Colorimetric and ratiometric fluorescent response for anthrax bio-indicator: A combination of rare earth MOF and rhodamine-derived dye.

Bacillus anthracis spores have a unique biomarker of calcium dipicolinate (CaDPA). In this work, we reported a composite nanostructure for the optical sensing of DPA, with Eu (III)-doped metal-organic framework (MOF) as supporting lattice, a rhodamine-derived dye as sensing probe, respectively. By means of XRD, IR, TGA and photophysical analysis, this composite structure was carefully discussed. It was found that rhodamine absorption and emission were enhanced by DPA, while Eu emission was quenched by DPA. As a consequence, two sensing skills were observed from this composite structure, which are colorimetric sensing based on absorption spectra and ratiometric fluorescent sensing based on emission spectra. Linear sensing response was observed for both sensing channels with a warning signal at DPA concentration higher than 140 μM. Good selectivity was confirmed with a low LOD value of 0.52 μM. The sensing mechanism was revealed as the combination of emission turn-on effect triggered by DPA-released protons and emission turn-off effect originated from electron-transfer from EuBTC to DPA. This composite structure showed its advantage of naked eye detection and two sensing skills with linear response.