Nanochitin: Chemistry, Structure, Assembly, and Applications

Abstract

Chitin, a fascinating biopolymer found in living organisms, fulfills current demands of availability, sustainability, biocompatibility, biodegradability, functionality, and renewability. A feature of chitin is its ability to structure into hierarchical assemblies, spanning the nano- and macroscales, imparting toughness and resistance (chemical, biological, among others) to multicomponent materials as well as adding adaptability, tunability, and versatility. Retaining the inherent structural characteristics of chitin and its colloidal features in dispersed media has been central to its use, considering it as a building block for the construction of emerging materials. Top-down chitin designs have been reported and differentiate from the traditional molecular-level, bottom-up synthesis and assembly for material development. Such topics are the focus of this Review, which also covers the origins and biological characteristics of chitin and their influence on the morphological and physical-chemical properties. We discuss recent achievements in the isolation, deconstruction, and fractionation of chitin nanostructures of varying axial aspects (nanofibrils and nanorods) along with methods for their modification and assembly into functional materials. We highlight the role of nanochitin in its native architecture and as a component of materials subjected to multiscale interactions, leading to highly dynamic and functional structures. We introduce the most recent advances in the applications of nanochitin-derived materials and industrialization efforts, following green manufacturing principles. Finally, we offer a critical perspective about the adoption of nanochitin in the context of advanced, sustainable materials.

Figure 1

(a) Timeline of discovery, analysis, extraction, use, and future direction of chitin and nanochitin. Red and purple dashed frames indicate chitin nanocrystal and nanofiber, respectively. Adapted with permission from ref (19). Copyright 2018 Elsevier. Summary of publications relating to (b) chitin, nanochitin, and nanocellulose and (c) comparison of chitin nanofibers and nanocrystals. The number of publications was collected from the Science and Engineering Indicators of the National Science Foundation between 2000 and 2020 using Scopus. The research was limited to titles, abstracts, and keywords as follows: chitin/cellulose (*) nanocrystal and nanofiber, including * nanowhisker, * whisker, * nanorod, * crystallite, nanocrystalline *, * nanofiber, * nanofibril, nanofibrillar *, and nanofibrillated *.

Figure 2

Structure of this Review, which includes a brief introduction (panel 1, section 1) and introduces the relationship between chitin chemistry (section 2) and its deconstruction into nanochitin (panel 3, section 3). The Review also covers the main interactions (panel 4, section 4) that rule the multiscale structuring of nanochitin in a variety of media, resulting in diverse applications. The latter take advantage of multidimensional phenomena (panel 5, section 5). Finally, we present the promise of nanochitin in relation to industrialization and to accelerate materials development in the context of the bioeconomy (panel 6, section 6).

Figure 3

Variety of organisms associated with chitin, including, to cite only a few examples: (a) Squid, typically in the beak and pens. Adapted with permission from ref (37). Copyright 2008 American Association for Advancement of Science. Arthropods including (b₁) crustaceans and (b2) insects and their larvae. (c) Shells, from terrestrial, river, or sea mollusks. Microorganisms including (d1) fungi and (d2₎ yeast. The dispersed mycelium in d1 is harvested from Aspergillus niger, and the yeast pseudohyphae in d2 is stained by Calcofluor white. Adapted with permission from refs (38) and (39). Copyright 2021 and 2002, respectively, Springer Nature.

Figure 4

Examples of multiscaled chitinous structures present in biological constructs. (a1) Mineral platelets organized within nacreous structures, e.g., in seashells, wherein the plates are cemented by calcite–protein–chitin complexes. Adapted with permission from ref (49). Copyright 2016 Springer Nature. (a2) Chitin fibril bundles within the protein matrix between inorganic plates. Adapted with permission from ref (60). Copyright 2021 American Chemical Society. (b) Multiscaled structures found in crustaceans. The monomeric unit (I) is polymerized into chitin polymers (II), which form bundles within proteins with specific binding sites (III). The bundles are arranged in a number of structures that optimize fracture deflection and strain-dependent response, herein exemplified with the honeycomb structure (V) and helicoidal Bouligand structures found at multiple scales within the cuticle (VI, VII). Adapted with permission from ref (44). Copyright 2010 John Wiley and Sons. (c1) Plane view of diabolical ironclad beetle, highlighting three distinct internal regions with variable spacing between organs and elytra. (c2) The jigsaw-type joints in the beetle yield a sequential manner, through delamination of the joints (right panel: computed tomography reconstruction of fractured suture), resulting in the highest resistance to compression in the animal kingdom. Adapted with permission from ref (53). Copyright 2020 Springer Nature.

