Antibacterial Pickering emulsions stabilized by bifunctional hairy nanocellulose.

HYPOTHESIS: Pickering emulsions, defined as emulsions that are stabilized by colloidal particles, provide dispersion stability by preventing coalescence of the dispersed phase. In this study, we used a bifunctional hairy nanocellulose (BHNC) bearing both aldehyde and carboxylic acid groups as an stabilizer. We hypothesize that these particles as Pickering stabilizers can effectively reside at the oil-water interface, better than hairy nanocelluloses containing only carboxyl groups or aldehyde groups, and provide long-term stability without the need of any surfactants. EXPERIMENTS: Varying concentrations of BHNC were tested to explore the optimal concentration that provides emulsion stability. The effects of various preparation conditions such as salt and pH were also studied. Finally, carvacrol, an antibacterial essential oil, was loaded in the oil phase to develop antibacterial emulsions. FINDINGS: It was shown that a 1% BHNC suspension provides 90% and 80% stability for a duration of 30 and 60 days, respectively. A theoretical model using nuclear magnetic resonance relaxometry data is developed to prove that only a monolayer of BHNC covers oil droplets. Increasing the concentration of BHNC decreased the size of oil droplets, which as a result increases the surface area available for monolayer coverage. It was also shown that the antibacterial emulsions are highly effective against Gram-negative (i.e. E. coli) and Gram-positive (i.e. S. aureus) bacteria. Accordingly, BHNC as a highly functionalized bio-derived colloidal particle opens new opportunities for engineering highly stable Pickering emulsions.

Cellulose-based dispersants and flocculants.

Natural dispersants and flocculants, often referred to as dispersion stabilizers and liquid-solid separators, respectively, have secured a promising role in the bioprocessing community. They have various applications, including in biomedicine and in environmental remediation. A large fraction of existing dispersants and flocculants are synthesized from non-safe chemical compounds such as polyacrylamide and surfactants. Despite numerous advantages of synthetic dispersants and flocculants, issues such as renewability, sustainability, biocompatibility, and cost efficiency have shifted attention towards natural homologues, in particular, cellulose-based ones. Within the past decade, cellulose derivatives, obtained via chemical and mechanical treatments of cellulose fibrils, have successfully been used for these purposes. In this review article, by dividing the functional cellulosic compounds into "polymeric" and "nanoscale" categories, we provide insight into the engineering pathways, the structural frameworks, and surface chemistry of these "green" types of dispersants and flocculants. A summary of their efficiency and the controlling parameters is also accompanied by recent advances in their applications in each section. We are confident that the emergence of cellulose-based dispersing and flocculating agents will extend the boundaries of sustainable green technology.

Hybrid Aerogel Nanocomposite of Dendritic Colloidal Silica and Hairy Nanocellulose: an Effective Dye Adsorbent.

In this study, a new type of silica-cellulose hybrid aerogel was synthesized through a green and facile chemical cross-linking process. In a first step, dendritic fibrous nanostructured (colloidal) silica particles (DFNS) were prepared by a simple hydrothermal technique. Then, the surface of DFNS particles was functionalized with amine groups using 3-aminopropyltriethoxysilane to produce DFNS-NH2. In a second step, bifunctional hairy nanocellulose (BHNC) particles were functionalized with both aldehyde and carboxylic groups. The aldehyde groups of BHNC and the amine groups of DFNS-NH2 chemically reacted through a Schiff base reaction to form a hybrid hydrogel nanocomposite. Therefore, no external cross-linker is required in the synthesis. This hybrid aerogel is very lightweight and highly porous with a density of 0.107 g mL-1 and a porosity of 93.0 ± 0.4%. It has a large surface area of 350 m2 g-1, a large pore volume of 0.23 cm3 g-1, and a small pore size of 3.9 nm. The developed aerogel contains both positively and negatively charged functional groups and is a highly efficient substrate for dye adsorption from water, for both cationic and anionic organic dyes. These aerogels were found to have an outstanding adsorption capacity toward methylene blue (MB) as a cationic dye and methyl orange (MO) as an anionic dye. The results show that the aerogels can adsorb MB and MO with a capacity of 270 and 300 mg dye/g adsorbent, respectively.

