Manganese-doped highly ordered mesoporous silicate with high efficiency for oxidation suppression

Article abstract of DOI:10.1002/chem.201202726

Herein, we demonstrate a facile approach to manganese-doped highly ordered mesoporous silicate with oxidation-suppression function. As biocompatible supports of guest ions, the ordered mesoporous silicate was synthesized by evaporation-induced self-assembly. The phase-transition from disordered to lamellar structures in the highly ordered mesoporous structure of these porosity-tuned materials was controlled by adjusting the concentration of a lab-made polystyrene-b-polyethylene oxide copolymer. Manganese was successfully incorporated as a guest in the hexagonally packed mesoporous silicate by using an ultrasound-assisted technique. The incorporation of manganese ions into the pores of a mesoporous silicate support could be induced for host-guest functional applications. Manganese-doped mesoporous silicate structures have been examined for their use as antioxidizing agents by electron spin resonance (ESR) measurements and radical-scavenging tests. The manganese atoms in the mesoporous structures could act in a free-radical-scavenging capacity, much like manganese nanoparticles. The high efficiency of their oxidation-suppression function is extended for application to catalytic products.

Full text of DOI:10.1002/chem.201202726

FULL PAPER
DOI: 10.1002/chem.201202726
Manganese-Doped Highly Ordered Mesoporous Silicate with High Efficiency
for Oxidation Suppression
Chang Woo Kim,[a] Minhyuk Kang,[b] Bongjin Moon,[b] Jinheung Kim,[c] and
Young Soo Kang*[a]
Abstract: Herein, we demonstrate a
facile approach to manganese-doped
highly ordered mesoporous silicate
with oxidation-suppression function.
As biocompatible supports of guest
ions, the ordered mesoporous silicate
was synthesized by evaporation-in-
duced self-assembly. The phase-transi-
tion from disordered to lamellar struc-
tures in the highly ordered mesoporous
structure of these porosity-tuned mate-
rials was controlled by adjusting the
concentration of a lab-made polystyr-
ene-b-polyethylene oxide copolymer.
Manganese was successfully incorporat-
tional applications. Manganese-doped
mesoporous silicate structures have
been examined for their use as antiox-
idizing agents by electron spin reso-
nance (ESR) measurements and radi-
cal-scavenging tests. The manganese
atoms in the mesoporous structures
could act in a free-radical-scavenging
capacity, much like manganese nano-
particles. The high efficiency of their
oxidation-suppression function is ex-
tended for application to catalytic
products.
ed as
a guest in the hexagonally
packed mesoporous silicate by using an
ultrasound-assisted technique. The in-
corporation of manganese ions into the
pores of a mesoporous silicate support
could be induced for host–guest func-
Keywords: doping · host–guest sys-
tems · manganese · mesoporous ma-
terials · radical scavengers
Introduction
served for enzymes with molecular weights above 40 kDa
because the enzyme could not be trapped inside the pores
of MCM-41. Although colloidal mesoporous silicas (mSiO2)
have been considered to be ideal stimuli-responsive carriers
for controlled-release systems, clogging of their pores with
nanoparticles results in the uncontrolled attachment of a
guest onto a host.[4] This research indicates that mSiO2 mate-
rials with fully opened porosity are needed as hosts for bio-
molecules. Also, the preservation of the chemical activity of
organic molecule can be enhanced by trapping the mole-
cules inside the pores of the mSiO2. For the immobilization
of functional organic molecules, the pore size is a critical
factor, because smaller pore sizes tend to limit the diffusion
of organic molecules.[5] In addition, the suppression of bioac-
tivity and the chemical activity of the organic molecules can
be carried out by using mSiO2, owing to their slow release
