Fatty acid carboxylate-and anionic surfactant-controlled delivery systems that use mesoporous silica supports

Article abstract of DOI:10.1002/chem.200903125

We report the preparation of a MCM-41 mesoporous material that contains the dye [Ru(bipy)₃]Cl₂ (bipy = bipyridine) inside the mesopores and functionalised with suitable binding groups at the entrance of the pores. Solids S1-S3 were obtained by the reaction of the mesoporous material with N-methyl-N'-propyltrimethoxysilylimidazolium chloride, N-phenyl-N-[3- (trimethoxysilyl)propyl]thiourea, or N-phenyl-N'-[3-(trimethoxysilyl)propyl] urea, respectively. A study of the dye delivery of these systems in buffered water (pH 7.0, 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid (HEPES), 10^-3 moldm^-3) in the presence of a family of carboxylate ions was carried out. In the interaction of the anions with the surface of the solids, the response depends on the characteristics of the binding groups (i.e., imidazolium, urea and thiourea) at the pore outlets and their specific interaction with the corresponding anion. The interaction of long-chain carboxylate ions with the binding sites at the surface of the solids resulted in a remarkable inhibition of the delivery of the dye. This inhibition was observed clearly for the dodecanoate anion, whereas the octanoate, decanoate, cholate, deoxycholate, glycodeoxycholate and taurocholate anions induced a certain pore blockage that varied according to the solid studied. The interaction of smaller anions, such as acetate, butanoate, hexanoate and octanoate, with the solids had no effect on the dye release process. The possible use of the gating system for the chromo-fluorogenic detection of anionic surfactants through selective dye delivery inhibition was also explored. Molecular dynamic simulations that use force-field methods have been made to theoretically study the capping carboxylate mechanism. The calculations are in agreement with the experimental results, thus allowing a representation of the dye delivery inhibition in the presence of long-chain carboxylate ions.

Full text of DOI:10.1002/chem.200903125

DOI: 10.1002/chem.200903125
Fatty Acid Carboxylate- and Anionic Surfactant-Controlled Delivery Systems
That Use Mesoporous Silica Supports
Carmen Coll,[a, b] Elena Aznar,[a, b] Ramꢀn Martꢁnez-MꢂÇez,*[a, b] M. Dolores Marcos,[a, b]
Fꢃlix Sancenꢀn,*[a, b] Juan Soto,[a] Pedro Amorꢀs,[c] Joan Cano,[d, e] and Eliseo Ruiz[e]
Abstract: We report the preparation of
a MCM-41 mesoporous material that
contains the dye [RuACHTNUGTRNEUNG(bipy)3]Cl2 (bipy=
solids, the response depends on the
characteristics of the binding groups
(i.e., imidazolium, urea and thiourea)
at the pore outlets and their specific in-
teraction with the corresponding anion.
The interaction of long-chain carboxyl-
ate ions with the binding sites at the
surface of the solids resulted in a re-
markable inhibition of the delivery of
the dye. This inhibition was observed
clearly for the dodecanoate anion,
whereas the octanoate, decanoate,
cholate, deoxycholate, glycodeoxycho-
late and taurocholate anions induced a
certain pore blockage that varied ac-
cording to the solid studied. The inter-
action of smaller anions, such as ace-
tate, butanoate, hexanoate and octa-
noate, with the solids had no effect on
the dye release process. The possible
use of the gating system for the
chromo-fluorogenic detection of anion-
ic surfactants through selective dye de-
livery inhibition was also explored.
Molecular dynamic simulations that
use force-field methods have been
made to theoretically study the capping
carboxylate mechanism. The calcula-
tions are in agreement with the experi-
mental results, thus allowing a repre-
sentation of the dye delivery inhibition
in the presence of long-chain carboxyl-
ate ions.
bipyridine) inside the mesopores and
functionalised with suitable binding
groups at the entrance of the pores.
Solids S1–S3 were obtained by the re-
action of the mesoporous material with
N-methyl-N’-propyltrimethoxysilylimi-
dazolium chloride, N-phenyl-N’-[3-(tri-
methoxysilyl)propyl]thiourea, or N-
phenyl-N’-[3-(trimethoxysilyl)propy-
l]urea, respectively. A study of the dye
delivery of these systems in buffered
water (pH 7.0, 2-[4-(2-hydroxyethyl)pi-
perazin-1-yl]ethanesulfonic
acid
Keywords: carboxylate ligands
·
(HEPES), 10À3 moldmÀ3) in the pres-
ence of a family of carboxylate ions
was carried out. In the interaction of
the anions with the surface of the
controlled release · hybrid materi-
als · mesoporous materials · molec-
ular dynamics
[a] C. Coll, E. Aznar, Prof. R. Martꢀnez-MꢁÇez, Dr. M. D. Marcos,
Dr. F. Sancenꢂn, Dr. J. Soto
[d] Dr. J. Cano
Instituto de Ciencia Molecular (ICMol) and
Instituto de Reconocimiento Molecular y Desarrollo Tecnolꢂgico
Centro Mixto, Departamento de Quꢀmica
Universidad Politꢃcnica de Valencia
Fundaciꢂ General de la Universitat de Valꢄncia (FGUV)
Universitat de Valꢄncia, Paterna, Valencia, E-46980 (Spain)
Instituciꢂ Catalana de Recerca i Estudis AvanÅats (ICREA)
Camino de Vera s/n. 46022, Valencia (Spain)
Fax : (+34) 96-387-93-49
E-mail: rmaez@qim.upv.es
[e] Dr. J. Cano, Dr. E. Ruiz
Departament de Quꢀmica Inorgꢅnica and
Institut de Quꢀmica Teꢆrica I Computacional
Universitat de Barcelona, Diagonal 647, 08028 Barcelona (Spain)
fsanceno@upvnet.upv.es
[b] C. Coll, E. Aznar, Prof. R. Martꢀnez-MꢁÇez, Dr. M. D. Marcos,
Dr. F. Sancenꢂn
CIBER de Bioingenierꢀa, Biomateriales y
Nanomedicina (CIBER-BBN)
[c] P. Amorꢂs
Institut de Ciꢄncia del Materials (ICMUV)
Universitat de Valencia
P.O. Box 2085, 46071 Valencia (Spain)
10048
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Chem. Eur. J. 2010, 16, 10048 – 10061

FULL PAPER
Introduction
system capped with boronic acid-functionalised gold nano-
particles that showed both a pH- and photo-switched release
of guests.[20]
The combination of chemical principles and solid structures
has recently resulted in the discovery of new synergic strat-
egies, a unique tunability of the properties of nanoscopic
solids and new perspectives of applicability to supramolec-
ular concepts.[1] In this interdisciplinary area of “hetero-
supramolecular” chemistry, one attractive concept is the de-
velopment of the so-called molecular gating systems. A
gating device can control mass transport and can ideally be
“opened” and “closed” by certain target external stimuli
(either chemical or physical). One appealing application of
such systems is the design of functionalised nanocontainers
with triggered structures for the development of advanced
functional controlled release supports. Apart from the use
of microcapsules,[2] micelles,[3] vesicles,[4] liposomes and so
forth as nanocarriers, the use of silica mesoporous supports
(SMPS) has recently attracted attention for use as systems
in which gating functional scaffoldings could be coupled.
