Credit: © 2008 Wiley

Until now, terbium has been considered the best rare-earth metal for detecting anthrax spores because it is bright and has a long fluorescence lifetime. Lehui Le and co-workers1 from the Chinese Academy of Sciences in Changcun now show that europium-based nanoparticles are two orders of magnitude more sensitive and can discriminate anthrax from a host of interfering compounds more effectively than terbium.

The team modified the surface of fluorescein-doped silica nanoparticles with the ethylenediamine tetraacetic acid (EDTA) ligand and then converted the ligand into a europium complex. Europium served as the sensing molecule and fluorescein acted as a reference dye in the particle core; covalent binding of the dye and the europium minimized leaching and allowed efficient sensing with a low background signal. On addition of calcium dipicolinate — the unique biomarker of anthrax spores — the fluorescence of the nanoparticles increased in a concentration-dependent manner and changed from green to orange. The particles could detect the biomarker for anthrax spores down to 0.2 nM, which is much lower than the infectious dose of 60 µM. Moreover, the change in colour could be identified from as low as 1 µM, and time-dependent studies show the detection was complete in just 30 s.

This new study suggests europium-based nanoparticles compete well with terbium-based sensors and have the potential for rapid and ultrasensitive detection of anthrax spores in solution, with good selectivity over interfering compounds.