Designing Cellulose Nanofibrils for Stabilization of Fluid Interfaces.

Particles of biological origin are of increasing interest for the Pickering stabilization of biocompatible and environmentally friendly foams and emulsions. Cellulose nanofibrils (CNFs) are readily employed in that respect; however, the underlying mechanisms of interfacial stabilization remain widely unknown. For instance, it has not been resolved why CNFs are unable to stabilize foams while efficiently stabilizing emulsions. Here, we produce CNFs with varying contour lengths and charge densities to investigate their behavior at the air-water phase boundary. CNFs adsorbing at the air-water interface reduce surface tension and form interfacial layers with high viscoelasticity, which are attributed to the thermodynamic and kinetic stability of CNF-stabilized colloids, respectively. CNF adsorption is accelerated and higher surface pressures are attained at lower charge densities, indicating that CNF surface charges limit both adsorption and surface coverage. CNFs form monolayers with ∼40% coverage and are primarily wetted by the aqueous phase indicating a contact angle <90°, as demonstrated by neutron reflectometry. The low contact angle at the air-water interface is energetically unfavorable for adsorbed CNFs, which is proposed as a potential explanation why CNFs show poor foaming capacity.

Nanostructural Properties and Twist Periodicity of Cellulose Nanofibrils with Variable Charge Density.

Cellulose nanofibrils (CNFs) are a renewable and facile to produce nanomaterial that recently gained a lot of attention in soft material research. The nanostructural properties of the fibrils largely determine their self-organizing functionalities, and the ability to tune the CNF nanostructure through control of the processing parameters is therefore crucial for developing new applications. In this study, we systematically altered the CNF production parameters (i.e., variation in cellulose source, chemical, and mechanical treatment) to observe their impact on the nanostructural properties of the resulting fibrils. Atomic force microscopy (AFM) allowed detailed topological examination of individual CNFs to elucidate fibril properties such as contour length, kink distribution and the right-handed twist periodicity of individual fibrils. Statistical analysis revealed a large dependency of the fibril properties on the industrial treatment of the cellulose source material. Our results furthermore confirm that the average charge density of the fibrils regulates both contour length and twist periodicity and, thus, has a very strong impact on the final morphology of CNFs. These results provide a route to tune the detailed nanostructure of CNFs with potential impact on the self-organization of these biological colloids and their optimal use in new nanomaterials.

Hybrid Amyloid Membranes for Continuous Flow Catalysis.

Amyloid fibrils are promising nanomaterials for technological applications such as biosensors, tissue engineering, drug delivery, and optoelectronics. Here we show that amyloid-metal nanoparticle hybrids can be used both as efficient active materials for wet catalysis and as membranes for continuous flow catalysis applications. Initially, amyloid fibrils generated in vitro from the nontoxic ß-lactoglobulin protein act as templates for the synthesis of gold and palladium metal nanoparticles from salt precursors. The resulting hybrids possess catalytic features as demonstrated by evaluating their activity in a model catalytic reaction in water, e.g., the reduction of 4-nitrophenol into 4-aminophenol, with the rate constant of the reduction increasing with the concentration of amyloid-nanoparticle hybrids. Importantly, the same nanoparticles adsorbed onto fibrils surface show improved catalytic efficiency compared to the same unattached particles, pointing at the important role played by the amyloid fibril templates. Then, filter membranes are prepared from the metal nanoparticle-decorated amyloid fibrils by vacuum filtration. The resulting membranes serve as efficient flow catalysis active materials, with a complete catalytic conversion achieved within a single flow passage of a feeding solution through the membrane.