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Extraction, Preparation and Characterization: Nanocellullose

Nanocellulose the organic nanomaterial is an emerging and promising material with exceptional properties and a broad spectrum of potential applications in numerous fields, i.e. pharmaceutics, biomedical, food ingredients, cosmetic additives, packaging materials, hygiene products, film and absorbance media, electronics, barrier films, membranes, nanocomposites and supercapacitors etc. Due to its unique structural features and remarkable physicochemical properties such as biodegradability, biocompatibility, renewability, low density, optical transparency, adaptable surface chemistry and improved mechanical properties nanocellulose has drawn tremendous attention in academia as well as in industrial sector. The main aim of this article is to provide an overview of recent research in the field of nanocellulose production from different pre treatments and methods.


Introduction
Cellulosic materials (cellulose, hemicelluloses and lignin) due to their unique characteristics viz. biodegradability and renewability, low cost as well as low carbon dioxide release (Abdul Khalil et al., 2016) have been categorized as sustainable, green and environmental friendly materials (Maleki et al., 2017). Utilization of these cellulosic materials generated from different agricultural wastes to produce nanocellulose is a feasible approach (Elias et al., 2017).
The nanocellulose obtained from such biomaterials are equipped with superior characteristics which are high surface area, nanoscale dimension, high crystallinity, optical properties and stiffness alongwith biodegradability and renewability (Siro 2010; Abraham et al., 2011; Trache et al., 2017) and they also combine significant properties of cellulose such as crystalline structure, hydrophilicity and broad chemical modification (Mahfoudhi & Boufi 2016). Reallocation from cellulose to nanocellulose based materials is a productive method so far (Kalia et al., 2014;Khalil et al., 2014).
Cellulose is the most abundant polymer which is renewable and biodegradable (Jabbar et al., 2017). Cellulose is a linear homopolysaccharide composed of β-D-glucopyranose units linked together by β-1-4-linkages ( Kumar et al., 2014). Each β-1-4 glucopyranose bears three hydroxyl groups where the primary OH group is located at the C-6 position and the secondary OH group is located at the C-2 and C-3 position. The secondary and the primary hydroxyl groups of the anhydro-glucose rings are the basis for extensive intra and intermolecular hydrogen bonding (Alemdar & Sain 2008) allowing the formation of highly ordered three-dimensional crystal structures; therefore making cellulose a semi-rigid polymer. As, Cellulose is linked by β (1→4) glycosidic linkage in a long chain therefore its properties are directly related to the chain length (Horn et al., 2012). One of the properties of cellulose is water insoluble, due to its glycosidic linkage between the monomers. Cellulose derived from plants has hemicellulose, lignin, pectin and other substances.
Hemicellulose is a branched amorphous polysaccharide that comprises about 20% of the biomass of most plants. In addition to glucose hemicellulose is derived from several sugars like mannose, galactose, rhamnose, arabinose and especially xylose ( Moran et al., 2008). Lignin the most abundant plant-derived amorphous polymer is an organic substance having dendridic network of phenyl propane which bind the cells, fibres and vessels which constitute wood and the lignified elements of plants. After cellulose, it is the most abundant renewable carbon source on earth. Lignin is a complex molecular structure containing crosslinked polymers of phenolic monomers especially pcoumaryl alcohol, coniferyl alcohol and sinapyl alcohol. Pectin is a heteropolysaccharide of 1-4 linked galacturonic acid with methyl esters of different sugar units (Klemm et al., 1998;Eichhorn et al., 2001). In plant cell wall structure, crystal and amorphous cellulose structure form nano/micro fibers, which together produce cellulose fiber (Eichhorn et al., 2010). Therefore, cellulose contains highly ordered crystalline alternative with less ordered amorphous segments (Sjostrom E 1993). The amorphous segments can be removed to produce highly crystalline cellulose (Azizi et al., 2005). Cellulose occurs in four major polymeric forms: cellulose I, II, III and IV (Giri & Adhikari 2013). Among them cellulose I is natural cellulose with structures I α and I β . Cellulose I α has a single chain triclinic structure and I β has two chain monoclinic structure (Kukle et al., 2011). Cellulose in regenerated cellulose fibers is cellulose II that can be formed whenever the lattice of cellulose I is destroyed as on swelling with strong alkali or on dissolution of cellulose (Kamel 2007). The conversion of cellulose I to cellulose II is irreversible, suggesting that cellulose I is metastable and cellulose II is stable. With various chemical treatments it is possible to produce the structures of cellulose III on heating cellulose I and II with ammonia and cellulose IV on heating cellulose III ( Park et al.,

