Research on bionanocomposites has been developed, while its application as food packaging is still being explored. They are usually made from natural polymers such as cellulose acetate, chitosan (CS), and polyvinyl alcohol. Bionanocomposite materials can replace traditional non-biodegradable plastic packaging materials, enabling them to use new, high-performance, lightweight, and environmentally friendly composite materials. However, this natural polymer has a weakness in mechanical properties. Therefore, a composite system is needed that will improve the properties of the biodegradable food packaging. The aim of this mini-review is to demonstrate recent progress in the synthesis, modification, characterization, and application of bionanocomposites reported by previous researchers. The focus is on the preparation and characterization of CS-based bionanocomposites. The mechanical properties of CS-based food packaging can be improved by adding reinforcement from inorganic materials such as organoclay. Meanwhile, the anti-bacterial properties of CS-based food packaging can be improved by adding nanoparticles such as Ag and ZnO.
Packaging is a key component in all phases of the food industry. However, its permeability is a major drawback in the conventional food packaging materials. There are no packaging materials that completely repel water vapor and atmospheric gases . In addition, the industry, along with the food supply chain, is looking for new, inexpensive, and environmentally friendly food packaging systems. However, they still pay attention to the main functions of food packaging, such as protecting and controlling food safety and quality . Therefore, research on innovative food packaging materials needs to be explored.
Most of the materials used in the packaging industry are oil-based plastic polymer materials. It accounts for about 8% of global gas production and fossil fuels are used to make non-degradable synthetic polymers . Further development of renewable packaging or green packaging has the potential to reduce the negative environmental impact of synthetic packaging using biodegradable materials such as biopolymers (e.g., chitosan (CS), cellulose, and starch) .
CS is a natural, non-toxic, and high molecular weight polymer that is biodegradable . CS is the second most abundant biopolymer on earth after cellulose. Unlike plant fibers, CS has a positive ionic charge (amino group), which gives it the ability to chemically bind to fats, lipids, cholesterol, metal ions, proteins, and negatively charged macromolecules . CS can be isolated from crustacean and shrimp shell waste, so that from an economic point of view, CS-based food packaging will produce cheaper products than oil-based plastic polymer materials. CS has gained increasing commercial interest as a raw material due to its excellent properties including biocompatibility, biodegradability, adsorption, and the ability to be spread as thin film and chelated with metal ions. As a result, CS is widely used in various industries such as pharmaceuticals, biochemistry, biotechnology, cosmetics, and food packaging .
In addition to these advantages, CS-based food packaging has a weakness, namely, low mechanical strength  Therefore, it needs to be reinforced with other materials, one of which is clay. The abundance of bentonite clay minerals is quite a lot in Indonesia, so the cheap clay prices will reduce the production cost of CS-based food packaging . The layered structure of bentonite clay improves the mechanical properties of CS-based food packaging. This is due to the possibility of bentonite clay structures exfoliating when reacting with CS. This exfoliating structure will be obtained by synthesizing organoclay first, which aims to make the organic sites of the clay easily interact with CS. Some biocomposite materials are developed to improve the functional properties of general food packaging, such as barrier performance, mechanical strength and thermal stability, and other nanomaterials can combine bacteriostatic agents, antioxidants, plant extracts, and enzymes to extend the shelf life of food products .
One of the functions of food packaging is to protect food from bacteria that can make food spoil easily. Therefore, it is necessary to add an antibacterial agent to the CS-based food packaging system. There are several kinds of antibacterial agents that can be used as food packaging additives based on CS, including natural antimicrobials (vegetable essential oils and lysozyme from egg cells) and nanostructured antimicrobials, both metal nanoparticles (NPs; Ag and Cu) and metal oxide NPs (ZnO, TiO2, and CuO) [11,12].
CS has a function as a reducing agent and capping agent in the synthesis of NPs, such as ZnO, Cu, Ag, and Au NPs [13,14]. The NPs that have low toxicity are AgNPs and ZnO NPs . Therefore, the addition of these two types of NPs is very important to increase the antibacterial properties of CS-based food packaging. Research results of Zhao et al.  showed that the CS layer enriched with grape seed extract-Ag NPs had the potential to maintain the quality and extend the shelf life of grapes. While the research results of Zheng et al.  showed that the contents of ZnONPs and essential oil of Artemisia annua were 5 and 8%, respectively, the physicochemical and biological properties of CS-based edible films could reach the optimum. Thus, the addition of AgNP and/or ZnONP additives into a CS-based bioplastic system is very promising for improving the quality of bioplastics.
