Biomedical Engineering Reference
In-Depth Information
proportional to changes in the refractive index and consequently to the mass concentra-
tion of the biomolecules at the surface of the metal, which is the basis of the use of SPR for
biosensing purposes (34).
The most common setup for SPR applications is the Kretschmann configuration, shown
in Figure 20.5. In this configuration, SPs are excited at the surface of a gold film deposited
on a prism. The thickness (
d
) of the gold film is crucial, as the efficiency of conversion of
bulk waves into SPs decreases with increasing thickness and decreasing transparency of
the gold film (39). In contrast, if the film is too thin, SPs are rapidly reconverted into bulk
waves, significantly changing the width of the resonance curve and reducing the sensitiv-
ity of the SPR measurements (39). The optimal thickness for a gold film using, for exam-
ple, a wavelength of 790 nm, is 45 nm (39).
As mentioned above, SPs occur at the interface between two media of dielectric perme-
abilities of opposite sign, such as between free electronlike metals including silver, alu-
minum and gold, and water. Given that the dielectric permeability of any material is a
function of the wavelength of the incident light, it follows that for any given interface, the
excitation of SPs will not be possible at all wavelengths (39). For metals, the dielectric con-
stant (
m
) is negative in the infrared-visible range of the spectrum, whereas it is positive
for water (
b
) within the same range. This therefore allows the existence of SPs at
metal-water interfaces (39). Owing to its optical and chemical properties, gold is the most
commonly used metal for SPR applications, a thin film of which is typically deposited
onto a glass substrate (39).
SP waves are distinguished by the fact that they have maximal intensity at the interface,
decaying exponentially in both the metal and the ambient medium, with distance from the
surface. The SP wave is thus specifically associated with the metal-dielectric interface and
is therefore different to a bulk wave. The propagation of electromagnetic waves is charac-
terized by a wave, or propagation, vector. The propagation vectors for the SP wave (
k
sp
)
and the bulk wave (
k
b
) are defined below (39) (see Figure 20.5):
12
(20.1)
kk
sp
[
(
)]
0
m
b
m
b
12
kk
b
(20.2)
0
b
where
k
0
is the propagation vector in a vacuum.
Glass prism (
n
p
)
I
k
b
I
n
p
k
b
sin
FIGURE 20.5
Kretschmann configuration for surface
plasmon resonance (SPR). See text for
details.
Metal film (
n
m
)
d
k
sp
