Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 37

3.3 Localized Surface Plasmon Resonance of Au

and Ag NPs

Electromagnetic surface waves can propagate along the interface
between conducting materials and a dielectric over a broad range of
frequencies, ranging from radio frequencies to the visible range. The
oscillation modes comprise an electromagnetic ield coupled to the
oscillations of conduction electrons and are called surface plasmons,
which are characterized by strong ield enhancement at the
interface, while the electric ield vector decays exponentially away
from the surface (in the nanometer range). Gold and silver belong
to a family of free electron metals that have a illed valence shell
but an unilled conduction band; thus, they display strong surface
plasmons. When the dimensions of the conductor are reduced,
boundary and surface effects become very important, and for this
reason, the optical properties of small Au and Ag NPs are dominated
by the collective oscillation (LSPR absorption) of the conduction
electrons in resonance with incident electromagnetic radiation. Au
and Ag NPs display unique optical properties as a result of this LSPR
absorption. The LSPR absorptions of Au and Ag NPs are dependent
on their size, shape, and composition. For example, aqueous
solutions of Au and Ag NPs of 20 nm diameters appear brilliant red
and yellow, respectively. The solution colors of Au-Ag nanorods
(45 nm in length and 28 nm in width) and boat-like nanomaterials
(50 nm in length and 30 nm in width) are red and green, respectively.
In addition, the dielectric properties of the medium and the distance
between NPs are important parameters when considering their
optical properties. 62,63 The inluence of the surrounding medium is
usually related to its refractive index; Mie theory predicts resonance
to occur when ε 1 ( ω ) = −2 ε m (where ε 1 ( ω ) is the real component of
the metal dielectric function at angular frequency ω and ε m is the
medium dielectric constant), but this equation is only valid for dilute
NP dispersions in nonabsorbing media. However, when concentrated
systems are considered, the NPs become closer to each other, and
interactions between neighboring particles can arise, and the
models for isolated particles do not hold any longer. The theoretical
modeling of concentrated systems requires the use of effective
medium theories, such as those derived in the early 20th century by
Maxwell-Garnett and Bruggemann, which allow for the calculation
of the average dielectric function for composite media with varying
concentrations of an iniltrated material.