DYNAMIC ADAPTATIVE AND SUPRAMOLECULAR
HYBRID MEMBRANES
Adinela CAZACU, Mathieu MICHAU, Carole ARNAL-HERAULT, Jihane NASR,
Andreea PASC, Anca MEFFRE, Yves-Marie LEGRAND, and Mihail BARBOIU*
Adaptative Supramolecular Nanosystems Group,
Institut Européen des Membranes
Place Eugène Bataillon CC047, 34095 Montpellier cedex 5, France
mihai.barboiu@iemm.univ-montp2.fr
Molecular self-organization and self-assembly to supramolecular structures is the basis
for the construction of new functional nanomaterials. Hybrid organic-inorganic materials produced
by sol-gel process are subject of various investigations especially to achieve nanostructured
materials from molecular and recently from self-organized supramolecular silsesquioxane hybrids.
The structure-directed function of hybrid materials and control of their build-up from
suitable units by self-organisation is of special interest. Toward this objective, our aim is to
develop functional hybrid membrane materials that form selective patterns so as to enable efficient
translocation events. Artificial systems, functioning as carriers or as channel-forming
superstructures in liquid and bilayer membranes, have been extensively developed during last three
decades. Our interest focus on functional biomimetic membranes in which the recognition-driven
transport properties could be ensured by a well-defined incorporation of receptors of specific selforganization functions, incorporated in a hybrid solid dense or a mesopourous siloxane inorganic
matrix.
Of particular interest are the potential ability of such thin-layer membrane films to
present polyfunctional properties such as solute molecular recognition and the generation of the
directional diffusion pathways by self-assembling at the supramolecular level.
The hierarchical generation of such functional hybrid materials has been achieved in two
steps. First, the supramolecular oligomers were generated by dynamic self-assembling of
monomers followed by a second sol-gel transcription (polymerization) step into tubular or
continual solid hybrid devices at the nanoscopic level (Figure 1).
Figure 1. Hierarchical
generation and sol-gel
transcription of tubular
self-assembled
hybrid
nanomembranes
58
Based on this strategy three heteroditopic classes of receptors have been recently
reported by our group: crown ether, amino acids conjugates and nucleobase ureido-silsesquioxane
derivatives 1-3. (Figure 2) They generate self-organized continual superstructures in solution and
in the solid state based on three encoded features: (1) the molecular recognition, (2) the
supramolecular H-bond directing interactions and (3) the covalently bonded triethoxysilyl groups.
The inorganic precursor moiety allows us, by sol-gel processes, to transcribe the solution selforganized dynamic superstructures in solid heteropolysiloxane material. This represents an
intermediate approach between the previously reported methods to form self-organized hybrid
materials using TEOS organogel template and appropriate silylated organic molecules.
Figure 2. Molecular structures of molecular receptors 1-3
The urea-based head-to-tail motifs were used for 1 and 2 as assembling H-bond
supramolecular interactions, assisted by - stacking. From the mechanistic point of view, we use
carriers which self-assemble in functional aggregates which would present combined (hybrid)
intermediate features between the former carrier-monomers and the resulted pseudo-channelforming structures. Thus, we therefore studied the membrane transport properties in solid materials
a of such supra-molecular and organic-inorganic hybrid polymers resulted by the dynamic selfassembly of the hydrogen-bonded urea-crown ethers or organic functional molecules. Our
publisehd results3,4 showed that the self-organization properties in the membrane phase may
provide the first evidence for the possible hybrid transport carrier vs. channel mechanisms in
correlation with self-assembly properties of the heteroditopic receptors.
These dynamic self-organized systems can be “frozen” in a polymeric hybrid matrix by
sol-gel process, allowing us to design a novel class of hybrid nanomembranes.4 The hybrid
membranes successfully formed transport patterns so as to enable efficient translocation events.
Moreover, this system has been the first example of a hybrid nanomaterial where the concept of
self-organization and a specific function (generation of specific translocation ionic pathways in a
hybrid solid) might in principle be associated. As an example, a schematic representation of the
dynamic self-assembling of crown ethers in solution and the sol-gel transcription in the hybrid
membrane is illustrated in Figure 3 below.
59
 Sol-gel transcription
Figure 3. Schematic representation of the hierarchical organized system 1: (top) selforganization in solution and (bottom) sol-gel transcription of encoded molecular features
into a hybrid heteropolysiloxane matrix
The second example concerns the ureido-silsesquioxane compounds bearing aromatic
moieties of natural aromatic amino acids. Intermolecular interactions involving aromatic rings are
key processes in both chemical and biological recognition. Among these interactions, cation-
interactions between positively charged species (alkali, ammonium and metal ions) and aromatic
systems with delocalized -electrons are now recognized as important noncovalent binding forces
of increasing relevance.5 The importance of interactions between alkali cations and aromatic side
chains of aromatic aminoacids has been known for many years and they are of particular biological
significance. New heterocomplex structures emphasizing particular K+ -  contacts with phenyl,
phenol and indole rings were recently reported by our group.6 These results are now focusing us in
designing functional amino acids derivatives as suitable molecular channels in hybrid membranes.
