Placeholder Strategy with Upconversion Nanoparticles−Eriochrome Black T Conjugate for a Colorimetric Assay of an Anthrax Biomarker

The timely warning of the germination of bacterial spores and their prevention are highly important to minimize their potential detrimental effects and for disease control. Thus, a sensitive and selective assay of biomarkers is most desirable. In this work, a nanoprobe is constructed by conjugating lanthanide upconversion nanoparticles (UCNPs) with sodium tripolyphosphate (TPP) and eriochrome black T (EBT). The nanoprobe, UCNPs−TPP/EBT, serves as a platform for the detection of the anthrax biomarker, dipicolinic acid (DPA). In principle, DPA displaces EBT from the UCNPs−TPP/EBT nanoconjugate, resulting in a color change from magenta to blue because of the release of free EBT into the aqueous solution. The binding sites on UCNPs are partly preblocked with TPP as the placeholder molecule, leaving a desired number of binding sites for EBT conjugation. On the basis of this dye displacement reaction, a novel colorimetric assay protocol for DPA is developed, deriving a linear calibration range from 2 to 200 μM with a detection limit of 0.9 μM, which is well below the infectious dose of the spores (60 μM). The assay platform exhibits excellent anti-interference capability when treating a real biological sample matrix. The present method is validated by the analysis of DPA in human serum, and its practical application is further demonstrated by monitoring the DPA release upon spore germination.