Illustration of the detection scheme of DNA based on the application of silver nanoparticles (labeled as Ag NP in the figure). (A) When no target DNA is present, the SERS primer is closed, which results in a low SERS signal. (B) In the case of the presence of complementary DNA, the loop structure is opened. Thus, the dye label is free to adsorb on the silver nanoparticle surface producing a high SERS signal. Reprinted with permission from van Lierop et al. . Copyright 2011 American Chemical Society.