Loïc Jierry

Spray‐Assisted Polyelectrolyte Multilayer Buildup: from Step‐by‐Step to Single‐Step Polyelectrolyte Film Constructions

The alternate deposition of polyanions and polycations on a solid substrate leads to the formation of nanometer to micrometer films called Polyelectrolyte Multilayers. This step-by-step construction of organic films constitutes a method of choice to functionalize surfaces with applications ranging from optical to bioactive coatings. The method was originally developed by dipping the substrate in the different polyelectrolyte solutions. Recent advances show that spraying the polyelectrolyte solutions onto the substrate represents an appealing alternative to dipping because it is much faster and easier to adapt at an industrial level. Multilayer deposition by spraying is thus greatly gaining in interest. Here we review the current literature on this deposition method. After a brief history of polyelectrolyte multilayers to place the spraying method in its context, we review the fundamental issues that have been addresses so far. We then give an overview the different fields where the method has been applied.

Strategies for covalently reticulated polymer multilayers

The past decade has seen a growing need for robust organic coating technologies in various fields going from biotechnology, sensing and electronic devices to membrane design. Polyelectrolyte multilayers constitute a major and versatile tool in this respect. Yet, these films usually suffer from chemical and mechanical weakness. Improvement of the chemical and mechanical stability of multilayer films can be brought about by covalent reticulation between the constituent polymers. Three main routes were developed for this purpose: post-reticulation after film buildup, step-by-step reticulation during film buildup and recently one-pot film construction. This review summarizes these main strategies with emphasis on the mechanisms of the chemical reactions used.

Asymmetric Epoxidation of trans-Olefins via Chiral Dioxiranes: A Possible Contribution of axial Approaches in the case of Tri- and Tetrasubstituted α-Fluoro Cyclohexanones

During the asymmetric epoxidation of stilbene and methyl p-methoxycinnamate with chiral dioxiranes (derived from 2,2′,5-tri- and 2,2′,5,5′-tetrasubstituted cyclohexanones with an axial fluorine at C2) a 26 to 30% increase in ee was observed upon desymmetrization of the axial face of the dioxirane (through disubstitution at C5 or introduction of a fluorine on the equatorial iPr-substituent located at C5), which suggests a contribution of the axial approach of the olefin to the dioxirane (in addition to the main equatorial approach). The two ketones 5a and 6a, which do not undergo Baeyer− Villiger oxidation, can be used in substoichiometric amounts and are fully recovered after the reactions, provide epoxides in high yield (75−90%) and high (95:5) to satisfying (83:17 to 87:13) enantiomeric ratios.

Efficient Dibenzo[c]acridine Helicene-like Synthesis and Resolution: Scaleup, Structural Control, and High Chiroptical Properties

Herein, we describe our recent expeditious synthesis of dibenzo[c]acridine helicene-like compounds on a large scale in pure enantiomeric form. This flexible synthesis allows for variation at several positions on the skeleton. Geometrical parameters related to these compounds have been obtained from monocrystal X-ray structure resolution. Additionally, chiroptical parameters have been recorded, highlighting the versatility of this family showing for example optical rotation at 589 nm varying between 135 and 150 deg g(-1)cm(2).

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.

Polysaccharide Films Built by Simultaneous or Alternate Spray: A Rapid Way to Engineer Biomaterial Surfaces

We investigated polysaccharide films obtained by simultaneous and alternate spraying of a chitosan (CHI) solution as polycation and hyaluronic acid (HA), alginate (ALG), and chondroitin sulfate (CS) solutions as polyanions. For simultaneous spraying, the film thickness increases linearly with the cumulative spraying time and passes through a maximum for polyanion/CHI molar charge ratios lying between 0.6 and 1.2. The size of polyanion/CHI complexes formed in solution was compared with the simultaneously sprayed film growth rate as a function of the polyanion/CHI molar charge ratio. A good correlation was found. This suggests the importance of polyanion/polycation complexation in the simultaneous spraying process. Depending on the system, the film topography is either liquid-like or granular. Film biocompatibility was evaluated using human gingival fibroblasts. A small or no difference is observed in cell viability and adhesion between the two deposition processes. The CHI/HA system appears to be the best for cell adhesion inducing the clustering of CD44, a cell surface HA receptor, at the membrane of cells. Simultaneous or alternate spraying of CHI/HA appears thus to be a convenient and fast procedure for biomaterial surface modifications.

