Document Type : Review Paper
Department of Microbiology, Indian Academy Degree College-Autonomous, Bengaluru-560043, Karnataka, India
Advances in material chemistry in the last few decades have played a prominent role in the development of nanoscience and nanotechnology which deals with the modification and utilization of particles of the order of one billionth of a meter (1 x 10-9 m) called nanomaterials. Nanoscience is a microscopic and molecular approach to the regulation of matter on larger scales, where the physiochemical properties of nanoparticles vary significantly from their larger counterparts. Unique characteristic features of nanomaterials have paved the way for new scientific inventions in the field of nanoscience and nanotechnology [1,2]. Nanomaterials have been categorized as organic (e.g., carbon-based nanomaterials) and inorganic (e.g., metal and metal oxide nanoparticles). Based on their overall dimension, nanomaterials are classified as zero-dimensional (e.g., quantum dots), one-dimensional (e.g., nanotubes), two-dimensional (e.g., nanosheets), and three-dimensional (e.g., nanoflowers). Further depending on their physicochemical features, they are subdivided into polymeric-based nanomaterials (e.g., nanobiocomposites), carbon-based nanomaterials (e.g., carbon nanotubes), lipid-based nanomaterials (e.g., liposomes), semiconductor-based nanomaterials (e.g., CdTe), layered nanomaterials (e.g., perovskites and LDH) and ceramic based nanomaterials (e.g., nano-oxides of Si, Al, Ti, and Zr) [3,4]. Among all the inorganic nanomaterials, transition metal and metal oxide-based nanoparticles such as Cu, Fe, Co, Mn, Zn, Cr, Ni, Ag, Ti, and Au are of the most important scientific concern due to their outstanding and diverse applications in various sectors such as chemical, photonics, electronic, food science, energy harvesting, environmental, biomedical, pharmaceutical, agricultural and industrial [5-7].
An increased thrust in the scientific realm influenced the researchers to develop two major approaches viz.: top-down and bottom for the synthesis of nanomaterials with desired properties (Fig. 1). In a top-down approach, the active bulk material is reduced to nano-sized nanomaterial under the influence of chemicals, radiation or mechanical shearing. Different techniques utilize the top-down principle to produce nanomaterials which include lithography, mechanical milling, sputtering, etching, pulse laser ablation, pulse wire discharge, evaporation condensation reaction, and ion implantation. However, top-down approaches suffer from limitations which include high cost, use of toxic chemicals, ineffective in producing nanomaterials with desired properties, and difficulties to control the size of nanoparticles. The bottom-up approach involves the pile-up of the atoms, ions, and molecules to form complex structures of nano dimension. A few techniques of the bottom-up approach are the chemical vapor deposition process, laser pyrolysis, sol-gel method, plasma arcing, wet synthesis, self-assembly process, electrochemical, sonochemical, metal organic decomposition, hydrothermal, solvothermal, and spinning .
Anthropogenic activities including combustion of fossil fuels, oil spills, agriculture, deforestation, mining, and various industrial sectors such as food, textile, paper, and pharmaceutical have been significant contributors to environmental contamination. Several research studies indicated the impact of high environmental contamination on living organisms and their positive relationship with increasing health problems . Myriad nanoparticles have found applications in environmental remediation as sensors, adsorbents, and photocatalysts for the elimination of toxic chemicals and pollutants. Recent advances in novel polymer fabricated nanomaterials employed for the treatment of industrial effluents and wastewater contaminated with radionuclides, metal ions, and chemical solutes were highlighted by several researchers [10,11].
In addition to the environmental applications, a plethora of engineered nanomaterials proved to be strategically effective antimicrobial agents complementary to traditional antibiotics, antifungal, and antiviral agents. Antimicrobial nanomaterials include diversified groups of inorganic metal and their oxides as they possess a range of intrinsic and chemical composition properties that influence innate antimicrobial mechanisms such as the disruption of the plasma membrane, diffusion into the cytoplasm, and the degradation of nucleic acids and enzymes. [12-14].
Herein, we provide a comprehensive view of scientific investigations made in the synthesis of inorganic nanoparticles using plants as a model for biological sources of nanoparticle synthesis and their diversified application in the field of environmental remediation and the biomedical industry.
