Biomedical Engineering Reference
In-Depth Information
and slope indicates poor agreement with a Förster mechanism
(1/
R
6
) and precise agreement with the (1/
d
4
) plot, indicating the
energy transfer process follows a SET route. To assess the trend in
distance independently from the theoretical predictions, different
values of
r
0
and
n
as itting parameters were used. For the six
replicated experiments, the average value of n was 4.0 with an average
r
0
of 92 Å at 50% quenching eficiency. These results are in good
agreement (<2% error) with the theoretically calculated values for a
SET mechanism (
d
0
= 94.3 Å) while the strong discrepancy between
the it and the FRET model in terms of slope and distance eliminates
FRET as the quenching mechanism. Therefore, the mechanism of
luorescence quenching for an organic dye near a metal NP is almost
entirely due to the energy transfer to the metal surface, and thus the
quenching eficiency is an 1/
d
4
distance dependence.
10.2.2 SEF
10.2.2.1
Surface plasmons
Use of the proper surface structures to resonantly couple the surface
plasmon polariton (SPP) modes into radiative emission and to
recover the energy is an important aspect of SEF. There are two main
momentum matching techniques that are used to couple the SPP
modes to photons. The irst increases the momentum of the photon
by using high index materials with speciic optical geometries. The
second technique involves breaking the translational invariance of a
smooth metallic surface through corrugated or roughened surfaces.
We discuss these techniques below.
10.2.2.1.1
Flat metallic surfaces
Coupling between propagative light and SPP on lat surfaces
enhances both the excitation and the emission processes of the
luorophores. Because the evanescent EM ield associated with SPP
has its maximum at the surface, excitation processes are enhanced.
Subsequently, the energy given to the SPP by near-ield coupling with
the luorophore is recovered due to the enhancement of the emission
processes. Therefore, luorophore coupling with propagating SP
plays a crucial role in SEF.
Using a Kretschmann coniguration, surface plasmons on the
front side of a metallic ilm (i.e., the luorophore side) can be excited
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