Using the same strategy presented before we have prepared hybrid membrane materials based on
the compounds of type 2.7 Successive H-bond urea self-assembling and sol-gel transcription steps
yield to preferential conduction pathways within the hybrid membrane materials. Crystallographic,
microscopic and transport data concludes in formation of the self-organized molecular channels
transcribed in solid dense thin-layer membranes The ionic transport across the organized domains
illustrates the power of the supramolecular approach for the design of continual hydrophilic
transport devices in hybrid materials by self-organization .
60
Figure 4. a) Dynamic self-assembly of molecular precursors; b) SEM cross section of the
hybrid membrane; c) Single crystal structure of the supramolmolecular channel; d)
Crystalline fields at the surface of the hybrid membrane (TEM image).
In the last part of our studies, a guanine derivative 3 was used as suitable molecular preinformed precursor for the molecular recognition of alkali metal ions as well as for its H-bond selfassembly in tubular stacked ion-channel superstructures. Among the natural nucleobases,
homooligomers of guanine derivatives self-assemble through Hoogsteen interactions into either
linear tapes or G-quartet macrocycles. Metal ions i.e. K+ template the formation of not only the
cyclic tetramers G4 but also the G-quadruplex, four-stranded column-like superstructures. In this
context in the last two decades, the G-quartet has been proposed as scaffold for building synthetic
ion channels. Only very recently, a new supramolecular dynamic strategy was successfully used to
generate a rich array of interconnverting ion-channel conductance states of G-quadruplex in lipid
bilayers.
Our efforts involve self-assembly of silylated monomers in a G4 configuration followed
by their fixation in an inorganic polysiloxane matrix.8 For example, the inorganic transcription of 3
gives rise to hexagonal helical-like architectures (Figure 3). X ray powder diffraction, microscopic
methods concludes in the formation of tubular helical superstructures which are successfully
transcribed in a hybrid material via sol-gel method. Works are in progress to prepare membranes
based on this very promising ion-channelling scaffold.
61
Figure 3. The guanine-silylated conjugate 3 self-associates in the presence of alkali-metal
cations to forme hexagonal nanotubes, presumably formed by G-quartet-based interactions.
Our interest focus also on hybrid solid
membranes in which the molecular recognitiondriven transport function could be ensured by a
dynamic incorporation of specific organic
receptors, non-covalently linked in a hydrophobic
dense siloxane inorganic matrix. Of particular
interest is the potential ability of such solid
membranes to combine functional properties such
as solute molecular recognition and generation by self-assembling of the directional conduction
pathways at the supramolecular level. New self-organized hybrid membranes have been prepared
(Figure 4) by embedding self-organized ureidocrown-ethers 15-crown-5, 1 or 18-crown-6, 2 into
silica mesoporous hybrid materials, regularly oriented along the pores of the Anodisc 47 (0.02m)
alumina membranes as support.
Figure 4: Schematic representation of the synthetic route to obtain functionnalised
mesostructured silica-receptor nanocomposite in the AAMs: (a) anodic alumina membrane
(pore diameter =  200 nm, thickness =  60 m, diameter of membrane = 47 mm),
mesostructured silica-surfactant before (b) and after (c) calcination, ODS-hydrophobized
silica before (d) and after (e) inclusion of the hydrophobic carriers 1 or 2.
In a first step, the selective recognition functions of alkali metal ions (Figure 5a) and self
organization inside regular nanochannels of about 40 Å (Figure5b) have been emphasized by using
NMR, FTIR and X-ray diffraction techniques. The MCM41-type mesostructured powders were
used as hydrophobic host matrix for physically or chemically entrapped 15-crown-5 and 18crown-6 self-organized receptors.
62
Figure 5: Schematic representation of the
hierarchical organized system 1: (left) selforganization in solution and (right) dynamic
transcription of encoded molecular features into
a hydrophobic heteropolysiloxane matrix
By this way, based on hydrophobic and specific hydrogen bonds such as urea-urea or
urea-anion interactions, molecular carriers can be non-covalently trapped in an inorganic matrix,
which allow us to prepare very promising dynamic molecular channels.
Subsequently, hybrid organic-inorganic membranes have been prepared by filling a
porous alumina membrane coupled with the sol-gel process. The MCM41-type functionalized
materials were successfully oriented along the alumina membrane pores and characterized by SEM
microscopy. These membranes have been tested in selective Na+/K+ transport.