CONVENTIONAL METHODS OF NANOPARTICLE SYNTHESIS
The efficiency of nanoparticles is greatly influenced by the shape, size, and surface topology which is further reliant on the method of synthesis. For several decades, conventional techniques falling under the category of physical or chemical methods such as hydrothermal, sol-gel, combustion, reactive grinding, mechanical activation, microemulsion, co-precipitation, microwave irradiation, laser ablation, sputtering, sonochemical reduction, polyol method, and thermal deposition have been utilized to synthesize the nanomaterials of specific dimension to control the properties of nanomaterials [15,16].
However, these methods are known to suffer from significant drawbacks as presented in Table 1. The effort to synthesize the nanomaterials of desired properties and dimensions involving sustainable approaches opened unique and new opportunities in this emerging field of research .
NANOTECHNOLOGY AND GREEN CHEMISTRY
Researchers have developed multiple routes for the synthesis of nature-friendly nanoparticles utilizing natural sources to provide the advantages of clean, nontoxic, and environmentally adequate synthesis methods. The green and biological synthesis of nanoparticles is an attractive practice that enables synthesis in an aqueous environment with minimum cost and low energy investment which could easily be scaled up to a higher level. Numerous studies have already showcased the greater efficiency of plants [18-20], edible and nonedible mushrooms [21,22], and microorganisms [23-25] for the synthesis of inorganic nanoparticles as shown in Fig. 2. The application of green chemistry principles in the synthesis of nanoparticles have paved the way for sustainable development in the field of nanotechnology.
ROLE OF PLANTS IN THE SYNTHESIS OF METAL AND METAL OXIDE NANOPARTICLES
The insight that plants could bioaccumulate and reduce metal ions has opened multiple options for considering their use as an alternate way of synthesizing nanoparticles. The plant-mediated synthesis of nanomaterials provides multiple advantages over the microbial synthesis approach in low cost, reliability, simplicity, non-pathogenicity, short reaction time, and control over the reaction. Several plants including Aloe barbadensis, Acalypha indica, Apiin, Avena sativa, Azadirachta indica, Camellia sinensis, Brassica juncea, Cinnamomum, Carica papaya, Coriandrum sativum, Emblica officinalis, Eucalyptus, Garcinia, Jatropha, Medicago sativa, Ludwigia adscendens, Mentha piperita, Nelumbo nucifera, Ocimum sanctum, Pelargonium roseum, Psidium guajava, Sedum alfredii, Tanacetum vulgare, and Terminalia catappa have been reportedly employed for the synthesis of inorganic nanoparticles [26,27]. The process of nanoparticle synthesis is initiated by the addition of metal ions solution to an aqueous extract of various parts of the plants such as roots, stems, bark, leaves, flowers, and fruits at different reaction conditions [28,29]. The phytochemicals (viz., aldehydes, alkaloids, flavonoids, ketones, organic acids, phenolic acids, and terpenoids) and bioactive compounds (complex terpenes, enzymes, vitamins, and minerals) in the plant extracts have greater potential to reduce metal ions into their corresponding metal and metal oxide nanoparticles. In addition, polysaccharides, proteins, enzymes, and amides in plant extracts play a dual role by acting as both reducing and stabilization agents [30,31]. Some examples of potential plants investigated for the synthesis of inorganic nanoparticles are shown in Fig. 3.
Flavonoids, a large group of polyphenolic compounds comprising anthocyanins, chalcones, flavones, flavanols, flavanones, and isoflavonoids are known for their chelation and bioreduction properties. The release of reactive hydrogen or oxygen atoms from the tautomeric transformation of flavonoids from enol-form to keto-form is believed to be involved in the bioreduction of metal ions. For example, Ocimum sanctum leaf extracts naturally contain high amounts of quercetin (flavonoid) containing OH- and keto groups. During the synthesis of Ag nanoparticles, quercetin reacts with AgNO3 as an acid using OH- groups attached to the carbon atoms of an aromatic ring that participates in the reduction of Ag ions to Ag nanoparticles and prevents agglomeration in the reaction medium . In addition, quercetin can chelate carbonyl and hydroxyl groups at the C3 and C5 site and catechol groups at C3 and C4 positions which are further involved in the chelation of several metal ions viz., Al3+, Cr3+, Co2+, Cu2+, Fe2+, Fe3+, Pb2+ and Zn2+ .
