Aerosol-synthesis of mesoporous organosilica nanoparticles with highly reactive, superacidic surfaces comprising sulfonic acid entities

Article abstract of DOI:10.1002/adfm.201302330

Combining high internal surface area with tailor-made surface properties is pivotal for granting advanced functional properties in many areas like heterogeneous catalysis, electrode materials, membranes, or also biomimetics. In this respect, organic-inorganic hybrid nanostructures and in particular mesoporous organosilica materials are ideal systems. Here, the preparation of mesoporous solids via a new sol-gel building block comprising sulfonic acid (R-SO₃H) is described. The degree of organic modification is not only maximal (100%), it is also proven that the novel material exhibits superacid properties. Furthermore, an aerosol assisted method is applied for generating this material in the form of mesoporous, spherical nanoparticles with substantial colloidal stability. Highly acidic, high surface area materials, like prepared here, are promising candidates for numerous future applications like in heterogeneous catalysis or for proton conducting membranes. However, first experiments addressing the antibacterial effect of the sulfonic-acid, mesoporous organosilica materials are shown. It is demonstrated that the superacid character is required for exhibiting sufficient antifouling activity. High surface area organosilica colloids with superacid properties are prepared from a novel sulfonic acid sol-gel precursor. An aerosol assisted method affords spherical mesoporous particles, which exhibit a high density of surface acid groups. Furthermore, initial antifouling studies are conducted.

Full text of DOI:10.1002/adfm.201302330

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Aerosol-Synthesis of Mesoporous Organosilica
Nanoparticles with Highly Reactive, Superacidic
Surfaces Comprising Sulfonic Acid Entities
Julia Gehring, David Schleheck, Martin Luka, and Sebastian Polarz*
micro-/nanostructured surfaces can be a
decisive factor.
Combining high internal surface area with tailor-made surface properties is
pivotal for granting advanced functional properties in many areas like het-
erogeneous catalysis, electrode materials, membranes, or also biomimetics.
In this respect, organic-inorganic hybrid nanostructures and in particular
mesoporous organosilica materials are ideal systems. Here, the preparation
of mesoporous solids via a new sol–gel building block comprising sulfonic
acid (R-SO3H) is described. The degree of organic modification is not only
maximal (100%), it is also proven that the novel material exhibits superacid
properties. Furthermore, an aerosol assisted method is applied for gener-
ating this material in the form of mesoporous, spherical nanoparticles with
substantial colloidal stability. Highly acidic, high surface area materials, like
prepared here, are promising candidates for numerous future applications
like in heterogeneous catalysis or for proton conducting membranes. How-
ever, first experiments addressing the antibacterial effect of the sulfonic-acid,
mesoporous organosilica materials are shown. It is demonstrated that the
superacid character is required for exhibiting sufficient antifouling activity.
Therefore, in the introduction section
of this article, we will firstly reflect on
known methods for preparing high sur-
face area materials furnished with tailor-
made organo-functional groups. Secondly,
we will discuss properties and potential
applications of nanoporous materials
comprising acidic functions. Further-
more, some contemporary approaches for
antifouling materials as a special case of
self-cleaning coatings will be addressed
briefly.
1.1. Porous Organosilica Materials
The vast majority of inorganic solids com-
prising organically modified surfaces rely
on silica chemistry. This is, because both
Si–C and Si–O–Si represent two very
stable linkages. The application of organosilane sol–gel precur-
sors, for example, R-Si(OEt)3 is an established technique for the
preparation of various organic/inorganic hybrid materials, also
known as ORMOSILs.[4] Huge activity in this field was driven
by the development of novel techniques for nanostructuring,
in particular the possibility to prepare periodically ordered
mesoporous silica (POS).[5] A silica sol–gel precursor like
Si(OEt)4 is hydrolyzed at certain pH conditions and in the pres-
ence of a structure directing agent (template). Soon attempts
were made to combine the areas POS and ORMOSILs.[6] Nice
overviews were given by Ying et al.[7] in 1999 and Froeba and
co-workers in 2006.[6] Starting from meso-SiO2, via grafting or
co-condensation using suitable organosilanes, one can achieve
materials comprising up to 25% functionalization degree.[8]
Much higher content of organic modification (up to 100%)
accompanied by maximization of the density of the functional
entities can be reached, when special sol–gel precursors with
a bridging organic group Rf ((R′O)3S-Rf-Si(OR′)3 with R′ = Me,
Et,isoPr) are used for the preparation of the so-called periodi-
cally ordered mesoporous organosilica materials (PMOs).[9] The
organic functionality becomes an inherent part of the matrix.
Since then, it has taken some time until PMOs with advanced
chemical functionality could be realized. The interested reader
is referred to one of the following, contemporary review arti-
cles.[10] Our group has concentrated on the so-called UKON
1. Introduction
Biological evolution has created some of the most advanced
functional systems known to date.[1] The precise knowledge
about guiding principles of operation is pivotal for transfer-
ring concepts from biology to materials science and technology.