Periodic mesoporous materials have attracted considerable attention during the last
decade because of their promising applications as catalyst support, or as hosts for nanostructured
materials. Many of these applications benefit from arrangements of preferentially aligned, ordered
arrays of certain mesostructures. The evaporation-induced self-assembly method has been
established as an efficient process for the preparation of thin films with mono-oriented materials.
However, the most frequently obtained films display hexagonally ordered channels that are aligned
in a nonfavorable parallel orientation to the surface of the substrate.
Recently, the synthesis of mesoporous materials within the regular 200 nm channels of
Anodisc alumina membranes (AAMs) has been explored, with the aim of attaining greater control
over the morphology (orientation) of the mesoporous system. It was then demonstrated that porous
anodic alumina can serve as support material to form silica-surfactant nano-composite with a
desirable orientation of nanochannels, perpendicular to the surface of the support and,
consequently parallel to (along) the alumina pores [5].
Therefore, this method was also applied by us for the preparation of our membranes, in
order to allow preferentially transport nano-paths for molecules. In the first step, the AAMs were
filled in with surfactant (CTAB)-template silica and then calcinated to remove of CTAB.
63
Afterwards silica was react with ODS followed by the incorporation of long chain hydrophobic
carriers.
In the absence of the silica-surfactant-receptor nanocomposite in the alumina membrane,
Na+ and K+ cations are transported through the membrane in a similar proportion. In contrast, the
hybrid crown-alumina membranes, including the silica-surfactant composite, shows a selective
transport of salts depending on the receptor selectivity 1 or 2, respectively.
Figure 6. Side-view SEM micrograph
of alumina membrane sample M4
D1 D2
D = diffusion coefficient
P = permeability
1 simple diffusion domain
2 transition diffusion domain
3 facilitate diffusion domain
concentration
Figure 4. Concentration vs
time profile and diffusion
regimes
D3
P1
P2
time
The Fick law diffusion model 1 allows us to determine the transport parameters such as
diffusion coefficients and permeability across the membrane (Table 1). Moreover, in every case,
we can distinguish two stages for the transport mechanism: 1) a simple and 2) a facilitate diffusion.
In the first one, the membrane are functioning like a “sponge”, and the simple rapid diffusion
through the membrane is accompanied with the selective complexation of the fittest cation (Na +
for 1 and K+ for 2, respectively); it is the so-called “membrane self-preparing step”. The selective
transport of the specific cation (Na+ for 1, M2 and K+ for 2, M3, respectively) occurs in the second
stage, much faster. Thus, one can conclude that the membrane with 15C5 receptor facilitate in the
second step the transport of Na+, whereas 18C6 receptor facilitate the transport of K +. These
experimental results suggest that the self-assembly of receptors inside the surfactant –templated
silica nanochannels of the columnar alumina pores can reorganise during the molecular transport,
the mechanism being characterized by an initial self-preparing step.
64
Table 1: Characteristics of Na+/K+ transport across mesoporous functionalized membranes.
A: CTAB/TEOS sol-gel filling of AAM (except of M4 where 1 was introduced in the sol-gel
solution precursor), B: thermal removal of the surfactant, C: octadecyltrichlorosilane (ODS)
fonctionalization of silica nanotubes, D: functionalize-tion with ureidocrown-ether derivatives 1
or 2, respectively.
Membrane
preparation
Permeability
(cm2/s 108)
Na+
K+
AAM
A
B
C
D
Diffusion coefficient
(cm2/s 107)
Na+
K+
P1
P2
P1
P2
D1
D2
D3
D1
D2
D3
S
-
-
-
-
8.1
0.7
25
0.8
300
20
46
552
24
48
M1
x
x
x
-
34
0.5
29
0.4
340
6
26
174
8
25
M2
x
x
x
1
40
1.5
45
1.1
3
0.003
0.09
383
25
22
M3
x
x
x
2
45
1.3
15
1.7
350
20
35
101
5
3
M4
x
-
-
-
8.8
0.2
26
0.3
310
11
18
367
16
21
We reported in this short review one rational approach for building molecular-channels
in hybrid organic-inorganic materials via the inorganic (sol-gel) transcription of dynamic selfassembled superstructures. The basic and specific molecular information encoded in the molecular
precursors (crown ether, amino acid and guanine ureido-silsesquioxanes) results in the generation
of tubular and continual superstructures in solution and in the solid state which can be “frozen” in
a polymeric hybrid matrix by sol-gel process. These systems have been successfully employed to
design solid dense membranes, functioning as ion-channels and illustrate how a self-organized
hybrid material performs interesting and potentially useful transporting functions.
