1 Introduction

Green synthesized metallic nanoparticles exhibit unique physical, thermal, chemical, and catalytic properties, [1, 2] and have found great application in health care as antiplasmodial [3], anticancer, and antimicrobial [4, 5], synthetic biology and cellular transportation [6]. The most important feature responsible for their wide range use is attributed to their large surface area to volume ratio, stability, biocompatibility and have a high surface energy as well as unique adsorption phenomenon [7-9]. In addition, metal nanoparticles are a few hundred nanometers smaller than comparable large biological molecules such as enzymes, receptors and antibodies [1]. The unique physical and chemical properties of metal nanoparticles may lead to effective interactions between itself, bacterial cell membrane and other biological entities [1, 10]. The antimicrobial activity of nanoparticles is not well understood, however, various mechanisms have been suggested. The nanoparticles have been reported to cause oxidative stress to the cell of the gram-negative bacteria by generating excess reactive oxygen species [11] as well as release of antimicrobial metal ions from the surface of the nanoparticle. The nanoparticles also have a membrane damaging abrasiveness which inherently gives them an antimicrobial property [12].

Plants exhibit high chemical diversity due to genetic variations, ecological and environmental factors [6]. This means that the plant extracts will contain different concentrations of natural reducing agents which can influence the characteristics of the nanoparticles. Green synthesized iron oxide nanoparticles from plants offers an alternative for exploitation of plants for pharmaceutical, water treatment, biomedical and catalytic applications [13-17]. Previous studies have shown that iron nanoparticles produced from plants have been used to catalyze the degradation of organic dyes such as bromothymol blue, methylene blue, aniline and Rhodamine B [14, 18]. The iron nanoparticles have shown much efficiency in the oxidation of organic dyes. This is because iron has two stable oxidation states which influence the efficiency of iron nanoparticles as photo catalysts [16, 19]. Its suggested that the mechanism for photocatalytic degradation involves oxidation of the dye by hydrogen peroxide, catalyzed by iron oxides and hydroxides present in the nanoparticles through a Fenton like mechanism. This involves corrosion of the surface of the iron oxide nanoparticle in an acidic environment producing ferrous ions that generate hydroxyl radicals. The radicals attack the dye molecules thus degrading them [14, 20].

In this study, plant mediated iron nanoparticles (FeNPs) were synthesized using extracts of local herbal plants Galinsoga parviflora, Conyza bonariensis and Bidens pilosa leaf extracts as reducing and stabilizing agents. These plants were chosen basing on their availability as well as the presence of large amounts of biodegradable and water soluble polyphenols which aid in the synthesis of the nanoparticles through reduction as well as stabilization [8, 21, 22]. The synthesis of iron nanoparticles was performed in a one-step by adding the plant extracts to 0.1 M of ferric chloride as the iron precursor leading to formation of a black precipitate of Galinsoga parviflora (GpNPs), Conyza bonariensis (CbNPs) and Bidens pilosa (BpNPs) iron particles. These were analysed using UV–Vis spectrophotometer (UV–VIS), Fourier Transform Infrared spectrophotometer, X-ray Fluorescence (EDXRF), X-ray diffractometer (XRD), and scanning electron microscope (SEM) to determine composition, structure and size. The iron nanoparticles were then used as catalysts in the degradation of methylene blue solution.

2 Materials and methods

2.1 Collection of plant extracts

Fresh leafs of Galinsoga Parviflora, Bidens pilosa and Conyza bonariensis were collected from Bungoma East Sub-County, Bungoma County in Kenya, washed thoroughly with running laboratory tap water and further with distilled water and air-dried at room temperature (20–25 °C) for 14 days to remove moisture [23]. Dry plant materials were ground finely using a milling machine.

2.2 Quantitative Phytochemical analysis

2.2.1 Total phenolic content

The total phenolic content of the crude extracts was evaluated by the Folin–Ciocalteau method with some modifications using Garlic acid as standard [24]. Dry and ground plant material (0.1 g) were extracted using 4.9 mL 80% methanol and filtered through a Whatman filter paper no.1 to make the stock sample. Absorbance was measured at 769 nm using compact, double-beam UV–Vis spectrophotometer (UV 1800, Shimadzu) [25, 26]. The total phenolic content expressed in mg of garlic acid equivalents (GAE)/g of dry weight extract (DW).