The latter idea has led to the emergence of the seminal field
“bionics”, respectively biomimetics.[2] One of the most promi-
nent cases is represented by the discovery of the construc-
tion principles for the lotus leaf and the resulting significant
research activities on superhydrophopic surfaces and self-
cleaning coatings in general.[3] The recognized paradigm is that
solely the presence of certain chemical entities is not sufficient
for granting a desired functionality, but the combination with
J. Gehring, M. Luka, Prof. Dr. S. Polarz
University of Konstanz
Department of Chemistry
D-78457, Konstanz, Germany
E-mail: sebastian.polarz@uni-konstanz.de
Dr. D. Schleheck
University of Konstanz
Department of Biology
D-78457, Konstanz, Germany
DOI: 10.1002/adfm.201302330
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materials containing a bridging phenyl entity modified with
various functional groups R in the 3-position of the aromatic
ring:[11] R = −Br (≅ UKON-1), –COOH (≅ UKON-2a), –NH2
(≅ UKON-2d), and so forth.
1.3. Antifouling Materials
Microbial biofilms are very serious competitors for keeping
surfaces clean and fouling of surfaces by microorganisms is
a serious issue in many medical, biotechnological and food-
technological settings, as well as in the shipping industry. In
the recent years, a great deal of research has gone into finding
novel chemical strategies (or biological strategies) for keeping
surfaces clean. Furthermore, the demand for environmentally
friendly approaches has increased significantly, since the eco-
logical and toxicological issues of organotin compounds were
taken much more serious.[26] An overview about chemical
antifouling coatings and strategies was given by Kane and co-
workers in 2011.[27] For example, a very promising approach
comprises the photocatalytic activity of titania surfaces.[28] It is
assumed that the photoreaction of TiO2 with the solvent (e.g.,
water) is responsible for the generation of reactive species (e.g.,
H+, OH·), which then attack all organic species on the surface.
It is also worth noting that the cytotoxicity of Ag+ ions can be
used, when silver nanoparticles are immobilized on surfaces
via suitable anchoring groups.[29] Furthermore some hydro-
philic polymers such as poly(ethylene glycol) (PEG),[30] polyoxa-
zoline polymers,[31] and zwitterion-containing polymers such
as poly[2-(methacryloyloxy) ethylphosphorylcholine (PMPC)[32]
have been tested as effective coatings as they suppress protein
adsorption.
One important prerequisite for the application and future
technological implementation of POS materials is the possi-
bility to allocate processable samples. For this purpose, it is
beneficial if a material of interest exists as a colloidal disper-
sion instead of an ill-defined powder. There has been some
effort on the preparation of mesoporous silica in the form
of colloidal nanoparticles. Two methods have been proven
to be extremely powerful. One approach is a modification
of the well-known Stoeber process.[12] Recently, Bein and
co-workers presented monodisperse, mesoporous silica col-
loids with particles sizes in the 100 nm regime.[13a,b] It is also
worth mentioning the papers by Jaroniec et al. and Froeba
et al. about PMO nanoparticles generated via a modified
Stoeber method.[13c,d] An alternative, highly innovative, aerosol
assisted approach for the generation of mesoporous silica
nanoparticles has been introduced by Brinker and co-workers
in 1999.[14] Only few papers describe the preparation of PMO
micro-/nanoparticles using bridging sol–gel precursors.[9c,15]
To the best of our knowledge there is no report that reports
about the synthesis of PMO nanoparticles via the aerosol-
assisted route.
Of special relevance for the present work are methods that
involve chemical surface modification via silica sol–gel coat-
ings.[33] For instance, Tang et al. describe the antifouling
behavior of hydrophobic organosilica xerogels in 2005.[34] Fur-
thermore, Mahltig et al. have demonstrated in 2008 that sur-
face OH groups of wood can be facilitated for an attachment
of organosilanes via sol–gel chemistry to exhibit antifouling
properties.[35] Whereas the focus of these examples was on the
hydrophobic nature of the surfaces, there is much less known
whether materials may exhibit antifouling properties via active
chemical triggers, such as strong acids like –SO3H.
Herein, we report the synthesis of mesoporous UKON nano-
particles prepared via an aerosol aided synthesis route. Fur-
thermore, a novel precursor comprising benzene sulfonic acid
is presented. The precursor was used for the preparation of a
highly acidic PMO material (UKON-2i). Finally, the UKON-2i
nanoparticles were tested concerning a potential application in
antifouling.
1.2. Porous Solid State Acids
Among the various functional groups, it has been shown that the
preparation of mesoporous solid-state acids is of large interest,
in particular for materials with strong acids like –SO3H.[16] All
current PMO materials comprising sulfonic acid have been
prepared by post-functionalization routes. The direct sulfona-
tion of a phenyl-bridged PMO was described by Inagaki et al.
in 2002.[17] The proton-conducting properties of this material
was reported very recently by Wark and team.[18] Kondo and co-
workers used the Diels-Alder reaction for the attachment of an
arylsulfonic acid to the π bond of an ethylene-bridged PMO.[19]
Mehdi and colleagues could convert disulfide bridges to two
terminal –SO3H functions in 2006.[20] There is currently no
example for a PMO material with 100% sulfonic acid derivatiza-
tion content, because this requires the availability of a –SO3H
modified sol–gel precursor.
