Quenching on Gold Nanoparticles
Ensheathed with Layer-by-Layer
Gre ´gory Schneider and Gero Decher*
Institut Charles Sadron, CNRS UPR022, 6 rue Boussingault, F-67083 Strasbourg
Cedex, France, and UniVersite ´ Louis Pasteur (ULP), 1 rue Blaise Pascal,
F-67008 Strasbourg Cedex, France
Nicolas Nerambourg, Raı 1ssa Praho, Martinus H. V. Werts,* and
Synthe `se et Electrosynthe `se Organiques, CNRS UMR6510, UniVersite ´ de Rennes 1,
Campus de Beaulieu, Ba ˆt. 10A, F-35042 Rennes Cedex, France
Received December 9, 2005; Revised Manuscript Received January 11, 2006
We report on the preparation, characterization, and photophysical study of new fluorescent core/shell nanoparticles fabricated by electrostatic
layer-by-layer assembly. On the basis of gold cores with a diameter of 13 nm, these nanocolloids possess different fluorescently labeled
polymer corona layers at various distances from the surface of the core metal using nonfluorescent polyelectrolytes as spacer layers. UV−
visible spectroscopy and transmission electron microscopy confirm that the particle suspensions of fluorescently labeled core/shell nanoparticles
are stable at all stages of their construction. Photophysical investigations reveal strongly distance-dependent fluorescence quenching in
these particle systems. The contribution of the metal core to this quenching can be assesed precisely after the gentle dissolution of the gold
cores by potassium cyanide. The photophysical measurements reveal clearly that the gold nanoparticles decrease the transition probability
for radiative transitions.
Metal nanostructures can have interesting and potentially
useful effects on the photoluminescence of nearby emitters.
Molecular chromophores situated in the vicinity of isolated
colloidal metal particles in suspension usually experience
quenching of their fluorescence,1-10whereas photolumines-
cence may be enhanced in more complex structures that arise
from the deposition of aggregated metal particles onto
surfaces.11-14In the latter case, the enhanced luminescence
appears to arise from certain “hot spots” where the metal is
structured such that the local electromagnetic field is highly
concentrated. An interesting challenge in this context would
be to try and see if these “photonic hot spots” may be self-
assembled in solution. For such an endeavor, it is important
to know in detail the photophysical behavior of the more
simple systems that may eventually constitute the building
blocks for such assemblies.
In this letter, we describe the use of layer-by-layer (LBL)
deposition15-18of oppositely charged polyelectrolytes onto
13-nm-diameter gold colloids to fabricate metal core-polymer
shell capsules in which fluorescent organic dyes fluorescein
and lissamine rhodamine B (in the following referred to as
lissamine or LISS, Scheme 1) are situated at various distances
from the gold core. LBL-coated gold nanoparticles have been
reported before,19,20but this earlier work only describes LBL-
shells of up to eight layers with unknown synthetic yields.
However, the work in the present manuscript requires the
assembly of 20 and more layers, including layers with
functional groups, which requires robust coating conditions
and a high stability of the nanoparticle dispersion like that
reported previously by us.21The limpid and optically dilute
suspensions of the dye-labeled core-shell particles can be
studied readily by the methods of molecular fluorescence
spectroscopy in solution. A useful property of the metal-
containing core-shell particles is that the gold core can be
dissolved gently by the addition of cyanide ions, which
allows for a direct and precise comparison of the photo-
physical behavior of the fluorophores in the presence and
absence of the metal nanocore. The photophysical investiga-
Vol. 6, No. 3
10.1021/nl052441s CCC: $33.50
Published on Web 01/27/2006
© 2006 American Chemical Society
tion of fluorophores positioned at well-defined distances near
metal nanoparticles is not only of interest for nanoscale optics
but especially also for the domain of functionalized metal
colloids,22,23which may for example find applications in
biomedical diagnostics,24,25where information is frequently
conveyed by fluorescence.
