Preparation of multifunctional mesoporous silica particles: The use of an amphiphilic silica precursor with latent amine functionality in selective functionalization of the inner surface

Article abstract of DOI:10.1039/c1jm10440c

The mesoporous silica particle has three different areas as a functionalization site, including an outer surface, a silica framework and an inner surface. We functionalized the inner surface selectively using an amphiphilic silica precursor with latent am

Full text of DOI:10.1039/c1jm10440c

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PAPER
Preparation of multifunctional mesoporous silica particles: the use of an
amphiphilic silica precursor with latent amine functionality in selective
functionalization of the inner surface
*
Min Soo Kim and Ji Young Chang
Received 28th January 2011, Accepted 8th April 2011
DOI: 10.1039/c1jm10440c
The mesoporous silica particle has three different areas as a functionalization site, including an outer
surface, a silica framework and an inner surface. We functionalized the inner surface selectively using
an amphiphilic silica precursor with latent amine functionality and prepared a multifunctional
mesoporous silica particle showing fluorescence resonance energy transfer (FRET). An
organotriethoxysilane consisting of a perylene diimide unit and two triethoxysilyl groups was
co-condensed with tetraethoxysilane and the amphiphilic organotriethoxysilane [(4-octylphenyl-3-
(triethoxysilyl)propylcarbamate)] in the presence of cetyltrimethylammonium bromide under basic
conditions. The resulting periodic mesoporous silica particle contained perylene diimide units in a silica
framework as a FRET donor. The hydrophobic part of the amphiphile was tethered to the pore wall
through a thermally reversible urethane bond. The surfactant molecules were extracted by reflux in
a mixture of 1,4-dioxane, HCl and water. In this process, hydrophobic groups in the pore were also
removed to produce amino groups on the inner surface. The rhodamine units were introduced into the
pore as a FRET acceptor by the reaction of the isothiocyanate group of Rhodamine B isothiocyanate
with the amino group on the inner surface, forming a thiourea linkage. The outer surface of the silica
particle could be also functionalized by grafting with 3-(trimethoxysilyl)propyl acrylate before
extraction of the surfactant molecules. A silica–polymer thin film was prepared where the silica particles
were embedded in a poly(2-hydroxyethyl methacrylate) matrix. Both the suspension of the silica
particles in ethanol and their polymer composite film showed the efficient FRET effect.
For example, Bein’s group presented the site-selective function-
Introduction
alization by sequential or delayed co-condensation of a func-
tional organosilica precursor and tetraethoxysilane (TEOS). The
organosilica precursor participated in the condensation reaction
at a different stage of the silica particle growth, resulting in the
preferential incorporation of the functional group into the core
or the shell of the silica particle. In a different approach, Land-
ry’s group used diffusion-based post-reaction for the selective
functionalization. Both outer and inner surfaces of mesoporous
silica were functionalized with 9-fluorenylmethyloxycarbonyl
(Fmoc)-protected aminoalkyl silanes, after which Fmoc groups
were removed by the reaction with a piperidine solution to
generate an amine functionality. Differential functionalization
was achieved by controlling the rate of deprotection.
Functionalized and ordered mesoporous silica particles have
diverse application opportunities in catalysis,1 separation,2
sensing,3 and delivery.4 As a functionalization site, the meso-
porous silica particle has three different areas including an outer
surface,
a silica framework and an inner surface. Two
approaches have been used for the functionalization:
co-condensation of a functional organosilica precursor with
a silica precursor, which allows the introduction of functional
groups into the silica framework, and grafting, which is widely
used for the surface functionalization. These two methods have
been used to introduce various organic functionalities into the
mesoporous silica particle to provide appropriate properties for
specific applications, but selectively functionalizing the three
areas of the silica particle remains very challenging.
