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SPR is a Quantum Mechanical
Phenomenon
Snell’s law describes what happens
when light is directed through a high refractive index
prism (e.g. Sapphire - refractive index 1.76) to a
surface of the prism in contact with a low refractive
index medium, for example physiological buffer (See
Figure 1). Light rays below the critical angle will
exit the prism bending toward the prism surface. Light
rays above the critical angle are totally internally
reflected back through the prism. Photons that are
totally internally reflected create an electric field
at the interface. Light is not coming out of the prism
but an electric field extends past the reflecting
surface. This field oscillates with the usual
characteristics of an electromagnetic mode. The
electrical component perpendicular to the interface
decays exponentially – this is called an evanescent
wave. This wave is bound to the surface and is used in
SPR to detect changes occurring at the surface.
SPR spectroscopy adds a thin metal
layer between the prism surface and the aqueous
compartment (See Figure 2). Free electrons in the
metal layer can act as a resonator. Energy for the
resonance comes from the evanescent wave produced by
the totally internally reflected photons.
When certain conditions, determined
by the wavelength, illumination angle, and refractive
index of the prism, metal and aqueous layers are met,
coupling/resonance occurs between the plasma
oscillations of the free electrons in the metal and
the bound electromagnetic field of the totally
internally reflected photons. This coupling is the
result of the momentum of the incoming light equaling
the momentum of the plasma electromagnetic field.
Photons are “absorbed” and converted to surface
plasmons. Since the photons are not reflected, a
“shadow” occurs in the reflected light.
Reichert Inc.’s SPR instruments are
optically configured to illuminate a spot on the gold
surface over a range of angles from about 58 to 85
degrees. When the aqueous compartment is e.g.
physiological saline solution the “shadow” or SPR
minimum occurs at about 66 Degrees. Mass changes at
the interface between the gold layer and the aqueous
compartment cause changes in the local refractive
index near the gold layer. This changes the
coupling/resonance angle. For example the if a protein
layer was added to the gold / aqueous interface the
coupling angle would be about 66.6 Degrees. Actual
reflectivity from the SR7000 (Angle vs. reflectivity)
before and after immobilizing a protein to the surface
is shown in Figure 3.
In the SR7000 and SR7000DC
Spectrometers the gold surface is illuminated with a
range of angles that encompass these shifts in the
coupling angle. This range of angles is continuously
monitored with a linear photodiode array detector.
Image analysis determines the angle at which the
reflectivity minimum occurs. This is continuously sent
to the data acquisition computer/software.
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“The more
success that quantum theory has, the sillier it
looks”.
-Albert Einstein to Heinrich
Zangger on Quantum Theory, May 20, 1912
“All these fifty years of conscious
brooding have brought me no nearer to the answer to
the question, 'What are light quanta?' Nowadays every
Tom, Dick and Harry thinks he knows it, but he is
mistaken. “
-Albert Einstein, 1954.
Figure 1
At incident angles below the
critical angle light is refracted out of the prism,
angle angles above the critical angle light is totally
internally reflected back through the prism.
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Figure 2
Adding a thin
gold layer creates a “resonator” for the incoming
light. This resonance occurs at an angle dependent
on the refractive index or mass in contact with the
solution side of the gold layer. As more mass is
added to the surface, e.g. during an antigen binding
to a surface immobilized antibody, the resonance
angle “shadow” increases.
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Figure 3
Reflected light image as viewed by
the SR7000DC Photodiode Array image detector. The
resonance angle increases as protein is added to the
surface. This angle change is monitored and recorded
in real-time. |