The distinctive characteristics of mesoporous supports, such
as high homogeneous porosity, easy surface functionalisa-
tion, inertness, stable structure, large surface area, biocom-
patibility and high load capacity, make these systems attrac-
tive scaffoldings for the development of systems for con-
trolled delivery studies.[5]
However, apart from light and pH-anions stimuli, perhaps
the most popular approach to delivery control in mesopo-
rous materials is the use of redox reactions. Lin and co-
workers developed mesoporous functionalised materials for
the controlled release of certain substances, such as drugs,
DNA and so forth, by using mesoporous scaffoldings capped
with CdS, gold, or magnetic nanoparticles.[21] The collabora-
tion between Zink and Stoddart has also produced several
examples of gated nanovalves triggered by redox inputs
based on the use of rotaxanes and pseudorotaxanes that
contain redox active stations, such as 1,5-dioxylnaphthalene
and tetrathiafulvalene,[22–25] and Fujiwara et al. used a disi-
lane–disulfide derivative to close the pores and opened
them through the simple addition of dithiothreinol, which
cleaves the disulfide bond.[26] Also, redox-induced release
has been reported by Feng and co-workers by using mesopo-
rous silica particles capped with a polymer network.[27]
Apart from the examples described above, which report
the use of redox-, light- and pH-triggered systems, additional
studies explore the use of other stimuli such as electrochem-
ical oxidation,[18] temperature,[28] or enzyme-catalysed reac-
tions.[29,30,31] In addition, we have also recently reported the
capping of mesoporous materials with antibodies and selec-
tive gate opening in the presence of the corresponding anti-
gen.[32] Most of these systems are designed for delivery ap-
plications, although we also consider that gating ensembles
could possibly be used in the development of new signalling
protocols, although little has been done so far in this direc-
tion.[33,34,35]
Despite these advances, the approach of gated smart sup-
ports for the development of real delivery scaffolds is still in
the incipient stage. In particular, the possibility of designing
the opening/closing of gated nanocontainers that could be
controlled by chemical rather than physical stimuli is highly
appealing and may open new perspectives of applicability to
controlled release scaffoldings. Furthermore, it is apparent
that few studies have been carried out on the development
of gated mesoporous materials capable of varying delivery
behaviour according to the anions present in the solution.
With this objective in mind, we report herein the design of
hybrid mesoporous materials that contain gatelike ensem-
bles based on imidazolium, thiourea and urea binding
groups, the solids being loaded with a suitable dye. The con-
trolled delivery of the entrapped dye was studied in the
presence of a family of carboxylic acids and surfactants in
pure water. The possibility of using these systems in signal-
ling protocols is also discussed. Part of this study has already
been recently reported.[36]
The first example of gatelike structures in mesoporous
materials was reported by Fujiwara et al., who used the re-
versible photodimerisation of coumarin to open/close the
pores of a mesoporous scaffolding.[6] Since these reports,
other photochemical gated systems, including the reversible
light-driven cis/trans isomerisation of azobenzene groups,[7–9]
association and light-operated dissociation of b-cyclodextrin
with azobenzene groups,[10] and spiropyran/merocynine pho-
toswitchable transformations,[11] have been reported.
The pH value has been another popular stimulus to con-
trol mass transport in mesoporous materials. We reported
the first gated hybrid system to operate in aqueous solution
controlled by modulation of the pH value and the presence
of certain anions.[12,13] The gated material consisted of a mes-
oporous silica support functionalised with polyamines on the
external surface. An approach that could be considered
complementary was reported by Xiao and co-workers by
using mesoporous surfaces functionalised with carboxylate
ions.[14] Zink and co-workers reported the design, synthesis
and functional behaviour of a pH-controlled supramolecular
gatelike scaffolding that consisted of a [2]pseudorotaxane
formation through the encirclement of dialkylammonium
cations with the macrocyclic polyether dibenzo[24]crown-
8,[15,16] by using ion–dipole interactions between curcubi-
t[6]uril and bis-ammonium stalks[17] or by using the interac-
tion between ferrocene derivatives and curcubit[7]uril.[18]
Kim and co-workers demonstrated the controlled release of
guest molecules by employing a pH-responsive gating en-
semble that used mesoporous materials based on a cyclodex-
trin/polyamine pseudorotaxane motif.[19] Other more sophis-
ticated dual controlled release systems have been devel-
oped. For example, we recently reported a gated hybrid
Results and Discussion
Gated material: As stated above, the functionalisation of
mesoporous scaffolds with certain gatelike ensembles has
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F. Sancenꢂn et al.
proved a suitable procedure for the design of systems to
study mass transport control (controlled delivery). In most
cases, the designed systems consist of a switchable ensemble,
able to respond to an external stimuli, anchored on a suita-
ble nanocontainer and loaded with an entrapped substance
to be delivered. For our study, we selected a mesoporous
MCM-41 material that contains mesopores in the 2–3 nm
range as the solid support. In relation to the molecular or
supramolecular ensemble, we selected anion binding sites
such as imidazolium, thiourea and urea derivatives. The de-
livery of a certain dye from the mesopores was studied in
the presence of a family of carboxylate ions that can display
a coordination interaction with the appended binding sites
and eventually close the gate (Scheme 1).
pores to the solution during grafting of the binding groups.
By additionally taking into account that the reaction of the
trimethoxysilyl derivatives with the silica surface is generally
a quick process, it is feasible to deduce that the final struc-
tures will contain coordinating groups (i.e., imidazolium,
urea or thiourea) anchored preferentially on the external
surface and external entrance of the pore voids. A similar
two-step synthetic procedure has recently been used both by
us and others to develop responsive operational gating
structures that contain a certain cargo in the pores and suit-
able switchable ensembles at the pore outlets. In all cases,
the final solids were isolated by filtration and meticulously
washed with different solvents and dried.
This procedure resulted in the preparation of solids that
contain the anion binding
groups imidazolium (S1), thio-
urea (S2) and urea (S3) anch-
ored on the surface and a cer-
tain amount (see below) of dye
entrapped in the mesopores.
The imidazolium ion has been
widely reported as a suitable
coordination site for anions
À
through electrostatic and (C
H)+···XÀ
ionic
hydrogen
bonds.[37] On the other hand,
the urea and thiourea moieties
have also been used widely as
anion-binding systems. It is
known that these groups can
act as double hydrogen-bond
donors to give bidentate Y-
shaped interactions with car-
boxylate ions.[38] These com-
plexes have been reported to
show appreciable stability, even
in polar solvents.
