Physical methods are based on the use of electrical fields and some other physical phenomena as the reducing agent during the synthesis of IONPs. The most reoccurring one is particle growth, which is based on physical processes and top-down approach. Examples of such techniques are sonochemical, lithography, sputtering, microwaves irradiation and laser ablation. The sonochemical technique encompasses the sonication of an aqueous ferric solution at ambient conditions and in the presence of air (Nisticò, 2021). Ultrasounds generate irregular compression and expansion acoustic waves which cause the oscillation of the microbubbles (Wu et al., 2015). When the bubbles collapse a localized hot spot is formed at high temperature around 4500 °C and pressure of about 1000 bar which enhanced the conversion of iron precursors into IONPs (Pinkas et al., 2008). The IONPs produced via this technique have high stability and remarkable magnetic properties. Despite the aforementioned advantages it is difficult to control the shape of IONPs produced from this technique (Ali et al., 2016).

Microwaves irradiation approach uses an electromagnetic source with wavelength in the range of 1–103 mm as reducing agent (Nisticò, 2017). The radiation during this process causes molecules to align with the external field to generate motion that produces internal heating. This process has the advantages of reduction in treatment time and energy consumption. Also purified IONPs are mostly obtained. Notwithstanding, a report has revealed that this technique is limited because IONPs produced have poor morphological features and low surface reactivity (Pascu et al., 2012).

The electrochemical technique encompasses the immersion of galvanic cell with two electrodes (usually made up of iron) into a saline solution (Nisticò, 2021). This process involves iron electro-oxidation and electrolysis of water at the anode, alongside water reduction at the cathode. The electrochemical technique is affected by parameters such as working distance between electrodes, pH, reaction time and temperature. These techniques allow easy control of particle size and IONPs with hydrophilic surfaces are usually obtained (Cabrera et al., 2008).

The chemical approach of synthesizing IONPs depends on the growth of iron oxides from the liquid phase via the use of some chemical reagents. Among the several protocols and techniques used for the chemical method of synthesis such as co-precipitation, micro-emulsion, sol-gel and polyol-mediated technique, co-precipitation technique is the most common and simplest (Lenders et al., 2016; Pang et al., 2016). Co-precipitation entails the stoichiometric mixture of iron precursor in presence of a basic conditions, following the given reaction pathway (Eq. 1).

(1)

Acidic iron ions (Fe³⁺ or Fe²⁺) when introduced into basic solution precipitates to form IONPs because magnetite is poorly soluble in basic condition. The formation of magnetite or maghemite NPs using the co-precipitation route depends on the ratio of Fe³⁺ or Fe²⁺ used (Nisticò et al., 2017a). The temperature requirement for the co-precipitation route is in the range of (20–250 °C) (Franzoso et al., 2017; Nisticò, 2017b). The following parameters iron precursor and ratio used, ionic strength, temperature, stirring rate and pH influences the size and shape of IONPs synthesized via the co-precipitation technique (Yazdani and Seddigh, 2016). Co-precipitation technique has been reported as the famous chemical methods for synthesizing IONPs with control size distribution and high yields. However, this process suffers from some disadvantages such as the use of hazardous chemical reagents and difficulties in controlling the shape IONPs (Nisticò et al., 2017a).

The micro-emulsion process is another form of chemical method. In this process iron precursor either in water or oil biphasic system is exploited in the presence of amphiphilic molecules in form of block copolymers or sometimes surfactants such as cetyl trimethyl ammonium bromide or polyvinyl pyrrolidone at the interface (Nisticò, 2017). As a result of the dual nature (i.e., occurrence of both hydrophilic head and hydrophobic tail) of the amphiphiles macro molecules, the species moved at the interface of the two immiscible phases form covalent bonds and also assemble themselves into supramolecular aggregates of various shapes (Nisticò, 2018). The micro-emulsion process has advantages of narrow size IONPs production and easy shape regulation. However, the process has some disadvantages, such as low yield, impure products and formation of agglomeration (Wu et al., 2015).

Sol–gel technique involves acid/based-catalyzed hydrolysis and condensation of precursors from colloidal solutions to produce condensed network of iron oxides (Nisticò et al., 2017b). Iron alkoxides or iron salts are the main precursor for this technique because they can easily react through hydrolysis or condensation to yield oxides (Lemine et al., 2012).