Figure 1 shows an example of the progress of research on CS-based food packaging, published from 2001 to 2022, which was taken on August 25, 2022 from the science direct website. This shows that there is an increase in research on CS-based food packaging. Therefore, research on CS-based food packaging is important to continue future benefits to be developed in future studies.
2 Materials for bionanocomposite preparation
In general, nanomaterials used in food packaging can be divided into two categories: inorganic and organic materials . The first category, the inorganic materials, such as metals, metal oxides, and clay NPs are incorporated into bionanocomposite films, and nanofibers can be considered. In addition to the common bacteriostatic AgNPs, some of the inorganic active ingredients, or oxidized NPs, such as CuO, ZnO, TiO2, MgO, and Fe3O4, have attracted great interest due to their resistance to harsh processing conditions and their strong inhibition against foodborne pathogens . Other materials, such as clays, can make them gasses and water vapor resistant and improve the mechanical strength of biopolymers . The second group, organic materials, include but not limited to phenols, halogenated compounds, quaternary ammonium salts, plastic polymers, and natural polysaccharides or protein materials such as CS, chitin, zein, and whey protein isolates, which have become very popular recently .
2.1 Polymer matrix in bionanocomposite
The polymer matrix in bionanocomposites for food packaging is a biopolymer that has good film-forming ability and is also biodegradable, namely, a biopolymer that is classified as a biomass product . Biomass products consist of polysaccharides, lipids, and proteins. Examples of polysaccharide groups are cellulose, alginate, starch, CS, pectin, gum, carrageenan, pullulan, or their derivatives. Examples of lipids are beeswax, oils, and free fatty acids. Examples of plant proteins are wheat gluten, soy, corn zein, and examples of animal proteins are collagen, whey, casein, and gelatin. Lipids and protein films show limited utilization in food packaging applications , so the best biopolymer matrix is polysaccharide groups. The largest abundance of polysaccharide groups is cellulose, but cellulose is still widely needed for other needs. While the abundance of the second largest polysaccharide group is CS which is mostly produced from shrimp shell waste . So, we prefer CS as a biopolymer matrix for food-based packaging.
CS is a modified natural carbohydrate polymer derived from chitin which is found in a variety of natural sources, such as seafood, shrimp, crustaceans, fungi, insects, and algae . The selected crustacean shells consisted of 30–40% protein, 30–50% calcium carbonate and calcium phosphate, and 20–30% chitin. Chitin is a linear chain of poly-(1 → 4)--linked N-acetyl-d-glucosamine (Figure 2a) whereas CS is a homopolymer of (1 → 4)-N-acetyl-d-glucosamine obtained by removing the acetyl group, (CH3–CO) of the bulk polymer (Figure 2b) . CS is soluble in acetic acid and dilute formic acid . In essence, the difference between chitin and CS is in the acetyl group. The deacetylation reaction can break the acetyl group in the chitin compound. If the acetyl group that has been released exceeds 75%, it can be called a CS compound . This value is called the degree of deacetylation (DD). The greater the DD value, the more reactive CS will be because there are more free amino groups.
Many CS-based food packaging is composed with other materials, both organic and inorganic . A brief overview of recent developments in the production of CS-based film for packaging applications is provided in Table 1. Table 1 shows that CS-based films for food packaging need to be added with additives to improve the properties of the films. These additives can be sourced from organic or inorganic materials. The properties of the film also vary depending on the target type of food as an application. The data in Table 1 cannot be compared in general because the CS used in these studies has different characteristics, such as the DD and molecular weight. The DD is the value of the loss of acetyl groups in the acetamide group of chitins or the number of free amino groups produced after the deacetylation process . The higher the DD, the more amine groups (−NH2) in the CS molecule, so that the CS is more reactive and increases its antibacterial properties .
