Role of Plant’s Metabolites in the Biomimetic Synthesis of Plant-Mediated Silver Nanoparticles: A Review
Ropisah Me1,2*, Muhammad Hafiz Istamam1, Nur Alyaa Syazwani Amir1, Rashidah Iberahim1, Alice Shanthi3, Noor Hidayah Pungot4, Nazlina Ibrahim5
1Faculty of Applied Sciences, Universiti Teknologi MARA, Cawangan Negeri Sembilan, Kampus Kuala Pilah, Negeri Sembilan, Malaysia
2Atta-Ur-Rahman Institute of Natural Product Discovery, Universiti Teknologi MARA, Kampus Puncak Alam, Selangor, Malaysia
3Academy of Languages Study, Universiti Teknologi MARA, Cawangan Negeri Sembilan, Kampus Seremban, Negeri Sembilan, Malaysia
4Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia
5Faculty of Science & Technology, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia
*Corresponding Author: [email protected],my
Accepted: 15 December 2020 | Published: 31 December 2020
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Abstract: Biomimetic synthesis of silver nanoparticles (AgNPs) has attracted interest due to its eco-friendliness, safe, cost effective and suitability for biomedical applications. Primary plants metabolites such as proteins, amino acids, vitamins and polysaccharides and secondary plants metabolites including terpenoids, flavonoids, polyphenol, ascorbic acid, alkaloids, saponins, steroids, tannin plays significant role in bioreduction and stabilization of the synthesized silver nanoparticles. In this study, several types of metabolites from plants that influence the biomimetic synthesizing of silver nanoparticles were reviewed. The present review indicates the presence of several phytochemical groups in Eluesine indica (Sambau) extract may contribute to the formation of AgNPs.
Keywords: primary metabolite, secondary metabolites, plant-mediated silver nanoparticles, reducing agent, Eleusine indica
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1. Introduction
Nanotechnology is an important area of modern research that deals with the design, synthesis and manipulation of particle structure in one dimension from about 1 nm to 100 nm (Panigrahi, 2013). Nanotechnology is gaining rapid prominence in areas such as healthcare, cosmetics, food, environmental safety, mechanics, optics, biomedical sciences, chemical industries, electronics, space industries, drug-gene delivery, energy sciences, optoelectronics, catalysis, reorography, single electron transistors, light emitters, nonlinear optical devices and photoelectrochemical applications (Panigrahi, 2013). The demand is growing at an exponential pace, due to their applicability in a wide sector of life.
Silver is one of the most commercialized nano-materials with an annual production of 500 tons of silver nanoparticles, which is expected to grow in the next few years (Ahmed et al., 2016).
Silver nanoparticles (AgNPs) are considered of great importance due to its high antiviral, antibacterial, antifungal and anticancer properties (Balashanmugam & Kalaichelvan, 2015).
Silver nanoparticles have shown good potential in biomedical applications for wound healing (Dhapte et al., 2014), anticancer activity (Abdel-Fattah & Ali, 2018), drug delivery system and osteogenesis activity (Burdusel et al., 2018). In addition, the application of AgNPs was also
found in other various fields including water treatment (Balavigneswaran et.al, 2014), optoelectronic devices (Poulose et al., 2014) and catalytic activities (Mosaviniya et al., 2019).
A number of physical and chemical methods have been used to synthesized AgNPs, but remains to be costly and require the use of hazardous chemicals (Lalitha et al., 2013).
Therefore, biomimetic synthesis or green synthesis of AgNPs using phytochemically active plant extracts has attracted considerable interest. A wide range of metabolites from plants play a significant role in reducing silver ions and also can act as stabilizing agent to form AgNPs (Panigrahi, 2013). Herein, we give an overview of the role of plant metabolites in the biomimetic synthesizing of silver nanoparticles.
2. Plant-Mediated Silver Nanoparticles
Previous techniques such as assisted radiation, thermal decomposition, sonochemical and electrochemical techniques used in AgNPs processing contributed to the presence of harmful chemical such as NaBH4 on the AgNPs surface and are incompatible for medical application (Din et al., 2015). Therefore, biomimetic synthesis approaches seems to be very fast, effective, non-toxic and environmental friendly (Rajeshkumar & Bharath, 2017). Numerous studies that reports on the use of plant extracts in synthesizing AgNPs are listed in Table 1.