Furthermore, none of the reports mentioned above reports
about superacidic properties of the respective, porous organo-
silica materials. The term superacid describes a system with
an acidity greater (respectively a pKa value smaller) than that of
pure sulfuric acid.[21] The number of known solid-state super-
acids is yet quite limited. The most prominent examples involve
sulfated zircona and some heteropolyoxometalate acids.[22] Also
some zeolites exhibit superacid properties.[23]
There are manifold potential applications for acidic, and in
particular superacidic, porous materials. They can act as hetero-
geneous catalyst materials for demanding organic transformat-
ions.[16b,24] Another interesting field is the application as novel
proton conducting membranes for future fuel cells.[25] Further-
more, it is recognized that solid-state acids could potentially
play a role also for self-cleaning surfaces.
2. Results and Discussion
2.1. UKON Precursor and PMO Powder Preparation
The preparation of known sol–gel precursors and the related
PMO materials containing bridging Ph-Br (UKON-1), Ph-
CO2H (UKON-2a), Ph-NH2 (UKON-2d) is described in previous
papers.[11] The pKa value of benzoic acid is 4.3. This means that
UKON-2a is expected to be a solid-state acid, but a rather weak
one. Therefore, it would be highly desirable to have a PMO
material available with much stronger acidity. Benzene sulfonic
acid (Ph-SO3H) is a promising candidate as a bridging organic
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Scheme 1. Synthesis of the novel PMO precursor.
entity since its pKa value is 0.7. In the current manuscript, we
present the required, novel PMO precursor containing ben-
zene sulfonic acid (Ph-SO3H) and the preparation of the cor-
responding mesoporous organosilica for the first time.
In analogy to the other systems reported by us in the past,[11]
the synthesis of the desired PMO precursor utilizes aromatic
derivatization chemistry starting from compound (1) with
bromine in 3-position (Scheme 1). Lithiation affords a stable
nucleophile, which can react further with various electrophiles.
Different attempts have been made for the introduction of the
sulfonic acid group. An overview is given in the Supporting
Information (S-1). Less successful were routes involving the
oxidation of a thiol-functionalized compound, or the reaction of
the lithiated species with SO3. However, referring to the litera-
ture,[36] 1,5-bis-tri(isopropoxysilyl)-benzene-3-sulfonyl chloride
(2) could be obtained in gram quantities using sulfuryl chloride
as an electrophile.
Like for most sol–gel precursors, due to the influence of
the alkoxy groups, it has not been possible to grow single-
crystals for X-ray structure determination. The successful
preparation and purity of (2) was proven by NMR spectroscopy
(1H, 13C, 29Si) and is documented in the Supporting Informa-
tion, S-2. In addition, electron spray ionization mass spectrom-
etry (ES-MS) was performed. The ES-MS pattern (given in S-2,
Supporting Information) contains several fragmentation prod-
ucts, which can all be assigned to (2). The most intense signal
and the simulated pattern for the corresponding fragment are
exemplarily shown in Figure 1.
Next, the novel sol–gel precursor (2) was used for the prep-
aration of PMOs referring to typical true liquid-crystal tem-
plating procedures reported in the literature.[37] Hydrolysis
and polycondensation take place under aqueous conditions.
Thus, it should be noted that in the case of compound (2) as a
precursor not only the alkoxysilane groups react, but the S-Cl
entity will also be hydrolyzed, yielding the desired sulfonic acid
Figure 1. Excerpt from the ESI-MS pattern of (2) (black line) and simu-
lated signal (grey) for the [SO2Ph(Si(OiPr)3)2]− species.
organosilica material (Scheme 2), accompanied by the forma-
tion of hydrochloric acid. The chemical nature of the resulting
material was analyzed using a combination of techniques. First,
solid-state NMR spectroscopy was applied. The 29S-NMR spec-
trum (Figure 2b) contains characteristic three, so-called T-sig-
nals at δ = −65 ppm for (HO)2RSi(OSi) ≅ T1, −73 ppm for (HO)
RSi(OSi)2 ≅ T2 and −82 ppm for RSi(OSi)3 ≅ T3.[38] Q-type sig-
nals (δ ≈ −110 ppm) indicating the presence of pure silica (SiO2)
parts are absent. The latter result proves that the S–C bonds of
the UKON precursor are stable during synthesis conditions.
Proving the presence of the –SO3H function is much more
difficult. This is because sulfur is hardly accessible via NMR,
and the chemical shifts of aromatic C’s attached to –SO3H are
Scheme 2. Generation of the sulfonic-acid PMO material.
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in a similar region (δ ≈ 135–140 ppm) as the signals of non-
substituted or bromo-substituted aromatic rings.[11a,39] How-
ever, the direct comparison of UKON-2i to the NMR spectra
of the precursor (2) and of UKON-1 (as a reference),[11a] shows
good evidence that the sulfonic acid group is still present in the
material. The signal at δ ≈ 116 ppm characteristic for Carom-Br
in UKON-1 is missing. Instead, a new signal can be found at
136 ppm, which is in good agreement to the NMR spectrum
of the starting compound (2) (Figure 2a). Similar effects have
been observed for IR spectroscopy (Figure 2c). In comparison
to the spectrum of UKON-1, there is one additional band at
1024 cm−1 superimposed by the S–O–Si vibration of the matrix
(1070 cm−1). Comparison to information from the literature
confirms that this new band can be assigned to the presence
of the aromatic sulfonic acid.[40] In addition, energy dispersive
X-ray spectroscopy (EDX) was performed (data given in S-3,
Supporting Information). EDX shows that there is significant
amount of sulfur present. The S:Si ratio equals 0.75:2, which
is slightly less than expected (1:2), but still is within the error
of the EDX method ( 20%). Furthermore, X-ray photoelectron
spectroscopy (XPS) was acquired (S-3, Supporting Information).