|Biopolymer matrix||Additive||Properties of films||Antibacterial activities||Type of food||Ref.|
|CS 0.8% (w/v)||Pullulan 0.4% (w/v) and carvacrol 1.5% (w/v)||2.99||82.06||—||0.68||P. fluorescens, L. monocytogenes, E. coli, P. putida, E. cloacae, and S. aureus||Chilled meat|||
|CS 2.5% (w/v)||Clove Oil was encapsulated into halloysite nanotubes (HNT)||—||—||—||—||B. mojavensis and E. coli||—|||
|CS 2% (w/v)||Pectin 2% (w/v) and piper betle 2% (w/v)||10.09||20.33||—||1.77||S. aureus, B. cereus, P. aeruginosa, and K. pneumoniae||Purple eggplant|||
|CS 2% (w/v)||Ramie fiber 20 wt% and lignin 20 wt%||57.8||55||—||—||—||Chicken breast and cherry tomato|||
|CS 2% (w/v)||Dioscorea hispida starch 1.5% (w/v) and lemongrass essential Oil 0.75% (v/v)||7.7||1.7||15.11||—||E. coli, S. typhi, S. aureus, and S. epidermidis||—|||
|CS 2% (w/v)||Inclusion complexes||20.1||68.7||—||—||E. coli, S. typhimurium, and S. aureus||—|||
|Gelatin 8% (w/v) and CS 1.6% (w/v)||3-Phenyllactic acid 2% (w/w)||—||—||15.35||—||S. enterica Enteritidis and S. aureus||Chicken and meat|||
|CS 1% (w/v)||Riboflavin 5% (w/w)||28.48||33.58||40.05||—||L. monocytogene, V. parahaemolyticus, and S. baltica,||Salmon|||
|Gelatin 1% (w/v) and CS 1% (w/v)||Capsaicin loaded FeIII-HMOF-5||46.05||23.5||58.74||—||E. coli||Apple|||
|Fish gelatin (FG) 2.3% (w/v) and chitosan 1% (w/v)||TiO2–Ag 0.5% (w/v)||9.014||72.71||30||0.94||E. coli, S. aureus, and B. cinerea||—|||
|Carboxymethyl-CS 1% (w/v)||MgO 1% (w/w)||9.4||25||18||—||L. monocytogenes and S. baltica,||Water-rich food|||
|CS 1% (w/v) and cellulose nanocrystals 50% (w/w)||25.3||60||—||5.6||C. albicans, Pseudomonas, and Enterobacteriaceae||Chicken and meat|||
|CS 2% (w/v) and polyvinyl alcohol (PVA) 2% (w/v)||Cu2O@NCs (CS NPs) 4% (w/v)||35.77||63.93||19.21||0.15||E. coli and S. aureus||Cherry tomatoes|||
|FG 2% (w/v) and CS 2% (w/v)||Shrimp and crab protein hydrolysates (SPH and CPH) 6% (w/w)||SPH: 6.18 and CPH: 9.03||SPH: 31.89 and CPH: 50.07||SPH: 89.7 and CPH:93.8||—||K. pneumoniae, S. enterica, S. typhimurium, Enterobacter sp. E. coli, S. aureus, B. cereus, and M. luteus||Fruits and vegetables|||
|CS 2% (w/v) and polyethylene glycol (PEG) (0.2%, w/v) Hydroxyethylcellulose (HEC) (0.2%, w/v)||Cerium oxide NPs 0.4% (w/v)||—||—||—||1.6||E. coli and S. aureus||—|||
|CS 0.5% (w/v), PVA 1.5% (w/v), and guar gum (GG) 0.2% (w/v)||Hydroxy citric acid (HCA) 0.2% (w/v)||42||12.5||10.8||0.19||E. coli and S. aureus||—|||
|CS 1% (w/v)||Satureja and Thyme essential oils 1% (w/w) and CS nanofibers (2 wt%)||13.7||5.5||22.3||2.81||E. coli||Fruits and vegetables|||
|CS 1% (w/v) and PVA 1% (w/v)||Boswellic acid 0.8%||40||10||80||—||E. coli, S. aureus, and C. albicans||—|||
|CS 3% (v/v) and poly(caprolactone) (PCL) 12% (v/v)||Chlorogenic acid loaded HNT (CGA@HNTs) 6 wt%||31.12||7.01||—||0.4||E. coli and S. aureus||—|||
|CS 1% (w/v)||Pine needle extract 20% (v/w)||65.04||26.94||19.60||0.178||S. aureus, B. subtilis, P. aeruginosa, and E. coli.||—|||
|CS 1.5% (w/v) and PVA 0.5% (w/v)||Ethyl vanillin 2% (w/v)||70||10||—||—||E. coli and S. aureus||—|||
|CS 1% (w/v)||Kojic acid 2% (w/v)||55||60||9.13||0.78||E. coli and S. aureus||—|||
|CS 1% (w/v)||Xylan 30% (w/w) and carvacrol 10% (w/w)||66.63||6.72||—||—||L. innocua and E. coli||—|||
TS (Tensile strength, MPa), EAB (Elongation at break, %), WS (Water solubility, %), WVP (water vapor permeability (g mm/h m2 kPa)).