Figure 5

Chitin structures and associated colors found in beetles. (a1) Range of structural colors in various beetles. Adapted with permission from ref (74). Copyright 2008 John Wiley and Sons. (a2) Highly scattering shell of Cyphochilus sp. yielding a superwhite appearance. Adapted with permission from ref (72). Copyright 2018 John Wiley and Sons. (a3) Preserved 40 million year old beetle cuticle showing light (Chrysomelids from Eckfeld). Adapted with permission from ref (66) . Copyright 2012 The Royal Society. (b1) Optical micrograph of the exoskeleton of beetle Chrysina gloriosa showing bright yellow reflections from the core of each cell and greenish reflection from the edges. (b2) AFM and (b3) SEM images for the orientation of the Bouligand structures into semispheres in the epicuticle. Adapted with permission from ref (63). Copyright 2009 American Association for Advancement of Science. (c1) Cross-sectional SEM and (c2) 3D reconstruction of structures of white reflecting epicuticle as displayed in a3. Adapted with permission from ref (72). Copyright 2018 John Wiley and Sons.

Figure 6

SEM images of the spatula- and spindlelike adhesive structures found in various insects (beetle, fly, and spider), indicating that heavier insect bodies exhibit finer adhesive structures. Adapted with permission from ref (76). Copyright 2003 National Academy of Sciences.

Figure 7

Three-dimensional schematic illustration of the structure of trans-membranous chitin synthase. The left panel is the crystal structure of the bacterial cellulose synthase complex from Rhodobacter sphaeroides. Here, the cellulose polymer is shown with gray spheres. The right panel corresponds to a computed 3D structure of the C-terminal parts of chitin synthase. The crystal structure of the bacterial cellulose synthase (left panel) was used as a template for structural predictions. The highly conserved amino acid sequences include QRRRW (product binding site) and EDR (saccharide binding site), found in other glycosyltransferases such as cellulose synthases. Adapted with permission from ref (94). Copyright 2017 MDPI.

Figure 8

(a) Schematic illustration of a typical cross-sectional structure of an insect cuticle in the premolt stage, showing the most important layers, with a fully developed endo- and exo-epicuticle. (b) Model of complex protein–chitin superstructures via post tanning/sclerotization, wherein proteins (dark and green tubes wrapping around the chitin crystals) specifically interact with the multiple faces of chitin crystals as well as with the amorphous domains. Note: different colors represent different proteins. Adapted with permission from ref (74). Copyright 2008 John Wiley and Sons. (c) Simplified structure of crustacean cuticle, highlighting the mineralized domains within the cuticle, wherein the network of minerals is considerably more continuous in crustaceans than in seashells. Adapted with permission from ref (119). Copyright 2017 Elsevier.

Figure 9

(a) Schematic illustration showing chitinous structures found in microorganisms. The chitin–glucan complex models include Saccharomyces cerevisae and Candida albicans for the yeast, Aspergillus fumigatus for the filamentous fungi, and Schizophyllum commune for the mushroom. Adapted from ref (135). Copyright 2020 American Chemical Society. (b) Three-dimensionally schematic illustration of the use of chitosome, a chitin-generating complex and its structure in fungi. Adapted with permission from ref (132). Copyright 2013 Elsevier.

Figure 10

Illustration of three main polymorphs of chitin. (a) AFM images of three polymorphs of ChNFs obtained from the same acid/base extraction process. α-Chitin can be extracted from crab shells (Potamon ibericum), β-chitin from squid pens (Sepia sp.), and γ-chitin from moth cocoons (Orgyia dubia). Adapted with permission from ref (151). Copyright 2017 Elsevier. The gray arrows indicate the orientation of chitin macromolecules within the crystalline domains, with the arrows pointing away from the reducing end. Molecular structure and hydrogen H-bonding in (b1) α-chitin and (c1) β-chitin. Note: the intersheet bonding is absent in the case of β-chitin. Adapted with permission from ref (152). Copyright 2009 Elsevier. Structure of (b2) α-chitin and (c2) β-chitin in different projection planes (ac, bc, and ab). Adapted with permission from ref (9). Copyright 2006 Elsevier.

Figure 11

Overview of chitin extraction from crustacean shells that involves demineralization (DM), deproteinization (DP), and discoloration (all shown in solid boxes) as well as alternative pretreatments (dashed box). New extraction methods are emerging given considerations of environmental impact and sustainability.

Figure 12

Overview of the methods used for isolation of (a) ChNF and (b) ChNC. ChNF containing crystalline and disordered structures is produced by (a1) mechanical treatment or mechanical treatment assisted by (a2) chemical modification and (a3) biological processing. The main goal of the chemical and biological modifications is to endow additional chemical features that facilitate mechanical fibrillation. ChNC is produced by surface exfoliation and by removal of disordered chitin structures using strong chemical processing with acids or oxidizing agents.