The signal found at an electron binding energy of 169.1 eV
is indicative for sulfur in oxidation state (+VI), in agreement
to R-SVIO3H.[41] It can be summarized, that the sol–gel process
has occurred as depicted in Scheme 2. The composition of the
organosilica material can be described as Si2O3(C6H3SO3H).
Neither C–Si nor C–S bonds were cleaved in course of the sol–
gel process.
Successful meso-structuring of the sulfonic organosilica
matrix required some unexpected, special measures described
in the following. Initially, we chose a standard procedure which
is well established for the synthesis of numerous PMO mate-
rials:[11] The precursor is dissolved in a solution of an amphi-
philic, structure-directing block-copolymer of the Pluronic type
in ethanol, and an appropriate amount of aqueous HCl (pH =
2) is added. Eventually pre-hydrolytic treatment is required (see
experimental section). After polycondensation and drying one
removes the template by liquid-liquid extraction. The character-
istics of the pore system is studied by the typical set of ana-
lytical techniques used for mesoporous materials:[42] Transmis-
sion electron microscopy (TEM), N2 physisorption measure-
ments and small angle X-ray scattering (SAXS) if appropriate.
Other than expected, the physisorption isotherm (type I)[43]
of the material prepared for those standard conditions is typ-
ical for microporous materials (Figure 3a). The latter result
was confirmed by TEM micrographs (given in S-4, Supporting
Information). Neither mesopores nor any ordered pore system
can be identified. The reason for the missing structuration
is that fragmentation of the PEO-PPO-PEO blockcopolymer
(≅ Pluronic) into polyethylene oxide (PEO) and polypropylene
oxide partitions has taken place. The latter was shown by time-
1
dependent H-NMR spectroscopy. The variances in the spectra
(given in S-5, Supporting Information) are consistent with
acid-catalyzed ether cleavage reactions. Once PEO and PPO
are separated from each other, any amphiphilic property is lost
and the formation of the liquid crystalline template is inhibited.
Instead, the PEO chains lead to the formation of micropores.[44]
The described result is a first indication for the enhanced reac-
tivity of the sulfonic acid groups in UKON-2i.
Figure 2. a) 13C- solid-state NMR spectrum of UKON-2i (black graph)
compared to the spectrum of UKON-1 (grey graph) and of the precursor
(2; measured in solution; light grey graph) as references. b) 29Si- solid-
state NMR spectrum of UKON-2i. c) Fingerprint IR region of UKON-2i
(black graph) compared to UKON-1 (grey graph) as a reference.
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Figure 4. TEM image of UKON-2i prepared using Pluronic 123 as a
template.
material. The difficulties in templating at higher pH values can
be explained by the insufficient interaction between the neu-
−
tral blockcopolymer template and the anionic -SO3 precursor
species. It is known from the literature that it is preferred for
successful templating using neutral block-copolymers, when
the silica species are neutral as well.[46] It can be concluded that
there is only a narrow pH window for the successful structura-
tion of UKON-2i, when using Pluronics as structure directing
agents.
Figure 3. a) N2 physisorption isotherms (adsorption and desorption) for
UKON-2i materials prepared at standard conditions (squares), pH = 1.5
(circles), pH = 1.9 (triangles), and pH = 2.6 (hashes). b) BJH pore-size
distribution of the UKON-2i material prepared at pH = 1.9.
Better results could be obtained for an amphiph-
ilic poly[(ethylene-co-butylene)-b-(ethylene oxide)] (KLE;
Mw = 8.1 kDa; 41% polyethylene oxide), due to its lower sensi-
tivity towards proton catalyzed ether cleavage. KLE has already
been applied successfully by others for the preparation of large-
pore mesoporous silica and metal oxide materials.[47] TEM
images of UKON-2i prepared with KLE as a structure directing
agent (S-6, Supporting Information) show a highly ordered
PMO material with an average pore size of ≈15 nm. The latter
is in agreement with SAXS data, which indicate a periodicity
of 17.2 nm (S-6, Supporting Information). The model used for
the simulation of the SAXS pattern using the program package
SCATTER[48] is shown in detail in S-6, Supporting Information.
However, because KLE blockcopolymers are not commercially
available, we still concentrate in the following on the applica-
tion of Pluronics as templates.