2.2 Filler in bionanocomposite
Filler in bionanocomposite is needed to improve the mechanical properties of CS-based food packaging . Many types of fillers can increase the mechanical strength. However, we need to pay attention to the abundance of these compounds in nature. Clay is an inorganic compound that is quite abundant, especially in Indonesia. Clay has a layered structure that is miscible with polymers. However, to facilitate the mixing, the clay needs to be modified with surfactants to become organoclay .
Clay (illustrated in Figure 3) is a layered aluminosilicate material that has strong coordinate-covalent bonds but weak van der Waals interaction between layers . As a result, this material can be cut parallel or extended to nanometer-thin sheets in the direction of the plane. The separation of large aggregates (booklets) into thin sheets is defined as exfoliation .
Montmorillonite (MMT), a smectite-type clay, has a 2:1 structural unit consisting of two tetrahedral sheets of silica sandwiching an octahedral alumina sheet . The general formula is (Na)0.7(Al3.3Mg0.7)Si8O20(OH)4·nH2O. Substituting the isomorph from Mg for Al in the octahedral alumina sheet and Al for Si in the tetrahedral sheet produces a charged layer in the MMT layer . The MMT layer has a cation exchange capacity which is balanced with a cation (e.g., Na+ and Ca2+) in the space between the MMT layers. Water molecules can enter the interlayer space and cause swelling of the MMT circulation because of its hydration property. In addition, the interaction forces between MMT layers are weak electrostatic forces and van der Waals forces. Because of these characteristics, MMT has the ability to exfoliate free-standing nanometer layers .
A more effective way of exfoliating MMT is by modifying the MMT intergallery through intercalation with cationic surfactants to become organoclay . Cationic surfactants commonly used are quaternary ammonium salts which have at least one long alkyl chain. The positive charge at the end of the surfactant chain will experience ion exchange with the cations on the MMT surface, thus changing the MMT properties to become organophilic . Thus, MMT can be used as a filler in certain hydrophobic materials, such as polymers .
There are three types of MMT/polymer composite structures that occur when there is an interaction between MMT and polymer, as illustrated in Figure 4. The tactoid structure occurs when the polymer interacts with the MMT surface, the intercalation structure occurs when the polymer enters the MMT interlayer, while the exfoliation structure occurs when the MMT layers are exfoliated. This occurs when the MMT is exfoliated and well dispersed in the polymer . Nanolayer in the polymer matrix can significantly improve the properties of nanocomposites. MMT/polymer nanocomposites indicate increasing mechanical strength and thermal stability .
The use of clay as a filler in nanocomposites as food packaging has been widely used, such as in cellulose  and starch polymers. However, the use of clay as a filler in CS has not been widely reported. A brief overview of recent developments in the production of CS-based films using clay as a filler for packaging applications is presented in Table 2.
|Biopolymer matrix||Additive||Properties of films||Antibacterial activities||Type of food||Ref.|
|CS (2%, w/v)||Na-MMT (3% w/w) and citric acid||3.1||64.2||—||—||—||Cherry tomatoes|||
|CS (2%, w/v)||MMT (0.1% w/w) and gelatin||89||7||7||0.828||—||Breads|||
|CS (2%, w/v)||Montmorillonite-thiabendazoluim (1:19)||4||—||—||—||E. coli, S. aureus, and P. aeruginosa||—|||
|CS 2% (w/v)||MMT–CuO nanocomposite (3% w/w)||32.5||58.7||16.5||3.6||S. aureus and B. cereus||—|||
|CS 2% (w/v) and PVA 5% (w/v)||MMT (15%, w/w) and sodium lactate (10%, w/w)||60.2||25.2||—||1.68||E. coli||—|||
TS (Tensile strength, (MPa)), EAB (Elongation at break (%)), WS (Water solubility (%)), WVP (water vapor permeability (g mm/h m2 kPa)).
