There are numerous techniques for nanoparticle synthesis, from physical methods to chemical procedures using various organic or inorganic chemicals or biological methods using living organisms. Each and every technique has its own advantages and drawbacks. Conventional techniques are more time-consuming and expensive. Synthesizing nanoparticles utilizing biological methods is non-toxic and eco-compatible and more appropriate than conventional techniques (Patra and Baek 2014). There are numerous parameters that affect the formation and action of microbial nanoparticles such as temperature, pH, pressure, concentration, raw materials, etc. (Baker et al. 2013).
The temperature of the media is the parameter that regulates the property of the nanoparticle designed in which the reaction takes place. Temperature affects the synthesis of nanoparticles in the physical which requires more than 350oC, the chemical method requires temperature between 100- 350oC as well as the biological method which requires less than 100oC. Patra and Baek in 2014 have reported the outcome of pH upon the dimension, composition and texture of nanoparticles genesis. The melting point of nanoparticles lowered the dimension of particles to nano-sized (Baker et al. 2013). Green technology used to synthesize nanoparticles is also under the control of the reaction time of incubation. Therefore, the potential and the quality of the generated nanoparticles can be prompted on the basis of exposure to light and its shelf life (Baer et al. 2011). For the most advanced implementation of nanoparticles, cost-effectiveness is another important factor. The advantage of synthesis through the chemical method is that it is less time consuming but requires more capital. The physical method is also an overpriced technique as well as demands equipment (Ding et al. 2015). The origination of nanoparticles involving biological techniques is more in demand as it requires less capital and can be employed at a massive level (Gour and Jain 2019). Intracellular and extracellular enzymes are released by different microbes specifically in varying amounts and also affect nanoparticle synthesis.
Therefore, the most accepted technique for bioremediation using nanoparticles to clean up the environment is green technology. Microbe synthesised nanoparticles transform harmful pollutants into non-toxic, soluble compounds. Proteins in microbes get attached to the metallic nanoparticle making it more stable. Thus, the stability of metallic nanoparticles synthesised by microbes is more than the chemically synthesized metallic nanoparticles (Balakrishnan et al. 2017). Also, the production cost can be reduced to 1/10th by the green method. The biosynthesized nanoparticles possess a large surface area along with a high catalytic reaction. The mechanism of the formation of bio nanoparticles can be either intracellular or extracellular. Biosynthesis through extracellular mode is of low cost and can be without involving downstream processes (Kapoor et al. 2021). The microbial cell takes up precursor metals and synthesizes corresponding nanoparticles and aids in decontamination activity (Mohseniazar et al. 2011). Bacteria, yeast, fungi, algae, marine microorganisms, and actinomycetes have been majorly utilized for the design of metallic nanoparticles. Advantages of using microorganisms are that they don’t require high energy, and no addition of cap or stabilizing agents, making it more cost-effective. Such advantages help to ease the production, scaling up the nanoparticles using biological agents as nano factories.
Bacteria Mediated Nanoparticle Synthesis:
Bacteria are very versatile organisms due to their adaptation to unfavourable environmental conditions and helping in bioremediation. That’s why, bacteria are also referred to as capable bioagents for nanoparticles synthesis such as silver, gold, palladium, platinum, titanium, etc. Some bacterial strains have the potentiality to convert noxious ions into insoluble harmless NPs (Fang et al. 2019). Thus, bacteria also serve as biofactories as they release enzymes and initiate biodegradation (Iqtedar et al. 2019). Bacterial cells are platforms that generate metal-linked nanoparticles as well as metalloids both intrinsically and extrinsically under variable physicochemical settings like exposure time, pH, temperature, bacterial concentration, and metal ions. In the intracellular process, functional groups present on the plasma membrane attaches metals and metalloids and react with the protein present inside the bacterial cell. Whereas, in an extracellular process, the biomolecules on the cell wall can reduce metal ion. Chemical processes for certain groups, like amines and carboxylic groups, can modify the cell surface with metal binding active sites, converting positive charge to negative charge. Extracellular reduction is shown to be more advantageous due to its simple extraction stages and high efficiency. Large amounts of nanoparticles (size 100–200 nm) can be created in refined form by the secretion of enzymes from bacteria extracellularly. Non-viable bacteria can also be utilized for the nanoparticle synthesis same as viable ones. Metals can be mobilized or immobilized by bacteria, and metal ions can be reduced or precipitated. The bacterial cell wall plays a very crucial role as metal ions penetrate into the cellular matrix and return back for extracellular secretion on the cellular wall (Fang et al. 2019). The benefit of bacteria in the manufacture of nanoparticles is a profitable technique because it eliminates the need for expensive and harmful chemicals in the synthesis and stabilization operations. Some reports from different bacterial strains demonstrate the solution to the environmental issue of the sites with manganese contamination. Bacillus sp. cells have the capability to synthesize corresponding oxide nanoparticles intracellularly which are manganese oxide NPs, while cleansing manganese from the culture media (Sinha et al. 2011). The genesis of gold nanoparticles utilizing bacteria Lactobacillus kimchicus has also been reported (Mughal et al. 2021).
