This fluorescence micrograph shows phage-quantum dot complexes (bright blue spots) bound to E. coli cells (cylindrical green shapes). The NIST/NCI method of tagging cells with quantum dots can be used to identify bacteria much faster than conventional methods. The fluorescence signal is strong and stable for hours, enabling scientists to count the number of phage viruses bound to a cell. A quantum dot (QD) is a nanocrystal made of semiconductor materials that is small enough to exhibit quantum mechanical properties. A fluorescence microscope is an optical microscope that uses fluorescence and phosphorescence instead of, or in addition to, reflection and absorption to study properties of organic or inorganic substances