Bruno Jean

Preparation By Grafting Onto, Characterization, and Properties of Thermally Responsive Polymer-Decorated Cellulose Nanocrystals

The grafting of thermosensitive amine-terminated statistical polymers onto the surface of cellulose nanocrystals (CNCs) was achieved by a peptidic coupling reaction, leading to unusual properties like colloidal stability at high ionic strength, surface activity, and thermoreversible aggregation. We have used a large variety of experimental techniques to investigate the properties of the polymer-decorated CNCs at different length-scales and as a function of the different reaction parameters. A high grafting density could be obtained when the reaction was performed in DMF rather than water. Infrared and solid-state NMR spectroscopy data unambiguously demonstrated the covalent character of the bonding between the CNCs and the macromolecules, whereas TEM images showed a preserved individualized character of the modified objects. Dynamic light scattering and zeta potential measurements were also consistent with individual nanocrystals decorated by a shell of polymer chains. Surface tension measurements revealed that CNCs became surface-active after the grafting of thermosensitive amines. Decorated CNCs were also stable against high electrolyte concentrations. A thermoreversible aggregation was also observed, which paves the way for the design of stimuli-responsive biobased nanocomposite materials.

Non-Electrostatic Building of Biomimetic Cellulose-Xyloglucan Multilayers

Layer-by-layer assembly was used to build thin films, consisting of multiple layers alternating cellulose nanocrystals and xyloglucan, benefiting from the strong non-electrostatic cellulose-xyloglucan interaction. Data from atomic force microscopy and neutron reflectivity showed that these well-defined films exhibited a thickness increasing linearly with the number of layers, without increase in surface roughness. These green nanocomposite films, reminiscent of plant cell wall, are composed of a regular stack of single layers of cellulose nanocrystals separated by very thin xyloglucan spacers. Such architecture differs from the one formed by cellulose/polycations multilayers, where the cellulose phase itself consists of a double layer.

Preparation, morphology and structure of cellulose nanocrystals from bamboo fibers

Bamboo cellulose nanocrystals (CNCs) were successfully prepared from the sulfuric acid hydrolysis of bamboo bleached fibers. Reaction parameters such as temperature, acid-to-pulp ratio and hydrolysis time were varied over a limited range and conditions for the preparation of individual bamboo nanorods with a yield of 30xa0% were identified. The characteristics of bamboo CNCs were compared with those of CNCs obtained from three other sources, namely eucalyptus, sisal and curauá, prepared using already reported conditions. The shape and size of all four types of CNCs were investigated by complementary techniques, namely transmission electron microscopy, atomic force microscopy, dynamic light scattering and X-ray diffraction. The surface charge of the nanocrystals was determined by conductometric titration and zeta potential measurements. In all cases, negatively charged ribbon-like nanoparticles constituted of a few laterally associated elementary crystallites were observed. The influence of the reaction parameters on the CNC characteristics was discussed and compared with the data reported in the literature.

Structural Details of Cellulose Nanocrystals/Polyelectrolytes Multilayers Probed by Neutron Reflectivity and AFM

Neutron reflectivity measurements and AFM observations were used as complementary techniques to investigate multilayered films consisting of alternating sheets of rigid cellulose nanocrystals and flexible poly(allylamine hydrochloride) (PAH) prepared by the layer-by-layer assembly technique. Both techniques showed that smooth films with a high load of cellulose could be obtained. After deposition, the cellulose component occurred as a double layer with different densities: 50% and 25% for the lower and upper layer, respectively. A linear growth of the multilayer and the presence of a Bragg peak on neutron reflectivity curves indicated the formation of a well-ordered structure resulting from entropy-driven assembly and smoothening effect of the flexible PAH macromolecules. The possible alignment of the nanocrystals when anisotropic suspensions were used is also shown and opens the route to an improved control of the architecture of these multilayers.

Self-Assembly of Amphiphilic Glycoconjugates into Lectin-Adhesive Nanoparticles

This work describes the synthesis and self-assembly of carbohydrate-clicked rod-coil amphiphilic systems. Copper-catalyzed Huisgen cycloaddition was efficiently employed to functionalize the hydrophilic extremity of PEG-b-tetra(p-phenylene) conjugates by lactose and N-acetyl-glucosamine ligands. The resulting amphiphilic systems spontaneously self-assembled into nanoparticles when dissolved in aqueous media, as evidenced by dynamic light scattering (DLS), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). The formation of highly monodisperse micelles having a mean diameter of 10 nm was observed for systems containing a PEG 900 core, and a decrease in the hydrophilic moiety (PEG 600) led to the formation of vesicles with a broader size distribution. The presence of carbohydrate residues on the surfaces of the micelles and their ability to establish specific interactions with wheat germ agglutinin (WGA) and peanut agglutinin (PNA) were further highlighted by light-scattering measurements, thus confirming the attractive applications of such sugar micelles in biosensor devices.

