Isophthalate and N‐Chelating Linkers Based Luminescent Cd‐MOF for ‘Turn‐On’ EtOH Sensing via RGB‐Assisted Smartphone Platform and ‘Turn‐Off’ Fe3+ Monitoring in Biodiesel Specimens

Monitoring trace solvent and metal adulteration in biodiesel is crucial for quality control and commercialization. This study explores a mesoporous Cd(II)‐MOF for dual‐mode fluorescence sensing: ‘turn‐on’ detection of EtOH and ‘turn‐off’ monitoring of Fe3+ in aqueous and biodiesel samples. Synthesized using N‐chelating, π‐conjugated 2,2'‐bipyridine and μ3‐η1:η2 bridging 5‐hydroxyisophthalic acid, the MOF exhibits high phase purity, lamellar rod morphology, and high thermal stability, attributed to its robust framework, reinforced by supramolecular H‐bonding and interlayer π–π stacking interactions. The MOF’s blue luminescence enables rapid detection of EtOH (6.27‐fold enhancement, LOD: 6.33 ppm, 40s response time) and Fe3+ (>90% quenching, LOD: ~1.17 µM, KSV: 1.318 × 105 M⁻1, 30s response time). The EtOH sensing ensues via photo‐induced electron transfer (PET) restriction in the MOF, aided by hydrogen bonding with MOF hydroxyls and polarity effects, while Fe3+ quenching arises from absorption competition quenching (ACQ) and electrostatic interactions. The sensor detects EtOH and Fe3+ in pre‐treated biodiesels derived from jatropha and waste cooking oil (recovery: 78–90% and 73–75%, respectively). Further, a 4.89‐fold ‘turn‐on’ for EtOH and ~81% quenching for Fe3+ was evidenced when analytes were directly added into untreated jatropha‐biodiesel. A smartphone‐based RGB calibration further enhances real‐time ethanol analysis, ensuring practical applicability.