In this study, we prepared multifunctional periodic meso-
porous silica particles, of which three areas were functionalized
with a different functional group. In particular, we were able to
selectively functionalize the inner surface using an amphiphilic
silica precursor with latent amine functionality. Fig. 1 shows
a schematic drawing of the multifunctional mesoporous silica
particle. The multifunctional silica particle was designed to show
fluorescence resonance energy transfer (FRET). FRET is
In recent years, several studies on the selective functionaliza-
tion of the mesoporous silica nanoparticle have been reported.5–7
Department of Materials Science and Engineering, College of Engineering,
Seoul National University, Seoul, 151-744, Korea. E-mail: jichang@snu.ac.
kr; Fax: +82-2-885-1748; Tel: +82-2-880-7190
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surface, surfactant molecules occupying the pores were removed
by heating under acidic conditions. In this process, an amino
group was also generated on the inner surface via dissociation of
the urethane bond. A FRET acceptor was attached covalently to
the pore wall by the reaction with the amino group.
Organotriethoxysilane Perylene-Si, comprised of a perylene
diimide unit and two triethoxysilyl groups, was prepared
according to the literature method (Scheme 2),9 and incorporated
into a silica framework as a FRET donor by co-condensation
with TEOS. Perylene diimides have high fluorescence efficiency
and photochemical stability.10 The amphiphilic organo-
triethoxysilane (OctylPh-Si) was synthesized from octylphenol
and 3-(triethoxysilyl)propyl isocyanate. OctylPh-Si could func-
tion as a surfactant as well as a monomer for the condensation
reaction. The fluorescent mesoporous silica particles (FMS) were
prepared by co-condensation of an amphiphilic organo-
triethoxysilane (OctylPh-Si), a fluorescent organotriethoxysilane
(Perylene-Si), and TEOS in the presence of cetyl-
trimethylammonium bromide (CTAB) under basic conditions.
Both amphiphiles, CTAB and OctylPh-Si, were dissolved in
a basic aqueous medium and allowed to form a micellar struc-
ture, where hydrophobic portions of the amphiphiles were
located inside the pore. The silica precursors, TEOS and Per-
ylene-Si, were allowed to react in this aqueous solution and
precipitated silica particles were isolated as reddish solids.
The outer surface of the silica particle as precipitated was
functionalized by grafting with 3-(trimethoxysilyl)propyl acry-
late. Since the pores of the silica particle were filled with
amphiphile molecules, we presume that the grafting reaction
occurred preferentially at the outer surface.11,12 However, it is
still possible that some silyl molecules diffused into the pore.
Several studies reported surfactant–silane exchange under
similar conditions, resulting in the reaction inside the pore.5a,13,14
Fig. 2 shows SEM images of the mesoporous silica particles as
precipitated and the grafted ones. The silica particles had
spherical and elliptical morphology with diameters of between
100 nm and 500 nm. Their shapes were unaffected by the grafting
reaction.
Fig. 1 Schematic drawing of the structure of the multifunctional mes-
oporous silica particle.
a distance-dependent interaction between the electronic excited
states of two chromophore molecules. A donor chromophore,
initially in its electronic excited state, transfers energy to an
acceptor chromophore in proximity through nonradiative
dipole–dipole coupling. In the multifunctional silica particle,
FRET donor and acceptor moieties were introduced into the
silica framework and the pore, respectively. The outer surface of
the silica particle was grafted with hydroxyalkyl groups. Here we
report the synthesis and FRET activity of the multifunctional
silica particle.
Results and discussion
Our approach to the preparation of the multifunctional meso-
porous silica particle is shown in Scheme 1. The overall synthetic
strategy was as follows. A FRET donor was introduced into
a silica framework by co-condensation of a functional silica
precursor bearing a FRET donor moiety, TEOS and amphiphilic
organotriethoxysilane having a thermally reversible urethane
bond. In the resulting periodic mesoporous silica particle, the
hydrophobic part of the amphiphile was tethered to the pore wall
through a urethane bond.8 After functionalizing the outer
The surfactant molecules were extracted by reflux in a mixture
of 1,4-dioxane, HCl and water. In this process, hydrophobic
octylphenoxy groups in the pore and acryl groups on the outer
surface were also removed to produce amino and hydroxyl
groups, respectively.15 Extraction was confirmed by the decrease
in the characteristic FT-IR absorptions of CTAB in the 3000–
2800 cmꢀ1 region (Fig. 3). In addition, the peak around
1735 cmꢀ1 disappeared, supporting the hydrolysis of the urethane
and ester bonds.
The structure of FMS-NH2 was further investigated by solid-
state 13C CP/MAS NMR spectroscopy (Fig. 4). The peaks cor-
responding to three different organic groups were observed as
Scheme 1 Preparation of the FRET mesoporous silica particles.