First experimental evidence of a giant permanent electric-dipole moment in cellulose nanocrystals

The existence of a permanent electric dipole in cellulose nanocrystals (CNCs) has been evidenced by designed rectangular reversing pulse experiments. Transient electric birefringence (TEB) was used to measure the orientation under electric fields of CNCs dispersed in an apolar solvent (toluene) at low volume fraction. We probed the static and the dynamic orientational behaviour of CNCs in order to evaluate both the permanent and induced electric-dipole contributions to the orientational order parameter S2. We demonstrated the presence of a permanent dipole of about Debye along the CNCs long axis. The existence of this permanent dipole can stem from the parallel arrangement of cellulose chains in a non-centrosymmetric crystallographic lattice within each CNC together with the dipole moment borne by each glucosyl monomer.

Coloured Semi‐reflective Thin Films for Biomass‐hydrolyzing Enzyme Detection

A new enzymatic activity detection assay based on colour change of the semi-reflective films is presented. The method is based on the preparation of multilayered thin films of controlled thickness obtained by sequential deposition of cellulose nanocrystals and xyloglucan. The hydrolysis of the films leads to a decrease in layer thickness that enables to detect enzyme activity, to the naked eye, from the resulting colour changes in a span of few minutes. The method allows direct, fast, highly sensitive, and easy-to-use characterization of enzymatic activities.

Xyloglucan-cellulose nanocrystal multilayered films: effect of film architecture on enzymatic hydrolysis.

Understanding the hydrolysis process of lignocellulosic substrates remains a challenge in the biotechnology field. We aimed here at investigating the effect of substrate architecture on the enzymatic degradation process using two different multilayered model films composed of cellulose nanocrystals (CNCs) and xyloglucan (XG) chains. They were built by a spin-assisted layer-by-layer (LbL) approach and consisted either of (i) an alternation of CNC and XG layers or of (ii) layers of mixed (CNC/XG) complexes alternated with polycation layers. Neutron reflectivity (NR) was used to determine the architecture and composition of these films and to characterize their swelling in aqueous solution. The films displayed different [XG]/[CNC] ratios and swelling behavior. Enzymatic degradation of films was then performed and investigated by quartz crystal microbalance with dissipation monitoring (QCM-D). We demonstrated that some architectural features of the substrate, such as polysaccharide accessibility, porosity, and cross-links, influenced the enzymatic degradation.

SANS Measurements of Semiflexible Xyloglucan Polysaccharide Chains in Water Reveal Their Self-Avoiding Statistics

We explored the behavior and the characteristics of xyloglucan polysaccharide chains extracted from tamarind seeds in aqueous media. The initial solubilization is achieved by using a 0.01 M NaOH solution. The absence of compact aggregates in the solution and the average molecular mass of the individual chains were unambiguously demonstrated by size exclusion chromatography with multi-angle light scattering detection. The composition and the stability of the solution were quantitatively checked over weeks by using liquid state nuclear magnetic resonance with DMSO as internal standard. The conformational characteristics of the chains were measured using nondestructive small-angle neutron scattering (SANS). The unambiguous determination of the Flory exponent (ν = 0.588) by SANS enabled us to directly prove that xyloglucan chains in water behave like semiflexible worm-like chains with excluded volume statistics (good solvent), contrary to most of the neutral water-soluble polymer chains that rather exhibit Gaussian statistics (θ-solvent). In addition to the Flory exponent, the persistence length l(p) and the cross section of the chains were also determined by SANS with utmost precision, with values of 80 and of 7 Å, respectively, which provides a complete description of the conformational characteristics of XG chains at all relevant length scales.

Self-assembled carbohydrate-based micelles for lectin targeting

Biocompatible low-polydispersity micelles designed for lectin targeting have been prepared by spontaneous self-assembly in water of macromolecular glycosylated amphiphiles. Propargyl-β-lactoside and N-acetyl-β-D-glucosaminide were conjugated by copper-catalyzed Huisgen cycloaddition to azide-terminated PEG 900 stearate. Upon dissolution in water, the resulting amphiphiles immediately self-assemble into highly regular micelles having a mean diameter of 10 nm. Dynamic Light Scattering (DLS), Transmission Electron Microscopy (TEM) and Small-Angle X-ray Scattering (SAXS) were used to investigate the structure of the self-assembled saccharidic amphiphiles micelles. The presence of the carbohydrate epitopes on the surface of the micelles and their bioavailability for lectin targeting were also demonstrated by light scattering measurements. Specific interaction of the GlcNac and Lac residues with Wheat Germ Agglutinin (WGA) and Peanut Agglutinin (PNA) respectively, unveils potential applications of such carbohydrate-derived surfactants as simple and site-specific vectorization systems for drug delivery.