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.

Abstract

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.