Metal Nanoparticles of Microbial Origin and Their Antimicrobial Applications in Food Industries

Introduction

Nanotechnology (“nanotech”) is the art and science of manipulating matter at the atomic or molecular scale and holds the promise of providing significant improvements in technologies for protecting the environment. The birth of nanotechnology is attributed to physicist Richard Feynman, who suggested the possibility of manipulation of individual atoms as a more powerful form of synthetic chemistry than those used at the time In 1974, Norio Taniguchi first used the word nanotechnology in the context of an ion sputter machine to refer production technology to get the extra-high accuracy and ultra-fine dimensions, that is the preciseness and fineness in the order of 1 nm (Chawla et al.. 2018). However, the term nano is adapted from the Greek word meaning “dwarf”. When used as a prefix, it implies 10~ A nanometre is one-billionth of a metre, or roughly the length of three atoms side by side. A DNA molecule is 2.5 nm wide; a protein, approximately 50 nm; and a flu virus, about 100 nm. A human hair is approximately 10,000 nm thick. The National Nanotechnology Initiative (NNI) defines nanotechnology as the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (Chawla et al., 2019). In the 21st century, nanotechnology has become one of the key technologies influencing science on a global scale. The engineering with nanomaterials is an enabling technology that has opened up new avenues of research and development in several fields, including medicine, cosmetics, agriculture, and food, and is being used as a means for comprehending how physicochemical characteristics of nano-sized substances can change the structure, texture, and quality of foodstuffs. Modern science is focused on exploiting some of the special properties expressed by nanomaterials to generate useful technologies for the benefit of humanity (Agarwal et al., 2020). Current applications of nanotechnology in the food sector, pharmaceutical, cosmetics, nutraceuticals, and water purification, among others, have made it prevalent as a commercial commodity in many aspects of modern life (Pathania et al., 2018). Accordingly, there is an essential requirement to develop high-yield, low-cost, non-toxic, simple, and environmentally benign procedures for the green synthesis of nanoparticles (NPs; Singh et al., 2020). Consequently, the biological approach for the synthesis of NPs is important, especially in employing microorganisms as novel green “nanofactories” (Dhull et al., 2019). The adoption of biological methods in the synthesis of NPs is expected to yield novel structural entities of the desired morphology.

Routes for the Synthesis of Nanostructures

There are different routes for synthesizing nanostructures which are mainly categorized into chemical routes and biological routes.

Chemical Synthesis

Chemical method of synthesis is valuable as it takes a tiny time for synthesis of a large number of NPs. Nevertheless, in this method, capping agents are necessary for the size stabilization of the NPs (Chawla et al., 2020). Following are some of the wet chemical techniques that are used for the synthesis of metal NPs by chemical means (Tieshi et al., 2009):

  • • Microemulsions
  • • Solvent-extraction reduction
  • • Chemical oxidation/reduction
  • • Sol-gel
  • • Coprecipitation
  • • Hydrothermal/solvothermal
  • • Ultraviolet (UV) irradiation
  • • Template-assisted

Biological Synthesis

The biosynthesis approach typically employs whole living organisms for the synthesis of bio-inorganic materials. Although solution-based chemical methods enjoy a long history dating back to the pioneering work of Faraday on the synthesis of aqueous gold colloids, biosynthesis is still largely in the “discovery phase” wherein different nanomaterials are synthesized using microorganisms like fungi, bacteria, algae, and plants (Bansal et al., 2012). Microorganisms have been employed for an eon of time towards remediation of toxic metals due to their inherent capability to withstand high concentrations of heavy metal ions through specific resistance mechanisms (Vails and De Lorenzo, 2002), but the possibility of exploring these organisms for nanomaterials synthesis is a relatively recent phenomenon. A few early reports in this area encompassed organisms known to create specific functional materials in natural habitats, for example silica in diatoms (Parkinson and Gordon, 1999), gold in algal and bacterial cells (Hosea et al., 1986), cadmium sulfide (CdS) in bacteria and yeast (Dameron et al., 1989), zinc sulfide (ZnS) in sulfate-reducing bacteria (Temple and Le Roux, 1964) and magnetite in magnetotactic bacteria (Blakemore et al., 1979). Although using diatoms or magnetotactic bacteria to synthesize nanomaterials in our laboratories might sound interesting, this is not necessarily highly appealing from a fundamental perspective since these organisms are already known to create these specific inorganic materials during their natural growth.

An important and even more interesting aspect of the biological synthesis is the use of living organisms for the synthesis of those inorganic materials, which these organisms are not known to encounter during their natural growth environments (e.g. gold, silver, oxide nanomaterials, etc.). These observations were the source of inspiration for using microorganisms for deliberate synthesis of a range of nanomaterials (intra- cellularly or extracellularly; Thakkar et al., 2010), including bacteria for the synthesis of gold (Kashefi et al., 2001), silver (Ramanathan et al., 2011), palladium (Windt et al., 2005), gold-silver (Au-Ag) alloy (Nair and Pradeep, 2002), CdS (Kang et al.,

2008). ZnS (Bai et al., 2006), iron sulfide (Mann et al., 1990) and magnetite (Mann et al., 1984); algae for the synthesis of gold and silver (Lengke et ah. 2007); and fungi for the synthesis of gold (Shankar et ah, 2003), silver (Balaji et ah, 2009), silica, tita- nia, zirconia, barium titanate (Bansal et ah, 2006), CdS (Ahmad et ah, 2002), and Au-Ag alloy NPs (Senapati et ah, 2005). One important aspect outlined in chemical synthesis routes is the ability to control shape and size, which confer unique properties to these particles. To compete with chemical methods, monodispersed gold NPs were synthesized using an extremophilic actinomycete, Thermonospora sp. (Ahmad et ah, 2003a, b). The synthesis of metal NPs using biological systems has been in vogue in the recent past, with substantial evidence proving its superiority over the chemical route. The formation of NPs by this method is extremely rapid, requires no toxic chemicals and the NPs are stable for several months (Tejo Prakash et ah, 2009).

 
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