Polyol-mediated technique entail the use of polyols as the reducing and stabilizing agents to aid the shape and size control during the synthesis of IONPs (Nisticò, 2021). This process is based on spreading iron containing precursors such as alkoxides in liquid polyols and heat to its boiling point. A report has shown that IONPs synthesized by polyol-mediated techniques are highly crystalline and can be easily dispersed into polar media due to their hydrophilic surfaces. Conversely, this technique has the limitation of generating toxic byproducts (Wu et al., 2015).

The biological method of synthesizing IONPs is classified into two major routes, namely microorganism and plant based IONPs synthesis.

2.3.2 Plant based IONPs synthesis

This is the process of synthesizing IONPs using extracts obtained from parts of plants, such as back, leaves, root, shoot, stems or the whole plant. In general, the desired plant part is obtained, sorted and washed to remove any impurities (solid or particles), air dried, chopped/mercerized to provide good surface area for extraction (Bolade et al., 2018). The extraction of their chemical constituents is accomplished by soaking the air-dried plant part at room temperature or boiled at elevated temperature to obtain the desired amount of extract (Bolade et al., 2020). This depends on the successful extraction of bioactive constituents in plant extracts. These bioactive constituents (phenols, tannins, saponins, alkaloids, organic acids, flavonoids and vitamins) function as reducing agents during the IONPs synthesis by reacting with the iron precursor (iron chloride, iron nitrate or iron sulphate) to produce IONPs, which are further stabilized by the chemical constituent present in the extract (Sorbiun et al., 2018). Several studies have reported the use of water as extracting solvent in the extraction of bioactive components of plants during the synthesis of IONPs (Prabhakar et al., 2017). Leaves, peel, bark and fruits of green plants have been studied for the eco-friendly synthesis of IONPs, as showed in Tabs. 1 and 2. The methods of synthesizing iron oxide NPs are illustrated in Fig. 1.

Table 1.
Characterization techniques of biosynthesized IONPs from some plant materials.

Note Thermogravimetric analysis (TGA), vibrating sample magnetometer (VSM), and atomic force microscopy (AFM).

Table 2.
Applications of IONPs synthesized from plants extracts.

Figure 1.
Green synthesis of iron oxide nanoparticles.

Three stages, namely activation, growth and termination stage, are involved in the synthesis of IONPs from plant sources. Plant extracts serve as reducing agents for the bio-reduction of iron ion due to the chemical constituents found in plant extract. During synthesis, iron ions react with chemical constituents such as flavonoids, polyphenols and tannins via reduction and oxidation reactions. The electron rich biomolecules with (-OH) functional groups has the efficiency of reducing the iron ions (Fe²⁺ or Fe³⁺) from divalent or trivalent oxidation state depending on the iron precursor used to metallic form Fe⁰. This zerovalent iron will then be converted to IONPs due to some chemical reactivity. Evidently, some reporters have deduced from the functional group identification via Fourier transform infrared spectroscopy (FTIR) analysis that OH functional group in the extract are responsible for bio-reduction (Mohamed et al., 2020). The growth phase occurs during the annealing and segregation of iron atoms which progressively combined to yield IONPs. Finally, the termination step encompasses the stabilization of synthesized IONPs, which occurs when the equivalent amount of function groups found in the extract bind with the surface of the IONPs. Another study has reported the three stages involved in the formation mechanism of IONPs as; complexing of iron cation with hydroxyl functional group, simultaneous bio-reduction of iron cation with hydroxyl functional group to form IONPs and capping of formed IONPs with hydroxyl containing functional group (Thilagavathi et al., 2016). The flow chart illustrating the synthesis of IONPs from plant material is presented in Fig. 1.

Result from previous study had shown different precursors adopted for the green synthesis of IONPs using plant extract. Examples of such precursors are ferric nitrate, ferric chloride, iron acetate, ferrous sulphate, ferric citrate, ammonium ferric citrate. Some mixture of precursors such as mixture of ferrous sulphate and ferric chloride, mixture of potassium ferricyanide and potassium ferrocyanide. The study conducted by Rajendran and Sen (2016) on the effect of precursors on the synthesis of IONPs showed that there was a huge variation in number of days taken for the synthesis of IONPs using different precursors. They stated that the solubility of precursors in water has influence on the reaction kinetics. Furthermore, they concluded that all the precursors showed different morphological features when the IONPs synthesized were compared. Another study had revealed that the nature of the precursor used for the green synthesis of IONPs determine its reaction time, shape and size (Sayed and Polshettiwar, 2015).

