Philippe J. Mésini

Polymer Multilayer Films Obtained by Electrochemically Catalyzed Click Chemistry

We report the covalent layer-by-layer construction of polyelectrolyte multilayer (PEM) films by using an efficient electrochemically triggered Sharpless click reaction. The click reaction is catalyzed by Cu(I) which is generated in situ from Cu(II) (originating from the dissolution of CuSO(4)) at the electrode constituting the substrate of the film. The film buildup can be controlled by the application of a mild potential inducing the reduction of Cu(II) to Cu(I) in the absence of any reducing agent or any ligand. The experiments were carried out in an electrochemical quartz crystal microbalance cell which allows both to apply a controlled potential on a gold electrode and to follow the mass deposited on the electrode through the quartz crystal microbalance. Poly(acrylic acid) (PAA) modified with either alkyne (PAA(Alk)) or azide (PAA(Az)) functions grafted onto the PAA backbone through ethylene glycol arms were used to build the PEM films. Construction takes place on gold electrodes whose potentials are more negative than a critical value, which lies between -70 and -150 mV vs Ag/AgCl (KCl sat.) reference electrode. The film thickness increment per bilayer appears independent of the applied voltage as long as it is more negative than the critical potential, but it depends upon Cu(II) and polyelectrolyte concentrations in solution and upon the reduction time of Cu(II) during each deposition step. An increase of any of these latter parameters leads to an increase of the mass deposited per layer. For given buildup conditions, the construction levels off after a given number of deposition steps which increases with the Cu(II) concentration and/or the Cu(II) reduction time. A model based on the diffusion of Cu(II) and Cu(I) ions through the film and the dynamics of the polyelectrolyte anchoring on the film, during the reduction period of Cu(II), is proposed to explain the major buildup features.

Polyelectrolyte multilayers capped with polyelectrolytes bearing phosphorylcholine and triethylene glycol groups: parameters influencing antifouling properties.

In this paper, we investigate the design of antifouling surfaces by the deposition of polyelectrolytes modified by grafting of antifouling groups onto a (PSS/PAH)n precursor multilayer film [PSS, poly(styrenesulfonate); PAH, poly(allylamine)]. Different polyelectrolytes and different antifouling moieties are investigated, in particular, (EO)3 and (EO)3PC moieties (EO, ethylene oxide; PC, phosphorylcholine group). We find that protein adsorption can strongly be reduced and even practically suppressed through the deposition of only one layer of polyelectrolyte modified with PC and/or (EO)3 groups. We discuss the influence of various parameters such as the nature of the polyelectrolyte backbone, the nature of the antifouling moiety, and the grafting ratio on the reduction of protein adsorption. We find in particular that (EO)3 and (EO)3PC moieties grafted on poly(acrylic acid) (PAA) totally prevent protein adsorption for grafting ratios of 25% or more, at least within the detection limits of the used quartz crystal microbalance and optical waveguide light mode spectroscopy devices. The mechanism that leads to the antifouling property is discussed and compared to that leading to the antifouling properties of ethylene oxide self-assembled monolayers. Finally, by incorporating biotin on top of the precursor film, we show that one layer of PAA-(EO)3PC is not sufficient to prevent interaction with streptavidin but a PAA-(EO)3PC/PAH/PAA-(EO)3PC multilayer largely protects the biotin from interacting with streptavidin.

Cyto-mechanoresponsive Polyelectrolyte Multilayer Films

Cell adhesion processes take place through mechanotransduction mechanisms where stretching of proteins results in biological responses. In this work, we present the first cyto-mechanoresponsive surface that mimics such behavior by becoming cell-adhesive through exhibition of arginine-glycine-aspartic acid (RGD) adhesion peptides under stretching. This mechanoresponsive surface is based on polyelectrolyte multilayer films built on a silicone sheet and where RGD-grafted polyelectrolytes are embedded under antifouling phosphorylcholine-grafted polyelectrolytes. The stretching of this film induces an increase in fibroblast cell viability and adhesion.

