## 3 Synthesis of Nanomaterials  
The synthetic processes for obtaining nanomaterials are classified into 'top-down' and 'bottom-up' methods (Figure 3). The former consists of obtaining nanostructures from the subdivision of material by physical methods to nanometer-sized particles, while the latter corresponds to the formation of nanostructures from the aggregation of atoms followed by growth in some dimension to obtain nanostructures. Its advantages include uniform size distribution and homogeneity, in contrast to the drawbacks, which include the high energy and costs and low yields. The advantages of green synthesis over physical and chemical methods remain in a nontoxic process thus it is environmentally friendly, pollution-free, low-cost, and sustainable [18]. The green synthesis classification includes biological synthesis (biosynthesis) and biomimetic synthesis. The former uses organisms (fungi, bacteria, algae, and viruses) or extracts thereof as reducing agents. The latter method refers to the use of biological molecules or principles of bioreduction for the formation of nanostructures [19; 20; 21; 22]. In these processes, with regard to the shape and size of nanostructures, factors such as temperature, pH, extract concentration, metal precursor concentration, and incubation time are important in the reduction process of metal ions [21].  
The synthesis of nanomaterials has generally focused on the use of materials such as metals (silver, copper, gold, platinum, aluminum, magnesium, titanium, and mixtures  
Figure 3: Differences between nanomaterials synthesis and new eco-friendly methods.  
of them); metal oxides (titanium dioxide, zinc oxide, copper oxide, magnesium oxide, calcium oxide, and aluminum oxide); biopolymers and ceramic materials such as alumina; or combinations of all these materials [14; 19; 20; 21; 22; 23].  
On the one hand, nanomaterials synthesized from metals stand out for their optical and electrical properties, which are attributed to their surface charge density; such nanomaterials are of interest in the development of biosensors and antimicrobial agents, among other things.  
On the other hand, the synthesis of metal-oxide-based nanomaterials increases their efficiency and reactivity and modifies the properties and characteristics of the metal in the presence of oxygen [24]. The electrical (changes in conductance or electrical resistance) and chemical (adsorption and desorption of gaseous compounds given by the surface of their structure) properties of nanomaterials are of interest in systems for the monitoring and detection of volatile organic compounds (VOCs) in fresh produce, as part of food packaging, and for their antimicrobial effect [25]. The use of biopolymers has generated significant interest, which is why they exist in great variety, among which are natural biopolymers (starch, cellulose, chitosan, alginates, gluten, collagen, etc.), chemically synthesized ones (glycolic acid, poly-\(\epsilon\)-caprolactone, polybutylene succinate, polyvinyl alcohol, and polylactic acid), microbial polyesters (polyhydroxyalkanoates, polyhydroxybutyrate, etc.), and non-biodegradable polymers (nylon, polyamide, polyurethane, etc.). Nanocomposites obtained with these polymers in particular have great potential in the development of food packaging due to their thermal, optical, mechanical, barrier, and, in some cases, biodegradation properties [26]. Concerning ceramic materials, it is known that they are practically inert materials; the most used are alumina or calcium phosphate, which serve as scaffolds mainly for their porosity and resistance. For this reason, they have been used together with polymers to generate structures that allow the immobilization of bacteria, for purification processes, and in the formation of ceramic bodies as gelling agents, which are helpful in the food industry in products such as ice cream, jelly, and jam, to mention a few [27; 28].