Studies have shown that concentration of precursor influence the time taken for the synthesis of IONPs. Increase in concentration of precursor causes an increase in the rate of synthesis while decrease in concentration of precursor has been linked with delay in bio-reduction process during IONPs. This phenomenon was attributed to inadequate proportion of biomolecules present in the extract to the precursor for growth and nucleation of nanocrystals in the solution (Zhu et al., 2012). The influence of concentration of precursor on particle size has been reported, decrease in concentration of precursor have been found to yield IONPs with decrease particle size and vice versa (Verma and Mehata, 2016). Nucleation and growth models of NPs have been adopted in the explanation of the effect of concentration of precursor on particle size claiming that the number of NPs formed in the solution increased with increase in precursor concentration. Aside nucleation rate, delay in particle growth was also noted as the concentration of precursor increases resulting to increase in the availability of elemental concentrations of precursor which cause increase in the size of particle (Cho et al., 2016).

The pH which determines the level of acidity and basicity of reaction medium has been detected to be an important factor that influences the synthesis of IONPs and other metal oxide NPs from plants materials. The influences of pH of solution medium on size and texture of NPs synthesized from plant extract has been documented (Jacob et al., 2019). Therefore, variation of the pH of solution has been adopted in regulation and control of shape and size of the synthesized NPs (Huang et al., 2015). The pH of basic medium (7 to 9) has been reported as the optimum condition for the synthesis of IONPs Aeromonas hydrophila (Lenders et al., 2016). The biosynthesis that occurs at pH 12 and 4 has been reported to completely retard the synthesis of IONPs. This indicated that extreme acidic and basic conditions do not favor the synthesis of IONPs using plant extract (Woźnica et al., 2003).

Temperature is one of the crucial parameters that influences the physical, chemical and biological method of synthesizing IONPs. The temperature requirement for the green synthesis of IONPs using plant extract is in the range of 25 to 100 °C (Patra et al., 2014). However, most researchers prefer the synthesis of IONPs at room temperature due to the volatility of some secondary metabolites of plants extract that are needed for bio-reduction of iron ions. Findings have shown that temperature of the reaction solution influences the morphological identity of NPs (Patra et al., 2014). Findings from the UV-visible analysis of IONPs synthesis from plant part at temperatures interval of 40 and 30 °C shown complete synthesis at 48 and 72 h, respectively. This indicated that rapid synthesis occurs at higher temperatures. This report also indicated that an increase in temperature beyond 40 °C led to poor synthesis of IONPs, which was attributed to the inactivation of biomolecules liable for the reduction of the iron precursor (Rajendran and Sen, 2016). However, a report has shown the successful synthesis of IONPs using Punica granatum seed extract at temperature of 70 °C (Bibi et al., 2019).

The influence of incubation time on NPs synthesized using plant extract have been examined to influence the morphological properties and qualities of NPs (Harlekar et al., 2014). Other factors, such as storage conditions, device used for storage and exposure to light also affect the reaction time of IONPs. Long time incubation period has been documented to cause aggregation and shrinkage of particles (Saif et al., 2016).

The synthesis of IONPs using plants extract is dependent on types of phytochemical found in the extract and the volume used (Devatha et al., 2018). The volume of plant extract used for the synthesis of NPs influence its duration of complete synthesis. When large volume of extract is being used, the rate of synthesis occurs rapidly because more chemical constituents are available in the solution which bind with the iron precursor to effect rapid bio-reduction and stabilization of IONPs while limited volume of extracts are being used the rate of IONPs formation decreases because there are insufficient biomolecules to bind with the metal precursor, which finally result into instability of NPs formed because the iron precursor would not be completely reduced to zerovalent form (Fazlzadeh et al., 2017). To attain an optimum condition for the green synthesis of IONPs, the ratio of the volume of plant extract must correspond to the concentration of iron precursor used because complete bio-reduction occur when there is equivalent amount of iron precursor and functional groups capable of causing reduction (Toledo et al., 2018). Also, the nature and kind of phytochemical available in plant extract influences the yield of IONPs produced (Gholami et al., 2018). The yield of IONPs depend largely on the volume of extract used for synthesis. High volume of extract has been reported induced to increased yield of synthesized IONPs when various volumes of plant extract were used, this was linked to the present of more functional groups, which react with the iron salt to produce improved absorption (Sumera et al., 2018). Findings have proved that volume and kind of extract used for NPs synthesis also affected their morphological properties and biological activities (Shen et al., 2017).

