Eu(III) Guests Sensitized by a Metal–Organic Framework for Sensing Cr(VI) in Strong Acids

Acidic industrial wastewaters pose challenges for applying coordination frameworks as sensors, e.g., for detecting toxic Cr(VI). For strong acids can not only impede the uptake of the analyte ions but also destroy the coordination scaffold itself. Here, we leverage a framework equipped with dense sulfonic group (−SO3H) arrays that anchor and sensitize Eu(III) guests to effectuate its strong fluorescence, achieving a recyclable probe with high selectivity and sensitivity for Cr6+ under harsh acidic conditions. Specifically, the metal–organic framework (MOF) solid ZrDMBD (2,5-dimercaptobenzenedicarboxylate) was H2O2-oxidized to form the sulfonate-equipped solid ZrDSBD (2,5-disulfoxylsulfobenzenedicarboxylate); the water-dispersible ZrDSBD readily takes up Eu(III) ions from water to form ZrDSBD-Eu, which offers excellent fluorescence stability for detecting CrO42– and Cr2O72– in the presence of many metal ions (e.g., Cr3+) and interfering anions. A limit of detection of 39 ppb was achieved, which is lower than the legal limit of 1 μM or 50 ppb for Cr6+ set by the World Health Organization in drinking water. This is the highest sensitivity reported of MOF materials for sensing Cr(VI) in strong acids (pH = 2 and below). The Cr(VI)-quenched ZrDSBD particles recover the fluorescence after a simple wash with water so as to be recycled for many rounds of sensing applications.