A smartphone integrated ratiometric fluorescent sensor for point-of-care testing of fluoride ions

In view of the high toxicity and widespread availability, it is of great importance to develop accurate, sensitive, and convenient assays for fluoride ion (F^-) detection. Herein, a ratiometric fluorescent system is established for point-of-care testing (POCT) of F^- with a smartphone as analyzer. The sensing system of calcein-QDs-Eu³⁺ contains two fluorescent probes of calcein (green emission) and ZnCdSe/ZnS QDs (red emission). The sensing system only presents red emission in that the calcein emission is quenched due to the combination between calcein and Eu³⁺. When F^- is introduced, the fluorescence of calcein is recovered due to the stronger interaction between F^- and Eu³⁺, which changes the emission from red to green. The ratiometric strategy offers an obvious fluorescence color change of the system, which eliminates interference and improves the detection accuracy. Specifically, the sensing system has excellent selectivity in that Eu³⁺ is more inclined to bind with F^- rather than other anions. The developed assay was further used to prepare a test paper and hydrogel for POCT. To further improve the detection sensitivity and realize quantitative analysis, a smartphone installed with a color scan app is integrated as signal reader and analyzer, which is used for POCT of F^- in real samples, showing great application potential in environmental protection and food safety evaluation.