Silver, gold, and copper colloids have been the major focus of interest because of their unique optical properties
determined by the collective oscillations of electron density termed plasmons, which give rise to an intense absorption
in the near UV–visible. Such strong absorption induces the brilliant color of these metal nanoparticles in colloidal
dispersion
. It is produced by the strong coupling of nanoparticles to the electromagnetic radiation of incident light,
leading to a collective excitation of all of the free electrons in the particles. As illustrated in Figure 8, the movement
of the electrons under the influence of the electronic field vector of the incoming light leads to a dipole excitation
across the particle sphere, the positive polarization charge acting as a restoring force, which makes the electrons oscillate.
Thus, the electron density within a surface layer, the thickness of which is about equal to the screening length
of a few angstroms, oscillates, whereas the density in the interior of the particle remains constant [“surface plasmon”
(SP)]. From one up to three SP bands may be observed, corresponding to three polarizability axes of the metallic
nanoparticles. A prerequisite for the formation of an intense plasmon absorption band is that 12 (the imaginary part of the
dielectric constant of the metal) is not too large [11]. Silver particles have the unique property that the excitation of the
collective oscillation (plasmon absorption,  = 380 nm) and of the interband transitions ( = 320 nm) occur in separate
wavelength regimes. The plasmon resonances possessed by gold and copper are in the visible (Au 520 nm; Cu 570 nm),
however, these resonances are superimposed by interband transitions.
In theory, the absorption spectra of metal particles smaller than the wavelength of incident light can be calculated
on the basis of the Mie equation for nanoparticles whose metallic dielectric function is known and which
are embedded in a medium of known dielectric constant [413–416]. The position and magnitude of the surface plasmon
absorption band are intensively dependent on the size of the particles, the refractive index of the solvent,
and the degree of aggregation. The surface plasmon resonance should shift slightly to high energy, and broaden
somewhat as the particle size is decreased; meanwhile, the intensity of absorption decreases remarkably until it is essentially
unidentifiable for crystallites of less than 2 nm effective diameter [417, 418]. For example, a symmetric and
comparatively narrow absorption peak at a shorter wavelength is usually indicative of relatively small, monodisperse,
and spherical metal nanoparticles. However, it has been demonstrated both theoretically [419–422] and experimentally
[423, 424] that the SP resonances of the coinage metal nanoparticles depend much more sensitively on the particle
shapes than on the sizes. Typically, in the case of gold nanorods, the absorption spectra are characterized by the
dominant SPl band (at longer wavelength, ca. 620–900 nm), corresponding to longitudinal resonance and a much weaker
transverse resonance than the SPt band (at shorter wavelength, ca. 520 nm).
In general, for bimetallic Ag/Au nanoparticles, the absorption bands of alloys fall between the maxima of the
respective constituent surface plasmon absorption bands, and core–shell structures manifest themselves in two distinct
bands [413, 425]. Morphologies and absorption spectra of composite Ag/Au nanoparticles were, however, found
to depend on the deposited metal atoms/surface ratio in a less than straightforward manner [426]. In contrast, several
experimental results show that what is observed in the core– shell structured nanoparticles is only the plasmon absorption
of pure metal of the core or shell constituent, probably due to particles deviating from the perfect core–shell model
[427, 428]. The wavelength of the plasmon resonance of the nanoshells that are the other kind of colloidal particles,
consisting of a small dielectric core covered by a thin metallic shell, can be tunable in the visible and near-infrared
regions by varying the core/shell radii. This allows for the design of nanoshells with plasmon resonance across a spectral
range from about 600 to 2500 nm [429]. The scattering from nanoshells adheres to the Mie scattering theory
[429–431].
The efficiency for the absorption and scattering of light by metal nanoparticles can surpass that of any molecular
chromophore.
In addition, both absorption and scattering properties can be considerably altered by surface
modification or by electronic coupling between individual nanoparticles. Together with an expectional resistance
to photodegeneration, such favorable optical features are stimulating the development of new applications in analytical
chemistry and in photophysics, making metal colloids attractive components for diagnostic, electronic, and photonic
devices.

from Noble Nano Particles