How to discreetly mark banknotes?

Question

Information Security

The question may be off topic. But it's urgent. We need a way to mark banknotes, which is not visible to the naked eye, but at the same time is not visible in the ultraviolet. I have no idea what to come up with.

Answer 1

There is ink that is only visible in IR. Under ultraviolet they are not visible, but a special IR camera is needed.

Answer 2

Put a few small dots in the form of a certain figure, for example, in the form of a constellation, in a certain place on the bill. No one will look at it that way, but the bills will be marked and invisible in the ultraviolet.

Answer 3

cm.