The detection is based on the fact that molecules of different substances interacting with light from a laser will scatter the light differently, providing a unique spectrum that can be used to identify the substance, much like a fingerprint. The key to detection is the enhancement that is provided by small clusters of silver nanoparticles, that concentrate the electric field of the light into 'hot-spots'. A molecule found in such a hot-spot will experience a concentrated electric field and give a very strong SERS signal. In our recent work, we used a microfluidic device to orchestrate the interactions between silver nanoparticles and methamphetamine molecules in saliva. The microfluidic device allows for the controlled introduction of the sample and the nanoparticles, and the subsequent aggregation of the nanoparticles into hot-spot rich clusters that allow us to detect minute amounts of the drug.