You can (not) be concerned about nothing in particular:
And you can (not) be concerned about a particular thing:
If something is the object of concern, then remember to use the preposition: concerned about something.
It's also possible to say the same thing using concern as a noun, where concern often collocates with another word (in this case, 'cause'):
And it's also possible to use the verb form (notice the auxiliary verb 'to do' in the construction of the negative!):
So which one to use? Adjective? Noun? Verb? Well if you can remember to put __ed onto concern and add about, it's probably best to use the adjective form!
Tell a friend about some of the things you're concerned about.