(not) one; (not) a single
Used with a negative predicate to mean 'not even one' or 'not a single thing'. Emphasizes complete absence.
何一つ問題はありません。
There is not a single problem.
彼は何一つ知らなかった。
He didn't know a single thing.
He doesn't know a single thing about the matter.
Even after ten years, nothing about this town has changed at all.
In the emptied room, not a single thing was left behind.