When a gold nanostructure encounters electromagnetic
radiation of an appropriate wavelength, the delocalized
conduction electrons of the metal will begin to oscillate
collectively relative to the lattice of positive nuclei with the
frequency of the incoming light. Fig. 1B illustrates this
phenomenon for a gold nanosphere. This process can be
divided into two types of interactions: scattering, in which
the incoming light is re-radiated at the same wavelength in all
directions, and absorption, in which the energy is transferred
into vibrations of the lattice (i.e., phonons), typically observed
as heat. Together, these processes are referred to as extinction
(extinction = absorption + scattering). In addition, LSPR
also generates strong electric near fields close to the surface of
the particle. All three of these interactions can be harnessed for
biomedical applications, which will be discussed in greater
detail in the second half of this review.