| The presence of pathogenic bacteria is a worldwide health concern; therefore, rapid identification is crucial to prevent widespread outbreaks of disease. Exhibiting size-dependent optical and electronic properties, semiconductor nanocrystal quantum dots (QDs) are progressing as improved fluorophores for biological labeling because they provide many advantages over the typical organic dyes used in current assays. CdSe QDs are the best understood quantum-confined system to date and are the most popular choice for biolabeling because their fluorescence can span the entire visible region of the electromagnetic spectrum.; Numerous sizes of high-quality CdSe QDs have been synthesized by various colloidal methods and optically characterized using UV-vis absorption spectroscopy and fluorometry. Synthesized with a surrounding hydrophobic organic matrix that can be replaced or exchanged, CdSe/ZnS core/shell QDs have been altered through either ligand exchange or phospholipid micelle encapsulation to obtain aqueous solubility and have been adapted using two different methods for the attachment of pertinent molecules that recognize specific receptors on the surface of Escherichia coli. The optical properties of the QD core were maintained, with solvent exchange and molecular attachment confirmed through solubility tests, optical spectroscopy, and Fourier transform infrared (FTIR) spectroscopy.; Used in standard biochemical reactions, streptavidin-functionalized CdSe/ZnS QDs can selectively label single cells of pathogenic E. coli O157:H7. Determined using fluorometry and epifluorescence microscopy, this simple assay is 2 orders of magnitude more sensitive and more photostable than an analogous method using a traditional dye, fluorescein isothiocyanate (FITC). These same QDs have also surpassed their FITC analogue in the realm of flow cytometry, exhibiting an order of magnitude higher fluorescence signals and lower detection limits (1%) for E. coli O157:H7 in bacterial mixtures containing E. coli DH5alpha. Therefore, semiconductor QDs are vastly improved fluorophores for selective and sensitive labeling of a pathogenic bacterium and have no deleterious effects on the biological molecules or the antibody-antigen and streptavidin-biotin interactions utilized in these studies. Potentially revolutionizing the field of cellular labeling through incorporation in multicolor detection schemes employing a single excitation source, QDs could eventually replace organic dyes entirely as fluorophores in biolabeling protocols for the purposes of pathogen detection. |
