I just made that sentence myself I think, it seemed kind of weird to you. Here's the orignal one. I picked this sentence from Cambridge dictionary.
"Is she especiallyunhappy at school?" "Not that I know of."
What does it mean in above sentence?
Is it similar to " I don't know about that" or not?
Another common way of saying “Not that I know of” is “Not as far as I know”. In the sort of context in which that fits, the related expression “to the best of my knowledge” might also be appropriate (No. To the best of my knowledge, that’s not the case).