text: The plasmon resonance displayed by nanoparticles, gold particles are most often used as an example, can be altered using the interfacial layer. When either anionic or cationic ligands bound to a nanoparticle made of gold for example are increased in length, the wavelength of the plasmon resonance will shift to red. 
  An example of another effect, that has recently been observed by Amendola et al. on small gold nanoparticles, of 10 nm or less, is that dense monolayers that consist of certain specific short chain ligands tend to dampen the surface plasmon resonance effects.
  Plasmon resonance can be used to analyze the surfactants of the nanoparticle. This principle is based on the so-called Fröhlich condition which states that the refractive index of the surrounding medium of a nanoparticle can be used to tune or alter the frequency of the surface plasmon resonance. The equation that relates both properties is as follows: 
  In which  is the wavelength at which the plasmon resonance frequency peaks,  is the refractive index of the environment, which relates to the dielectric constant of the medium as follows: . Furthermore  is the frequency of the plasmon resonance and  is the speed of light in vacuum.
  The relation between the wavelength and the refractive index of the environment is not strictly linear but for small values of  the theoretical predictions align with experimental results. 
  This relation can thus be used to analyse the environment of the nanoparticle, i.e. the interfacial layer, by measuring the wavelength of the plasmon resonance.