Anti-plastic

Anti-plastic was a substance used by the ninth and tenth incarnations of the Doctor against the Nestene Consciousness.

It was a blue liquid, created by various combinations of chlorofluorocarbons, vinyl acetates, copolymers, maleic anhydride, and ethyl alcohol. Anti-plastic interacted with the molecular structure of plastic and caused it to disintegrate. When put in water, it bonded with the polar hydrocarbon molecules, adhering to them and replicating itself throughout the water. (PROSE: Autonomy)

The Doctor carried the liquid around in a vial in his pocket. He had originally planned to use the anti-plastic as a threat and a bargaining chip to make the Consciousness leave Earth, which the Consciousness refused to do. Discovering the Doctor's TARDIS, the Consciousness had an Auton take the Doctor's anti-plastic from him.

When Rose Tyler rescued the Doctor, she knocked over the Auton holding the vial, who fell into the Consciousness. When it did so, the substance reacted explosively with the Nestene Consciousness, which led a halt of the attempted Auton invasion. (TV: Rose)

The Seventh Doctor also defeated the Nestenes utilising a similar liquid, although he didn't refer to it as anti-plastic. (COMIC: Plastic Millenium)

Antiplastica