Chapter 1: Hairy Cellulose Nanocrystals: From Synthesis to Advanced Applications in the Water–Energy–Health–Food Nexus

Cellulose is a polysaccharide¹  made up of repeating β-D-glucose monomer units, which is considered the most inexhaustible raw material in the world that may replace non-renewable and oil-based products.²  This polysaccharide was first discovered in 1838 by the French chemist Anselme Payen³  and later identified as the most abundant biopolymer on Earth, being found in grass, wood, and agricultural residues.^4,5  Cellulose is also produced by some species of algae, fungi, tunicates,⁶  and bacteria (e.g. Acetobacter xylinum). The biorenewability,⁷  biodegradability,^8,9  biocompatibility,¹⁰  low surface roughness,¹¹  mechanical properties,⁸  and abundance^2,9  of cellulose have rendered this material an attractive candidate for environmental, medical, food, and energy applications.²  Cellulose has provided a raw material in the past 150 years¹²  to produce textiles, paper, coatings, laminates, optical films, adsorbents, and pharmaceuticals.²  Sources of celluloses include wood fibers (WF), plant fibers (PF), tunicate cellulose (TC), algae cellulose (AC), and bacterial cellulose (BC).¹³  Wood cellulose is one of the most frequently used sources of cellulose, which, in its native form, is a composite material made up of non-carbohydrate (lignin) and carbohydrate biopolymers, such as pectin, cellulose, and hemicellulose isolated through industrial chemical pulping, separation, and purification.¹²  Bacterial cellulose is different from celluloses of plant origin due to a high degree of polymerization (DP ∼ 2000–8000), high crystallinity (60–90%), and high water content (>90%).²  BCs have been used as biomaterials to fabricate artificial blood vessels for microsurgery,¹⁶  veterinary medicine,²  tissue engineering,^17,18  and cosmetics.² 

Nanostructured celluloses (nanocelluloses) have been one of the focuses of sustainable nanotechnology¹⁹  for developing advanced materials and devices, including nanocomposites^8,20  for biomedicine,^10,21  printable electro-conductive composites,^22,23  sensors,²⁴  and microfluidic devices.²⁵  Nanocelluloses are typically produced from cellulose fibers,²⁶  lignocellulosic fibers,²⁷  algae, and bacteria.²⁶  There are three major forms of nanocelluloses: (i) cellulose nanofibrils (CNFs), also known as microfibrillated cellulose (MFC) or nanofibrillated cellulose (NFC), (ii) cellulose nanocrystals (CNCs), also referred to as nanocrystalline cellulose (NCC) or cellulose nanowhiskers, and (iii) hairy cellulose nanocrystals (HCNCs), also known as hairy cellulose nanocrystalloids, with CNFs and CNCs referred to as conventional nanocelluloses.^13–15,28 

CNFs are produced through the mechanical disintegration of pretreated WF or PF pulps using high-pressure homogenization,²⁹  grinding,³⁰  cryocrushing,³¹  high-intensity ultrasonication,³²  or microfluidization.³³  Enzymatic and chemical pretreatments reduce the energy consumption of CNF production by 20–30-fold.³⁴  Common pretreatment enzymes are cellobiohydrolases (CBH) and endoglucanases (EG),³³  and chemical pretreatments include carboxymethylation, oxidation, sulfonation, acetylation, and silylation.³³  2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation is the most common pretreatment of cellulose fibers in which TEMPO reacts with the primary alcohol (hydroxyl) group on the C6 of cellulose, producing carboxyl groups.³⁵  CNF size is typically on the order of 0.5 to tens of microns in length and 1–100 nm in width.¹³ 

CNCs are whisker-shaped nanoparticles typically produced through the strong acid hydrolysis of WF, PF, or CNF.^36–38  Cellulose becomes hydrolyzed as protons (H⁺) attack the β(1 → 4)-glycosidic linkage, mainly reacting with the amorphous regions to produce oligosaccharides and glucose.³⁹  The process of hydrolysis is commonly conducted using sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid,³⁹  and maleic acid.⁴⁰  Reactants affect both crystalline and amorphous regions; however, the facile accessibility of disordered regions leads to the disintegration of cellulose fibrils from the amorphous regions, causing a rapid decrease in the degree of polymerization (DP).³⁹  As amorphous regions are broken down and CNCs are produced, a seemingly constant DP results, which is called the leveling off degree of polymerization (LODP).³⁹  Understanding surface characteristics and modifications of these nanocrystals is considered paramount for synthesizing advanced, multifunctional, and hierarchical materials.⁴¹  Surface modifications via oxidation, esterification, amidation, carbamation, nucleophilic substitution, and etherification have been conducted to impart varying functionalities and colloidal properties (e.g. electrostatic⁴²  and/or steric^43–45  stabilization) to CNCs.⁴¹  Surface modifications of CNCs enable their applications in membranes, films, catalysts, and drug delivery, as well as templating agents.^41,46  CNC size strongly depends on the cellulose source. For example, CNCs prepared from wood are typically on the order of 100–200 nm in length and less than 10 nm in width.¹³  Two grand challenges associated with CNCs are their limited crystal area for functionalization and colloidal stability, which hinder their applications in several fields, such as high-efficiency treatment of environmental or physiological media prone to high ionic strengths.

