Fluorescence On–Off Response of Hydrogen-Bonded Viologen-Polyoxometalate Supramolecular Structure for the Determination of Ag⁺ in Beverages and Daily Necessities

The remarkable sensitivity of viologen ligands to external stimuli leads to discoloration, which makes them suitable for visual detection. However, the photobleaching of the viologen ligand under prolonged light irradiation has always been a challenging issue. Herein, a novel viologen-polyoxometalate (POM)-based supramolecular compound, [(HMSBP)₂(β-SiMo₁₂O₄₀)]·2H₂O (<b>V-POM</b>) (MSBP = 1-(4-methanesulfonyl-benzyl)-[4,4']bipyridinyl-1-ium), is synthesized using a hydrothermal method. There are abundant hydrogen bonds in the units of <b>V-POM</b>, which facilitate the formation of a tightly packed two-dimensional (2D) reticular structure through a layer-by-layer assembly process. The 2D <b>V-POM</b> structure is rich in S═O and C═N─H, which makes it possible to detect Ag⁺ through coordination interaction (LOD = 26.7 nM). Moreover, <b>V-POM</b> exhibits satisfactory recovery rates in detecting Ag⁺ in various water sources, beverages, and daily necessities. The <b>V-POM</b>-enriched hydrogel is developed, which can quickly and accurately detect Ag⁺ in beverages and daily necessities by RGB and HSV analysis. Additionally, a fluorescent ink capable of dual information encryption is also created by <b>V-POM</b>. The supramolecular self-assembly of POM and a viologen ligand offers a novel approach to enhance fluorescence stability and visual detection.