Schematic diagram of an optofluidic device: (A) side view, (B) top view, (C) side view of an optofluidic device with aggregated nanoparticle-SERS active clusters at the step structure. The depth of nanochannel is smaller than that of nanoparticles. Thus, nanoparticles are trapped and aggregated. (D) Fluorescent image of polystyrene nanoparticles trapped at the step boundary of the optofluidic device. Reprinted with permission from . Copyright 2007 American Chemical Society.