Aldehyde Approach to Hydrophobic Modification of Chitosan Aerogels

One pot synthesis of hydrophobic nanochitin aerogel via tert-butyl alcohol/water binary solvents as antibacterial and renewable oil superabsorbent

In this study, hydrophobic nanochitin aerogels are synthesized via one-pot synthesis strategy and subsequent freeze-drying technique, employing nanochitin, hexanal and formaldehyde as primary components. The tert-butyl alcohol (TBA)/water binary solvents are found efficient for well mixing of hydrophilic nanochitin and hydrophobic hexanal, which is fundamental for fabricating hydrophobic aerogels with water contact angle as high as 105°. Schiff base reaction between amino groups in nanochitin and aldehyde groups in hexanal is believed to be the main reason for the successful hydrophobization of nanochitin aerogels. Additionally, formaldehyde is employed to enhance the mechanical properties of aerogels via ice templated crosslinking technique. Nanochitin aerogels prepared in this work possess surface area as high as 237 m2 g-1, which are believed benefiting from the TBA/water binary solvents with lower density, smaller ice crystal and convenience in freeze-drying. The ultralow density, ultrahigh porosity, and hydrophobicity nature also lead to the advanced oil adsorption (as high as 210 g g-1) of nanochitin aerogels. The simple preparation process, nature sustainability and excellent adsorption performance is believed rendering nanochitin aerogels as a viable alternative for the remediation of oil spills.

Construction of a multifunctional dual-network chitosan composite aerogel with enhanced tunability

Typically, the tailorable versatility of biomass aerogels is attributed to the tunable internal molecular structure, providing broad application prospects. Herein, a simple and novel preparation strategy for developing multifunctional dual-network chitosan/itaconic acid (CSI) aerogel with tunability by using freeze-drying and vacuum heat treatment techniques. By regulating the temperature and duration of amidation reaction, electrostatic interactions between chitosan (CS) and itaconic acid (IA) was abstemiously converted into amide bond in frozen aerogel, with IA acting as an efficient in-situ cross-linking agent, which yielded CSI aerogels with different electrostatic/covalent cross-linking ratios. Heat treatment and tuning of the covalent cross-linking degree of CSI aerogel changed their microstructure and density, which led to enhanced performance. For example, the specific modulus of CSI1.5-160 °C-5 h (71.69 ± 2.55 MPa·cm3·g-1) increased by 119 % compared to that of CSI1.5 (32.73 ± 0.718 MPa·cm3·g-1), converting the material from superhydrophilic to hydrophobic (124° ± 3.6°), exhibiting favorable stability and heat transfer performance. In addition, part of -NH3+ of CS was retained in the electrostatic cross-linked network, endowing the aerogel with antibacterial properties. The findings of this study provide insights and a reliable strategy for fabricating biomass aerogel with good comprehensive performance via ingenious structural design and simple regulation methods.

Chitosan/silica hybrid aerogels with synergistic capability for superior hydrophobicity and mechanical robustness

Chitosan aerogels could be applied potentially in thermal insulation for energy-saving buildings, separation/adsorption, and catalysis. However, disadvantages of chitosan aerogels include their hydrophilicity and low insufficient mechanical strength. Here we propose a silica-phase hybriding route to create chitosan/silica hybrid aerogels with a synergistic capability for favourable hydrophobicity and superior mechanical strength, demonstrating an emergent finding (hydrophobicity optimised with the improved mechanical strength). The aerogels exhibit low drying shrinkage (as low as 13.41 %), lightweight (lowest to 0.149 g cm-1), high-efficient thermal insulation (thermal conductivity as low as to 0.024 W m-1 K-1 at room temperature and normal pressure) either under cryogenic (-196 °C) or high-temperature conditions, exceptional fire-retardancy (self-extinguishing in 1.8 s) and environmentally friendly characteristic (initial mineralisation after 10 d). High hydrophobic property (water contact angle up to 142°) of the aerogels were achieved depending upon 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane of vapor deposition, presenting a discovery concerning substantial improvement of mechanical properties (up to 0.188 MPa at 5 % strain, increased by 25 %). Furthermore, we demonstrate that a plausible mechanism for simultaneous hydrophobic and mechanical enhancement is depending upon the modulation of networking skeletons at the nanoscale.

