Isophthalate and N‐Chelating Linker‐Based Luminescent Cd‐MOF for “Turn‐On” EtOH Sensing via RGB‐Assisted Smartphone Platform and “Turn‐Off” Fe 3+ 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,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, K_SV: 1.318 × 10⁵ M^{- 1}, 30 s response time). The EtOH sensing ensues via restriction of photo-induced electron transfer in the MOF, aided by hydrogen bonding with MOF hydroxyls and polarity effects, while Fe³⁺ quenching arises from absorption competition quenching and electrostatic interactions. The sensor detects EtOH and Fe³⁺ in pretreated 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⁺ is evidenced when analytes are directly added into untreated Jatropha biodiesel. A smartphone-based RGB calibration further enhances real-time ethanol analysis, ensuring practical applicability.