Surface plasmons are collective oscillations of the free electron gas at the surface of metals that can be induced by an external electric field, including electromagnetic radiation (Figure 1). When the size of the metal structures is reduced to nanoscale dimensions, the electron oscillations are confined in the nanostructures creating a phenomenon known as localized surface plasmon resonance (LSPR). The excitation of LSRP gives rise to a strong enhancement of electromagnetic fields in the near-field region and the ability to concentrate electromagnetic radiation into particles of nanoscale dimensions has made plasmonic materials attractive for a large variety of applications, including molecular sensing.
For most types of silver and gold nanostructures, the resonance frequency is in the visible region of the electromagnetic spectrum, which gives rise to distinct colors for nanoparticle ensembles. The colors emanates from the extensive absorption and scattering (extinction) of light by the nanoparticles at the LSPR frequency. The resonance conditions are highly dependent on a number of parameters, including i) the nature of the nanostructures i.e. their size, shape, and composition ii) their interaction with other, closely spaced metal nanostructures and iii) the dielectric properties of the surrounding medium.