Biopolymer-Polysiloxane Double Network Aerogels

A new series of transparent aerogels of biopolymer-polysiloxane double networks is reported. Biopolymer aerogels have attracted much attention from green and sustainable aspects but suffered from strong hydrophilicity and difficulty to make homogeneous structures in nanoscale; these drawbacks are overcome by compositing with a polysiloxane network. Alginate-polymethylsilsesquioxane aerogel has high optical transparency, water repellency, comparable superinsulation property and improved bending flexibility compared to pure polymethylsilsesquioxane aerogel. The nanoscale homogeneity is realized by separating the crosslinking steps for two networks in a sequential protocol: condensation of siloxane bonds and metal-crosslinking of biopolymer. The crosslinking order, biopolymer-first or siloxane-first, and universality/limitation of biopolymer-crosslinker pairs are discussed to construct fundamental chemistry of double network systems for their further application potentials.

Antimicrobial Properties of Chitosan-Modified Cotton Fabric Treated with Aldehydes and Zinc Oxide Particles

Chitosan is a natural biopolymer with a proven ability to impart textile materials with antimicrobial properties when loaded onto them. The mechanism of its bacteriological activity depends on the contact between the positive and negative charges of the amino groups located on the surface of the microbes. Unfortunately, the type of microorganisms and pH influence this action-shortcomings that can be avoided by chitosan modification and by loading its film with substances possessing antimicrobial properties. In this study, chitosan was modified with benzaldehyde and crosslinked with glutaraldehyde to form a film on the surface of cotton fabric (CB). Also, another material was obtained by including zinc oxide particles (CBZ) synthesized in situ into the chitosan coating. The performed analyses (contact angle measurement, optical and scanning electron microscopy, FTIR, XRD, and thermal analysis) evidenced the modification of the cotton fabric and the alteration of the film properties after zinc oxide inclusion. A comparison of the antimicrobial properties of the new CB with materials prepared with chitosan without benzaldehyde from our previous study verified the influence of the hydrophobicity and surface roughness of the fabric surface on the enhancement of antimicrobial activity. The microbial growth inhibition increased in the following order: fungal strain Candida lipolytica >Gram-positive bacteria Bacillus cereus >Gram-negative bacteria Pseudomonas aeruginosa. The samples containing zinc oxide particles completely inhibited the growth of all three model strains. The virucidal activity of the CB was higher against human adenovirus serotype 5 (HAdV-5) than against human respiratory syncytial virus (HRSV-S2) after 60 min of exposure. The CBZ displayed higher virucidal activity with a Δlog of 0.9 against both viruses.

Benzoxazine-grafted-chitosan biopolymer films with inherent disulfide linkage: Antimicrobial properties

Synthesis and fabrication of naturally sourced biopolymers, especially chitosan, grafted with renewable small molecules have recently attracted attention as efficient antimicrobial agents and are highly desired for sustainable material development. Advantageous inherent functionalities in biobased benzoxazine extend the possibility of crosslinking with chitosan which holds immense potential. Herein, a low-temperature, greener facile methodology is adopted for the covalent confinement of benzoxazine monomers bearing aldehyde and disulfide linkages within chitosan to form benzoxazine-grafted-chitosan copolymer films. The association of benzoxazine as Schiff base, hydrogen bonding, and ring-opened structures enabled the exfoliation of chitosan galleries, and such host-guest mediated interactions demonstrated outstanding properties like hydrophobicity, good thermal, and solution stability due to the synergistic effects. Furthermore, the structures empowered excellent bactericidal properties against both E. coli and S. aureus as investigated by GSH loss, live/dead fluorescence microscopy, and morphological alteration on the cell surface by SEM. The work provides the benefits of disulfide-linked benzoxazines on chitosan, offering a promising avenue for general and eco-friendly usage in wound-healing and packaging material.