Design of Nanohybrid Systems from a Partially Fluorinated Organogelator and Syndiotactic Polystyrene Thermoreversible Gel.

Nanohybrid systems are prepared from organogels of a partially fluorinated molecule and from thermoreversible gels of syndiotactic polystyrene. The thermodynamic behavior, morphology, and structure are investigated by using differential scanning calorimetry, atomic force microscopy, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The outcomes of these investigations suggest that the fibrils of the organogel coil around the sPS fibrils, probably through a heterogeneous nucleation process. These systems therefore differ from previously investigated sPS/OPV systems (oligo vinylene phenylene) where OPV fibrils pervade the sPS network.

Self-assembled nanotubes and helical tapes from diacetylene nonionic amphiphiles. Structural studies before and after polymerization.

We synthesized new amphiphiles comprised of a single diacetylenic chain and an oligoethylenoxide polar chain linked by an amide bond. In aqueous medium, they are not soluble at room temperature but form weak gels. Electron microscopy studies have shown that they self-assemble into helical tapes or nanotubes with lengths of several micrometers, and inner and outer diameters of 50 ± 1 and 59 ± 1 nm, respectively. The wall has a thickness of 10 ± 1 nm for both kinds of objects and has an amphiphile bilayer structure. The hydrophobic chains are ordered, and the amide groups are linked with each other by H-bonds. The dissociation of the tubes is a first-order transition with an enthalpy of ca. 40 kJ mol(-1). The nanotubes were photopolymerized to yield purple solutions consisting of helical tapes and almost flat ribbons. The polymers exhibit irreversible thermochromism upon heating.

Bioaffinity sensor based on nanoarchitectonic films: control of the specific adsorption of proteins through the dual role of an ethylene oxide spacer.

The identification and quantification of biomarkers or proteins is a real challenge in allowing the early detection of diseases. The functionalization of the biosensor surface has to be properly designed to prevent nonspecific interactions and to detect the biomolecule of interest specifically. A multilayered nanoarchitecture, based on polyelectrolyte multilayers (PEM) and the sequential immobilization of streptavidin and a biotinylated antibody, was elaborated as a promising platform for the label-free sensing of targeted proteins. We choose ovalbumin as an example. Thanks to the versatility of PEM films, the platform was built on two types of sensor surface and was evaluated using both optical- and viscoelastic-based techniques, namely, optical waveguide lightmode spectroscopy and the quartz crystal microbalance, respectively. A library of biotinylated poly(acrylic acids) (PAAs) was synthesized by grafting biotin moieties at different grafting ratios (GR). The biotin moieties were linked to the PAA chains through ethylene oxide (EO) spacers of different lengths. The adsorption of the PAA-EOn-biotin (GR) layer on a PEM precursor film allows tuning the surface density in biotin and thus the streptavidin adsorption mainly through the grafting ratio. The nonspecific adsorption of serum was reduced and even suppressed depending on the length of the EO arms. We showed that to obtain an antifouling polyelectrolyte the grafting of EO9 or EO19 chains at 25% in GR is sufficient. Thus, the spacer has a dual role: ensuring the antifouling property and allowing the accessibility of biotin moieties. Finally, an optimized platform based on the PAA-EO9-biotin (25%)/streptavidin/biotinylated-antibody architecture was built and demonstrated promising performance as interface architecture for bioaffinity sensing of a targeted protein, in our case, ovalbumin.

