<p>The detection of metal ions such as Al³⁺, Cr³⁺, Fe³⁺, Pb²⁺, and Hg²⁺ is crucial due to their significant environmental and biologic impacts. While Fe³⁺ and Cr³⁺ are essential in trace amounts, Pb²⁺ and Hg²⁺ are highly toxic even at low concentrations, necessitating the development of rapid and sensitive detection methods. Based on this condition, a zinc porphyrin-based metal–organic framework (MOF), Chem_CUJ_MOF2, was synthesized via coordination polymerization of Fe²⁺ with Zn-metalated <i>meso</i>-tetra(<i>p</i>-carboxyphenyl)porphyrin (ZnTCPP). The resulting MOF was thoroughly characterized using UV–Visible spectroscopy, FTIR, PXRD, TGA, BET, and SEM–EDX analyses. Chem_CUJ_MOF2 exhibits strong fluorescence properties and acts as an efficient “turn-on” fluorescent probe for metal ion detection in aqueous media. The addition of trivalent ions (Al³⁺, Cr³⁺, and Fe³⁺) results in significant fluorescence enhancement, with up to a 23-fold increase for Cr³⁺ and ~ 13-fold increases for both Al³⁺ and Fe³⁺, demonstrating higher sensitivity toward M³⁺ ions compared to divalent ions (Pb²⁺ and Hg²⁺), which show relatively weaker responses. A linear relationship between relative intensity (I/I₀) at 607&#xa0;nm and metal ion concentration was observed in the 5–25&#xa0;μM range for Al³⁺, Cr³⁺, and Fe³⁺, with correlation coefficients (R²) of 0.9908, 0.9414, and 0.9818, respectively. The limits of detection (LOD) were found to be remarkably low, 1.41&#xa0;nM for Al³⁺, 3.51&#xa0;nM for Cr³⁺, and 1.91&#xa0;nM for Fe³⁺ demonstrating the exceptional sensitivity of Chem_CUJ_MOF2. In addition to fluorescence sensing, UV–Visible studies revealed similar spectral profiles for all tested metal ions with only minor variations in absorbance intensity. However, a distinct visual color change was observed exclusively in the presence of Fe³⁺, indicating a limited but noticeable colorimetric response. These results highlight the potential of Chem_CUJ_MOF2 as a high-performance “turn-on” fluorescent sensor for the rapid and quantitative detection of trivalent metal ions in environmental and analytical applications.</p>