Scheme 1. A representation of the sensory hybrid materials and the interaction of the gatelike ensemble with
a) butanoate (the gate remains “open”) and b) dodecanoate anions (the gate is “closed”)
Characterisation of the hybrid
For release protocols, the solid support must contain an
entrapped guest (the dye) in the pores, whereas the molecu-
lar gating mechanism should ideally only be anchored on
the outer surface (the pore outlets). The mesoporous MCM-
41 support was synthesised with tetraethyl orthosilicate
(TEOS) as a hydrolytic inorganic precursor and the surfac-
tant hexadecyltrimethylammonium bromide (CTAB) as a
porogen species. Calcination of the mesostructured phase
produced the starting solid. The dye was added to a suspen-
sion containing the MCM-41 scaffolding, which was stirred
for 24 h with the aim of completely loading the pores of the
mesoporous support. The trimethoxysilyl derivatives N-
methyl-N’-propyltrimethoxysilylimidazolium chloride, N-
phenyl-N’-[3-(trimethoxysilyl)propyl]thiourea, or N-phenyl-
N’-[3-(trimethoxysilyl)propyl]urea were added to this mix-
ture to prepare the solids S1–S3, respectively. This reaction
was carried out in the presence of a high concentration of
the dye to disfavour the diffusion of the latter from the
materials: Figure 1 shows powder X-ray diffraction (XRD)
patterns of the as-synthesised MCM-41, calcined MCM-41
and the hybrid material S1. XRD studies of the as-synthes-
ised MCM-41 shows four angle reflections typical of the
hexagonal array that can be indexed as (100), (110), (200)
and (210) Bragg peaks. A significant shift of the (100) re-
flection and broadening of the (100) and (200) peaks in the
powder XRD study of the calcined MCM-41 sample is clear-
ly appreciated in curve (b), thus corresponding to a cell con-
traction of approximately 6–8 ꢈ due to the condensation of
silanol during the calcinations step. Figure 1 also shows a
curve that corresponds to S1. Reflections (110) and (200)
are lost for this solid, most likely related to a decrease in
contrast because of the filling of the pores with the rutheni-
um complex. Similar behaviour was observed for S2 and S3
(not shown). Nevertheless, the clear presence of the d100
peak in the XRD patterns suggests that the loading process
with the [RuACTHNUTRGNEUNG
(bipy)3]2+ complex and the additional function-
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Controlled Dye Delivery as a Molecular Gating System
FULL PAPER
tent of the dye and imidazolium, thiourea and urea groups
in the final solids S1–S3 are shown in Table 1.
Table 1. Amount of the imidazolium, thiourea and urea groups and dye
present in the S1–S3 sensory materials.
Solid
Binding groups
[wt%]
Dye
ACHTUNGTRENNUNG[wt%]
U
S1
S2
S3
9
7
5
10
19
15
The solids were also characterised by UV/Vis spectroscop-
ic measurements. The UV/Vis diffuse reflectance spectrum
of S2 shows an intense absorption band at l=470 nm that
corresponds to the [RuACTHNUTRGNEUNG
(bipy)3]2+ complex inside the pores
(Figure 3). Similar spectra were obtained for S1 and S3
(data not shown).
Figure 1. Powder X-ray patterns of the a) as-synthesised MCM-41, b) cal-
cined MCM-41 and c) the final solid S1 that contains the dye and the
anchored imidazolium derivatives.
alisation with the imidazolium, thiourea and urea derivatives
did not substantially modify the mesoporous MCM-41 sup-
port. The presence of the mesoporous structure in the fun-
cionalised solids is also confirmed by TEM analysis, in
which the typical hexagonal porosity of the MCM-41 matrix
can clearly be observed (Figure 2).
Figure 3. Diffuse reflectance UV/Vis spectra of S2.
The nitrogen adsorption–desorption isotherms of the cal-
cined MCM-41 material shows a typical curve for these mes-
oporous solids; that is, an adsorption step at intermediate
value (0.3). This curve corresponds to a type IV isotherm, in
which the observed step can be related to nitrogen conden-
sation inside the mesopores by capillarity. The absence of a
hysteresis loop in this interval and the narrow pore distribu-
tion suggests the existence of uniform cylindrical mesopores
(0.90 cm3 gÀ1). Application of the BET model[39] gave a value
for the total specific surface of 1371.6 m2 gÀ1. From XRD
studies, porosity and TEM studies, the cell parameter ao,
pore diameter and wall thickness (39.97, 23.2 and 16.76 ꢈ,
respectively) can be calculated. The volume and pore size
were estimated by using the Barret–Joiner–Haselda (BJH)
method.[40] The isothermal nitrogen adsorption/desorption
behaviour of S1–S3 were also studied (Figure 4). In contrast
to the MCM-41 starting materials, S1–S3 show almost flat
curves, thus indicating a remarkable decrease of porosity
(Table 2). This result could be expected, bearing in mind the
Figure 2. TEM image of S1 that shows the typical porosity of the MCM-
41 matrix.
After functionalisation, the IR spectra of S1–S3 show the
expected features, namely, intense bands due to the silica
matrix (n˜ =1250, 1087, 802, 462 cmÀ1) and vibrations of
water molecules (n˜ =3420, 1620 cmÀ1). As the funcionalisa-
tion of the solids is low, it is not possible to observe the
characteristic bands of the anchored organic moieties. How-
ever, a drop in intensity of the band at n˜ =952 cmÀ1 related
to the vibration of the silanol group was observed, thus indi-
cating surface functionalisation.
Quantification of the content of the binding group and
the [Ru
ric analysis (TGA) and elemental analysis. The typical con-
ACHTUNGTRENNUNG
(bipy)3]2+ dye was accomplished by thermogravimet-
high content of the [RuACTHNUTRGNEUNG
(bipy)3]2+ complex that fills the
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F. Sancenꢂn et al.
livery of the dye was directly monitored through the spin-al-
lowed metal-to-ligand change-transfer (MLCT) transition
band of the [Ru
aqueous phase.
(bipy)3]2+ dye centred at l=454 nm in the
ACHTUNGTRENNUNG
The delivery of the dye from the pore voids for S1–S3 in
the presence of carboxylate ions is shown in Figure 5, which
plots the difference in absorbance at l=454 nm for the dye
delivered from the solid and for the corresponding solid/car-
boxylate ion system. Values near zero indicate that there
was no pore blockage, whereas higher values mean that dye
delivery was inhibited.
The chromogenic response is most likely related to the
ability of the positively charged imidazolium ion and the
neutral thiourea and urea moieties to form complexes with
carboxylate ions. The small carboxylate ions do not have
any effect, whereas the longer ions induce partial or total
delivery inhibition (Figure 5). Solids S2 and, especially, S3
show a gradual increase in release inhibition as the length of
the carboxylate ion increases. In contrast, there is no gradu-
al pore blockage for S1, but rather an “on/off” behaviour;
that is, complete dye delivery for small carboxylate ions
(from acetate to octanoate), which can not “close” the gate
and complete blockage for longer carboxylate ions, which
act as molecular taps in the gatelike ensemble. Coordination
of a small or medium carboxylate ion, such as octanoate or
smaller, does not inhibit dye release from the pores to the
bulk solution. On the other hand, on coordination with the
imidazolium ion, the dodecanoate ion would form a highly
hydrophobic layer around the entrance to the pores that
would inhibit the delivery of the otherwise rather hydrophil-
ic ruthenium(II) dye from inside the pores to the water
phase. The small differences in the behaviour of S1–S3 in
the presence of carboxylate ions are modulated and are
likely due to the differences in the nature of the binding
sites.
Figure 4. Nitrogen adsorption/desorption isotherms for a) the mesoporous
MCM-41 material, b) S1, c) S2 and d) S3.
Table 2. BET specific surface values, pore volume and pore size calculat-
ed from the nitrogen adsorption/desorption isotherms for S1–S3.
Sample
SBET
[m2 gÀ1
BJH pore[a]
[nm]
Total pore volume[b]
[cmÀ3 gÀ1
G
]
N
]
MCM-41
1371
122
266
172
2.3
2.0
2.1
2.2
0.91
0.08
0.19
0.12
S1
S2
S3
[a] Pore estimated by using the BJH model applied on the adsorption
branch of the isotherm. [b] Total pore volume according to the BJH
model.
pores. We had previously observed similar behaviour in
other gated materials. Therefore, the total lack of porosity
provides direct evidence of the high efficiency of the dye
loading.