toward the outside of their fully opened pores and their hin-
dered structures. In particular, the antioxidant ability of the
organic molecules, such as tocopherol and vitamin C, has at-
tracted a lot of attention for applications in radical-scaveng-
ing and catalytic products. For the suppression of the degra-
dation of such sensitive organic functional materials, antiox-
idizing elements, such as manganese, can be used to pre-
serve these molecules for an extended period of time.[6]
Herein, we report the phase-transition of porosity-tuned
materials by adjusting the concentration of a lab-made poly-
styrene-b-polyethylene oxide (PS-b-PEO) diblock-copoly-
mer surfactant. The controlled phase-transition from disor-
dered to lamellar structures proceeded through a hexagonal-
ly close-packed structure. In the controlled mSiO2 structure,
the size and shape of the pores in the hexagonally close-
Structure-directing agents (SDAs) have allowed porous ma-
terials to become promising candidates for use in applied in-
dustrial engineering because they endow these materials
with many advantages, such as tunable porosity and the ca-
pability of modifying their surface characteristics by doping
the surface with organic molecules.[1] Recently, porous mate-
rials were synthesized in a controlled manner by using a
nanocasting technique with mesoporous silica by Vinu and
co-workers and by Zhao and co-workers.[2] As a biocompati-
ble application of porous materials, the immobilization of
enzymes inside MCM-41 was first reported by Diaz and
Balkus.[3] According to their study, no adsorption was ob-
[a] Dr. C. W. Kim, Prof. Y. S. Kang
Korea Center for Artificial Photosynthesis
Department of Chemistry
Sogang University
Seoul, 121-742 (Korea)
Fax : (+82)2-701-0967
E-mail: yskang@sogang.ac.kr
[b] Dr. M. Kang, Prof. B. Moon
Department of Chemistry
Sogang University
Seoul, 121-742 (Korea)
[c] Prof J. Kim
Department of Chemistry and Nano Science
Ewha Womans University
Seoul, 120–750 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202726.
Chem. Eur. J. 2013, 19, 135 – 140
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
135

packed mSiO2 material were tuned so that the pores were
completely accessible to functional organic and inorganic
molecules. The hexagonally close-packed mSiO2 was doped
with manganese ions by using an ultrasound-assisted techni-
que and it was examined for the preservation of organic an-
tioxidizing agents, such as vitamin C. To achieve the sup-
pression of vitamin C, we dispersed manganese ions inside
the pores of mSiO2; the size of the metal particles was con-
trolled by using various concentrations of manganese ions.
In particular, the Mn-incorporated mSiO2 was measured for
their free-radical-scavenging and catalytic effects by room-
temperature X-band electron spin resonance (ESR) spectro-
scopy.
thermal motion of the spherical micelles at low concentra-
tions of the surfactants resulted in an irregular spherical
shape of the porous silica, as shown in Figure 1a. As the
concentration of polymer surfactant was increased, the
number of spherical micelles also increased, which resulted
in an increase in the order of the pores in the silica matrices.
At a concentration of 0.0033 mmol of the PS-b-PEO diblock
copolymer, highly ordered mesoporous silica was obtained,
as shown in Figure 1c. Thereafter, the order in the porous
silica only changed slightly, owing to the disrupted state, as
shown in Figure 1d. Finally, at higher polymer-surfactant
concentrations (0.0080 mmol), the phase changed into a la-
mellar mesophase, as shown in Figure 1e.
Figure 2a shows a SEM image along the y axis of the
mSiO2 mesophase that was obtained by hydrothermal treat-
ment in a concentration of block-copolymer surfactant of
0.0033 mmol at 908C for 72 h and calcination at 6508C for
6 h. An orthogonal view of the sample is shown in Fig-
ure 2b, which shows a highly ordered mSiO2 structure.