Phenolic acids are plant-derived essential biomolecules containing phenolic rings and functional groups of esters, glycosides, or carboxylic acids. Their benzene ring plays a significant role in the reduction and metal chelation. Various studies demonstrated the reducing ability of caffeic acid, chlorogenic acid, cinnamic acid, coumaric acid, ellagic acid, gallic acid, ferulic acid, and protocatechuic acid [33,34]. Propyl gallate, an ester derivative of gallic acid, has been applied as a reducing and stabilization agent for the synthesis of Ag nanoparticles by Ping and Nian, . The hydrogen bonds of propyl gallate-coated Ag nanoparticles deliver a chain structure that results in a plasmon resonance peak in the IR wavelength region which allows the determination of major antioxidants.
Terpenoids are volatile organic components constituting 90 % of essential oils produced by plants as secondary metabolites. Isoprenoids, a basic unit of terpenoids, act as the building blocks of other metabolites including plant chlorophyll, carotenoids, hormones, sterols, and turpentine. Mono and sesquiterpenoids are identified to play a significant role in the synthesis of nanoparticles . Leaf extracts of Leucas martinicensis have produced spherical and crystalline Ag nanoparticles with the action of terpenoids . In addition, the ability of A. indica in the synthesis of ZnO nanoparticles was attributed to the strong reduction or oxidation reaction carried out by terpenoids present in the leaf extracts .
Monosaccharides such as glucose and fructose-containing keto groups are capable of undergoing a series of tautomeric transformations involving the development of nanoparticles. The reducing ability of polysaccharides is greatly dependent on the ability of monomeric sugars to adopt an open chain form within an oligomer to provide access for metal ions to an aldehyde or keto group . Various amino acids serve as excellent capping and stabilization agents in the development of inorganic nanoparticles. Several researchers reported the inherent ability of amino acids in directing and assembling superstructures [40,41]. Courrol and Matos , conducted the experiments using 21 amino acids for the synthesis of silver nanoparticles where authors observed that five amino acids viz., tryptophan, tyrosine, methionine, cysteine, and histidine were proficient in the reduction and stabilization of nanoparticles. In another study, photo-reduced gold nanoparticles were synthesized using arginine, aspartic acid, valine, threonine, and tryptophan. A relationship between polarizability and the oxidation potential of an amino acid was reported during the nanoparticle formation upon irradiation.
APPLICATIONS OF BIOGENIC METAL AND METAL OXIDE NANOPARTICLES
In recent years, nanomaterials have found applications in almost all disciplines due to their improved properties at the nanoscale. Some of the major applications of nanotechnology are fundamental fields, mainly environmental remediation, agriculture, electronics, and medicine. With the scientific community on a constant lookout for highly biocompatible and sustainable ways of environmental pollution mitigation, the toxic effects of xenobiotics have put nanoscience and nanotechnology as the front runners [43-45]. The applications of biogenic nanoparticles in the elimination of environmental pollutants and their antimicrobial abilities have been briefly reviewed.
Several noble and transition-based nanoparticles synthesized using plants are utilized for water purification due to their high reactivity and photocatalytic characteristics owing to the narrow band gap. They are also known for their extraordinary absorption capability exhibiting advantages such as fast kinetics and high regeneration ability. Numerous metal and metal oxide nanoparticles have been investigated for the remediation of several contaminants, but most studies have been dedicated to the removal of heavy metals and chlorinated pollutants from an aqueous environment . Several nanomaterials have been frequently utilized for environmental remediation since they are flexible toward both in-situ and ex-situ applications. Table 2 provides an overview of inorganic nanoparticles synthesized from plant extracts and their reported environmental applications.