Figure 13

Microscopic images of ChNF prepared by typical mechanical nanofibrillation. SEM images of (a) ChNF isolated from chitin via ultrasonication in water and (b) ChNF isolated from crab shell α-chitin after one-pass grinding in acetic acid medium. Adapted with permission from ref (247). Copyright 2007 AIP Publishing LLC. Adapted from ref (250). Copyright 2009 American Chemical Society. TEM micrographs of (c) ChNF produced from squid pen β-chitin by one-pass microfluidization in acetic acid medium and (d) low-protein ChNF produced from lobster exoskeletons by microfluidization. Adapted with permission from ref (258). Copyright 2019 The Royal Society of Chemistry. Adapted with permission from ref (259). Copyright 2014 Elsevier.

Figure 14

Schematic illustration of the reaction mechanism that takes place at the molecular level following (a) partial deacetylation and (b) TEMPO-mediated oxidation on the surface of chitin prior to fibrillation. Adapted with permission from ref (19). Copyright 2018 Elsevier. The molecular structure of chitin is included to indicate the reaction sites in the different processing steps. (c) Self-exfoliation pathway and mechanism of native chitin assisted by the pseudosolvent treatment. Adapted with permission from ref (288). Copyright 2021 John Wiley and Sons. (c1) Schematic illustration of self-exfoliation of chitin into ChNF using pseudo-solvent swelling and subsequent homogeneous surface ionization. (c2) Two-step sequential chemical and structural evolution of chitin during the self-exfoliation process, wherein step I involves deprotonation of chitin in and step II corresponds to ionization of chitin with maleic anhydride. (c3) Model showing of pseudo-solvent-assisted maleic anhydride intercalation into chitin molecules. (d) Enzymatic deacetylation of chitin surface via bacterial chitin deacetylase. Adapted with permission from ref (290). Copyright 2019 The Royal Society of Chemistry.

Figure 15

Images of ChNF prepared by mechanical nanofibrillation assisted by chemical treatment. TEM images of ChNF isolated by (a) ultrasonication and (b) microfluidization of partially deacetylated α-chitin from crab shells in acidic water (acetic acid). Adapted with permission from ref (272). Copyright 2010 Elsevier. Adapted from ref (273). Copyright 2019 American Chemical Society. (c) TEM image of ChNF produced from TEMPO-oxidized tubeworm β-chitin in water. Adapted with permission from ref (279). Copyright 2009 Elsevier. (d) SEM image of ChNF isolated via grinding of maleic anhydride-esterified α-chitin in water. Adapted with permission from ref (283). Copyright 2016 Elsevier. (e) TEM micrograph of ChNF self-exfoliated from squid pen β-chitin by pseudo-solvent swelling and maleic anhydride intercalation. Adapted with permission from ref (288). Copyright 2021 John Wiley and Sons. (f) TEM image of ChNF mechanically disintegrated from chitin deacetylase-processed α-chitin in acidic water. Adapted with permission from ref (290). Copyright 2019 The Royal Society of Chemistry.

Figure 16

TEM images of HCl-hydrolyzed ChNC from crab shell α-chitin dialyzed in suspension at (a1) pH 3 and (a2) pH 6 followed by 10 min of ultrasonication. TEM images of HCl-hydrolyzed ChNC from (b) crab shells and (c) Riftia tubes. Adapted from refs (306) and (312). Copyright 2003 and 2002, respectively, American Chemical Society. TEM images of ChNC isolated from crab shell α-chitin using (d) TEMPO-mediated oxidation and (e) ammonium persulfate treatment. Adapted from ref (320). Copyright 2008 American Chemical Society. Adapted with permission from ref (327). Copyright 2017 Elsevier.

Figure 17

(a) Schematic illustration of nanofibrillation of deacetylated chitin clusters upon acid hydrolysis, generating single, individual ChNC. (b1) Cryo-TEM and (b2) AFM images of individual ChNC obtained by 90 min acid hydrolysis. (c1) Cryo-TEM of 30 min acid-hydrolyzed ChNC after dialysis and in the absence of ultrasonication. (c2) TEM image of 60 min acid-hydrolyzed ChNCs after dialysis and 30 s ultrasonication. The dashed lines in (c1) are added to indicate loosely bound chitin nanofibrils. Adapted from ref (315). Copyright 2020 American Chemical Society. (d) Proposed pathway for the accelerated deacetylation, oxidation, and degradation of nonordered domains in chitin upon periodate oxidation, resulting in soluble compounds containing carboxylic groups. This process demonstrates the capability of periodate to produce ChNC. Schematic representation of the (e1) production of ChNC by selective alkaline periodate oxidation of the nonordered chitin domains while keeping the ordered domains intact and (e2) restricted deacetylation and alkaline periodate oxidation on ChNC surface upon disintegrating chitin into ChNC, leading to zwitterionic nanocrystals. (f) TEM image of ChNC obtained from shrimp chitin. Adapted with permission from ref (328). Copyright 2021 The Royal Society of Chemistry.