Hydrophobization of aerogels based on chitosan, nanocellulose and tannic acid: Improvements on the aerogel features and the adsorption of contaminants in water

Hydrophobic chitosan aerogels are promising adsorbents for immiscible contaminants such as oils and organic solvents. However, few studies have reported the application of hydrophobic aerogels as adsorbent for organic contaminants dissolved in water. Herein, novel highly hydrophobic chitosan (CS) beads containing cellulose nanocrystals (CNC) and hydrophobized tannic acid (HTA) composite were prepared with different CS and CNC-HTA content to achieve an optimized adsorbent to remove emerging contaminants from water in batch and fixed-bed assays. The CS@CNC-HTA beads properties were assessed by FTIR, XRD, SEM, XPS, Micro-CT, WCA, and zeta potential. Supramolecular interactions and physical interlacements between CS and CNC-HTA enabled the formation of CS@CNC-HTA beads with high porosity (98.6%), great volume of open pore space (10.16 mm³) and hydrophobicity (121.8°). The 1:1 CS@CNC-HTA beads showed the best performance for removing the pharmaceutical sildenafil citrate, the basic blue 26 dye, and the surfactant cetylpyridinium chloride, reaching adsorption capacities of 86 (73%), 375 (84%), and 390 (90%) mg.g^-1, respectively. The 1:1 CS@CNC-HTA beads efficiently removed sildenafil citrate, basic blue 26 and cetylpyridinium chloride in fixed-bed experiments with exhaustion times of 890, 300, and 470 min, respectively. Theoretical calculations and adsorption assays indicate that the main attractive interactions are pyridinium-π, π-π, electrostatic and hydrophobic.

Hydrophobic Cellulose Acetate Aerogels for Thermal Insulation

As naturally derived material, cellulose aerogels have excellent thermal insulation properties due to their unique high porosity and three-dimensional mesoporous structure. However, its hydrophilic properties limit its application in the field of building insulation. Here, we propose a method to prepare high hydrophobicity by adopting the sol-gel method and chemical vapor reaction strategy using cellulose acetate type II as raw material and 2,4-toluene diisocyanate as the cross-linking agent. Thermal properties of cellulose acetate aerogels (CAAs) were measured, where pyridine was the catalyst, acetone was the solvent, and perfluorodecyltriethoxysilane (PFDS), hexamethyldisilazane (HMDS), and methyltriethoxysilane (MTES) were used as hydrophobic agents (by process hydrophobic test). Compared with MTES-modified cellulose acetate aerogels (M-CAAs) and HMDS (H-CAAs)-modified cellulose acetate aerogels, PFDS-modified (P-CAAs) cellulose acetate aerogels are the most hydrophobic. By implementing hydrophobic modification of PFDS both inside and outside the structure of cellulose acetate aerogels, the water contact angle can reach up to 136°, strongly demonstrating the potential of PFDS as a hydrophobic agent. The results show that the thermal conductivity and compressive strength of cellulose acetate aerogel with the best hydrophobic properties are 0.035 W m−1 K−1 at normal pressure and 0.39 MPa at 3% strain, respectively. This work shows that the highly hydrophobic cellulose acetate aerogel has potential as a waterproof material in the field of building thermal-insulation materials.

Neat Chitosan Porous Materials: A Review of Preparation, Structure Characterization and Application

This review presents an overview of methods for preparing chitosan-derived porous materials and discusses their potential applications. This family of materials has garnered significant attention owing to their biocompatibility, nontoxicity, antibacterial properties, and biodegradability, which make them advantageous in a wide range of applications. Although individual porous chitosan-based materials have been widely discussed in the literature, a summary of all available methods for preparing materials based on pure chitosan, along with their structural characterization and potential applications, has not yet been presented. This review discusses five strategies for fabricating porous chitosan materials, i.e., cryogelation, freeze-drying, sol-gel, phase inversion, and extraction of a porogen agent. Each approach is described in detail with examples related to the preparation of chitosan materials. The influence of the fabrication method on the structure of the obtained material is also highlighted herein. Finally, we discuss the potential applications of the considered materials.