2.2.2 Total flavonoid content

The total flavonoid content was determined using the Aluminium chloride method with Rutin as the standard according to [27] and the absorption readings done at 510 nm using compact, double-beam UV–Vis spectrophotometer (UV 1800, Shimadzu). The calibration curve was used to determine the total flavonoid content and was expressed as mg of Rutin equivalent (RE)/g of dry weight of sample.

2.2.3 Synthesis of iron nanoparticles from leaf extract

Twenty grams of the dried and ground Galinsoga parviflora, Conyza bonariensis and Bidens pilosa were transferred into different labeled 250 mL volumetric flasks. 200 mL of distilled water added and the mixture boiled at 70 °C with constant stirring for 45 min. The cooled extract was collected by filtering three times with cotton wool and further through Whatman No.1 filter paper to get a clear extract [7, 13, 28]. 0.1 M FeCl3·6H2O solution (working standard) was prepared by weighing 13.52 g of FeCl3·6H2O and dissolving in 100 mL of distilled water and further topped to 500 mL with enough distilled water [29]. Synthesis of Iron nanoparticles was done by adding 30 mL of plant extracts to 10 mL of 0.1 M Ferric Chloride dropwise using a burette [30, 31] with constant stirring at room temperature and pressure. The black precipitate was washed several times with distilled water, centrifuged at 6000 rpm for 10 min (Centurion 6000). The obtained nanoparticles were dried at 60 °C in the oven [29].

2.2.4 Characterization of iron nanoparticles

UV–Vis absorption spectra were recorded using compact, double-beam UV–Vis spectrophotometer (UV 1800, Shimadzu) in the range of 200–800 nm [32]. The Shimadzu FTIR spectrophotometer (FTS—8000, Japan) was used to analyze functional groups present on the plant extract as well as the iron nanoparticles by the standard KBr method, with spectral resolution set at 4 cm–1 and the scanning range from 400 to 4000 cm–1 [33]. The elemental composition was done using the X-ray fluorescence machine Bruker model at the Mines and Geological Department laboratories, Kenya. The crystallinity phase identified using STOE STADIP P X-ray Powder Diffraction System (STOE & Cie GmbH, Darmstadt, Germany). The X-ray generator was equipped with a copper tube operating at 40 kV and 40 mA and the sample irradiated with a monochromatic Cu Kα radiation with a wavelength of 1.5409 nm and 2θ range of 2–90 °C at 0.05 °C intervals. Morphological analysis was performed using Tescan Mira3 LM FE Scanning electron microscope, Germany operated at an accelerating voltage of 3 kV. The samples were gold sputtered before observation to avoid the charging effect.

2.2.5 Methylene blue degradation

Methylene blue degradation was established using previous methodology [34]. The reagent was prepared by diluting 1 mL of it to 50 mL with distilled water. Two spectral scans were done on 3 mL of the solution in the range of 800–300 nm in 5 min intervals using double-beam UV–Vis spectrophotometer (UV 1800, Shimadzu). 1 mL Hydrogen peroxide was added and two more scans were allowed, after which 50 mg of the iron nanoparticles were sprinkled on the solution and allowed to run for 10.5 h in 5 min intervals. A spectral scan for the degradation of methylene blue solution using hydrogen peroxide without the nanoparticles and nanoparticles alone as a control was done under the same conditions for 1 h.

3 Results and discussion

3.1 Biosynthesis of iron nanoparticles

Figure 1 shows the schematic representation of biosynthesis of iron nanoparticles. The colour changes indicated the formation of iron nanoparticles [7].

Fig. 1
figure 1

Flow diagram for synthesis of iron nanoparticles

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The results indicated a relatively higher concentration of the total flavonoids as compared to total phenolic content. Conyza bonariensis had the highest total phenols (45.50 ± 0.59) while Bidens pilosa had the highest total flavonoids (126.27 ± 0.01). Polyphenols in plants are responsible for the antioxidant behaviour of most plants and offer the potential of using plants as nutraceuticals as well as food additives. As discussed in the mechanism of nanoparticles formation [35, 36], the phytochemicals present offer a complementary role in the reduction as well as stabilization of the iron nanoparticle [37]. The postulate underlying the green synthesis perspective is that the phytochemicals present in the plant naturally reduce as well as stabilize the nanoparticle [36–39] (Table 1).

Table 1 Quantitative total phenolic and total flavonoid content of Galinsoga Parviflora, Bidens pilosa and Conyza bonariensis leaf extracts
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3.2 UV–Vis absorption spectra

UV–Vis absorption spectra of an aqueous solution of Galinsoga Parviflora, Bidens pilosa and Conyza bonariensis leaf extracts, synthesized iron nanoparticles and iron III chloride Figs. 2, 3, 4.

