Tools to Probe Nanoscale Surface Phenomena in Cellulose Thin Films: Applications in the Area of Adsorption and Friction - The Nanoscience and Technology of Renewable Biomaterials - page 104

Detector

Light
source

II

I

Reflected
light

I I

Polarized
Incident light

Angle

II

II

Prism

II

Chip with metal
thin film

I

Time

Flow channel

Figure4.8 Schematicsofsurfaceplasmonresonance.

interact with free electrons available in the metal surface. This photon energy is then
transferred to a charge density wave and can be observed as a sharp dip in the reflected
light intensity. The angle at which the sharp dip happens is called 'SPR angle'. Outside
the metal surface there is an evanescent electric field which decays exponentially. This
evanescent field interacts with the close vicinity of the metal. The SPR signal arises under
conditions of total internal reflection and depends on the refractive index of solutions in
contact with the surface. Molecules in solution exhibit changes in refractive index and
thus give rise to a measurable SPR signal if specific interactions occur.

A schematic

illustration of SPR is shown in Figure 4.8.
The refractive index near the sensor surface changes because of the binding of poly-
mers to the surface. As a result, the SPR angle will change according to the amount
of bound material. The thickness of the adlayer can be estimated from Equation (4.3)
(Bailey, Kanazawa et al . 2001), which assumes that the thickness of the dielectric film
is much smaller than the wavelength of the probing laser:

nλ √ − ε m ε s (ε s − ε m )
2 π

ε m + ε s
ε m ε s

2

ε f

d f = ( sin θ c ) (4.3)

(ε f − ε s )(ε f − ε m )

where d f is the thickness of adlayer; n is the solvent refractive index; λ is the wavelength
of the incident laser; ε f is the dielectric constant of the film; ε s is the dielectric constant
of the solvent; ε m is the real part of the dielectric constant of the metal; and θ c is the
critical resonant angle on the plasmon resonance curve. So for a given system with
known solvent and metal (usually gold), θ c is the only variable. Equation (4.3) can be
simplified as:

d f = k( sin θ c ) (4.4)

where k is a factor that can be obtained after a calibration. In most cases, θ c is
very small and there is a linear relationship between the amount of bound material