Hafner and his colleagues borrowed bioconjugate chemistry techniques, coating the bipyramids with antibodies and then adding antigens that strongly bind to them. Then the antigens were rinsed off. Whenever one was released from its bond to the bipyramid antibody, the researchers detected a slight shift toward the blue in the red light naturally scattered by gold bipyramids.
The process is "label-free," meaning the molecule itself is being detected, rather than a fluorescent tag that requires modification of the molecule, Hafner said. Also, the dielectric property being detected is permanent, so molecules could be tracked for more than 10 hours, as compared with only minutes with current methods.
"The ability to measure over long time scales opens the possibility to study systems with strong affinity at the single-molecule limit, such as lectin-carbohydrate interactions responsible for cell recognition and adhesion," Hafner said. "Other single-molecule methods based on fluorescence are limited by photo bleaching, and those based on force measurements are limited by radiation damage and mechanical instabilities."
Work needs to be done before LSPR becomes an ideal biological sensor, he said. The team plans to tweak the bipyramids and will test other particles.
"With this bipyramid, we went a little too red," he said. "It's a compromise. Make them long and they're really sensitive, but so red that we don't get much signal. Make them shorter, they're somewhat less sensitive but you have more signal.
"If we can get the signal-to-noise ratio up by a factor of two or three, we think it will be a powerful method for biological research."
|Contact: David Ruth|