ReviewRevisiting the mechanistic pathways for bacterial mediated synthesis of noble metal nanoparticles
Introduction
Nanotechnology has emerged as a progressive and interdisciplinary science during the last few decades. The prefix “nano” indicates one billionth or 10−9 units. It is widely known that nanoparticles (NPs) range in size from 1 to 100 nm (Jafar Ali and Ali, 2015). Nanomaterials usually display unique biological, physical and chemical properties compared to their bulk matter (Ahmad et al., 2013; Wang et al., 2018a, Wang et al., 2018b). Synthesis of noble metal NPs has drawn the much attention due to potential applications in electronics, photonics, catalysis, nanomedicine, biofuel cells, biomedical engineering and biological recovery of metals (Daraee et al., 2016; Park and Na, 2015; Rana et al., 2017; Wang et al., 2015). Remarkably, silver nanoparticles (AgNPs) possess inhibitory and bactericidal effects. Antibacterial characteristics of AgNPs have enabled to minimize the antibiotic resistance, which has emerged as a major health problem in recent years (Jain et al., 2009; Musarrat et al., 2010; Prabhu and Poulose, 2012). Recently there has been an upsurge of interest in the microbial reduction of metallic ions to metal nanoparticles (NPs). Significant applications of the nanomaterials are usually size dependent, thus controlled size synthesis of nanomaterials is highly desired (Jiang et al., 2008; Narayanan and Sakthivel, 2010; Wang et al., 2017).
Various physical and chemical strategies could be employed to synthesize well-defined nanomaterials (Daraee et al., 2016; Shah et al., 2019). But these conventional synthesis protocols are not preferred due to high cost and involvement of hazardous materials (Jha et al., 2009; Narayanan and Sakthivel, 2010; Starowicz et al., 2006). Moreover, the large scale synthesis also face many issues such as low stability and less monodispersity (Manoj et al., 2018). Henceforth there is a growing demand for the employment of environmentally benign processes for the synthesis of stable NPs (Fayaz et al., 2010). In this regard elucidation of molecular mechanisms may play a critical role in the green synthesis of noble metal NPs with controlled dimensions and high monodispersity (Ali et al., 2016). Various mechanistic theories about bio-synthesis of NPs have been presented in literature (Ali et al., 2017; Ali et al., 2016). Majority of studies have speculated the nitrate reductase as a principle reducing agent along with its stabilizing feature. However, a plausible bio-synthesis mechanism may involve more than one cellular components. Pathways involved in the green synthesis of nanomaterials are of prime importance for commercialization of nanotechnology and also for environmental sustainability.
Synthesis mechanisms will also improve the bioremediation and biomineralization processes for environmental contaminants. Biomineralization is the utmost process of controlling the ultimate fate in biogeochemical cycling and ecological impacts of heavy metals (Diaz et al., 2015). A better understanding of microbial transformation pathway at the genetic level may lead to develop new genetic tools for accelerating the bioremediation strategies (Kang et al., 2017; Wu et al., 2016). Assumingly, biological transformation pathways of heavy metals can be explored further. An escalating number of publications about nanomaterials during the last decade illustrates the potential of this active domain. A thorough and comprehensive review of literature is needed to provide facts about the green synthesis. The fundamental insight of enzyme-metal interaction is elaborated here which, will enable the biotransformation of toxic heavy metals hence providing the detoxification effect (Liu et al., 2016; Venkatachalam et al., 2017).
Although several detailed reviews about the synthesis and applications as antimicrobial agents have been published (Davis and Shin, 2008), but very few studies have focused on mechanistic pathways involved in the green synthesis of NPs (Zhao et al., 2013). We have tried to summarize the mechanisms for green synthesis of NPs which have been reported during the last few years. Moreover, we have highlighted various biomolecules involved in the reduction of metals into their NPs. We have also described the potential remedial prospects based on these mechanisms. It is anticipated that the current study will improvise the understandings of molecular routes involved in biochemical processes at complex interfaces.
Section snippets
Mechanisms for the synthesis of noble metal nanoparticle
The specific mechanisms of NPs formation varies from organism to organism. However, the synthesis of NPs follows a generalized scheme wherein; metal ions are either entrapped into the microbial cells or on the microbial surface in the presence of an enzyme, metal ions get reduced to NPs (Yin et al., 2016). Initially, algal biomass of Verticillium sp. formulated the intracellular NPs of gold and silver, the exact pathway was not identified but NPs synthesis on the mycelial surface have
Conclusion
Green synthesis of metal NPs has resulted in efficient, cost-effective and eco-friendly fabrication methodologies. Applications of nanomaterials are highly size dependent. Exploring the underlying molecular mechanism of NPs formation is very necessary for controlled size synthesis and enhanced applications. Although several studies have presented the underlying mechanisms of NPs formulation, generally nitrate reductase is considered as principle reducing agent. Role of catalytic proteins and
Future prospects
Nanotechnology has emerged as a promising domain of modern science. Enzymatic pathways are mainly contributing to the biosynthesis of metal NPs. Mechanistic insights will accelerate the synthesis processes of controlled morphology, stabilized nanomaterials for enhanced applications. Biological synthesis of metal NPs has also contributed to remediate the environment contaminants (Prabhu and Poulose, 2012). Heavy metals contamination has become a significant concern due to non-biodegradable
Conflict of interest
Authors declare the no conflict of interest.
Acknowledgments
This work was supported by a University of Chinese Academy of Sciences (UCAS) Scholarship for International Students (to Jafar Ali), the National Key R&D Program of China (2017YFA0207203), National Natural Science Foundation of China (21407160), Strategic Priority Research Program of the Chinese Academy of Sciences (XDA09030203) and Program of Research and Demonstration of the Large Lake Control Technology for Algal Blooms and Endogenous Pollution Control (Y7A0011001) and Quaid-i-Azam University
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