Angewandte
Chemie
DOI: 10.1002/anie.201203374
Fluorescent Nanoparticles
White Fluorescence from Core–Shell Silica Nanoparticles**
Jeremy Malinge, Clꢀmence Allain,* Arnaud Brosseau, and Pierre Audebert*
Silica nanoparticles (NPs) are versatile materials with unique
properties. Since the first reports of their colloidal fabrica-
tion[1,2] and functionalization[3–5] with light-emitting mole-
cules, their photophysical properties have been extensively
studied.[6,7] One of the most fascinating advances in this area
of research deals with the incorporation of multiple dyes in
one nanoparticle, which allows the formation of ultrabright
nanoobjects.[8] From the accumulation of different photo-
responsive molecules inside the silica core, complex photo-
physical events may arise, such as excimer formation,[9,10]
photoswitchable fluorescence emission,[11] or resonance
energy transfer.[12,13] As a result, new luminescent nanoobjects
with multiple color emission under a single excitation wave-
length are accessible and find applications in sensors,[14,15]
biolabeling,[16] and emitting displays.
properties of these nanoobjects.[29] Another recent study from
our group[30] showed that a naphthalimide chromophore
covalently linked to a tetrazine was able to act as an antenna,
increasing the brightness of the tetrazine. Herein we report
a new white-light-emitting system based on bichromophoric
silica NPs doped by a naphthalimide dye in the inner core and
by a tetrazine derivative on the outer shell.
Reaction between 1,8-naphthalic anhydride and ethanol-
amineprovided N-(hydroxyethyl)-1,8-naphthalimide 1rapidly
and in good yield. Intermediate 1 was reacted with trie-
thoxy(3-isocyanatopropyl)silane, providing the desired
silane-functionalized naphthalimide derivative (designated
Napht). The corresponding tetrazine derivative was obtained
as described in our previous study (Scheme 1; see also the
Supporting Information).
In the meantime, much attention has been directed to the
fabrication of white-light-emitting devices. In most devices,
the white fluorescence arises from the mixing of various dyes
having blue, green, and orange fluorescence simultane-
ously.[17–22] Such displays require a careful control of each
color contribution and of the energy transfer between the
different dyes to obtain the pure white color according to the
CIE standards. This issue has been overcome by Park et al.[23]
by blocking any energy transfer between two linked fluoro-
phores with blue and yellow emission. Multiple dye-doped
silica NPs may be an interesting alternative to prepare such
white displays, as many different light-emitting fluorophores
can be hosted in the same unit.
s-Tetrazines are aromatic heterocycles that have a struc-
ture similar to a benzene ring, where four carbon atoms are
replaced by nitrogen. We[24] and others[25] have explored the
photophysical and electrochemical properties of these mole-
cules. Indeed, some tetrazines substituted with heteroatoms[26]
showed a high fluorescence quantum yield, a good photo-
stability, and a long fluorescence lifetime. Furthermore, s-
tetrazines are extremely electron-deficient and can be rever-
sibly reduced, leading to nonfluorescent species.[27] These
photophysical and electrochemical properties make these
fluorophores attractive candidate for sensing purposes.[28] In
a recent study, we attached tetrazine derivatives onto the
surface of silica nanoparticles and evaluated the sensing
Scheme 1. Naphthalimide (Napht) and tetrazine (Tz) derivatives.
The co-functionalized NPs are prepared in two steps. First,
the naphthalimide dye is incorporated inside the silica matrix
following an existing method,[12] which involves the prehy-
drolysis of Napht before the onset of colloid formation, to
provide naphthalimide doped silica nanoparticles (denoted
NP0). After the reaction, the samples were washed by
successive centrifugations and redispersions in dry CH3CN.
The NPs thus obtained present a diameter of 80 nm measured
by dynamic light scattering (DLS). A value of 0.18 mmol of
Napht per gram of silica was calculated from TGA. In the
second step, a NP0 suspension in dry CH3CN is reacted with
the dye Tz in the presence of acetic acid, as described in our
previous work.[29]
[*] J. Malinge, Dr. C. Allain, A. Brosseau, Prof. P. Audebert
PPSM, CNRS UMR 8531, Ecole Normale Supꢀrieure de Cachan
61, avenue du Prꢀsident Wilson, 94235 Cachan cedex (France)
E-mail: clemence.allain@ppsm.ens-cachan.fr
Aliquots of the grafting reaction mixture were isolated
after 2, 24, and 40 h, providing three different sets of NPs,
which are respectively denoted NPA, NPB, and NPC. The
amount of tetrazine grafted as well as the DLS size
distributions are shown in Table 1.
[**] This work was supported by the DGA (Direction Gꢀnꢀrale de
l’Armement) for a Ph.D. to J.M. Many thanks to Dr. Gilles Clavier
and Dr. Rachel Mꢀallet-Renault for fruitful discussions.
The photophysical properties of the new silane derivative
Napht were investigated in CH3CN. The absorption spectrum
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!