TITANIUM DIOXIDE NANOPARTICLES
Titanium dioxide has been documented for water treatment and air purification due to its characterized photocatalytic, semiconducting, low cost, nontoxicity, and energy-converting properties [47-50]. TiO2 nanoparticles are easily activated by photons producing highly reactive oxidants like OH-, thus frequently employed for the elimination of organic contaminants from various media. Goutam et al., , described the synthesis of TiO2 nanoparticles by using leaf extracts of Jatropha curcas and their application for the degradation of tannery wastewater in self-designed fabricated parabolic trough reactors. At the end of photocatalytic treatment, 76.4 % removal of Cr and 82.6 % removal of COD from wastewater were reported. Crystalline anatase TiO2 nanoparticles synthesized from extracts of Diospyros ebenum at 600 ºC were evaluated for the mineralization of crystal violet under UV light irradiation . The results suggested the improved photocatalytic activity of nanoparticles due to the fine crystallite size and higher surface area available for catalysis.
Another frequently utilized nanoparticle for the elimination of environmental pollutants is silver and its oxides. Silver nanoparticles are well known to exhibit unique size and shape which provides them with diverse optical and electrical properties [53-56]. Chand et al.  developed a novel route to synthesize silver nanoparticles using three plants viz., onion, tomato, and acacia catechu. The obtained spherical nanoparticles showed complete degradation of MO, MR, and CR within 20 min and 15 min respectively. In another study, Ag nanoparticles synthesized from Ficus hispida Linn reduced 4-NP into 4-Aminophenol in 15 min of exposure . Mehata et al. , applied Ag nanoparticles synthesized using a medicinal plant, Kalanchoe pinnata which is also known as Bryophyllum pinnatum for the photocatalysis of RhB under dark conditions. Ag nanoparticles including bimetallic nanoparticles have found applications as sensors in the sensing and detection of pollutants. For instance, green Ag-doped Ce nanoparticles synthesized from Cissus quadrangularis were highly efficient in the detection of Cd2+ ions even in presence of other heavy metals such as Co2+, Pb2+, Hg+, Ni2+, As3+, Mn2+, Zn2+ and Fe3+ .
PLATINUM AND GOLD NANOPARTICLES
In addition to silver, other noble metals such as Pt-based nanoparticles and Au-based nanoparticles were successfully synthesized by many researchers using various plant extracts . Eltaweil et al. , proposed a promising route of the green synthesis of Pt nanoparticles with the size 1 to 3 nm using leaf extract of Atriplex halimus and further investigation suggested the high catalytic activity for the complete degradation of MB in 5 min at 100 ppm of dye concentration. To investigate the efficacy of reducing agents in the degradation of organic compounds, Gupta et al.  explored the use of Sn(II) to enhance the degradation of MB by Au nanocatalysts synthesized from green tea leaves. It has been demonstrated that SnCl2 acts as a reducing agent facilitating the electron transfer and Au nanoparticles as a catalyst in the reaction. Furthermore, encapsulation of nanoparticles using natural polymers is important to overcome concerns regarding aggregation and recoverability. Nabikhan et al. , fabricated Au nanoparticles synthesized from an aqueous extract of Avicennia marina using sodium alginate and used as a heterogenous catalyst against 4-NP which was proved to be a potent, eco-friendly nano biocomposites catalyst for the remediation. A fever report in the literature suggested the unexplored efficiency of these nanoparticles for environmental applications.
Iron-based nanosorbents are particularly attractive due to their inherent magnetic property that favors their easy separation from the reaction medium [65,66]. Iron and its oxide-based nanoparticles synthesized from plant extracts are studied for the elimination of several contaminants such as antibiotics, heavy metals, and textile dyes. Xiao et al. , reported the synthesis of iron nanoparticles with increased stability using 15 different plant extracts viz., N. indicum, A. moluccana (L.) Willd, C. camphora (L.) Presl., P. orientalis (L.) Franco, G. robusta, B. variegata L., Black tea, Oolong tea, A carambola L., D. regia, E. citriodora, L. speciosa Pers., S. aromaticum, S. jambos (L.) Alston and D. longan Lour. The major phytochemicals responsible for the reduction and stabilization of Fe nanoparticles were also determined. Nanoparticles synthesized by S. jambos (L.) Alston extract exhibited significant adsorption capacity with 698.6 mg Cr (VI) per g of iron. Similarly, for the synthesis of zero-valent iron nanoparticles, Conde-Cid and his co-workers  employed two natural aqueous extracts, black tea leaves, and vineyard pruning residues. It was concluded that the prepared nanoparticles were able to eliminate 58 % of sulfadiazine via an adsorption mechanism whereas 69 % through a catalytic degradation mechanism.