When the sample is tilted at an angle of 458 along the x, y,
z axes, the magnified SEM and TEM images (Figure 2c,d)
show a long-axis column structure along the x-axis and very
ordered columns with a diameter of 26.6 nm. An illustration
of the micelle formation by the PS-b-PEO diblock copoly-
mer and hexagonally close-packed micelles of mSiO2 with a
pore size of 25 nm and a wall thickness of 10.8 nm is shown
in the Supporting Information, Figure S1. SAXS patterns of
mSiO2 in 0.0033 and 0.0080 mmol solutions of the SDA are
presented in the Supporting Information, Figure S2. Com-
pared to a typical lamellar pore structure, which show index-
ing from the (100) and (200) planes (see the Supporting In-
formation, Figure S2b), the Supporting Information, Fig-
ure S2a shows one first-order and two higher-order diffrac-
tion peaks from the (100), (110), and (200) planes of the
mesoporous lattice, which correspond to the formation of
Results and Discussion
As demonstrated by the pioneering work by Antonietti and
co-workers, in which polybutadiene-b-poly(vinylpyridinium)
block copolymers were used as a template to make mSiO2,
block-copolymer self-assembly and micro-phase separation
are solvent-dependent processes.[7] Contrary to low-molecu-
lar-weight surfactants, which usually rely on water and hy-
drophobic effects, fully opened porosity is typically achieved
by replacing the majority of the solvent by a metal or metal-
oxide precursor with similar polarity and condensing this
precursor around the aggregates. This process allows the
synthesis of mSiO2 materials with fully opened pores, which
are preferable hosts for the incorporation of biomolecules
and metal ions.
To control the size and shape of the mSiO2 pores, a
phase-transition from spherical micelles into a lamellar
structure through a hexagonal close-packed mesophase was
accomplished by controlling the concentration of a lab-
made PS-b-PEO diblock polymer (Figure 1). The random
Figure 1. Phase-transition of mSiO2 and TEM images of porosity-tuned mSiO2, which was prepared by using different concentrations of the PS-b-PEO di-
block copolymer: a) 0.0010 mmol, b) 0.0020 mmol, c) 0.0033 mmol, d) 0.0057 mmol, and e) 0.0080 mmol per 0.5 mL of TEOS (scale bar: 100 nm).
136
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Chem. Eur. J. 2013, 19, 135 – 140

Manganese-Doped Highly Ordered Mesoporous Silicate
FULL PAPER
Figure 2. a) Low- and c) high-resolution SEMs and TEMs in b) the 100
plane and d) the cross-sectional 010 plane of hexagonally packed mSiO2
with 0.0033 mmol of the SDA.
Figure 3. a) TEM image, b) Mn (green spots) elemental-mapping image,
and c) EDS data of Mn-doped hexagonally packed mSiO2.
hexagonally ordered pore structures. Herein, the mSiO2
sample that was prepared at 0.0033 mmol of the diblock co-
polymer showed a surface area of 419 m2 gꢀ1 and a pore di-
ameter of 26.6 nm (determined from the BET surface area,
BJH method, and SAXS measurements; see the Supporting
Information, Figures S2–S4). These results show that the
narrow size distribution of the pores, even with smaller di-
ameters, could lead to a larger probability of doping organic
and inorganic materials into the pores of mSiO2. Compared
with other mesophases, mSiO2, with its hexagonal pore
structure, is promising for high-temperature reactions, sepa-
ration, and large-molecule catalysis.[8] The hexagonally
packed mSiO2 could be used as an efficient carrier of the or-
ganic antioxidizing agents by doping manganese ions. The
manganese ion was doped into the pores of mSiO2 by using
an ultrasound-assisted technique in an aqueous solution of
manganese ions.
Doping processes by using this ultrasound-assisted techni-
que with water-soluble metal salts at an optimized concen-
tration, reaction time, and temperature are a useful synthet-
ic tool.[9] In our previous report, we demonstrated that
metal nanoparticles were formed on the surface of SiO2
nanoparticles through deprotonation of the hydroxy ligands,
followed by the reaction of electrophilic materials, such as
metal ions, at the nucleophilic sites.[9b,c] This ultrasound-as-
sisted technique allows well-dispersed Mn ions to approach
the surface of mSiO2 and to attach onto partially localized
surface hydroxy groups. The doping of Mn ions into the
pores of mSiO2 was confirmed by TEM and elemental map-
ping. After manganese-ion doping, the empty pores on the
silica became partially blocked by the manganese ions, as
shown in Figure 3.