Enhancing the antibacterial property of chitosan through synergistic alkylation and chlorination

Chitosan exhibits moderate antimicrobial properties. Here, we enhanced the antimicrobial properties of chitosan through alkylation and chlorination and evaluated the effect of alkylation on chitosan's hydrophobicity, bacterial attachment, chlorination, biocidal property, and stability. First, chitosan films were prepared through casting and were then immersed in a hexanal solution of different concentrations. The aldehyde groups of hexanal reacted with the amino group in chitosan through a Schiff base reaction. Next, the hexanal-modified chitosan films were soaked in 10 % bleach to form N-halamine. The results demonstrated that the surface became more hydrophobic, and chitosan films with increased hexanal-grafting concentrations exhibited less bacterial attachment. However, the degree of chlorination decreased as the degree of alkylation increased, further reducing the diameter of the zone of inhibition. Nevertheless, all chlorinated samples could kill ~5 log of Staphylococcus aureus and Escherichia coli within 30 min. Unlike previous results for chlorinated chitosan, in this study, alkylation before chlorination enhanced antibacterial properties and bactericidal ability and decelerated the degradation of chlorinated samples. The results of a systematic evaluation indicated that a hexanal-grafting concentration of approximately 80 mM maintains the equilibrium of the various properties of chitosan. Alkylated and chlorinated chitosan has considerable potential application as mask filter layers.

Performance improvement of PES membrane decorated by Mil-125(Ti)/chitosan nanocomposite for removal of organic pollutants and heavy metal

The Mil-125(Ti)-CS nanocomposite was successfully synthesized and characterized by using scanning electron microscopy (SEM) images, Fourier-transform infrared (FTIR) analysis and X-ray diffraction (XRD). Then, to improve the membrane performance, the synthesized Mil-125(Ti)-CS nanocomposite was embedded into the polyethersulfone (PES) membrane matrix. The nanofiltration membranes were fabricated via phase inversion method. Presence of chitosan in the structure of Mil-125(Ti) has increased the compatibility of nanoparticles with the polymer and also improved the hydrophilicity of the resulted membranes. The water contact angle of bare membrane (58°) was reduced to 40° by blending of 1 wt% nanocomposite led to increasing the pure water flux. However, the incorporation of more than 1 wt% of the nanocomposite caused the accumulation of nanocomposites and this was reduced the pore radius and permeability. The membrane containing 1 wt% nanocomposite was displayed the highest flux recovery ratio (FRR) ∼ 98% in bovine serum albumin (BSA) filtration. The membranes containing Mil-125(Ti)-CS also showed good performance against fouling. The performance of membranes was evaluated by treatment of six reactive dyes, antibiotic (cefixime), heavy metal, NaCl and Na₂SO₄ solutions. Addition of Mil-125(Ti)-CS NPs at low concentrations resulted in membranes with high pure water flux, higher separation efficiency, and remarkable anti-fouling behavior.

Bio-aerogels: Fabrication, properties and food applications

Traditional inorganic aerogels sustainability, biodegradability, and environmental safety concerns have driven researchers to find their safe green alternatives. Recently, interest in the application of bio-aerogels has rapidly increased in the food industry due to their unique characteristics such as high specific surface area and porosity, ultralow density, tunable pore size and morphology, and superior properties (physicochemical, mechanical, and functional). Bio-aerogels, a special category of highly porous unique materials, fabricated by the sol-gel method followed by drying processes, comprising three-dimensional networks of interconnected biopolymers (e.g., polysaccharides and proteins) with numerous air-filled pores. The production of bio-aerogels begins with the formation of a homogeneously dispersed precursor solution, followed by gelation and wet gel drying procedures by employing special drying techniques including atmospheric-, freeze-, and supercritical drying. Due to their special properties, bio-aerogels have emerged as sustainable biomaterial for many industrial applications, i.e., encapsulation and controlled delivery, active packaging, heavy metals separation, water and air filtration, oleogels, and biosensors. Bio-aerogels are low-cost, biocompatible, and biodegradable sustainable material that can be used in improving the processing, storage, transportation, and bioavailability of food additives, functional ingredients, and bioactive substances for their health benefits with enhanced shelf-life.