COPPER BASED NANOPARTICLES
Other prevalent transition metal nanoparticles are Cu and their respective oxide-based nanoparticles which are generally used against environmental remediation [69-76]. Green synthesized CuO nanoparticles prepared using extracts of mint leaves and orange peels were utilized for the removal of heavy metals. Maximum uptake capacity followed the order of Pb(II) > Ni(II) > Cd(II) which was recorded at an adsorbent dosage of 0.33 g/L and pH 6.0 within 60 min of contact time . Cu nanoparticles synthesized using Psidium guajava extract were reported as an excellent heterogeneous catalyst exhibiting the maximum degradation (93 % and 81 %) of industrial dyes NB and RY-160 respectively . In another article, Madhuca longifolica-based CuO nanoparticles demonstrated 77 % of photocatalytic degradation of MB dye after 150 min of visible light irradiation suggesting the durability of the green nanoparticles .
ZINC OXIDE-BASED NANOPARTICLES
ZnO nanoparticles possess unique physiochemical properties, due to dimensional characteristics and surface morphology. These are known to exhibit good adsorptive and catalytic behavior, making them suitable materials in the field of environmental remediation [80,81]. Fazlzadeh et al  demonstrated the synthesis of ZnO nanoparticles using powdered Peganum harmala seed extract (ZnO) and the synthesized nanoparticles were coated with powdered activated carbon of Peganum harmala seed (PPAC) for enhancing adsorption capacity. ZnO/PAC showed the highest adsorption efficiency (68.4 mg/g) for the Cr(VI) followed by PPAC and bare ZnO. The study revealed the significance of surface modification of nanoparticles for enhanced removal of heavy metals. MB, a widely used thiazine dye was removed from aqueous samples with ZnO nanoparticles synthesized using an aqueous extract of Ulva lactuca. The degradation of dye was carried out using natural sunlight. Under optimum conditions, the process showed 90.4 % degradation of dye. The photo-nanocatalyst proved to be effective for the oxidation and degradation of MB which proceeded at high reaction rates . Vidya et al. , reported the synthesis of ZnO nanoparticles utilizing leaf extracts of Artocarpus heterophyllus with an average size of 15 to 25 nm. The green synthesized ZnO nanoparticles showed outstanding photocatalytic degradation efficiency (> 80 % within 1 h) against Rose Bengal dye, the main water-pollutant released by the textile industries.
Several investigations have demonstrated the utilization of bimetallic nanoparticles as a means of overcoming some of the drawbacks associated with monometallic nanoparticles. Mixed metallic oxide nanomaterials synthesis from the green route has also been investigated by several researchers for their efficiency in environmental remediation. For instance, Sahoo et al. , reported the preparation of ZnO-ZnFe2O4 mixed nanocomposites using leaf extracts of Psidium guajava and evaluated wastewater remediation. Results suggested significant adsorption of CR and MB from wastewater with the maximum adsorption capacity of 120.3 mg/g and 90.3 mg/g respectively. The study also revealed the prominent role played by the mixed nanocomposite in improving plant immunity in addition to the growth and development of the plant. In another study, Cu-Ag and Cu-Ni bimetallic nanoparticles prepared using ginger rhizome powder were utilized as nano-catalysts for the reduction of 2-NP, 4-NP, MO, CR, and RhB. In addition to their outstanding ability to degrade, the nano-catalysts also showed excellent stability and reusability , Ismail, et al., 2018.
ANTIMICROBIAL APPLICATIONS OF NANOPARTICLES
As a result of rapid evolution through genetic mutations, several microorganisms have established resistance against various antimicrobial agents. Thus, many researchers have devoted themselves to developing new potential therapeutic agents to fight against resistant pathogenesis which is one of the major challenges in recent years. Biogenic inorganic nanoparticles have shown remarkable antimicrobial action against multi-drug resistant microorganisms via multiple mechanisms targeting cell membranes, proteins, and genetic material (Table 3). Similarly, noble metals reduced to nano dimension are also being coated on wound dressing as a preventable measure of microbial infection in wounds [86-88]. There are several examples where metallic and metal oxide nanoparticles synthesized from plants have been explored for antimicrobial activity against many microbial pathogens. Table 4 summarizes the various inorganic nanoparticles investigated for their antimicrobial activity.
