Detection of Bacillus anthracis spores: comparison of quantum dot and organic dye labeling agents

Organic dyes are routinely used in labeling assays and sensing of biological molecules. Semiconductor quantum dots (QDs) have attractive features, particularly good resistance to photobleaching and narrow emission bands, which make them potential replacements for organic dyes. Using a previously identified synthetic peptide, a QD, and the R-phycoerythrin (RPE) dye, we have examined various labeling strategies for detecting Bacillus anthracis spores. The RPE dye performed well, but we were unable to demonstrate successful binding of the QD by flow cytometry or confocal microscopy. Using a related conjugation strategy, Jurkat T-cells were labeled via a monoclonal antibody (mAb) that binds to the CXC chemokine receptor, CXCR4. It was noted that both QD and RPE labeled the cells. Based on electron microscopy data, we propose that a dense network of fibers on the Bacillus anthracis spore surface prevented the QD, but not RPE, from binding. These results suggest that the nature of the biological surface must be considered when using QDs as replacements for organic dyes in biological sensing.