*2.1. Fundamental of Nanocellulose*

Cellulose, which was first extracted from wood by Enselme Payen in 1838, is a polysaccharide consisting of β-1,4-linked anhydroglucopyranoside units, in which every monomer unit is corkscrewed at 180◦ compared to its neighbors [5,57]. The annual production of this abundant biopolymer is estimated to be between 1010 and 10<sup>11</sup> tons-per-year. It can be isolated from different sources such as wood, herbaceous plants, grass, crops and their byproducts, bacterial, algae and animal sources, among others [58–61]. The properties of this biomacromolecule are closely related to the natural source, its maturity and origin, pretreatment methods, processing approaches and reaction conditions. Typically, lignocellulosic biomass necessitates the removing of non-cellulosic constituents such as extractives, lignin and hemicellulose [62]. Trache et al. have recently reviewed the common pretreatment and processing methodologies, which allow the obtaining of pure cellulose from lignocellulosic [13]. As depicted in Figure 1, cellulose is semi-crystalline in nature, hence it contains both crystalline and amorphous regions. This latter is susceptible to hydrolysis; it can steadily be eliminated to generate crystalline parts upon phase segregation and is more prone to react with other molecular groups [57,63,64]. The intra- and intermolecular chemical groups (Figure 1) confer to this fascinating polymer its specific features such as infusibility, chirality, hydrophilicity, insolubility in several aqueous media and ease of chemical functionalization [65]. Cellulose can be classified into various polymorphs, that is, cellulose I, II, IIII, IIIII, IVI and IVII, which can be converted from one form to another through chemical or thermal treatments [42].

In their nano-size form, cellulose nanomaterials, also known as nanocellulose (NC), display outstanding physical, chemical, biological, magnetic, electrical and optical characteristics compared to the bulk materials [1,11,66]. Conceptually, NC can be produced by a top-down hydrolysis methodology through different steps, that is, (i) pretreatment processes of lignocellulosic biomass employing physical approaches concerning crushing, screening, washing and cooking to eliminate coarse particles, oily content and dust from the material surface, (ii) removing extractive/hemicellulose/lignin via chemical, physical, physicochemical, biological or the combination of two or more treatments, (iii) fragmentation and cleavage of cellulosic elementary fibrils or micro-fibrils to generate nanofibers through various approaches and (iv) post-treatments such as solvent removing, dialysis, sonication, centrifugation, surface modification, stabilization and drying. The three latter steps have received great interest from the scientific community for designing products with desired features [12,13,23,27,38,67–73]. NC possesses excellent useful properties such as renewability, eco-friendliness, biocompatibility,

non-toxicity, hydrogen-bonding capacity, tunable crystallinity, high chemical resistance, tailored aspect ratios (100–150), low thermal expansion coefficient, reactive surface, low density (1.6 g/cm3), high specific surface area (100–200 of m2/g), high tensile strength (7.5–7.7 GPa) and elastic modulus (130–150 GPa) [10,30]. This promising polysaccharide has received tremendous attention during the last two decades in a wide range of applications such as sensors and biosensors, energy storage systems, oil and gas drilling and cementing, papermaking, filtration, decontamination, adsorption, separation, wood adhesives, Pickering emulsifiers, medical and nanocomposites, to cite a few [13,14,16,20,35,36,42,74–76]. Depending on the isolation method, morphology and size, NC is principally categorized into: (i) cellulose nanostructured materials such as cellulose microfibrils and microcrystalline cellulose and (ii) cellulose nano-objects, also known as nanofibers, such as cellulose nanocrystals (CNC), cellulose nanofibrils (CNF) and bacterial nanocellulose (BC).

**Figure 1.** Hypothetical schematic of dilute acid pretreatment process to extract the crystalline regions of cellulose from amorphous domains. In the middle, the configuration of cellulose repeating unit with the β-(1,4) glycosidic linkage under the effect of intra/intermolecular hydrogen bonding is denoted. Reproduced with permission from Reference [36]. Copyright ©2019, Elsevier.

In contrast to nanostructured materials, nanofibers present more uniform particle size distribution, high specific surface area, amphiphilic nature, barrier properties, high crystallinity and tend to produce more stable self-assembled structures such as hydrogels and films [1,30,77]. In recent years, other types of nanofibers appeared such as amorphous nanocellulose, cellulose nanoyarn and cellulose nanoplatelets [17]. Several in-depth reviews with detailed discussions dealing with NC-based materials have been reported over the past few years, covering the nanocellulose sources, isolation methods, structure modification, potential uses, advantages and shortcomings [11,16,20,23,35,36,38,40,41,78–83]. In the following, we concisely go through current extraction methods of CNC and describe their outstanding features.