Delivery in the presence of carboxylate ions: A study was
made of the effect of certain anions on the control of the
dye delivery. In the first step, the capacity of S1–S3 to deliv-
er the ruthenium(II) dye in the presence of different carbox-
The release kinetics at neutral pH was studied in more
detail for S1 and two carboxylate ions (ccarboxylate
=
10À3 moldmÀ3), for which a clearly different behaviour was
observed; that is, the octanoate and dodecanoate ions, for
which a remarkably different dye delivery process is seen
(Figure 6). It also can be observed that maximum dye libera-
tion is found after approximately 30–40 minutes. A similar
kinetic release (not shown) to that of the octanoate ion was
found for only S1 in the presence of shorter carboxylate
ions, such as acetate, butanoate and hexanoate.
ylate ions (CH3ACHTUNGTRENNUNG
(CH2)nCOOÀ; n=0, 2, 4, 6, 8, 10) was tested
in aqueous solution. The longer carboxylate ions (n>10)
were poorly soluble in water and were not studied. In the
preliminary scanning, the response of the solids in the pres-
ence of carboxylate ions was very similar in the range pH 5–
8. Lower pH values were not studied because of the lower
solubility of the protonated carboxylic form, whereas higher
pH values were avoided because of possible attack by the
OHÀ ion on the silica matrix. A neutral pH value was finally
selected for the study (pH 7.0, 2-[4-(2-hydroxyethyl)pipera-
zin-1-yl]ethanesulfonic acid (HEPES), 10À3 moldmÀ3). In a
typical release experiment, the solid (10 mg) was suspended
in water (25 mL) that contained the corresponding carboxyl-
ate ion (ccarboxylate =10À3 moldmÀ3). The suspension was
The different behaviour displayed by the octanoate versus
dodecanoate ion can also be seen in Figure 7, which shows
adsorption versus Àlogccarboxylate curves for these carboxylate
ions with S1 in water. The dodecanoate ion can completely
inhibit dye delivery from S1 at the millimolar level, whereas
the octanoate ion does not have any effect on delivery, even
at very high concentrations of up to 10À2 m.
An interesting question related to this system is the evalu-
ation of the imidazolium/carboxylate ion interaction. It is
evident from Figure 6 that whereas the dodecanoate ion can
inhibit the delivery of the dye, the octanoate ion can not dis-
play the same behaviour. This outcome may be related to
stirred until complete diffusion of the [RuACTHNUTRGNEUNG
(bipy)3]2+ complex
from the pore voids into the bulk solution was observed.
Maximum delivery was detected after approximately 30 mi-
nutes (see below). Finally, the mixture was filtered. The de-
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Controlled Dye Delivery as a Molecular Gating System
FULL PAPER
Figure 6. Colorimetric dye release kinetics for S1 in water (pH 7.0,
HEPES, 10À3 moldmÀ3) in the presence of octanoate ( ) or dodecanoate
&
) ions (ccarboxylate =10À3 moldmÀ3) and without carboxylate ions (&).
!
(
Figure 7. Variation in the absorption (l=454 nm) versus the logarithm of
!
&
the concentration for dodecanoate ( ) , decanoate (&) and octanoate ( )
ions with the sensory material S1 in water. t=1 h.
certain carboxylate ion is added to S1, the TGA and ele-
mental analysis clearly show an increase in organic material,
thus indicating that self-assembly of the corresponding car-
boxylate ion occurs on the imidazolium surface. However, it
turned out to be rather difficult to obtain reliable quantita-
tive results of the anchored carboxylate ion through interac-
tion with the imidazolium ion from TGA and elemental
analysis. As the possible cause of error, we suggest the pres-
ence of the [RuACTHNUTRGNEUNG
(bipy)3]2+ dye at the pores, which is partially
delivered even in short reaction times in the presence of the
carboxylate ions. Moreover, for MCM41-type solids, below
3008C water loss is typically observed due to the condensa-
tion of an undetermined number of silanol groups, thus
making quantitative interpretation rather complex. Due to
this reason, we opted to use a simpler solid support to carry
out studies on the imidazolium/carboxylate ion interactions.
Thus, a simple fumed-silica support was prepared (similar to
the MCM-41 material with no mesopores or the dye) and
functionalised with the imidazolium binding sites to make
Figure 5. Colorimetric response of a) S1, b) S2 and c) S3 materials in the
presence of carboxylate ions of different length at neutral pH in water.
The difference in the absorbance for the solid system alone A0 and the
absorbance for the corresponding solid/carboxylate ion system AI is
shown. t=1 h
the possibility that the dodecanoate ion can interact with
the imidazolium ions and that this interaction may not be
possible for the octanoate ion. Additional experiments were
carried out to rule out this possibility. When an excess of a
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F. Sancenꢂn et al.
suitable assays. This material was readily obtained by reac-
tion of the N-methyl-N’-propyltrimethoxysilylimidazolium
chloride derivative in fumed silica in acetonitrile at room
temperature to produce solid FS-Im, which was character-
ised by using conventional solid-state techniques. The imida-
zolium content was determined by TGA and elemental
analysis. The molar ratio between the imidazolium binding
molecule and SiO2 was 5.3ꢉ10À2 (0.67 mmol per g solid),
which provides a relatively dense monolayer of imidazolium
cations with an average distance between neighbouring imi-
dazolium ions of approximately 10 ꢈ. We used this FS-Im
support to study the interaction with two different guests,
that is, octanoate and dodecanoate ions. For this purpose,
aqueous suspensions of FS-Im were titrated with these two
carboxylate ions and the titrations were followed by con-
ductimetry. Figure 8 shows the conductivity of the solution
as a function of the number of equivalents of carboxylate
ion added (equivalents with respect to the equivalents of
imizadolium ions in the solid; i.e., 0.67 mmol per g solid).
From these titration profiles, it can be seen that a significant
change in conductivity is observed for both carboxylate ions
at one equivalent of added carboxylate ion. The results un-
doubtedly show that for both the octanoate and dodeca-
noate ions a monolayer of carboxylate ions is formed coor-
dinated to the imidazolium-functionalised surface. By ex-
trapolating this result to S1, it seems clear that both carbox-
ylate ions coordinate on the imidazolium surface at the pore
boundaries and in both cases the substitution of a chloride
group by the corresponding carboxylate ion would surely be
complete, as with FS-Im. This behaviour means that the dif-
ference observed between the dodecanoate and octanoate
ions (Figures 6 and 7) is not only due to different densities
of the S1/carboxylate ion at the pore outlets, depending on
the carboxylate used, but is most likely due to the “gatelike”
effect.
Finally, we were particularly concerned with the possibili-
ty that the observed behaviour was caused by a simple inter-
action of the carboxylate ion with the silica surface. To rule
out this possibility, the delivery experiments were repeated
using a MCM-41 support loaded with the dye but not func-
tionalised with the imidazolium ion. This solid shows no se-
lectivity and has similar dye release kinetics for all the car-
boxylate ions. Therefore, it can be concluded that the colori-
metric sensing discrimination observable to the naked eye
of the long-chain carboxylate ions only occurs in solids that
contain both nanoscopic pores and a coordinating group at
the entrance to the pores.
