Nanoscopic probes are bio-weapons used by Slade. They are made of metal and have Slade's initials located on the top of them. They also have little spikes on the bottom of the probe so it can latch on to blood cells. How the probes function and how exactly they can kill the Titans is unknown, as they latch onto the blood cells and not the actual veins in their bodies. It is possible that the probes obstruct actual blood flow, as Beast Boy when under the influence of the probes grips his heart and lurches over.
In the episodes "Apprentice - Part 1" and "Apprentice - Part 2", Slade used the probes as leverage to get Robin to be his apprentice, and he does so by infecting Starfire, Cyborg, Beast Boy, and Raven with the probes, that are connected to a switch that Slade has attached to his wrist. Whenever Robin disobeyed any of Slade's commands, Slade would press the button on his switch, causing the infected Titans to glow orange and yellow, apparently in great pain. Only when Robin seems to give up, or when he infects himself, does Slade release the Titans, bringing them back to normal but with the nanoprobes still running through their bodies. They later purged themselves of the probes.