of the chemosensors Ru(N₂P₂phen) and Ru(N₃P₃phen) studied herein is the possibility to immobilize them on titanium oxide surfaces without extra-synthetic costs using phosphonate substituents as anchoring groups. After activation with TMSBr, both complexes were successfully grafted on hydrated mesoporous TiO₂ (S_BET = 650 m² g⁻¹) performing reactions in DMF solutions at room temperature. Strong covalent binding of chemosensors Ru(N₂P₂phen) and Ru(N₃P₃phen) to the mesoporous TiO₂ support provides high chemical stability to the functionalized solids Ru(N₂P₂phen)@TiO₂ and Ru(N₃P₃phen)@TiO₂ but only one of them gives an optical response in the presence of copper(II) ions. The Ru(N₃P₃phen)@TiO₂ material thus prepared allows for the luminescent detection of cupric ions in aqueous solutions displaying LOD lower than 10⁻¹³ M. Such a high sensitivity is unprecedented and cannot be obtained using soluble chemosensors in solutions. This solid-state sensor can be regenerated and reused at least three times.