Size-Selective 2D Ordering of Gold Nanoparticles Using Surface Topography of Self-Assembled Diamide Template

Size-selective organization of ~2 nm dodecanethiol stabilized gold nanoparticles (AuNPs) into periodic 1D arrays by using the surface topographical features of a soft template is described. The template consists of micrometer length nanotapes organized into nanosheets with periodic valleys running along their length and is generated by the hierarchical self-assembly of a diamide molecule (BHPB) in cyclohexane. The AuNP ordering achieved simply by mixing the preformed template with the readily available ~2 nm dodecanethiol stabilized AuNPs is comparable to those obtained using programmable DNA and functional block copolymers. The observed periodicity of the AuNP arrays provided valuable structural clues about the organization of nanotapes into nanosheets. Self-assembling BHPB molecules in the presence of AuNPs by heating and cooling the two components led to a comparatively disordered organization because the template structure was changed under these conditions. Moreover, the template could not order larger AuNPs (~5 nm) into a similar 1D array, owing to the steric restriction imposed by the dimension of the valleys on the template. Interestingly, this geometric constraint led to AuNP size sorting when a polydisperse sample (2.5 ± 0.9 nm) was used for organization, with AuNPs attached to the template edges being larger (≥2.2 ± 0.9 nm) than those associated to the inner valleys (1.6 ± 0.8 nm). This is a unique example of size-sorting induced by the surface topographical features of a soft template.

Origin of Invariant Gel Melting Temperatures in the c-T Phase Diagram of an Organogel.

Binary c-T phase diagrams of organogelators in solvent are frequently simplified to two domains, gel and sol, even when the melting temperatures display two distinct regimes, an increase with T and a plateau. Herein, the c-T phase diagram of an organogelator in solvent is elucidated by rheology, DSC, optical microscopy, and transmitted light intensity measurements. We evidence a miscibility gap between the organogelator and the solvent above a threshold concentration, cL. In this domain the melting or the formation of the gel becomes a monotectic transformation, which explains why the corresponding temperatures are nonvariant above cL. As shown by further studies by variable temperature FTIR and NMR, different types of H-bonds drive both the liquid-liquid phase separation and the gelation.

Investigation of the interactions involved in the formation of nanotubes from organogelators.

Investigations into the formation of nanosized structures, particularly nanotubes, by a diamide ester compound are reported. Two aspects are concurrently examined: the role of the solvent and the role of the alkyl chain. The former is addressed by using a benzene derivative (o-xylene) and a totally saturated double ring (trans-decahydronaphthalene) whereas the latter is achieved by replacing the hydrogenous alkyl chain with its fluorinated counterpart while keeping the overall architecture the same. The thermodynamic behavior by differential scanning calorimetry, the morphology by transmission electron microscopy, and the structure by X-ray scattering and small-angle neutron scattering are studied. Despite the identical architecture, the fluorinated molecule does not produce any nanotubes, unlike its totally hydrogenous counterpart. Also, o-xylene prevents the hydrogenous molecule from forming nanotubes, while nanotapes are produced instead. Conversely, the fluorinated molecule produces regularly twisted protostructures in either solvent. Neutron scattering experiments show that the fluorinated alky chain is located within the core of this structure. This suggests that the prerequisite for forming nanotubes relies on the necessity of the alkyl group to point outward.

Supramolecularly Engineered π-Amphiphile

This article describes self-assembly of supramolecularly engineered naphthalene-diimide (NDI)-derived amphiphiles NDI-1 and NDI-2. They have the same hydrophobic/hydrophilic balance but merely differ by a single functional group, amide or ester. They exhibit distinct self-assembly in water; NDI-1 forms hydrogel, which upon aging forms crystals, whereas NDI-2 forms micelles as revealed by in-depth structural analysis using cryo-TEM, dynamic light scattering, and small-angle X-ray scattering studies. These results suggest that the H-bonding among the amide groups fully regulates the self-assembly by overruling the packing parameters. Further, the present study elucidates sharp lower critical solution temperature exhibited by these π-amphiphiles, which has been extensively studied for many important applications of water-soluble polymers but hardly known in the literature of small-molecule surfactants. Control experiments with the same water-soluble hydrophilic wedge did not show such a property, confirming this to be a consequence of the supramolecular polymerization by extended amide-amide H-bonding and not inherent to the structure of the hydrophilic wedge containing oligo-oxyethylene chains.