Green process for chemical functionalization of nanocellulose with carboxylic acids.

An environmentally friendly and simple method, named SolReact, has been developed for a solvent-free esterification of cellulose nanocrystals (CNC) surface by using two nontoxic carboxylic acids (CA), phenylacetic acid and hydrocinnamic acid. In this process, the carboxylic acids do not only act as grafting agent, but also as solvent media above their melting point. Key is the in situ solvent exchange by water evaporation driving the esterification reaction without drying the CNC. Atomic force microscopy and X-ray diffraction analyses showed no significant change in the CNC dimensions and crystallinity index after this green process. The presence of the grafted carboxylic was characterized by analysis of the "bulk" CNC with elemental analysis, infrared spectroscopy, and (13)C NMR. The ability to tune the surface properties of grafted nanocrystals (CNC-g-CA) was evaluated by X-ray photoelectron spectroscopy analysis. The hydrophobicity behavior of the functionalized CNC was studied through the water contact-angle measurements and vapor adsorption. The functionalization of these bionanoparticles may offer applications in composite manufacturing, where these nanoparticles have limited dispersibility in hydrophobic polymer matrices and as nanoadsorbers due to the presence of phenolic groups attached on the surface.

Antibacterial Cellulose Nanopapers via Aminosilane Grafting in Supercritical Carbon Dioxide.

In this work, we present an innovative strategy for the grafting of an antibacterial agent onto nanocellulose materials in supercritical carbon dioxide (scCO2). Dense cellulose nanofibril (CNF) nanopapers were prepared and subsequently functionalized in supercritical carbon dioxide with an aminosilane, N-(6-aminohexyl)aminopropyltrimethoxysilane (AHA-P-TMS). Surface characterization (X-ray photoelectron spectroscopy, contact angle, ζ-potential analysis) evidenced the presence of the aminosilane. The results show that the silane conformation depends on the curing process: a nonpolycondensed conformation of grafted silane with the amino groups facing outwards was favored by curing in an oven, while the curing step performed in scCO2 yielded CNF structures with the alkyl chain facing outwards. The grafted nanopapers exhibited antibacterial activity, and no antibacterial agent was released into the media. Furthermore, these materials proved to benefit from low cytotoxicity. This study offers a proof of concept for the covalent grafting of active species on nanocellulose structures and the control of aminosilane orientation using a green and controlled approach. These newly designed materials could be used for their antibacterial activity in the biomedical field. Thus, perspectives for topical administration and design of wound dressing could be envisaged.

Adsorption of xyloglucan and cellulose nanocrystals on natural fibres for the creation of hierarchically structured fibres.

Green treatment of natural fibres is a major issue in paper, textile and biocomposites industries to design innovative and eco-friendly products. In this work, hierarchical structuring of flax woven fabrics by the adsorption of xyloglucan (XG) and cellulose nanocrystals (CNC) is studied. Indeed, CNC have high mechanical properties, high specific surface area and great potential for functionalization. The adsorption of XG and CNC has been investigated in terms of localization by confocal and scanning electron microscopy (SEM) and quantification through adsorption isotherms. Adhesion force measurements have also been performed by Atomic Force Microscopy (AFM). XG and CNC are homogeneously adsorbed on flax fabric and adsorption isotherms reach plateau values around 20 mg /gfibres for both. The pre-adsorption of XG on flax fabric influences the amount of adsorbed CNC in the high concentrations and also creates entanglements and strong interactions between XG and CNC with the formation of an extensible network.

Cellulose phosphorylation comparison and analysis of phosphorate position on cellulose fibers.

Chemical modifications of cellulose fibers as pretreatment for cellulose nanofibrils (CNF) production have been investigated to improve the production process and the quality of obtained cellulosic nanomaterial. In this study, phosphorylation of cellulose fibers was done in anticipation of a future nanofibrillation. Different phosphate salts, namely NH4H2PO4, (NH4)2HPO4, Na2HPO4, NaH2PO4 and LiH2PO4 with different constants of solubility (Ks) were used to increase the efficiency of the modification. Phosphorylated cellulose pulps were analyzed using elemental analysis, solid-state 13C and 31P NMR, or conductimetric titration method. No effect of Ks was observed whereas a counterion effect was pointed out. The study also reported the effect of pH, cellulose consistency, temperature and urea content in phosphorylation efficiency. Finally, chemical functionalization and penetration of phosphorylation reagents in the cellulose fibers were evaluated using XPS, SEM-EDX, ToF-SIMS and solid-state NMR.

The surface chemistry of a nanocellulose drug carrier unravelled by MAS-DNP.

Cellulose nanofibrils (CNF) are renewable bio-based materials with high specific area, which makes them ideal candidates for multiple emerging applications including for instance on-demand drug release. However, in-depth chemical and structural characterization of the CNF surface chemistry is still an open challenge, especially for low weight percentage of functionalization. This currently prevents the development of efficient, cost-effective and reproducible green synthetic routes and thus the widespread development of targeted and responsive drug-delivery CNF carriers. We show in this work how we use dynamic nuclear polarization (DNP) to overcome the sensitivity limitation of conventional solid-state NMR and gain insight into the surface chemistry of drug-functionalized TEMPO-oxidized cellulose nanofibrils. The DNP enhanced-NMR data can report unambiguously on the presence of trace amounts of TEMPO moieties and depolymerized cellulosic units in the starting material, as well as coupling agents on the CNFs surface (used in the heterogeneous reaction). This enables a precise estimation of the drug loading while differentiating adsorption from covalent bonding (∼1 wt% in our case) as opposed to other analytical techniques such as elemental analysis and conductometric titration that can neither detect the presence of coupling agents, nor differentiate unambiguously between adsorption and grafting. The approach, which does not rely on the use of 13C/15N enriched compounds, will be key to further develop efficient surface chemistry routes and has direct implication for the development of drug delivery applications both in terms of safety and dosage.