Fluorescence\nVisualization Quantitative Detection\nof Tetracycline and Nitrofurantoin in Food and Natural Water by Zn<sup>2+</sup>@Eu-bpdc Composite

Quantitative detection of tetracycline (TC) and nitrofurantoin\n(NFT) in food and water is of importance for food safety and environmental\nprotection. Herein, Zn<sup>2+</sup> was introduced into a europium\nmetal–organic framework Eu-bpdc (H<sub>2</sub>bpdc = 2,2′-bipyridyl-5,5′-dicarboxylic\nacid) to prepare a composite of Zn<sup>2+</sup>@Eu-bpdc, which was\ndeveloped as a fluorescence sensor for TC and NFT. The fluorescence\nmechanism concerns with bpdc<sup>2–</sup> ligand-to-Eu(III)\ncharge transfer, and the detection mechanism is the inner filter effect.\nZn<sup>2+</sup>@Eu-bpdc is a ratiometric fluorescence sensor for TC\nwith the linear fitting equation of <i>I</i><sub>520</sub>/<i>I</i><sub>618</sub> = 1.94 × 10<sup>4</sup> M<sup>–1</sup><i>C</i><sub>TC</sub>, whose limit of detection\n(LOD) is 0.148 μmol·L<sup>–1</sup> (μM); it\nis also a fluorescence “turn-off” sensor for NFT with\nthe fitting equation of (<i>I</i><sub>0</sub>–<i>I</i>)/<i>I</i> = 3.62 × 10<sup>4</sup> M<sup>–1</sup><i>C</i><sub>NFT</sub> and LOD = 0.0792\nμM. Zn<sup>2+</sup>@Eu-bpdc can detect TC or NFT in lake water,\nhoney, and milk with high accuracy. The emission color changes of\npaper-based Zn<sup>2+</sup>@Eu-bpdc depending on <i>C</i><sub>TC</sub> or <i>C</i><sub>NFT</sub> reveal the visualization\ndetections of TC and NFT. With the red and green values as input signals,\nsmartphone-assisted on-site detection is utilized to recognize the\nantibiotic residuals of TC and NFT by a self-programmed APP. Zn<sup>2+</sup>@Eu-bpdc is promising in a smartphone-assisted intelligent\nplatform for on-site detection of TC and NFT.