Ultraviolet visible spectroscopy is a technique used for the confirmation of the formation of metal NPs and metal oxide NPs, such as IONPs (Madubuonu et al., 2020). This is carried out by the measurement of its surface plasmon resonance and estimation of the oscillations of conduction band electrons obtained in the electromagnetic radiation (Madubuonu et al., 2020). Ultraviolet analysis has been used to study the stability, size, aggregation and structure of the NPs (Aisida et al., 2020a). Each metal oxide and metal NPs have specific absorbance wavelength and this is obtained from the UV spectrum when incident rays of light encounter the conduction band electrons on the surface of the metal NPs (Akintelu et al., 2020b). The specific absorbance band for IONPs is in the range of 280 to 450 nm on the UV spectrum (Aisida et al., 2020b). Findings obtained from recent studies on the use of UV analysis for the confirmation of IONPs formation are summarized in Tab. 1.

Fourier transform infrared spectroscopy is useful for the identification of the functional groups present in the plant extract and IONPs (Kumar et al., 2016). It is equally used in the determination of the functional groups that participated in the bio-reduction of the iron precursors (Mirza et al., 2018). Fourier transform infrared spectroscopy analysis can produce the absorption and infrared emission spectrum of solid, liquid and gas. The uniqueness in the combination of atoms of biomolecules enhance the identification of functional groups present in synthesized IONPs using the spectra obtained from FTIR analysis (Vasantharaj et al., 2019). The spectrum obtained from FTIR spectroscopic analysis presents fingerprint containing the absorption peaks that correspond to the wavelength of vibrations within the bounds of atoms of the NPs (Ibraheem et al., 2019; Sneha and Karthikeyan, 2019). Fourier transform infrared spectroscopy analysis have been used by numerous researchers to confirm the presence of some biomolecules, such as flavonoids, tannins, alkaloids, saponins in the extract of plants used for the bio-reduction of iron precursors during the formation of IONPs (Rahmani et al., 2019). The prominent functional groups that are responsible for the reduction process during the synthesis of IONPs obtained from previous studies are presented in Tab. 1.

Various investigations have been carried out to enhance the antimicrobial activities of available antibiotic drugs and also to develop novel antimicrobial agents to reduce/eliminate the microbial resistance towards readily available antibiotics and antiseptic (Ansari et al., 2017). Reports from the in vitro antimicrobial studies of metal and metal oxide NPs on numerous microbial species showed that metal oxide and metal NPs demonstrated remarkable inhibition against the growth of tested microbial species when compared with commercially available antibiotics and antiseptic (Rana et al., 2019). However, the antimicrobial activities of IONPs and other NPs depends upon majorly on particle size and material used for synthesis. Over the past decades, IONPs functionalized with therapeutic agents, such as antimicrobials, have gained scientific and industrial attention because of their impressive results of antimicrobial and antibiofilm activities (Holban, 2015). When IONPs agglomerate, their surfaces are modified with important biological molecules and other polymer, IONPs have been coated with several antimicrobial agents to prevent IONPS agglomeration in attempt to extend the utility of engineered IONPs in biomedical applications (Seabra et al., 2017). The utilization of metal NPs have been recommended as the most promising approach for eradicating microbial drug resistance because of their multiple reaction mechanisms with microbial cells (Seabra et al., 2017). Some of the reaction mechanisms are generation of pits in the bacterial cell, which causes fragmentation of the cell wall, denaturation of the outermost membrane of microbes especially bacterial, and reaction with the disulfide groups of enzymes to obstruct metabolic processes, which causes cell death (Rai et al., 2013). Like other metal and metal oxide NPs such as silver, gold, zinc oxide and copper oxide. The IONPs also possess potential antimicrobial activities (Taghizadeh et al., 2019). Patra et al. (2017) reported that IONPs synthesized using corn plant extract exerted synergistic antibacterial activities against tested bacterial species. Previous study on the bactericidal action of IONPs synthesized using the fruit extract of Couroupita guianensis revealed that particles exhibited effective bactericidal potency on tested human pathogens (Gao et al., 2017). Findings have equally shown that the growth of Proteus mirabilis and Escherichia coli can be limited by IONPS obtained using leaf extract of Argemone mexicana L. (Arokiyaraj et al., 2013). The potential of IONPs in combating both gram positive and gram-negative bacteria have been reported effective and further strategies to develop synergistic IONPs platform which could function as carrier system for the treatment of microbial infection in future have been devised (Nehra et al., 2018). Some results obtained from recent studies on the antimicrobial potency of IONPs synthesized using plant extracts against the growth of some deadly human pathogens are shown in Tab. 2.

