Luminescent Metal–Organic Frameworks for pH Sensing: A Critical Overview from Molecular Design to Point-of-Care Diagnostics

Conventional pH probes suffer from mechanical fragility, dye leaching, aggregation-caused quenching, and single-intensity dependence. Luminescent metal–organic frameworks (LMOFs) address these limitations through structural tunability, multiple emission centers, permanent porosity, and ratiometric self-calibration. This review is organized around three axes: emission origin, signal output mode, and response mechanism. LMOF advantages are critically examined alongside limitations including guest leaching, limited reversibility, 40–60% sensitivity loss in complex matrices, and insufficient toxicity data. Analysis of over 60 systems reveals an inverse sensitivity–stability trade-off and establishes design rules across pH 1–14. Ratiometric mixed-lanthanide and dye@MOF architectures are highlighted for self-calibration, visible color changes, and smartphone readout. We identify four research directions: flexible wearables, smartphone diagnostics, machine learning-assisted discovery, and externally programmable MOFs. We propose the minimum reporting standard (MRS-MOF-pH) reporting standard and advocate for safe-by-design synthesis using endogenous metals (Zn2+, Mg2+, Fe3+). This review provides a unified design framework, an application-driven decision matrix, and a translational roadmap for point-of-care diagnostics, environmental monitoring, and food safety.