Green Preparation of Robust Hydrophobic β-Cyclodextrin/Chitosan Sponges for Efficient Removal of Oil from Water

A relatively straightforward green method to fabricate robust hydrophobic sponges for effective removal of oil pollutants and other organic contaminants was developed. These sponges were constructed from bio-sources: citronellal and palmitic acid-modified aminoethyl cyclodextrin-sodium phytate-chitosan (ACCTCS). The modified sponge exhibited desirable mechanical properties and strong hydrophobicity with a water contact angle (WCA) of 147.8°. Scanning electron microscopy showed that the ACCTCS sponge had a highly porous structure that was particularly suitable for organic component absorption. The sponge exhibited excellent absorption capacities for n-hexane, trichloromethane, vacuum pump oil, and peanut oil (47.9, 32.3, 32.6, and 32.2 g/g, respectively). The removal rate of oil was more than 80% (>26.2 g/g) after 10 absorption-desorption cycles. The ACCTCS sponge also showed good oil/water and organic components/water separation performance. The bio-source materials, green preparation method, and new absorbed-oil recovery strategy provided a novel pathway to construct multifunctional absorbents for oil/water separation in industrial wastewater.

Chitosan Functionalization: Covalent and Non-Covalent Interactions and Their Characterization

Chitosan (CS) is a natural biopolymer that has gained great interest in many research fields due to its promising biocompatibility, biodegradability, and favorable mechanical properties. The versatility of this low-cost polymer allows for a variety of chemical modifications via covalent conjugation and non-covalent interactions, which are designed to further improve the properties of interest. This review aims at presenting the broad range of functionalization strategies reported over the last five years to reflect the state-of-the art of CS derivatization. We start by describing covalent modifications performed on the CS backbone, followed by non-covalent CS modifications involving small molecules, proteins, and metal adjuvants. An overview of CS-based systems involving both covalent and electrostatic modification patterns is then presented. Finally, a special focus will be given on the characterization techniques commonly used to qualify the composition and physical properties of CS derivatives.

Fabrication of chitin nanofiber-PDMS composite aerogels from Pickering emulsion templates with potential application in hydrophobic organic contaminant removal

Natural polymers have aroused increasing attention in water treatment but their application in removing hydrophobic organic contaminants (HOCs) was limited due to their hydrophilicity. Herein, hydrophobic aerogels were successfully fabricated from Pickering emulsions stabilized by chitin nanofibers (ChNF) with polydimethylsiloxane (PDMS) as dispersed phase and glutaraldehyde as a crosslinking agent, and their performance in HOCs removal were evaluated. The Pickering emulsions with PDMS ratios of 2.5-20% v/v showed high stability, demonstrating great potential as aerogel templates. The solidified PDMS droplets were evenly distributed within the matrix, contributing to homogeneous and permanent hydrophobicity. The composite aerogels with water contact angles of over 130° could selectively remove non-aqueous phase HOCs from water. The CCl₄ adsorption capacity was 521-2820 wt%, depending on PDMS contents. Meanwhile, the mechanical resilience of the composite aerogels was significantly improved, facilitating the adsorbent regeneration by simple mechanical squeezing. The adsorption capacity remained above 85% for 24 cycles. Moreover, the aerogels could also remove dissolved HOCs from water with a maximum adsorption capacity of 1.34 mg/g for 10 mg/L TCE. This work reveals the potential of Pickering emulsions in the fabrication of composite hydrophobic materials from natural biopolymers with promising application in HOCs related water treatment.