Because the hydrolysis of the S-Cl group in precursor (2)
induces a significant drop of the pH-value, we checked, if con-
trol of pH using a buffer system leads to an improvement in
structuring. Indeed, the emergence of an isotherm type char-
acteristic for mesoporous materials (type IV), increased pore-
volume and increased surface area (320 → 360 → 405 m2 g−1)
has been observed, when pH was adjusted to 1.5, respectively
1.9 (Figure 3a). The Barret, Joyner, Halenda (BJH)[45] pore-
size distribution function is shown for the sample prepared at
pH = 1.9 (Figure 3b). The observed pore-size of 3.5 nm is in
the mesoporous range, but it is much smaller than for other
mesoporous materials prepared using Pluronic P-123 as a
template (Dpore = 5–6 nm). TEM investigation of the sample
(Figure 4) shows that a mesoporous material has formed, in
agreement to physisorption analysis. Unfortunately, there is a
worm-hole type pore-system with low periodic order.
2.2. Aerosol Synthesis of Colloidal UKON Nanoparticles
The subtle sensitivity of the UKON-2i system regarding
pH value during synthesis can also be seen from the N2 iso-
therm obtained for a material prepared at pH = 2.6 (Figure 3a).
Again a type-I isotherm is seen, representing a microporous
For the preparation of mesoporous organosilica materials in
nanoparticle shape, we found that a modified Stoeber process
cannot be adopted for the UKON system.[13,15e,49] This can be
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Scanning electron microscopy (SEM) investigation shows that
the samples contain numerous spherical particles in the size-
range 100 nm–1 μm (see Figure 5b). The polydispersity is high,
which is typical for particles obtained via aerosol routes.[51]
All materials were further characterized by TEM, SEM,
N2-physisorption analysis and solid-state NMR. This set of ana-
lytical data is summarized in S-7–10, Supporting Information.
Exemplarily TEM images are shown in Figure 6. It is seen that
all samples can be described as mesoporous materials with
wormhole pore-system. The latter conclusion is supported by
physisorption analysis (S-7–10, Supporting Information). The
textural data (BET surface ABET are and BJH pore-size distribu-
tion maximum Dpore) are summarized in Table 1.
It is important to note that the higher temperatures applied
during the aerosol method (compared to the sol–gel process
conducted in the liquid phase) does not lead to cleavage of the
S–C bond, proven by the absence of Q-type signals in 29S-NMR
(data given in S-7–10, Supporting Information), and also not to
detachment of the functional group in 3-position proven by 13C-
NMR, respectively by direct comparison to the corresponding
data presented before for the (ordinary) powder samples.
2.3. Properties of Colloidal UKON-2i Nanoparticles
In the following, we emphasize some of the advanced proper-
ties of the spherical, mesoporous organosilica particles com-
prising the bridging benzene sulfonic acid entity (UKON-2i)
(see also Figure 6d).
First, we tested if dispersions of those particles can be pre-
pared in water. As the aerosol process affords also particles
larger than λvis ( = 400–800 nm) (see also Figure 7b), these dis-
persions appear turbid, which can be used to follow the sedi-
mentation with the bare eye (Figure 7). Whereas conventional
UKON powders (see above) sediment within seconds due to
gravitational force, the effect is much slower for the porous
nanoparticles prepared via the aerosol method. Furthermore,
there is no flocculation. The latter observations represent good
indication for the colloidal nature of the aerosol particles. The
colloidal stability of the particles appears to be lower at low pH-
values. A significant amount of particles have moved to the
bottom of the vial within 100 min in a dispersion at pH = 1.
This effect is reasonable, because the sulfonic acid becomes
partially protonated (–SO3H), which reduces the charge of the
particles and hence leads to less electrostatic repulsion/sta-
bilization. After 1000 min only the sample with pH = 9 still
contains some dispersed particles. Similar effects can also be
found for UKON-2a nanoparticles.
It is time to characterize the acidity of UKON-2i and in par-
ticular the related mesoporous nanoparticles in more detail.
The results were compared to mesoporous UKON-2a nano-
particles with –COOH groups and pure SiO2 nanoparticles as
references. An established way for determining the acidity of
solid-state acids beyond the Brønstedt window of water is to
determine the Hammett acidity function H0 by using appro-
priate indicator dyes (see Experimental Section).[52] In essence,
one uses pH indicator dyes (D) with known pKa values and
investigates in organic solvents to what extend these dyes are
protonated (DH+) if the acidic substance of interest is present.
Figure 5. a) Illustration of the setup used for the preparation of
mesoporous UKON-particles via an aerosol assisted process, and
(b) SEM image of resulting spherical UKON-2i particles. Scale bar ≅ 2 μm.
attributed to two factors: The lower hydrolysis rate of the iso-
propoxysilyl groups (in comparison to ethoxy groups present in
common sol–gel precursors) and the enhanced hydrophobicity
of the precursor due to the aromatic ring hamper the growth of
particles with defined porosity (results not shown).
Therefore, we have decided to test the aerosol-assisted
method presented by Brinker et al.[14] A sol containing a pre-
hydrolyzed UKON precursor, aqueous HCl, ethanol and
an amphiphilic block-copolymer of the Pluronic type as a
structure-directing agent is prepared (see Experimental Sec-
tion). An aerosol containing small droplets of this mixture is
prepared and is passed through a tube oven (Figure 5a). Eth-
anol evaporates, inducing liquid crystal formation due to the
increased concentration of the polymer (evaporation induced
self-assembly).[50] Polycondensation takes place, and the final
mesostructured organosilica particles can be separated. Finally,
after template extraction one obtains the desired mesoporous
nanoparticles. Besides compound (2) (the precursor for UKON-
2i; see Scheme 2), also the sol–gel precursors for UKON-1,
UKON-2a, and UKON-2d have been used in analogous experi-
ments in order to show the broad applicability of the method.