ANTIBACTERIAL EFFECT OF NANOPARTICLES
Metal and metal oxide nanoparticles often exhibit high antibacterial properties against bacterial pathogens. Through vast analysis of literature, it has been observed that nanoparticles can be bacteriostatic or/and bactericidal depending on their dimension, concentration, and capping method against both gram-positive and gram-negative [89,90]. In the case of gram-positive bacterial strains, the cell wall composition shows a thick layer of peptidoglycan with a strongly negative surface charge that enables the electrostatic interaction between nanoparticles and cells. This facilitates the penetration of nanoparticles allowing the entry of negatively charged superoxide radical anions and peroxide ions to ensure cell destruction at relatively low concentrations [91,92]. Unlike, gram-positive bacteria, gram-negative strains contain a thin peptidoglycan polymer covered by an outer polysaccharide membrane with structural lipopolysaccharides. This enables the generation of reactive oxygen species and oxidative stress resulting in bacterial cell destruction and inhibition [93-95]. Sundrarajan et al.  stated that ZnO nanoparticles are involved in oxidative stress via the generation of reactive oxygen species and damage the structural protein of bacterial strains. Nanoparticles may also interfere with genetic material and destroys the respiration chain and thus inhibit cell respiration .
Ag nanoparticles synthesized using aqueous extracts of Cissus quadrangularis exhibited antibacterial activity against Escherichia coli, Bacillus subtilis, Streptococcus pneumoniae, and Staphylococcus aureus at 60 g/ml. . Similarly, Ag nanoparticles synthesized from Leucas martinicensis leaf extract inhibit S. aureus, B. subtills, S. typhi, and E. coli with a zone of clearance of about 11.4 mm, 13.0 mm, 9.4 mm, and 11.5 mm respectively. Several researchers attributed the antibacterial properties of silver to the existence of Agº core . However, Ag nanoparticles accumulate at the microbial membranes forming aggregates and causing perforation leading to death [100-102]. Eltaweil et al.  reported the antibacterial activity of Pt nanoparticles synthesized using an aqueous extract of Atriplex halimus leaves against K. pneumonia with a zone of inhibition of about 17 mm. Mohammed et al. , investigated the efficiency of biologically synthesized zinc nanoparticles against Salmonella typhimurium ATCC 14028, B. subtilis ATCC 6633, and Micrococcus luteus ATCC 9341 and compared them with chemically synthesized zinc nanoparticles.
Nanoparticles of metallic compounds and metallic oxides have also shown potent antibacterial efficacy. Visible light-driven S-doped TiO2 nanoparticles exhibited antibacterial activity by interfering with the cell integrity of S. typhimurimin with a MIC value of 25 mg/ml . ZnO nanoparticles synthesized from U. lactuca are effective against a set of gram-positive and gram-negative bacterial strains and significantly showed the reduction of B. licheniformis, B. pumilis, E. coli, and P. vulgaris by 90 %, 89 %, 90 %, and 91 % respectively under visible light exposure . Research on CuO and Cr5O12 nanoparticles also revealed their antibacterial activity against E. coli, S. aureus, B. subtilis, and Enterobacter [79, 106].
ANTIFUNGAL EFFECT OF NANOPARTICLES
Fungi display the versatility of adaptation to any medium and are capable of colonizing different substrates or media in precarious environmental conditions. This characteristic of fungal species has been significantly contributing to the ever-increasing infection morbidity and mortality rate. Fungistatic and fungicidal activities of several metals and metal-derived nanoparticles have been studied to control outbreaks caused by pathogenic fungi. Spherical Ag nanoparticles synthesized from the medicinal plant Acanthospermum austral of size 14 nm are proven to have the potent antimycotic property tested against Microsporum canis, M. gypseum, Epidermophyton floccosum, Trichophyton rubrum, T. mentagrophytes, T. tonsurans, Malassezia furfur, M. sympodialis, M. globose, M. restricta, Candida albicans, C. krusei, C. tropicalis, C. parapsilosis and C. glabrata with a MIC value ranging from 2.0 µg/ml to 32.0 µg/ml . Similarly, Ag nanoparticles derived from another medicinal plant Acalypha indica have exhibited antifungal activity against 3 Aspergillus species with an IC50 value of 5 mg/ml . The fungicidal property of nanoparticles is associated with their ability to attach the sulfur-containing proteins of the cell membrane causing irreversible damage to the cytoplasmic membrane. In addition, metallic nanoparticles often produce reactive oxygen species and hydroxyl radicals, disrupting the mitochondria and leading to the death of fungal cells.