Desulfovibrio desulfuricans and Shewanella oneidensis showed their role in the reduction of Pd divalent to Pd zerovalent ion (Bio-Pd) nanoparticles (Hennebel et al. 2009). From Escherichia sp. SINT7 copper nanoparticles were synthesized, which are resistant to copper. The biogenic nanoparticles were found to mineralize inorganic ions like chloride and phosphate ions, azo dye and textile effluent. The data of breakdown of complex synthetic colourants such as reactive black-5 dye, direct blue-1, congo red, and malachite green reported was 83.61 %, 88.42 %, 97.07 %, and 90.55 %, respectively at 25 mg/l of the concentration, whereas, at 100mg/l concentration, the reduction shifted to 76.84 %, 62.32 %, 83.90 %, and 31.08 %, respectively. It has been reported that iron-sulfur NPs were capable of detox Naphthol Green B dye extracellularly mediated by electron transfer without using extra sulfur (Cheng et al. 2019). Synthesis of nanoparticles through Pseudoalteromonas sp. CF10-13 contributes an environment-compatible procedure for bio-decontamination. The synthesis of nanomaterials also lowered the formation of toxic gases and intermediate cation complex compounds. The performance of such potential biogenic nanoparticles prompts an economical and sustainable approach to remediate industrial effluents (Noman et al. 2020; Mandeep and Shukla 2020).
Fungi Mediated Nanoparticle Synthesis
The filamentous fungi are very potent for the development of nanoparticles as the fungal mycelium possesses large surface area which secretes enough enzymes, proteins, and metabolites (Mohanpuri et al. 2008; Fouda et al. 2018). On comparing with bacteria, the fungal synthesis of nanoparticles is better because of the release of metabolites. The growth of filamentous fungi is rapid and their sustentation at laboratory conditions is also easy. So, the development of nanoparticles by fungi is economically feasible.
Also, fungi have a high regenerative ability which help in the fabrication and generation of metallic nanoparticles in proper quantity (Bansal et al. 2005). An aqueous solution of AuCl4 (tetachloraurate) with NADH-enzyme catalysed reaction, Fusarium oxysporum librates reducing agents and generates gold nanoparticles. Senapati and team (2005) found F. oxysporum exhibits the property of creation of gold-silver nanoparticles extracellularly when it is reacted with an equivalent combination of silver nitrate and tetra-chloroauric ion by molarity. Also, it can produce Pt nanoparticles when reacted with hexachloroplatinic acid (Riddin et al. 2006). The biomass of Fusarium oxysporuma also helped to produce silver NPs (Karbasian et al. 2008). Because of the protein binding capability provided by the connection of cysteine and lysine residues, nanoparticles have long-term stability (Das et al. 2017). Some other fungi namely, Aspergillus niger, Aureobasidium pullulans, Cladosporium resinae, Ganoderma lucidum, Penicillium species, Rhizopus arrhizus and Trametes versicolor were also used for the synthesis of nanoparticles and mediated the absorption of heavy metals out-of polluted sites (Kapoor et al. 2021). Non-living Hypocrea lixii biomass can uptake of Cu (II) and formation of copper nanoparticles (Salvadori et al. 2014). Hypocrea lixii was also capable to create NiO nanoparticles (Salvadori et al. 2015).
The genesis of NiO nanoparticles (5.89 nm size) from dead biomass of Aspergillus aculeatus (Salvadori et al. 2014). 9 nm sized silver nanoparticles were synthesized by Aspergillus flavus because their plasmid had ‘sil genes’ and it can also reduce silver ions (Vigneshwaran et al. 2007). A report on the intracellular nano sized creation of gold particles by fungal strain, Verticillium luteoalbum when it was treated to the solution of chloroaurate (Gericke and Pinches 2006). From various reports, it was found that filamentous fungi can uptake metal very efficiently. It can easily be cultured and slight genetic modifications can enhance the potency of nanoparticle synthesis.
