Hello everyone! I have synthesize carbon dots using hydrothermal reactor with PTFE liners. And there were black residue appear on the surface of PTFE liner which contact with my reactant. I have try to remove it with vatious way. I have been using Piranha solution, hot piranha solution, base piranha solution, aqua regia, 1 M NaOH in EtOH, even ultrasonic probe with detergent solution. But it still failed.

So, I would like to ask you all for an advice about the way to clean it.

Thank you.