Figure 18

Chemical strategies used to tailor the surface chemistry of nanochitin. Hydroxyl groups: (a) TEM image of acetylated ChNC. (b) X-ray diffraction patterns of ChNC and acetylated ChNC. (c) Dispersibility of ChNC (after 30 min standing at 4 °C) and acetylated ChNC (after 12 h standing at 4 °C) in tetrahydrofuran. Adapted with permission from ref (337). Copyright 2014 The Royal Society of Chemistry. Amino groups: (d) SEM image of naphthaloylated ChNF with 14% substitution. (e) Transmittance spectra (0.1 w/v% in water) of deacetylated ChNF (dashed line) and naphthaloylated ChNF in dimethyl sulfoxide (solid line). Adapted with permission from ref (347). Copyright 2014 Elsevier. Grafting: (f) SEM micrographs of AA-grafted ChNF. (g) UV–vis transmittance spectra of AA-grafted ChNF in basic water. n refers to the molar ratio of grafted AA against an N-acetyl glucosamine unit of a ChNF. Adapted with permission from ref (354). Copyright 2012 Elsevier.

Figure 19

Hierarchal structures (blue boxes) and challenges for processing and assembly of (nano)chitin at different length scales, as noted. Experimental techniques that can be used for the design and characterization of nanochitin assemblies are introduced as a function of the characteristic length scale: NMR, nuclear magnetic resonance; TEM, transmission electron microscopy; SEM, scanning electron microscopy; AFM, atomic force microscopy; Cryo-EM, cryogenic electron microscopy; QCM-D, quartz crystal microbalance with dissipation; SPR, surface plasmon resonance; DLS, dynamic light scattering; TGA, thermogravimetric analysis; DMA, dynamic thermomechanical analysis; DSC, differential scanning calorimeter; FTIR, Fourier transform infrared spectroscopy; XRD, X-ray diffraction.

Figure 20

(a) Schematic illustration of average structures of simulated chitin nanofibril systems using color-coded building blocks to highlight structural features. The number of nonzero digits in the name is the number of chitin chains in the ab plane, whereas the numeral at a given position is the number of chains in the corresponding ac plane. Number of (b₁) nonspecific and (b₂) specific H-bonds emerging during simulation using the (1110) model. Lines represent running averages over 5 ns time intervals, and vertical bars are variances in the same time interval. (c) Selected structures of the (1110) system observed at 0 (left panel) and 250 ns (right panel) during the MD simulations with highlighted H-bonding. Nonspecific H-bonding shows green and specific ones show orange. The chain backbones are shown as gray tubes. Adapted with permission from ref (316). Copyright 2016 The Royal Society of Chemistry.

Figure 21

(a) Tomography reconstructions and respective chirality profiles obtained from Cryo-TEM images of a ChNC based on principal component analyses of nanocrystals showing left-handed (Individualized ChNC #1) and nonchiral (Individualized ChNC #2) twist profiles along the axial direction. (b) Schematic diagram showing a standard model used for analyzing the twisting features along the axial direction of the nanocrystal. (c) Chiral twist (±99% confidence interval) of ChNCs as a function of axial length (±standard deviation) of the cross-sectional segments in (b). The inset in (c) summarizes the number of ChNC samples with different twisting behavior. The total number of analyzed ChNC samples was 28. Adapted from ref (315). Copyright 2020 American Chemical Society.

Figure 22

Schematic illustration of DLVO profiles of (a) spherical nanoparticles subjected to interactions at the colloidal scale, including van der Waals (vdW) forces, electrostatic, and depletion interactions and (b) rodlike ChNCs in aqueous suspensions of given ionic strength (1, 10, and 80 mM NaCl), considering parallel and cross orientations and under relevant forces operating at the colloidal scale (vdW and electrostatic). Adapted with permission from refs (366) and (370). Copyright 2020 and 2017, Elsevier.

