Life in extreme environments : lanthanide-based detection of bacterial spores and other sensor design pursuits

Abstract

Bacterial spores, or endospores, are produced by certain genera of bacteria under stress and are considered to be one of the most resilient forms of life on Earth. Detection of endospores is vital in areas ranging from bioburden reduction to homeland security. Rapid bacterial spore detection is achieved by targeting dipicolinic acid (DPA), a chemical marker unique to endospores. An improvement on the current bacterial spore detection assay based on sensitized lanthanide luminescence is presented through the implementation of a dipicolinate-specific Tb^(3+) receptor site. The use of a chelating ligand such as DO2A (1,4,7,10-tetraazacyclododecane-1,7-bisacetate) can increase both the sensitivity and selectivity of the assay. The luminescent series of Ln(DO2A)(DPA)- complexes (Ln = Sm, Eu, Tb and Dy) is fully characterized in terms of structure, photophysics and stability, and the Tb(DO2A)^+ binary complex in particular is investigated as a sensing complex for bacterial spores. The ‘ligand enhancement’ observed in all cases improves dipicolinate binding affinity by approximately one order of magnitude over the lanthanide ion alone. Binding of the DO2A ligand also appears to generate a ‘gadolinium break’ effect, creating a discrepancy in binding affinity in the lanthanide series and rendering the terbium complex the most effective dipicolinate receptor site of all investigated. We have also extended the application of this receptor site design technology to the targeted detection of other aromatic analytes of biological relevance, such as salicylates and catecholamines. Our work indicates that construction of effective receptor site complexes is not governed by net electrostatic considerations, and that local charge variations from the ligand-induced perturbation of lanthanide electron density may play a significant role. This work sets the stage for the development of the next-generation terbium(macrocycle) complex for bacterial spore detection, with the aim of constructing a solid-state endospore microsensor for applications ranging from sterilization validation to life detection in extreme environments.