If you are looking for an App that will turn your I-Phone into a stand-alone scanner, that won't happen. Cell phones use a dedicated set of frequencies in a digital format. They do not have the ability to "listen" to other frequencies like a scanner does.
About all you can do is use an App that has a feed from a scanner in the area you want to monitor.
"...the first rule of law enforcement: At the end of your shift make sure you go home alive." - Sean Connery, The Untouchables.
21 Club Member #4500.