Nanoparticles Based on Origin
Apart from dimensions, NPs can be classified based on origin as natural or synthetic- based (Jeevanandam et al., 2018).
1.4.1 Natural Nanoparticles
NPs produced by biological sources or by anthropogenic activities are considered as natural NPs. The natural occurrence of NPs can be found on Earth in all three spheres, namely hydrosphere, atmosphere, and lithosphere (Hochella et al., 2015).
1.4.2 Synthetic Nanoparticles
Synthetic NPs are generally produced by a variety of man-made methods, such as mechanical grinding, exhaust, and smoke produced by engines, or synthesis by either physical, chemical, biological, or combination of all in the form of the hybrid method. Synthetic NPs can lead to a category of engineered NPs that exhibit distinctive properties, based on the source, methods, and condition utilized during synthesis, that significantly differ from that of natural NPs (Wagner et al., 2014; Sharma et al., 2015).
- 1.4.3 Nanoparticles Based on Material
- 1.4.3.1 Organic Nanoparticles
Dendrimers, liposomes, micelles, and ferritin are generally know'n as organic NPs or polymers; these are biodegradable and non-toxic, whereas some, such as micelles and liposomes, have a hollow cavity similar to fullerenes, which are also known as nanocapsules which are generally sensitive to thermal and electromagnetic changes. All these exclusive characteristics can be harnessed in term of drug delivery either in an absorbed form or an entrapped form (Wiener et al., 1994; Tiwari et al., 2008). Dendrimers are a well-researched subject as due to their being highly specialized, encapsulating a targeted drug and its delivery (Li et al., 2007).
1.4.3.2 Inorganic Nanoparticles
Inorganic NPs are those which are mainly void of carbon; generally, metal-and metal oxide-based NPs are categorized in this category.
Metal-based NPs are those which are directly synthesized from metal to NP sizes either by mechanical destruction or biological and chemical formation. These commonly used metal-based NPs are aluminium, cadmium, cobalt, copper, gold, iron, lead, silver, and zinc. These have distinctive properties as having a smaller size, ranging from 10 to 100 nm, and surface characteristics like high surface-to-volume ratio, charge, shape, color, and amorphous structure (Salavati-Niasari et al., 2008).
Metal oxide-based NPs are synthesized to modify the property from their respective metal-based NP; these are synthesized because of their higher reactivity and efficiency due to the oxide molecules’ availability. In terms of metal oxide-based NPs, they have incomparable properties to their counterpart metal-based NPs. Some of the metal oxide-based NPs are aluminium oxide, iron oxide, silicon dioxide, titanium oxide, and zinc oxide (Tai et ah, 2007).
Quantum dots (QDs) are similar to NPs but contain tiny droplets of electrons. Quantum drops are nanocrystals ranging from 2 to 10 nm in size and are capable as semiconductors. They can be synthesized from various semiconductor material via colloidal synthesis or electrochemistry. The most common being cadmium selenide (CdSe), cadmium telluride (CdTe), indium phosphide (InP) (Pal et ah, 2011).
Nanoparticle Use in Antimicrobials (Basics)
Nano-reinforcement could be used to enhance the antimicrobial properties of the films, which ultimately extends the service life of the product. Moreover, antimicrobial agents in the nano form when reinforced into the polymer matrix hold improved activity owing to their high surface-to-volume ratio (Llorens et ah, 2012). The selection of a suitable antimicrobial agent is based on its effect on the food product and the associated microorganism. Several studies have been conducted on nano-reinforcement of antimicrobials into the polymer matrix both in individual and combined forms (Jin and Gurtler, 2011; Zohri et ah, 2013). To improve the antimicrobial properties, essential oils are also incorporated with other reinforcements. The addition of essential oils may affect the mechanical, physical, and barrier properties of the film. For instance, the addition of essential oil in hydroxypropyl methylcellulose decreased water activity by 20% and increased water vapour permeability and oxygen permeability by 38% and 40%, respectively (Klangmuang and Sothornvit, 2016). Apart from the antimicrobials discussed earlier, some inorganic nanofillers such as zinc oxide (Mallakpour and Behranvand, 2016), copper oxide (Oun and Rhim, 2017), silver (Franci et ah, 2015), and copper (Chatterjee et ah, 2014) also possess antimicrobial properties.