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Table 1. Textural data for the mesoporous UKON spheres.
BET/ [m2g−1
]
Sample
DPore/ [nm]
A
a)
UKON-1100
474
3.5
4.1
7.0
3.5
3.5
3.8
5.5
UKON-2a100
UKON-2d100
UKON-2i100
UKON-2i50
UKON-2i10
SiO2
424
268
720
412
550
635
a)The subscript indicates the organic derivatization degree. It is 100, if the bridging
sol–gel precursor, e.g., compound (2) has been used in pure, undiluted form.
Lower values indicate, that a mixture of the PMO precursor and Si(OEt)4 (→SiO2)
has been used during aerosol synthesis.
measurements, followed by application of Lambert-Beer law.
The measurements performed with 1-fluoro-4-nitrobenzene
(FNB) (pKa = −12.4) and 1-fluoro-2,4-dinitrobenzene (FDNB)
(pKa = −14.5) are shown in Figure 8. When UKON-2i is added
to a solution of FNB one sees that its absorption band at λmax
=
256 nm drops in intensity due to protonation of the dye. This
demonstrates that the acidity of UKON-2i is sufficient to pro-
tonate FNB to certain extend. Application of Equation 1 gives
a value of H0 = −12.53 for the Hammett acidity of the sulfonic
acid containing PMO material. In comparison, the less basic
FDNB cannot be protonated by UKON-2i. The spectra before
and after addition of the PMO material are identical (Figure 8).
It was mentioned before that sulfuric acid (H0 = −12) represents
an important reference mark. Therefore, one can conclude that
Figure 6. TEM images of mesoporous organosilica spheres prepared via the
aerosol assisted method. a) UKON-1, b) UKON-2a, c) UKON-2d, d) UKON-2i.
c(B)
c(BH+)
(1)
H = pKa +
0
For determining the concentration of the protonated spe-
Figure 7. Photographic images showing the time-dependency of the sedi-
mentation of UKON-2i nanoparticles as a function of pH-value.
cies c(BH+) and deprotonated species one facilitates optical
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SiO2, one sees that a higher ratio of the protons become avail-
able, but at the same time IEC and proton density drop (see
Table 2).
It was mentioned in the introduction that porous, solid-
state acids may have numerous applications, such as in fuel
cells or as catalysts, or as antifouling agents. Here, we show
a first proof-of-principle for the antifouling properties of the
UKON nanoparticles and evidence that the strong acidity of the
UKON-2i material is mandatory to maximize the (anti-) biolog-
ical activity.
The potential antifouling activity of the UKON nano-
particles was evaluated as the inhibition of bacterial surface
colonization on agar plates. Therefore, the opportunistic
human pathogen and biofilm model organism Pseudomonas
aeruginosa was inoculated onto agar plates and examined
for its ability to grow through a barrier of nanoparticles
(50 nmol), that is, from the inside of a circle of nanoparticles that
had been placed onto the agar plate, to the outside (Figure 9).
Bacterial colonies encircled by pure SiO2 nanoparticles
grew well through the barrier within 24 h, as expected, but
the growth of the colonies through the barriers of UKON
nanoparticles was retarded, or even prevented. In particular,
UKON-2i nanoparticles showed the strongest effect on bacte-
rial surface colonization (Figure 9). Whereas bacterial growth
could be confined using UKON-2a nanoparticles for 36 h, in
the case of UKON-2i the bacteria could not cross the barrier
even after 48 h. Hence, these results show promise for future
biological studies on the antifouling activity of UKON mate-
rials, in particular for UKON-2i.
Figure 8. Optical absorption spectra of 1-fluoro-4-nitrobenzene before
(black squares) and after exposure to UKON-2i (black line), and of
1-fluoro-2,4-dinitrobenzene before (grey squares) and after exposure to
UKON-2i (grey line).
UKON-2i is slightly more acidic than H2SO4, and thus, belongs
to the class of superacids. In comparison, UKON-2a (H0 = 2.38);
see S-11, Supporting Information) or unmodified mesoporous
silica particles (H0 = 3.16) are expectedly much less acidic. The
results for UKON-2a concerning its weak acidity are in agree-
ment with data obtained for tritration with 1 M NaOH per-
formed in aqueous solution (see S-11, Supporting Information).
The amount of available protons per gram of UKON-2i
nanoparticles, respectively per surface unit, can be determined
via the ion exchange capacity (IEC); see also the experimental
part.[53] The maximum IEC value can be obtained for UKON-
2i nanoparticles prepared using the undiluted PMO precursor
(UKON-2i100; see Table 2).