This journal is ª The Royal Society of Chemistry 2011
Scheme 2 Synthesis of organotriethoxysilane molecules.
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Fig. 2 SEM images of (a) FMS and (b) FMS-Acr.
Fig. 5 (a) Nitrogen adsorption–desorption isotherm (inset: pore size
distribution), (b) X-ray diffractogram, (c) SEM image and (d) TEM
image of FMS-NH2.
distribution indicated uniform mesopores with a diameter of
3.3 nm. The X-ray diffraction (XRD) pattern of FMS-NH2
ꢀ
showed two resolved peaks with d-spacings of 46.93 and 27.07 A,
corresponding to (100) and (110) reflections of a hexagonal
ꢀ
Fig. 3 FT-IR spectra of FMS-Acr and FMS-NH2.
structure with a lattice parameter of a ¼ 54.19 A, respectively
(Fig. 5b).
The visual images of the multifunctional particles were
obtained by scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM) (Fig. 5c and d). The SEM
image showed that the morphology of the silica particles was not
altered during the pore modification. The TEM image showed
a densely packed hexagonal pore structure with an average pore
diameter of about 3 nm, which was in good agreement with the
values calculated from the XRD and BJH measurements.
In FMS-NH2, perylene diimide units were embedded in the
silica matrix between the pores as a FRET donor. We chose
Rhodamine B isothiocyanate (rhodamine) as a FRET acceptor
chromophore because its absorption wavelength was well
matched with the emission wavelength of Perylene-Si as shown in
Fig. 6. A strong overlap of the absorption band of the acceptor
with that of the emission band of the donor is required to achieve
efficient FRET. In an ethanol solution, Perylene-Si showed three
intense absorption bands at 454, 488 and 520 nm and
Fig. 4 29Si and 13C CP/MAS solid NMR spectra of FMS-NH2.
expected. The peaks for the aminomethylene carbon on the inner
surface and the hydroxymethylene carbon on the outer surface
appeared at 42 and 64 ppm, respectively. The peak at 163 ppm
was assigned to the carbonyl carbon of Perylene-Si. The
aromatic carbon peaks appeared at 120–135 ppm. The peak for
the silylmethylene carbon of the propyl groups appeared at
7 ppm.
29Si CP/MAS solid NMR spectroscopy also supported the
structure of FMS-NH2 (Fig. 4). The 29Si CP/MAS NMR spec-
trum of FMS-NH2 exhibited relatively strong two signals at ꢀ59
and ꢀ68 ppm corresponding to T2 [RSi(OSi)2OH] and T3 [RSi
(OSi)3] resonances, respectively, suggesting that organic groups
were covalently attached to the silica framework.16
The N2 adsorption/desorption isotherms of FMS-NH2 were of
type IV, indicating that the pore sizes were in the mesopore range
(Fig. 5a). The Barrett–Joyner–Halenda (BJH) pore size distri-
bution function calculated from the adsorption branch of the
isotherm is shown in Fig. 5a (inset).17 The FMS-NH2 samples
had a Brunauer–Emmett–Teller (BET) surface area of 861 m2 gꢀ1
and pore volume of 0.77 cm3 gꢀ1. The narrow BJH pore size
Fig. 6 Normalized UV-vis (dashed-line) and PL (solid-line) spectra of
Perylene-Si (lex ¼ 488 nm) and rhodamine (lex ¼ 548 nm) obtained at
a concentration of 1 ꢁ 10ꢀ5 mol Lꢀ1 in ethanol.
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fluorescence emission bands at 535 and 575 nm when excited at
488 nm. The absorption band of rhodamine appeared around
548 nm.
The rhodamine units were introduced into the pore by the
reaction of the isothiocyanate group of Rhodamine B iso-
thiocyanate with the amino group on the inner surface, forming
a thiourea linkage.5,18 The loading amount of rhodamine was
calculated by assuming all the reactions occurred stoichiometri-
cally. The sample was labeled with respect to the molar ratio of
rhodamine to the amino functional group on the pore wall as
FMS-NH2 (no rhodamine), FMS-Rh(0.5) (0.5 molar ratio) and
FMS-Rh(1.0) (1.0 molar ratio). After introduction of the
rhodamine unit, a decrease in a BET surface area was observed.