Impact of Chitosan-Genipin Films on Volatile Profile of Wine along Storage

Chitosan-genipin films have been proposed for preservation of white wine, maintaining their varietal key odorants and organoleptic characteristics of sulfur dioxide treated wines. Nevertheless, these wines showed aroma notes that slightly distinguish them. It is possible that during the contact of films with wine for at least 2 months, after fermentation and prior to bottling, interactions or chemical reactions are promoted. In this work, wine model solutions with volatile compounds in contact with chitosan-genipin films were performed to evaluate their evolution along time. To complement these analyses, the volatile compounds of white and red wines kept in contact with chitosan-genipin films during 2 and 8 months were also studied. The results obtained allowed us to conclude that the contact of chitosan-genipin films with both white and red wines tend to retain long carbon chain volatile compounds, such as ethyl hexanoate and octan-3-one. It also promoted the formation of Maillard reaction products, such as furfural by dehydration of pentoses and Strecker aldehydes, such as 3-methylbutanal and phenylacetaldehyde, by degradation of amino acids. This study reveals that the use of chitosan-genipin films for wine preservation is also able to promote the formation of compounds that can modulate the wines aroma, maintaining the varietal notes.

Solvents, CO2 and biopolymers: Structure formation in chitosan aerogel

The functionality of biopolymer aerogels is inherently linked to its microstructure, which in turn depends on the synthesis protocol. Detailed investigations on the macroscopic size change and nanostructure formation during chitosan aerogel synthesis reveal a new aspect of biopolymer aerogels that increases process flexibility. Formaldehyde-cross-linked chitosan gels retain a significant fraction of their original volume after solvent exchange into methanol (50.3 %), ethanol (47.1 %) or isopropanol (26.7 %), but shrink dramatically during subsequent supercritical CO₂ processing (down to 4.9 %, 3.5 % and 3.7 %, respectively). In contrast, chitosan gels shrink more strongly upon exchange into n-heptane (7.2 %), a low affinity solvent, and retain this volume during CO₂ processing. Small-angle X-ray scattering confirms that the occurrence of the volumetric changes correlates with mesoporous network formation through physical coagulation in CO₂ or n-heptane. The structure formation step can be controlled by solvent-polymer and polymer-drying interactions, which would be a new tool to tailor the aerogel structure.

High-Capacity Reusable Chitosan Absorbent with a Hydrogel-Coated/Aerogel-Core Structure and Superhydrophilicity under Oil for Water Removal from Oil

In this work, inspired by the self-cleaning surfaces of fish scales, we prepared a porous chitosan aerogel (CSA) through a simple freeze-drying process. With the three-dimensional interconnected microstructure, the aerogel was highly porous (porosity > 98.16%) and ultralight with a density ranging from 10.19 to 36.05 mg/cm³. The core/shell structure of the CS-hydrogel-coated/CS-aerogel core (CSHA) was fabricated through a simple spray process. The aerogel with low-adhesion CS-hydrogel-coating exhibited superoleophobicity (θ_oil ∼ 162°) under water and superhydrophilicity (θ_water ∼ 0°) in oil. The hydrogel coating as a switch of the absorbent resists the oil phase and induces permeation of the water phase into the aerogel easily and quickly. The dry aerogel core with a porous structure has become a huge storage space. Taking advantage of this structure, an absorption capacity of 147 times could be obtained for water. The unique water absorption process along with switching between the aerogel and hydrogel gives the CSHA incredible potential for oil purification applications on site. Using the CSHA for oil purification, the purity of the obtained oil can be as high as 99.8%. Importantly, two facile approaches, including redissolving and drying, were applied to recycle the aerogels. The natural hydrophilic aerogels are made from dissolution and regeneration of chitosan powder, which is green, low-cost, simple and easy to scale-up. Using the as-obtained high-capacity recyclable CSA for oil/water separation, the mixture can be separated with high efficiency, making it a favorable candidate for applications in large-scale separation of oil-water mixtures in the future.

