**3. Cellulose nanofibers (CNF)**

#### **3.1 Background**

The cell wall of plants contains cellulose, hemicelluloses and lignin. The lignin component accounts for 10–25% by dry weight and acts as a binder between cellulose and hemicelluloses. It is lignin that enhances rigidity, durability, and protects the cell wall thereby acting as a binder. The other two major components of the plant cell wall are cellulose and hemicelluloses that comprise 35–50% and 20–35%, respectively, of the dry weight of lignocellulosic biomass. Cellulose is the linear polysaccharide with repeating units of cellobiose (disaccharide *d*-glucose) units linked by β-1,4 linkage. Hence, there are strong intramolecular or intermolecular hydrogen bonds between adjacent glucose units in similar or different chains through the open hydroxyl groups present in glucose monomer units. The hemicellulose in plant cell walls is mainly composed of xylan and glucomannan, which are pentose and hexose monomers linked by short or branched chains. The compact structures of hydrogen bonding are tightly packed networks in cellulose fibers that provide antibacterial properties, toughness, strength, and water or solvent impermeability to the plant cell wall. Nanocellulose is a fiber or crystal-based form of cellulose measuring a few micrometers in length and diameter < 100 nm and typically extracted from plant, animal, mineral and other natural sources. It is biodegradable, lightweight with a density of about 1.6 gm/cc with 10 GPa of high tensile strength similar to cast iron. It also accommodates reactive hydroxyl groups which are ideal for surface functionalization for various applications. The three types of nanocellulose materials are cellulose nanofibers (CNF), cellulose nanocrystals (CNC) and bacterial nanocellulose (BNC) [36]. **Table 2** shows the chemical composition of lignocellulosic materials derived from various agricultural wastes.

#### **3.2 Methods of cellulose nanofibers extraction**

Numerous studies on the use of agricultural waste (biomass) as sources of CNF have been widely discussed over decades. The processes for extracting


**Table 2.**

*Chemical composition of lignocellulosic materials from various agricultural wastes.*

nanocelluloses (NCs) from biomass are categorized into two main procedures: biomass pre-treatment before NC extraction and method of isolation of NC [36].