2.3 NP as antimicrobial agents in bionanocomposite
Metal NPs that are commonly used as fillers for CS-based antibacterial food packaging films are AgNPs and ZnONPs . AgNPs and ZnONPs have high antibacterial properties and low toxicity properties, so they are best used as additives in antibacterial food packaging [14,66]. Table 3 summarizes the published work on CS-based packaging with AgNPs and ZnONPs as filler.
|Biopolymer matrix||Additive||Properties of films||Antibacterial activities||Type of food||Ref.|
|CS (2.5%, w/v)||Covalent organic frameworks and immobilized AgNPs||34||17||—||0.95||E. coli and S. aureus||Fish|||
|CS (2%, w/v)||Grape seed extract and AgNP||—||—||—||—||A. niger and P. chrysogenum||Grape|||
|CS (2%, w/v)||Ag@MOF||27.67||250||25.78||0.361||E. coli and S. aureus||—|||
|CS (2%, w/v)||Nano ZnO 5% and Artemisia annua essential oil 8%||32.33||37.37||—||—||Pork|||
|CS and starch||Pineapple leaf fiber and ZnO||0.9||26.5||—||—||—||Bread|||
|CS (2%, w/v)||ZnONPs loaded Gallic-acid (ZnO@gal) (0.07%, w/v)||54.83||52.17||16.34||4.23||B. subtilis and E. coli||Fruits|||
|CS (1%, w/v) and PVA (1%, w/v)||ZnONPs (0.1%, w/v), anthocyanins (240 mg/g), roselle calyx (RE), and purple potato powder (PPE)||RE: 17.22 and PPE: 20.68||RE: 17.50 and PPE: 21.50||RE: 23.38 and PPE: 22.38||RE: 0.28 and PPE: 0.26||E. coli and S. aureus||Shrimp|||
|CS 1.5% (w/v)||ZnO particles (0.03% (w/v), Melissa essential oil (0.5% (w/v))||100||2.5||16||4.68||E. coli||—|||
|CSCS and polyurethanes (PU) (PU/CS 0.75:0.25)||ZnONP 5% (w/w)||8.1||2.156||—||—||E. coli and S. aureus||Carrot|||
|CS (3%, w/v) and GG (3%, w/v)||RE–ZnO nanocomposites (5%, w/v)||67||75||—||—||B. cereus, S. aureus, P. aeruginosa, L. monocytogenes, E. coli, Y. enterocolitica, S. typhimurium, A. niger, A. flavus and A. terries||Ras cheese|||
|CS (2%, w/v)||AgNPs green synthesis mediated by 8 mL of (0.1%, w/v) tea polyphenols||53.68||12.61||21.13||2.4||E. coli and S. aureus||—|||
|CS (2%, w/v)||AgNPs 5% w/v and essential oils (10%, w/v)||58.1||61.9||—||1.5||E. coli, L. monocytogenes, S. typhimurium, and A. niger||Straw-berries|||
|CS (2%, w/v) and lignocellulosic biomass (2.7%, w/v)||AgNPs (0.03%, w/v)||30||15||13.5||—||E. coli and S. aureus||—|||
|CS (1%, w/v)||AgNPs (1:1 (v/v))||15.63||25.70||14.49||—||S. enterica, P. aeruginosa, Enterobacter sp., S. aureus, M. luteus, L. Monocytogenes, and B. cereus||—|||
|FG (2.3%, w/v) and CS (1%, w/v)||TiO2–Ag (0.4%, w/v)||10.97||68.96||5.8||0.94||E. coli, S. aureus, and B. cinerea||—|||
TS (Tensile strength, (MPa)), EAB (Elongation at break (%)), WS (Water solubility (%)), WVP (water vapor permeability (g mm/h m2 kPa)).