Yeast Mediated Nanoparticle Synthesis
Yeast is another potent organism that has bioremediation properties and can synthesise nanoparticles as well. Yeast cells are also referred as quantum semiconductor crystals due to their capability to amalgamate semiconductor nanoparticles (Dameron et al. 1989). Some of the yeast strains synthesising nanoparticles have been reported. Williams (1996) reported the formation of quantum crystallites by two strains of yeast, Candida glabrata and Schizosaccharomyces pombe. These cells were cultured on medium with cadmium salts. Gericke and Pinches (2006) have reported the formation of gold nanoparticles by Pichia jadinii yeast cells. The enzyme secreted by the cells reduced the gold ions. Another yeast, Yarrowia lipolytica synthesised gold nanoparticles and also helped in the breakdown of hydrocarbons and heavy metals which is beneficiary participation in the treatment of contamination of the environment (Bankar et al. 2009).
The dead biomass of Saccharomyces cerevisiae lowered the cost-price of nanoparticles significantly. Lead sulfide nanoparticles were synthesized by Rhodosporidium diobovatum intracellularly (Seshadri et al. 2011). Silver nanoparticles developed by Candida albicans, Candida utilis, and Saccharomyces boulardii were reported (Soliman et al. 2018). Yeast strain, Magnusiomyces ingens LHF1 helped in the biosynthesis of selenium nano-sized particles (Lian et al. 2019). The dead biomass of Rhodotorula mucilaginosa generated magnetic nickel/nickel oxide spherical nanoparticles (Salvadori et al. 2016). Magnetic nanoparticles exhibit economic importance such as induction of the uptake of toxic metals from liquid waste results and results in the detoxification of industrial effluents. Most of the yeast genera have been updated with the aggregation of a significant number of heavy metal ions. The detoxification ad degradation mechanism occurs in yeast cells by the proteins such as glutathione, metallothioneins and phytochelatins, etc which acts as a chelator of heavy metals. Because of the peptide coating, the particles did not clump together and displayed very high stability when compared to chemically produced nanoparticles.
Algae Mediated Nanoparticle Genesis
Aquatic oxygenic photoautotrophs which are also known as algae, are also used in the generation of complex inorganic nanoparticles intracellularly and extracellularly as well (Castro et al. 2013). The biogenic development of gold, palladium, silver, platinum, zinc, copper and their oxide nanoparticles for the bioremediation has been reported in many of the works (Kapoor et al. 2021).
In 1986, Hosea and his co-workers marked the assemblage of gold nanoparticles of varying size from 9–20 nm by dried algal suspension of Chlorella vulgaris. Intracellular genesis of silver nanoparticles by some species of the algae such as Chlorella vulgaris, Dunaliella salina, and Nannochloropsis oculata (Mohseniazar et al. 2011). Tetraselmis kochinensis has been outlined to generate gold nanoparticles intracellularly (Mughal et al. 2021). Similarly, the synthesis of gold nanoparticles by Rhizoclonium fontinales intracellularly. Lyngbya majuscula and Spirulina subsalsa synthesized gold nanoparticles extracellularly (Chakraborty et al. 2009). In other finding, the aggregation of gold sulfide (AuS) nanoparticles on the cellular wall of green alga, Pediastrum boryanum in aqueous AuCl3 solution and Cyanobacteria conferred the reduction of AuCl3 to Au along with the generation of an intermediate AuS (Lengke et al. 2006). Accumulation happens by electrostatic bonding of metal ions to functional groups on the outer wall or via enzymes that drive nuclei formation and development with metal ion reduction (Parial et al. 2012).
Actinomycetes Mediated Nanoparticle Synthesis:
Along with bacteria and fungi, actinomycetes also play the significant part in the synthesis of metallic cation nanoparticles. Rhodococcus and Thermomonospora species synthesised gold nanoparticles of 5 nm - 15 nm under extreme temperature and alkaline environmental conditions (Ahmad et al. 2003). Other genera of actinomycetes such as Nocardia farcinica, Streptomyces viridogens, S. hygroscopicus and Thermoactinomyces sp. synthesized gold nanoparticles (Składanowski et al. 2016). Actinomycetes are good agents for the biogenesis of nanoparticles as it provides a large surface area and releases secondary derivatives and also no harm occurs to the cell after the synthesis. Metallic nanoparticles can be produced by actinomycetes both within and outside the cell. Comparatively the nanoparticles quantity is high on the outer wall rather than on the cell membrane (Hassan et al. 2018). El-Gamal and his teammates (2018) reported that many other metals like copper, manganese, silver, zinc nanoparticles were also formed by employing Streptomyces species.
