Statement of Novelty

The development of facile and low-cost methods to synthesize ultrasmall (2–20 nm) maghemite nanoparticles with naturally occurring plant materials is an emergent and timely necessity for scientific community worldwide. In this study, single phase γ-Fe2O3 NPs was synthesized using fruit extract of Ficus carica; confirmed by various spectroscopic and microscopic techniques. It also showed considerable antioxidant and catalytic activity.

Introduction

Iron (Fe) is a 4th abundant metal in the earth’s crust representing the essential elements which are absolutely critical to the biological processes [1]. It exists mostly in the form of Fe0, and oxides of Fe2+/Fe3+ as wustite (FeO), magnetite (Fe3O4), hematite (α-Fe2O3) and maghemite (γ-Fe2O3) forms. Since the early 1990s, the ultrafine iron oxide nanoparticles (FeONPs) has been taken special attention by researchers from various fields due to their outstanding multifunctional properties such as small size, high surface area to volume ratio, high magnetism, thermal stability, biocompatibility and low toxicity compared to other nanoparticles [2]. Therefore, FeONPs are frequently assessed for their potential nanotechnology related applications in magnetic fluids, magnetic recording devices, high-frequency switch modes, electromagnetic absorbers [3], catalysis, dye/antibiotic degradation, [4], nano-biomedicines, cosmetics, diagnostics, sensing [5], heavy metal absorption, waste water treatment, material sciences, etc. [6, 7]. Among different iron oxides, γ-Fe2O3 is a very common ferrimagnetic mineral on the earth's surface, stable to aqueous conditions, enhanced adsorbing abilities, chemical stability, nontoxicity and exhibit the cubic spinel structure [1]. The use of nanoparticles in different field has escalated 25 fold from 2005 to 2010 [8]. Various chemical and physical synthetic approaches widely used for preparing maghemite nanoparticles (γ-Fe2O3NPs) including aerosol/ vapour (pyrolysis), gas deposition, sol–gel, co-precipitation, electrochemical, microemulsion, flow injection, bulk solution, hydrothermal and solvo-thermal method, etc. [9, 10]. But, only few solutions-based synthetic routes are recommended and scaled up for industrial-scale production due to their low operating cost, biological compatibility and safety issue [11, 12]. However, controlled synthesis of γ-Fe2O3 NPs with such specific morphology is still challenging. Generally, FeONPs are chemically robust and can be oxidized easily with air. Therefore, it is necessary to maintain their stability by promising protection tactics and to coat their surface with biomolecules, organic molecules, polymers, or inorganic layer [13].

In recent years, the one-pot biosynthesis of γ-Fe2O3 NPs using extracts of natural products as reducing and capping agent has been considered as a simple, cost effective and environmentally friendly alternative to chemical and physical methods of nanoparticle production [14, 15]. Phytochemicals in plant extract simply convert Fe2+/3+ to Fe0/FeO NPs due to difference between the standard reduction potential of phytochemicals (0.534 V) and metallic iron (− 0.44 V), and therefore the reduction process is spontaneous [15]. While there are many studies concerning on the biosynthesis of γ-Fe2O3 NPs using various plant materials such as (a) leaf extracts of Castanea sativa [14], Omani mango [16], Tea [17] and Oolong tea [18], Tridax [19], Peltophorum pterocarpum [20], Iranian Mango [21], Rhamnella gilgitica [22], Sageretia thea [23]; (b) fruit extracts of Hyphaene thebaica [24] Ficus carica [25], (c) dried bark of Cinnamomun verum and (d) dried pods of Vanilla planifolia [26] has been already reported. Singh et al. reported superparamagnetic starch functioned γ-Fe2O3 NPs for the removal of Chromium [27], whereas Duman et al. used blooming microalgae mediated synthesis γ-Fe2O3 NPs for biofuel production [28]. Therefore, the development of facile and low-cost methods to synthesize ultrasmall (2–20 nm) γ-Fe2O3 NPs with naturally occurring plant materials is an emergent and timely necessity for scientific community worldwide.

Ficus carica (F. carica), is native to South-West Asian countries, and the Mediterranean region [29]. Fruits are edible, axillary on leafy branchlets, pear shaped, often get cracked on ripening. The interior portion of fruit is a white, inner ring containing a seed mass bound with sweet jelly-flesh. Seeds are numerous in number and vary in size [30]. Fruit contains flavonoids, coumarins, and major phytochemicals of anthocyanin class, namely, glycosides of cyanidin, pelargonidin and peonidin [31].