The proton density can be calculated considering the surface
area of the respective material (Table 1). The value obtained for
UKON-2i (= 1.26 protons per nm2) is in good agreement to the
density of functional groups found for other PMO materials
(=1.2 nm−2).[11a] However, comparing to the theoretical amount
of protons that should be present in 1 g UKON-2i (considering
its molecular mass, Mw = 260.29 g mol−1), it seems that only
40% of the protons are available directly. When the sulfonic
acid functionality becomes diluted with non-modified, pure
3. Conclusion
The generation of novel high surface material with interfaces
characterized by a high density of reactive organic groups paves
the way towards advanced applications in various areas. In the
current paper, we have concentrated on the preparation of mate-
rials with acidic properties. A novel sol–gel alkoxide silane pre-
cursor comprising benzene sulfonic acid as a bridging, organic
entity was utilized for the preparation of mesoporous organo-
silica materials. A more refined morphology, more precisely
spherical nanoparticles, was achieved via an aerosol assisted
method. These nanoparticles exhibit specific surface area of up
to 720 m2 g−1 and pore-sizes in the range 3.5–5 nm. It could be
shown that the mentioned, porous solid belong to the class of
superacids with a very high surface density of 1.3 × 1018 acid
Table 2. Ion exchange capacity for UKON-2i samples.
Sample
IEC
[mmol g−1
Proton density
ΔIEC
[H+ nm−2
]
[%]a)
]
UKON-2i100
UKON-2i50
UKON-2i10
SiO2
1.51
0.81
0.31
40%
43%
1.26
1.18
81%
0.34
-
2.1 × 10−3
0.002
a)ΔIEC was obtained by dividing the experimental IEC value by the amount of protons determined by stoichiometry.
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4. Experimental Section
All chemicals were received from Sigma-Aldrich. Prior to use they
were carefully purified and dried, when applicable. All reactions on the
precursor state were performed under inert conditions using Schlenk
technique. The synthesis of 1,3-bis-tri(isopropoxy)silyl-5-bromobenze
(1), 1,3-bis-tri(isopropoxy)silyl-5-aniline and 3,5-bis-ti(isopropoxy)
silylbenzoic acid have been described previously.[11a,b,e]
Synthesis of 1,5-bis-tri(isopropoxysilyl)-benzene-3-sulfonyl chloride (2):
tBuLi (7 mL, 1.5 M, 10.6 mmol) was added dropwise to a solution of
3 g of 1,3-bis-tri(isopropoxy)silyl-5-bromobenze (7.95 mmol) in 100 mL
dry Et2O. The mixture was stirred for 30 min and then added slowly
to SO2Cl2 in 100 mL dry Et2O. After additional stirring for 10 min at
−78 °C the solution was warmed to room temperature and the solvent
was removed under vacuum. 100 mL dry pentane were added and non-
soluble residues are removed via centrifugation. Finally, 1.3 g of product
(2) (42%; 2.22 mmol) are obtained after vacuum distillation at 140 °C.
i
1H NMR (400 MHz, CDCl3): δ/[ppm]: 1.19 (d, 36H, 3J = 6.1 Hz, Pr-
i
CH3); 4.25 (sept, 6H, 3J = 6.1 Hz, Pr-CH); 7.74 (m, 2H, o-arom. H);
7.87 (m, 1H, p-arom. H). 13C NMR (100.61 MHz, CDCl3): δ/[ppm]: 25.6
(iPr-CH3); 65.4 (iPr-CH); 127.0 (S-arom. C); 131.9 (p-arom. C); 136.7
(o-arom. C); 141.7 (S-arom. C).
Preparation of UKON-2i Under Standard Conditions: A total of 0.5 g of
precursor (2) (0.85 mmol) and structure directing agent (0.39 g Pluronic
P123, or 0.21 g KLE-25) were dissolved in 1 g EtOH. 0.26 g aqueous HCl
(1 M) were added dropwise while stirring. The sol was pre-hydrolized for 3 h
at 60 °C and aged for 2 days at room temperature. The resulting monolithic
pieces were dried at 100 °C for 24 h. Template removal occured by liquid
extraction using 25 mL of H2O and 25 mL of H2SO4 (conc.) at 90 °C and
then 25 mL EtOH and 25 mL HCl (conc.) at 60 °C within 2–4 days.
Preparation of UKON-2i Under pH Control Using Buffer Systems: The
materials were prepared similar to the method described for standard
conditions. However, precursor (2) and the template were dissolved in
1.5 g buffer solution.
Aerosol Synthesis of Mesoporous UKON-2a Nanoparticles: 2.84 g of
3,5-bis-ti(isopropoxy)silylbenzoic acid (5.4 mmol) were dissolved in
3.52 g EtOH. A total of 0.19 g H2O and 2.7 μL HCl (0.1 M) were added
and the solution was refluxed at 60 °C for 20 h. The sol was diluted with
7.54 g EtOH followed by addition of 0.68 g H2O, 120 μL HCl (1 M) and
0.7 g Pluronic P123 as surfactant (pH = 2.1). The final reactant mole
ratios (prec: EtOH: H2O: HCl: P123) were 1: 44: 10: 2.022: 0.0224. The
spherical mesoporous nanoparticles were obtained using an aerosol
reactor (TSI Inc., Model 3076) at a volumetric flow rate of 2.6 L min−1.
The aerosol was dried at room temperature for 2.8 s followed by heating
at 500 °C for 4.5 s and finally collected on a PTFE filter (average pore
size 450 nm). The as received particles were extracted with 15 mL EtOH
and 15 mL HCl conc. at 60 °C for 4 days. Full characterization occured
via IR, TEM, SAXS, N2 physisorption, and 13C and 29Si solid state NMR
(see Supporting Information).