We were also interested in studying the behaviour of
these solids in the presence of other anions and found in
preliminary studies that the interaction of S1–S3 with the
cholate, doexycholate, glycodeoxycholate and taurocholate
anions resulted in a partial pore blockage. These ions are
the corresponding anions of primary bile acids and play a
fundamental role in the metabolism of cholesterol.[41] Fur-
ther assays were carried following experimental procedures
similar to that described above with the corresponding solid
(10 mg) in water at neutral pH. Capping of the solids in the
presence of the cholate, deoxycholate, glycodeoxycholate
and taurocholate ions was evaluated by monitoring the visi-
ble absorption band when the dye was released into the so-
lution. The results for S2 and S3 are shown in Figure 9,
which also includes the response of this solid to the dodeca-
noate ion for comparative purposes. The capping ability of
the anions tested on S2 is on the order dodecanoate~deoxy-
cholate~glycodeoxycholate>cholate>taurocholate and
a
similar trend was observed for S3 and S1 (not shown). Of
the anions tested, the taurocholate ion shows the lowest cap-
ping ability, most likely due to the weaker interaction of the
sulfonate unit of the taurocholate anion with the binding
sites on the solids relative to the analogous carboxylate ions.
It is also interesting to note that small differences in the
structure of the anions and the anchored binding groups on
the solids give slightly different final responses. To sum up,
the studies carried out with S1–S3 demonstrated that a
family of carboxylate ions can modulate the delivery of the
entrapped guest through interaction with the binding groups
Figure 8. Conductivity versus the number of equivalents of carboxylate
ions added in the titration experiments of solid FS-Im with a) octanoate
and b) dodecanoate ions. The difference in the conductivity of an aque-
ous suspension of FS-Im in the presence of a certain amount of carboxyl-
ate ion and an aqueous solution that contains the same quantity of car-
boxylate ion is shown.
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Figure 10. PCA score plot for the acetate, butanoate, hexanoate, octa-
noate, decanoate, dodecanoate, cholate, deoxycholate, glycodeoxycholate
and taurocholate ions with S1–S3 (the data shown are from three differ-
ent trials). The PC axes are calculated to lie along lines of diminishing
levels of variance in the data set.
zone at positive values of PC1. The low discrimination is in
agreement with the very similar response found for all these
species (no pore blockage). The remaining anions are
placed at medium and negative values of PC1. A clear dif-
ferentiation between the clusters for the decanoate and do-
decanoate ions is observed. Also, different overall behaviour
was found for the cholate and taurocholate ions, whereas
the deoxycholate and glycodeoxycholate ions form a unique
cluster in the PCA space.
Figure 9. Colorimetric response of a) S2 and b) S3 in the presence of the
cholate, deoxycholate, glycodeoxycholate and taurocholate ions at neu-
tral pH in water. The difference in absorbance for the solid system alone
A0 and the absorbance for the corresponding solid/carboxylate system AI
is shown. For comparative purposes, the dye delivery from the solids in
response to the dodecanoate ion is also included. t=1 h.
Signalling applications—sensing of anionic surfactants: One
suggested application for stimuli/responsive systems is their
use as triggered structures for the development of advanced
functional controlled release supports. These materials are
promising, for example, in the design of functional tailor-
made nanodevices for on-command drug/gene/protein deliv-
ery, self-healing and so forth. From a different point of view,
we have also suggested that these materials can be used for
signalling applications.[33–35] In our case, the hybrid material
has been loaded with a dye and the system is designed to be
uncapped. The addition of the solid to a solution that con-
tains the target analyte results in capping of the pores and
inhibition of the dye delivery. With regard to closing the
pores as a consequence of the selective interaction with the
gatelike binding sites (as in our case, for example, the inter-
action of the large carboxylate ions with the binding site
groups), the recognition process becomes a selective optical
response. This remarkable colorimetric “on/off” discrimina-
tion of long-chain carboxylate ions in water is, as far as we
know, difficult to achieve by using conventional molecular
probes. In fact, colorimetric sensors for long-chain carboxyl-
ate ions are scarce and many of these probes show nonspe-
cific chromo-fluorogenic responses to the carboxylate func-
tional ion. There are also few examples of the colorimetric
discrimination of individual members or subgroups of car-
boxylate families in organic solvents[43] or water.[44]
anchored on the external surface of the mesoporous materi-
al. In addition, the degree of capping of the anions studied
depends on the characteristics of both the anion and binding
groups. Therefore, the response of a certain solid in the
presence of a certain anion depends on the intrinsic chemi-
cal nature of the binding site, shape and size of the anion,
possibility of densely packed monolayers and so forth.
These subtle differences are reflected in the specific final
delivery behaviour of each solid–anion ensemble.
To study the different response of the solids with different
anions, the responses of S1–S3 could be combined to form
ensembles for the pattern recognition of anions in an at-
tempt to find selectivity fingerprints. This approach can be
carried out, for example, when the data matrix is analysed
by principal component analysis (PCA) algorithms.[42] The
PCA score plot of the results for S1–S3 in the presence of
the anions studied over three different trials is shown in
Figure 10. Recognition patterns can be identified clearly for
some of the anions studied. A detailed look at the PCA plot
suggests that there is some correlation between the position
of the anion clusters on the PC1 axis and their size. Thus,
HEPES (no anion) and the carboxylate ions acetate, buta-
noate, hexanoate and octanoate appear together in the same
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F. Sancenꢂn et al.
After the detailed study of the interaction of imidazolium,
urea, or thiourea moieties with different carboxylate ions
and the effect of this interaction on the delivery of the dye
from the mesoporous scaffoldings, we further explored the
possible use of the gating system for the selective chromo-
fluorogenic detection of anionic surfactants. It was envi-
sioned that the very same inhibition effect observed in the
presence of the dodecanoate ion with S1, which contained
imidazolium ions, would be operative in the presence of
anionic surfactants, which also contain an anionic groups
and a hydrophobic tail. We have previously demonstrated
that anionic surfactants have an affinity for forming com-
plexes with imidazolium-functionalised surfaces.[45] Accord-
ingly, we considered that the interaction between an imida-
zolium derivative and an anionic surfactant could be con-
veniently applied to the colorimetric determination of these
environmentally important anions through dye delivery in-
hibition in functionalised nanoporous solids such as S1. At
this point, it might be worth commenting that the easy de-
tection of anionic surfactants in water is still an unresolved
goal. For example, many commonly reported methods re-
quire time-consuming procedures (e.g., titrimetry,[46] chro-
matography[47]), need time-consuming sample preparation
(e.g., spectrophotometry,[48] spectrofluorimetry[49]), use large
amounts of harmful solvents (e.g., CH3Cl in the standard
well-known spectrophotometric “methylene blue” method),
or show problems related to reproducibility and signal sta-
bility (typically found in ion-selective electrodes).[50] In addi-
tion, many of these methods are also not generally suitable
for quantitative or semiquantitative assays in situ.
tems for the selective chromo-fluorogenic detection of
anionic surfactants in real samples. In this respect, S1
(10 mg) was suspended in water (25 mL; pH 7.0, HEPES,
10À3 moldmÀ3) containing phosphate, nitrate, acetate, buta-
noate and LSÀ ions (canion =10À3 moldmÀ3). The colorimetric
response of S1 under these conditions is shown in Figure 11.
The dye release inhibition in a mixture containing LSÀ and
the interferents is nearly the same relative to LSÀ alone.
Figure 11. Colorimetric response of S1 in the presence of anionic (LSÀ,
DPÀ), cationic (CTA+) and neutral (Triton X-100) surfactants and inor-
ganic cations and anions (c=10À3 moldmÀ3). The difference in absorb-
ance for the solid system alone A0 and the absorbance for the corre-
sponding S1–anion/cation system AI is shown. t=1 h. a) A mixture con-
taining phosphate, nitrate, acetate, butanoate and LSÀ ions (c
=
anions
10À3 moldmÀ3).