Current challenges associated with the structural limitations of conventional nanocelluloses may be addressed via the preferential functionalization of amorphous regions, taking advantage of the accessibility of disordered chains without fully removing them.²⁸  Increasing the solubility of cellulose through the chemical and/or physical treatment of amorphous regions allows the cellulose fibrils to be cleaved at these regions.^47–50  When the amorphous regions are solubilized and a sufficient number of cellulose chains are cleaved, nanocelluloses that comprise a crystalline body sandwiched between two chemically modified amorphous cellulose layers are produced. These cellulose nanoparticles are referred to as HCNCs,^48–50  which are synthesized via the periodate-mediated oxidation of cellulose fibrils, followed by heating and/or chemical treatment. Periodate oxidation stereospecifically converts the C2–C3 bonds of β-D-glucose monomer units of cellulose⁵¹  to 2,3-dialdehyde units,⁵²  which yields a cellulose derivative intermediate at room temperature called dialdehyde modified cellulose (DAMC, a copolymer of glucose and dialdehyde glucose).^53,54  The majority of crystal surface and amorphous regions of DAMC are decorated with dialdehyde groups.¹⁴  Heating and/or chemical treatment of DAMC solubilize the amorphous regions, disintegrating the fibrils into HCNCs that are electrically neutral (SNCC, sterically stabilized nanocrystalline cellulose), anionic (ENCC, electrosterically stabilized nanocrystalline cellulose), or cationic (CNCC, cationic nanocrystalline cellulose).^14,55  Note that CNCC is also a type of ENCC. In this chapter, we use the terminology originally adopted for the HCNCs in the literature; however, to acknowledge their key differences, the following terminology may also be used: anionic HCNC (a-HCNC), electrically neutral HCNC (n-HCNC), and cationic HCNC (c-HCNC).

This chapter provides a comprehensive overview of the synthesis, applied aspects, and future directions of HCNCs, a newly emerged family of nanocelluloses. The synthesis of anionic, cationic, and electrically neutral HCNCs will be discussed in detail. The chapter also summarizes the recent advances in developing HCNC-based soft material platforms, including colloidal dispersions, hydrogels, aerogels, films, coatings, and membranes for advanced applications pertinent to the water–energy–health–food nexus.

HCNCs are synthesized through a multistep chemical modification of cellulose fibrils, as presented in Figure 1.1. Cellulose fibrils are first oxidized by periodate (IO₄⁻) to produce DAMC fibrils, which are then heated and/or chemically treated, yielding neutral or charged hairy nanoparticles. This section outlines the reaction conditions for producing HCNCs, including reactants, main and side reactions, reaction time, mixing conditions, and cosolvent-assisted separation. We begin with DAMC preparation, followed by the synthesis of electrically neutral (SNCC), anionic (ENCC), and cationic (CNCC) HCNCs. Heating and/or chemical treatments result in approximately a 1 : 1 ratio of HCNCs (SNCC, ENCC, and CNCC) to their respective fully solubilized biopolymer counterparts (DAMC, dicarboxylated cellulose [DCC], and quaternary ammonium modified cellulose [QAC], respectively), which are then separated through cosolvent addition.^14,28 

DAMC is the intermediate for HCNC synthesis. To prepare DAMC fibrils, cellulose pulp is soaked in water for a few hours, followed by rigorous mechanical disintegration, for example, using a blender. The wet pulp is then oxidized using periodate (IO₄⁻) to form DAMC fibrils via converting the C2–C3 alcohol groups (vicinal diols) into dialdehydes, presented in reaction (eqn (1.1)).^48–50,68  Note that, for simplicity, all the cellulose reactions in this chapter are shown based on a glucose unit, not a cellobiose unit. This reaction requires negatively charged periodate ions to react with negatively charged pores of the cellulose fibril wall.⁵⁶  The main concern with reactions in nanopores, for example, cellulose fiber walls, is the limited accessibility of reaction sites, resulting in slow kinetics and low yields, especially for pores and reactants with the same charge.⁵⁶  Due to the Donnan equilibrium, the concentration of periodate ions inside the nanopores is lower than the bulk.⁵⁷  Inert salts, such as sodium chloride (NaCl), may shift the Donnan equilibrium, increasing the reaction rate and yield by increasing the concentration of periodate ions inside the nanopores.⁵⁶  During oxidation, the reactor is covered to prevent light-induced deactivation of periodate.^48–50  The iodate produced can be converted to periodate with hypochlorite (ClO⁻) under alkaline conditions, allowing for the recovery of periodate as presented in eqn (1.2).⁵⁸  After cellulose fibril oxidation, ethylene glycol is added to the reaction medium to quench the unreacted periodate, and the remaining chemicals are removed from the intact DAMC fibrils through repeated rinsing with deionized (DI) water and vacuum filtration.^48–50  DAMC fibers can be stored at 4 °C for later use.⁵⁰ 

A typical recipe for producing DAMC fibrils is as follows: 4 g of softwood kraft pulp (containing ∼25 mmol of C₆H₁₀O₅ units) is soaked in water for 3 days, followed by the mechanical disintegration of pulp fibers.⁴⁸  The disintegrated pulp is vacuum filtered and reacted with 5.33 g (24.9 mmol) NaIO₄ and 15.6 g (266 mmol) NaCl in 266 mL of total DI water (final volume) inside a beaker.⁴⁸  The reaction continues for 96 h at ambient temperature. The beaker is covered with aluminum foil to prevent light-induced periodate deactivation.⁴⁸  After the reaction is completed, 1 mL of ethylene glycol is added to the reaction medium and incubated for a few minutes. The intact DAMC fibers are the only undissolved product, which then go through several rinses with DI water and vacuum filtration to remove the chemicals.⁴⁸ 

Heating DAMC fibrils disintegrates them into a mixture of fully solubilized DAMC polymers and dispersed SNCC, comprising CNCs sandwiched between two uncharged, amorphous DAMC layers (hairs) protruding from both ends, which in turn impart steric stabilization to this type of HCNC.⁵⁰  Heating fully oxidized DAMC fibers may be conducted at 80 °C for 4 h to break down the fibrils from their amorphous DAMC regions.⁵⁵  Centrifugation is used to separate non-fibrillated fibers,⁵⁰  yielding a transparent dispersion containing SNCC and dissolved DAMC polymer chains.⁵⁰  The addition of propanol, a poor solvent for DAMC, reduces the steric stability, causing the hydrodynamic size of colloidal SNCC to first decrease due to DAMC chain shrinkage and then increase due to colloidal aggregation.⁵⁰  Further addition of propanol to the solution causes the DAMC polymer chains to aggregate and precipitate.^50,59  Based on the theory of sterically stabilized colloids, there is a critical flocculation point for suspensions.⁶⁰  Flocculation of colloids begins when the Flory–Huggins colloid–solvent interaction parameter χ is greater than (bad solvent) or equal to 0.5 (θ−solvent).⁶¹  The critical χ parameter of colloidal suspensions χ_c = 0.5 + f(n), where n represents the number of repeating monomer units, and f(n) is a positive function of n (monotonically decreasing).^61,62  Increasing propanol concentration increases the χ parameter, and SNCC precipitates out of the dispersion before DAMC due to a lower χ_c.⁵⁰  The cosolvent method results in a dispersion of SNCC particles free from solubilized DAMC.⁵⁰  A further increase in the cosolvent concentration precipitates the DAMC chains (Table 1.1).

