Tb-based Metal–Organic Framework-Referenced Fluorescence Assay for Distinguishing Hydroquinone from Its Isomers and Subsequent Quantitative Visual Detection of Cu2+

Hydroquinone (HQ) and copper ions (Cu²⁺) are categorized as environmental pollutants that are severely limited in water. Designing a selective assay for discriminating HQ from its two isomers and the convenient determination of Cu²⁺ is of great importance. Herein, a Tb-based metal–organic framework (Tb-MOF) and HQ are assembled innovatively into a ratiometric fluorescence nanoprobe to selectively distinguish HQ and subsequent quantitative visual detection of Cu²⁺. The native blue emission of HQ at 338 nm is used as a response signal, while Tb-MOF with green fluorescence offers a reference signal at 545 nm. Notably, neither resorcinol (RC) nor catechol (CC) exhibits obvious emission under the same experimental conditions, which enables discriminating HQ from its isomers. Thus, a ratiometric fluorescence method has been designed for the selective detection of HQ with the fluorescence intensity ratio F₃₃₈/F₅₄₅ as the readout. The redox reaction between HQ and Cu²⁺ induces fluorescence quenching of HQ and no change to that of Tb-MOF, resulting in a noticeable color variation from blue-green to green via the naked eye. Furthermore, sensitive visual detection of Cu²⁺ is achieved with a low detection limit of 1.67 μM using a smartphone. The satisfactory recoveries and good repeatability of quantitative visualization determined in spiked water samples make this sensing platform suitable for on-site monitoring of environmental samples.