Lauryl sulfate (LSÀ) and dodecylphosphate (DPÀ) were
selected as anionic surfactants and S1 was chosen as the
sensing solid for these studies. Analysis of the response was
tested simply by using S1 (10 mg) suspended in water
(25 mL; pH 7.0, HEPES, 10À3 moldmÀ3) that contained the
corresponding anion (canion =10À3 moldmÀ3) and stirring the
suspension for 30 minutes. For the sake of comparison, the
response was also studied in the presence of the cationic ce-
tyltrimethylammonium (CTA+) and the zwitterionic Triton
X-100 surfactants and compulsory water-present anions and
Computational studies: As we have discussed above, cap-
ping of the pores is most likely related to the interaction of
long-chain carboxylate ions with the anchored binding
groups on the surface of the mesoporous solids. In addition,
to study this interaction in further detail, the carboxylate-
controlled modulation of the gate ensemble was studied by
molecular dynamic simulations with force-field methods by
selecting the solid functionalised with imidazolium ions.
With this goal, a mesoporous silica structure was constructed
by taking the plane (1À11) of the b-cristobalite structure as
a base on which large hexagonal nanopores were included
(see the Experimental Section). This model shows a hexago-
nal supercell with the following parameters: a=b=
40.503 ꢈ, a=b=90.08 and g=120.08. The size of this “su-
percell” was chosen to generate pores and walls with similar
dimensions (d=22.9, t=15.5 ꢈ) to those dimensions experi-
mentally found in the prepared MCM-41-based solids (d=
23.3, t=13.1 ꢈ). The corresponding imidazolium derivatives
were anchored on this scaffolding to the external surface.
Because of the difficulty in finding the global energy mini-
mum, molecular dynamic simulations were performed to
cover the most important parts of the potential-energy sur-
face by using thermal energy to escape from local minima
(see the Experimental Section). The starting system was
built by placing the carboxylate molecules on the nanopore
of the mesoporous surface. Dynamic simulations and subse-
quent geometry optimisation on the model showed that the
3À
2À
cations, such as Na+, K+, Ca2+, CO32À, PO4 and SO4
.
The colorimetric response of S1 to these species is shown in
Figure 10, which plots the absorbance difference at l=
454 nm for the dye delivered from S1 and the corresponding
solid/anionic surfactant system. The anionic surfactants LSÀ
and DPÀ show a quite similar capping ability relative to the
dodecanoate ion, whereas the cationic and zwitterionic sur-
factants can not inhibit the dye release. Also, except for
some response to phosphate, S1 shows a poor response to
inorganic cations and anions. Figure 10 suggests that S1 or
similar systems could be used for the development of
screening probes for the colorimetric detection of anionic
surfactants in water through dye delivery inhibition by using
mesoporous ensembles containing binding sites on the pore
outlets. Further analytically oriented studies are being car-
ried out in our laboratory. We carried out a competitive ex-
periment with S1 to study the possibility of using these sys-
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Controlled Dye Delivery as a Molecular Gating System
FULL PAPER
carboxylate ion moved in proximity to the imidazolium ions
influenced by electrostatic interactions. At the starting
point, two models were tested in which the carboxylate mol-
ecules were placed so that the carboxylic or alkyl portions
of the molecule pointed towards the pore (Figure 12a,b),
length of the carboxylate ion (for example, Figure 13 shows
a parallel perspective views of the surfaces of the models
with the acetate and decanoate anions). The change in the
Figure 13. Parallel perspective views of the surfaces of models with
a) acetate and b) decanoate. Pale yellow, blue, grey, red and pink have
been employed for the mesoporous base, nitrogen, carbon, oxygen and
hydrogen atoms, respectively.
Figure 12. Starting points of the two models tested (see text) at which the
carboxylate ions were placed pointing towards the pore with the a) car-
boxylic or b) alkylic portions of the molecule and the possible final inter-
actions (c,d).
size of the nanopore (d; calculated with the simulation
model) with the length of the alkyl chain of the carboxylate
ion is displayed in Figure 14. In addition, an estimation of
thus expecting that the final conformation may be different
depending on the initial situation of the carboxylate ions
(Figure 12c,d). However, it was observed that the studied
carboxylate ions moved in all cases to final positions similar
to those shown in Figure 12c in which the attached imidazo-
lium rings interact with the carboxylate ions that are situat-
ed in the outer part of the mesopores. Although not the cen-
tral issue of this study, it is also noteworthy that geometry
optimisation carried out on surfaces not completely covered
by imidazolium ions suggested a strong interaction of the
carboxylate ions with the positive charge on the silicon
atoms. In fact natural-bond-orbital (NBO) analysis carried
out on the B3LYP wave function showed that the charge
presented by the silicon atoms at the surface is larger than
the charge observed on the imidazolium ring, which delocal-
ises the charge over the rest of ligand. Thus, the nitrogen
atoms in the ring have a negative charge and the positive
charge on the ring is mainly placed on the hydrogen atoms.
When dynamic simulations and subsequent geometry op-
timisation on the interaction of imidazolium ions on the sur-
face was carried out for a family carboxylate ions (CH3-
the molecular diameter for the [RuACHTUNGTRNEUNG
(bipy)3]2+ complex (dRu
ꢀ12 ꢈ) can be calculated from the Van der Waals radii by
supposing a spherical geometry for this complex. Moreover,
it has to be taken into account that the model is a dynamic
system in which the pore diameter continuously changes,
ACHTUNGTRENNUNG
Figure 14. Dependence of the nanopore diameter d [ꢈ] as a function of
the number of carbon atoms in the CH3ACTHNUTRGNEUNG
(CH2)nCOOÀ ions.
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10057

F. Sancenꢂn et al.
but with averaged values that are close those calculated
values. Thus, the theoretical results must be considered only
in a qualitative way. Nevertheless, it can be concluded from
a comparison between the values of the d and dRu parame-
ters that the ruthenium complex can be released for alkyl
carboxylate ions with carbon atoms of ꢁ8 (octanoate),
which is in agreement with the experimental observation. In
summary, the model shows that carboxylate ions that inter-
act with imidazolium ions at the pore outlets can significant-
ly decrease the pore diameter, thus eventually resulting in
pore blockage and dye release inhibition, which is in agree-
ment with the experimental observations when using long-
chain carboxylate ions.
Experimental Section
Methods: XRD, TGA, elemental analysis, TEM, nitrogen adsorption/de-
sorption and UV/Vis spectroscopic techniques were employed to charac-
terise the synthesised materials. TGA was carried out on a TGA/SDTA
851e Mettler Toledo balance using an oxidant atmosphere (air;
80 mLminÀ1
) with a heating program that consisted of a ramp of
108CminÀ1 from 393 to 1273K. X-ray measurements were performed on
a Bruker AXS D8 Advanced diffractometer using CuKa radiation. Nitro-
gen adsorption/desorption isotherms were recorded on a Micromeritics
ASAP2010 automated sorption analyser. The samples were degassed at
1208C in a vacuum overnight. The specific surface area was calculated
from the adsorption data in the low-pressure range by using the BET
model. The pore size was determined following the BJH method. UV/Vis
spectroscopic analysis was carried out with a Lambda 35 UV/Vis spec-
trometer (Perkin–Elmer Instruments). The UV/Vis spectra of the solids
were recorded using
a spectrophotometer (Model Perkin–Elmer
Lambda) equipped with a diffuse reflectance accessory (Model RSA-PE-
20, Labsphere, Inc., North Sutton, NH, USA). The measurements were
conducted at room temperature over a wavelength range of l=350–
800 nm with a wavelength step of 1 nm. The reflectance data were trans-
formed using the Kubelka–Munk function.[51]
Conclusion
The development of suitable mass-control systems is a
promising area of research with a wide range of applications
from complex controlled delivery systems to novel sensing
paradigms. A study was carried out on the delivery of the
Reagents and solutions: For the synthesis of the mesoporous material,
tetraethylorthosilicate (TEOS), n-cetyltrimethylammonium bromide
(CTAB) and triethanolamine (TEAH3) were obtained from Aldrich.