One-pot morphogen driven self-constructing films based on non-covalent host–guest interactions

The construction of films with complex architectures through one-pot reactions taking place exclusively on a surface remains a challenge. Recently, to address this problem, we introduced a concept based on morphogen-driven film buildup. We used Cu(I) as morphogen and the Huisgens click-reaction between azide and alkyne groups on polymers as film building blocks. Here, we extend this concept to films whose integrity is based exclusively on non-covalent host–guest interactions that are reversible allowing much broader tunability of the film properties. We trigger electrochemically the self-construction of films based on clickable host (cyclodextrin) and guests (ferrocene or adamantane) both functionalized by alkyne functions and poly(acrylic acid) bearing azide groups. Under voltammetry cycles, where Cu(I) is formed in situ from Cu(II), the film builds up by the non-reversible covalent grafting of the host and guest molecules to poly(acrylic acid) chains through triazole formation and by the subsequent formation of reversible host–guest interactions which entirely support the film cohesion. This process leads to the continuous self-construction of a nanometre size film whose thickness increases with the application time of the electrochemical stimulus. The growth rate of the film can be tuned by changing in the buildup solution either the relative ratio in concentration of host and guest or through the competition between clickable and non-clickable guest molecules. The effect of different stimuli leading to the dissolution of the film is also reported.

Simultaneous spray coating of interacting species: general rules governing the poly(styrene sulfonate)/poly(allylamine) system.

Simultaneous spraying of two solutions of interacting species onto a substrate held vertically leads to the formation of nanometer-sized coatings. Here we investigate the simultaneous spraying of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) solutions leading to the formation of a film composed of PSS/PAH complexes. The thickness of this film increases linearly with the cumulative spraying time. For a given spraying rate of PAH (respectively PSS), the growth rate of the film depends strongly upon the PSS/PAH ratio and passes through a maximum for a PSS/PAH ratio lying between 0.55 and 0.8. For a PSS/PAH ratio that is maintained constant, the growth speed of the film increases linearly with the spraying rate of polyelectrolyte of both solutions. Using X-ray photoelectron spectroscopy, we find that the film composition is almost independent of the PSS/PAH (spayed) ratio, with composition very close to 1:1 in PSS:PAH film. The 1:1 PSS:PAH composition is explained by the fact that the simultaneous spraying experiments are carried out with salt-free solutions; thus, electroneutrality in the film requires exact matching of the charges carried by the polyanions and the polycations. Zeta potential measurements reveal that, depending on whether the PSS/PAH spraying rate ratio lies below or above the optimal spraying rate ratio, the film acquires a positive or a negative excess charge. We also find that the overall film morphology, investigated by AFM, is independent of the spraying rate ratio and appears to be composed of nanometer-sized grains which are typically in the 100 nm range.

Unlimited growth of host–guest multilayer films based on functionalized neutral polymers

Multilayer polymer films based on host–guest interactions were formed on gold surfaces covered with self-assembled monolayers terminated by β-cyclodextrin groups using poly(N-hydroxypropylmethacrylamide) derivatives bearing ferrocene or β-cyclodextrin moieties.

Asymmetric oxidation of silyl enol ethers using chiral dioxiranes derived from α-fluoro cyclohexanones

Asymmetric oxidation of silyl enolethers derived from tetralone, 2-methyl-tetralone, propiophenone and deoxybenzoin using chiral dioxiranes generated in situ from oxone and new chiral α-fluorinated cyclohexanones or fructose-derived ketone have been studied. It was observed that tetrasubstituted silyl enolethers are poor substrates, that substitution at C8 of the fluoro-ketones has a significant effect on the enantioselectivities obtained and that the fructose-derived-ketone provides higher enantioselectivities. The absolute configuration of the major hydroxy ketones obtained can be rationalized using a spiro model proposed for epoxidation of olefins.