Figure 23

(a1) Schematic representation of the bonds formed from ChNC suspension via confined evaporation-induced self-assembly (EISA). The dashed box indicates the long-range order of the lamellar structure in (a1), image obtained by polarized optical microscopy (POM). (a2) Schematics showing a POM image of the lamellae formation and distribution of ChNCs along the bond under capillary flow. Adapted with permission from ref (384). Copyright 2021 The Royal Society of Chemistry. (b) Order parameter S_eff of ChNC/siloxane suspension inferred from I = f(ψ) traces as a function of duration of the applied electric field. The inset corresponds to azimuthal intensity profiles I = f(ψ) at a different time intervals after applying the field and revealed (SAXS) a gradual alignment of ChNCs. Adapted from ref (395). Copyright 2013 American Chemical Society. (c1) TEM micrograph of a mesoporous ChNC/siloxane nanocomposites with aligned pores induced by self-assembled ChNCs. (c2–c4) Series of representative polarized-light micrographs of the composite in (c1) for different orientations with respect to the directions of both the magnetic field B and the polarizers (P for polarizer and A for analyzer). Homogenous birefringence of uniaxially oriented sample is revealed by a 4-fold increase in light intensity after a 45° rotation (c2 to c3). (c4) Blue color obtained after introducing a first-order λ retardation plate (γ = slow axis direction), indicating the aligned structure along n perpendicular to the direction of the magnetic field B. Adapted with permission from ref (397). Copyright 2010 John Wiley and Sons.

Figure 24

(a1) Schematic illustration of the adsorption of spherical particles at the oil/water interface according to the contact angles (θ). Adapted with permission from ref (407). Copyright 2002 Elsevier. (a2) Adsorption of particles with a contact angle < 90° in an oil-in-water (O/W) system. The desorption of the particles from the interfaces is prevented if the energy barrier for detachment is higher than the thermal energy. Adapted from ref (428). Copyright 2021 American Chemical Society. (b) Three-phase contact angle of ChNF films spin-coated from ChNF suspensions of varying pH (3–6). θ was measured by injecting a sunflower oil droplet on a solid ChNF film in contact with water. Adapted from ref (410). Copyright 2020 American Chemical Society. (c1) Surface (air/water) and interfacial (oil/water) tension of ChNF suspensions at given concentrations. The inset shows the shape of the bubble or droplet during measurement. (c2) Interfacial dilatational rheology of air/water interfaces using a 10 μL bubble oscillating at 0.5 Hz frequency, demonstrating the interfacial adsorption of ChNFs in a 0.5 wt % suspension. Adapted from ref (273). Copyright 2019 American Chemical Society. (d) SEM image of ChNF-stabilized polystyrene sphere showing a high ChNF surface coverage. Adapted from ref (417). Copyright 2021 American Chemical Society. (e) Fluorescent images of ChNF/CNF-stabilized sunflower O/W Pickering droplets at different ChNF concentrations. CNF/NCh complexes were stained using Calcofluor white. Adapted from ref (417). Copyright 2021 American Chemical Society. (f1) SEM image of a ChNC-stabilized Pickering foam after drying. (f2) Polarized optical micrograph of a ChNC-based foam in liquid state. Adapted with permission from ref (425). Copyright 2015 The Royal Society of Chemistry. (g) Images of water-in-air liquid marbles (10 μL of water, dyed with methylene blue) wrapped by superamphiphobic ChNC powder on the surfaces of glass, paper, plastic, and water. Adapted from ref (339). Copyright 2020 American Chemical Society.

Figure 25

(a) Lyotropic liquid crystalline transitions of ChNC as a function of concentration, showing a dark, upper isotropic phase and a bright, bottom nematic phase. The left- and right-hand side schematics illustrate the disordered organization and ordered liquid crystals of ChNCs, respectively. Adapted with permission from ref (439). Copyright 2006 IOP Publishing. (b) POM images of ChNC dispersions of different concentrations as droplets suspended in silicone oil. Adapted from ref (442). Copyright 2018 American Chemical Society. (c) Concentration dependence of G′ and tan δ of ChNC suspensions with 0.1% strain at 20 °C. Adapted with permission from ref (443). Copyright 2019 Elsevier. (d) POM image of fingerprint pattern of the anisotropic phase formed from a ChNC suspension (5 wt %) stored for 1 day after treatment. The cholesteric texture exhibits periodic lines with spacing of ∼30 μm, corresponding to half of the cholesteric pitch. Adapted with permission from ref (438). Copyright 1993 Elsevier. SEM image of the cross section of the films prepared by drying a nematic ChNC suspension imaged at (e1) low and (e2) high magnifications, showing helicoidal architecture and Bouligand arches. The inset is a transparent film producing by evaporating the ChNC nematic phase. (e3) Schematic illustration showing the assembly of disordered ChNCs in suspension to ordered nematic organization, fitting the cholesteric pitch in d2. (e4) Chiral nematic pitch of ChNCs in suspension and in solid state at varied HCl and NaCl concentrations. Adapted from ref (304). Copyright 2019 American Chemical Society. (f) Illustration of the formation of a helically ordered ChNC/PAA hybrid via photopolymerization of ChNC/AA LCPs. (g) POM image of a fingerprint texture of ChNC/AA LCPs. SEM image of the cross section of cross-linked ChNC/PAA hybrid at (h1) low and (h2) high magnifications. The inset is the visual appearance of a typical composite film. Adapted with permission from ref (456). Copyright 2015 John Wiley and Sons.

