Ratiometric fluorescence detection of melamine in milk by a zirconium-based metal-organic frameworks composite

Melamine has biological toxicity and is forbidden to be used in food processing and food additives. The long-term ingestion of melamine will cause renal malfunction and metabolic problems of the urinary system, so it is of great significance for the rapid and selective detection of melamine. Herein, a zirconium-based metal–organic frameworks (Zr-based MOFs) encapsulated tris(2,2-bipyridyl)ruthenium (II) chloride hexahydrate ([Ru(bpy)3]2+) composite, donated as UiO-66-NH2@Ru, was reported and further studied its sensing performance for melamine based on ratiometric fluorescent method. The UiO-66-NH2@Ru was prepared from ZrCl4, 2-amino-terephthalic acid (TPA-NH2) and [Ru(bpy)3]2+ by one pot reaction through solvothermal method, which exhibited dual emission signals at 445 and 595 nm from single excitation of 350 nm. The corresponding two peaks showed different responses to melamine, that was, the obvious enhancement at 445 nm and the slow decrease at 595 nm, thus achieving the “on–off” ratiometric fluorescence detection. When the concentration of melamine was in the range of 0.27 to 110 μM, the ratio of fluorescence intensity F445/F595 had a good linear relation with the correlation coefficient R2 = 0.995 and the detection limit of 90 nM. The synthetic UiO-66-NH2@Ru probe implemented rapid determination of melamine in real sample of milk powder with high sensitivity and selectivity. Compared with the reported carbon dots, quantum dots, metal nanoclusters, nanocomposites and other fluorescent sensors, MOFs-based sensor have broadened the application of nanomaterials in melamine detection for the first time. This study also demonstrates the Zr-based MOFs as a platform to encapsulate fluorescent dye for constructing ratiometric sensor in the determination of melamine is feasible and exciting.