Transparent, Aldehyde-Free Chitosan Aerogel

Aldehyde-free, transparent chitosan aerogel is reported. The aerogel was prepared by thermal decomposition of urea to induce gelation of a chitosan solution, followed by solvent exchange to ethanol, and supercritical drying. Low urea concentrations (≤ 25 g L^-1) result in transparent and highly mesoporous aerogels, while higher urea concentrations (≥ 30 g L^-1) produce opaque, more macroporous aerogels. The high surface areas of > 400 m² g^-1, large mesopore volumes up to 3.5 cm³ g^-1, and optical transparency of the low-urea aerogels indicate a high structural homogeneity at the mesoscale, and the properties comparable to previously reported transparent chitosan aerogels prepared with formaldehyde crosslinking. The macroscopic size changes of the wet gels indicate that microstructure formation is controlled by the timing of chitosan coagulation, which depends among others on urea concentration. The aldehyde-free, microstructure-tunable process provides a new series of transparent biopolymer aerogels with "true aerogel" mesoporous structures.

Strong, Machinable, and Insulating Chitosan-Urea Aerogels: Toward Ambient Pressure Drying of Biopolymer Aerogel Monoliths

Biopolymer aerogels are an emerging class of materials with potential applications in drug delivery, thermal insulation, separation, and filtration. Chitosan is of particular interest as a sustainable, biocompatible, and abundant raw material. Here, we present urea-modified chitosan aerogels with a high surface area and excellent thermal and mechanical properties. The irreversible gelation of an acidic chitosan solution is triggered by the thermal decomposition of urea at 80 °C through an increase in pH and, more importantly, the formation of abundant ureido terminal groups. The hydrogels are dried using either supercritical CO₂ drying (SCD) or ambient pressure drying (APD) methods to elucidate the influence of the drying process on the final aerogel properties. The hydrogels are exchanged into ethanol prior to SCD, and into ethanol and then heptane prior to APD. The surface chemistry and microstructure are monitored by solid-state NMR and Fourier transform infrared spectroscopy, scanning electron microscopy, and nitrogen sorption. Surprisingly, large monolithic aerogel plates (70 × 70 mm²) can be produced by APD, albeit at a somewhat higher density (0.17-0.42 g/cm³). The as prepared aerogels have thermal conductivities of ∼24 and ∼31 mW/(m·K) and surface areas of 160-170 and 85-230 m²/g, for SCD and APD, respectively. For a primarily biopolymer-based material, these aerogels are exceptionally stable at elevated temperature (TGA) and char and self-extinguish after direct flame exposure. The urea-modified chitosan aerogels display superior mechanical properties compared to traditional silica aerogels, with no brittle rupture up to at least 80% strain, and depending on the chitosan concentration, relatively high E-moduli (1.0-11.6 MPa), and stress at 80% strain values (σ₈₀ of 3.5-17.9 MPa). Remarkably, the aerogel monoliths can be shaped and machined with standard tools, for example, drilling and sawing. This first demonstration to produce monolithic and machinable, mesoporous aerogels from bio-sourced, renewable, and nontoxic precursors, combined with the potential for reduced production cost by means of simple APD, opens up new opportunities for biopolymer aerogel applications and marks an important step toward commercialization of biopolymer aerogels.

One-Step Dynamic Imine Chemistry for Preparation of Chitosan-Stabilized Emulsions Using a Natural Aldehyde: Acid Trigger Mechanism and Regulation and Gastric Delivery