Nitroaromatic compounds are the most common organic pollutant that exists in industrial and agricultural wastewaters. 4-nitrophenol (4-NP) and 2-nitrophenol (2-NP) are examples of toxic substance that are stable in aqueous media and have been classified as a priority toxic pollutant by the U.S. Environmental Protection Agency (EPA) [32]. Nitrophenolic wastes are carcinogenic, and cause hepatotoxicity and mutagenesis in living organisms so they need to be removed from aqueous media. The removal of nitrophenol from aqueous media or catalytic transformation into valuable compounds such as amino phenols have been widely used with NaBH4 through catalytic reduction reactions by the colloidal metallic nanoparticles like gold, silver, iron, cobalt, nickel and copper [33, 34].

In this report, we have developed a facile, low-cost and ecofriendly method for the one-pot biosynthesis of single phase γ-Fe2O3 NPs using unripened fruit extracts of F. carica as a natural resource.The rich phytochemicals of the plant extracts is responsible for reducing and stabilizing the nanoparticles. The obtained γ-Fe2O3 NPs have been examined by various analytical tools including U.V.-Vis spectroscopy, XRD, FTIR, TGA, TEM-SAED and DLS. In addition to this, the synthesized γ-Fe2O3 NPs were used as an antioxidant for scavenging free radical against DPPH and catalyst for the reduction of 4-NP to 4-AP.

Experimental

Materials

All chemicals were reagent grade and used without any purification. FeSO4·7H2O (99.0%) and 4-Nitrophenol (> 99.5%, 4-NP) were purchased from Spectrum, USA. 1, 1-diphenyl-2 picrylhydrazyl (DPPH·, > 99.5%) and Sodium borohydride (NaBH4, > 99.5%) were purchased from Sigma-Aldrich, USA. Milli-Q water was used in all experiments. All glassware was washed with acid/base solution followed by Milli Q water and finally dried in a hot air oven.

Preparation of Aqueous Extract of Ficus carica Fruit (FCF)

In this experiment unripened F. carica fruit was collected from the garden of Playa Chica 1 near Universidad de las Fuerzas Armadas ESPE, Sangolquí, Ecuador and washed with Milli Q water to remove dust particles. Aqueous extract of F. carica fruit (FCFE) was prepared as per our earlier report [35]. About 20 g of thoroughly washed unripe F. carica fruit was chopped finely (0.5–0.8 cm) and heated in 100 mL of Milli-Q water for 60 min at 65–70 °C. The light yellow extract was filtered through a Whatmann paper No. 2 to eliminate particulate substance. The clear filtrate was used for the synthesis of nanoparticles and stored at 4 °C for subsequent experiments.

Biosynthesis of γ-Fe 2 O 3 NPs

For the biosynthesis γ-Fe2O3 NPs via co-precipitation, 25 mL of FeSO4·7H2O (4 mM, 27.8 mg) was mixed with 10 mL of FCE extract (68.2279 µg GAE, Gallic acid equivalents) and pH is adjusted to 9–10 using 0.1 M KOH solution. After that, as-prepared greenish-blue colour reaction mixture was kept under vigorous magnetic stirring for 2 h at 78–80 °C. The visual colour change from greenish-blue to brownish-black indicated that the γ-Fe2O3 NPs were formed (Fig. 1a). The resulting γ-Fe2O3 NPs were centrifuged at 8000 rpm for 2 × 15 min and washed several times with 1:1 mixtures of distilled water, and absolute methanol. The Purified γ-Fe2O3 NPs powder was dried at 90 °C for 12 h and stored in an airtight bottle for further analytical characterization by UV–Vis, XRD, FTIR, TGA, TEM, and DLS.

Fig. 1
figure 1

a Photograph of γ-Fe2O3 NPs and b UV–Vis spectrum of FCF extract and as-synthesized γ-Fe2O3NPs

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Morphology and Physicochemical Characterization of γ-Fe 2 O 3 NPs

The optical properties of samples containing FCF extract and γ-Fe2O3 NPs were recorded by using UV–Visible spectrophotometer, SPECORD® S600 from Analytik Jena, Germany. Crystal structure identification of nanoparticle was achieved by X-ray diffraction (XRD) analyses with a PANalytical brand θ–2θ configuration (generator-detector) X-ray tube, copper λ = 1.54 Å and EMPYREAN diffractometer. The FTIR- attenuated total reflection (ATR) spectra were collected in the transmission mode (4000–400 cm−1) using a Spectrum 100 IR spectrometer (Perkin Elmer, USA) to determine the possible involvement of functional groups for the synthesis of nanoparticles. The thermogravimetric analysis (TGA) was carried out (TGAQ500, TA Instruments, USA) by heating 10–15 mg of the sample up to 500 °C in a platinum sample cup at the rate of 20 °C/min in the presence of N2 gas. The samples were analyzed in TGA for degradation of organic matter with increase in temperature. Dried powder sample of nanoparticles was used to carry out XRD, FTIR, and TGA analysis. The pH measurements of reaction solution were performed by using pH meter (Seven Easy pH, METTLER TOLEDO AG, 8603, Switzerland). The hydrodynamic size distribution of the centrifuzed γ-Fe2O3 NPs sample was analyzed in a dynamic light scattering (DLS) instrumentation from HORIBA, LB-550 Size Distribution Analyzer. Morphology and selected area electron diffraction (SAED) pattern of as-synthesized γ-Fe2O3 NPs were determined using transmission electron microscopy (TEM) by FEI Tecnai, G2 Spirit Twin.

