Molecular beacons are:

"artificial single-stranded oligonucleotides designed with stem-loop structures, which comprise a fluorophore and a quencher moiety at two opposite ends.  Without target molecules, the base pairs of the stem portion hybridize to hold the fluorophore and quencher close, and the fluorescence is quenched.  In the presence of target, the loop DNA region can bind to it and cause the stem-loop structure to open, which would spatially separate the fluorophore from the quencher, and the fluorescence increases"¹

 
1. Wang, H.; Yang, R.; Yang, L.; Tan, W. Nucleic Acid Conjugated Nanomaterials for Enhanced Molecular Recognition.  ACS Nano 2009, 3.