Luminescent β-diketonate coordinated europium(III) sensor for rapid and sensitive detection of Bacillus Anthracis biomarker

The hazardous B. Anthracis are harmful pathogens causes anthrax could be potentially used as biothreat agents or bioweapons and have severe consequences to public health. These bacterial endospores contain dipicolinic acid (DPA) as calcium dipicolinate (CaDPA) as a primary cell wall constituent (1 M, 1015 molecules/spore). The anthrax attacks in USA by shipping spores in postal mails have triggered the hunt for a simple, rapid, sensitive, and selective detection of Bacillus Anthracis spores. Therefore, a fast and sensitive sensing platform for anthrax spores is critical to contain such biothreats and outbreaks of infection. Herein, we intend to develop a sensitive and selective optical detection strategy for anthrax spore biomarker based on DPA-sensitized lanthanide luminescence. The anionic hard DPA2− strongly binds with hard Ln3+ forming thermodynamically stable complex and sensitizing unique LnIII time-resolved luminescence originated from f-f transitions with a high S/N ratio. Here, we employed and repurposed a simple luminescent EuIII complex: [Eu(TTA)3(H2O)2] (1) (TTA: 3-thenoyltrifluoroacetonate) for the detection of the DPA2− biomarker. Complex 1, when titrated with DPA, displayed instant Switch-ON optical response by 10-fold enhancement in the time-resolved luminescence (TRL) intensity of the 5D0→7F2 transition, which is sensitive to the changes in the symmetry of the EuIII primary coordination sphere. Upon binding with DPA, we also observed shifting of the excitation wavelength (λex) from ∼345 nm to 275 nm in the presence of ∼3.0 mol equivalent of dipicolinic acid. After stoichiometric reaction with DPA, the resulting complex has a long-lived, highly stable excited state of the EuIII ion with a lifetime of 2100 μs in solution compared to only 260 μs for the original probe. We proposed possible displacement of the TTA− and H2O from the primary coordination sphere of the EuIII ion by potent chelating antenna ligand DPA2−. This reaction results in the formation of nine-coordinated [Eu(DPA)3]3- complex in solution confirmed by ESI-MS analysis. The luminescence response of the complex 1 is found highly selective for DPA in the presence of various interfering carboxylic acids and quantitatively able to detect up to 1.20 μM.