Highly Absorbent Antibacterial and Biofilm-Disrupting Hydrogels from Cellulose for Wound Dressing Applications.

In this study, a carboxyl-modified cellulosic hydrogel was developed as the base material for wound dressings. ε-poly-l-lysine, a natural polyamide, was then covalently linked to the hydrogel through a bioconjugation reaction, which was confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). The antibacterial efficacy of the hydrogel was tested against two model bacteria, Staphylococcus aureus and Pseudomonas aeruginosa, two of the most commonly found bacteria in wound infections. Bacterial viability and biofilm formation after exposure of bacteria to the hydrogels were used as efficacy indicators. Live/Dead assay was used to measure the number of compromised bacteria using a confocal laser scanning microscope. The results show that the antibacterial hydrogel was able to kill approximately 99% of the exposed bacteria after 3 h of exposure. In addition, NIH/3T3 fibroblasts were used to study the biocompatibility of the developed hydrogels. Water-soluble tetrazolium salt (WST)-1 assay was used to measure the metabolic activity of the cells and Live/Dead assay was used to measure the viability of the cells after 24, 48, and 72 h. The developed antibacterial hydrogels are light weight, have a high water-uptake capacity, and show high biocompatibility with the model mammalian cells, which make them a promising candidate to be used for wound dressing applications.

A Review on Surface-Functionalized Cellulosic Nanostructures as Biocompatible Antibacterial Materials.

As the most abundant biopolymer on the earth, cellulose has recently gained significant attention in the development of antibacterial biomaterials. Biodegradability, renewability, strong mechanical properties, tunable aspect ratio, and low density offer tremendous possibilities for the use of cellulose in various fields. Owing to the high number of reactive groups (i.e., hydroxyl groups) on the cellulose surface, it can be readily functionalized with various functional groups, such as aldehydes, carboxylic acids, and amines, leading to diverse properties. In addition, the ease of surface modification of cellulose expands the range of compounds which can be grafted onto its structure, such as proteins, polymers, metal nanoparticles, and antibiotics. There are many studies in which cellulose nano-/microfibrils and nanocrystals are used as a support for antibacterial agents. However, little is known about the relationship between cellulose chemical surface modification and its antibacterial activity or biocompatibility. In this study, we have summarized various techniques for surface modifications of cellulose nanostructures and its derivatives along with their antibacterial and biocompatibility behavior to develop non-leaching and durable antibacterial materials. Despite the high effectiveness of surface-modified cellulosic antibacterial materials, more studies on their mechanism of action, the relationship between their properties and their effectivity, and more in vivo studies are required.

Dye Removal Using Hairy Nanocellulose: Experimental and Theoretical Investigations.

Adsorption is a common technique for the treatment of dye-contaminated wastewater. Achieving a high dye removal capacity is a common challenge with sustainable, low-cost adsorbents. Recently, a class of easily functionalized, biorenewable cellulose nanoparticles called hairy nanocellulose has been developed. Electrosterically stabilized nanocrystalline cellulose (ENCC), which can be synthesized from wood pulp through a two-step oxidation by periodate and chlorite, is a form of hairy nanocellulose with a high negative charge density, and thus has the potential for a high adsorption capacity. In this work, the adsorption of methylene blue, a cationic dye, by ENCC was shown to occur up to charge stoichiometry (1400 mg dye/g adsorbent), at which point aggregation of ENCC-dye complexes is observed. A model is developed to show that the adsorption can be described by an ion-exchange mechanism and is influenced by the presence of other ions. Equilibrium dye removal is reduced at both high ionic strengths and low pH. To facilitate handling, composite hydrogel beads of sodium alginate and ENCC (ALG-ENCC beads) are developed, and their methylene blue removal capacity is shown to maintain a high removal capacity (1250 mg/g). ALG-ENCC beads provide a facile way to employ these nanoparticles on a larger scale, providing a potential means for the removal of dyes and other contaminants at larger wastewater volumes.