Figure 26

(a1–a4) Cryo-TEM images of β-ChNFs that underwent self-assembly at pH 8 in four different stages (followed by time of observation). (a1) is the start of the assembly, while (a2) to (a4) occurred after 30 s. The visualization of the self-assembly after longer time periods was not possible given the increased fiber thickness that prevented electron transmission. Adapted rom ref (460). Copyright 2019 American Chemical Society. (b) Schematic illustration showing the dispersion state and nanostructured surface interactions of ChNC and bentonite under alkaline (pH 9), neutral (pH 7), and acidic (pH 3.4) conditions. The assembly of ChNC can be tuned by the conformation of bentonite at different pH values. Adapted from ref (462). Copyright 2018 American Chemical Society. (c) Magnitude of heat signal and schematics of interactions for ChNF and CNF under different pH conditions. The heat signal upon deprotonation decreases as the pH is reduced, and equilibrium shifts to favor the protonated form of NCh (purple area). The pH is adjusted due to the high acidity of the ChNF suspension. The heat signal for ionic complex formation initially increases as the ionization of ChNF is increased, and the driving force for complexation improves (yellow area); meanwhile, the heat signal subsequently decreases as the charge groups on CNF are consumed. Adapted with permission from ref (463). Copyright 2021 Elsevier.

Figure 27

(a) Effect of the coupling of monovalent Na and multivalent anions on swelling/deswelling properties of ChNF films supported on a quartz crystal microbalance sensor. The black (h) and red (Δh) dashed lines are drawn to show the thicknesses of ChNF film before and after exposure to electrolytes, respectively. Adapted from ref (466). Copyright 2021 American Chemical Society. (b1) Photographs of partially deacetylated ChNF (PD-ChNF) in the dry state (left panel) and redispersed in water (right panel) after the addition of different salts into the initial aqueous dispersions. (b2) Photographs of redispersed TEMPO-oxidized ChNF (TEMPO-ChNF) suspension at 0.7 and 3.0% concentration, respectively. Adapted from ref (278). Copyright 2018 American Chemical Society.

Figure 28

Roadmap toward multidimensional materials built from nanochitin with multiscale design principles, including current applications (blue circles) related to 0D emulsions, 1D fibers, 2D films, and 3D gels, and future perspectives (red circle) in 0D bioplastics, 1D smart wires, 2D intelligent textiles, and 3D engineered biolubricant. Here, “0D” refer to objects that make use of nanochitin as a primary building block. Adapted with permission from ref (478). Copyright 2017 John Wiley and Sons. Adapted from ref (479). Copyright 2018 American Chemical Society.

Figure 29

(a) Fluorescent microscopy images of Pickering emulsions stabilized by CNC and ChNF. Adapted with permission from ref (376). Copyright 2018 The Royal Society of Chemistry. Adapted from ref (273). Copyright 2019 American Chemical Society. (b) Visual appearance and optical microscopy images of pH-reversible, paraffin-in-water Pickering emulsions prepared at different pH using zwitterionic ChNF. Adapted with permission from ref (280). Copyright 2017 The Royal Society of Chemistry. (c1) Fluorescent microscopy images of HIPPE solely stabilized by ChNF. (c2) Cryo-SEM image of the HIPPE droplets (emulsions of 88% oil fraction) showing the distribution of ChNFs on the droplet surface and in the continuous phase. (c3) 3D-printed object produced from HIPPEs. In (a) and (c1), the nanopolysaccharides are stained blue and the oil phase is dyed red. Adapted from ref (410). Copyright 2020 American Chemical Society. (d1) Foamability (upper panel) and foam stability (bottom panel) of liquid foams stabilized by a mixture of a nonionic surfactant (Tween 20, T20) and ChNF at different concentrations (mg/mL). The T20 concentration was 0.5 wt % in all the samples. (d2) Confocal image of the liquid foam stabilized by FITC-labeled ChNF (7.5 mg/mL). (d3) SEM image of the cross section of ChNF-stabilized foam upon drying. The inset shows a photograph of a piece of solid foam on the leaf of a bracket plant. Adapted from ref (426). Copyright 2018 American Chemical Society. (e) Reduced free fatty acid (FFA) release of ChNF-stabilized Pickering emulsion under simulated small intestinal conditions. The emulsions named as “ChNF-Tween 80” or “Tween 80-ChNF” refer to the main droplet stabilizer, either ChNF or the nonionic surfactant (Tween 80), respectively. Adapted with permission from ref (498). Copyright 2020 Elsevier.