The combined features of structural adaptation in a specific hybrid nanospace and of
dynamic supramolecular selection process make the membranes presented here of general interest
for the development of a specific approach toward nanomembranes of increasing structural
selectivity.
From the conceptual point of view these membranes express a synergistic adaptative
behavior: the addition of the fittest alkali ion drives a constitutional evolution of the membrane
toward the selection and amplification of a specific transport crown-ether superstructure in the
presence of the solute that promoted its generation in a first time. It embodies a constitutional selfreorganization (self-adaptation) of the membrane configure-tion producing an adaptative response
in the presence of its solute. This is the first example of dynamic “smart” membranes where a
solute induces the upregulation of (prepare itself) its own selective membrane.
Acknowledgments
This work, conducted as part of the award “Dynamic adaptive materials for separation
and sensing Microsystems” (M.B.) made under the European Heads of Research Councils and
European Science Foundation EURYI (European Young Investigator) Awards scheme in 2004,
65
was supported by funds from the Participating Organizations of EURYI and the EC Sixth
Framework Program. See www.esf.org/euryi.
References
[1]
M. Barboiu, C. Guizard, N. Hovnanian, J. Palmeri, C. Reibel, C. Luca, L. Cot. J. Membrane
Sci., 172, 91 (2000)
[2] M. Barboiu, G. Vaughan, A. van der Lee Org. Lett., 5, 3073 (2003)
[3] M.Barboiu. J. Incl. Phenom. Mol Rec, 49, 133 ( 2004)
[4] M. Barboiu, S. Cerneaux, G. Vaughan, A. van der Lee, J. Am. Chem. Soc. 126, 3545 (2004)
[5] a) D.A. Dougerthy Science 271, 163 (1996), b) R. MacKinnon, Angew. Chem. Int. Ed. 43, 4265
(2004); c) A. M. Meyer, R. K. Castellano, F. Diederich, Angew. Chem. Int. Ed. 42, 1210
(2003); d) J. C. Ma, D.A. Dougherty, Chem. Rev. 97, 1303 (1997).
[6] C. Arnal-Herault, M. Barboiu, E. Petit, M. Michau, A. van der Lee, New J. Chem., , 29,
1535-1539 (2005).
[7] M. Michau, C. Arnal-Herault, A. Pasc-Banu, M. Barboiu, Chem.Eur.J (2007).
[8] A. Cazacu, M. Michau, C. Arnal-Herault, A.Pasc-Banu, M. Barboiu, manuscript in
preparation (2006)
Further reading
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
66
C. Arnal-Herault, A. Pasc-Banu, M. Michau, D.Cot, E. Petit, M. Barboiu Functional GQuartet Macroscopic Membrane Films, Angew. Chem. 2007, 119, 8561-8565 ; Angew.
Chem. Int. Ed. 2007, 46, 8409-8413.
C. Arnal-Hérault, M. Barboiu, A. Pasc, M. Michau, P. Perriat, A. van der Lee, Constitutional
Self-Organization of Adenine-Uracil-derived Hybrid Materials, Chem. Eur.J. 2007, 13,
6792-6800.
C. Arnal-Herault, A. Pasc-Banu, M. Barboiu M. Michau, A. van der Lee, Amplification and
transcription of the dynamic supramolecular chirality of the G-quadruplex, Angew. Chem.
2007, 119, 4346-4350; Angew. Chem. Int. Ed. 2007, 46, 4268-4272. in press. (Cover
Communications Picture, June 2007)
M. Barboiu, A. Cazacu, M. Michau, R. Caraballo, C. Arnal-Herault, A. Pasc-Banu,
Functional Organic- inorganic
hybrid membranes, Chem. Eng. Proc. 2007, doi
10.1016/j.cep.2007.07.018.
A. Cazacu, M. Michau, R. Caraballo, C. Arnal-Herault, A. Pasc-Banu, A Ayral, M. Barboiu,
Ann. Chim. Sci. des Mat., 2007, 32(2) 127-139.
A. Cazacu, A. Pasc-Banu, M. Barboiu, Molecular and supramolecular dynamics- a versatile
tool for self-organization of polymeric membranes systems, Macromol. Symposia, MACRO
2006, 2006, 245-246, 435-438.
A. Cazacu, C. Tong, A. van der Lee, T.M. Fyles, M. Barboiu, Columnar Self-Assembled
Ureidocrown-ethers – an Example of Ion-channel Organization in Lipid Bilayers, J. Am.
Chem. Soc., 2006, 128(29), 9541-9548.
A. Cazacu, M. Michau, C. Arnal-Herault, A. Pasc-Banu, A. Meffre R. Caraballo, A. Pasc,
and M.Barboiu, Hybrid supramolecular dynamic membranes as selective information
transfer devices, Desalination, 2006, 199, 521-522.