2.4 Applications of Cellulose Nanoparticles
Evolution of materials based on CNCs with indulging properties and their ability to be modified makes it flexible to be used in a wide array of applications in our daily lives.
2.4.1 Bio-based applications
Cellulose nanocrystals can be used as an antibacterial agent when properly derivatized. Common method used is by hydrolysing cellulose in strong acids, such as sulphuric acid. Sulphuric acid reacts with the surface of cellulose comprising of hydroxyl groups and produces sulphate ions, which then improve the colloidal stability. Due to the presence of negative charged surface, it also enables the conjugation of sulphated CNCs with cationic biomolecules through electrostatic interactions (Sunasee et al., 2016). As a result, CNCs could be further modified and tailored for highly desirable biological applications.
Silver nanoparticles (AgNPs) is known for its antibacterial properties. It is widely used for water purification (Dankovich & Gray, 2011), food preservation (Mohammed Fayaz et al., 2009), wound dressing (Rujitanaroj et al., 2008) and cosmetics (Kokura et al., 2010). It also has low toxicity, high thermal stability and low volatility (Durán et al., 2007; Z. Shi et al., 2015). CNC modified with AgNPs can be used to inhibit viral infection However, they tend to clump together when introduced to aqueous solution (Sunasee et al., 2016). To ensure that the nanoparticles are stable in the CNC matrix, the surface of CNC need to be modified (Z. Shi et al., 2015). To enable the adsorption for most metallic nanoparticles is enabled.
Besides AgNPs, rosins are used to bind with CNCs. Rosin and its derivatives have been utilised in paper sizing agents, emulsifiers, surface coatings, chewing gums, insulating materials, additives for printing inks and microencapsulating materials (de Castro et al., 2016; Lee & Hong, 2002; Mandaogade et al., 2002; Wilbon et al., 2013). The preparation of an antimicrobial CNC by grafting rosin on its surface is reported by Castro et al. (2016). They tested the rosin modified CNC for its antimicrobial properties on E. coli and B. subtilis. A promising result against the Gram-positive B. subtilis is observed. However, it was less effective against the Gram-negative E. coli due to different interactions of their cell walls.
Porphyrin functionalised cellulose nanocrystals can also be used in photobacterial applications. When porphyrin is exposed to white light, it makes a singlet oxygen which can be an effective bactericide (Grishkewich et al., 2017). The advantage of a covalently bonded porphyrin to the surface of CNCs is that it will be able to add a longer lasting or permanent antimicrobial properties to the cellulose and reduce the leaching of biocidal agent towards the environment (Feese et al., 2011). These modified porphyrin CNCs could be applied as photobactericide on fabrics, coatings, papers, food packaging and healthcare products (Grishkewich et al., 2017)
2.4.2 Drug delivery
Common forms of nanocellulose-based drug carriers can be mainly divided into microparticles, hydrogels and membrane films (Lin & Dufresne, 2014). The presence of CNC in alginate-based microparticles showed a more consistent swelling patterns, better encapsulation efficiency and a sustained release profile of the drug.
Charged particles such as ion-exchange resins are favourable because they can bind to acidic or basic drugs that release at an extremely fast rate due to the presence of their counterions in the body fluid (Lam et al., 2012). For example, chitosan contains amine groups in its structure. It is often used as in the drug delivery system because the amine groups can bind strongly to the negatively charged drugs, such as antisense and oligonucleotides once protonated (Lam et al., 2012). CNC is still being tested as a possible nanomaterial for targeted drug delivery on account of its outstanding physical properties suitable for internalisation within the cells.
Nanocellulose is suitable for photonic applications due to the liquid crystalline behaviour of CNCs which gives rise to iridescent films of defined optical character (Abitbol et al., 2016). CNCs can form chiral nematic, iridescent coloured films simply by evaporation of aqueous suspension (Abitbol et al., 2016; Mu & Gray, 2014, 2015). Besides that, nanocellulose can be made compatible with both hydrophilic and hydrophobic components, acting as a host for optically active nanoparticles (Abitbol et al., 2016).