Fig. 2
figure 2

UV–Visible absorption spectrum of iron III chloride (Fe3+), Conyza bonariensis (Cb) leaf extracts and synthesized iron nanoparticles

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Fig. 3
figure 3

UV–Visible absorption spectrum of iron III chloride (Fe3+), Bidens pilosa (Bp) leaf extracts and synthesized iron nanoparticles

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Fig. 4
figure 4

UV–Visible absorption spectrum of iron III chloride (Fe3+), Galinsoga Parviflora (Gp) leaf extracts and synthesized iron nanoparticles

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The leaf extracts were brown in colour and had higher absorption peaks at 267 nm, 288 nm and 286 nm for Galinsoga parviflora, Bidens pilosa and Conyza bonariensis respectively. These indicates that the leaf extracts contain electron rich polyphenols, amino acids as well as sugars which are responsible for the reduction of Fe3+ [18, 40]. Fe3+ spectra displayed peaks at absorption peaks of 203 nm and 295 nm. Upon the addition of the extract into the 0.1 M Ferric chloride solution, black colored colloidal solutions developed gradually. The strong absorption peaks of the extract, and the Fe3+ in the range of 200 to 350 nm disappeared, and a broad absorption was observed, which demonstrate the formation of poly dispersed iron nanoparticles through electron transfer and surface Plasmon resonance [41]. The change in color is associated with the excitation of electrons as the Fe3+ ions are reduced to Fe0 [18].

3.3 FTIR characterization of ZVIN

The functional groups present in the nanoparticles as well as the aqueous leaf extracts were determined using FTIR and Figs. 5 and 6 shows the spectra obtained respectively.

Fig. 5
figure 5

FTIR spectra of Conyza bonariensis (Cb extract), Galinsoga parviflora (Gp extract) and Bidens pilosa (Bp extract)

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Fig. 6
figure 6

FTIR spectra of Conyza bonariensis iron nanoparticles (CbNPs), Galinsoga parviflora iron nanoparticles (GpNPs) and Bidens pilosa iron nanoparticles (BpNPs)

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Based on the spectra obtained (Fig. 5), the Conyza bonariensis, Galinsoga parviflora and Bidens pilosa extracts depicted O–H stretching of phenolic groups at 3309 cm−1 3329.7 cm−1 and 3336 cm−1 respectively. This is suggested to be responsible for the bioreducton of the ferric ions [42]. The O–H stretching vibration in CbNPs, GpNPs and BpNPs shifted to 3406 cm−1, 3438.3 cm−1 and 3402.6 cm−1. The O–H stretch in the nanoparticles is due to the secondary metabolites such as polyphenols, sugars, amino acids which have been reported to be responsible for chelation and bio reduction of Fe3+ and Fe2+ to Fe0 [7, 14, 29, 40, 43]. These also explains their roles as capping agents as they are also observed in spectra of iron nanoparticles, though there was a slight shift in the vibrational frequencies observed. The peaks at 1027 cm−1, 1037.7 cm−1 and 1031 cm−1 in CbNPs, GpNPs and BpNPs and at 1000 cm−1 for the extracts respectively are due to C–O-C vibrational frequencies of glycosidic linkage in the extracts [44]. Those in 1400 cm−1 for the nanoparticles are assigned to the C–C and C = C for aromatic stretching [7]. The bands at 1631, 1630 and 1627.4 cm−1 in BpNPS, CbNPs and GpNPs respectively indicate the presence of C=C vibrational mode for unsaturated metabolites such as terpenoids. The iron-oxygen bond observed at 671 cm−1, 621 cm−1 and 636 cm−1 (Fig. 6) [13, 28, 29, 45-48]. The FTIR bands for the synthesized iron nanoparticles matched well with those for their respective plant extracts, which demonstrate that the components present in the extracts capped the synthesized iron nanoparticles. However, shifts in the bands between the nanoparticles and the extracts, weakening in the transmittance as well as disappearance designate that water soluble biomolecules such as reducing sugars and polyphenols are responsible for bio-reduction of Fe3+ [28, 49]. The spectra of the iron nanoparticles after degradation of methylene blue in the presence of H2O2 are shown in Fig. 7. It was observed that the OH stretch peaks for the used CbNPs, GpNPs and BpNPs nanoparticles shifted from 3406 cm−1, 3438 cm−1 and 3402 cm−1 to 3337 cm−1, 3320 cm−1 and 338 cm−1 respectively. The shifts in peak intensity of the nanoparticles after degradation is due to the photo excitation of the catalyst, which is brought about by the exothermic reaction between the dye, and the hydroxyl groups attached on the surface of the nanoparticles [50, 51].