Marine Mediated Nanoparticle Synthesis
Microorganisms living in the marine environment also have the capability to synthesize nanoparticles. These microbes such as marine bacteria, cyanobacteria, yeast, fungi, and algae can reduce large number of inorganic elements and mediate nanoparticle synthesis (Kapoor et al. 2021). Penicillium fellutanum, a marine fungus, was isolated out of sediments of coastal mangroves. It was capable to synthesize silver nanoparticles when placed in silver nitrate solution extracellularly. Another fungal species, Aspergillus niger AUCAS 237 and bacterium, E. coli AUCAS 112 also produced silver nanoparticles (Kathiresan et al. 2009). Thraustochytrids another marine fungus possesses poly-unsaturated fatty acids and exhibit the property of extracellular biogenesis of silver as well as lipid nanoparticles (Gomathi 2009). Intracellular biogenesis of silver nanoparticles of varied sizes ranging from 20-100 nm was also reported from Pseudomonas sp. (Muthukannan and Karuppiah 2011). Silver nanoparticles were produced by proteins of a marine yeast, Pichia capsulate and marine bacteria Oscillatoria willei (Manivannan et al. 2010). Spirulina platensis synthesized silver, gold and bimetallic nanoparticles. Seshadri et al. (2011) reported Rhodosporidium diobovatum has a sulfur-rich amino acid that served as a coating medium in the manufacture of lead sulfide nanoparticles. Govindaraju and his team (2009) showed the formation of silver nanoparticles in a brown seaweed, Sargassum wightii, as well as gold nanoparticles by the same seaweed, extracellularly (Singaravelu et al. 2007).
Seaweed, Sargassum crassifolium, fabricated gold nanoparticles. Gu and team (2018) Cystoseira trinodis formed CuO nanomaterials and showed its antioxidant prospects and degradation of methylene blue dye. Gelidiella acerosa, a red seaweed, aided the genesis of silver nanoparticles (Vivek et al. 2011), another weed, Ulva fasciata (Rajesh et al. 2012), Hypnea musciformis (Roni et al. 2015), Phormidium fragile (Satapathy and Shukla 2017). Table 1 shows the list of a few microorganisms mediating the formation of metallic nanoparticles.
Table 1 Genesis of Metallic Nanoparticles by Microbes
S. No.
|
Sources
|
Nanoparticles
|
Size (nm)
|
References
|
1
|
Escherichia coli
|
Silver
|
10-100
|
Ghorbani 2013
|
2
|
B. methylotrophicus
|
Silver
|
10-30
|
Wang et al. 2016
|
3
|
Pseudomonas putida
|
Silver
|
60
|
Thamiselvi and Radha 2013
|
4
|
P. fluorescens
|
Silver
|
10-60
|
Syed et al. 2016
|
5
|
Lactobacillus sp.
|
Titanium
|
40-60
|
Prasad et al. 2007
|
6
|
B. amyloliquefaciens
|
Titanium oxide
|
22-97
|
Khan and Fulekar 2016
|
7
|
Aspergillus niger
|
Silver
|
20
|
Gade et al. 2008
|
8
|
Fusarium oxysporum
|
Silver
|
5-30
|
Hussinay et al. 2015
|
9
|
Fusarium solani
|
Silver
|
5-35
|
Ingle et al. 2009
|
10
|
Trichoderma viride
|
Silver
|
5-50
|
Fayaz et al. 2010
|
11
|
Rhizopus stolonifera
|
Gold
|
9.47
|
AbdelRahim et al. 2017
|
12
|
S. cerevisiae
|
Gold; Titanium Oxide
|
12-15
|
Jha et al. 2009; Sen et al. 2011
|
13
|
Rhodotorula mucilaginosa
|
Nickel/Nickel Oxide
|
5.5
|
Slavadori et al. 2016
|
14
|
Sargassum tenerrimum
|
Gold
|
5-10
|
Ramakrishna et al. 2016
|
15
|
Bifurcaria bifurcata
|
Copper Oxide
|
5-45
|
Abboud et al. 2014
|
16
|
Streptomyces capillispiralis
|
Copper
|
3.6-5.9
|
Hassan et al. 2018
|
17
|
Streptomyces hygroscopicus
|
Gold
|
10-20
|
Waghmare et al. 2014
|
18
|
Fucus vesiculosus
|
Gold
|
20-50
|
Mata et al. 2009
|
19
|
Rhodosporidium diobovatum
|
Lead
|
2-5
|
Sheshadri et al. 2007
|
20
|
Ulva fasciata
|
Silver
|
28-41
|
Rajesh et al. 2012
|