Two-Piece Imaging Probes

By wrapping a squaraine dye in a molecular cloak, chemists at the University of Notre Dame, Indiana, have managed to shield the reactive dye compound from nucleophiles and improve its stability, thereby increasing its viability as a near-infrared fluorescent-imaging probe for cellular and other biological studies (J. Am. Chem. Soc. 2007, 129, 15054).

For a molecule to be a good near-IR fluorescent-imaging probe, it needs to have two things going for it: It must fluoresce brightly, and it has to be long-lived enough to shine through the duration of an imaging experiment. Squaraine dyes possess an intense fluorescence emission, but their electron-deficient squaric acid core makes them prone to nucleophilic attack.

A team led by Bradley D. Smith, Jeremiah J. Gassensmith, and Easwaran Arunkumar found that by threading a squaraine dye molecule through an anthracene-containing tetralactam macrocycle, they could stave off nucleophilic attack while still preserving the molecule's fluorescence. The inclusion complex's emission is red-shifted from that of the lone squaraine, making it easy to differentiate the two-piece probe from its individual components.

Furthermore, the Notre Dame team found that the complex readily self-assembles within the complicated molecular environment of a living cell. Next the researchers plan to modify the system so that it can be used as a molecular probe for cell and whole-animal imaging.