Europium(III)-Modified Silver Nanoparticles as Ratiometric Colorimetric and Fluorescent Dual-Mode Probes for Selective Detection of Dipicolinic Acid in Bacterial Spores and Lake Waters

Detection of Bacillus anthracis spores is of great importance for biosecurity and prevention of disease outbreaks. In this paper, a ratiometric colorimetric and fluorescent dual-mode platform for rapid, sensitive, and visual detection of dipicolinic acid (DPA), a biomarker of anthrax spores, was developed based on Eu3+-modified silver nanoparticles (AgNPs). Eu3+-modified AgNPs can induce the aggregation of AgNPs, its localized surface plasmon resonance (LSPR) absorption at 392 nm is decreased, and a new absorption peak at 524 nm emerged. Interestingly, upon the addition of DPA, it can effectively inhibit the aggregation of AgNPs–Eu3+, and the solution color changes from pink to yellow, while the red characteristic fluorescence of Eu3+ is turned on owing to the energy transfer from DPA to Eu3+ by the antenna effect. A new insight that AgNPs can also enhance the fluorescence of Eu3+ was found. Eu3+-modified AgNPs as a dual-mode platform for ratiometric colorimetric and fluorescence detection of DPA were thus fabricated. The good linearity is in the range of 1.0–25 μM with a LOD of 0.31 μM for the ratiometric colorimetric assay and 0.25–45 μM with a LOD of 17 nM for the fluorescent assay, which is greatly lower than the dose of DPA (60 μM) released by the contagious number of spores to cause anthrax. The ratiometric colorimetric and fluorescent probes were successfully applied for the detection of DPA in lake waters and Bacillus subtilis, and as low as 1.8 × 104 spores were detected.