FMS-Rh(1.0) showed a BET surface area of 501 m2 gꢀ1 and pore
Fig. 8 (a) Photograph of the FMS-Rh(1.0)-embedded PHEMA film and
(b) PL spectra of the film and the FMS-Rh(1.0) suspension in ethanol
(lex ¼ 488 nm).
volume of 0.60 cm3 gꢀ1
.
Fig. 7 shows the PL spectra of the suspension of the silica
particles in ethanol. FMS-Rh showed absorption bands corre-
sponding to perylene diimide and rhodamine units around 500
and 550 nm, respectively. When excited at 488 nm, the intensity
of the perylene diimide emission peak at 543 nm decreased with
increasing rhodamine content, while the intensity of the rhoda-
mine emission peak at 575 nm increased. These results strongly
suggest that the excitation energy of perylene diimide units in the
silica framework was transferred to the rhodamine moieties
inside the pore. Similar FRET effects were reported by Inagaki
and coworkers, where FRET acceptors were introduced into the
pore inside by physical adsorption.19
Fig. 9 Confocal laser scanning microscopy (CLSM) images of the FMS-
Rh(1.0)-PHEMA film. Images (a) and (b) were obtained by detecting the
emission at 505–530 nm and above 560 nm, respectively.
We prepared a free standing nanocomposite film in which the
functional silica particles were embedded in a poly(2-hydroxy-
ethyl methacrylate) (PHEMA) matrix. Free radical polymeriza-
dispersed in the PHMMA film obtained by using the 488 nm line
of an argon-ion laser for excitation. The emission was detected at
505–530 nm and above 560 nm. At 505–530 nm, weak green
fluorescence was observed from perylene diimide units in the
silica particles (Fig. 9a). When detected above 560 nm, the
distinct red fluorescent particle image was obtained, indicating
the strong emission of rhodamine units due to the FRET effect
(Fig. 9b).
tion of
a dispersion of FMS-Rh(1.0) in 2-hydroxyethyl
methacrylate (HEMA) was carried out in between glass plates to
yield a film with a thickness of about 260 mm.20 The film was
transparent and slightly reddish (Fig. 8a), indicating a homoge-
neous dispersion of the silica particles in the polymer matrix. We
presume that hydroxypropyl groups located on the outer surface
of the silica particle enhanced the compatibility between the silica
particle and PHEMA.
The PL spectrum of the film (Fig. 8b) showed a similar pattern
of emission to that of the FMS-Rh(1.0) suspension in ethanol
with a little red shift of about 16 nm. Fig. 9 shows the confocal
laser scanning microscopy (CLSM) images of FMS-Rh(1.0)
Conclusions
We investigated the synthesis of multifunctional mesoporous
silica particles by selective and sequential functionalization. The
designed multifunctional silica particle contained FRET donor
and acceptor moieties inside the silica framework and the pore,
respectively, and hydroxypropyl groups at the outer surface. A
free standing, multifunctional, silica–polymer thin film was
prepared where the silica particles were embedded in the
PHEMA matrix. Both the suspension of the silica particles in
ethanol and their polymer composite film showed the efficient
FRET effect. We believe that this selective and sequential func-
tionalization method will help in synthesizing advanced multi-
functional mesoporous silica materials for diverse applications.
Experimental
Measurements
1H and 13C NMR spectra were recorded on a Bruker Avance
DPX-300 (300 MHz) and an Avance 500 (125 MHz)
Fig. 7 PL spectra of FMS-Rh suspended in ethanol (1 mg per 10 mL)
(lex ¼ 488 nm).
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spectrometer, respectively. Solid-state 13C CP/MAS and 29Si CP/
MAS NMR spectra were obtained on a Bruker Avance DSX-400
(400 MHz) spectrometer equipped with the CP-MAS probe.
Samples were spun in air at approximately 7 kHz. Powder X-ray
diffractograms were obtained with the use of a Bruker Nanostar
Small Angle X-Ray Scattering (SAXS) system (Cu Ka radiation,
ethanol was added and the reaction mixture was continuously
ꢂ
stirred for another 48 h at 40 C. Grafted silica particles (FMS-
Acr) were isolated by filtration, washed with ethanol several
ꢂ
times, and dried in a vacuum oven at 25 C for 3 days.