Table 1: Plants extract used in synthesizing of AgNPs
Plant extracts References
Ephedra intermedia Gul et al., 2017 Bauhinia acuminate Hu et al., 2019
Ficus hispida Ramesh et al., 2018
Skimmia laureola Ahmed et al., 2015 Salvia miltiorrhiza Zhang et al., 2019 Moringa oleifera Moodley et al., 2018
Carica papaya Jain et al., 2009
Camellia sinensis Loo et al., 2012 Azadirachta indica Ahmed et al., 2016 Datura stramonium Gomathi et al., 2017 Thevetia peruviana Oluwaniyi et al., 2016 Ocimum sanctum Ramteke et al., 2013 Callistemon citrinus Layaretan et al., 2019
Retama extract Alfalluos, 2017
Nepeta deflersiana Al-Sheddi et al., 2018 Cranberry extract Ashour et al., 2015 Ramalina dumeticola Din et al., 2015
Cassia roxburghii Balashanmugam & Kalaichelvan, 2015
2.1 Type of Plant’s Metabolites in the Formation of Plant-Mediated Silver Nanoparticles
One of the major benefits of using plant extracts for the synthesis of AgNPs is the availability of plant products, non-toxicity and number of metabolites that can cause silver reduction (Gumel et al., 2019). Plant extract containing proteins, enzymes, alkaloids, tannins, flavonoids, phenolics, quinines and oils can be used for the synthesis of nanoparticles (Borase et al., 2014).
Table 2 shows several plant metabolites used in the green synthesis of AgNPs from various plant species.
Table 2: Plant metabolites from various plant species involving in biomimetic synthesizing AgNPs
Plant species Plant metabolites References
Ocimum sanctum Protein, terpenoids Mallikarjuna et al., 2011 Capparis spinosa Carbohydrates, glycosides, flavonoid Benakashani et al., 2016
Elettaria cardamomom Flavones Gnanajobitha et al., 2012
Azadirachta indica Alkaloid, terpenoids Ahmed et al., 2016
Coleus aromaticus Flavanoids Vanaja & Annadurai, 2013
Nigella arvensis Flavonoids, alkaloids Chahardoli et al., 2017 Lantana camara Flavonoids, terpenoids, alkaloids Ajitha et al., 2015 Dioscorea bulbifera Polyphenols, flavonoids Ghosh et al., 2012 Datura stramonium Proteins, polyphenols, carboxylic acid Gomathi et al., 2017 Ficus hispida Polyphenols, glucose and fructose Ramesh et al., 2018 Parmotrema prasorediosum Phenolics, glycosides Mie at al., 2013 Cranberry extract Polyphenolic compounds Ashour et al., 2015
Salvia miltiorrhiza Diterpenes Zhang et al., 2019
2.2 Role of Plant’s Metabolites in the Formation of Plant-Mediated Silver Nanoparticles The biomimetic synthesis of plant-mediated silver nanoparticles promoted via plant extracts occurs in three different steps involving induction phase, growth phase and termination phase.
The induction phase is where ion reduction and nucleation of metallic seeds into small, reactive and unstable crystals occurs. Then the ion is spontaneously aggregated and transformed into large aggregates (growth phase), and when the sizes and shapes of the aggregates become energetically favourable, some biomolecules will act as capping agents to stabilize the nanoparticles (termination phase) (Marchiol et al., 2014). According to Makarov et al. (2014), plants are able to reduce metal ions on their surface as well as in different organs and tissues remote from the ion penetration site. Metabolites in plant extracts, include enzymes, proteins, amino acids, vitamins, polysaccharides and organic acids such as citrates, can potentially reduce ions in metals. Some metabolites groups such as flavonoids, phenolic, carbohydrates, sugars and proteins, can serve as both reduction agents and stabilisation agents in nanoparticles synthesis (Chahardoli et al., 2017; Mahakham et al., 2017). Eleusine indica, a species of the Poaceae family have huge amounts of metabolites including flavonoids, terpenoids, phenolics, steroids, alkaloids and glycosides (Iberahim et al., 2015;2018). The presence of these metabolites in E. indica extract have potential to act as a reducing and stabilizing agent in the formation of AgNPs.
2.3 Plant Metabolites as Reducing Agent
Plant metabolites responsible for silver ion reduction include tannins, terpenoids, flavonoids, ketones, aldehydes, amides and carboxylic acids (Prabhu & Poulose, 2012). Secondary metabolites have a polyhydroxy group that is responsible for its strong antioxidant activity and free radical scavenging properties can also play a major role in reducing metal ions to nanoparticles (Sahu et al., 2016). Terpenoids are a class of diverse organic polymers synthesized in five-carbon isoprene plants that exhibit strong antioxidant activity that plays a key role in converting silver ions into nanoparticles (Makarov et al., 2014). Flavonoids are a broad group of multi-class polyphenolic compounds: anthocyanins, isoflavonoids, flavonols, chalcones, flavones, and flavanones that can actively chelate and reduce metal ions into nanoparticles (Makarov et al., 2014). Jain & Mehata (2017) was found that the hydroxyl groups in flavonoids such as quercetin are responsible for reducing silver ions to AgNPs as a result of tautomeric transformation of flavonoids from enol to keto type, which may release reactive hydrogen atoms that reduce silver ions to silver nanoparticles.