Gold nanoparticles were synthesized using the reduction
of tetrachloroauric acid by trisodium citrate.26,27Several
primer layers of low molecular weight poly(allylamine
hydrochloride) (PAH, MW) 15 000 g/mol) and poly(stryrene
sulfonate) (PSS, MW ) 13 400 g/mol) were adsorbed
consecutively in a layer-by-layer fashion, to obtain Au
nanoparticles coated with 2, 10, and 20 layers (Au(PAH/
PSS)n).21Fluorescein isothiocyanate (FITC) and lissamine
rhodamine B sulfonyl chloride (LISS) were covalently
attached to PAH to obtain PAH-FITC and PAH-LISS,
respectively (see the Supporting Information). Only a small
fraction (approximately 1%) of the available amino groups
was labeled. PAH-FITC or PAH-LISS were adsorbed onto
the Au(PAH/PSS)n core-shell particles, followed by the
adsorption of a final PSS layer (see Scheme 1). After each
of the deposition steps, the particles were washed thoroughly
by successive centrifugation/resuspension cycles.
Transmission electron microscopy (TEM) investigations
on Au(PAH/PSS)n(PAH-FITC) revealed mean distances
between the surface of the Au core and the fluorescent layer
of about 1.5 ( 0.3 nm (n ) 1), 4.0 ( 0.5 nm (n ) 5), and
7.9 nm ( 0.7 nm (n ) 10), respectively; for the case of
Au(PAH/PSS)n(PAH-LISS), the corresponding distances
were 1.8 ( 0.3 nm (n ) 1), 3.9 ( 0.5 (n ) 5), and 7.7 (
0.6 nm (n ) 10), respectively. TEM micrographs of
individual coated dye-labeled nanoparticles are shown at the
right of Figure 1 for 1, 5, and 10 spacer pairs of PAH/PSS
layers. The LBL-assembled shells were stained with uranyl
acetate in order to enhance the contrast of the images. The
left side of Figure 1 shows overviews of samples of such
particles that are composed of a majority of single particles
but also of a few aggregates with two or more Au cores.
The statistical evaluation of the aggregation state derived
from the TEM images is reported in the Figure 1C for 1, 5,
and 10 primer pairs of layers with PAH-FITC and PAH-
LISS. Because aggregation may take place during the
preparation of the TEM grids, the statistics obtained through
TEM measurements are to be considered as a “worst-case”
representation of the distribution in the suspensions on which
the photophysical measurements are done. In all suspensions,
over 60% of the core-shell particles exist as isolated entities.
UV-visible absorption spectra taken during layer build-
up (see the Supporting Information, Figure S1) indicate that
the incorporation of fluorescent polyelectrolytes (PAH-FITC
Core-Shell Nanoparticles Containing Fluorescent Corona
Layer-by-Layer Assembly for the Construction of
aThe distance between the metal cores and the fluorescent layers
is conveniently adjusted by varying the number of nonfluorescent
layers (in black/white) between the gold nanoparticle core and the
fluorescent layer (in green), thereby fine-tuning the photophysical
properties of these nanodevices. The chemical structures of the
fluorescently labeled poly(allylamine) and the dyes are shown in
the bottom part.
Figure 1. (A) Left: Transmission electron micrograph (TEM)
overview on particles bearing 10 primer pairs of PAH/PSS layers
and further coated with PAH-FITC, Au(PAH/PSS)10(PAH-FITC).
Right: TEM of LBL-coated individual gold nanoparticles en-
sheathed with n ) 1, 5, and 10 pairs of PAH/PSS spacer layers,
respectively, with a last layer of PAH-FITC, Au(PAH/PSS)n-
(PAH-FITC), before deposition of the terminating PSS layer. All
samples were stained with uranyl acetate. (B) Left: TEM overview
on lissamine-functionalized Au(PAH/PSS)10(PAH-LISS). Right:
TEM of LBL coated individual Au(PAH/PSS)n(PAH-LISS) nano-
particles, with n ) 1, 5, and 10. (C) Statistical evaluation of the
aggregation state derived from TEM images of 2000 Au cores.
Nano Lett., Vol. 6, No. 3, 2006531
and PAH-LISS) does not significantly perturb layer growth
or the aggregation state of the particles as illustrated by the
absence of spectral changes due to aggregation even when
only a few polymer layers are adsorbed (case n ) 1). This
is explicitly stated here because this is the first report in
which functionalized polymers (in this case carrying fluoro-
phores) are used in the deposition sequence. Additionally,
one can safely continue to deposit at least one layer of PSS
on top of the fluorescent layer that generates an increase of
the PSS absorption band at 225 nm. After several layers (n
> 5), the surface plasmon (SP) resonance appears to reach
a fixed intensity and position at 534 nm in the case of PAH-
FITC and near 533 nm for PAH-LISS. This dependence of
the position and the intensity of the SP band on the number
of deposited layers is due to dielectric screening of the SP
by the adsorbed polymers.
Because of the low level of dye labeling of the outer
polymer layer, the UV-visible absorption spectra of the
core-shell particles (Figure 2A and C) do not show any
clearly distinguishable dye-related absorption bands, being
completely dominated by the intense SP absorption of the
gold nanocore. The presence of the dyes, however, becomes
evident through the observation of their typical emission
spectra under excitation at their characteristic absorption
wavelengths (Figure 2B and D). Fluorescence excitation
spectra (not shown) confirm their identity because they reveal
the typical absorption profile of each dye.
Attempts to isolate the absorption bands of the dyes
through numerical subtraction of the absorption spectra of
unlabeled particles did not yield unambiguous and conclusive
results. However, the fact that metallic gold is used as the
nanoparticle core allows for the removal of the surface
plasmon band by dissolving the gold with the help of cyanide
ions. This is an elegant procedure, which, however, requires
highly stable particle suspensions and low cyanide concen-
trations in order to avoid particle aggregation caused by
electrostatic screening. In previous work on this type of
core-shell material, the removal of the gold core was shown
to result in empty nanospheres.19,21In the present experi-
ments, the dissolution of gold is started by adding a small
volume (typically 20 µl) of a stock solution (10 g L-1) of
KCN in water to the particle suspension contained in the
fluorescence cuvette to bring the concentration of KCN in
the cuvette to 1 mM. The evolution of the UV-visible
absorption and fluorescence emission spectra was followed
As shown in Figure 2A and C, dissolution of the gold
core by cyanide leads to a gradual disappearance of the gold
surface plasmon absorption. After completion of this process,
a weak absorption band remains, which corresponds to light
absorption by the dye, the fluorescent dye remaining chemi-
cally unchanged after this mild dissolution of the core. The
dissolution of the metallic cores is accompanied by a strong
increase of the observed fluorescence (Figure 2B and D).
All fluorescence spectra have been fully corrected, including
a correction for inner-filter effects (see the Supporting
Information). Thus, they represent the intrinsic emission
spectra and intensities of the dye-labeled capsules without
any contibutions from experimental factors such as wave-
length-dependence of the detector response or reabsorption
of emitted light by the solution itself.
The etching reaction results in the formation of “lost
template” fluorescent nanospheres independent of the number
of primer layers, as shown in Figure 3. The dark spot
observed in the inside of the particles may be due to the
presence of uranyl acetate in the empty core of the sphere.
Figure 2. (A) Evolution in time of the absorption spectrum of a solution of Au(PAH/PSS)5(PAH-FITC/PSS) in 1 mM TRIS buffer upon
addition of 1 mM KCN. The first spectrum was taken before addition of KCN. The complete disappearance of the gold nanocore takes
approximately 4 h. The inset shows a close-up of the residual dye absorbance after 1 night. (B) Corresponding emission spectra (λexc) 490
nm). (C) Evolution of the absorption spectrum of Au(PAH/PSS)5(PAH-LISS/PSS) in water upon addition of 1 mM KCN. (D) Emission
spectra corresponding to C (λexc) 563 nm).
Nano Lett., Vol. 6, No. 3, 2006
This spot is not visible when particles are not stained and
may be related to the hollowness of these nanospheres. The
overview TEM micrograph shown in Figure 3B shows little
to no changes in the aggregation state of the nanoparticles
with a majority of single particles and few doubles or triples
like those reported in Figure 1.
The dye absorption bands remaining after dissolution of
the cores enable a reliable determination of the relative dye
concentration in each sample, irrespective of fluctuations in
polymer layer deposition efficiencies, which may introduce
batch-to-batch variations in the level of dye loading of the
core-shell particles (UV-visible spectrometry might actu-
ally be used to study these fluctuations). The information
about the dye concentration obtained after dissolution of the
gold core was used to scale the fluorescence emission spectra
of the initial metal-core polymer-shell capsules. The scaled
spectra are shown in Figure 4. The areas under the spectra
represent the relative brightness of dye fluorescence for each
type of core-shell capsule. The relative brightness is
proportional to the product, ?Φ, of the extinction coefficient
and the fluorescence quantum yield of the incorporated dye.
It is expressed on a “per molecule” (or “per molar”) basis
and conveys how the fluorescent behavior of an average dye
molecule changes as it is positioned nearer and nearer to
the gold nanocore. We note that in this context the number
of fluorescent dye molecules per particle is not important.
For completeness, we estimated these numbers on the basis
of the UV-visible absorption data. The number of fluores-
cein dye molecules per core-shell particle Au(PAH/PSS)n-
(PAH-FITC/PSS) are estimated to be 60 (n ) 1), 62 (n )
5), and 43 (n ) 10). The numbers for Au(PAH/PSS)n(PAH-
LISS/PSS) are 68 (n ) 1), 85 (n ) 5), and 74 (n ) 10).
This justifies that approximately 40-60 PAH chains adsorb
per nanoparticle at these stages of deposition.
There is a clear rise in relative brightness as the number
of spacer layers separating the dye and the metal core
increases, which confirms the distance dependence of the
fluorescence quenching. Instead of considering the fluores-
cence quantum yield, Φ, alone, it is more appropriate to
discuss the photoluminescence spectra of these metal-
containing capsules in terms of brightness (?Φ). We will
see below that the fluorescence quenching is partly due to a
decrease in the probability of spontaneous emission, in which
case there is likely to be an effect on the transition probability
of absorption as well, potentially leading to a change in
extinction coefficient.28,29Thus, changes in observed fluo-
rescence intensity may depend on both changes in light
absorption and emission probabilities.
In the case of the empty nanospheres obtained upon
cyanide-induced disolution of the gold core, we have direct
access to the absorbance spectrum, which enables us to
measure the fluorescence quantum yields of the dyes in these
empty polymer capsules using quinine bisulfate in 0.5 M
H2SO4(Φ ) 0.546) as a reference. The quantum yield data
for the empty capsules (Table 1) suggest that the polymer
matrix also has an effect on the fluorescence of the
incorporated dye. Compared to the free dyes, the quantum
yields of the polymer-bound dyes and the dyes incorporated
in the shell of empty capsules are diminished. This is not
unexpected; it is known that fluorescence quenching can
occur through electron transfer from nearby free amino
groups. Variations in the local pH, which may affect
fluorescein and are known to exist in LBL structures,30are
unlikely to play a role because the suspensions were buffered
at pH 8.3, a pH significantly above the first pKa of the
fluorescein chromophore, 6.4. This is substantiated by the
UV-visible spectra of the empty capsules, which show that
the fluorescein chromophore is in its dianion form. The
presence of less fluorescent, protonated forms of fluorescein
Figure 3. (A) Transmission electron micrographs (TEM) of uranyl-
stained LBL nanospheres consisting of 2, 10, and 20 layers of
alternating PAH and PSS ((PAH/PSS)n(PAH-FITC/PSS) with n
) 1, 5, and 10) after complete gold core dissolution by an etching
reaction with potassium cyanide. The bar corresponds to 10 nm.
(B) Overview TEM micrograph of the empty nanospheres after
dissolution of the gold core by cyanide and staining with uranyl
acetate (left). Enlarged picture on two individual “lost template”
Figure 4. Fully corrected emission spectra of core-shell nano-
particles of structure Au(PAH/PSS)n(PAH-FITC/PSS) (top, λexc
) 490 nm) and Au(PAH/PSS)n(PAH-LISS/PSS) (bottom, λexc)
563 nm) in aqueous suspension. Spectra have been scaled to equal
concentrations of dye, determined through the residual UV/vis
absorption band after complete etching of the gold core by 1 mM
KCN. Thus, their areas represent the relative brightness (propor-
tional to the product of extinction coefficient and fluorescence
quantum yield) of the dyes near the metal core.
Nano Lett., Vol. 6, No. 3, 2006533
would show up in the absorption spectra by the appearance
of an additional absorption band at shorter wavelengths.
Compared to fluorescein, lissamine appears to be particularly
affected by the incorporation in the layer-by-layer polymer
capsules. It may be that this dye is more sensitive to electron
transfer quenching by amino groups than fluorescein. The
polymers inside the layer-by-layer structure are probably
more compact than the free polymers in solution, leading to
a larger proximity of quenching amino groups.
The effect of the metal nanocore on the fluorescence of
the dyes can be quantified by evaluating the ratio of the areas
under the fluorescence spectra before (Ifull) and after (Iempty)
the core dissolution: η ) Ifull/Iempty. Because these two
fluorescence intensity measurements are done on the same
sample, the concentration of the dye is rigorously the same
(the cyanide only attacks the gold leaving the dyes and the
polymer shells intact, as evidenced by optical spectroscopy,
Figure 2, and TEM, Figure 3). Thus, the ratio, η, can be
measured with good certainty. Moreover, the dyes are kept
in the same polymer environment, the only difference being
the presence or absence of the metal core, and η reflects the
effect of the gold nanocore alone. This is important because
it allows us to study the effect of the metal core alone,
canceling out any effects of the polymer matrix. Figure 5
contains a plot of ratio η versus the number of spacer layers
separating the dye and the core. It shows that the metal core-
induced fluorescence quenching becomes significantly stron-
ger as the dye approaches the metal.
Insight of the origin of the quenching is obtained by
combining the steady-state value, η, with fluorescence
lifetime data. Fluorescence decay traces were obtained for
Au(PAH/PSS)n(PAH-FITC/PSS) and the corresponding
empty capsules using time-correlated single photon counting
(TCSPC) under excitation with a nanosecond flash lamp.
Unfortunately, TCSPC measurements for the lissamine
particles proved to be impossible using our current setup
because of the absence of sufficient excitation intensity at
the wavelength required. The fluorescein particles already
required long aquisition times. For all samples, a mono-
exponential fluorescence decay was observed (see the
Supporting Information, Figure S2). The fluorescence life-
times do not change drastically on going from a particle
containing a metal core to the corresponding empty capsule,
even when the overall fluorescence intensity is greatly
affected (Table 2).
The observed fluorescence decay time, τ, is determined
by the radiative rate constant, kr, and its nonradiative
counterpart, knr: τ ) (kr + knr)-1. It is related to the
fluorescence quantum yield through
Often, molecular fluorescence is quenched by nonradiative
processes that augment knrbut do not change kr. As a result,
changes in fluorescence quantum yield will be accompanied
by a proportional change of the observed decay time. In the
case of the Au(PAH/PSS)n(PAH-FITC/PSS) particles, there
is no such proportionality, suggesting that there the metal
core induces a change in kr. The ratio, η, of the fluorescence
intensity of a solution of gold-core-containing particles and
that of the same solution after complete removal of the gold
nanocores is equal to the ratio of the brightnesses, ?Φ, of
the full and empty capsules where ? is the molar extinction
where ?′ and kr′ are the extinction coefficient and the radiative
rate constant for the gold-containing capsules. The change
in radiative rate induced by the gold nanocore may be
Effect of the Polymer Matrix on Dye Fluorescencea
dye ) FITCdye ) LISS
free dye in solution
PAH-bound dye in solution
aAbsolute fluorescence quantum yields (Φ) of the free dyes in solution,
the polymer bound dyes in solution, and the dyes incorporated in the empty
polymer capsules, after dissolution of the gold core by 1 mM KCN. The
estimated error in the quantum yield values is (15% for the free dye and
the polymer. It amounts to (30% for the capsules because of the weakness
of UV/vis absorption of the solutions. The fluorescein (FITC) samples were
measured in 1 mM TRIS + 1 mM KCN (pH 8.3); the lissamine (LISS)
samples were measured in 1 mM KCN.bFree fluorescein (not FITC) in
Figure 5. Fluorescence intensity of core-shell Au(PAH/PSS)n-
(PAH-dye/PSS) nanoparticles relative to the intensity of the
corresponding empty nanocapsules after removal of the gold core
as a function of the number of polymer layers separating the dye-
doped polymer layer from the core of the capsules.
Lifetimes upon Removal of the Gold Nanocore from
Fluorescein-Labeled Au(PAH/PSS)n(PAH-FITC/PSS) Capsulesa
Variations of the Fluorescence Intensities and
n ) 1
n ) 5
n ) 10
0.11 ( 0.01
0.24 ( 0.02
0.29 ( 0.02
3.2 ( 0.2
3.3 ( 0.1
3.1 ( 0.1
3.3 ( 0.2
3.7 ( 0.1
4.0 ( 0.1
0.34 ( 0.02
0.52 ( 0.02
0.61 ( 0.02
aη ) Ifull/Iempty. τfulland τemptyrepresent the fluorescence lifetimes of
the full core-shell particles and empty capsules, respectively, measured
by TCSPC. R is the factor by which the radiative rate of the chromophore
is changed in the presence of the golden nanocore, R2) ητempty/τfull.
Φ ) krτ
Nano Lett., Vol. 6, No. 3, 2006
represented by an empirical factor, R, such that kr′ ) Rkr. A
change in this spontaneous emission transition probability
might be accompanied by a change in the probability for
absorption. Here we tentatively assume that ?′ ) R?,28,29
although we are aware that the Einstein relation between
absorption and spontaneous emission is defined for isotropic
media, which is not the case here. Theoretical models
describing light emitters and absorbers near a metal particle
more explicitly may be applicable.31Assuming that the
Einstein relation holds, the factor R can be obtained from
our experimental data through
All three parameters in eq 3 (η, τempty, and τfull) can be
measured with relative ease and reliability on the same
solution, before and after exposure to 1 mM KCN. In a
certain sense, the sample serves as its own reference. The
values of R for different fluorescein-gold nanocore separa-
tions are in Table 2. As the fluorescein dye approaches the
core surface, its radiative transition probabilities appear to
drop to 34% of its original value at a distance of 1.5 nm.
The observed reduction in radiative rate may be reflected in
changes of the extinction coefficient,28,29but reliable deter-
mination of the dye absorption bands in the presence of the
metal core has not been possible here because of the low
dye-labeling levels applied. The effect might be observed
in particles having higher dye loadings, such that the
absorption band of the dyes may be more readily distin-
guished from the surface plasmon absorption band. Profound
changes in the absorption spectrum of J aggregates of a
cyanine dye adsorbed directly onto the surface of silver and
gold nanoparticles have been observed recently.32
Studies on the direct effect of the metal particle core on
molecular light absorption may clarify whether the transition
probabilities for absorption and emission are affected in the
same way. If this is not the case, then the R2term in eq 3
needs to be replaced by the product (RabsRem) of the changes
in absorption and spontaneous emission probabilities. Direct
access to the absorption spectrum of the dyes in the presence
of the metal core may also shed more clarity onto the
apparent discrepancy between the fact that the spectral
position of the surface plasmon practically ceases to shift
after deposition of 10 layers (5 pairs) of polymers, whereas
the reduction of photoluminescence by the particle core
occurs for even twice that distance. It may be that the
presence of the (very weak) dye absorption partially obscures
further shifts of the SP band. In previous work,21the SP band
of gold nanoparticles coated using unlabeled PAH and PSS
continues to shift slightly even after the deposition of 20
In conclusion, the gold nanocore quenches the fluorescence
of the fluorescein and lissamine dyes situated in the outer
polymer layers of the core-shell nanoparticles. Even with
20 spacer layers (i.e., 10 pairs of PAH/PSS, corresponding
to a dye-nanocore distance of 8 nm), this quenching is
significant. Nonetheless, the fluorescence of the core-shell
particles remains sufficiently bright for potential applications
as diagnostic or sensing devices. Combining the measured
relative brightness and observed fluorescence decay times
of the set of fluorescein-containing core-shell nanoparticles,
we find clear indications that an important part of the
fluorescence quenching is due to a reduction of the radiative
rate, which is in agreement with very recent findings by
Dulkeith et al.10Quantitave experimental examples of the
effect of isolated metal nanoparticles on the radiative rate
of nearby photoluminescent emitters are scarce. In this Letter,
the distance between dye and metal core is controlled reliably
through LBL assembly, and the fluorescence measurements
give a direct access to the effect of the metal core on the
The layer-by-layer method is, in principle, applicable to
other core materials, according to classical rules of colloidal
chemistry.33-35It may thus be possible to change the shape,
size, and composition of the metal core. LBL deposition
enables precise control of the distance separating the photo-
active units and the metal core and provides the flexibility
for the facile integration of different chromophores and other
functionalities. The “lost template” photoactive nanospheres
obtained after dissolution of the gold core constitute in
themselves interesting objects that can form the basis of
multifunctional fluorescent sensors or perhaps even photo-
controllable delivery agents. From a more fundamental point
of view, the physicochemical behavior (e.g., molecule and
ion permeability etc.) of these objects may be an interesting
subject of further studies.
Acknowledgment. Financial support from Rennes Me ´tro-
pole, Universite ´ de Rennes 1 (BQR), and the Ministe `re de
l′Education Nationale, de la Recherche et de la Technologie
is gratefully acknowledged. Annette Thierry and Marc
Schmutz are also acknowledged for their invaluable help and
support in transmission electron microscopy.
Supporting Information Available: Preparation of the
materials and details of spectroscopic measurements. This
material is available free of charge via the Internet at http://
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