Biodetection Based on Resonance Light Scattering of Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 160

where N / V is the number of aggregates per unit volume. The real
and imaginary parts of α are related to each other so that when α i is
maximized in some wavelength range, α r also behaves anomalously
in this region; hence, | α | 2 is at a maximum in the absorption band,
which also results in increased scattering. Under normal conditions,
this increased scattering is dificult or impossible to detect because
of the increased absorption and the weakness of the enhanced
scattering effect. However, when this effect is investigated for
aggregates of chromophores, the enhanced RLS can be enormous.
The absorption depends on the irst power of the polarizability,
which in turn depends linearly on the volume of the aggregate. Thus,
a solution with a ixed concentration of the aggregating component
exhibits no change in A as aggregation occurs because the product
of N / V and α i remains constant. However, the amount of scattering
depends on the square of the volume of the aggregate, and thus it
increases as a consequence of aggregation. RLS is therefore extremely
sensitive to aggregates.

8.2.1 RLS of Plasmonic NPs

When a metallic NP is exposed to an electromagnetic wave, the elec-
trons in the metal (plasmons) oscillate at the same frequency as the
incident wave. Subsequently, the oscillating electrons radiate elec-
tromagnetic radiation with the same frequency as the oscillating
electrons. This re-radiation of light at the same incident wavelength
is often referred to as plasmon scatter. 23,24 If the colloid is illumi-
nated with light ( λ ) that matches the principle plasmon absorption
maxima ( λ p ), the NPs can then both absorb and scatter light outside
of their physical cross-sections. For wavelengths longer than λ p , light
is only inluenced over the physical constraints of its cross-section.
Because the nature, size, and shape of NPs are important parame-
ters for λ p , they also affect scattering properties. For example, C scat =
8 × 10 -12 cm 2 for a 30 nm diameter Ag NP at 380 nm, which is about
2000-fold larger than that of polystyrene NP of 30 nm. 23 One 60 nm
Au NP has the same scattering intensity as 3 × 10 5 luorescein mol-
ecules. When compared to four of the most commonly used metal
NPs (Ag, Au, Cu, and Al), Ag shows the most eficient plasmon scat-
ter, which is about 10-fold higher than Au and 20-fold higher than
Cu and Al. 23 Although Au NPs are less eficient at scattering than
Ag NPs, their use is also popular, mainly because they are stable and
easy to prepare and conjugate.