Nanoparticles for in-vitro and in-vivo biosensing and imaging

40 Principles
by an additional new term Γ m (Fig. 2.4). In this case the quantum yield and lifetime of
the fluorophore near the metal surface are given by
Q m =
Γ + Γ m
Γ + Γ m + k nr
(2.3)
τ m = (Γ + Γ m + k nr ) −1 (2.4)
Figure 2.4: Modified Jablonski diagrams which include metalfluorophore interactions.
arrows represent increased rates of excitation and emission. Ref. [12]
The thicker
These equations result in unusual predictions for the fluorophore emission near a
metal surface. As the value of Γ m increases, the quantum yield increases while the lifetime
decreases. We may observe then a variety of favorable effects due to metal particles, such
as increased fluorescence intensities, increased photostability 1 , and increased distances
for FRET. We refer to these favorable effects as metal-enhanced fluorescence (MEF).
The most dramatic relative changes are found for fluorophores with the lowest quantum
yields. If Q = 1.0, then changing Γ m has no effect. If Q is low, such as 0.1 in Fig. 2.5,
the metal-induced rate Γ m increases the quantum yield. At sufficiently high values of
Γ m , the quantum yields of all fluorophores approach 1.0. Examination of Fig.2.5 reveals
that larger values of Γ m /Γ are required to change the lifetime or quantum yield of low
quantum yield fluorophores. This effect occurs because, for the same unquenched lifetime
τ 0 , lower quantum yields imply larger values of k nr . Larger values of Γ m are required to
compete with the larger values of k nr .
1 less time at the excited state and as consequence lower bleaching