Green Synthesis of Phytochemical Nanoparticles and their Antimicrobial Activity, A Review Study

which are major phytoconstituents of the plant extracts, and stabilizing agents such as polysaccharides and proteins, stable metal NPs can be easily synthesized using the plant extracts. Green synthesis provides advancement over chemical and physical method as it is cost effective, environment friendly, easily scaled up for large ABSTRACT Rapid industrialization, urbanization and population explosion are resulting in deterioration of earth atmosphere and a huge amount of hazardous and unwanted substances are being released. It is now high time to learn about the secrets that are present in the nature and its natural products which lead to advancements in the synthesis processes of NPs. Traditionally nanoparticles were produced only by physical and chemical methods. Often, chemical synthesis method leads to presence of some of the toxic chemical absorbed on the surface that may have adverse effect in the medical applications. This is not an issue when it comes to biosynthesized nanoparticles via green synthesis route. So, in the search of cheaper pathways for nanoparticles synthesis, scientist used microbial enzymes and plant extracts like phytochemicals. In this review, Ag metal is used because of its specific properties like larger surface area to volume ratio, less reactive, more bioactive, potent and stable. Different phytochemical compounds of plants such as flavonoids, saponins, alkaloids, phenolic compounds, tannins, terpenoids etc. are mostly bound with Ag+ ions to form complex that give good antimicrobial effect. It is investigated that the zone of inhibition of plant extracts against microorganisms are less than the zone of inhibition of AgNps. So, it can be said that AgNps are more bioactive and potent than the plant extracts and have good antimicrobial activity. Future studies will probably focus on obtaining other nanoparticles with antimicrobial effects at its maximum level and toxicity at minimum.


Introduction
Incorporation of green chemistry techniques and methodologies into nanotechnology is of great interest which has gained much attention over the past decade [1]. Furthermore, NPs are widely applied to human contact areas and there is a growing need to develop processes for synthesis that do not use harsh toxic chemicals [2]. The nanoparticles synthesized from chemical and physical methods generally require high temperature, pressure, expensive equipment, toxic chemicals, and reagents and most importantly capping agents for the stabilization of nanoparticles; thus, these methods are toxic to environment and nonecofriendly [3]. With their antioxidant or reducing properties they are usually responsible for the reduction of metal compounds into their respective nanoparticles [4]. The conventional methods for the production of NPs are expensive, toxic, and non-environment friendly. To overcome these problems, researchers have found the precise green routes like the naturally occurring sources and their products that can be used for the synthesis of NPs [5].
Therefore, green/biological synthesis of NPs is a possible alternative to chemical and physical methods [6]. Biological methods of synthesis have thus paved way for the "greener synthesis" of nanoparticles and these have proven to be better methods due to slower kinetics [7]. Recently, green methods using plant extracts have been developed as an alternative for common chemical and physical methods to synthesize noble metal NPs. Due to the presence of reducing agents like alkaloids, polyphenols, and flavonoids which are major phytoconstituents of the plant extracts, and stabilizing agents such as polysaccharides and proteins, stable metal NPs can be easily synthesized using the plant extracts. Green synthesis provides advancement over chemical and physical method as it is cost effective, environment friendly, easily scaled up for large

ARTICLE INFO ABSTRACT
Rapid industrialization, urbanization and population explosion are resulting in deterioration of earth atmosphere and a huge amount of hazardous and unwanted substances are being released. It is now high time to learn about the secrets that are present in the nature and its natural products which lead to advancements in the synthesis processes of NPs. Traditionally nanoparticles were produced only by physical and chemical methods. Often, chemical synthesis method leads to presence of some of the toxic chemical absorbed on the surface that may have adverse effect in the medical applications. This is not an issue when it comes to biosynthesized nanoparticles via green synthesis route. So, in the search of cheaper pathways for nanoparticles synthesis, scientist used microbial enzymes and plant extracts like phytochemicals. In this review, Ag metal is used because of its specific properties like larger surface area to volume ratio, less reactive, more bioactive, potent and stable. Different phytochemical compounds of plants such as flavonoids, saponins, alkaloids, phenolic compounds, tannins, terpenoids etc. are mostly bound with Ag+ ions to form complex that give good antimicrobial effect. It is investigated that the zone of inhibition of plant extracts against microorganisms are less than the zone of inhibition of AgNps. So, it can be said that AgNps are more bioactive and potent than the plant extracts and have good antimicrobial activity. Future studies will probably focus on obtaining other nanoparticles with antimicrobial effects at its maximum level and toxicity at minimum. scale synthesis [8]. Green  Therefore, synthesis of AgNPs is an emerging area and interesting subject. In green synthesis, a solvent (usually water) is chosen and employed in step one. A non-toxic reducing and stabilizer agents are utilized in steps two and three, respectively. In this method, solvents, reducing, and stabilizers agents are selected from natural non-toxic and eco-friendly substances without any adverse effects on the environment.     (Tables 1 & 2) Many research papers reported the synthesis of silver nanoparticles using plant extracts such as Croton sparsiflorus (Ban tulasi) [1]; Chlorophytum borivilianum (Musli) [5]; Musa paradisiaca (Banana) [6]; Aloe vera [7]; Enteromorpha flexuosa (Green alga) [8]; salvinia molesta (Giant salvinia or exotic weed) [9]; Cissus quadrangularis (Veldt grape) [10]; Ficus benghalensis (Banyan) [11]; Azadirachta indica (Neem) [11]; Cocos nucifera (Coconut) [12]; Pithophora oedogonia (Green alga) [13]; Aegle marmelos (Bael) [14]; Dalbergia spinosa [15]; Lythrum salicaria (Purple loosestrife) [16]; Euphorbia confinalis (Spurge) [17];

