Multifunctional POM-based inorganic-organic hybrids for proton conduction and selective sensing of Fe<sup>3+</sup> ions

Polyoxometalates (POMs), famous for their structural diversity and versatile properties, offer an attractive platform for constituting multifunctional materials through covalent assembly with lanthanide (Ln) ions, integrating optical, electrical, and magnetic functionalities. Herein, four three-dimensional (3D) POM-based inorganic-organic hybrids were successfully synthesized by hydrothermal method, [Ln₄(HIDC)₄(H₂O)₈(HBW₁₂O₄₀)]·2H₂O (Ln = La (<bold>1</bold>), Ce (<bold>2</bold>); H₃IDC = 4,5-imidazoledicarboxylic acid) and [Ln₄(HIDC)₄(H₂O)₆(HBW₁₂O₄₀)]·8H₂O (Ln = Tm (<bold>3</bold>), Eu (<bold>4</bold>)). Among these hybrids, compound <bold>4</bold> emerges as a bifunctional material, exhibiting both proton conduction and fluorescence sensing capabilities. For proton conduction, compound <bold>4</bold> achieves a conductivity of 1.53 × 10^‒⁵S·cm^‒¹ at <sc>65 °C</sc> and 95% relative humidity. To enhance conductivity and practical applicability, compound <bold>4</bold> was incorporated into a polyvinyl alcohol/glycerol (PVA/Gly) matrix, yielding the hydrogel Eu@PVA/Gly-15%. This hydrogel displays remarkable proton conductivity (1.01 × 10^‒² S·cm^‒¹ at <sc>75 °C</sc> and ambient humidity) and flexibility, enabling the assembly of strain sensors with high sensitivity for detecting human joint movements. Simultaneously, compound <bold>4</bold> exhibits excellent aqueous stability and characteristic Eu³⁺ luminescence, facilitating selective detection of Fe³⁺ ions in aqueous media <italic>via</italic> fluorescence quenching. This work shows the versatility of POM-Ln hybrids in developing advanced materials for flexible electronics and chemical sensing.