SPPs result from collective excitations of electrons at the
boundary between a metal and a dielectric. The strong sensitivity
of the plasmon-propagation constant to the permittivity of a nanoscale
layer of material (bioreceptors) deposited on the metallic
surface has led to the monitoring of surface plasmon resonance
(SPR) to detect (bio)chemical changes occurring between molecules.
As a result, biochemical reactions are measured with SPR by
monitoring the evolution of its effective refractive index. In practice,
the so-called Kretschmann–Raether approach realizes this by
launching light beams from a high refractive index prism to a thin
metallic interface at an angle such that light is totally reflected [1].
In doing so, an evanescent wave extends in the metal overlay.
When the component of the propagation constant of the light
along the interface matches the one of a plasmon wave at the
other side of the metal layer, part of the light couples to the
plasmon, which decreases the reflection. The interrogation is
made either by varying the wavelength and keeping the incidence
angle constant or by using monochromatic light and modifying the
angle. In both cases the polarization state of the light has to be
parallel to the incidence plane so that the plasmon wave is normally
polarized to the interface. Biochemical reactions occurring
above the metal surface affect the effective refractive index of the
plasmon wave, which is detected through a shift of the SPR. The
sensitivity to the surrounding refractive index often ranges in the order of 106
–107
.
Numerous transducing mechanisms have been developed,
bringing additional assets compared to the Kretschmann–Raether
prism, which remains exploited in most commercial systems. In
this aspect, optical fiber based sensors are particularly attractive.
With their lightweight, compactness and ease of connection, they
provide remote operation in very small volumes of analyte. And
with their continuous development and optimization, they appear
perfectly suited for in situ or even possibly in vivo diagnosis
while bringing the potential to assay different parameters
simultaneously.
SPPs result from collective excitations of electrons at theboundary between a metal and a dielectric. The strong sensitivityof the plasmon-propagation constant to the permittivity of a nanoscalelayer of material (bioreceptors) deposited on the metallicsurface has led to the monitoring of surface plasmon resonance(SPR) to detect (bio)chemical changes occurring between molecules.As a result, biochemical reactions are measured with SPR bymonitoring the evolution of its effective refractive index. In practice,the so-called Kretschmann–Raether approach realizes this bylaunching light beams from a high refractive index prism to a thinmetallic interface at an angle such that light is totally reflected [1].In doing so, an evanescent wave extends in the metal overlay.When the component of the propagation constant of the lightalong the interface matches the one of a plasmon wave at theother side of the metal layer, part of the light couples to theplasmon, which decreases the reflection. The interrogation ismade either by varying the wavelength and keeping the incidenceangle constant or by using monochromatic light and modifying theangle. In both cases the polarization state of the light has to beparallel to the incidence plane so that the plasmon wave is normallypolarized to the interface. Biochemical reactions occurringabove the metal surface affect the effective refractive index of theplasmon wave, which is detected through a shift of the SPR. Thesensitivity to the surrounding refractive index often ranges in the order of 106–107.Numerous transducing mechanisms have been developed,bringing additional assets compared to the Kretschmann–Raetherprism, which remains exploited in most commercial systems. Inthis aspect, optical fiber based sensors are particularly attractive.With their lightweight, compactness and ease of connection, theyprovide remote operation in very small volumes of analyte. Andwith their continuous development and optimization, they appearperfectly suited for in situ or even possibly in vivo diagnosiswhile bringing the potential to assay different parameterssimultaneously.
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