Chitosan is a polysaccharide widely used as a structuring agent in foods and other materials because of its positive charge (amino groups). At present, however, it is difficult to form and stabilize emulsions using chitosan due to its high hydrophilicity. In this study, oil-in-water emulsions were prepared using a one-pot green-chemistry method. The chitosan and aldehyde molecules were in situ interfacially conjugated during homogenization, which promoted the adsorption of chitosan onto the oil droplet surfaces where they created a protective coating. The universality of this method was verified by using chitosan with different molecular weights and four kinds of natural aldehydes [cinnamaldehyde (CA), citral (CT), citronella (CN), and vanillin (VL)]. Chitosan with higher molecular weight facilitated the formation of emulsions. By harnessing the dynamic covalent nature of imine bonds, chitosan emulsions with an imine link display dynamic behavior with acid-catalyzed hydrolysis. The aldehyde structure could control the pH point of trigger for breakdown of emulsions, which was 1.0, 3.0, 4.0, and 4.0 for CA emulsion, CT emulsion, CN emulsion, and VL emulsion, respectively. At pH 6.5, aldehyde helped to decrease the interfacial tension of chitosan to about 10 mN/m, while this value would increase if the pH decreased by adding acid during the measurement. Chemical kinetics studies indicated that the hydrophobicity and conjugation effect of the aldehyde together determined the trigger points and properties of the emulsion. Finally, we used the optimized emulsions to encapsulate and control the release of curcumin. The gastric release behavior of the curcumin depended on aldehyde structure: VL > CN > CT ≈ CA. Hence, a tailor-made trigger release emulsion system can be achieved by rational selection and design of aldehyde structure to control hydrophobicity and conjugation effect of aldehydes.

Formation of Nanofibrous Structure in Biopolymer Aerogel during Supercritical CO2 Processing: The Case of Chitosan Aerogel

Supercritical drying is widely considered as the gold standard to produce aerogels that preserve the microstructure of the gels, but we have found this is not always the case. Chitosan aerogel, one of the emerging biopolymer aerogels, was prepared by chemical cross-linking gelation, followed by solvent exchange with methanol and supercritical drying using CO₂. Small-angle X-ray scattering analysis shows that the structure of the wet gel, which consists of Gaussian chains of individual molecular strands, converts into a nanofibrous network during CO₂ processing. In situ observation reveals a drastic shrinkage of the gel in CO₂, demonstrating that physical coagulation caused by the low affinity between chitosan and CO₂ is the main structure-forming step. These results challenge the common perception of supercritical drying: it is no longer an inactive drying method, but rather an active nanostructure forming a tool to produce porous biopolymer materials with tailored structure and properties.

Facile Preparation of Metal-Organic Framework (MIL-125)/Chitosan Beads for Adsorption of Pb(II) from Aqueous Solutions

In this study, novel composite titanium-based metal-organic framework (MOF) beads were synthesized from titanium based metal organic framework MIL-125 and chitosan (CS) and used to remove Pb(II) from wastewater. The MIL-125-CS beads were prepared by combining the titanium-based MIL-125 MOF and chitosan using a template-free solvothermal approach under ambient conditions. The surface and elemental properties of these beads were analyzed using scanning electron microscopy, Fourier transform infrared and X-ray photoelectron spectroscopies, as well as thermal gravimetric analysis. Moreover, a series of experiments designed to determine the influences of factors such as initial Pb(II) concentration, pH, reaction time and adsorption temperature was conducted. Notably, it was found that the adsorption of Pb(II) onto the MIL-125-CS beads reached equilibrium in 180 min to a level of 407.50 mg/g at ambient temperature. In addition, kinetic and equilibrium experiments provided data that were fit to the Langmuir isotherm model and pseudo-second-order kinetics. Furthermore, reusability tests showed that MIL-125-CS retained 85% of its Pb(II)-removal capacity after five reuse cycles. All in all, we believe that the developed MIL-125-CS beads are a promising adsorbent material for the remediation of environmental water polluted by heavy metal ions.

Water-soluble chitosan-derived sustainable materials: towards filaments, aerogels, microspheres, and plastics

Bioinspired materials have aroused great interest as their inherent biocompatible and structural characteristics have given rise to sustainable applications. In this work, we have reported the phase and morphology transformation of chitosan from crystalline nanofibrils into amorphous sheets for fabricating sustainable materials. Acetylation-induced aqueous dissolution of native chitosan nanofibrils affords water-soluble chitosan as a biopolymeric liquid. Water-soluble chitosan macromolecules self-aggregate into amorphous sheets on solidification, presenting an interesting way to inspire new structures of chitosan assemblies. Through control over gelation, lyophilization, and self-assembled confinement of water-soluble chitosan, we have fabricated novel chitosan materials including filaments, aerogels, microspheres, and plastics that are promising for sustainable use.