Metal Nanoparticles—Overview

Due to distinctive properties, structures, and extraordinary characteristics exhibited by NPs, metal oxides are extensively studied class of inorganic solids. Transition metal oxides exhibit several industrial applications. Nano-powders, nanotubes, and NPs hold remarkable applications in household uses, bioremediation, medicine, and industries. To explore morphology and scale-dependent applications, ID nanostructures of metal oxides are ideal systems. This is the main area of focus in nanotechnology and nanoscience due to the easy availability and presence of metal oxides in a variety of shapes, structures, composition, and physical and chemical properties (Zhai et al., 2009).

Synthesis of Nanoparticles

For the synthesis of NPs, numerous methods can be employed and these are widely distributed into two major categories, that is (1) the top-down approach and (2) the bottom-up approach (Wang and Xia 2004; Iravani 2011). These methods are further categorized into various subcategories based on the adopted protocols, operations, and reaction conditions. To date, in chemical and physical fields, several methods are utilized for NPs synthesis, but these are expensive and involve the use of toxic chemicals, which is why the use of biological synthesis is preferred more. Plant, fungal, and bacterial extracts are also used for the synthesis because these kinds of methods are cost-effective, very reliable, and non-toxic (Singh et al„ 2015). Out of all the suggested methods for synthesis of NPs, two methods, that is chemical reduction and biological synthesis, were widely accepted because of their benefits in controlling the size and morphology of NPs perfectly.

Top-down and bottom-up pathways are the most commonly used processes in the synthesis of NPs. In the top-down pathway, with the help of various lithographic approaches like chemical etching and ball milling/chemical milling, the material in bulk is converted into smaller nanoscale particles. The biggest limitation of this procedure is that it adds imperfections in the structure’s surface, and NPs are greatly dependent on their surface structure (Gudikandula and Maringanti, 2016). However, in the bottom-up pathway, oxidation and bio-reduction procedures are used to prepare NPs from smaller striating materials. The possibility of having any flaw is reduced, as in this procedure, atoms aggregate to form nuclei range at the nanoscale. In the biological procedure, stabilizing and capping mediators (flavonoids, phytochemicals like phenolics, co-factors, and terpenoids) that provide greater stability are used (Korbekandi et al„ 2009; Thakkar et al., 2010). In more advanced studies considering the factors like ease of synthesis, cost-efficacy, stable NPs, the function of microbes in green synthesis are also considered (Duran et al., 2007; Huang et al., 2007). From simple prokaryotic bacterial cells to compound eukaryotes like angiosperms, a wide range of life forms are used in the biogenic synthesis (Ahmad et al., 2003).

Methods Commonly Used for Nanoparticles Synthesis

Synthesis of Nanoparticles by the Physical Method

NPs synthesis by physical methods includes sonochemistry, laser ablation, radiolysis, and ultraviolet (UV) irradiation, among others. During the physical synthesis process, the vaporization of metal atoms takes place, followed by condensation on a variety of supports, which result in the rearrangement of the metallic atoms and finally aggregated as a small group of metallic NPs (Hurst et al., 2006). These methods required extremely refined instruments, chemicals, and radiative heating, as well as high power expenditure, which lead to a high functioning price.

Synthesis of Nanoparticles by the Chemical Method

NP synthesis is a declination of metallic ions in the solution using chemicals. In the circumstances of the reaction mixture, metal ions may help in the process of nucle- ation or aggregation to the appearance of a small group of metals. The chemicals generally used as reducing agents are sodium borohydride, hydrogen, and hydrazine (Egorova and Revina, 2000). Stabilizing agents like natural or synthetic polymers, such as natural rubber, cellulose, co-polymers micelles, and chitosan, are also utilized. These chemicals are hydrophobic so that they need the count of several organic solvents such as dimethyl, ethane, toluene, formaldehyde, and chloroform. These chemicals are poisonous and are non-biodegradable, which limits the manufacture extent. Some of the poisonous chemicals may also pollute the surface of NPs and make them inappropriate for certain biomedical applications (Patel et al., 2015). In this circumstance to eradicate all these drawbacks of chemical and physical methods and researchers are focusing on the substitute process of synthesis of metal NPs (Figure 6.1).

Different methods utilized for the metal-based nanoparticles synthesis

FIGURE 6.1 Different methods utilized for the metal-based nanoparticles synthesis.

Synthesis of Nanoparticles by the Biological Method

In recent years, the biogenic synthesis process of metallic NPs has concerned substantial attention. The biogenic NP synthesis process is achieved by using microorganisms and plants (Mukunthan and Balaji, 2012). Biosynthesis can offer NPs of a better-defined size and morphology compared to some of the other physico-chemical methods of manufacturing (Narayan and Sakthivel, 2008). It has been found that the microbial-based synthesis process is readily eco-friendly and well suited to the use of products for pharmacological applications, but synthesis by the use of microorganisms is often costlier than production with plant materials. The major assistance of the plant-based synthesis approach over the physical and chemical process is that plant-based synthesis is cheaper, eco-friendly, and easily extend the process for the synthesis of NPs in large-scale, and there is no need to use pressure, high temperature, and toxic chemicals (Shankar et al., 2004).

