An europium functionalized carbon dot-based fluorescence test paper for visual and quantitative point-of-care testing of anthrax biomarker.

We reported Eu(III) functionalized carbon dots (CDs-Eu) as a novel ratiometric fluorescent probe for point-of-care testing (POCT) of dipicolinic acid (DPA), a biomarker of anthrax. The probe was synthesized simply by coordination interaction between Eu(III) ions and the CDs with carboxyl and amino groups. Upon addition of DPA, the intrinsic red luminescence of Eu(III) ions can be sensitized due to the presence of energy transfer when DPA chromophore coordinated with Eu(III) ions. The fluorescence peaking at 530 nm as a reference remained essentially constant, leading to ratiometric fluorescence response toward DPA. This approach exhibited a linearity range from 0.5 nM to 5 μM and a detection limit of 0.8 nM. More Importantly, the combination of probe-based test paper and smartphone provided a convenient approach for visual POCT of DPA with a detection limit of 67 nM, which was lower than other reported test paper for DPA detection. The color of fluorescence gradually changed from green to red as the DPA increases, which can be captured by a smartphone and further quantitatively analyzed with a built-in Color Analyzer App. The satisfactory result of detecting DPA accurately and reliably in real samples by the test paper certified its practicality in POCT of DPA.