Cellulose nanocrystal research; A personal perspective.

This contribution to the special issue of Carbohydrate Polymers commemorating the 100th Anniversary of the Cellulose and Renewable Materials Division of the American Chemical Society is a personal account, from a research chemist's point of view, of some aspects of the discovery, development and utilization of nanocellulosic materials. The main focus is on cellulose nanocrystals stabilized by sulfate half-ester surface charges.

In Situ Preparation of Silver Nanoparticles in Paper by Reduction with Alkaline Glucose Solutions.

Percolation of contaminated water through paper sheets containing silver nanoparticles is a promising way to provide emergency drinking water. The silver nanoparticles are deposited by the in situ reduction of silver nitrate on the cellulose fibers of an absorbent blotting paper sheet. Sodium borohydride has been used as the reductant but is toxic and expensive.

Surface Charge Influence on the Phase Separation and Viscosity of Cellulose Nanocrystals.

A series of four cellulose nanocrystal (CNC) suspensions were prepared from bleached softwood kraft pulp using different conditions of sulfuric acid hydrolysis. The CNCs were identical in size (95 nm in length × 5 nm in width) but had different surface charges corresponding to the harshness of the hydrolysis conditions. Consequently, it was possible to isolate the effects of surface charge on the self-assembly and viscosity of the CNC suspensions across surface charges ranging from 0.

Order and gelation of cellulose nanocrystal suspensions: an overview of some issues.

Cellulose nanocrystals (CNCs) are polydisperse rod-shaped particles of crystalline cellulose I, typically prepared by sulfuric acid hydrolysis of natural cellulose fibres to give aqueous colloidal suspensions stabilized by sulfate half-ester groups. Sufficiently dilute suspensions are isotropic fluids, but as the concentration of CNC in water is increased, a critical concentration is reached where a spontaneously ordered phase is observed. The (equilibrium) phase separation of the ordered chiral nematic phase is in competition with a tendency of the CNC suspension to form a gel.

Twist-Bend Stage in the Relaxation of Sheared Chiral Nematic Suspensions of Cellulose Nanocrystals.

Aqueous suspensions of cellulose nanocrystals (CNCs), prepared from natural cellulose by sulfuric acid hydrolysis, form stable chiral nematic suspensions above a critical CNC concentration. The chiral nematic organization may be preserved in films prepared from the suspensions by evaporation. However, shrinkage, gelation, and shear during film formation impair the optical properties of the dry film.

Recent Advances in Chiral Nematic Structure and Iridescent Color of Cellulose Nanocrystal Films.

One unique property of cellulose nanocrystals (CNC) is their property of forming suspensions with chiral nematic order. This order can be preserved in films cast from the suspensions, raising the possibility of applications as photonic materials and templates. However, it has proved difficult to generate uniform, well-ordered chiral nematic materials from CNC.

Chiral Nematic Structure of Cellulose Nanocrystal Suspensions and Films; Polarized Light and Atomic Force Microscopy.

Cellulosic liquid crystalline solutions and suspensions form chiral nematic phases that show a rich variety of optical textures in the liquid crystalline state. These ordered structures may be preserved in solid films prepared by evaporation of solvent or suspending medium. Film formation from aqueous suspensions of cellulose nanocrystals (CNC) was investigated by polarized light microscopy, optical profilometry and atomic force microscopy (AFM).

Formation of chiral nematic films from cellulose nanocrystal suspensions is a two-stage process.

The evaporation of aqueous suspensions of cellulose nanocrystals (CNCs) gives iridescent chiral nematic films with reflection colors at visible wavelengths. A key problem is controlling the chiral nematic pitch, P, and hence the reflection colors of CNC films. By adding D-(+)-glucose to the suspension, we show that the change in P during evaporation occurs in two distinct stages.

A(3)-Coupling catalyzed by robust Au nanoparticles covalently bonded to HS-functionalized cellulose nanocrystalline films.

We decorated HS-functionalized cellulose nanocrystallite (CNC) films with monodisperse Au nanoparticles (AuNPs) to form a novel nanocomposite catalyst AuNPs@HS-CNC. The uniform, fine AuNPs were made by the reduction of HAuCl4 solution with thiol (HS-) group-functionalized CNC films. The AuNPs@HS-CNC nanocomposites were examined by X-ray photoelectron spectroscopy (XPS), TEM, ATR-IR and solid-state NMR.

Bactericidal paper impregnated with silver nanoparticles for point-of-use water treatment.