Preparation of nanocellulose
Generally, preparation of nanocellulose is carried out in two steps. The first step involves the pretreatment of raw material (depending upon the source) to produce purified individual cellulosic fibres that can be further processed. The second step is the transformation of individual cellulosic fibres into microfibrils, nanofibres or nanocrystals (Garcia et al., 2015). Various methods have been suggested by different researchers to synthesize nanocellulose.

Enzymatic Hydrolysis
To understand the enzymatic hydrolysis of cellulose, most significant is (i) the structural behavior of cellulosic biomass and (ii) biological barriers for enzyme interactions. Enzymes introduced desired functional groups on the surface of nanocelluloses (Karim et al., 2015). Enzymes are employed to alter hemicelluloses and lignin contents while maintaining cellulose portion (Janardan & Sain 2006 ; Khalil et al., 2014). Enzymatic treatments facilitate the fabrication of cellulose nanocrystals with reduced energy consumption. The enzymes involved are cellobiohydrolases and endoglucanases. Cellobiohydrolases (the main enzymes) are able to attack highly crystalline cellulose and endoglucanases require some disorder in the structure to perform cellulose degradation. These enzymes show strong synergistic effects (Henriksson et al., 2007).

Oxidation
The TEMPO-mediated (2,2,6,6-tetramethyl-1-piperidine-1-oxyl) oxidation is the most widely used method to isolate nanofibres or nanocrystals. The carboxylate and aldehyde functional groups are put into the C 6 glucose unit of cellulose structure under moderate conditions (Iwamoto et al., 2011). A primary oxidizing agent (sodium hypochlorite) and an additional catalyst sodium bromide or an iodine are employed at pH 9-11 (Saito et al., 2009).

Electrospinning
Electrospinning is simple, efficient and cost effective processes which formulate nanofibres from non-woven to the order of few nanometers with advanced mechanical properties, large surface area and ease of functionalisation. UV irradiation in concentrated crystalline liquid solutions containing isotropic solvents like acetone, ethanol, methanol, dimethylformamide or water are generally used (Christoforou 2010). Over the past few years, there has been a tremendous intensification of the research activities to explore electrospinning for nanofibers formation involving a large variety of materials. It operates on the principle that under the action of a high electric field a solution is extruded electrospun. Once the voltage is sufficiently high, a charged stream of matter is ejected following a rather complicated loop and 3D spiral deployment trajectory (Rebouillat & Pla 2013). During this, the solvent evaporates leaving behind randomly oriented nanofibers accumulating on the collector (Fang et al., 1997). For visualizing morphological features of the electrospun nanofibers different parameters such as electric field strength, tip-to-collector distance, solution feed rate and composition are used. To produce cellulose nanofibres by electrospinning technique have been used by ( Frey & Joo., 2005) where cellulose fibers have been dissolved in solvents, such as ethylene diamine, with a salt selected from the group consisting of potassium thiocyanate, potassium iodide and mixtures thereof, the salt being present at their saturation points.

Mechanical Processes
Different mechanical processes have been used to produce microfibrillated cellulose and cellulose nanofibres. These include refining followed by high-pressure homogenization, cryocrushing, grinding, and high intensity ultrasonic treatments.

Refining and High pressure homogenization
Refining is a pretreatment before high pressure homogenization (Stenstad et al., 2008). For refining to be carried out, a disk refiner is generally used in which a diluted cellulosic suspension is forced through a gap between a rotor and stator. The disks surfaces are grooved and fitted with bars to subject the fibers to repeated cyclic frictional stresses. This mechanical treatment leads to irreversible alterations in the fibres, increasing their bonding potential by modification in their morphology and size (Herrick et al., 1983).
High pressure homogenization is a process in which a large pressure drop facilitated the micro fibrillation and fed through a spring loaded valve. As this valve opens and closes at a fast rate, the fibers are exposed to a large pressure drop with shear and impact forces which alongwith high velocity reduce the size of fibers to nanoscale (Frone, et al., 2011). High pressure homogenization is among the proficient methods of refining cellulosic fibres because of its high efficiency, simplicity and without organic solvents requirement (Keeratiurai & Corredig, 2009).

