Eu-MOF and its composites as turn-off fluorescence sensors for p-nitrophenol with applications in monitoring catalytic reduction reactions

Abstract

p-Nitrophenol (p-NP) is one of the toxic nitrophenol pollutants; detection methods for these pollutants are of importance. A new europium metal–organic framework (Eu-MOF) was prepared with 5-(4H-1,2,4-triazol-4-yl)isophthalic acid (H₂TIPA) and 2-aminoterephthalic acid (H₂NBDC). The XPS and TEM analyses support the distribution of Pd nanoparticles (Pd NPs) on the surface of Eu-MOF with a particle size of 5.7 ± 1.8 nm. MeOH solutions of Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Mn²⁺, Cl⁻, and SO₄²⁻ do not greatly affect the fluorescence quenching of Eu-MOF by p-NP. The sensitivity test shows a linear fitting equation of (I₀ − I)/I = 2.45 × 10⁵C_p-NP (C_p-NP = p-NP concentration) at C_p-NP = 1–10 with LOD = 80.1 nM. The detection mechanism is the inner filter effect based on a pseudo-first-order physical adsorption and a Freundlich model for the adsorption of p-NP by Eu-MOF and Pd@Eu-MOF@SA. The changes in the absorbance at 400 nm and the emission intensity at 450 nm occur synchronously with C_p-NP in the catalytic process of the reduction p-NP by Pd@Eu-MOF@SA. Pd@Eu-MOF@SA maintains high durability over eight adsorption–desorption cycles. Pd@Eu-MOF@SA acts as a fluorescent indicator for the catalytic reduction reaction of p-NP by NaBH₄ with an obvious emission color change. This work provides a new fluorescence sensor for p-NP detection and visualization monitoring for the reduction reaction of NaBH₄ and p-NP.