We have engineered aptamers that contain fluorescent reporters and that signal the presence of cognate ligands in solution. Two different anti-adenosine "signaling aptamers", one made from RNA and one from DNA, can selectively signal the presence of adenosine in solution. Increases in fluorescence intensity reproducibly follow increases in adenosine concentration, and can be used for quantitation. The facile methods we have developed can potentially be used for generating a wide variety of signaling aptamers for use in sensor arrays.