Biodetection Based on Fluorescence Quenching and Surface-Enhanced Fluorescence Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 228

of the nonradiative rate, higher multipole orders are necessary.
A multiple multipole method is useful to correctly describe the
observed variation of the luorescence rate with respect to the
particle-molecule distance. 66 The luorescence enhancement of Nile
blue dye achieves a maximum at a distance of 5 nm from the surface
of 80 nm diameter Au NPs. When a laser with λ = 637 nm irradiates
the Au NPs, the luorescence enhancement of Nile blue is strongest
at its luorescence wavelength of 680 nm and weakest at another of
its luorescence wavelengths of 485 nm, showing the redshift of the
excitation frequency with respect to the localized surface plasmon
resonance of the Au NPs.

10.2.2.2.2 Multiple metal NPs

When two or more NPs are close to each other, the overall system
is similar to an ensemble of elementary NPs. Contrary to a complex
object having a spherical symmetry, polarization of the incident
light is crucial when more than two NPs are close. 67,68 When two
NPs are close enough to each other, the dipole of each NP induces
a dipole on the other. This dipole-dipole interaction essentially
hybridizes the elementary localized surface plasmon resonance. To
achieve a narrow width of the localized surface plasmon resonance
and increase the ield enhancement, use of several NPs forming a
chain-like structure is essential. The enhancement of the electric
ield can be calculated by using the electrostatic approximation. 69 If
each particle enhances its applied ield by a factor α , the cumulative
effect of the chain of particles results in an enhancement factor of α n
where n is the number of NPs in the chain. However, it is dificult to
prepare such a chain-like structure.
For speciic shapes of NPs, the electromagnetic coupling inside
the gap can lead to huge enhancements of 4 orders of magnitude. 70-72
For example, Au NPs have been used for producing nonlinear, optical
four-wave mixing. By decreasing the interparticle distance from a
large separation to that of touching contact, we can increase the 4
four-wave mixing yield by 4 orders of magnitude mainly because
of the shift of the localized surface plasmon resonance to infrared
wavelengths as the dimer forms. In a similar way as antennas in
the radio wave regime, nanostructures are designed to increase
the radiation eficiency of the luorescent particle. For example,
the interaction of a single quantum dot with a bowtie antenna
enhances visible light. 73 When scanned over the quantum dot,