FMS-NH2. FMS-Acr (0.6 g) was stirred in a mixture of HCl
(35 wt%, 10 g), deionized water (50 g) and dioxane (150 g) at
120 ꢂC for 24 h. The product was filtered, washed with dioxane,
methanol and acetone, and dried in a vacuum oven at 25 ꢂC for
3 days.
ꢀ
l ¼ 1.54 A). SEM images were obtained by using a JEOL JSM-
6330F microscope. Transmission electron microscopy (TEM)
images were obtained by a Tecnai F20 operating at 200 kV. TEM
samples were dispersed in chloroform and a drop of the mixture
was placed on a carbon-coated copper TEM grid. N2 adsorption/
desorption measurements were carried out using Belsorp-Max
(BEL Japan, Inc.), and the pore size distribution was calculated
using the Barrett–Joyner–Halenda (BJH) model on the adsorp-
tion branch. Fluorescence measurements were performed using
FMS-Rh. A certain molar ratio of Rhodamine B iso-
thiocyanate to the amino functional group on the pore wall was
introduced into FMS-NH2 according to the method in the
literature.5,18 A typical procedure is as follows: FMS-NH2 (50
mg, 2.44 ꢁ 10ꢀ3 mmol of NH2) was suspended in ethanol (10
mL). Rhodamine B isothiocyanate (1.51 mg, 2.82 ꢁ 10ꢀ3 mmol)
in EtOH (15 mL) and a catalytic amount of triethylamine were
added to the suspension. After stirring the reaction mixture in the
dark for 24 h at room temperature, the product was isolated by
filtration and washed with ethanol several times to remove the
unreacted dye.
a
Shimadzu RF-5301PC spectrofluorometer. Fluorescence
images were obtained by using Carl Zeiss-LSM510 confocal laser
scanning microscope. UV-vis spectra were obtained with the use
of a SCINCO S-3150.
Synthesis
4-Octylphenyl-3-(triethoxysilyl)propylcarbamate (OctylPh-Si).
To a solution of 4-octylphenol (4.13 g, 20 mmol) in tetrahydro-
furan (10 mL) were added 3-(triethoxysilyl)propyl isocyanate
(14.84 g, 60 mmol) and DBDU (2 mL) at room temperature. The
reaction mixture was stirred for 24 h at 75 ꢂC. After evaporation,
the product was isolated by column chromatography on silica gel
(ethyl acetate : n-hexane ¼ 1 : 14 v/v): yield: 71%. 1H NMR
(DMSO-d6, 300 MHz): d 7.14 (d, J ¼ 8.3 Hz, Ar–H, 4H), 6.97 (d,
J ¼ 8.3 Hz, Ar–H, 4H), 7.72 (t, J ¼ 5.6 Hz, NH), 3.74 (q, J ¼
7.0 Hz, 6H), 3.03 (q, J ¼ 6.7 Hz, 2H), 2.54 (t, J ¼ 7.4 Hz, 2H),
1.49–1.57 (m, 4H), 1.26 (m, 10H), 1.15 (m, 9H), 0.85 (t, J ¼
6.2 Hz, 3H), 0.57 (t, J ¼ 8.5 Hz, 2H); 13C NMR (DMSO-d6,
125 MHz): d 154.4, 149.1, 138.6, 128.5, 121.4, 57.6, 43.1, 34.5,
31.3, 28.7, 22.8, 18.1, 13.7, 7.2; IR (KBr, cmꢀ1): 3342, 2973, 2857,
1732, 1538, 1501, 1457, 1390, 1218, 1080, 1020, 956, 785, 641.
Anal. Calcd for C24H43NO5Si: C, 63.54; H, 9.55; N, 3.09. Found:
63.37; H, 9.61; N, 2.94%.
Composite film. FMS-Rh(1.0) (2 mg) was dispersed in
2-hydroxyethyl methacrylate (5 mL), and then ADMV (1 wt%)
was added to the suspension. A free standingꢂfilm was prepared
by polymerization between glass plates at 40 C for 2 days.
Acknowledgements
This research was supported by grants from the Mid-career
Researcher Program through NRF grant (no. 2010-0017552)
funded by the Ministry of Education, Science and Technology,
Republic of Korea.
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