Metal–Ligand Ratio-Controlled Synthesis of Two Zn(II)–Eu(III) Coordination Polymers for Ratiometric Fluorescence Sensing of Sodium Salicylate

Sodium salicylate (SS) is widely used in medical and food applications closely related to daily life. Accurate detection of SS is crucial for human health and food safety. Herein, two novel Zn(II)-Eu(III) heterometallic coordination polymers (CPs) were synthesized and explored as ratiometric fluorescence sensors for sensing of SS. By controlling the metal-ligand ratios, two Zn(II)-Eu(III) CPs with distinct structures were constructed, namely, [Zn₂Eu₂(DCA)₁₀(phen)₂]<i>_n</i> (<b>1</b>) and {[Zn(phen)₃][Eu₄(DCA)₁₄]}<i>_n</i> (<b>2</b>) (DCA = 3,5-dichlorobenzoic acid, phen = 1,10-phenanthroline). <b>1</b> presents a zero-dimensional (0D) tetranuclear Zn₂Eu₂ cluster, while <b>2</b> displays an ion-pair with 0D [Zn(phen)₃]²⁺ and 1D [Eu₄(DCA)₁₄]^2- bicomponent. Both <b>1</b> and <b>2</b> were used as on-off ratiometric fluorescence sensors for sensing SS with simultaneous fluorescence of Eu(III) and SS. The limits of detection (LODs) are 1.73 and 1.37 μM, respectively. They can also effectively detect SS in milk. After the addition of SS, the color change from red to blue can be observed due to the synergistic effect of the inner filter effect (IFE) and photoinduced electron transfer (PET). Moreover, a smartphone-assisted platform based on RGB analysis was developed. Fluorescence films were also fabricated for the visual detection of SS, enhancing their practical application for the real-time detection of SS.