Dye Immobilization Behavior of Polyallylamine Hydrogels

Abstract

A poly(allylamine) (PAA) hydrogel cross-linked with thiourea units was synthesized from PAA and CS2, and its ability to immobilize various pH indicators and a universal indicator was investigated. Owing to its cationic nature in water, the PAA hydrogel effectively immobilized anionic dyes through ionic interactions. All four tested pH indicators were successfully immobilized and exhibited reversible, indicator-specific color changes in aqueous media. The immobilization capacity varied depending on dye acidity and charge, demonstrating selective uptake of anionic dyes. The universal indicator containing multiple dyes was also immobilized, and the resulting hydrogel showed continuous color transitions over pH 3–9. MALDI-TOF MS analysis confirmed the incorporation of all dye components. Further, the universal indicator–loaded hydrogel showed acid-induced swelling, indicating coupled optical and mechanical responses. These results highlight the potential of PAA hydrogels as simple, reusable pH sensors and versatile stimuli-responsive devices.