2.5 Treatment of Nanocellulose
Treatment of cellulose fibres boost accessibility, increase inner surface, improve crystallinity, breaks hydrogen bond and stimulates reactivity of cellulose (Kargarzadeh et al., 2017; Khalil et al., 2014; Šturcová et al., 2005). It is a crucial modification step as it can alter the structural organization, crystallinity and polymorphism of cellulose (Kargarzadeh et al., 2017; Mariano et al., 2014).
Grinding is a treatment that breaks down cellulose into nanosized fibres (Abdul Khalil et al., 2014). The objective of a fibrillation mechanism in grinder is to separate the hydrogen bonds and cell wall by shear forces (Khalil et al., 2014; Siró & Plackett, 2010). Lignocellulosic biomass such as plants are composed of complex and rigid cell walls which must be repeatedly ground to achieve uniform-sized nanofibrils (Iwamoto et al., 2007; Nair et al., 2014).
2.5.2 Alkali treatment
In an alkali treatment, cellulose fibres are exposed to concentrated strong base such as sodium hydroxide solution (Acharya et al., 2011; Majeed et al., 2013; Ng et al., 2015). The main purpose of this treatment is to eliminate certain amount of hemicellulose and other contaminants covering the exterior cell wall (Littunen et al., 2013; Ng et al., 2015; Zainuddin et al., 2013).
Removal of hemicellulose along with hydroxyl groups of fibres ensures a production of finer fibrillar structures with new reactive sites (Ng et al., 2015), resulting in a better interfacial adhesion between the fibres and polymer matrix (Hernandez & Rosa, 2016). Moreover, the alkalization of cellulosic materials induce crystallinity (Majeed et al., 2013; Ng et al., 2015), decrease surface tension (Hernandez & Rosa, 2016) and elevate the number functional sites (Hernandez & Rosa, 2016; X. Li et al., 2007).
Bleaching removes residual materials, mainly lignin (Ng et al., 2015; Panaitescu et al., 2013; J. Shi et al., 2011). It is often repeated to ensure lignin is completely removed. The removal of components from fibres affect their thermal stability and the end-product directly since the cellulose will be more accessible to acid attack during the subsequent treatment (Chen et al., 2015; Malucelli et al., 2017; Q. Wang et al., 2014; X. Wang et al., 2014). However, chemicals involved in bleaching, such as hydrogen peroxide, is dangerous and harmful to the environment (Malucelli et al., 2017).
2.6 Ammonium persulfate (APS) oxidation
Ammonium persulfate is a chemical that is widely-used as a strong oxidising agent in polymer chemistry and as a cleaning or bleaching agent (Mascheroni et al., 2016). It is an oxidant with low long-term toxicity, high water solubility and low cost (Leung et al., 2011).
Cellulose nanocrystals produced through APS oxidation have shown higher charge densities, crystallinity, clarity of solution, leading to improved transparency of the coating (Mascheroni et al., 2016). This is attributed to the carboxylate groups formed during the process which are effective in binding with metal ion-exchange reaction(Ifuku et al., 2009; Lu et al., 2016; Matsumoto et al., 2006; Saito & Isogai, 2005)
APS treatment is able to remove lignin, hemicellulose, pectin and other non-cellulosic contents because of free radicals formed during heating. Hydrogen peroxide is also formed under acidic conditions (pH 1.0). Both SO4- and HSO4- aid in breaking down the amorphous region, isolating CNCs as well as decolourising and isolating the material by opening the aromatic rings of lignin (Leung et al., 2011). Carboxylate cellulose nanocrystals can be used as coatings for high-performance and sustainable flexible packaging materials (F. Li et al., 2013; Mascheroni et al., 2016)
S2O82- + heat 2SO4-
S2O82- + 2H2O 2HSO4- + H2O2