Sodium hydroxide was purchased from Scharlab. For the synthesis of the
imidazolium derivative, N-methylimidazole was obtained from Acros Or-
ganics and 3-(chloropropyl)trimethoxysilane from Fluka. For the prepara-
tion of the urea and thiourea derivatives, 3-aminopropyltrimethoxysilane,
phenyl isothiocyanate and phenyl isocyanate were obtained from Sigma–
Aldrich and were used without any further purification. The dye tris(2,2’-
dye [RuACHTUNGTRENNUNG
(bipy)3]2+ from mesoporous materials that contained
anion binding sites, such as imidazolium, thiourea and urea,
on the pore outlets in the presence of certain anions. Large-
scale dye delivery was obtained in the presence of small car-
boxylate ions (from acetate to octanoate) that could not
“close” the gate, whereas complete blockage was observed
for longer carboxylate ions, which act as molecular taps in
the gatelike ensemble. For longer carboxylate ions, the de-
livery inhibition effect is attributed to the formation, on co-
ordination with the binding groups, of a highly hydrophobic
layer around the entrance to the pores that inhibits the de-
livery of the otherwise rather hydrophilic ruthenium(II) dye
from inside the pores to the water phase. Small differences
in the behaviour of S1–S3 in the presence of the carboxylate
ions are modulated, probably due to the differences in the
nature of the binding sites. The partial delivery of the dye
from the functionalised solids was also observed in the pres-
ence of the bile anions cholate, deoxycholate, glycodeoxy-
cholate and taurocholate. A possible use of the gating
system for the chromo-fluorogenic detection of anionic sur-
factants was also explored. A selective dye delivery inhibi-
tion from S1 in water was observed in the presence of the
anionic surfactants lauryl sulphate and dodecylphosphate,
whereas no response was found for the cationic CTA+ and
zwitterionic Triton X-100 surfactants. Finally, the experimen-
tal carboxylate control of mass transport has also been stud-
ied by means of a theoretical model based on the imidazoli-
um-functionalised solid S1. This study was carried out by
using molecular dynamics calculations with force-field meth-
ods and clearly shows that the interaction of the imidazoli-
um-functionalised surface with long-chain carboxylate ions
significantly decreases the pore size in agreement with the
dye delivery inhibition observed experimentally.
bipyridyl)ruthenium(II) chloride hexahydrate [RuACTHUNTRGNE(UNG bipy)3]Cl2·6H2O was
purchased from Sigma–Aldrich. All the used sodium carboxylate salts
were purchased from Sigma Aldrich.
Synthesis of N-methyl-N’-propyltrimethoxysilylimidazolium chloride (1):
This compound was prepared following a slightly different procedure to
that recently reported by Valkenberg et al.[52] A mixture of N-methylimi-
dazole (6.57 g, 80 mmol) and 3-(chloropropyl)trimethoxysilane (15.89 g,
80 mmol) were stirred in a dry 100 mL flask under nitrogen flow at 958C
for 24 h. After cooling at room temperature, the resulting liquid product
was extracted with diethyl ether. The final compound was obtained as a
yellow liquid.
Synthesis of N-phenyl-N’-[3-(trimethoxysilyl)propyl]thiourea (2): This
product was prepared following a reported procedure[53] by the reaction
between 3-(aminopropyl)trimethoxysilane and phenyl isothiocyanate at
508C in CH3Cl (50 mL) in the presence of a few drops of triethylamine.
Synthesis of N-phenyl-N’-[3-(trimethoxysilyl)propyl]urea (3): This com-
pound was synthesised by the reaction between 3-(aminopropyl)trime-
thoxysilane (3.49 mL, 20 mmol) and phenyl isocyanate (2.17 mL,
20 mmol) in anhydrous acetonitrile (15 mL) with a few drops of triethyla-
mine as a catalyst. The mixture was stirred in a dry 50 mL flask under
argon flow at 808C for 15 h. After the mixture had been at room temper-
ature, the solvent was removed by evaporation. The final compound was
obtained as a yellow liquid. 1H NMR (300 MHz, CDCl3, 258C): d=0.58
(t, 2H; CH2Si), 1.56 (t, 2H; CH2CH2Si), 3.16 (t, 2H; NHCH2CH2), 3.49
(s, 9H; CH3O), 5.19 (s, 1H; CONHCH2), 6.79 (,1H; C6H5NHCS), 6.99 (s,
1H; CSNHC6H5), 7.22 ppm (m, 4H; C6H5NHCS); 13C NMR (75 MHz,
CDCl3, 258C): d=7.6 (CH2Si), 23.7 (CH2CH2Si), 42.1 (NHCH2CH2), 50.5
(CH3O), 119.8 (C6H5NH), 122.7 (C6H5NH), 128.9 (C6H5NH), 139.3
(C6H5NH), 157.1 ppm (NHCONH); HRMS: m/z: calcd for
C13H22N2O4Si: 298.1348; found: 298.1357.
MCM-41: The mesoporous MCM-41 support was synthesised by follow-
ing the so-called “atrane route”,[54] a simple preparative technique based
on the use of complexes that include TEAH3-related ligands (i.e., in gen-
eral “atranes” and silatranes for the silicon-containing complexes), TEOS
as hydrolytic inorganic precursors and a surfactant as a porogen species.
The molar ratio of the reagents in the mother liquor was fixed at
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Controlled Dye Delivery as a Molecular Gating System
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TEAH3/TEOS/CTAB/NaOH/H2O=7:2:0.52:0.5:180. In a typical synthe-
sis, TEAH3 (25.79 g, 0.173 mol) and NaOH (0.49 g, 0.012 mol) dissolved
in water (2 mL) were stirred, heated to 1208C and cooled to 708C.
TEOS (11 mL, 0.049 mol) was added to the reaction mixture and heated
to 1208C to remove the ethanol released during formation of the atrane
complexes. CTAB (4.68 g, 0.013 mol) was gradually added. The liquid
was cooled to 708C and water (80 mL, 4.4 mol) was added with vigorous
stirring. After a few minutes, a white suspension was formed. The mix-
ture was subsequently aged at room temperature overnight. The resulting
powder was collected by filtration, washed with water and dried at 708C,
thus producing the as-synthesised MCM-41. The as-synthesised solid was
calcined at 5508C in an oxidant atmosphere for 5 h to remove the tem-
plate phase, thus yielding the final MCM-41 porous material.
