A new class of nano structured plasmonic chemical sensors has been developed in recent years. Gold and silver nanoparticles are frequently studied because they exhibit a strong surface plasmon resonance in visible wavelength. The aggregation of these nanoparticles will result in a significant color change that serves as the basis of sensitive chemical detection. FOr example, the surface of Au nanoparticles may be derivatized with oligonucleotide and in the presence of complementary target DNA, Au nanoparticles will aggregate and change their color from red to blue. A similar approach was applied to trinitrotoluene detection where Au nanoparticles prepared from trisodium citrate were first treated with cysteamine.Upon addition of TNT solution, the Au nanoparticles solution turned from red to violet blue owing to TNT-induced Au nanoparticle aggregation. Although these chemical sensing techniques wre demonstrated in solution, it is likely that solid-state thin-film methods will be developed in the future.