Cryocrushing
Cryocrushing is an alternative method for producing cellulose nanofibers (

Grinding
Grinding is another approach to liberate cellulose nanofibers by rupturing cellulose. The cellulose suspension is passed between the static and the rotating grinding stones of the grinder set at ~1500 rpm. In this process, the cell wall structure and hydrogen bond is

High intensity ultrasonication
High intensity ultrasonication is a mechanical process which promotes the isolation of cellulose nanofibres with oscillating power and hydrodynamic forces of ultrasound (Cheng et al., 2009). In this process, cavitation leads to a powerful mechanical oscillating power and when molecules absorb ultrasonic energy (Chen et al., 2013) high intensive wave aids in formation and expansion of microscopic gas bubbles.

Microfluidization
Microfluidization is the process similar to high pressure homogenization which can be used to produce cellulose nanofibres. Microfluidizer includes intensifier pump to increase the pressure and interaction chamber to defibrillate the fibers using shear and impact forces against colliding streams and the channel walls (Ferrer et al., 2012). Aspect ratio of cellulose fibers increases when they pass through the microfludizer (Lee et al. (2009).

Steam explosion
Steam explosion is a thermo mechanical processe. Due to the high pressure, steam penetrates into the cellulose fibers and shear force is generated by sudden pressure release which hydrolyze the glycosidic and hydrogen bonds resulting in the formation of cellulose nanofibers (Giri & Adhikari, 2013: Cherian et al., 2010).

Chemical Treatment
Chemical treatment is the most efficient and easy method to isolate cellulose nanofibres. Alkali treatment, bleaching treatment and acid hydrolysis together disintegrate the cellulosic fibres to nanofibres. Different alkalis remove hemicelluloses and lignin thereby purifies the cellulose. Bleaching agents remove remaining hemicellulose and lignin (Kaur et al., 2017). Oxidative fragmentation of lignin takes place and some of the lignin form lignin chloride which is easily dissolvable. Highest percent of cellulose content is observed after bleaching. Acid hydrolysis induces individualization of microfibrils from the cell wall structure. With H 2 SO 4 , surface charged sulfates are formed, promoting the dispersion of nanocrystals ( Revol et al., 1992). With H 2 SO 4 aggregation of Cellulose nanofibres has been observed. Combination of H 2 SO 4 and HCl during hydrolysis generates spherical nanoparticles due to less sulfate groups on their surface with improved thermal stability (Wang et al., 2007).

Characterization
Conventionally, microscopic methods have been used as advantageous tool to analyze the appearance, morphology, sizes and shapes of microfibres and cellulose nanofibres produced from various cellulosic raw materials (  The surface morphology of cellulose nanofibres is generally analysed by scanning electron microscopy (SEM). In this technique samples are placed on the aluminium stub and incubated in the oven at 60 0 C. Then the samples are coated with gold using a vaccum sputter coater. The accelerating voltage taken as 15kV. FE-SEM imaging can also be achieved by using low acceleration voltages without the conductive layer. The morphology of the regenerated cellulose nanofibres can also be carried out by transmission electron microscopy (TEM). A drop of a dilute aqueous suspension (0.1 wt%) is deposited on the surface of a copper grid coated with a thin carbon film. The accelerating voltage to be kept as 100-120 kV. Surface topography and morphology can also be determined by atomic force microscopy (AFM) using tip broadening effect where usually a drop of dilute suspension is placed on a mica surface and left to dry. The structural properties of the cellulose nanofibres like size and crystallinity index can be characterized using x-ray diffraction. The thermal stability of cellulose nanofibres can be determined by thermogravimetric analysis. Decomposition of fibres occurs at different temperatures. Fourier transform infrared spectroscopy measurements are carried out with finely ground sample is mixed with potassium bromide, KBr which is then compressed to

Conclusion
Nanotechnology has the ability to manipulate and acquaintance at microscopic stratum. Bio-based nanocellulose has been widely considered as one of the most promising nanomaterials, which has phenomenal applications in industrial sector and in different fields. Nanocellulose is an environmental-friendly material that could serve as an indispensable renewable resource for progressive revitalization which has no harmful effects on human's health and on our environment. This review has provided an explanation of several aspects that are important for understanding of various nanomaterials.