Visualization of enzymatic hydrolysis of cellulose using AFM phase imaging

The effect of pre-treatment and process conditions on the gas barrier properties of fibrillated cellulose films and coatings: A review

Microfibrillated cellulose (MFC) is a bio-material produced by disintegrating cellulose fibres into fibrillar components. MFC could offer a sustainable solution to packaging needs since it can form an excellent barrier to oxygen. However, a comprehensive understanding of how MFC characteristics impact barrier properties of MFC films or coatings is required. This article critically reviews how the extent of separation of fibres into fibrils-and any resulting changes to the crystallinity and degree of polymerisation of cellulose-influences gas barrier properties of MFC films or coatings. Findings from publications investigating the barrier performance of MFC prepared through different processes intending to increase the effectiveness of fibrillation are evaluated and compared. The effects of processing conditions or chemical pre-treatments on barrier properties of MFC films or coatings are then discussed. A comparison of reported results showed that morphology and size polydispersity of the cellulose strongly influence the barrier properties of MFC. However, changing the MFC production process to decrease fibril diameter and polydispersity can result in changes to cellulose crystallinity; reduction in fibril length; introduction of bulky functional groups; or increased fibril surface charge: all of which could have a negative impact on the barrier properties of the final films or coatings.

Real-Time Visualization of Cellulase Activity by Microorganisms on Surface

A variety of methods to detect cellulase secretion by microorganisms has been developed over the years, none of which enables the real-time visualization of cellulase activity on a surface. This visualization is critical to study the interaction between soil-borne cellulase-secreting microorganisms and the surface of plant roots and specifically, the effect of surface features on this interaction. Here, we modified the known carboxymethyl cellulase (CMC) hydrolysis visualization method to enable the real-time tracking of cellulase activity of microorganisms on a surface. A surface was formed using pure CMC with acridine orange dye incorporated in it. The dye disassociated from the film when hydrolysis occurred, forming a halo surrounding the point of hydrolysis. This enabled real-time visualization, since the common need for post hydrolysis dyeing was negated. Using root-knot nematode (RKN) as a model organism that penetrates plant roots, we showed that it was possible to follow microorganism cellulase secretion on the surface. Furthermore, the addition of natural additives was also shown to be an option and resulted in an increased RKN response. This method will be implemented in the future, investigating different microorganisms on a root surface microstructure replica, which can open a new avenue of research in the field of plant root-microorganism interactions.

Investigation of the Dissociation Mechanism of Single-Walled Carbon Nanotube on Mature Amyloid-β Fibrils at Single Nanotube Level

Amyloid fibrils originating from the fibrillogenesis of misfolded amyloid proteins are associated with the pathogenesis of many neurodegenerative diseases, such as Alzheimer's, Parkinson's, and Huntington's diseases. Carbon nanotubes have been extensively applied in our life and industry due to their unique chemical and physical properties. Nonetheless, the details between carbon nanotubes and mature amyloid fibrils remain elusive. In this study, we explored the interplay between single-walled carbon nanotubes (SWCNTs) and preformed amyloid-β (Aβ) fibrils by atomic force microscopy at the single SWCNT level, together with ThT fluorescence, cellular viability assays, infrared spectroscopy, and molecular dynamics (MD) simulations. The results demonstrated that SWCNTs could partially destroy the preformed Aβ fibrils and form the Aβ-surrounded-SWCNTs conjugates, as well as reduce the β-sheet structures. Peak force quantitative nanomechanical measurements revealed that the conjugates have lower Young's modulus than fibrils. Furthermore, our MD simulation demonstrated that the dissociation ability was dependent on the binding sites of Aβ fibrils. Overall, this study provides an insight into the dissociation mechanism between SWCNT and Aβ fibrils, which could be beneficial for the study of bionanomaterials and the development of other potential drug candidates for amyloidosis.

Measurement of nanoscale molten polymer droplet spreading using atomic force microscopy

We present a technique for measuring molten polymer spreading dynamics with nanometer scale spatial resolution at elevated temperatures using atomic force microscopy (AFM). The experimental setup is used to measure the spreading dynamics of polystyrene droplets with 2 μm diameters at 115-175 °C on sapphire, silicon oxide, and mica. Custom image processing algorithms determine the droplet height, radius, volume, and contact angle of each AFM image over time to calculate the droplet spreading dynamics. The contact angle evolution follows a power law with time with experimentally determined values of -0.29 ± 0.01, -0.08 ± 0.02, and -0.21 ± 0.01 for sapphire, silicon oxide, and mica, respectively. The non-zero steady state contact angles result in a slower evolution of contact angle with time consistent with theories combining molecular kinetic and hydrodynamic models. Monitoring the cantilever phase provides additional information about the local mechanics of the droplet surface. We observe local crystallinity on the molten droplet surface, where crystalline structures appear to nucleate at the contact line and migrate toward the top of the droplet. Increasing the temperature from 115 °C to 175 °C reduced surface crystallinity from 35% to 12%, consistent with increasingly energetically favorable amorphous phase as the temperature approaches the melting temperature. This platform provides a way to measure spreading dynamics of extremely small volumes of heterogeneously complex fluids not possible through other means.

