A radio-nuclide was a substance that could be locked onto by a transporter.
In 2372, while discussing how to keep a transporter lock on Odo while the Changeling participated in the Great Link, Benjamin Sisko suggested he be injected with radio-nuclides. Miles O'Brien vetoed the suggestion, saying the nuclides could disperse throughout the Link, ultimately resulting in their beaming up multiple shapeshifters. (DS9: "Broken Link")