One of the challenges in looking online, even with resources such as Trulia, is that information can be outdated by the time you see it. The other challenge if you are looking for a one bedroom condo in downtown Chicago is the critical importance to find a real estate professional who is able to assist you to sift through the listings in Streeterville, Gold Coast, and possibly River North to find a place that a) you like and b) that has the potential to work well as an investment. Here's the thing - there are so many buildings to choose from, but far fewer buildings that have what you (should) want in a building - strong finances combined with location are perhaps the top two features that matter.
Having said all of this, please feel free to reach out to me via my profile if you are interested in finding a place in downtown Chicago.
All the best,
The Real Estate Lounge Chicago with @ properties