Lack of selective targets and multidrug resistance have made the effective treatment of tumor and cancer an abortive and worrisome issue (El-Boubbou, 2018). The advancement in nanotechnology and nanoscience over the years have shown some effective contribution of NPs in cancer treatment due to their exceptional features and mechanism of reaction with cancerous cells (Sathishkumar et al., 2018). Among the existing metal oxide NPs, IONPs have been recommended as good anticancer therapy due to their large surface area to graft targeting substrates and moieties, great resistance to in vivo degradation and potential synergistic activity in influencing the sensitivity of drugs towards the treatment of cancer (Bahrami et al., 2017). The cytotoxicity effect of biosynthesized IONPs against human HepG2 liver cell lines reveal that IONPs is capable of inhibiting the growth of cancer cell as the concentrations of IONPs increases (Rajendran et al., 2015). It was deduced from an investigation conducted to determine the cytotoxicity efficiency of lead oxide NPs and IONPs against HepG2 cells that IONPs exhibited higher cytotoxicity efficiency of 38.49% against HepG2 cells while the cytotoxicity efficiency of lead oxide NPs was 20.88%. The difference in the efficiency of the metal oxides in inhibiting the growth of HepG2 cells was linked with the smaller particle size of IONPs (Muhammad et al., 2019). The result obtained from the in vitro cytotoxicity evaluation of various concentrations of IONPs synthesized using flaxseed against MCF-7 cells revealed high toxicity efficacy at concentrations of 4.7 μg mL^–1 and above (Rahmani et al., 2020). The high cytotoxicity efficiency of IONPs against the MCF-7 cell line was attributed to the ability of IONPs in causing breakdown of MCF-7 cells membrane by interacting with the phospholipid molecules in the cell layer (Berry et al., 2004). An outstanding cytotoxicity effect has been documented from the action of Psoralea corylifolia mediated IONPs against significant anticancer activity against renal tumor cells (Nagajyothi et al., 2017).

The wide use of anionic and cationic dyes in textiles, plastic, pharmaceuticals, leather, printing, and paper milling industries had led to their gigantic demand and supply across the globe (Fowsiya et al., 2016). Findings have shown that after manufacturing processes in the aforementioned industries over 20% of the total dyes used are wasted and are discharged into the environment where they resulted into various forms of environmental pollution (Ratna and Padhi, 2012). The discharge of dye and other toxic waste into the ecosystem have result to the death of many aquatic animals, water turbidity and several human health menace (Jin et al., 2018). However, the management and effective control of effluents containing dyes have been a daunting challenge. The implication of the environmental challenges associated with industrial effluents have led to the investigation of the catalytic degradation and oxidation of dyes by metal and metal oxide NPs (Thandapani et al., 2018). Interestingly, metal oxides, such as zinc, copper, titanium and iron oxides, have showed good dye degradation efficiency (Gonawala and Mehta, 2014; Stan et al., 2015). The photodegradation of dyes by IONPs have been attributed to the easy adsorption of dyes linked with the high surface area to mass ratio and large number of surface reactive sites of metal oxide NPs (Dutta et al., 2014). Catalyst loading, pH, temperature, and time have reported as the major factors affecting the photocatalytic activities of IONPs (Ahmed et al., 2020). Several results obtained from the applications of IONPs on environmental control of toxic effluent containing dyes and inorganic substance are documented in Tab. 2.

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