Computational details
Three requirements must be accomplished by the silica crystal structure
chosen to build a suitable two-dimensional model of a mesoporous silica
material of the MCM-41 family: 1) one of the surfaces must only present
terminal oxygen atoms; 2) the opposite surface should only show termi-
nal silicon atoms that will act as anchoring points for the imidazolium;
3) the morphology of the mesopore must be close to a cylindrical ideal
geometry. By attending these conditions, we selected the crystal structure
of b-cristobalite. A cleavage of the crystal structure parallel to the (1 À1
1) plane allowed us to obtain a mesoporous model with large quasi-cylin-
drical nanopores. This model can be described as a hexagonal supercell
with the following parameters, a=b=40.503 ꢈ, a=b=90.08 and g=
120.08. The size of this “supercell” was chosen to generate pores and
walls with similar dimensions (diameter (d): 22.9 ꢈ, thickness (t): 15.5 ꢈ,
respectively) to those experimentally found in MCM-41 family of solids
(d=23.3, t=13.1 ꢈ). The deepness of the pores in this two-dimensional
model is 28.7 ꢈ. The terminal oxygen atoms in the surfaces and inside
the nanopore were protonated. In our system, imidazolium-substituted
alkyl ions are anchored in the surface and its chemical function must be
to interact with carboxylate ions to limit the free motion of the last mole-
cules that will be in charge of the closing of the pore. Therefore, we la-
belled the imidazolium substituents as receiver groups. These receiver
groups were anchored on the mesoporous surface modelled with the
Cerius2 package.[55] All the possible anchoring positions were not used to
avoid problems caused by steric effects between neighbouring receivers.
On this model, which was considered by us to be a multi-receiver cell
(MRC), different carboxylate ions were added to study the mechanism
that tunes the closing of the nanopore.
S1: In a typical synthesis, template-free MCM-41 (1.0 g) and the dye
tris(2,2’-bipyridyl)ruthenium(II) chloride (0.6 g, 0.8 mmol) were suspend-
ed in anhydrous acetonitrile (40 mL) inside a round-bottomed flask con-
nected to a Dean–Stark trap apparatus in an inert atmosphere. The sus-
pension was heated to reflux (1208C) in an azeotropic distillation and
10 mL of liquid was collected in the trap to remove the adsorbed water.
The suspension was stirred for 24 h at room temperature to load the
pores of the MCM-41 scaffolding. An excess of N-methyl-N’-propyltrime-
thoxysilylimidazolium chloride (2.805 g, 10 mmol) was added to the reac-
tion mixture and the suspension was stirred for 5.5 h. The final orange
solid S1 was removed by filtration, washed with acetonitrile and dried at
708C for 12 h.
A similar material was prepared for comparison as a control solid with a
fumed-silica support, which lacks the mesoporous structure of the MCM-
41-based solids. In a typical synthesis, fumed silica (1.0 g) was suspended
in anhydrous acetonitrile (40 mL) inside a round-bottomed flask connect-
ed to a Dean–Stark trap apparatus in an inert atmosphere. The suspen-
sion was heated to reflux at 1208C in an azeotropic distillation to remove
the adsorbed water. An excess of N-methyl-N’-propyltrimethoxysilylimi-
dazolium chloride (2.805 g, 10 mmol) was added to the reaction mixture
and the suspension was stirred for 5.5 h. The final white solid FS-Im was
removed by filtration, washed with acetonitrile and dried at 708C for
12 h. FS-Im consists of a “flat” surface (i.e., without the presence of
nanoscopic pores) with anchored imidazolium ions.
Due to the huge size of the models needed for this kind of study, the cal-
culations were carried out by using force-field methods. For this purpose,
the universal force field (UFF) suggested by Rappe et al. was em-
ployed.[56] To find the global energy minimum and because of the pres-
ence of many local minima, molecular dynamics simulations were carried
out to cover the more important parts of the potential-energy surface by
using thermal energy to escape from the local minima. These molecular
dynamics simulations were carried out within the canonical ensemble
(number of particles, volume and temperature were maintained at a con-
stant) for 10 ps with a time step of 1 fs. Among the conformations ob-
served during the molecular dynamics simulation, the most stable geome-
try was taken as starting point for a later geometry optimisation. The ge-
ometry optimisations and molecular dynamics simulations were per-
formed with the Cerius2 package.[55] In the evaluation of the nanopore di-
ameter, the Van der Waals radii provided by the Cerius2 package for
each element were used. In fact, this parameter represents the maximum
diameter of a sphere that moves freely into and out of the nanopore.
S2: In a typical synthesis, template-free MCM-41 (1.00 g) and tris(2,2’-bi-
pyridyl)ruthenium(II) chloride (0.6 g, 0.8 mmol) were suspended in anhy-
drous acetonitrile (40 mL) inside a round-bottomed flask connected to a
Dean–Stark trap apparatus in an inert atmosphere. The suspension was
heated to reflux at 1208C in an azeotropic distillation and 10 mL of
liquid was collected in the trap to remove the adsorbed water. The sus-
pension was stirred for 24 h at room temperature to load the pores of the
MCM-41 scaffolding. An excess of N-phenyl-N’-[3-(trimethoxysilyl)pro-
pyl]thiourea (3.14 g, 10 mmol) was added to the reaction mixture and the
suspension was stirred for 5.5 h. The final orange solid was removed by
filtration, washed with acetonitrile and dried at 708C for 12 h.
DFT calculations were carried out on the singlet spin states of the imida-
zolium-substituted alkyl molecule with the hybrid B3LYP method,[57] as
implemented in the Gaussian 03 program,[58] with the triple-z quality
basis sets proposed by Ahlrichs and co-workers.[59] The electronic-density
data were obtained from NBO analysis.[60]
S3: In a typical synthesis, template-free MCM-41 (1.00 g) and tris(2,2’-bi-
pyridyl)ruthenium(II) chloride (0.6 g, 0.8 mmol) were suspended in anhy-
drous acetonitrile (40 mL) inside a round-bottomed flask connected to a
Dean–Stark trap apparatus in an inert atmosphere. The suspension was
heated to reflux at 1208C in an azeotropic distillation and 10 mL of
liquid was collected in the trap to remove the adsorbed water. The sus-
pension was stirred for 24 h at room temperature to load the pores of the
MCM-41 scaffolding. An excess of N-phenyl-N’-[3-(trimethoxysilyl)pro-
pylurea (2.66 g, 10 mmol) was added to the reaction mixture and the sus-
pension was stirred for 5.5 h. The final orange solid S3 was removed by
filtration, washed with acetonitrile and dried at 708C for 12 h.
Acknowledgements
We thank the Spanish Ministerio de Educaciꢂn
y Ciencia (Project
CTQ2006-15456-C04-01/BQU) and the Generalitat Valenciana (Project
GV06/101) for their support. C.C. is grateful to the Universidad Politꢃcn-
ica de Valencia for the award of a doctoral fellowship. E.A. thanks the
Ministerio de Ciencia e Inovaciꢂn for an FPU fellowship. We are obliged
to the Servicio de Microscopꢀa de la UPV for the use of their TEM facili-
ties.
Dye release studies: Carboxylate recognition studies were carried out
using the solid (S1–S3 and FS-Im; 10 mg) suspended in aqueous solution
(25 mL; buffered at pH 7.0 with HEPES, 10À3 moldmÀ3) that contained
the corresponding carboxylate ion (ccarboxylate =10À3 moldmÀ3). The sus-
pension was stirred for some minutes to allow maximum dye delivery
and removed by filtration with a teflon filter. The release of the [Ru-
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Chem. Eur. J. 2010, 16, 10048 – 10061

Controlled Dye Delivery as a Molecular Gating System
FULL PAPER
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