Fig. 7
figure 7

FTIR spectra of BpNPs, CbNPs and GpNPs after degradation of methylene blue solution using hydrogen peroxide

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3.4 X-ray fluorescence analysis

The composition of iron nanoparticles using Galinsoga parviflora, Conyza bonariensis and Bidens pilosa extracts were evaluated using EDXRF and the results are depicted in Fig. 8.

Fig. 8
figure 8

Elemental composition of the synthesized iron nanoparticles analysed using energy-dispersive X-ray spectrometer of iron oxide nanoparticles

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Fig. 9
figure 9

X-ray diffractogram of BpNPs

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Fig. 10
figure 10

X-ray diffractogram of CbNPs

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Fig. 11
figure 11

XRD diffractogram of GpNPs

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Figure 8 shows the percentage elemental composition obtained from EDXRF quantification of the synthesized plant iron oxide nanoparticles. Plant mediated FeNPs of Galinsoga parviflora, Conyza bonariensis and Bidens pilosa had 51, 47 and 44% of iron respectively. The synthesized Fe particles revealed highest percentage of iron than other elements in the total weight of Fe3O4-NPs [13]. The higher percentage of chlorine must have originated from chloride present in aqueous extracts used in the synthesis protocol [30]. Therefore, the elemental analysis of the iron nanoparticles confirmed the synthesis of FeNPs using Galinsoga parviflora, Conyza bonariensis and Bidens pilosa aqueous extracts with impurity peaks of P2O5, K2O and CaO.

3.5 X-ray diffractograms

The X-ray diffractograms obtained for the FeNPs synthesized using Bidens pilosa, Conyza bonariensis and Galinsoga parviflora extract is shown in Figs. 9, 10, 11 respectively.

BpNPs displayed a number of diffraction peaks at 2θ values of 16.03°, 21.44°, 32.73°, 36.45°, 43.75°, 49.46°, 58.39°, and 68.69° while GPNPs had diffraction peaks at 2θ values of 29.89°, 34.20°, 43.13°, 49.02°, 53.17° and 57.95° which are due to a mixture of iron oxides such as magnetite and maghemite [52]. Shoulders depicted at the range of 2θ = 10°–20° at around 20° relates to the presence of polyphenols from leaf extracts as capping and stabilizing agents caused by organic matter [53-55]. Those from 20° to 58° corresponds to the characteristic peaks of a mixture of iron oxides. A typical peak at 36° in BpNPs shows presence magnetite (Fe3O4) [14, 40, 48, 52, 56]. CbNPs had an amorphous nature while GpNPs with poor crystallinity due to the amorphous nature hence the diffraction patterns do not depict any distinctive diffraction peaks suggesting that the green synthesis approach resulted in the production of amorphous iron nanoparticles [54, 55, 57].

3.6 SEM micrographs

The SEM micrographs of iron nanoparticles synthesized from Conyza bonariensis, Bidens pilosa, and Galinsoga parviflora aqueous extracts are shown in Fig. 12.

Fig. 12
figure 12

SEM images of synthesized GpNPs (a), CbNPs (b), and BpNPs (c) iron nanoparticles (FeNPs)

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From the SEM micrographs, GpNPs, CbNPs and BpNPs nanoparticles were agglomerated due to presence of various biomolecules, chloride, and potassium compounds present in the leaf extracts which influence the morphology of the nanoparticles [14] and the gold used in sample preparation for SEM analysis [58]. Polyphenols present in the aqueous extracts of Conyza bonariensis, Bidens pilosa, and Galinsoga parviflora play a vital role as reducing agents and control the agglomeration of the nanoparticles through van der Waals interaction [48, 59].

3.7 Degradation of methylene blue

The nanoparticles were investigated for their ability to catalyze degradation of methylene blue dye and the results are depicted in Figs. 13, 14, 15 [50]. The catalytic activity of the hydrogen peroxide, the nanoparticles without hydrogen peroxide as controls, were also evaluated for 40 min each as shown in Figs. 15, 16.