5-one and beta sitosterol-α-D-glucose) of Ficus benghalensis (family:
Moraceae) with size 40nm and analyzed by UV-spectrophotometer, DLS, Fe-SEM, AFM and ATR-FTIR [11]. Various phytochemicals like nimbin, nimbinin, deacetyl nimbin, nimbinene, 6-deacetyl nimbinene, nimbandiol, polysaccharides G1A, G1B, G2A, and G3A, and NB-2 peptidoglucan were synthesized from the bark extract of Azadirachta indica and used in the synthesis of Ag-Nps with 50nm size and showed antimicrobial activity against B. subtilis, E. coli, P. aeruginosa and V. cholerae [11]. The synthesized silver nanoparticles showed maximum activity by using leaf extract of coconut tree against K. pneumoniae, P. shigelloides, V. Alginolyticus, Salmonella paratyphi and Bacillus subtilis and produced 22nm size with spherical shape nanoparticles. The synthesized nanoparticles were characterized by UV-visible spectroscope, FTIR and TEM analysis [12].  [15]. The biosynthesized silver nanoparticles were characterized by UV-Vis spectroscopy and TEM analysis. The antibacterial activity of these nanoparticles against Pseudomonas aeruginosa was measured by disc diffusion method, agar cup assay and serial dilution turbidity measurement assay [19]. The phytochemical screen¬ing of Chrysanthemum indicum revealed the presence of flavonoids, terpenoids, and glycosides, suggesting that these compounds act as reducing and stabilizing agents. The spherical and hexagonal Ag-NPs were also synthesized by using Chrysanthemum indicum flower extract with an average particle size from 37.71-71.99nm and characterized by using UV-Vis spectroscopy, XRD, TEM, and EDX. The antimicrobial effect of the synthesized AgNPs revealed a significant effect against the bacteria Klebsiella pneumonia, Escherichia coli, and Pseudomonas aeruginosa [20].
The biosynthesis of silver nanoparticles using Taraxacum officinale floral extract showed the formation of nanoparticles of spherical shaped with a size of 545nm ± 5nm) upon addition of 1 mM silver nitrate. AgNPs synthesized from floral extract of T.officinale showed good antibacterial activity against selected pathogns such as Enterococcus faecalis and Pseudomonas aeruginosa by disc diffusion assay [21].
The spherical-shaped AgNPs were synthesized by using Phoenix dactylifera seed extract as stabilizing agent and characteristics of particles were studied by using UV-Vis spectroscopy, SEM, HR-TEM, and DLS. The antibacterial activities were found to be increased with the increasing concentration of AgNPs. The zone of inhibition was greater (24mm) at highest concentrations (500 g/ml) of AgNPs, while smaller (11mm) at lowest concentrations (7.8 g/ml) [22]. The silver nanoparticles (AgNPs) synthesized using hot water olive leaf extracts as reducing and stabilizing agent is evaluated for antibacterial activity against drug resistant bacterial isolates. The silver nanoparticles were with an average size of 20-25 nm and mostly spherical.

Conclusion
In summary, it is concluded that during the last decade many efforts have been made for the development of green synthesis.
Green synthesis gives headway over chemical and physical methods as it is cost-effective, eco-accommodating and effectively It is understood that the variety of natural compounds that are present in plant extracts can act as both reducing and stabilizing agents for synthesis of AgNPs. Plants mediated AgNPs are stable due to the presence of natural capping agents such as proteins which prevent the particles from aggregation. Green synthesis of AgNPs using plant extracts has several advantages such as ecofriendliness, biocompatibility and cost-effectiveness. It is concluded that due to these unique properties, AgNPs will have a key role in many of the nanotechnology-based processes.