Spectral Shifts: Blue-shifting/Red-shifting

For many types of nanoparticles spectral peaks can shift due to changes in the size, shape, coating, or aggregation state of the plasmonic nanoparticles.  Blue-shifting refers to an electromagnetic response that is shifted towards shorter wavelengths (higher frequencies, higher energies).  Red-shifting refers to a peak that shifts to longer wavelengths.  For example, as the aspect ratio of  agold nanorod was decreased, the peak extinction wavelength would blue shift from 800 nm to 600 nm.  Encapsulating a spherical silver nanoparticle with a silica shell will red shift the plasmonic peak of the silver sphere.  Aggregation typically results in a red shift in the spectral characteristics of the nanoparticle suspension.