A europium metal-organic framework as a sensitive fluorescence turn-off sensor for 4-nitrophenol detection in water and soil

The detection of 4-nitrophenol (4-NP) is of significance for environment safety and human health. This work presented a 3D europium-based metal-organic framework (MOF) material, Eu-NDC, as a fluorescence turn-off sensor for monitoring 4-NP with high sensitivity, anti-interference, and durability. Its detection conditions were optimized as Eu-NDC concentration = 0.2 mg mL⁻¹, fluorescence response time = 2 min, and the solvent being water. The sensitive test revealed a linear fitting equation of (I₀–I)/I = 3.541 × 10⁴ M⁻¹C_4-NP with 4-NP concentration (C_4-NP) ranging 9.9–90.9 μM, whose limit of detection and limit of quantitation were calculated to be 0.170 and 0.565 μM, respectively. Fluorescence quenching efficiency of Eu-NDC is beyond 99 % as sensing 4- NP with the presence of common ions and five nitro-containing aromatic compounds in water, supporting a high anti-interference. A high durability can be demonstrated that the emission intensity at 618 nm of Eu-NDC can stay at least 90 % in the first five fluorescence quenching-regeneration cycles. The presented fluorescence assay is reliable for determining 4-NP in lake water, tap water and soil with all measured C_4-NPs close to the spiked C_4-NPs and all fluorescence recoveries around 100 %. The detection mechanism of Eu-NDC sensing 4-NP is interpreted as an inner filter effect.