Europium Ion-Based Magnetic-Trapping and Fluorescence-Sensing Method for Detection of Pathogenic Bacteria

Europium ions (Eu³⁺) have been utilized as a fluorescence-sensing probe for a variety of analytes, including tetracycline (TC). When Eu³⁺ is chelated with TC, its fluorescence can be greatly enhanced. Moreover, Eu³⁺ possesses 6 unpaired electrons in its f orbital, which makes it paramagnetic. Being a hard acid, Eu³⁺ can chelate with hard bases, such as oxygen-containing functional groups (e.g., phosphates and carboxylates), present on the cell surface of pathogenic bacteria. Due to these properties, in this study, Eu³⁺ was explored as a magnetic-trapping and sensing probe against pathogenic bacteria present in complex samples. Eu³⁺ was used as a magnetic probe to trap bacteria such as Staphylococcus aureus, Escherichia coli, Enterococcus faecalis, Acinetobacter baumannii, Bacillus cereus, and Pseudomonas aeruginosa. The addition of TC facilitated the easy detection of magnetic Eu³⁺-bacterium conjugates through fluorescence spectroscopy, with a detection limit of approximately ∼10⁴ CFU mL^-1. Additionally, matrix-assisted laser desorption/ionization mass spectrometry was employed to differentiate bacteria tapped by our magnetic probes.