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 we describe a new platform to identify structure - switching dna beacon aptamers , which detect small molecules in a specific manner . by clonally amplifying a dna library designed to fluoresce in response to binding events onto microbeads , 
 aptamer beacons can be selected by stringent fluorescence - assisted sorting . 
 we validated this method by isolating known and novel anti - steroid aptamers from two separate dna libraries that were structurally enriched with three - way junctions . 
 importantly , aptamers were retrieved in only a few ( three ) rounds of selection by this approach and did not require further optimization , significantly streamlining the process of beacon development . 
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