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Chapter 7: Theory
7.1 Localized Surface Plasmon Resonance
OpenSPR™ is a label free biosensing platform that utilizes localized surface plasmon resonance
(LSPR) to make extremely sensitive measurements of biomolecular interactions. LSPR is an optical
phenomenon that occurs when light is absorbed by noble metal nanoparticles. These particles
are typically on the order of 10 – 100 nm in size. The absorbed light exhibits an extinction
maximum in the visible spectrum that is dependent on the size, shape, composition of the
nanoparticles as well as the dielectric environment. These features combine to create a highly
sensitive biosensor capable of measuring local refractive index changes. When a biomolecule
binds near the surface of the nanoparticle, it changes the local refractive index, this shift is
reported by the nanoparticle sensor in the form of a peak shift. The magnitude of that shift is
governed by the following equation
1
:
Where m is the bulk refractive index sensitivity (RIS) of the nanoparticles [nm/refractive index
unit (RIU)], Δn is the refractive index difference between the analyte and the buffer solution, d
1
is the thickness of the capture layer, d
2
is the thickness of the target analyte and l
d
is the effective
electromagnetic (EM) field decay length, which takes into account the finite distance over which
the EM field decays outside the particle. The nanoparticle shape, size and material determine the
values of m and l
d
, while the capture layer and target biomarker determine d
1
, d
2
, and Δn. This
equation is the basic transfer function used to predict the output of an LSPR sensor.
The RIS of the nanoparticles is an important parameter that helps define the performance of the
sensor. It is defined as
1
:
Where λ
p
is the peak position and n is the bulk refractive index.
Another important parameter that helps define LSPR sensor performance is the LSPR figure of
merit (FOM) defined as
1
:
Where Δλ is the spectral line width or full width at half maximum of the LSPR peak.
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