To characterize the nature of the pores in these samples,
N2-adsorption/desorption isotherms of Mn-doped and un-
doped mSiO2 were recorded (Figure 4a). The isotherms of
Mn-doped and undoped mSiO2 clearly revealed different
isotherms at a relative pressure of 0.4. Whereas Mn-doped
mSiO2 revealed a type-II isotherm at higher relative pres-
sures, because of its monolayer/multilayer adsorption onto
the open surface of the closed pores, a type-IV isotherm (by
IUPAC classification)[10] was observed for undoped mSiO2.
Because the type-IV isotherm for the undoped mSiO2
sample is associated with capillary condensation in its meso-
pores, the difference between both isotherms clearly shows
surface-structural differences, owing to the doping of man-
ganese onto the surface of mSiO2. Figure 4b shows high-res-
olution XPS spectra of Mn-doped mesoporous silicate,
which shows deconvoluted Mn 2p3, Si 2p3, and O 1s signals.
Taking the Fermi level as a reference (Ef =0), emission
bands that are related to Mn 2p3 in Mn-doped mSiO2 should
be observed at above 641.9 eV.[11] Two peaks at 642.8 and
648.0 eV are typical of a Mn 2p spectrum, owing to the 2p1/2
and 2p3/2 levels of Mn ions.[12] As-synthesized Mn-doped
mesoporous silicate revealed O 1s emission with compo-
nents at about 530.7 and 534.7 eV, which corresponded to
ꢀ
the typical binding energies of oxygen in Mn O bonds, and
components at about 533.2 and 534.3 eV, which correspond-
ꢀ ꢀ
ed to the typical binding energies of oxygen in Si O Si and
ꢀ
in Si OH bonds. Such high-energy shifts in the binding en-
ergies of the valence electrons in the O 1s and Mn 2p3 orbi-
ꢀ
ꢀ
tals are due to the formation of Mn O and Si O chemical
bonds. Taken together, these energy-dispersive X-ray spec-
troscopy data, N2-adsorption/desorption isotherms, and
high-resolution XPS spectra are all consistent with the for-
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137

Y. S. Kang et al.
mation of Mn-doped mSiO2,
for which the relative quantity
of manganese ions was deter-
mined to be 13.99 wt.%.
The homogeneous distribu-
tion of Mn ions within the
pores of mSiO2 is clearly
shown in the elemental-map-
ping images, as shown in
Figure 3. This result is compa-
rable to the elemental-mapping
data of Si and O in mSiO2 at
various manganese concentra-
tions from 1–14 wt.% at room
temperature (Figure 5a). The
room-temperature ESR spec-
trum of MnII (14 wt.%)-doped
mSiO2 is shown in Figure 5a).
Because MnII is well-dispersed
as
throughout the mSiO2 frame-
work, typical sextet-split
ESR signal that is produced
from 55Mn (I=5/2) with
g value of 2.0165 and isotropic
hyperfine coupling (Aiso
9.65 mT) is dominant in Fig-
a
paramagnetic guest
a
a
Figure 4. a) N2-adsorption/desorption isotherms of Mn-doped and undoped mSiO2. b–d) High-resolution XPS
spectra of Mn-doped mSiO2, which shows a Mn 2p3 signal, and the corresponding peak deconvolutions.
=
ure 5a, compared with non-paramagnetic mSiO2, which indi-
cates octahedral symmetry of the MnII species. Interestingly,
the broad singlet for mSiO2 with 1% Mn indicated inhomo-
geneous aggregation on the external surface of the mSiO2.
