A boronate hydrogel film containing organized two-component dyes as a multicolor fluorescent sensor for heavy metal ions in water

Abstract

A multicolor fluorescent hydrogel sensor containing two types of fluorescent dyes with an amidoquinoline receptor unit and a rhodamine chromophore covalently incorporated into a polyvinyl alcohol–boronate matrix was prepared. Although the gel film exhibited red emission due to the rhodamine chromophore, the addition of a small amount of Zn²⁺ ions induced a multicolor fluorescence response, which included white-light emission, of the film in water. The change in emission color is ascribed to fluorescence enhancement of the amidoquinoline moiety upon binding to Zn²⁺ ions with an association constant of (3.5 ± 0.2) × 10⁵ M⁻¹. In contrast, a negligible change in the emission color of the gel film was observed in the presence of Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Hg²⁺, Al³⁺, and Pb²⁺ ions. The selectivity of the gel film enabled visual detection of Zn²⁺ ions in tap water. Moreover, the film could be reused as a sensor strip after treatment with ethylenediaminetetraacetic acid.