Instructions for Preparing and Transferring Final Short

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

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 toachieve 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 focusonfunctional biomimeticmembranes in which the recognition-driven transport properties could be ensured by a well-defined incorporation of receptors of specific self-organization functions, incorporated in a hybrid solid dense or a mesopourous siloxane inorganic matrix.

Of particular interest arethe 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-assemblingof monomers followed by a second sol-gel transcription (polymerization) step into tubular or continual solid hybriddevices at the nanoscopic level (Figure 1).

Figure 1. Hierarchical generation and sol-gel transcription of tubular self-assembled hybrid nanomembranes

Based on this strategy three heteroditopic classes of receptors have been recentlyreported by our group: crown ether, amino acids conjugates and nucleobase ureido-silsesquioxane derivatives1-3. (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-bonddirecting interactions and (3) the covalently bonded triethoxysilyl groups. The inorganic precursor moiety allows us, by sol-gel processes, to transcribe the solution self-organized 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

Theurea-based head-to-tail motifs were used for 1 and 2 asassembling 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-channel-forming 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 self-assembly of the hydrogen-bonded urea-crown ethers or organic functional molecules. Our publisehd results3,4showed 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.

 Sol-gel transcription

Figure3. Schematic representation of the hierarchical organized system 1: (top) self-organization 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.5The 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 structuresemphasizing particular K+ -  contacts with phenyl, phenol and indole rings were recently reported by our group.6These 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 type2.7Successive H-bond urea self-assemblingand sol-gel transcription steps yieldto preferential conduction pathways within the hybrid membranematerials. Crystallographic, microscopic and transport data concludes in formation of theself-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 .

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 derivative3 was used as suitable molecular pre-informed precursor for the molecular recognition of alkali metal ions as well as for itsH-bond self-assembly in tubular stacked ion-channel superstructures.Among the natural nucleobases, homooligomers of guanine derivativesself-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 G4but 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.

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 recognition-driven 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 intosilica 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 =  200nm, thickness =  60m, diameter of membrane = 47mm), 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 40Å (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 18-crown-6 self-organized receptors.

Figure 5: Schematic representation of the hierarchical organized system 1: (left) self-organization 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. 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

Figure 4. Concentration vstime profile and diffusion regimes

The Fick law diffusion model1 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, M2and 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.

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.

AAM / Membrane preparation / Permeability
(cm2/s 108) / Diffusion coefficient
(cm2/s 107)
A / B / C / D / Na+ / K+ / 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 self-assembled superstructures. The basic and specific molecularinformation 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 self-reorganization (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, was supported by funds from the Participating Organizations of EURYI and the EC Sixth Framework Program. See

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