Stimuli-responsive chitosan as an advantageous platform for efficient delivery of bioactive agents.

Despite a diverse range of active pharmaceutical agents currently at our disposal, high morbidity rate diseases continue to pose a major health crisis globally. One of the important parameters in this regard is the controlled cargo delivery at desired sites. Among a variety of synthetic and natural macromolecular systems, chitosan, an abundant biopolymer, offers a platform for tailored architectures that could have high loading capacity of cargo, target and deliver. Stimuli directed accumulation of vehicles and drug release is an area of direct relevance to biomedical applications. In this review, we highlight essential characteristics of modified chitosan that present themselves for efficient response through an internal (glutathione, reactive oxygen species, pH, temperature, enzymes, and chemical/electrical potential gradient), and external stimuli (ultrasound, light, mechanical stimuli, magnetic and electrical fields). With a brief review of the pertinent properties of chitosan that are relevant to biology, the design and critical evaluation of varied chitosan-based platforms is discussed. Future directions in exploiting important features of chitosan in this area can be derived from the presented comparative evaluation of the current literature in drug delivery.

Dendrimer directed assembly of dicarboxylated hairy nanocellulose.

HYPOTHESIS: Dendrimer-directed assembly of electrosterically stabilized nanocrystalline cellulose (ENCC) to form network structures was studied. ENCC is a member of the family of hairy nanocelluloses and consists of a crystalline rod and dicarboxylated cellulose chains ("hairs") protruding from both ends, which are very reactive. We hypothesized that covalent linking of ENCC and dendrimers should lead to self-assembled hybrid network structures in which dendrimeric nodes connect cellulose nanorods. EXPERIMENTS: Polyamidoamine (PAMAM) dendrimers were covalently linked to ENCC by a bioconjugation reaction with different ratios of ENCC to PAMAM. To control the self-assembly process and prevent aggregation, acid hydrolysis of ENCC was performed to obtain crew-cut ENCC with shorter hairs and less negative charge. The formation of self-assembled structures from different PAMAM concentrations were analyzed using atomic force microscopy. FINDINGS: It was observed that depending on the concentration of PAMAM, various linear, star-shaped, and closed-loop structures were formed. Also, networks were formed with dendrimers acting as the nodes, connecting long cellulose rods, thus producing a network with a characteristic length of around 100-200 nm, which is difficult to obtain otherwise. We have demonstrated that the reactions of dendrimers with ENCC are solely occurring at the hairs and not at the crystalline regions.

Developing Antibacterial Nanocrystalline Cellulose Using Natural Antibacterial Agents.

We used hairy nanocrystalline cellulose functionalized with aldehyde groups, otherwise known as sterically stabilized nanocrystalline cellulose (SNCC), to facilitate the attachment of the antibacterial agents lysozyme and nisin. Immobilization was achieved using a simple, green process that does not require any linker or activator. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses showed successful attachment of both nisin and lysozyme onto the SNCC. The efficacy of the conjugated nanocellulose against the model bacteria Bacillus subtilis and Staphylococcus aureus was tested in terms of bacterial growth, cell viability, and biofilm formation/removal. The results show that the minimum inhibitory concentration of the conjugated nanocellulose is higher than that of lysozyme and nisin in free form, which was expected given that immobilization reduces the possible spatial orientations of these proteins. We observed that free nisin is not active against S. aureus after 24 h of exposure due to either deactivation of free nisin or development of resistance in S. aureus against free nisin. Interestingly, we did not observe this phenomenon when the bacteria were exposed to antibacterials immobilized on nanocellulose, suggesting that immobilization of antibacterial agents onto SNCC effectively retains their activity over long time periods. We suggest that antibacterial SNCC is a promising candidate for the development of antibacterial wound dressings.