A nanosized metal–organic framework for visual detection of fluoride ions with smartphone via colorimetric test kit

Fluoride ion (F⁻) is an essential micronutrient in the human body, however, an abnormal large intake of F⁻ can result in adverse effects, such as skeletal fluorosis or DNA damage. Therefore, it is of great significance to develop convenient assays for the visual detection of F⁻ with high sensitivity. We rationally developed the nanosized iron-based metal organic frameworks (Fe-MOFs) by using 3,5-dicarboxybenzeneboronic acid as organic linkers and Fe³⁺ as metal ions through the solvothermal method, which can be applied for detecting F⁻ with high sensitivity and selectivity owing to the specific recognition of boronic acid groups towards F⁻ and the strong binding affinity between F⁻ and Fe³⁺. The Fe-MOFs possess excellent peroxidase-like activity, which can be effectively inhibited by F⁻. Moreover, a portable test kit for the determination of F⁻ was developed by embedding Fe-MOFs into hydrogel, and the visual measurement of F⁻ was realized based on smartphone by identifying the color signal of the oxidation of 3,3',5,5'-tetramethylbenzidine dihydrochloride. The output signal (R/G) was linear with F⁻ concentration in the range of 10.0–200.0 μM. The Fe-MOFs-based colorimetric strategy is easy-operation, low-cost and equipment-free, which has been further applied to detect F⁻ in the real water samples.