DAMC fibrils can readily be functionalized with alcohols,^63,64  sulfides,^65,66  alkylamines,⁶⁷  carboxyl groups^69,71 , or poly-L-glutamic acid⁷⁰  due to the high reactivity of dialdehyde groups.^52,63,70  Chlorite oxidation of DAMC fibrils results in the formation of dicarboxylated cellulose (DCC) throughout the surface of crystalline regions as well as the amorphous parts, peeling the crystalline regions and breaking down the amorphous regions to yield CNCs sandwiched between two layers of DCCs, referred to as negatively charged electrosterically stabilized nanocrystalline cellulose (ENCC).⁴⁸  In theory, ENCC can be produced through the chlorite oxidation of SNCC; however, in a simplified method, the chlorite oxidation of DAMC fibrils is conducted to eliminate the heating step.⁴⁸  Never-dried DAMC fibrils are suspended in an aqueous solution containing sodium chlorite, sodium chloride, and hydrogen peroxide.⁴⁸  Sodium chloride is added to shift the Donnan equilibrium during the chlorite oxidation inside the DAMC fibril nanopores,⁵⁶  and hydrogen peroxide converts the hypochlorous acid produced during the chlorite reaction to hydrochloric acid, water, and oxygen gas, as shown in eqn (1.3).⁷¹  The pH is maintained at ∼5 to obtain optimum reaction kinetics for the chlorite oxidation.⁴⁸  Over time, the DAMC pulp slurry turns transparent after the formation of a sufficient number of carboxyl groups (≥3 mmol g⁻¹), breaking down the fibrils into ENCC and DCC.⁴⁸  The suspension is then centrifuged to separate any remaining fibers, followed by cosolvent-assisted separation using ethanol to obtain a dispersion of ENCC particles.⁴⁸  Further addition of ethanol precipitates the DCC chains.

A typical recipe for producing ENCC from DAMC fibrils is the following: 1 g (dry basis) of never-dried DAMC fibrils, 2.93 g of NaClO₂ (32 mmol), NaCl (50 mmol), and H₂O₂ (86 mmol) are added to water (total water= 50 mL). The pH is maintained at 5 for the first 5 h, and the reaction is allowed to proceed for 24 h. Subsequently, the dispersion is centrifuged at 27 000 g to remove the remaining large fibers. Ethanol is then added to the reaction medium, precipitating ENCC, followed by centrifuging the solution at 3000 g. After decantation, the gel-like precipitate, ENCC, can be resuspended in water(Table 1.2).⁴⁸ 

Cationic HCNC can be produced via functionalizing DAMC fibrils with cationic moieties, such as quaternary ammonium, using a variety of chemistries (e.g. Schiff base reaction). This process yields cationic (electrosterically stabilized) nanocrystalline cellulose (CNCC). In theory, CNCC can be produced from ENCC, SNCC, or more simply, directly from DAMC fibrils.²⁸  ENCC can be turned into CNCC through a bioconjugation reaction where the carboxyl groups of ENCC are reacted with cationic moieties via the addition of an activator[e.g. 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride, EDC].⁷²  SNCC can be turned into CNCC through Schiff base reaction with cationic moieties. However, the first process requires producing ENCC, and the second requires a heating step. More simply, CNCC is produced via reacting DAMC fibrils with a reactant containing a cationic moiety and a primary amine (NH₂) through a Schiff base reaction, followed by heating.⁴⁹  As an example, DAMC fibrils are reacted with Girard's reagent T ((2-hydrazinyl-2-oxoethyl)-trimethylazanium chloride) that contains a cationic moiety and a primary amine, as shown in eqn (1.4).⁴⁹  Sodium chloride is used to shift the Donnan equilibrium at a pH ∼ 4.5.⁴⁹  This reaction yields cationic dialdehyde modified cellulose fibers (CDAMC).⁴⁹  The solution is then heated at 60 °C to solubilize the amorphous regions and obtain a transparent solution of CDAMC, CNCC, and quaternary amine (ammonium) modified cellulose (QAMC).⁴⁹  Centrifugation is used to remove CDAMC and other non-fibrillated fibers, then cosolvent separation with propanol is conducted to separate CNCC from QAMC.⁴⁹  The gel-like precipitate of CNCC can be freeze-dried or dispersed in water for later use.⁴⁹ 

A typical recipe for producing CNCC from DAMC fibrils is as follows:⁴⁹  0.5 g (never dried) DAMC pulp is added to 0.5 g of Girard reagent T (2.93 mmol) and 1.2 g of NaCl (20.5 mmol) solution containing 40 g of total DI water. The pH is adjusted to 4.5 using HCl, and the reaction is stirred for 24 h.⁴⁹  The CDAMC fibers are separated by filtration. A slurry of 0.2 g (never dried) CDAMC fibers and 20 g of water is heated at 60°C for 30 min. The solution is centrifuged at 5250 g to remove any non-fibrillated fibers. Then, propanol is added to the solution, followed by centrifugation at 2050 g, producing a gel-like CNCC precipitate (Table 1.3).