Fig. 13
figure 13

Kinetics of methylene blue degradation using CbNPs/H2O2. Insert time dependent UV–Vis absorption spectra of methylene blue degradation

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Fig. 14
figure 14

Kinetics of methylene blue degradation using GpNPs/H2O2. Insert time dependent UV–Vis absorption spectra of methylene blue degradation

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Fig. 15
figure 15

Kinetics of methylene blue degradation using BbNPs/H2O2. Insert time dependent UV–Vis absorption spectra of methylene blue degradation

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Fig. 16
figure 16

Kinetics of methylene blue degradation using H2O2. Insert time dependent UV–Vis absorption spectra of methylene blue degradation

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Hydrogen peroxide, GpNPs, CbNPs and BpNPs degraded the methylene blue by 6.85%, 0.4710%, 0.549% and 2.129% respectively. Their rate constants were found to be 5.60 × 10–4, 1.35 × 10–4, 1.70 × 10–5, and 2.82 × 10–4 respectively. The absorption rate of methylene blue (M.B) remained the same for about 20 min, but upon addition of hydrogen peroxide, there was a small drop in absorption spectra due degradation of the dye by hydrogen peroxide. Addition of the iron nanoparticles provoked further methylene blue degradation for variable contact times. Since absorbance is directly proportional to concentration, the percentage degradation was calculated using the formula: \((({A}_{0}-A)/{A}_{0})*100\%\). The order of reaction was determined using the plots of \(\mathrm{ln}A/{A}_{0}\) against time. The degradation rate constant (K) is the slope of the graphs. AO is the initial absorbance while A is the final absorption. The CbNPs, BpNPs and GpNPs degraded methylene blue by 86.2%, 84.3% and 92.9% for 10.5 h, 5.5 h and 4.3 h and the first order rate constants obtained from a plot of \(\mathrm{ln}A/{A}_{0}\) against time were 3.77 × 10–3, 8.35 × 10–3, and 1.456 × 10–3 per minute respectively [60]. The use of iron nanoparticles alone had no appreciable effect on the absorbance while use of hydrogen peroxide caused a slight change. The catalytic activity of the iron nanoparticles is due to the Fenton and Fenton-like mechanisms. The ferric cations can be reduced to ferrous cations through reaction with the electrons produced from photoreactions. The ferrous ions are reoxidized back to ferric by the hydrogen peroxide resulting in production of (·OH) radicals through photo Fenton reaction. Hydroxyl radicals (·OH) are generated due to the presence of iron ions and hydrogen peroxide. The ·OH radicals together with the H+ and ·O2− play the role of degradation of the methylene blue dye when coupled as H+  + ·O2− and H+  + (·OH) [61].

$${\text{Fe}}^{3 + } + {\text{H}}_{2} {\text{O}}_{2} /{\text{e}}^{ - } \to {\text{Fe}}^{2 + } + {\text{OH}}^{ - } + \cdot {\text{OH}}$$
(1)
$${\text{Fe}}^{2 + } + {\text{H}}_{2} {\text{O}}_{2} \to {\text{Fe}}^{3 + } + {\text{OH}}^{ - } + \cdot {\text{OOH}}$$
(2)
$$\cdot {\text{OH}}\;{\text{or}}\; \cdot {\text{OOH + Organics}} \to {\text{CO}}_{2} + {\text{Water}}$$
(3)

Generation of the radicals involves the electronic transitions between iron and oxygen in the particles which leads to an exciton emission (electron) in the valence band, consisting of Fe (3d) and O (2p) being excited to the conduction band that consists of Fe (4s and 3d) and O (2p). These create a positively charged hole in the valence band and negatively charged conduction band, which reacts with hydrogen peroxide and water producing reactive hydroxyl radicals The radicals break down the organic compounds into carbon dioxide and water. The intermediates iron produced such as Fe 2+, Fe 3+, their hydroxides are thermodynamically unstable and much active. These also contributes to the catalytic activity [61–65]. Therefore, the radicals react with methylene blue resulting in dye degradation [34, 50, 60].

4 Conclusion

Aqueous extracts of Galinsoga Parviflora, Bidens pilosa and Conyza bonariensis leaves can be used for the biosynthesis of iron oxide nanoparticles. Functional group analysis, and elemental composition of the particles revealed that the nanoparticles were largely composed of iron with oxygen bearing functional groups. Application of the synthesized nanoparticles as a catalyst against methylene blue degradation revealed that while the particles were able to degrade the dye, the rate of degradation was dependent on the extracts used during synthesis. Different nanoparticles had different degradation rates, but they all followed first order rate reaction. Hence, the aqueous extracts from the three plants can be used to synthesize iron nanoparticles which can be used as catalysts in the degradation of dyes with hydrogen peroxide at different reaction rates.