Phenols, flavonoids, and triterpenoic acid contain active oxygen that can contribute electrons into AgNPs to reduce silver precursors (Ramesh et al., 2018). Benakashani et al. (2016) reports that the presence of carbohydrates, glycosides, and flavonoids in leaf extract Capparis spinosa play important role in Ag+ reduction. The presence of reducing sugar and terpenoids in leaf concoction of Azadirachta indica plays a role in reduction of silver ions (Firdhouse & Lalitha, 2015). Monosaccharides such as glucose which have free aldehyde group can act as reducing agents. Proteins such as amino acids have differed in their ability to bind and reduce metal ions (Makarov et al., 2014). The presence of aromatic amine, amide (I) groups, phenolic groups and secondary alcohols in Coleus aromaticus leaf extract, can act as reducing agents (Vanaja &
Annadurai, 2013). In addition, according to Jha & Prasad (2010), the process of nano- transformation may have resulted due to redox activities of ascorbic/dehydroascorbic acid and flavones and involvement of ascorbates/glutathiones/metallothioneins, resulting in the reduction of the silver ions present in the Cycas beddomei leaf solution.
2.4 Plant Metabolites as Stabilizing Agent
Nanoparticles of metals have high surface energy which makes them less stable. Therefore, stabilizer is necessary to control the formation and dispersion stability of metal nanoparticles during the process of synthesizing nanoparticles (Ahmad et al., 2011). The presence of hydrocarbons such as nonacosane and heptacosane could potentially affect the reduction and stabilization cycle of AgNPs (Roopan et al., 2013). Spectroscopic analysis on this material containing functional groups of C=O and C=N has been indicating that the amide and polypeptide groups was served as capping agents which stabilizes the aggregation of the nascent metallic nanoparticles (El-Seedi et al., 2019). In Rhododendron dauricum extract, the oxidation of aldehydrate groups of terpenoids in carboxylic acid molecules result in the simultaneous reduction of silver salt accompanied by capping on the surface of synthesized nanoparticles thus providing stability to the nanoparticles (Mittal et al., 2012). The enzymes in the Ocimum sanctum leaf extract was combined with silver ions form a complex enzyme substrate with a transfer of charges between quercetin and Ag+ resulting in the formation of protein-capped silver nanoparticles, thus providing the stability against agglomeration (Jain &
Mehata, 2017).
Previous reports state that biosynthesis of AgNPs using Tithonia diversifolia (Tran et al., 2013), Ziziphora tenuior (Sadeghi & Gholamhoseinpoor, 2015), Vigna radiata (Kumari et al., 2017) and Azadirachta indica (Panigrahi, 2013) were produced amino acid residue and protein carbonyl group that have a greater ability to bind metal, meaning that the proteins may potentially form metal nanoparticles (i.e., silver nanoparticles capping) to avoid agglomeration and thereby stabilize the medium. This statement is also supported by findings from a study conducted by Logeswari et al. (2013) who found that proteins can bind AgNPs via free amine groups in proteins, and thus making it possible to stabilize the AgNPs by surface-bound proteins. Electrostatic stabilization is based on the creation of a charged layer by adsorption of ionic groups present in the medium to the surface of AgNPs, thereby generating a repulsive force between them to prevent aggregation (Mashwani et al., 2016). Therefore, it is assumed that these biomolecules and other proteins are responsible for capping, stabilizing and reducing Ag+ to AgNPs (Kumar et al., 2014).
4. Discussion and Conclusion
Plant extracts have a potential to reduce silver ion into AgNPs. Among the numerous biological routes that have been developed, methods of synthesis using plant extracts have many advantages over others as it does not involve any cell culture and the process can easily be
scaled up for synthesis at large scale (Roy et al., 2015). Plants are abundant of primary metabolites (proteins, amino acids, vitamins, polysaccharides) and also contain a huge number of secondary metabolites (terpenoids, flavonoids, polyphenol, ascorbic acid, alkaloids, saponins, steroids, tannins) which prove to serve as both reducing and stabilizing agents, and inhibit the aggregation and agglomeration of novel metallic nanoparticles by non-hazardous means (El-Seedi et al., 2019). Thus, there is a need to investigate other potential plants that can be used in synthesis of silver nanoparticles. Plant extracts are eco-friendly, cost and energy efficient and pharmaceutically compatible over physical, chemical and microbial synthesis (Jyoti et al., 2016). Eleusine indica or sambau extract is one of the plants with wide variety of metabolite groups such as flavonoids and terpenoids, phenolics and glycosides (Iberahim 2015;2018). Furthermore, the antivirus activity previously reported by the plant extract provides interesting potential for formation of AgNPs.
Acknowledgement
The authors would like to acknowledge Ministry of Education for the financial support under Fundamental Research Grant Scheme (FRGS/1/2019/STG01/UITM/02/10) and Universiti Teknologi MARA (UiTM), Cawangan Negeri Sembilan, Kampus Kuala Pilah, Negeri Sembilan; Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam, Selangor;
Faculty of Science & Technology, Universiti Kebangsaan Malaysia, Bangi, Selangor for providing the research facilities.
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