Active record and tsearch2, works?

I have a search form; I grab the value and use it in a fulltext query:

:conditions => [“idxfti @@ '” + search_string + "’::tsquery and
…]

Characters in search_string like “?”, “:”, bomb active record. I tried
a
single tick and “” to escape those, but active record still bombs (same
queries via psql work).

Suggestions?
-Tate