Figure 30

(a1) Photograph of mesoporous nitrogen-doped carbon films produced from nematic LCPs coassembled from ChNC and silica. (a2) SEM image of the cross section of the carbon film in (a1), showing a layered structure with embedding carbon nanorods in each layer. (a3) Galvanostatic charge/discharge curves of a typical carbon film in 1 M H₂SO₄ recorded at the different current densities. Adapted with permission from ref (458). Copyright 2014 The Royal Society of Chemistry. (b1) Schematic illustration of the preparation and application of the smart printable inks wherein ChNF was loaded in the aqueous phase as an additive to adjust the rheological behavior of the inks. (b2) (left panel) Viscosity at 0.1 s^–1 and (right panel) yield stress of the inks at different ChNF loading levels (wt %). Adapted with permission from ref (516). Copyright 2020 Elsevier. (c) Schematic illustration of the solvent exchange process that leads to the nucleation and growth of lignin nanoparticles in the presence of shrimp-derived ChNF, used as templating support. Adapted from ref (520). Copyright 2021 American Chemical Society.

Figure 31

(a1) Schematic illustration of the preparation of chitin-based macrofibers or filaments by extrusion of ChNF hydrogels into a coagulation bath and drying. (a2) SEM and digital images of the as-prepared macrofiber. (b1) Schematic illustration of structure of the hybrid macrofibers that were extruded from a mixture of ChNF and metal ions. (b2) Cryo-TEM image of metal nanoparticles bound to ChNF aggregates in the gel state. Adapted from ref (527). Copyright 2012 American Chemical Society. (c1) Schematic drawing for wet-stretching device into which a macrofiber (as shown in a1) is clamped in water and stretched under controlled strain rates. (c2) SEM images of a cross section of unstretched and stretched ChNF-base macrofibers. The stretching ratio was set to 0.3. Influence of stretching ratio on (c3) the tensile mechanical properties and (c4) orientation index and Young’s modulus (E) of ChNF-based filaments. Adapted from ref (532). Copyright 2014 American Chemical Society.

Figure 32

(a) Dry-spinning produce filaments by direct drawing of a viscous interface generated by contact of a ChNF suspension (red) and an alginate solution (blue). (b) Schematic illustration of the mechanism responsible for interactions upon interfacial complexation of ChNF and alginate. SEM images of (c1) surface shell and (c2) cross section core of dry composite filament. The dashed squares correspond to a higher magnification, and the arrow in the magnified image in (c2) shows the drawing direction. (d1) Tensile tests of filaments produced from ChNF with varying aspect ratios, in dry and wet conditions. ChNF-S, -M, and -H stand for ChNF with small, medium, and high aspect ratio, respectively. (d2) Visual appearance of a single filament (red dashed square) immersed in water and under load, demonstrating wet stability. Adapted from ref (537). Copyright 2020 American Chemical Society.

Figure 33

(a) Visual appearance of an optically transparent and flexible ChNF film produced from α-ChNF. Adapted with permission from ref (245). Copyright 2013 Elsevier. (b) Optical appearance of 80 g/m² nanopapers with a thickness between 60 and 80 μm produced from Cancer pagurus and Agaricus bisporus stalk, cap, and whole mushroom (from left to right), respectively. Adapted from ref (555). Copyright 2019 American Chemical Society. (c1) UV–vis transmittance and (c2) AFM images of self-standing films (25 μm thickness) of TEMPO-oxidized ChNC, deacetylated ChNF, HCl-hydrolyzed ChNC, and squid pen ChNF. Adapted with permission from ref (557). Copyright 2012 Elsevier. (d) Comparison of tensile strength and Young’s modulus of insect-derived ChNF and wood-derived CNF, indicating comparable mechanical properties of ChNF and CNF. Adapted from ref (257). Copyright 2020 American Chemical Society. (e) Time-dependent evaluation of vertical flame tests for different ChNF samples (CNF sample used as a reference). Note: the shaded areas correspond to the confidence intervals from triplicated tests. The image shows the burning height of ChNF-phosphate nanopaper after exposure to a flame for 3 s. Adapted from ref (465). Copyright 2019 American Chemical Society. (f1) Cycling performance of cells with using polypropylene, polypropylene-SiO₂, cellulose nonwoven, and ChNF separators operated at 17 mA g^–1 for the first five cycles (used for activation) and 85 mA g^–1 for the following cycles. (f2) Charge/discharge voltage profiles of the LiFePO₄/Li half-cells using different separators under 58.5 mA g^–1 rate at 120 °C. Adapted from ref (574). Copyright 2017 American Chemical Society.