Metallic oxides of various nanoparticles synthesized from plant extracts are also possessed antifungal properties against several members of the Saccharomycetaceae and Trichocomaceae families. Previous studies on antifungal activities of ZnO nanoparticles revealed that the nanoparticles disrupt the cell wall and alter the membrane permeability of C. Albicans and C. tropicalis through the generation of hydrogen peroxide and superoxide radicals at an MFC ranging from 6.25 µg/ml to 50 µg/ml . Helmy et al.  claimed that TiO2 nanoparticles produced from Malva parviflora extract at a concentration of 100 µm/ml inhibit the growth of C. Albicans. Oxidative stress on yeast cells by reactive oxygen species formation led to physical and chemical damage to the intracellular components and genetic materials.
CONCLUSION AND FUTURE INSIGHTS
Improvement of eco-friendly and reliable processes for the synthesis of inorganic nanoparticles is a significant step in the field of applied nanotechnology and nanoscience. Green nanotechnology presents a simple and nontoxic protocol of nanoparticle synthesis and it is of enormous interest due to economic prospects and feasibility. Several regulatory bodies are starting to devote additional attention to nanomaterials to differentiate the nanoparticles produced by green chemistry and classical chemistry. However, this flourishing technology needs to be optimized to identify the exact phytochemicals accountable for the synthesis of nanoparticles; there should be a thorough evaluation of the toxic effects of long-term exposure to the biogenic nanoparticles on flora and fauna. Another challenge is achieving high reproducibility, the levels of phytochemicals in the plants are easily influenced by environmental factors such as soil pH, water stress, and change in climate and location. Hence, it is certain that the properties of plant extracts may vary from batch to batch, which will influence the physiochemical properties of synthesized nanoparticles. In addition, efforts should be geared by researchers toward the cost-benefit analysis for commercial purposes as there is no data available to date. Future research and development of prospective green nanoparticle synthesis should be directed toward extending laboratory-based work to an industrial scale by considering traditional/present issues, especially health and environmental effects. Accordingly, ample possibilities remain for the exploration of new applications of biogenic nanoparticles.
The authors have no conflict of interest to declare.
The authors gratefully acknowledge Indian Academy Degree College-Autonomous for their support.
CdTe: Cadmium telluride; LDH: Layered double hydroxides; Si: Silica; Al – Alumina; Ti: Titania; Zr: Zirconia; Cu: Copper; Fe: Iron; Co: Cobalt; Mn: Manganese; Zn: Zinc; Cr: Chromium; Ni: Nickel; Ag: Silver; Au: Gold; OH-: Hydroxyl ions; AgNO3: Silver nitrate; C: Carbon; Al3+: Aluminium ions; Cr3+: Chromium ions; Co2+: Cobalt ions; Cu2+: Copper ions; Fe2+: Ferrous ions; Fe3+: Ferric ions; Pb2+: Lead ions; Zn2+: Zinc ions; IR: Infrared rays; ZnO: Zinc oxide; TiO2: Titanium oxide; SnCl2: Stannous chloride; COD: Chemical Oxygen Demand; MO: Methyl Orange; MR: Methyl Red; CR: Congo Red; 4-NP: 4-Nitrophenol; RhB: Rhodamine B; Ce: Cerium; Cd2+: Cadmium ions; Hg+: Mercury ions; Ni2+: Nickel ions; As3+: Arsenic ions; Mn2+: Manganese ions; Pt: Platinum; MB: Methylene Blue; ICP-OES: Inductively coupled plasma-optical emission spectrometry; NB: Nile Blue; RY-160: Reactive Yellow 160; 2-NP: 2-Nitrophenol; CeO2: Cerium (IV) oxide; Cr5O12: Chromium oxide; CV: Crystal Violet; SDZ: Sulfadiazine; RB: Rose Bengal; MG: Malachite Green; DG: Direct Green; NR: Neutral Red.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest.