Sol-gel emulsion synthesis of biphotonic core-shell nanoparticles based on lanthanide doped organic-inorganic hybrid materials

Article abstract of DOI:10.1039/c2jm15470f

Eu³⁺ and Tb³⁺ complexes based on an organo-alkoxysilane derived from 4-azido dipicolinic acid by "click chemistry" were coated onto silica nanoparticles previously prepared by the water in oil emulsion method. Structural and optical properties of the resulting luminescent core-shell monodisperse (30 nm) nanohybrids were fully characterized by infrared and Raman spectroscopies as well as by electron microscopy. Fluorescence spectroscopy evidenced that an efficient ligand-to-Ln³⁺ energy transfer - both in the mono- and biphotonic modes - occurs. Furthermore, surface modification by amino groups was realized, paving the way for nanotags of biological interest. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm15470f

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PAPER
Sol–gel emulsion synthesis of biphotonic core–shell nanoparticles based on
lanthanide doped organic–inorganic hybrid materials†
ac
bc
Xianmin Guo,bc Jean-Louis Canet,ac Damien Boyer,
Arnaud Gautier
and Rachid Mahioubc
*
*
Received 26th October 2011, Accepted 9th January 2012
DOI: 10.1039/c2jm15470f
Eu3+ and Tb3+ complexes based on an organo-alkoxysilane derived from 4-azido dipicolinic acid by
‘‘click chemistry’’ were coated onto silica nanoparticles previously prepared by the water in oil emulsion
method. Structural and optical properties of the resulting luminescent core–shell monodisperse (30 nm)
nanohybrids were fully characterized by infrared and Raman spectroscopies as well as by electron
microscopy. Fluorescence spectroscopy evidenced that an efficient ligand-to-Ln3+ energy transfer—
both in the mono- and biphotonic modes—occurs. Furthermore, surface modification by amino groups
was realized, paving the way for nanotags of biological interest.
luminescence is to design and prepare conjugated organic ligands
Introduction
(e.g. b-diketonates, aromatic carboxylates or heterocyclic
systems) that possess an excited triplet state T1 well matching the
high excited states of the central Ln3+ ion.6 Among them, dipi-
colinate (DPA) derivatives constitute a class of popular ligands
presenting strong absorption coefficients and efficient transfer of
absorbed energy to the central Ln3+ ions.7 Their maximum
absorption wavelengths may be tuned by changing the attached
group in position C-4 (Fig. 1) as we and others have recently
demonstrated through a simple ‘‘click chemistry’’ process.8 Click
chemistry, particularly thanks to the copper-catalyzed azide–
alkyne cycloaddition (CuAAC), brought new vitality to various
fields of research since its concept was introduced in 2001 by
Meldal and Sharpless.9 Its prominent and popular features
include simple and mild reaction conditions, high yield, easy
isolation and few byproducts thus opening a wide range of novel
and exciting opportunities for new materials design. To date, this
‘‘click’’ reaction has been used for the preparation of functional
Within the past ten years, the development of fluorescent silica-
based nanoparticles has aroused great scientific and technolog-
ical interest for bioanalysis and biotechnological applications
since they exhibit many more advantages in comparison with free
fluorophores, quantum dots or fluorescent polymer-based
nanoparticles (NPs).1 Indeed, the use of a silica matrix to
encapsulate a luminescent dye prevents any problems of
bleaching, blinking, leaching, hydrophobicity or biocompati-
bility.2 Regarding surface functionalization of such luminescent
NPs, specific functional groups (e.g. amine, thiol, carboxylic
acid) can be easily grafted using various organoalkoxysilanes
allowing us to link a broad range of biorecognition agents (such
as nucleic acids, antibodies or proteins).3
Among the various types of fluorescent sources available
today, lanthanide (Ln3+) complexes appear as the most attractive
since they exhibit outstanding optical properties:4 long lumines-
cence lifetime (hundreds of microseconds), large Stokes shift
(>150 nm) and narrow emission bands (<10 nm).4 However, as
Laporte selection rules forbid f–f transitions of Ln3+ ions, organic
ligands are required to attain high luminescence quantum effi-
ciencies. This latter characteristic results from the intense
absorption band of organic chromophores and an efficient
organic ligand-to-Ln3+ energy transfer by the so-called antenna
effect.5 Consequently, the key factor to obtain strong
aClermont Universiteꢀ, ENSCCF, Institut de Chimie de Clermont-Ferrand,
BP 10448, F-63000 Clermont-Ferrand, France. E-mail: damien.boyer@
ensccf.fr; Fax: +33 (0)4-73-40-71-08; Tel: +33 (0)4-73-40-76-47
bClermont Universiteꢀ, UBP, Institut de Chimie de Clermont-Ferrand, BP
10448, F-63000 Clermont-Ferrand, France. E-mail: Arnaud.Gautier@
univ-bpclermont.fr; Fax: +33 473-40-77-17; Tel: +33 473-40-76-46
cCNRS, UMR 6296, ICCF, F-63177 Aubieꢁre, France
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm15470f
Fig. 1 Europium and terbium complexes of DPA derivatives.
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polymers, modification of surfaces, diversity of drugs, and so
on.10 However, to the best of our knowledge, the fabrication of
DPA derivatives based on the CuAAC reaction and their
applications in the field of lanthanide luminescence have been
rarely reported.8
In this manuscript we propose luminescent hybrid materials
prepared by covalently grafting the lanthanide complexes
Ln(DPAE–Si)3 ({1-[2,6-di(methoxycarbonyl)pyridin-4-yl]-1H-
1,2,3-triazol-4-yl}methyl 3-(triethoxysilyl) propyl carbamate;
Ln ¼ Eu and Tb; Fig. 1) onto the surface of pure silica NPs
previously synthesized by reverse sol–gel emulsion. Further
surface functionalization with amino groups (–NH2) was carried
out. The structural, morphological and optical properties of
different materials were discussed in detail. Fluorescence
microscopy upon ultraviolet (UV) and infrared (IR) excitation
was achieved to assess the potentialities of these luminescent
hybrid materials for biolabelling applications.
Fig. 3 SEM images of (a) SiO2@Eu(DPAE–Si)3 and (b) SiO2@Tb(D-
PAE–Si)3.
The TEM images of SiO2@Ln(DPAE–Si)3 (Ln ¼ Eu3+ and
Tb3+) show spherical particles with uniform size distributions
around 30 nm, in accordance with SEM investigation results.
Although the core–shell structures for both materials could not
be confirmed by the above-mentioned characterization methods
owing to the limits of equipment, the luminescent properties
coming from the characteristic Ln3+ ions evidence that
Ln(DPAE–Si)3 (Ln ¼ Eu3+ and Tb3+) complexes have been
successfully grafted onto the surface of silica nanospheres.
Furthermore, surface functionalization with amino groups could
be realized using aminopropyl triethoxysilane (APTES). The
Raman spectrum of APTES modified SiO2@Eu(DPAE–Si)3
nanoparticles (Fig. 5a) exhibits typical stretching vibrations of
primary amines’ N–H bonds located between 3250 and 3400
cmꢀ1. Furthermore, vibration bands ascribed to aliphatic (–CH3)
and acyclic (–CH2–) groups are much stronger in Fig. 5b.
Results and discussion
Structural characterizations
Fig. 2 presents the FTIR spectra of DPAE–OH (a), DPAE–Si
(b), SiO2@Eu(DPAE–Si)3 (c) and SiO2@Tb(DPAE–Si)3 (d).
Compared to Fig. 2a, new characteristic vibration bands are
observed in Fig. 2b which are assigned to –OCONH– groups (C–
N stretching and N–H bending at 1540 cmꢀ1) and C–Si–OEt
(1391, 1104, 1078 and 795 cmꢀ1) groups, proving that the organic
ligand DPAE–OH was successfully functionalized by 3-iso-
cyanatopropyltrimethoxysilane (ICPTES), in good agreement
with 1H NMR results. In Fig. 2c and d, the formation of the Si–
O–Si framework is confirmed by the broad absorption band
centered at 1100 cmꢀ1 (nas, Si–O) and the narrower one located at
795 cmꢀ1 (ns, Si–O). Moreover, the characteristic band at 1540
cmꢀ1 attributed to the –OCONH– groups can also be seen in
both hybrid materials, suggesting that DPAE–Si remains intact
in the whole procedure. SEM images of SiO2@Ln(DPAE–Si)3
(Ln ¼ Eu3+ and Tb3+) are displayed in Fig. 3.
Photoluminescence studies
As discussed in the Introduction, the effective ligand-to-Ln3+
intramolecular energy transfer is one of the key factors for design
of Ln-based luminescent hybrid materials. Fig. 6 exhibits the UV
absorption spectra of the parent ligand DPAE–OH, functional-
ized ligand DPAE–Si, the resultant core–shell hybrid materials
SiO2@Ln(DPAE–Si)3 (Ln ¼ Eu3+ and Tb3+) in ethanol, and the
excitation spectra of SiO2@Ln(DPAE–Si)3 (Ln ¼ Eu3+ and Tb3+)
as solid samples obtained by monitoring the emission wave-
lengths of Eu3+ and Tb3+ ions at 616 nm and 546 nm respectively.
Two absorption bands from 190 to 300 nm are easily observed
in the UV absorption spectra of all samples (Fig. 6a–d), which
can be attributed to transitions from the ground state (p) S0 to
the excited level (p*) S1 of organic ligands. Compared to the
ligand DPAE–OH, the two absorption peaks for DPAE–Si
separately shift from 216 and 265 nm to 213 and 260 nm. This
It can be easily observed that both samples exhibit uniform
spherical morphologies with an average diameter of about
30 nm. The corresponding TEM images further support the
above statement, as shown in Fig. 4.
Fig.
2
FTIR spectra of (a) DPAE–OH, (b) DPAE–Si, (c)
Fig. 4 TEM images of (a) SiO2@Eu(DPAE–Si)3 and (b) SiO2@Tb(D-
SiO2@Eu(DPAE–Si)3 and (d) SiO2@Tb(DPAE–Si)3.
PAE–Si)3.
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efficient than through direct excitation in highly excited energy
levels of Eu3+ ions.
Fig.
7
exhibits the normalized emission spectra of
SiO2@Eu(DPAE–Si)3 and SiO2@Tb(DPAE–Si)3 as solid
samples recorded at room temperature upon excitation at the
maximum excitation wavelengths namely 300 nm and 289 nm
respectively. As shown in Fig. 7a, the emission spectrum of
SiO2@Eu(DPAE–Si)3 is composed of the typical fluorescent lines
of the Eu3+ ion at ca. 579, 591, 615, 651, and 695 nm, corre-
7
sponding to the intra shell-4f6 5D0 / FJ (J ¼ 0, 1, 2, 3, and 4)
5
7
transitions respectively, with the red D0 / F2 emission as the
dominant band. Moreover, a broad band with two peaks is also
detected in the range of 410–570 nm (magnified in Fig. 7), which
are attributed to the emissions arising from the triplet states of
/
the organic ligands and the Eu3+ 5D1 7F1–2 transitions,
respectively. However, the emission intensity in the blue-green
Fig.
5 Raman spectra of (a) SiO2@Eu(DPAE–Si)3 and (b)
SiO2@Eu(DPAE–Si)3@NH2.
spectral region is much weaker in comparison with those of the
5D0
/
7F0–4 transitions. As a consequence, the negligible
intensity of this band clearly reveals that an efficient energy
5
transfer occurs from the organic ligands to D0-Eu3+ ions in the
resultant core–shell hybrid materials.
In the case of SiO2@Tb(DPAE–Si)3, upon excitation in the
absorption band of organic ligands, four characteristic emission
5
7
bands of Tb3+ ions assigned to the D4 / F3–6 transitions are
observed, which locate respectively at 488, 543, 583, and 620 nm.
At the same time, the 5D4 / 7FJ (J ¼ 2, 1, and 0) transitions at
Fig. 6 UV-Vis absorption spectra of (a) DPAE–OH, (b) DPAE–Si, (c)
SiO2@Eu(DPAE–Si)3 and (d) SiO2@Tb(DPAE–Si)3 at 1 ꢁ 10ꢀ4 mM in
ethanol, respectively; excitation spectra of (e) SiO2@Eu(DPAE–Si)3 and
(f) SiO2@Tb(DPAE–Si)3 were recorded at room temperature from solid
samples by monitoring the red and the green fluorescence at 616 and
544 nm respectively.
weak blue-shift is probably due to a change in electron distri-
bution in p and/or p* states of the functionalized ligand of
DPAE–Si. Furthermore, the spectral shapes of SiO2@Eu(D-
PAE–Si)3 and SiO2@Tb(DPAE–Si)3 (Fig. 6c and d) are quite
similar to those of free ligands (Fig. 6a and b), indicating that
coordination of the Ln3+ ion to organic ligands does not have
a remarkable influence on the p–p* energies. However, intro-
duction of Ln3+ ions produces an increase of the crystal field
manifested by a more pronounced splitting between the two
bands. It should be noticed that an overlap clearly exists between
the absorption band of DPAE–Si and the excitation bands of
SiO2@Ln(DPAE–Si)3 (Ln ¼ Eu3+ and Tb3+) (Fig. 6e and f),
suggesting that Ln3+ ions can be efficiently sensitized by the
surrounding ligands through an ‘‘antenna effect’’.
For the excitation spectrum of SiO2@Eu(DPAE–Si)3, in
addition to a broad excitation band of organic ligands between
250 and 400 nm, a sharp peak at 391 nm is also observed and
assigned to intra-shell F0 / L6 absorption transition of Eu3+
ions. Compared to the absorption intensity of organic ligands,
this transition is much weaker, demonstrating that luminescence
sensitization through excitation of the ligands is much more
7
5
Fig. 7 Emission spectra of (a) SiO2@Eu(DPAE–Si)3 (lex ¼ 300 nm) and
(b) SiO2@Tb(DPAE–Si)3 (lex ¼ 289 nm) recorded at room temperature.
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7
648, 666 and 678 nm can also be detected in the amplificatory
spectral range from 630 to 710 nm. Similarly to Eu3+, the emis-
sion spectrum in the blue region of 410–475 nm was magnified
and exhibits very weak Tb3+ 5D3 / 7F2,3 transitions and a weak
contribution of the organic ligand indicating that an efficient
energy transfer occurs from this latter to 5D4-Tb3+ ions in
SiO2@Tb(DPAE–Si)3.
temperature by monitoring the Eu3+ 5D0 / F2 transition at
616 nm (Fig. 9).
The spectral repartition of this uncorrected spectrum from the
response of the dye laser solution used fits quite well with the one
photon excitation spectrum (OPE) recorded upon UV excitation
monitoring the same fluorescence. Since the excitation values are
twice the ones of the OPE spectrum, we assume that a two
photon excitation (TPE) mechanism via the ligands takes place
leading to the observation of the red fluorescence of the NPs.
This observation indicates clearly the capability of such NPs to
be utilized as TPE fluorescent probes.
The luminescence decay times (sexp) of the D0-Eu3+ excited
5
level for SiO2@Eu(DPAE–Si)3 and of the 5D4-Tb3+ excited level
for SiO2@Tb(DPAE–Si)3 were recorded at room temperature
using an excitation wavelength of 346.5 nm and by monitoring
the most intense emission lines at 616 and 543.5 nm, respectively
(Fig. 8).
Both decay curves are well fitted by a bi-exponential function
with a long and a short time contribution like in DPAE:Eu3+ and
DPAE:Tb3+ complexes. Such a behaviour indicates that Ln3+
ions (Ln3+ ¼ Eu3+ or Tb3+) exhibit two different local environ-
ments in each sample. The corresponding luminescence lifetimes
were independently determined to be 0.60 ms and 0.27 ms for
SiO2@Eu(DPAE–Si)3 and, 0.92 ms and 0.19 ms for
SiO2@Tb(DPAE–Si)3. The long components for both samples fit
well with those derived from the complexes while the short
components are twice higher. Time resolved fluorescence line
narrowing spectroscopy coupled to structure description is
planned to explain these features and will be the subject of
further experiments.
Fluorescence microscopy analyses
In order to explore the potential applications of SiO2@Eu(D-
PAE–Si)3 and SiO2@Tb(DPAE–Si)3 in biological fields as
nanotags, the resultant core–shell nanoparticles were imaged by
fluorescence confocal microscopy with the available CW excita-
tion at 405 nm (Fig. 10).
As expected, the images given in Fig. 10 exhibit strong red
and green fluorescence with high signal-to-noise ratio, demon-
strating that they fulfil the requirements for bio-labeling
applications.
Conventional fluorescent labels, including semiconductor
quantum dots and organic fluorophores, are mostly based on
down-conversion, i.e. they are excited by ultraviolet (UV) or
short wavelength visible (VIS) radiations. Hence, they have
a very limited penetration in tissues because of scattering and
absorption of optical photons. Furthermore, under UV or VIS
excitation, a biological medium may produce strong auto-
fluorescence from chromophores (e.g., collagens, porphyrins,
etc.), which decreases the sensitivity of detection. Finally, UV
radiations have potential cytotoxic and carcinogenic effects,
and are therefore not an attractive option for live cell/tissue
imaging.
In order to assess the capabilities as a fluorescent probe for
multiphoton microscopy, the near infrared (NIR) excitation
spectrum of SiO2@Eu(DPAE–Si)3 was recorded at room
In contrast, fluorescent labels that are excited by NIR radia-
tions give rise to minimal autofluorescence because of the lack of
efficient endogenous absorbers in the NIR spectral range. Pho-
todamage to cells and tissues is thus strongly reduced, and
penetration depths are increased (notably in the windows 0.7–1.1
mm) by several orders of magnitude. As a result confocal and
biphotonic microscopy is of great interest for medical applica-
tions. By using this technique, the image in Fig. 11 was recorded
from SiO2@Eu(DPAE–Si)3 nanoparticles upon excitation at
Fig. 8 Luminescence decay curves of (a) SiO2@Eu(DPAE–Si)3 and (b)
SiO2@Tb(DPAE–Si)3 recorded at room temperature.
Fig. 9 NIR excitation spectrum of SiO2@Eu(DPAE–Si)3 recorded at
7
room temperature by monitoring the Eu3+ 5D0 / F2 transition at 616
nm.
Fig. 10 Fluorescence microscopy images of (a) SiO2@Eu–DPAE and
(b) SiO2@Tb–DPAE recorded upon laser excitation at 405 nm.
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UV-Vis absorption measurements were carried out on
a Nicolet Evolution 500 spectrophotometer with ethanol as the
solvent.
The continuous wave (CW) photoluminescence spectra were
collected using as excitation source a 450 W xenon lamp CW
monochromatized through TRIAX 180 from Jobin-Yvon/
Horiba and analyzed by a TRIAX 550 Jobin-Yvon/Horiba
monochromator equipped with either a R928 Hamamatsu pho-
tomultiplier or a nitrogen-cooled CCD camera (Jobin-Yvon
Symphony LN2 series) as a detector. CW-excitation and emission
spectra were obtained by monitoring the detector response while
scanning the appropriate monochromator. The luminescence
decays were obtained by pulsed excitation using a pulsed dye
laser (Continuum ND60) pumped by a Continuum Surelite I-
SL10 doubled Nd:YAG laser (10 ns pulse, 0.1 cmꢀ1 band width,
and 10 Hz repetition rate). The dye laser was followed by a H2-
Raman cell (for IR excitation) or a KDP frequency doubler (for
UV excitation). LDS 698 used as dye solution provides energy up
to 400 mJ in the doubled selected UV region or 4 mJ in the H2-
Raman Stokes shifted IR region. The frequency doubled or
Raman shifted dye laser beam is spatially isolated from the
fundamental dye laser beam by two Pellin–Brocca prisms asso-
ciated with an iris diaphragm. In addition adequate sets of low or
long pass-band filters are used to block any parasitic laser radi-
ation. The red or green fluorescence of respectively Eu3+ and Tb3+
is analyzed through a Jobin-Yvon HR 1000 monochromator
(focal length: 1 m, 1200 groove mmꢀ1 grating and a band-pass of
Fig. 11 Fluorescence microscopy image of SiO2@Eu–DPAE recorded
upon laser excitation at 750 nm in the NIR range.
750 nm demonstrating a contrast high enough for bio-labeling
purposes.
Conclusions
A novel DPA derivative, DPAE, was synthesized via the ‘‘click
chemistry’’ route and further silylated with ICPTES with high
yield and simple reaction conditions. Based on DPAE–Si as
a linker, luminescent core–shell nanoparticles SiO2@Ln(DPAE–
Si)3 (Ln ¼ Eu3+ and Tb3+) were prepared by covalently grafting
lanthanide complexes Ln(DPAE–Si)3 onto the surface of pure
silica nanoparticles. Surface functionalization with amino groups
was achieved by the sol–gel process leading to SiO2@Ln(DPAE–
Si)3@NH2 (Ln ¼ Eu3+ and Tb3+) nanoparticles. The resultant
materials present excellent luminescence and efficient energy
transfer from the organic ligands to Ln3+ ions. At the same time,
fluorescence of SiO2@Eu(DPAE–Si)3 was evidenced by confocal
microscopy upon NIR excitation, demonstrating a great poten-
tiality as biphotonic probes for biomedical imaging.
ꢀ1
ꢂ
8 A mm slits). The detector was a Hamamatsu R1104 photo-
multiplier tube. Fluorescence decays were measured with a Lec-
roy 9310A-400 MHz digital oscilloscope.
Regarding fluorescence microscopy, a Zeiss LSM 510 confocal
scanning microscope was used to collect the images upon exci-
tation at 405 nm while for the biphotonic study, a Leica SP5
confocal laser scanning microscope was utilized.
Further linkages to biocompatible and biorecognition agents
are underway prior to performing biological tests.
Synthesis of core–shell luminescent nanoparticles
SiO2@Ln(DPAE–Si)3 (Ln ¼ Eu3+ and Tb3+)
Methods
Silica nanoparticles were prepared according to a ternary reverse
emulsion method based on the sol–gel process.11 Firstly, 4.05 g
(9.2 mmol) of polyoxyethylene(5) nonylphenyl ether (NP-5) were
mixed in 93 mL (0.863 mol) of cyclohexane under magnetic
stirring at room temperature. After the dissolution of the non-
ionic surfactant for nearly 15 minutes, 420 mL (6.2 mmol) of
concentrated ammonia were added to achieve the water-in-oil
nanoemulsion. After further 15 minutes stirring, 2.3 mL
(10.4 mmol) of tetraethylorthosilicate (TEOS) were added to
initiate the reaction. The reaction was allowed to continue for 24
hours under stirring. To stop the reaction and isolate the parti-
cles, approximately 50 mL of acetone were slowly introduced
into the emulsion: a white precipitate formed within the solution.
Then the nanoparticles were collected by centrifugation at
a speed of 3500 rpm for 20 minutes. Thereafter, the nanoparticles
were washed twice with absolute ethanol in order to remove all
the remaining surfactant. The silica particles were then dried in
Dimethyl 4-azidopyridine-2,6-dicarboxylate and DPAE–Si were
synthesized according to the literature.8a All other chemicals and
solvents were of analytic grade and used as received from Aldrich
or Fluka.
Fourier transform infrared (FTIR) spectra were measured on
a Perkin-Elmer 16PC spectrometer within the wavenumber range
4000–400 cmꢀ1 at a resolution of 4 cmꢀ1 with the KBr pellet
technique. Raman spectra within the wavenumber range 3000–
400 cmꢀ1 were measured on a T64000 Jobin Yvon confocal
microRaman spectrometer equipped with an Olympus micro-
scope and a CCD detector cooled with liquid nitrogen. The
514.532 nm line of a Spectra Physics Stabilite 2017 Ar Laser
operating at 100 mW was used as an excitation source.
Observations by transmission electron microscopy were per-
formed on a Hitachi H-7650 at an acceleration voltage of 120 kV
while SEM micrographs were recorded by means of a ZEISS
Supra 55VP scanning electron microscope operating in high
vacuum between 4 and 15 kV, using a secondary electron
detector (Everhart–Thornley detector).
ꢂ
an oven at 80 C for 24 hours. The mass yield of nanoparticles
was found to be around 50% with regard to the initial TEOS
quantity. Nanoparticles with an average diameter of 30 nm and
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Subsequently, Ln(DPAE–Si)3 solution (0.01 M) was synthe-
sized by dissolution of DPAE–Si and Ln(NO3)3$6H2O in
anhydrous ethanol (5 mL) with a molar ratio of Ln3+ : DPAE–
Si ¼ 1 : 3 under argon at room temperature and further stirring
for 10 h. Ultimately, the above solution (1.5 mL) was added
into silica nanoparticles (0.060 g) dispersed in anhydrous
ethanol (20 mL) at 80 ꢂC. The mixture was refluxed for 10 h
under argon at this temperature. The core–shell luminescent
nanoparticles were collected by centrifugation at a speed of
3500 rpm for 20 min, washed with acetone several times until
the supernate did not present any lanthanide characteristic
fluorescence under UV irradiation and dried at 60 ꢂC in an
oven for 20 h.
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Synthesis of core–shell SiO2@Ln(DPAE–Si)3@NH2
nanostructures (Ln ¼ Eu3+ and Tb3+)
To 20 mL of ethanol were added 10 mL of aqueous solution
containing SiO2@Ln–DPAE (0.070 g) and 1.8 mL of
ammonia. The solution was stirred for 30 min at room
temperature. Then APTES (0.15 mL) dispersed in ethanol (20
mL) was added into the above solution and the mixture was
further stirred for 5 h. The white precipitate was isolated by
centrifugation, washed with water and ethanol four times, and
dried at 60 ꢂC for 10 h.
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6122 | J. Mater. Chem., 2012, 22, 6117–6122
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