Physicochemical properties of HCNCs have been characterized through microscopy techniques[e.g. atomic force microscopy (AFM) and transmission electron microscopy(TEM)], spectroscopy techniques[e.g. Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), electro-acoustic spectroscopy (EAS), solid state carbon-13 nuclear magnetic resonance (NMR), and ultraviolet–visible (UV–vis) spectroscopy], light-scattering techniques[e.g. dynamic light scattering (DLS) and electrophoretic light scattering (ELS)], and other techniques[e.g. conductometric titration, thermogravimetric analysis (TGA), acoustic attenuation, rheometry, and in vitro assays]. These techniques have been utilized to analyze the size, morphology, mobility, surface chemistry and charge density, crystallinity, viscosity, tensile strength and Young's modulus, and cytotoxicity of ENCC,^{48,75,77,86,90,91}  SNCC,^50,59  and CNCC,⁴⁹  which were thoroughly discussed in the book“Cellulose Nanoparticles: Chemistry and Fundamentals” in the chapter entitled“Hairy Cellulose Nanocrystals: Chemistry and Fundamentals.”

HCNCs can be physically and/or chemically processed to produce soft materials, such as films, hydrogels, aerogels, membranes, and nanocomposites for applications in biomedicine, water, energy, and possibly food industries. Potential biomedical applications of HCNCs include biomimetic mineralization, nanocarriers for biomacromolecule/drug delivery, wound dressing, tablet additives, and 3D bioprinting, which are yet to be explored. Water- and environment-related applications encompass absorbents/adsorbents (e.g. for metal ion and dye removal), flocculants, and antiscalants. In paper industries, HCNCs may be used as paper additives and retention agents. In electronics, humidity switches, conductors, and insulators may be fabricated based on HCNCs. These nanomaterials may also be engineered to develop advanced food additives, an area that has not been explored yet. In this section, we review the emerging applications of HCNC-based soft material platforms.

Transparent composite films can be fabricated using chemically treated cellulose fibers with tunable physical and optical properties, such as water absorbency, tensile strength, Young's modulus, transparency, density, and barrier properties.⁹²  Such films have potential applications in wound dressing, disposable/disappearing packing, moisture sensing, and metal ion removal, which may be post-treated for tuning water uptake capacity, wet strength, hydrophobicity, and gas/liquid barrier properties.^92,93  When dried through vacuum filtration, the protruding DCC chains of ENCC entangle, producing highly transparent films (Figure 1.2A).⁷⁵  Depending on the concentration and drying process, these films may be stronger and more flexible than films prepared from conventional CNCs.⁷⁵  Mechanical properties of these films, such as tensile strength and Young's modulus, depend on the aldehyde content of DAMC fibers and the counter ion of carboxyl groups. Increasing the aldehyde content of DAMC fibers, regardless of the counterion, increases the tensile strength of produced films. However, films formed with sodium as the counterion are stronger, more transparent (by 10%), and have higher contact angles than those with proton as the counterion.⁷⁵  Dried ENCC can be redispersed in water as the hydrogen bonds between the nanoparticles are weakened and eventually broken by the DCC-mediated electrosteric repulsion.²⁸  Accordingly, these films can be used for high-security packing, such as packages that disappear when immersed in water. The ENCC transparent films may also have applications in biodegradable packaging and other high-end products.⁷⁵ 

The formation of hydrophobic, non-hygroscopic cellulose fibers has potential applications in flexible packing, self-cleaning films, and polymer composite reinforcement. Hydrophilic cellulose fibers become hydrophobic through the periodate oxidation of cellulose pulp, followed by sequential Schiff base reactions with 1,12-diaminododecane and butylamine. Before butylation, fibers are cross-linked with 1,12-diaminododecane to protect them from alkylation-induced fiber disintegration. Hydrophobic cellulose fibers have improved thermal stability (increased derivative weight loss from ∼325 °C to ∼500 °C) and increased water contact angle to >140° (nearly superhydrophobic) compared with cellulose fibers.⁹⁴  Drying wood (kraft) pulp using ethanol results in a foam-like pulp, which can then undergo vapor phase reaction with trichloromethylsilane (TCMS) to produce superhydrophobic foams with silane coverage of ∼100% and a water contact angle of ∼150°.⁹⁵  Superhydrophobic ENCC films can also be prepared through cross-linking by dipping ENCC films into TCMS, followed by drying.⁷⁵  This process produces films with a water contact angle ∼160°.⁷⁵  ENCC films dipped into an adipic acid dihydrazide and EDC (ADH/EDC) solution also yield superhydrophobic substrates.⁷⁵ 

Hydrogels are hydrophilic polymer networks that hold water up to several orders of magnitude higher than their dry weight.⁹⁶  Polymers/nanoparticles can be physically and/or chemically cross-linked to form physical, chemical, or dual-network gels (gels formed through physical and chemical cross-linking).⁹⁷  Hydrogels have been extensively used to form biodegradable materials for drug delivery, tissue engineering, sensing, contact lenses, and purification applications.⁹⁶  Concentrated dispersions of CNC (e.g. >10 wt%)⁹⁸  and ENCCs (e.g. ∼11 wt%)¹⁴  form physical hydrogels, presented in Figure 1.2B. Before gel formation, CNC dispersions form chiral nematic liquid crystals above∼4.5 wt%;⁹⁸  however, ENCCs are unable to form a liquid crystalline phase.¹⁴  Chemical cross-linking of HCNCs can also form hydrogels. As an example, cross-linking aldehyde-modified HCNCs with poly(d-glucosamine) (chitosan) through a Schiff base reaction yields imine bonds and spontaneously produces a hydrogel.⁸⁰  These hydrogels, shown in Figure 1.2C, can be freeze-dried to form multifunctional aerogels (Figure 1.2D).⁸⁰  Metal-based cross-linkers, such as ammonium zirconium carbonate (AZC) have been used to assemble soft networks of HCNCs and from nanocomposite metallogels with controlled rheological properties and gel formation dynamics.⁹⁹  Hydroxyethyl cellulose (HEC) has also been used to convert ENCC and CNC dispersions into gels via shifting the gel formation point to a low nanoparticle concentration.¹⁰⁰  These diverse cross-linking techniques enable facile and fast HCNC-based hydrogel formation at the ambient condition without requiring any post-purification step, which can be used for numerous applications, for example, in the food industry, biomedicine, and as superabsorbents.²⁸ 