A lot of research has been done on the biological synthesis of metal NPs employing microorganisms such as fungi, bacteria, plants, and algae (Figure 6.2). This is due to their reducing or antioxidant potential that is dependable for the reduction of metal NPs. It is found that microbe-mediated synthesis is not valid for large-scale manufacturing, because it needs high septic conditions and special preservation so that the synthesis of NPs from the plant source is more valuable over microbes due to the simple scale-up process and no extra requirement of maintaining cell culture (Dhuper et al., 2012). Employing plant extract for NP synthesis also reduces the additional requirement of microorganism isolation and preparation of culture medium which increases the cost-competitive capability over the synthesis of the NP by microbes. Plant-mediated production is a one-step procedure towards synthesis, whereas microbes during the course of time may lose their ability to produce NPs by mutation; thus, research on plants is expanding fast. Several synthesis processes have been developed, including the chemical decline of metal ions in aqueous solutions with or without stabilizing agents, thermal decayed in organic solutions, and so on.

Biological method for the synthesis of metal-based nanoparticles

FIGURE 6.2 Biological method for the synthesis of metal-based nanoparticles.

Synthesis of Nanoparticles From Fungi

Fungi have secondary metabolites and active biomolecules that are very necessary for the NPs synthesis. Fungal species, for example. F. oxysporum, exude polymers, proteins, and enzymes that help in the production of metal NPs (Riddin et al., 2006). These components improve the yield and constancy of NPs. In other studies, it was found that several fungal species can synthesize NPs using extracellular amino acid residues. For example, the yeast surface contains aspartic acid and glutamic acid that reduces silver ions into silver metal in the occurrence of a sufficient quantity of light (Nam et al., 2008). Ahmad et al. (2003) observed that fungal species like F. oxysporum has reductase enzyme in the cytosol which reduces silver ions into silver metal in the existence of nicotinamide adenine dinucleotide (NADFP), a reducing component (Ahmad et al., 2003). Phytochelatins are a group of compounds that are found mostly in fungus and have a high capability of reducing silver ion into silver metal (Lee and Jun, 2019). Sanghi and Verma (2009) utilized supernatant obtained from fungal species Coriolus versicolor to make AgNPs. In this study. Fourier-transform infrared (FTIR) data established the presence of hydroxyl groups in the mycelium of a fungus which contributes electrons to silver ion and condensed it into silver to form AgNPs. It also confirmed the presence of aromatic and aliphatic amines and some proteins in the extract of fungus that act as capping agents to stabilize the formed AgNPs. Moreover, it was confirmed that these compounds stabilized the silver metal by joining with protein through amide bonds. Tan et al. (2002) also reported the participation of SH group-containing protein from the fungal extract in the capping and stabilization of AgNPs. Das et al. (2009) had used mycelia of R. for the synthesis of nano conjugate of gold NPs (AuNPs) through in situ reductions of chloro- auric acid (HAuC14) in an acidic medium (pH 3). Verticillium fungus is also a good intermediary for the synthesis of AgNPs. Recently, it is reported that biomass of fungi intracellularly synthesized NPs on exposure to silver nitrate (AgNO,) in an acidic medium (pH 5.5-6) (Mukherjee et al., 2001b).

Synthesis of NPs From Bacteria

The synthesis of NPs from bacteria includes two approaches: intracellular and extracellular approaches. Extracellular NPs synthesis has more benefits than intracellular synthesis because it is less time-consuming and it does not require any downstream process for setting of NPs by microorganisms (Singh et ah, 2016). Reductase enzymes occur inside the cell of bacteria that speed up the decline of metal ions into metal NPs. D. mdiodurans bacteria have vast antioxidant properties and are extremely resistant to rays and oxidative stress (Li et ah, 2016). It makes it positive for use in the green synthesis of AuNPs by ionic appearance. The fictitious AuNPs were constant for a longer time and showed superior antimicrobial potential. Kunoh et ah (2017) used a bacterial strain, Leptothrix, for the production of AuNPs by sinking gold salt in an aqueous standard (Kunoh et ah, 2017). The utilization of guanine residues of RNA molecules and 2-deoxy guanosine was done for reducing the gold salt.

Synthesis of NPs by Plant Source

The synthesis of NPs by plant source is extra capable in conditions of obtaining an advanced production than the microorganism. Flora encloses several metabolites and biochemicals (e.g., polyphenols) that can be used as reducing agents in biogenic NPs synthesis. NPs synthesized by a plant source are eco-friendly (avoid the use of poisonous substances) and cost effective. The stability of these NPs is higher compared to those which are synthesized from microorganisms (Singh et ah, 2016). Synthesis of NPs from plant sources can be categorized into three groups such as intracellular, extracellular, and using phytochemicals. The extracellular way is utilized when a plant extract is used as the raw material for the synthesis of NPs. Synthesis of NPs from an intracellular manner occurred inside the plant tissue cells by the exploitation of cellular enzymes. Post synthesis, the NPs are improved by rupture the plant cell wall. The occurrence of a higher quantity of phytochemicals in plant extract is linked to the synthesis of NPs in high yield (Mohammadinejad et ah, 2019). Synthesis of NPs by phytochemicals is not a general method as it requires information of the exact phytochemical present, which is needed for the stabilized NPs synthesis.

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