Silica-Based Nanoparticles: Design and Properties - Advanced Fluorescence Reporters in Chemistry and Biology - page 242

of the fluorophores [ 70 - 77 ]. The interaction between the metal and the fluorophore
is unique and complicated. A generally accepted mechanism is that the metal
plasmon plays an important role in assisting energy transfer from the metal to the
fluorophores. To investigate this effect, a core-shell structure composed of metal
and a DDSN has been employed in some recent studies. This core-shell structure is
a proper model for studying plasmon-controlled fluorescence enhancement.
Figure 5 shows two typical core-shell structures; (a) contains a metal core and a
dye doped silica shell [ 30 , 32 , 33 , 78 - 85 ] and (b) has a dye doped silica core and a
metal shell [ 31 , 34 ]. There is a spacer between the core and the shell to maintain the
distance between the fluorophores and the metal to avoid fluorescence quenching
[ 30 , 32 , 33 , 78 - 80 , 83 ]. Usually, the spacer is a silica layer in this type of
nanostructures. Various Ag and Au nanomaterials in different shapes have been
used for fluorescence enhancement. Occasionally, Pt and Au-Ag alloys are selected
as the metal. A few fluorophores have been studied in these two core-shell
structures including Cy3 [ 30 ], cascade yellow [ 78 ], carboxyfluorescein [ 78 ],
Ru(bpy) 3 2+ [ 31 , 34 ], R6G [ 34 ], fluorescein isothiocyanate [ 79 ], Rhodamine 800
[ 32 , 33 ], Alexa Fluor 647 [ 32 ], NIR 797 [ 82 ], dansylamide [ 84 ], oxazin 725 [ 85 ],
and Eu 3+ complexes [ 33 , 83 ].
The geometry of the nanoscaled metals has an effect on the fluorescence
enhancement. Theoretically, when the metal is introduced to the nanostructure,
the total radiative decay rate will be written as

G m corresponds to
the radiative decay rate close to the metal surface. So, ( 1 ) and ( 2 ) should be
modified and the quantum yield and lifetime are represented as:

G þ G m , where

G þ G m
G þ G m þ

g r G
g r G þ

Q ¼

k nr ¼

(3)

k nr

1
G þ G m þ

1
g r G þ

t ¼

k nr ¼

(4)

k nr

). In a fluorescence enhancement system,
interactions between the metal and dye molecules result in shortening of the
excited-state lifetime, thus improving the photostability of the dye.

Here, g r is a factor of (1 þ G m / G

Fig. 5 Cores-shell structure
of metal and DDSN
complexes. (a) Metal core/
silica shell, (b) silica core/
metal shell