Carbon nanotubes (CNTs) have widely been used in composite materials and films as supercapacitors and electrostatic charge dissipators, and in photovoltaics due to their excellent electrical conductivity.^101–103  Forming cellulose–CNT composites have potential applications in optoelectronics and vapor sensing.^104,105  Electrical properties of cellulose-based nanocomposite materials depend on the concentration of water vapor.¹⁰⁶  Increasing the moisture content typically causes an increase in the conductivity of cellulose-based films.¹⁰⁷  CNT–CNC nanocomposites with 1% CNT loading have a relatively constant direct current (DC) conductivity ∼10 mS cm⁻¹ that slightly increases when relative humidity (RH) increases.¹⁰⁸  CNT–ENCC-based composites perform as effective insulators at RH below 75% with a DC conductivity of ∼ 5 µS cm⁻¹, which is ∼ 3–4 orders of magnitude lower than that of CNT–CNC nanocomposites.¹⁰⁸  However, at RH above 75%, CNT–ENCC composites behave as conductors with a DC conductivity of ∼1 S cm⁻¹, and ∼5–6 orders of magnitude increase.¹⁰⁸  The moisture uptake of ENCC and DCC is low compared to CNC at low and moderate RH; however, above RH ∼ 80%, DCC chains show a significant increase in moisture uptake (from <1.5% to >8%).¹⁰⁸  The abrupt change in conductivity and moisture uptake indicates that the DCC chains perform as insulators below RH ∼ 75%, and above this RH, they become hygroscopic and conductors.¹⁰⁸  This can be attributed to the hydrogen bond formation between carboxyl and hydroxyl groups on ENCC (particularly DCC). At a high RH (>75%), hydrogen bonds between the carboxyl and hydroxyl groups of DCC chains on ENCC break, causing a sudden increase in conductivity. Further engineering CNT–ENCC nanocomposites may set the stage for developing humidity sensitive devices.^108,109 

Nature forms elaborate, synergistic, and highly functional hierarchical structures from molecular/nanoscale building blocks, such as peptides, proteins, and carbohydrates.¹¹⁰  Bottom-up fabrication of materials via biomimetic approaches may leverage the development of advanced functional materials, such as modular hydrogels, aerogels, films, fibers, and nanocomposites.¹¹¹  Self-assembly of nanocelluloses may have applications in drug delivery,¹¹²  energy storage, and environmental remediation.¹¹¹  HCNCs can undergo physical and/or chemical self-assembly. Multivalent ions, such as Cu(ii), induce pole–pole attachment of ENCCs at the carboxyl-rich ends (hairs).⁷⁶  At a relatively low multivalent ion concentration, for example,∼100 ppm Cu(ii) per 1000 ppm of ENCC with 5.5 mmol COO⁻ per gram, the diffuse double layer is not completely screened, leading to the formation of star-like aggregates that are partially electrosterically stabilized, and at higher ion concentrations, raft-like aggregates are formed from fully neutralized ENCCs.⁷⁶  Functionalizing the DCC chains of HCNCs provides a bottom-up route for the chemical end-to-end assembly of nanocrystalline fibers with tailored length and aspect ratios.⁷⁸  Three chemical pathways are (i) copper-catalyzed click reaction between alkyne and azide functionalized hydrolyzed ENCCs (HENCC), (ii) diamine-mediated bioconjugation of HENCCs, and (iii) dendrimer-mediated cross-linking of HENCCs.^78,84  The first method involves forming two HENCC derivatives through carbodiimide-mediated modification of HENCC with alkyne (HENCC–AK) and azide (HENCC–AZ), respectively, followed by the copper-catalyzed click reaction to cross-link these two HENCC derivatives.⁷⁸  In the second method, HENCCs functionalized with a diamine moiety (e.g. ADH) undergo carbodiimide-mediated cross-linking with non-functionalized HENCCs.⁷⁸  The third approach involves the activation of carboxyl groups on HENCC with EDC and N-hydroxysulfosuccinimide (sulfo-NHS) and a subsequent reaction with polyamidoamine (PAMAM) dendrimers to produce HENCC–PAMAM, which self-assemble into various linear, closed-loop, and star-shaped structures in the presence of negatively charged polystyrene nanoparticles.⁸⁴  For efficient end-to-end assembly, hydrolysis of ENCC is required to reduce the steric hindrance of DCC chains and decrease the interparticle repulsion. Such structures may have applications in drug delivery/capturing and metal ion sequestration, enabling facile removal post sorption.⁸⁴ 

HCNCs with negatively/positively charged or electrically neutral biopolymeric hairs are able to scavenge cations, anions, or possibly organic pollutants, respectively. This class of nanocelluloses has provided a new, sustainable, and highly efficient benchmark for environmental remediation. SNCC decorated with a high concentration of aldehyde groups may be an effective nano-adsorbent for sequestering non-ionic species, such as organic pollutants. ENCCs bearing a high density of dicarboxylate groups have been used as biorenewable, low-cost, and environmentally friendly adsorbents for removing contaminants, such as heavy metal ions (e.g. copper) and dyes (e.g. methylene blue) from water/wastewater. ENCC adsorbs a stoichiometric amount of cations equivalent to its carboxyl content and undergoes raft-like aggregate formation, which readily sediment out of the ion solution without requiring centrifugation(Figure 1.3A). ENCC has a significantly higher maximum removal capacity (∼185 mg g⁻¹, Figure 1.3B) than common industrial adsorbents, such as activated carbon (4.4 mg g⁻¹).⁷⁶  Subsequent desorption is required to recover the adsorbed materials from the nano-adsorbents. In industrial water/wastewater treatment or ion-recovery plants, packed beds of ENCC in the forms of hydrogels or aerogels as well as ENCC-coated surfaces may be used.²⁸  HCNC-based aerogels, developed by freeze-drying hydrogels that were produced from the Schiff base-mediated cross-linking of bifunctional (aldehyde and carboxyl modified) hairy nanocrystalline cellulose (BNCC) with carboxymethylated chitosan (Figure 1.3C) enabled the adsorption of 785 mg of methylene blue dye per gram of the aerogel (Figure 1.3D,).⁸⁰  These aerogels swell in water and adsorb positively charged contaminants, such as methylene blue, at a stoichiometric ratio.⁸⁰  ENCCs, which have a higher charge content than BNCCs, have been able to remove 1400 mg of methylene blue dye per gram.¹¹³  Composite hydrogel beads of ENCC and sodium alginate, cross-linked using calcium ions, have been developed to remove methylene blue dye (Figure 1.3E). These hydrogel adsorbents were able to remove 1250 mg of methylene blue dye per gram (Figure 1.3F), which may be used for the large-scale treatment of dye-containing aqueous environments.¹¹³  In addition, CNCCs are able to remove ∼250 mg of tartrazine dye per gram, which may be used to separate anionic compounds.