Figure 34

(a) Visual appearance and flexibility of transparent ChNF composite film containing a methacrylic resin. Adapted with permission from ref (578). Copyright 2011 The Royal Society of Chemistry. (b) Coefficient of thermal expansion of a series of acetylated ChNF and ChNF composite films. Adapted from ref (336). Copyright 2010 American Chemical Society. (c1) SEM micrograph of UV-cured acrylated epoxidized soybean oil droplet that is emulsified using ChNC in the presence of a photoinitiator. (c2) Cross-sectional SEM image of UV-cured ChNC composite polymer film fractured in liquid nitrogen, showing a well-preserved, spherelike beads. (c3) Visual appearance of UV-cured ChNC composite film. Adapted rom ref (592). Copyright 2021 American Chemical Society. (d1) Schematic illustration of the fabrication of different ChNF nanopaper-based composites via embedding/immobilizing various types of nanoparticles/components, which are suitable for a range of optical (bio)sensing applications. (d2) Visual appearance and (d3) flexibility of a ChNF nanopaper-based sensing platform, created with an office laser printer via direct-printing the hydrophobic ink onto a ChNF nanopaper, endowing hydrophobic and hydrophilic test zones. Adapted from ref (551). Copyright 2020 American Chemical Society. (e1) Schematic illustration showing a radiofrequency (RF) identification tag used for card reading through (e2) ChNC/CNF- and (e3) aluminum-coated PET film indicating the capability of ChNC-based nanocoating to transmit information. Adapted from ref (605). Copyright 2019 American Chemical Society.

Figure 35

(a1) Structure of a scaffold used as a reverse template to form ChNF hydrogels. (a2) Visual appearance of the dissolution in alkaline media of the sacrificial template to form the ChNF hydrogel. (a3) Photograph and (a4) SEM image of ChNF hydrogel scaffold with a 10 mm edge length. Adapted with permission from ref (618). Copyright 2015 John Wiley and Sons. (b) Gelation process involving ChNFs in the presence of ammonia vapor during given processing time. Following a limited gelation period (5 h), a clear interface between the upper hydrogel phase and the bottom dispersion phase coexists (SEM image, bottom left); after ChNF dispersion is fully gelled (16.5 h), a network structure is formed by the nanofibers (SEM image, bottom right). Adapted with permission from ref (621). Copyright 2020 Springer Nature. (c) SEM images at higher magnification of a ChNC aerogel where the upper panel shows a porous network and nanocrystal aggregation with the bottom panel showing a highly porous network. Adapted with permission from ref (614). Copyright 2013 John Wiley and Sons. (d) Schematic illustration of the directional freezing used to prepare nanochitin cryogels. (e) SEM images of the cross section of freeze-dried ChNF cryogels, with the upper and bottom panels indicating ChNF cryogels obtained by freezing at −80 °C and under liquid nitrogen, respectively. Adapted rom ref (554). Copyright 2014 American Chemical Society.

Figure 36

(a) SEM image (upper panel) and fluorescent micrograph (bottom panel) of osteoblast cells on alginate-containing ChNC hydrogel (ChNC/alginate ratio of 1). Adapted from ref (624). Copyright 2015 American Chemical Society. (b) Representative photos of cutaneous wounds treated with BMSCs alone and BMSC-encapsulated ChNF hydrogel, indicating the accelerated healing at all time points after incorporating BMSCs in the ChNF hydrogel. Adapted with permission from ref (644). Copyright 2018 John Wiley and Sons. (c1) Schematic showing of the preparation of a polyacrylamide-ChNF hydrogel that is suitable as the electrolyte in solid-state ZIB containing a VO₂ cathode. (c2) Ragone plot of a Zn//VO₂ solid-state battery using polyacrylamide-ChNF hydrogel electrolyte compared with other vanadium-based cathodes for an aqueous ZIB. The results indicate the high energy density of the current system. (c3) Galvanostatic discharging and chemical self-charging (oxidation for 6 h) cycles of solid-state ZIB. Twenty cycles were performed continuously, showing excellent self-rechargeability. Adapted with permission from ref (652). Copyright 2021 John Wiley and Sons. (d) Visual appearance of dyed water before and after treatment with a ChNF hydrogel. Reactive Blue 19 (90 mg/L) was fully adsorbed by the ChNF hydrogel (pH 1.5 after 12 h contact at 150 rpm stirring). Adapted from ref (620). Copyright 2016 American Chemical Society.

Figure 37

Industrial processing of (a) raw crab shells into (b) decolored, purified chitin flakes using (c) a pilot production system that removes the minerals, proteins, and surface pigment components of the crab shells. Adapted with permission from ref (661). Copyright 2017 MDPI.