Aerosol Synthesis of Mesoporous UKON-2i Nanoparticles: 3.02 g of
1,5-bis-tri(isopropoxysilyl)-benzene-3-sulfonyl chloride (2) (5.14 mmol)
were dissolved in 3.35 g EtOH and 0.18 g H2O. 2.5 μL HCl (0.1 M) were
added (pH = 5.2) dropwise and the solution was refluxed at 60 °C for
20 h. The sol was diluted with 7.17 g buffer solution pH 1.8 (EtOH: H2O;
4:1) followed by 0.71 g tetra-butylammonium chloride (2.57 mmol) and
0.66 g Pluronic P123 as surfactant (pH = 2.1). The final reactant mole
ratios (prec: EtOH: Bu4NCl: H2O: P123) were 1: 44: 10: 0.5: 0.0224.
According to the procedure described above the spherical mesoporous
nanoparticles UKON-21 are obtained by using an aerosol reactor (TSI
Inc., Model 3076) and collected via a PTFE filter system. The volumetric
flow rate is 1.5 L min−1 and the aerosol was dried at room temperature
for 0.7 s followed by heating at 500 °C for 25 s. The as received particles
are extracted with 15 mL EtOH and 15 mL HCl conc. at 60 °C.
Figure 9. Representative illustration of the inhibition of bacterial sur-
face colonization as observed on blood agar plates by the application
of UKON materials as a ‘barrier’ surrounding the bacterial inoculum.
iI) SiO2 nanoparticles (control), iii) UKON-2a nanoparticles, and
i) UKON-2i nanoparticles were applied onto the plates in a circular line
and Pseudomonas aeruginosa was inoculated into the center of each circle
(see Experimental Section). Green areas indicate bacterial growth.
groups per m2, respectively 9.4 × 1020 surface acid groups per
gram. First proof of principles experiments were performed
towards antifouling applications. The comparison to materials
with either none acid groups (mesoporous silica particles) or
weak acidic groups (mesoporous organosilica particles con-
taining benzoic acid) shows that only the strong solid-state acid
(with the sulfonic acid) exhibit sufficient inhibition of bacterial
growth. Further potential applications of the novel material
reported herein are in the areas of heterogeneous catalysis of
for proton conducting membranes.
Ion Exchange Capacity: To determine the ion exchange capacity (IEC)
of the materials a classical titration method was used. A small amount
of dried material was suspended in 0.1 M NaCl solution for 48 h. The
remaining solution was titrated with 0.01 M NaOH solution in order to
neutralize the exchanged proton. The IEC was calculated according to:
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Nat. Mater. 2003, 2, 301–306; e) T. L. Sun, L. Feng, X. F. Gao, L. Jiang,
Acc. Chem. Res. 2005, 38, 644–652; f) A. Fujishima, X. T. Zhang,
D. A. Tryk, Surf. Sci. Rep. 2008, 63, 515–582.
V × M
m
IEC =
(2)
V is the titrant volume at the equivalent point in mL; M is the molar
concentration of the titrant (g mol−1); m is the dried sample weight in g.
Hammett Acidity: To define the acidity strength of the acidic materials
the Hammett acidity method was used. Using UV-vis technique in
organic solvents the protonation state of suitable indicator dyes with
different pKa values was monitored. For this purpose, p-fluoroanilin (pKa
= +2.4; weak acid), anthraquinone (pKa = −8.2), p-fluoronitrobenzene
(pKa = −12.4), and 2,4-ninitrofluorobenzene(pKa = −14.5) were used. The
basic indicators were dissolved in dry hexane. The absorbance of the
pure, non-protonated form was recorded. Then, the solid acids, Ukon-2a,
Ukon-2i, and SiO2 (as reference) were added to the indicator solutions.
After stirring for 30 min, the solid materials were centrifugated and the
absorbance of the supernatant was measured.
Analytical Characterization: NMR-spectra were acquired on a Bruker
Avance III 400 spectrometer (CDCl3 as solvent). Solid-state NMR
spectra were performed on a Bruker DRX 400 spectrometer. Using a
Bruker Esquire 3000 Plus spectrometer with a flow rate of 1 μg mL-1
the ES-MS data were recorded. The SEM images and the EDX data
were received by a Zeiss 249 CrossBeam 1540XB scanning electron
microscope. The Zeiss Libra 120 at 120 kv acceleration voltage
performed the TEM images. FT-IR spectra were recorded by using a
Perkin Elmer Spectrum 100 spectrometer using ATR unit. Small-angle
X-ray scattering (SAXS) measurements were carried out with a Bruker
AXS Nanostar. N2-physisorptions measurements were conducted on a
Micromeritics Tristar. Acquiring a Varian Carey 100 spectrometer the as
received UV/VIS spectra were recorded.
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft (DFG) for
funding with in the collaborative research program SPP 1570. The
authors also thank Dr. H. Schlaad (Max-Planck institute of Colloids and
Interfaces) for the donation of KLE-25.
Received: July 11, 2013
Revised: September 4, 2013
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