In all types of NPs that have been developed and characterized, silver-based NPs have taken an important place because of their innate ability of antimicrobial activity even in solid-state samples and therefore have been used as bacteriostatic agents since ancient times . Silver-based particles in the nanoscale include AgNPs, nanocluster of silver (AgNC), and silver-based alloy materials. AgNP is one of the most widely studied and applied antimicrobial agents because of the vast spectrum of antimicrobial activity against microorganisms .
On the other hand, ZnO is one of the zinc compounds considered as generally recognized as safe (GRAS) by the FDA (21CFR182.8991) . The application of ZnO in packaging materials can help in a variety of ways because the film is antibacterial and a source of zinc in zinc supplements as well. Several authors have reported the use of ZnONPs as fillers in composite polymers for active food packaging with the aim of reducing the growth of pathogenic bacteria and extending food shelf life .
3 Fabrication strategies for preparing CS-based films
There are four types of CS-based film preparation methods, namely, solution casting, layer-by-layer assembly (LBL), extrusion, and coating/spraying . The most widely used method is solution casting because this method is the easiest one. However, this method is generally carried out on a laboratory scale .
The steps taken in the solution casting method are to dissolve the CS in a weak acid solution. After the CS is completely dissolved, this solution can be directly molded or added with functional materials or fillers (such as antimicrobials, antioxidants, plasticizers, etc.) First, until homogeneous and then molded with molds as needed. Then, the solution in the mold evaporates at a certain temperature and time. After all the solvent has evaporated completely, the thin layer can be peeled off from the mold .
LBL is a film fabrication technique commonly used for multicomponent films that do not require sophisticated instruments such as the solution casting method. However, this method requires a relatively longer time because of the gradual drying process at a certain temperature until it dries. The first layer is dried first, then continued by adding a second layer and leave to dry and so on until a multilayer layer is formed .
The extrusion method is widely used for the production of conventional commercial plastic packaging films. Extrusion is often preferred over the solution casting method due to faster processing times and lower energy consumption. The extrusion process often provides films with acceptable mechanical properties and good thermal stability. Although extrusion is a promising approach for film fabrication, there are a number of studies related to the use of this technology in CS films .
Coating or spraying techniques are usually used for fresh foods such as fruit, vegetables, meat, fish, etc., to extend their shelf life. This technique has the advantage of being easy to perform and does not require sophisticated tools. However, a certain preparation is needed for the target food to be coated. The usual steps are: formulation of raw materials using CS and fillers in appropriate proportions; disinfection of food samples using sodium hypochlorite solution; spreading the CS-based composite solution into the food to form a uniform layer using a sterile spreader; and drying under certain conditions. Spraying technique involves similar steps as coating technique except that the spraying is carried out using a compressed air assisted sprayer .
CS-based films reinforced with organoclay, AGNPs, and ZnONPs can produce good interactions . Based on the material properties of the CS-based film, it can be estimated that an illustration of the interaction between organoclay, AgNPs and ZnONPs and CS can be estimated. Figure 5 shows that CS as a biopolymer can act as a film matrix and as a capping agent for AgNPs and ZnONPs. Meanwhile, organoclay is a layered material that can be intercalated or exfoliated when interacting with biopolymers.
4 Characterization of bionanocomposite for food packaging
4.1 Characterization of CS
4.1.1 FTIR Spectrophotometric method
The FTIR spectrophotometer method is used to determine the DD and identify functional groups that are lost or experience changes during the deacetylation process . The degree of CS deacetylation can be calculated using equation (1) with the baseline (a) .
where A 1,655 and A 3,450 are absorbance values at 1,655 and 3,450 cm−1 from the amide and hydroxyl groups which can be estimated from the baseline FTIR spectra. The baseline (b) proposed by Baxter et al.  is a modification of the method using equation (2).
Chitin with a DD equal to or greater than 75% is called CS .
4.1.2 Viscometric method
The viscometric method is used to determine the molecular weight of CS using intrinsic viscosity. Subsequent samples were tested using a viscometer. Determination of molecular weight begins with determining the value of intrinsic viscosity through the Huggins equation:
where η sp: specific viscosity or type viscosity, k′: Huggins constant, c: concentration, η: viscosity of sample solution, η o: solvent viscosity, and [η]: intrinsic viscosity.
The value of [η] can be calculated by making the η sp curve vs concentration (c). Extrapolation to zero concentration results in value [η].
