**2. Synthesis of nanoparticles**

In broader terms, nanoparticles can be synthesized either by (i) Top- down approach, or (ii) Bottom- up approach [15]. Based on the reaction conditions and operation, these two classes can be further categorized as physical, chemical and biological methods [15].

i.Top- down approach

In this method, larger molecules are broken down into smaller units which are then transformed into suitable nanoparticles [15] According to a study, the synthesis of the spherical magnetite nanoparticles that uses

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*Antimicrobial Efficacy of Biogenic Silver and Zinc Nanocrystals/Nanoparticles to Combat…*

duces particle sizes ranging from 20 nm to 50 nm [15].

natural iron oxide is performed through top- down method which pro-

This method works in reverse to the top- down approach as nanoparticlessynthesised using this approach are formed from smaller and relatively simpler substances that form clusters and are subsequently converted into desired nanoparticles. This technique is also known as building up approach. Sedimentation and reduction technique fall under this category which include sol gel, green synthesis, spinning and biochemical synthesis [15].

Green synthesis of nanoparticles refers to the synthesis of nanoparticles through biological routes such as those with the help of microorganisms, enzymes, fungus plants or using various plant products [16, 17] Conventional physical or chemical methods of nanoparticle synthesis often produce byproducts that are hazardous to the environment which is one of the key reasons to opt for a more suitable alternative, that is, the green synthesis or green technology [16]. Other aspects by which green synthesis is more superior than the physical and chemical methods are that

Bottom- up approach is employed in biological- based synthesis of nanoparticles that requires the use of stabilizing and reducing agents [16]. The process of biologically synthesizing nanoparticles is basically divided into three steps: (i) the choice of a suitable solvent medium used, (ii) the choice of a suitable reducing agent that is eco- friendly and environmentally benign, and (iii) the choice of a non- toxic

Prokaryotes as well as eukaryotes are used in the green synthesis of metallic nanoparticles such as silver, gold, platinum, iron, and metal oxides such as zinc

*Bacteria:* Prokarytic bacteria and actinomycetes are widely used in the synthesis of metal and metal oxide nanoparticles as they have the potential to reduce metal ions and therefore, are suitable candidates for the preparation of nanoparticles [18] . The fact that it is relatively easier to manipulate bacteria is a key point in employ-

*Fungi:* Another popular choice for the biological synthesis of metal and metal oxide nanoparticles is fungi as they behave as better biological agents because they have diverse intracellular enzymes [18]. It is also reported that fungi can comparatively synthesize more amounts of nanoparticles than bacteria which could also be because of the fact that fungi have various enzymes/proteins/reducing components

*Yeast: Saccharomyces cerevisiae* has found to be quite effectively employed in the synthesis of silver and gold nanoparticles as reported in numerous studies [18]. *Plants:* The most simple, efficient, cost effective and feasible method of biosynthesis of metal and metal oxide nanoparticles is using plants and plant extracts as biological agents. Biomolecules such as carbohydrates, proteins and coenzymes extracted from plants are employed to reduce metal salt into nanoparticles [18].

*DOI: http://dx.doi.org/10.5772/intechopen.99200*

**3. Green synthesis of nanoparticles**

oxide and titanium oxide [17].

they are cost efficient and consume less energy [16].

**3.1 Biological components for green synthesis**

ing them in nanoparticle synthesis [18].

on the surface of their cells [18].

capping agent that can stabilize the synthesized nanoparticles [16].

ii.Bottom- up approach

natural iron oxide is performed through top- down method which produces particle sizes ranging from 20 nm to 50 nm [15].

ii.Bottom- up approach

*Materials at the Nanoscale*

wide range of applications including biosensors, organic marking, cancer therapy, textiles, household and industrial applications etc. Silver nanoparticles AgNPs are mostly used in wound dressings, care of the eye, oral hygiene, biomaterials of bone substitutes, antimicrobial and anti-inflammatory drugs as well as in the coating of catheter products as anti-inflammatory and antimicrobial agents [2]. Silver is a stable and non-hazardous antibiotic agent used for centuries [3]. Most antimicrobials have many disadvantages including low stability, environmental toxicity and the lack of specificity towards the target microorganisms [4]. Few other antimicrobials

Silver has the unusual property of binding cellular components that are far larger than nuclear acids [6]. AgNPs may be synthesized employing physical, chemical and biological processes. The majority of the physical and chemical processes of synthesizing nanoparticles have many disadvantages such as low yield, strong reducing agents, energy-intensive mechanisms, uneven particle size and aggregate instability, hazardous waste production, difficulty to scale up and expensive organo-metallic precursors are required [7]. Biological approaches for the synthesis of nanoparticles are regarded as more stable and efficient [8]. For several nanoparticles like gold, silver, platinum and palladium, titanium dioxide, magnetite and cadmium sulphide, the most possible bio-factories are bacteria. Bacteriamediated AgNPs synthesis is preferred in comparison with other techniques.

Furthermore, bacteria mediated AgNPs are simpler to grow and environment friendly. Both intracellular (biomass) as well as extracellular (cell extracts) synthesis of silver nanoparticles can be performed. Intracellular approaches include the release of synthesized nanoparticles through ultrasonication and additional reactions with specific detergents. It is therefore essential that the AgNPs are synthesized with extracellular methods because of their easy downstream processing that supports large-scale development [7, 9, 10]. There is now a prevalence of multiple tolerance to antibiotics by various clinical infections and pathogens of the urinary tract, caused by excess antibiotics and by an accumulation of antibiotics in the system. This kind of resistance is exhibited by *Staphylococcus* sp., *Streptococcus* sp., *Klebsiella* sp., *Enterococcus* sp., *Proteus* sp., *Pseudomonas* sp. and *E. coli* due to their biofilm-forming potentials [11]. The use of antimicrobial silver nanoparticles will eliminate the multiple-drug resistance, which is a suitable option for antibiotics [12]. Biofilm formation has been regarded as the global barrier in avoiding catheterrelated infections in the field of medicine [13]. The conversion of nanoparticles into therapeutic agents, however, involves a detailed knowledge of the physicochemical particularities, results *in vitro* and *in vivo*, biodistribution, pharmacokinetics and pharmacodynamics, apart from the suitable methods of their synthesis [14].

In broader terms, nanoparticles can be synthesized either by (i) Top- down approach, or (ii) Bottom- up approach [15]. Based on the reaction conditions and operation, these two classes can be further categorized as physical, chemical and

In this method, larger molecules are broken down into smaller units which are then transformed into suitable nanoparticles [15] According to a study, the synthesis of the spherical magnetite nanoparticles that uses

are extremely irritating and expensive to develop [5].

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**2. Synthesis of nanoparticles**

i.Top- down approach

biological methods [15].

This method works in reverse to the top- down approach as nanoparticlessynthesised using this approach are formed from smaller and relatively simpler substances that form clusters and are subsequently converted into desired nanoparticles. This technique is also known as building up approach. Sedimentation and reduction technique fall under this category which include sol gel, green synthesis, spinning and biochemical synthesis [15].