Polymer-mediated flocculation and bridging of colloidal dispersion/suspensions have a myriad of applications, including wastewater pretreatment.¹¹⁴  HCNCs have an excellent bridging capability for the flocculation of colloids as a result of their DCC chains (hairs). ENCCs and SNCCs flocculate precipitated calcium carbonate (PCC), and CNCCs have been used to flocculate microalgae and clay particles.^79,87,89,115  The rate of flocculation and floc size depend on the dosage of flocculants and flocculation mechanism, for example, charge neutralization and/or bridging. ENCCs added to PCC dispersions induce flocculation through charge neutralization with maximum flocculation at the isoelectric point (IEP). DCC added to PCC dispersions adsorbs to the surface of PCC particles and induces flocculation through charge neutralization. When both ENCC and DCC are introduced to PCC dispersions, the flocculation rate attains a second maximum value beyond the IEP because ENCC replaces adsorbed DCC on PCC particles, suggesting a complementary flocculation mechanism due to the bridging effect (Figure 1.4A, i).⁷⁹  SNCCs can adsorb to PCC particles using DAMC chains to induce flocculation.¹¹⁵  The flocculation rate increases with an increasing aldehyde content of SNCCs. Accordingly, pure DAMC chains yield the highest flocculation rate and floc size.¹¹⁵  SNCCs induce flocculation via DAMC chain-mediated bridging of PCC particles, resulting in the reduction of steric repulsion among SNCCs, followed by aggregate formation. However, SNCC is a less effective flocculant than ENCC: for flocculating 1 g of PCC, ∼10 mg of SNCC and only∼0.15 mg of ENCC are required (Figure 1.4A, ii).¹¹⁵ 

CNCC can flocculate microalgae (Chlorella sorokiniana) suspensions with a flocculation mechanism and floc strength that are highly dependent on the CNCC dosage.⁸⁹  There are two main microalgal attachment configurations, presented in Figure 1.4B: initially, as CNCC is added, tangential adsorption onto the microalgae surface is favored, and by increasing the CNCC concentration, orthogonal adsorption becomes prevalent due to the electrostatic repulsion between the tangentially adsorbed CNCCs and the new CNCCs. Orthogonal adsorption is mainly responsible for intercellular attachment. At IEP (27.5 mg L⁻¹ CNCC per 0.4 g L⁻¹ microalgae), there is a mixture of CNCC nanoparticles that adsorb tangential and orthogonal to the surface of microalgae. At high doses (above the IEP), mainly orthogonal adsorption of CNCCs takes place. Aggregates are formed at doses below and above IEP, suggesting that the flocculation is not solely governed by charge neutralization.⁸⁹  The ζ-potential of negatively charged microalgae monotonically increases from ∼ − 10 mV to∼10 mV as CNCC is added to the suspension (Figure 1.4C).

CNCCs have applications in the aggregation of other organic- or inorganic-based suspensions.⁸⁹  As an example, CNCC induces the flocculation of clay suspensions through charge neutralization in a kaolinite model system, studied using ζ-potential (Figure 1.4D).⁸⁷  The highest flocculation rate and maximum floc size are obtained near the IEP, suggesting that the CNCCs flocculate the non-clay through charge neutralization. Fractal dimension (D_f) of clay-CNCC flocs can be estimated by a regression model to describe the flocculation process. The D_f suggests that the relationships between CNCC dosage and floc conformation can be categorized into three main groups: (i) CNCC dosage below 7.5 mg g⁻¹ (D_f = 2) gives rise to the small flocs (<120 µm) of CNCCs adsorbed tangentially to the surface of clay far from IEP, (ii) CNCC dosage between 10–25 mg g⁻¹ (D_f = 1.6) yields larger flocs (>200 µm) with orthogonally adsorbed CNCCs to the clay particles, and (iii) CNCC dosage above 25 mg g⁻¹ (D_f ∼ 1.9) results in more compact flocs (<200 µm) with CNCCs adsorbed orthogonally to the surface of clay particles, as presented in Figure 1.4E.⁸⁷  Accordingly, CNCC dosage and addition intervals may be engineered to tailor the industry-specific properties of flocs, for example, for wet end retention aids.