Intrinsic viscosity and molecular weight are related to the Mark–Houwink equation:
|Molecular weight of CS (Da)||Group of CS|
|50,000–190,000||Low molecular weight|
|190,000–310,000||Medium molecular weight|
|310,000–375,000||High molecular weight|
4.2 Characterization of organoclay
4.2.1 X-Ray diffraction (XRD) method
Crystal phase identification and crystallinity of the material were carried out using a diffractogram XRD analysis . The analyzed samples were CS, organoclay, and plastic. Crystal phase identification and crystallinity of the materials were carried out using a diffractogram XRD analysis.
One of the parameters for the success of surfactants to enter the clay interlayer is through the shift of the XRD peak at 2θ ± 5.5° to a smaller value of 2θ. Figure 6 shows an example of diffraction pattern of organoclay reported by Juwono et al. This can be calculated using Bragg’s law: λ = 2d sin θ, where λ is the wavelength of the X-ray beam, d is the distance between the clay interlayer, and θ is the diffraction angle.
4.2.2 FTIR spectrophotometric method
FTIR spectrophotometer can also be used to determine the CS-based clays and plastic functional groups [91,93] and evaluate the change in hydrophilicity of clay after being modified to organoclay . The FTIR spectra of organoclay show the addition of new absorptions at wavelengths of 2,851 and 2,928 cm−1 which were symmetric –CH2 strain and asymmetric −CH2 strain, respectively  and the reduction in band and peak that attributed to water absorption (ν –OH at 3,700–3,600 cm−1 and δ H–O–H at 1,620 cm−1). This new feature indicates that an interaction between the clay and surfactant has taken place  and the surface of organoclay becomes organophilic.
4.3 Characterization of NPs
4.3.1 XRD method
The phase and structural parameter of the prepared samples were explored by XRD pattern. For ZnONPs, all the diffraction peaks appeared at 2θ values of 31.8, 34.3, 36,3, 47.6, 56.3, 63.0, 66.1, 67.9, 69.2, 72.7, and 76.8° corresponding to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202), respectively.  While for AgNPs, all the diffraction peaks appeared at 2θ values of 38.5, 44.6, 65.1, and 78.1° corresponding to (111), (200), (220), and (311), respectively .
4.3.2 FTIR spectrophotometric method
CS functions as a capping agent for AgNPs and ZnONPs. The FTIR spectra of AgNPs show a peak near 3,431 cm−1 corresponding to the stretching vibration of the OH group. The band at 1,635 cm−1 corresponds to amide I due to carbonyl stretching in CS. The peak at 1,390 cm−1 is denoted as CH3 symmetrical deformation vibration. The presence of primary amino groups in interaction with metal surfaces and amino groups acting as cover sites for the stability of AgNPs can be identified from this spectrum . While ZnONPs showed wide vibrational bands at 3,445, 3,419, 3398.7 and 3,392 cm−1 which were associated with water hydroxyl (O–H) stretching in CS/ZnO nanocomposites. The hydroxyl group becomes wider and shifts to a lower wave number indicating a strong interaction between the hydroxyl group and ZnO. The absorptions at 2,946 and 2873.5 cm−1 were determined by the asymmetric stretching vibrations of CH3 and CH2 of CS which confirmed the adsorption of CS on the ZnO surface. Meanwhile, the 1,654 and 1,069 cm−1 vibration bands were associated with the bending and strain of (C–O)–NH2 group of vibrations. The band at 896 cm−1 originates from the weak vibrations of ZnO. The observed vibrational absorption at 500 cm−1 was determined for Zn–O lattice vibrations in CS/ZnO nanocomposites .
4.3.3 UV-Visible (UV-Vis) spectrophotometry method
UV-Vis spectra have been shown to be sensitive to the formation of AgNPs through the appearance of absorption peaks caused by the phenomenon of surface plasmon resonance (SPR) in metals. The SPR peak for AgNPs appears at wavelengths between 398 and 448 nm,  while absorption of ZnO was observed in the wavelength range of 350–380 nm . Colloidal solution from the synthesis of AgNPs was directly measured for its absorbance without dilution at wavelengths of 200–800 nm, while plastic samples were put directly into the cuvette and measured at the same wavelength.