There is an urgent need for cheap point-of-use methods to purify drinking water. We describe a method to deactivate pathogenic bacteria by percolation through a paper sheet containing silver nanoparticles. The silver nanoparticles are deposited by the in situ reduction of silver nitrate on the cellulose fibers of an absorbent blotting paper sheet.

Direct surface force measurements of polyelectrolyte multilayer films containing nanocrystalline cellulose.

Polyelectrolyte multilayer films containing nanocrystalline cellulose (NCC) and poly(allylamine hydrochloride) (PAH) make up a new class of nanostructured composite with applications ranging from coatings to biomedical devices. Moreover, these materials are amenable to surface force studies using colloid-probe atomic force microscopy (CP-AFM). For electrostatically assembled films with either NCC or PAH as the outermost layer, surface morphology was investigated by AFM and wettability was examined by contact angle measurements.

Surface grafting of cellulose nanocrystals with poly(ethylene oxide) in aqueous media.

Aqueous suspensions of poly(ethylene oxide)-grafted nanocrystalline cellulose (PEO-grafted NCC) were prepared in order to achieve steric instead of electrostatic stabilization. A two-step process was employed: in the first step NCC suspensions prepared by sulfuric acid hydrolysis were desulfated with sodium hydroxide, and in the second step the surfaces of the crystals were functionalized with epoxy-terminated poly(ethylene oxide) (PEO epoxide) under alkaline conditions. The PEO-grafted samples were analyzed by conductometric titration, ATR-IR, solid-state NMR, MALDI-TOF MS, SEC MALLS, and AFM.

Induced phase separation in low-ionic-strength cellulose nanocrystal suspensions containing high-molecular-weight blue dextrans.

Spontaneous entropic phase separation phenomena occur in a wide range of systems containing highly anisotropic colloidal particles. Among these are aqueous suspensions of negatively charged cellulose I nanocrystals produced by sulfuric acid hydrolysis of native cellulose, which phase separate into isotropic and chiral nematic liquid-crystalline phases. Phase separation of an isotropic phase from a completely ordered nanocrystal suspension may be induced by the addition of salts or nonadsorbing macromolecules.

Morphological and optical characterization of polyelectrolyte multilayers incorporating nanocrystalline cellulose.

Aqueous layer-by-layer (LbL) processing was used to create polyelectrolyte multilayer (PEM) nanocomposites containing cellulose nanocrystals and poly(allylamine hydrochloride). Solution-dipping and spin-coating assembly methods gave smooth, stable, thin films. Morphology was studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and film growth was characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry, and optical reflectometry.

Friction and forces between cellulose model surfaces: a comparison.

Four different cellulose model surfaces, and one silica surface, have been studied by means of atomic force microscopy (AFM). The normal interactions have been found to consist of a longer range double layer force with a short range steric interaction, the nature of which is extensively discussed. Both the surface charge and range of the steric force depend on the type of cellulose substrate used, as does the magnitude of the adhesion.

Surface forces measurements of spin-coated cellulose thin films with different crystallinity.

A systematic study of the surface forces between a cellulose sphere and cellulose thin films of varying crystallinity has been conducted as a function of ionic strength and pH. Semicrystalline cellulose II surfaces and amorphous cellulose films were prepared by spin-coating of the precursor cellulose solutions onto oxidized silicon wafers before regeneration in water. Crystalline cellulose I surfaces were prepared by spin-coating wafers with aqueous suspensions of sulfate-stabilized cellulose I nanocrystals.

Parabolic focal conics in self-assembled solid films of cellulose nanocrystals.

Suspensions of cellulose nanocrystals form colloidal chiral nematic phases. The liquid crystalline order in these suspensions can be captured in solid films by slow evaporation of the liquid. Studies of the microstructure of such chiral nematic solid films revealed parabolic focal conic (PFC) defects, a symmetric form of focal conic defects in which the line defects form a pair of perpendicular, antiparallel, and confocal parabolas.

Effect of reaction conditions on the properties and behavior of wood cellulose nanocrystal suspensions.

Sulfuric acid hydrolysis of native cellulose fibers produces stable suspensions of cellulose nanocrystals. Above a critical concentration, the suspensions spontaneously form an anisotropic chiral nematic liquid crystal phase. We have examined the effect of reaction time and acid-to-pulp ratio on nanocrystal and suspension properties for hydrolyzed black spruce acid sulfite pulp.