Table 3 The extraction methods of nanocellulose using top-down approach

Mechanical

High-pressure homogenization process

It entails utilizing force to drive a cellulose pulp into a vessel through a very small nozzle using a piston under high pressure and then subjected to a pressure drop to atmospheric conditions upon exiting the valve, resulting in significant shear stresses on the fiber surface.

PothulaLalithaKumar et al. (2016); Kargarzadeh et al. (2017); Etuk et al. (2018)

Grinding

High energy input is necessary to decrease small particles with mechanical force to even smaller sizes. The cellulose slurry is repeatedly transferred between a static husking stone and a rotating husking stone spinning at around 1500 rpm until the necessary dimensions in the nano-range are reached and further size reduction is no longer possible.

PothulaLalithaKumari et al. (2016); Khalil et al. (2014); Etuk et al. (2018)

Sonication

It is a method of agitating particles or discontinuous fibers in a liquid by using sound energy; because ultrasonic frequencies (more than 20 kHz) are commonly employed, the technique is also known as ultrasonication. Ultrasonic waves are produced in a liquid suspension by either directly sonicating the suspension with an ultrasound probe or “horn” or by submerging the sample container containing the suspension in a bath of an ultrasonic-wave-propagating liquid (indirect sonication).

Chung D. (2016); Sandhya et al. (2021); Taurozzi et al. (2012)

Microfluidization

The microfluidizer, unlike the homogenizer, functions at a constant shear rate rather than a constant pressure. To increase pressure, an intensifier pump is employed, and an interaction chamber is employed to defibrillate the fibers by providing shear and impact pressures against colliding streams and channel walls.

Kargarzadeh et al. (2017); Khalil et al. (2014); Kaur et al. (2021)

Cryo-crushing

Cryocrushing is an alternative method for producing nanofibers in which fibers are frozen using liquid nitrogen and high shear forces are then applied. Cellulosic fibers are soaked in liquid nitrogen before being shattered with a crusher and pestle. A standard cryo-crusher has two husking stones, a stator, and a rotor that can rotate at 1500 rpm.

Chirayil et al. (2014); PothulaLalithaKumari et al. (2016); Frone et al. (2011); Kargarzadeh et al. (2017); dos Santos et al. (2016); Etuk et al. (2018)

Chemical

Acid hydrolysis

In the process of acid hydrolysis, the amorphous portions are typically hydrolyzed, whilst the crystalline parts remain unaffected by the acid treatment. This characteristic is caused by the crystalline areas’ high tolerance to acid treatment than the amorphous parts.

Chen et al. (2019); dos Santos et al. (2016); Onu and Mbohwa (2021)

Alkali Hydrolysis

Similar to acid hydrolysis, alkali hydrolysis involves an enzymatic attack on amorphous areas of the cellulose substrate while staying in the crystalline sections. These treatments are typically performed with diluted NaOH (1–10%) concentrations at low or high temperatures, and concentrated NaOH solutions above 10% at low temperatures only. In rare cases, NH₄OH and anhydrous NH₃ (gas or liquid) are also used to activate organic molecules, resulting in an increase in hydrolytic breakdown.

Hubbe et al. (2008); Hua et al. (2016); Frone et al. (2011)

Enzymatic hydrolysis

Enzymatic hydrolysis involves the employment of enzymes to aid the breaking of bonds in organic molecules, which is followed by the addition of water molecules. These enzymes are fungal in origin and are known as cellulase in the case of cellulose.

dos Santos et al. (2016); Etuk et al. (2018); Madhu (2022)

Mechanical-chemical

The chemical–mechanical method combines one or more chemical pretreatment procedures with mechanical disintegration techniques.

Etuk et al. (2018)