Paper has a wide range of applications, spanning from food packaging to flexible electronics.^116,117  However, challenges such as the low mechanical integrity and resilience of paper over a wide range of relative humidifies limit their effectiveness in industries that demand mechanically robust substrates.¹¹⁸  Paper additives (e.g. latex¹¹⁹  and chitosan) have been used to improve the wet and dry strength of paper.¹¹⁸  ENCCs have been used as an environmentally friendly substitute for petroleum-based coating materials, such as latex, to produce reinforced paper nanocomposites. These nanocomposites were formed through the AZC-mediated cross-linking of starch nanoparticles (EcoSphere^® biolatex^®) and ENCCs under mild conditions (ambient pressure and 80 °C). The ENCCs provided mechanical reinforcement while the AZC cross-linking improved the paper resistance to humidity at elevated temperatures. Interestingly, ENCC maintains the low viscoelasticity of coating dispersions during the temperature-mediated cross-linking process and results in a more uniform, crack-free doping upon drying with up to∼10-fold enhancement in the mechanical properties of paper (Figure 1.5). Improved mechanical properties of the nanocomposites are observed within a broad range of temperatures and relative humidity at low concentrations of ENCC (e.g.1.5 wt%). The ability of ENCC to enhance the mechanical properties of paper may have applications in paper-based microfluidics, lab-on-a-chip platforms, flexible electronics, sensors, actuators, and energy-storage devices that require mechanically robust substrates.⁸² 

An interesting application of HCNCs is in retention aids. The wet end of the papermaking process involves a slurry of fibers, fillers, and other additives. Wet end additives are usually used to improve operation conditions (e.g. filler retention and drainage) and the properties of final paper, such as mechanical strength.¹²⁰  Retention additives are often cationic materials,¹²¹  including coagulants, polyacrylamides (PAM), bentonite, and starch. Chemical flocculation is integral in the wet end papermaking process for simultaneously achieving high retention and drainage rate.¹²²  CNCCs (e.g. 20 mg g⁻¹) have been used as retention agents in recycled paper production, improving the retention by 77% and decreasing the drainage time by ∼78% without detrimental effects on the mechanical properties of paper (<2% reduction in tensile strength).⁸⁸  Compared with other retention agents, such as cationic PAM (cPAM), polyvinylamine (PVA), and chitosan that decrease drainage time by more than 84%,¹²³  CNCCs had a lower reduction in drainage time. However, CNCCs yielded a higher filler retention (>98%) compared to cPAM (85%), PVA (82%), and chitosan (75%).^88,123  The ability of CNCCs to improve the total retention of fibers/fillers and drainage rate and to preserve the mechanical strength of paper may open new opportunities for developing single-component retention aids to simplify the wet end processes.

Biomineralization is a process in which living organisms produce minerals, such as calcium carbonate, with unique structural and/or morphological features.¹²⁴  Biomimetic mineralization is, in simplicity, following nature's designer templates to fabricate minerals that resemble naturally occurring polymorphs,¹²⁵  such as superior inorganic nanostructures and hybrid inorganic–organic nanocomposites.¹²⁶  Controlling shape and crystal structure plays a key role in designing biomimetic minerals.²⁸  As an example, some sea animals, such as teleost fish,¹²⁷  produce the least thermodynamically stable crystalline polymorph of calcium carbonate, that is, vaterite.¹²⁸  While adding CNC to a supersaturated solution of calcium and carbonate ions does not affect the formation of calcite, the most thermodynamically stable polymorph of calcium carbonate at ambient condition, ENCC/DCC-mediated mineralization of calcium carbonate yields microscale lenticular particles and macroscale nacre-like sheets of vaterite, as presented in Figure 1.6A and B.¹²⁶  Vaterite is produced when ENCC or DCC is mixed with a saturated solution of calcium and carbonate ions to form dicarboxyl–calcium complex that decreases the ζ-potential from ∼ − 70 mV to∼0 mV(Figure 1.6C).⁸¹  This binding prevents the formation of otherwise thermodynamically stable polymorphs of calcium carbonate, such as calcite, even at a low carboxyl-to-calcium concentration (as low as ∼0.0009 mol mol⁻¹).¹⁴  DCC concentrations as low as 0.33 ppm (charge density of ∼5.7 mmol g⁻¹) can stabilize vaterite even in the supersaturated solution of calcium carbonate ([COO⁻]/[Ca²⁺] ∼ 0.001).¹²⁹  ENCC and DCC introduce a promising benchmark for biomimetic mineralization with a ppm-level efficiency under ambient conditions, enabling excellent polymorph selectivity and macroscale mineralization.¹²⁶ 

The formation of sparingly soluble salts (scales), such as calcium carbonate, and their adhesion to unit operations in numerous industries, such as oil, gas, and water treatment plants, leads to economic losses and sometimes environmental hazards.¹³⁰  Scaling typically occurs in aqueous solutions containing high levels of hardness and alkalinity. Antiscalants are additives designed to inhibit the formation and precipitation of scales in pipes, heat exchangers, desalination facilities, and other unit operations in contact with water.¹³⁰  Common industrial antiscalants used for inorganic scale inhibition are not environmentally friendly, leading to extensive efforts for designing green alternatives, such as polymaleates (PMAs), polyaspartates (PASPs), and polyepoxysuccinates (PESAs).¹³⁰  HCNCs have been engineered to develop threshold antiscaling additives, coatings, and membranes.^81,83,129  As an example, the addition of less than 10 ppm of ENCC results in near complete scale inhibition in supersaturated calcium carbonate solutions.¹²⁹  Increasing ENCC concentration and carboxyl content increases the antiscaling efficiency of ENCCs (Figure 1.6D).¹²⁹  A quartz crystal microbalance with dissipation monitoring (QCM-D) study showed that coating plain silica surfaces with ENCC/DCC can result in scale-resistant interfaces, with ∼700% enhancement in resistance against calcium carbonate scaling. Low (ppm) concentrations of ENCC or DCC additives almost completely inhibit scale deposition on the QCM-D sensors submerged in saturated calcium carbonate solutions ([COO–]/[Ca²⁺] < 0.001), as shown in Figure 1.6E.⁸³  It has been shown that the addition of ENCC to polymer matrices may mitigate the scaling of calcium carbonate and increase the membrane lifetime by 300%, even at harsh electrochemical conditions.^81,129  It was hypothesized that the addition of ENCC (e.g. 0.4 wt%) to a cellulose acetate (CA, 2 mg mL⁻¹) membrane casting solution provided anionic sites at the membrane–electrolyte interface, attracting cations and then anions to stabilize calcium carbonate polymorphs, while impairing further cation diffusion from the bulk solution toward the interface, as schematically presented in Figure 1.6F.⁸¹ 

