Dual-mode luminescence and colorimetric sensing for Al3+ and Fe2+/Fe3+ ions in water using a zinc coordination polymer.

A zinc(II) coordination polymer, [Zn(H₂dhtp)(2,2'-bpy)(H₂O)]_n (1), has been utilized as a dual-mode luminescence-colorimetric sensor (H₂dhtp^2- = 2,5-dihydroxy terephthalate and 2,2'-bpy = 2,2'-bipyridine). The presence of hydroxyl groups in H₂dhtp^2- can promote excited-state intra- and intermolecular proton transfer (ESIPT) phenomena. Therefore, compound 1, which displays high stability in aqueous environments, exhibits a strong green-yellow photoluminescence. This luminescence signal can be considerably enhanced and blue-shifted upon the addition of Al³⁺ ions with a limit of detection (LOD) of 0.15 μM, and it demonstrates significant resistance to interference from several competing metal ions. To demonstrate a practical application, 1@paper strips were fabricated that can visually detect the Al³⁺ ion under a UV lamp. Moreover, 1 can detect either Fe²⁺ or Fe³⁺ ions in aqueous solutions by a visible color shift. Upon the incremental addition of Fe²⁺ or Fe³⁺ ions, the solution color changed from colorless to pink, exhibiting a pronounced absorption band at around 521 nm. The LODs were determined to be 1.55 and 0.34 μM for Fe²⁺ and Fe³⁺, respectively. Furthermore, compound 1 was used for the determination of Fe³⁺ ions in the real water samples, which can be evaluated on-site in real-time via a smartphone color-scanning application. The detection efficacy of 1 toward Al³⁺ and Fe²⁺/Fe³⁺ maintains significant luminescence stability and reusability.