Fluorescent detection of dipicolinic acid as a biomarker for anthrax using Tb ion-coordinated Fe3O4 nanoparticles

Abstract

Anthrax is a fatal disease caused by bacteria called bacillus anthracis. The bacteria can be used as a bioterror agent due to its high toxicity and resistance to the external environment of its spore. Dipicolinic acid (DPA) is released when the spore germinates; therefore, sensitively detecting DPA as a biomarker of the spores is focused on by many researchers [1]. Among them, nano-sensors including trivalent lanthanide ions have been used to detect DPA sensitively and promptly because the trivalent lanthanide ion such as europium (Eu3+) and terbium (Tb3+) shows unique fluorescent properties originated from their energy levels, and the intensity can be enhanced by chelating with DPA called antenna effect [2]. However, nanomaterials such as polymers or inorganic nanoparticles containing fluorescent substances usually quench the fluorescence by absorbing the excitation and emission light concurrently [3]. Therefore it is necessary to separate the fluorescent signal from the nanomaterials to improve its sensitivity.Herein, we develop terbium ion-coordinated iron oxide (Fe3O4-Tb) nanoparticles and detect DPA via measuring fluorescent signals from the Tb-DPA complexes [4]. The Fe3O4-Tb nanoparticles have a magnetite (Fe3O4) core and the surface is enveloped by Tb3+ that is coordinated with polyacrylate. Because of the small crystallite size of the Fe3O4-Tb nanoparticle, it has a trivial coercivity; therefore, Fe3O4-Tb nanoparticles do not agglomerate each other and reacts DPA with high reproducibility [5]. When the Fe3O4-Tb nanoparticles contact with DPA, Tb3+ is disassociated from the nanoparticle surface and chelated with DPA. The formed Tb-DPA complexes have a green fluorescence under certain ultraviolet (UV) wavelengths. We separate the iron oxide core via a permanent magnet to prevent quenching of the nanoparticles, then the clear signal shows a lower detection limit (5.1 nM) than the infectious dosage of anthrax (60 μM) [6]. The detecting method also has a high selectivity when compared with other aromatic carboxylic acids, which have a molecular structure similar to DPA.  Fig.1. Detection mechanism for DPA with Fe3O4-Tb nanoparticles and the fluorescent intensity dependent on the concentration of DPA.