Electrochemical Assay for a Total Cellulase Activity with Improved Sensitivity

Electrochemical methods allow fast and inexpensive analysis of enzymatic activities. Here, we report a simple and yet efficient electrochemical assay for the total activity of cellulase, a hydrolytic enzyme widely used in food and textiles industries, and for production of bioethanol. The assay exploits the difference in electrochemical signals from a soluble redox indicator, ferricyanide, on nitrocellulose films treated by cellulases. Ferricyanide electrochemistry is totally inhibited on graphite electrodes modified with an insulating nitrocellulose film and is evoked after the cellulase treatment. Ferricyanide voltammetric responses correlate with the increased permeability of the films and electrochemically active surface area of electrodes becoming accessible to the ferricyanide reaction after nitrocellulose digestion by cellulase. Trichoderma and Aspergillus niger cellulases activities were determined in a 5 min assay with a sensitivity of 10^-8 U per assay, being 10³-10⁴-fold more sensitive than the standard commercially available optical assays. That makes the developed electrochemical approach the most prospective cost-effective alternative both for research and automated industrial applications.

Impact of surfactant type for ionic liquid pretreatment on enhancing delignification of rice straw

This work describes an environmentally friendly method for pretreating rice straw by using 1-Allyl-3-methylimidazolium chloride ([AMIM]Cl) as an ionic liquid (IL) assisted by surfactants. The impacts of surfactant type (including nonionic-, anionic-, cationic- and bio-surfactant) on the ionic liquid pretreatment were investigated. The bio-surfactant+IL-pretreated rice straw showed significant lignin removal (26.14%) and exhibited higher cellulose conversion (36.21%) than the untreated (16.16%) rice straw. The cellulose conversion of the rice straw pretreated with bio-surfactant+IL was the highest and the lowest was observed for pretreated with cationic-surfactant+IL. Untreated and pretreated rice straw was thoroughly characterized through SEM and AFM. In conclusion, the results provided an effective and environmental method for pretreating lignocellulosic substrates by using green solvent (ionic liquid) and biodegradable bio-surfactant.

Interaction Dynamics in Inhibiting the Aggregation of Aβ Peptides by SWCNTs: A Combined Experimental and Coarse-Grained Molecular Dynamic Simulation Study

The aggregation of amyloid-β peptides (Aβ) is considered as the main possible cause of Alzheimer's disease (AD). How to suppress the formation of toxic Aβ aggregates has been an intensive concern over the past several decades. Increasing evidence shows that whether carbon nanomaterials can suppress or promote the aggregation depends on their physicochemical properties. However, their interaction dynamics remains elusive as amyloid fibrillation is a complex multistep process. In this paper, we utilized atomic force microscopy (AFM), electrostatic force microscopy (EFM), ThT/fluorescence spectroscopy, and cell viability measurements, combined with coarse-grained molecular dynamic (MD) simulations to study the dynamic interaction of full length Aβ with single-walled carbon nanotubes (SWCNT). At the single SWCNTs scale, it is found that the presence of SWCNTs would result in rapid and spontaneous adsorption of Aβ1-40 peptides on their surface and stacking into nonfibrillar aggregates with reduced toxicity, which plays an important role in inhibiting the formation of toxic oligomers and mature fibrils. Our results provide new clues for studying the interaction in amyloid/SWCNTs system as well as for seeking amyloidosis inhibitors with carbon nanomaterials.