CNCs are able to form nano-/microscale structures with various sizes, shapes, and surface properties in fluids, which affect the rheological properties of dispersions/suspensions. Molecular interactions between the nanocrystals and medium as well as the colloidal interactions among the nanocrystals can be engineered to tune rheological properties. HCNCs and conventional nanocelluloses with varying surface functional groups regulating colloidal interactions can be engineered to tailor the interparticle attractive or repulsive forces, as shown in Figure 1.7A. HCNCs (e.g. ENCC and SNCC) and conventional nanocelluloses (e.g. CNC and CNF) have been used to tune the rheological properties (e.g. storage and loss moduli) of metal-based[e.g. AZC] hydrogels by several orders of magnitude (Figure 1.7B and C). Steric and electrosteric interactions between functionalized polymer chains (hairs) on HCNCs cause colloidal repulsion between nanoparticle-adsorbed zirconium carbonate (ZC), impairing network formation, which yield ultrasoft hydrogels with up to five decades decrease in the viscoelastic moduli using only 0.5 wt% of HCNCs compared with the HCNC-free gel. HCNCs with higher charge content produce weaker gels due to stronger electrostatic repulsion among the particles; however, cleaving polymer chains reduces electrostatic repulsion, allowing for improved ZC–ZC cross-linking, which forms stiffer networks with a much faster cross-linking initiation (activation) time (Figure 1.7D). Conventional nanocelluloses have a significantly lower surface charge than charged HCNCs and are less affected by colloidal repulsion, allowing attractive forces to dominate ZC–nanocellulose network formation, which results in an increase in the viscoelastic moduli. CNFs modified with hydrogen donors/acceptors, such as amidoethanol, can increase the viscoelastic moduli by a factor of 40 using 5 wt% of nanoparticles. Together, HCNCs and/or conventional nanocelluloses with tailored surface functional groups can be used in industries that rely on the rheological modification of fluids (e.g. reducing viscosity for enhanced transport/injection or increasing it for better casting/3D printing), providing a nano-toolbox for the colloidal-assisted engineering of macromolecular systems with finely tuned viscoelasticity.

HCNCs have been engineered to produce long-lasting, non-leaching antibacterial nanostructures.¹³¹  Antibacterial SNCCs have been prepared through the immobilization of antibacterial agents, such as enzymes (lysozyme) and polycyclic antibacterial peptides (nisin) onto the nanoparticles via the aldehyde–amine Schiff-base reaction without requiring any linker or activator.⁷³  The minimum inhibitory concentration (MIC) of conjugated SNCCs against pathogenic bacteria Bacillus subtilis and Staphylococcus aureus is higher than free lysozyme and nisin (e.g. 4 ppm and 12 ppm for nisin and SNCC-nisin, respectively, against B. subtilis).⁷³  Interestingly, while free nisin becomes ineffective against S. aureus after 24 h, the immobilization of this antibacterial agent onto SNCCs retains its activity.⁷³  Some micro-/nanocelluloses have shown antibacterial properties without functionalization with antibacterial agents.¹³¹  Dialdehyde microcrystalline cellulose, prepared by the NaIO₄ oxidation of microcrystalline cellulose, has shown some antibacterial activity against Gram-negative and Gram-positive bacteria.¹³²  Increasing the aldehyde content of these microcrystalline celluloses resulted in an increase in antibacterial activity with a MIC as low as 15 mg mL⁻¹ for S. aureus, B. subtilis, and E. coli, and 30 mg mL⁻¹ for S. typhimurium, at an aldehyde content of 6.5 mmol g⁻¹.¹³²  Carboxyl-modified CNCs do not have antibacterial properties; however, they may be utilized for biomacromolecule immobilization/delivery and as carriers for antibacterial agents.^21,133  ENCC can potentially be utilized for this purpose due to its high surface charge, which increases colloidal stability in physiological conditions.¹³¹  CDAMC fibers, produced by reacting Girard's reagent T with dialdehyde-modified nanofibrillated cellulose have been used to produce foams with antibacterial activity against E. coli.¹³⁴  These cellulose nanostructures may have applications in water purification, antibacterial wound dressing, filters, and coatings.¹³¹ 

Nanocelluloses have been used as biorenewable, biodegradable, non-toxic, and abundant building blocks to develop a broad range of advanced materials and devices. This class of nanomaterials, categorized in three main groups (CNFs, CNCs, and HCNCs), is typically isolated from cellulose fibrils via physical and/or chemical routes. This chapter reviews the synthesis and applications of HCNCs, a newly emerged class of nanocelluloses made up of a CNC-like crystalline body sandwiched between highly functionalized amorphous cellulose chains (“hairs”). While conventional CNCs are produced from the strong acid hydrolysis of cellulose fibrils, HCNCs are synthesized via the controlled oxidization and cleavage of fibrils’ amorphous regions. Depending on the functional groups of hairs, HCNCs are categorized into three groups: electrically neutral (SNCC, sterically stabilized nanocrystalline cellulose), anionic (ENCC, electrosterically stabilized nanocrystalline cellulose), and cationic (CNCC, cationic-electrosterically stabilized-nanocrystalline cellulose) HCNCs. The hairs are more accessible than the inner crystalline regions, providing HCNCs with more reactive sites than CNCs that can undergo facile functionalization under mild conditions. The biopolymeric hairs on HCNCs enable higher functional group density and colloidal (steric/electrosteric) stability compared with conventional nanocelluloses (CNCs and CNFs). In addition, highly functionalized hairs facilitate the physical and/or chemical assembly of HCNCs, often used to develop 2D/3D soft material platforms, such as films, hydrogels, and aerogels. Moreover, the hairs inhibit the liquid crystal self-assembly, improve flocculation, and significantly alter the rheological properties of HCNC dispersions and enable scale inhibition and biomimetic mineralization. HCNCs are ideal candidates for surface modification, rendering them suitable for environmental remediation, healthcare applications, food security, and energy storage. This chapter has provided a comprehensive overview of HCNC synthesis and emerging applications pertinent to the water–energy–health–food nexus.

A. S. would like to acknowledge the startup fund from The Pennsylvania State University.