Smartphone-assisted ratiometric fluorescence detection of sodium salicylate based on a dual-ligand Eu(III)-MOF

Sodium salicylate (SS) is a derivative of aspirin, which plays important roles for anti-inflammatory, it is also a preservative used in food. However, excessive intake of SS will pose a threat to human health. Therefore, accurate and portable detection of SS is crucial for human health and food safety.

Herein, a new dual-ligand Eu(III) metal-organic framework (MOF) was synthesized and applied as ratiometric fluorescence sensor for sensitive detection of SS, namely, {[Eu(1,5-NDS)_0.5(Pim)(H₂O)]₂·3H₂O}_n (MOF 1) (1,5-NDS^2- = 1,5-naphthalenedisulfonate, Pim^2- = pimelate). In the structure, the Eu(III) centers are bridged by sulfonic and carboxylic groups to form the one-dimensional (1D) [Eu(SO₃)(COO)₂]_n chains, which are further connected by Pim^2- and 1,5-NDS^2- to generate a 3D framework with a trinodal (4,4,6)-connected topology. Due to the inner filter effect (IFE) and photoinduced electron transfer (PET), the blue emission of SS at 431 nm is enchanced while the red emission of Eu(III) at 623 nm is quenched after addition of SS, thus resulting in the ratiometric fluorescence response. A good linear relationship between I₄₃₁/I₆₂₃ with the SS concentrations from 1 to 60 μM can be obtained with the detection limits of 51.74 nM. Remarkably, the smartphone-assisted RGB method was developed for quantitative detection of SS.

The established ratiometric fluorescence and RGB methods can be applied to detect SS in real honey samples. This work is expected to establish an convenient and reliable strategy for SS detection.