Biological Methods Used in the Synthesis of Nanomaterial
Recently, there has been a growing interest in implementing green chemistry in the synthesis of nanomaterials, in order to reduce waste products and obtain processes that are sustainable. The use of biological methods in the synthesis of nanomaterials plays a crucial role in medicine as well as technology; for example, the use of magnetotactic bacteria in the preparation of magnetic nanoparticles.1211 In previous decades, only prokaryotic members were used for their ability to reduce toxic, insoluble metal ions to nontoxic, soluble metal salts. However, recently, eukaryotic members such as plants, diatoms, and algae have been used for the same purpose as prokaryotes, as their ability to reduce metal ions to metal nanoparticles was recognized.1211
The advantages of using biological approaches in the synthesis of nanomaterials include improved biocompatibility, fast synthesis of nanomaterials in neutral pH and under ambient temperatures. In addition to this, biological methods are less toxic and produce nanoparticles of a controlled size and morphology.1211 Biomaterials possessing reducing potential are used in the biodirect synthesis of metal nanoparticles. Biomaterials such as phytochemicals, enzymes, or moieties derived from natural sources such as plants, yeast, fungi, bacteria, and actinomy- cetes are used for both their reducing potential and stabilizing capability.121-231
Microorganisms produce nanoparticles by taking in the desired metal ion from the surrounding, reducing metal ions to elemental metals through the action of various metabolites and cellular enzymes. The nanoparticles produced by microbes are classified as intracellular or extracellular according to the site of production in the microorganism. Intracellular as well as extracellular inorganic nanoparticles can be produced by both unicellular and multicellular organisms (Figure 2.2).121221
The top-down approach is classified into three main categories, which are: grinding system, mechanochemical, and mechanical alloying method. The grinding system is further divided into dry grinding and wet grinding.1221
Mechanical grinding is a good example of a top-down approach that is used in the synthesis of nanomaterials. Mechanical attrition is based on the principle of breaking down coarse material into finer particles as a result of severe plastic deformation. Mechanical grinding is the most popular method used in the synthesis of nanomaterials as it is simple, requires relatively inexpensive equipment, and can be applied for almost all classes of materials.1221
The main advantage of using mechanical grinding is that it is easy to scale up. However, the main disadvantage of mechanical
FIGURE 2.2 Classification of methods of synthesis of nanomaterials.
FIGURE 2.3 Schematic representation of mechanical grinding.
grinding is contamination of the prepared nanomaterial from the atmosphere or milling media. The problem of contamination can cause this method to be dismissed and overlooked, at least for some materials.1231
The mechanism of mechanical milling is based on the principle that energy is transferred from the steel ball or refractory to the powder, which in turn, results in shear stress which is responsible for the formation of nanomaterials. This energy is generated by high-energy planetary ball, shaker, or tumblers. The transferred energy depends on the vibrational or rotational speed of the balls, the number and size of the balls, as well as the ball size-to-powder mass ratio. In addition to this, it also depends on the milling atmosphere and duration of milling (Figure 2.3).1231
Dry Grinding Process
In dry grinding, the bulk material is broken down into finer particles using compression force, shock, or by friction using methods such as hammer milling, jet milling, roller milling, shear milling, shock shear milling, tumbling milling, and using a ball mill. In the dry grinding process, pulverization is accompanied by particle condensation, which in turn makes it difficult to produce particles sizes of less than 3 pm.1241
Wet Grinding Process
In wet grinding process, equipment such as tumbling ball mill, planetary ball mill, centrifugal fluid mill, vibratory ball mill, agitation bead mills, annular gap beads mill, fluid conduit bead mill, and a wet jet mill are used to produce the nanomaterials. The wet grinding process is favored over the dry grinding process as it prevents condensation of the formed nanoparticles and hence, helps in producing highly dispersed nanoparticles.1241