Notably, this broad powder-patterned ESR signal changed
into a hypersplit sextet as the concentration of Mn ions in-
creased. Thus, that Mn ions started to become homogene-
ously dispersed throughout the pores of mSiO2 above
1 wt.% MnII-ion concentration. This result confirms the
strong hyperfine sextet ESR signal at a MnII concentration
of 14 wt.%.
Based on these XPS and ESR results, the antioxidizing ef-
fect[9c,13] of manganese Mn-doped mSiO2 was investigated
for its application as a radical scavenger. The free-radical-
scavenging capacity in Mn-doped mSiO2 follows Equa-
tion (1).
RꢀOO þMnðxÞþHþ ! ROOHþþMnðxþ1Þ
C
ð1Þ
To understand its free-radical-scavenging ability, the anti-
oxidizing effects of Mn were measured as a suppression of
the degradation of vitamin C (l-ascorbic acid) by free radi-
cals. The free-radical-scavenging capacity was measured by
analysis of the wt.% of preserved vitamin C. Figure 5b
shows that larger amounts of vitamin C were preserved by
Mn-doped mSiO2 and that this preservation was aging and
temperature dependent: 92.2 wt.% vitamin C was preserved
after 1 week at 508C and 78.4 wt.% was preserved after
4 weeks at 508C. Compared with the wt.% of preserved vi-
tamin C with un-doped mSiO2, the vitamin C-preservation
Figure 5. a) X-band ESR spectra of mSiO2 and Mn-mSiO2 with various
Mn concentrations of 1–14 wt.% at room temperature. b) Plot of the
amount of preserved vitamin C (%) of commercial cosmetics with and
without Mn-mSiO2 to show the effect of Mn incorporation on the sup-
pression of the oxidation of vitamin C in the mSiO2.
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Manganese-Doped Highly Ordered Mesoporous Silicate
FULL PAPER
doped manganese ions and their homogeneous dispersion are very impor-
tant. MnCl2·4H2O (0.3 g) was added into a solution of EtOH and deion-
ized water (10 mL, 7:3 v/v). Next, the previously synthesized mSiO2
(0.03 g) was added into this solution and ultrasonication afforded homo-
geneously dispersed manganese ions inside the pores. The mixture was
aged under ultrasound treatment for 30 min in an ice bath. After washing
with EtOH and deionized water, the product was dried in an oven at
908C. Ultrasound treatment was performed twice more.
efficiency improved by 7.8% after 4 weeks at 508C with
Mn-doped mSiO2. This result indicates that doped manga-
nese serves as a radical scavenger in mSiO2. Notably, this
functional guest element in the mSiO2 holds promise for bio
and catalytic applications, considering that both manganese
as a guest and SiO2 as a host are nontoxic and biocompati-
ble materials.
Test of the antioxidizing effect of manganese towards vitamin C: Tests of
the antioxidizing effect of Mn and the preparation of skin toner as a cos-
metic were performed at the research center of The Face Shop Ltd.
(Korea). To measure the amount of preserved vitamin C, the basic skin
toner was prepared by using a basic mist, which included purified water
and glycerin. Vitamin C and a desired amount of the Mn-doped mesopo-
rous silica were added into the basic mist and the skin toner was aged for
various lengths of time and at various temperatures. The skin toners with
and without Mn-doped mesoporous silica were compared in terms of the
wt.% of preserved vitamin C to determine their free-radical-scavenging
capacity. Their wt.% of preserved vitamin C was measured by HPLC.