Antimicrobial and antioxidant surface modification of cellulose fibers using layer-by-layer deposition of chitosan and lignosulfonates

To confer cellulose fibers antimicrobial and antioxidant activities, chitosan (CS)/lignosulfonates (LS) multilayers were constructed on fibers surfaces through layer-by-layer deposition technique. The formation of CS/LS multilayers on cellulose fibers surfaces was verified by X-ray photoelectron spectroscopy (XPS) and zeta potential measurement. The surface morphologies of CS/LS multilayers on fibers surfaces were observed by atomic force microscopy (AFM). The results showed that characteristic element (i.e. N and S element) content increased with increasing bilayers number, the surface LS content increased linearly as a function of bilayers. Zeta potential of modified fibers was inversed after deposition of each layer. AFM phase images indicated that the cellulose microfibrils on fibers surfaces were gradually covered by granular LS aggregate. The antimicrobial testing results demonstrated that CS/LS multilayers modified fibers with CS in the outermost layer exhibited higher antimicrobial activity against Escherichia coli. The antioxidant testing results showed that antioxidant activity of CS/LS multilayers modified fibers was better than that of original fibers under the same oxidation conditions.

Identification of novel bacterial expansins and their synergistic actions on cellulose degradation

Novel expansins, non-catalytic proteins which induce weakening of the rigid cellulose structure, have been identified in this study. A pipeline of bioinformatics was implemented for sequence and structure-based prediction of putative bacterial expansin-like group × family from NR databases. All putative expansins had no detectable activity against cellulosic and hemicellulosic substrates but showed varying degrees of synergy (2.0-7.6 folds) with the commercial Trichoderma reesei cellulase (Celluclast™ 1.5L) on degradation of filter paper in order of BpEX ≈ CmEX > MaEX > PcEX > SaEX. A mixture design with full cubic model predicted optimal formulation comprising Celluclast™: CmEX from Clavibacter michiganensis = 72.4%: 27.6%, with no synergy of β-glucosidase on degradation of alkaline pretreated rice straw. Under these conditions, the reducing sugar yield was 163.6% compared with the reaction containing cellulase alone. This work demonstrated the potential benefit of novel bacterial expansins on enhancing cellulose degradation efficiency in lignocellulosic biomass degradation.

Cellulases dig deep: in situ observation of the mesoscopic structural dynamics of enzymatic cellulose degradation

Enzymatic hydrolysis of cellulose is key for the production of second generation biofuels, which represent a long-standing leading area in the field of sustainable energy. Despite the wealth of knowledge about cellulase structure and function, the elusive mechanism by which these enzymes disintegrate the complex structure of their insoluble substrate, which is the gist of cellulose saccharification, is still unclear. We herein present a time-resolved structural characterization of the action of cellulases on a nano-flat cellulose preparation, which enabled us to overcome previous limitations, using atomic force microscopy (AFM). As a first step in substrate disintegration, elongated fissures emerge which develop into coniform cracks as disintegration continues. Detailed data analysis allowed tracing the surface evolution back to the dynamics of crack morphology. This, in turn, reflects the interplay between surface degradation inside and outside of the crack. We observed how small cracks evolved and initially increased in size. At a certain point, the crack diameter stagnated and then started decreasing again. Stagnation corresponds with a decrease in the total amount of surface which is fissured and thus leads to the conclusion that the surface hydrolysis “around” the cracks is proceeding more rapidly than inside the cracks. The mesoscopic view presented here is in good agreement with various mechanistic proposals from the past and allows a novel insight into the structural dynamics occurring on the cellulosic substrate through cellulase action.

Cellobiohydrolase hydrolyzes crystalline cellulose on hydrophobic faces

Biodegradation of plant biomass is a slow process in nature, and hydrolysis of cellulose is also widely considered to be a rate-limiting step in the proposed industrial process of converting lignocellulosic materials to biofuels. It is generally known that a team of enzymes including endo- and exocellulases as well as cellobiases are required to act synergistically to hydrolyze cellulose to glucose. The detailed molecular mechanisms of these enzymes have yet to be convincingly elucidated. In this report, atomic force microscopy (AFM) is used to image in real-time the structural changes in Valonia cellulose crystals acted upon by the exocellulase cellobiohydrolase I (CBH I) from Trichoderma reesei. Under AFM, single enzyme molecules could be observed binding only to one face of the cellulose crystal, apparently the hydrophobic face. The surface roughness of cellulose began increasing after adding CBH I, and the overall size of cellulose crystals decreased during an 11-h period. Interestingly, this size reduction apparently occurred only in the width of the crystal, whereas the height remained relatively constant. In addition, the measured cross-section shape of cellulose crystal changed from asymmetric to nearly symmetric. These observed changes brought about by CBH I action may constitute the first direct visualization supporting the idea that the exocellulase selectively hydrolyzes the hydrophobic faces of cellulose. The limited accessibility of the hydrophobic faces in native cellulose may contribute significantly to the rate-limiting slowness of cellulose hydrolysis.