4.3.4 Transmission electron microscopy (TEM) method
TEM images can show morphology and approximate particle size of Ag and ZnO . The morphology of Ag and ZnONPs produced from the synthesis using CS as a capping agent is spherical in shape with a size of 10–60 nm [53,99]. It can also be seen if there is agglomeration of particles in the sample .
4.4 Characterization of bionanocomposite
4.4.1 UV-Vis spectrophotometry method
Transparency indicates film compatibility and material distribution uniformity . The difference in thickness caused by the preparation process will have a specific impact on light transmission. UV-Vis spectroscopy was used to investigate the optical transparency of films by recording the transmittance spectrum in the UV-Vis region of 200–800 nm . Highly transparent films are suitable for food packaging . Also, the lower the visible light transmittance of the film, the less transparent it is; the higher the opacity of the film, the greater the light barrier function it has .
4.4.2 TEM method
The tactoid, intercalation, and exfoliating structures of the organoclay interacting with the polymer in the bionanocomposite can be seen from the TEM images . The size and morphology of the metal NPs added to the bionanocomposite can also be seen from the TEM image . Figure 7 indicates that the polymer system possesses an intercalated structure, while intercalation and exfoliation occurred in the polymer system. Moreover, the TEM images show a typical layer pattern of nanocomposite structure .
4.4.3 Test mechanical properties
Mechanical properties of bionanocomposite films for food packaging plastic applications, including tensile strength, elongation at break, and thickness, are determined . The plastic was tested using a universal testing machine to obtain the percentage of elongation and TS. In addition, the thickness of the plastic was also analyzed using a micrometer. As a food packaging material, CS plastic is expected to have moderate mechanical properties (TS = 10–100 MPa, percentage of elongation = 10–20%) [35,44,45,46,48,50,74,77,79].
WS is a measure of the solubility of a material in water at a certain temperature . The WS (%) of the film samples were calculated as follows:
where M1 = initial sample weight and M2 = sample weight after drying.
The international standard ASTM D570-98 (standard test method for water absorption plastics) states that conventional polypropylene plastic has a WS of 0.01%. However, the solubility properties of CS film cannot use conventional plastic standards. The solubility value of CS film also depends on the amount of plasticizer concentration added [36,37,38,39,40,41,42,75,107].
WVP is a material’s ability to allow water vapor to pass through it . The WVP value can be obtained from the following formula:
where W = weight gain (g) of film samples, A = permeation area (m2), x = thickness (m) of the film, and t = lapsed time (s) for the weight gain of film.
4.4.6 Biodegradable test
Test specimens of chitin and CS films were cut into 5 cm × 5 cm squares to evaluate the biodegradation. For the biodegradation test they were buried in the soil (outdoors) 10 cm below the surface. Two kinds of soils, paddy soil and red clay, were chosen. The burying period of the test samples was 1, 1.5, and 2 months .
5 Conclusion and future perspective
This review summarizes recent studies on the synthesis, modification, and characterization of CS-based bionanocomposites and their application in antibacterial food packaging. The addition of organoclay, AgNPs and ZnONPs at appropriate concentrations can improve the overall mechanical, barrier, and antimicrobial properties of the CS-based films. However, the physicochemical properties of CS–organoclay nanocomposite are strongly related to the three basic structures (tactoid, intercalation, or exfoliation). Therefore, future research needs to study the most optimal method in making exfoliating structures that can produce bionanocomposite films that have good mechanical properties. It is also necessary to study the technique of combining AgNPs and ZnONPs so that the bionanocomposite film still has good mechanical and barrier properties but with increased good antibacterial properties for food packaging.
LJM greatly acknowledges Indonesian Ministry of Education and Culture for granting Domestic Postgraduate Education Scholarships and Universitas Mathla’ul Anwar Banten for on leave approval to pursue her PhD.
Funding information: This review was supported by the Directorate of Research and Development, Universitas Indonesia (Doctoral International Indexed Publications: NKB-616/UN2.RST/HKP.05.00/2020).
Author contributions: Lisna Junaeni Muiz: writing-original draft and visualization. Ariadne Lakshmidevi Juwono: supervision and writing-review & editing. Yuni Krisyuningsih Krisnandi: methodology, conceptualization, funding acquisition, writing-review & editing, and supervision.
Conflict of interest: No conflict of interest.
Ethical approval: The conducted research is not related to either human or animal use.
Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
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