Conclusion
In summary, the phase-transition of mSiO2 from disordered
to lamellar structures, which proceeded through a highly or-
dered mesophase, was achieved by controlling the concen-
tration of a diblock copolymer. This research demonstrates
that hexagonally packed mSiO2 can be employed as a host
for biomolecules and metal ions because of its larger pores
and its highly ordered structure compared with other meso-
phase mSiO2. In contrast to the aggregation and blocking of
the mesophase pores by guest molecules in previous reports,
in this case, the guest molecules were well-dispersed
throughout the fully opened framework by using ultrasound
treatment. Thus, the penetration of guest molecules into the
pores of the mSiO2 material enhanced the preservation of
the chemical activity of organic molecule by trapping these
molecules. In particular, the suppression of the bioactivity
and chemical activity of l-ascorbic acid was achieved by
using Mn-doped mSiO2 with a hexagonally packed structure
and completely opened pores, owing to the use of antioxid-
izing agents, such as Mn, to preserve the molecules of l-as-
corbic acid, with their slow release toward the outside of the
mesopores. This guest-doped mSiO2 holds promise for appli-
cations in anti-aging or anti-wrinkling cosmetics. Moreover,
this strategy could be applied to other catalytic products.
Characterization: Thermogravimetric analysis (TGA) was carried out on
a Thermal Advantage Instrument TGA-2050 analyzer from 50–6008C
under a nitrogen atmosphere or in air at a heating rate of 108Cminꢀ1
.
For TEM observations, the samples were prepared by placing one drop
of the colloidal solution onto carbon-coated copper grids (mesh size:
200) and dried for a few minutes. The porosity of the sample was evaluat-
ed from their TEM micrographs. To determine the crystallinity and struc-
ture of the synthesized samples, Rigaku D’Max 2200 V (CuKa radiation,
l=1.5406 ꢁ) wide-angle X-ray diffraction (WAXD) and
a Rigaku
D’Max 2500 18 K small-angle X-ray scattering (SAXS) systems were
used. Nitrogen-adsorption/desorption isotherms were measured at 77 K
on an automated QUADRASORB ’SI’ analyzer (Quantachrome Instru-
ments). High-resolution X-ray photoelectron spectroscopy (XPS) spectra
of Mn-doped mesoporous silicate were recorded on an X-ray photoelec-
tron spectrometer (THERMO VG SCIENTIFIC, MultiLab2000) at Pu-
kyong National University, Korea. ESR spectra of mSiO2 with various
manganese content were recorded on a JEOL (Japan) JES PX 2300 FT-
ESR with a 1.4 T electromagnet and an output frequency of 8800–
9600 MHz (X-band) at Pukyong National University, Korea.
Acknowledgements
This work was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) and by a
grant from the Ministry of Education, Science and Technology of Korea
(MEST) for the Center for Next Generation Dye-sensitized Solar Cells
(No. 2012–0000591).
Experimental Section
Synthesis of mesoporous silicate as a support for guest ions: Poly(styr-
ene-block-ethylene oxide) (PS-b-PEO) was synthesized according to our
previously reported procedure[14] and that reported by Zhao and co-
workers, with some modification.[15] The PS-b-PEO diblock copolymer
was prepared by atom-transfer radical polymerization. An evaporation-
induced self-assembly (EISA) strategy was carried out in THF by using
tetraethyl orthosilicate (TEOS) as an inorganic precursor and the PS-b-
PEO diblock copolymer as a SDA. In a typical synthesis, a desired
amount of the PS-b-PEO diblock copolymer was stirred in THF
(2.8 mL). After a clear solution was obtained, 0.1m HCl (0.3 mL) and
TEOS (0.5 mL) were added and the mixture was stirred for 20 min to
form a homogeneous solution. Next, the solution was moved into glass
Petri dish and then aged for 24 h. After the THF had evaporated, trans-
parent powders were obtained on the bottom of the Petri dish. Next, the
powders were hydrothermally treated in a 1.0m solution of HCl (30 mL)
at 908C for 72 h to recrystallize the mesoporous silica framework. The
silica–polymer composite was separated by centrifugation, washed with
distilled water, and dried under ambient conditions. The polymer tem-
plate was removed in air at 6508C for 6 h to produce the mesoporous
silica.
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