Mechanism of plant-mediated synthesis of silver nanoparticles – A review on biomolecules involved, characterisation and antibacterial activity
Introduction
Nanotechnology encompasses the designing of materials at nanoscale level to achieve exclusive properties, which can be suitably employed for the required applications [1]. Throughout the past decade, a vast potential in nanotechnology has been recognised due to the effectiveness of various metal nanoparticles against several pathogenic microorganisms such as bacteria [2], fungi [3], algae, yeast [4] and virus [5]. Nanoscale materials thought of unique antimicrobial agents having a high area to volume quantitative relation to unravel the matter of the emergence of microorganism multidrug resistance [6]. Several types of metal nanoparticles like magnesium [7], Iron [8], gold [9], copper [10], zinc [11], alginate [12] and silver [13] have come up, however, AgNPs have been established to be simplest because they have sensible antimicrobial activity against various microorganisms.
Almost 5000 years ago, Romans, Greeks, Egyptians and Indians used silver in several forms to preserve the food products [14]. In ancient period usage of silver utensils for eating and drinking and preservation of eatable and drinkable items perhaps due to the awareness of antimicrobial action [15]. In the recent decade, AgNPs have been received the huge attention of the scientists due to their remarkable defence against various pathogenic microorganisms. AgNPs and Ag-based compounds are extremely cytotoxic to several microorganisms, displaying robust biocidal effects on various species of bacteria, including Escherichia coli (E.coli) [16], Staphylococcus [17] and Streptococcus mutans [18]. AgNPs have been widely used for bacterial diseases [19], incorporated into dental materials [20], treatment of severe skin burns [21] and water purification [22] etc.
Conventional approaches such as physical and chemical methods are proposed for the synthesis of AgNPs [23]. However, these methods are associated with the use of heavy equipment, huge amount of energy input, highly toxic and dangerous chemical compounds that generate biological hazards and most of the times these methods are not ecological and safe [24]. Apart from these methods, Plant-mediated synthesis of AgNPs seems to be very rapid, simple, dependable, non-toxic and eco-friendly [25]. The synthesis of metal nanoparticles using plant extracts deliver beneficial over other biological synthesis methods which are associated with very difficult procedures such as maintaining microbial cultures. After synthesis, AgNPs characterisation is essential to investigate their characteristic features such as surface area, morphology, size, shape, aggregation and solubility, etc [26]. Since the physical and chemical properties of a nanoparticles might have a substantial influence on their biological properties [27]. Characterisation of synthesised AgNPs is necessary before evaluating their toxicity [28]. Several analytical techniques have been used for the characterisation of nanoparticles, such as Ultraviolet-visible spectroscopy (UV-vis), Transmission electron microscope (TEM), Scanning electron microscopy (SEM), Atomic force microscopy (AFM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), Energy Dispersive Analysis (EDAX) of X-rays, etc [29].
Steps associated with Plant-mediated synthesis of AgNPs and their characterisation techniques were shown in the Fig. 1. Antibacterial effects of AgNPs against bacterial cells are complicated [30]. However, direct morphological analysis by SEM or TEM provides structural modification of the bacterial cell [31]. It could provide us helpful info for understanding bactericide activity of AgNPs against bacterial cells. Furthermore, exact antibacterial mechanism of the AgNPs is still mysterious. Thus the antibacterial activities and mechanisms of AgNPs against several bacteria were reported. The present review recapitulates various plant-mediated methods for AgNPs synthesis, characterizations, and predicted antibacterial activity against various bacteria.
Section snippets
Mechanism of plant-mediated synthesis of AgNPs
The use of plants for the production of AgNPs facilitating a lot of interest due to its quick, ecological, non-pathogenic, inexpensive procedure [32]. Plant-mediated synthesis of AgNPs enables advancement over chemical and physical methods and easily scaled up for large-scale synthesis [33]. The very first article on the Plant-mediated synthesis of AgNPs using Alfalfa (Medicago sativa) has been reported in 2003 a step toward Plant-mediated nanotechnology [34]. Specific parts in the plants such
Characterisation techniques of silver nanoparticle
Characterisation is an essential step to finding out the AgNPs by their size, shape, morphology, structure, surface chemistry, surface charge, dispersity and surface area. Various techniques are employing to characterise the AgNPs, which are stated below.
Antibacterial activity of AgNPs
Silver has been extensively used as a therapeutic for several diseases since from ancient times [143]. Before the establishment of antibiotics treatment, silver was used as an antiseptic agent for the treatment of burns and open wounds [144]. The antibacterial activity of AgNPs on gram negative and gram positive bacteria is not similar but competes for one over the other [145]. There are contradictory conclusions regarding the antibacterial activity of AgNPs against gram negative and gram
Conclusion
Predominantly AgNPs are synthesised by wet chemical methods, where the chemicals used are associated with toxicity and biological hazards. So there is a requirement of plant-mediated AgNPs synthesis method. Green nanotechnology could achieve this essential by synthesising the AgNPs without utilising any toxic chemical as a reducing agent. It is understood that bio-molecules that are existing in plant extracts can act as both reducing and stabilising agents for synthesis of AgNPs. Plant-mediated
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