However, it is important to bear in mind that both the surface plasmon resonance maximum (λspr) and the absorbance (A(λ)) are dependent on the real refractive index of the surrounding medium (nmed) and on the complex refractive index of a coating (ncoat), which may be present on the particles’ surface.
As an example, the influence of nmed and ncoat on the extinction efficiency at the surface plasmon resonance (Qext-spr) and on the position of λspr is shown in Figure 9 for a GNP having a diameter of 20 nm.
Figure 9a shows Qext-spr as a function of the coating thickness (tcoat) in water (nmed ) 1.333, solid line) and in toluene (nmed ) 1.50,24 dotted line) where the ncoat value was chosen to be typical for an optically transparent organic compound (ncoat ) 1.42, 0).
In the case of water as the surrounding medium (solid line in Figure 9a) Qext-spr increases by ∼13% when the coating is 4 nm thick (compared to the uncoated particle). Not surprisingly, Qext-spr approaches a value for thicker coatings, which is typical for an uncoated particle in a medium having a real refractive index of 1.42.
In the case of toluene as a surrounding medium Qext-spr decreases from a relatively large value at tcoat ) 0 by ∼11% for a coating thickness of 4 nm and approaches a value of Qext-spr for thicker coatings which is typical for an uncoated particle in a
medium having a real refractive index of 1.42.
Similar behavior is observed for λspr, which increases with increasing tcoat when water
is the surrounding medium (solid line in Figure 9b) and decreases when toluene is the surrounding medium (dotted line in Figure 9b) to reach the same value for both cases at large coating thickness.
This example demonstrates that the influence of a coating and of the surrounding medium on Qext and hence A cannot be neglected.
As a result of that, the equations and/or the fit parameters derived here have to be modified for other dielectric environments.