Free radical Scavenging Activity

Antioxidant capacities of FCF coated γ-Fe2O3 NPs were determined using DPPH assay as described earlier by Kumar et al. [36] with minor modification. The purified and dried γ-Fe2O3 (5 mg) was resuspended in 50 mL water. After that, 1000–200 μL of γ-Fe2O3 NPs solution and 1000–1800 μL of H2O were taken in different test tubes. Then, 2.0 mL of 0.2 mM DPPH fresh solutions (in 95% ethanol) was added and vortexed vigorously. The reaction mixture incubated in the dark for 30 min at room temperature. The absorbance of the reaction mixture was measured spectrophotometrically at 517 nm. The lower absorbance of the reaction mixture indicated a higher percentage of scavenging activity, and the percentage of inhibition of free radicals was determined by the following formula:

$$ {\text{Scavenging}}\,{\text{activity}}\,{\text{(\% )}} = (A_{0} - A_{1} )/A_{0} \times 100 $$
(1)

where A0 is the absorbance of the control (blank, or γ-Fe2O3 NPs) and A1 is the absorbance in the presence of the FeONPs. The final result was expressed as the percentage of DPPH free radical scavenging activity (μg).

Estimation of Total Polyphenolic Content (TPC) in FCFE

The TPC of was estimated by the colorimetric Folin–Ciocalteu’s (FC) method, according to Kumar et al. [37]. The 200–800 μL of FCFE, 800–200 μL of H2O and 2.0 mL of 0.02 N FC reagent were added in a separate glass tube and then mixed well using a Vortex mixer. After that, the reaction mixture was allowed to react for 3 min then 1 mL of 1 N Na2CO3 solution was added and mixed well using a Vortex. The solution was incubated in the dark for 120 min at room temperature (23–25 °C). Immediately after completion of reaction, the absorbance was measured at 725 nm using a spectrophotometer and the results were expressed in gallic acid equivalents (GAE; mg/100 g fresh mass) using a gallic acid (0–1 µg/mL) calibration curve. Additional dilution was done if the absorbance value measured was over the linear range of the standard curve. Each TPC assay was repeated thrice from the same extract in order to determine their reproducibility.

Catalytic activity of γ-Fe 2 O 3 NPs

The catalytic efficiency of γ-Fe2O3 NPs, was assessed by reduction of 4-NP in the presence of NaBH4 as a model reaction. Briefly, 0.2 mL of 4-NP (2 mM), 20 µg (Set 1) and 100 µg (Set 2) of as-synthesized γ-Fe2O3 NPs and 3.6 mL of H2O were mixed in a 4 mL standard quartz cuvette or (200 µL and 1000 µL of 1 mg/10 mL γ-Fe2O3 NPs added in 3.4 and 2.6 mL of H2O). To this reaction mixture, 0.2 mL of NaBH4 (0.05 mM) solution was added and UV–vis absorption spectra were recorded at different time intervals, leading to a colour change from pale yellow to colourless. Control reactions were also monitored for 30 min in the absence of γ-Fe2O3 NPs keeping other parameters constant.The observed rate constant (k) of the reduction process was determined by analyzing the change in an intensity of the peaks at 400 nm as a function of time.The concentration of 4-NP at time t is denoted as Ct, and the initial concentration of 4-NP at t = 0 is regarded as C0. The C0/Ct is measured from the relative intensity of absorbance (A0/At).The linear relationship of ln (A0/At) versus time (t) indicates that the reduction of 4-NP by γ-Fe2O3 NPs follows the first order kinetics [38].

Results and Discussion

Visual and UV–Vis Spectroscopy

The UV–Vis spectra of diluted FCF extract and as prepared γ-Fe2O3 NPs are presented in Fig. 1. The absorption spectra of FCF extract was taken as a baseline and as-synthesized γ-Fe2O3 NPs were evaluated. Iron oxide nanoparticles as γ-Fe2O3 NPs has been successfully synthesized by using an aqueous extract of Ficus carica fruit through green method. During one pot biosynthesis of γ-Fe2O3 NPs, a gradual change in greenish-blue colour to brownish-black reaction mixture is observed under vigorous stirring (Fig. 1a) at 75–80 °C, pH 10–11 for 2 h. This clearly indicates the formation of iron oxide nanoparticles/γ-Fe2O3 NPs in the reaction mixture. We assume that the FCF extract comprises flavonoids, polyphenolic and coumarins as an antioxidants may be act as primary complexing and reducing agent for the synthesis of γ-Fe2O3 NPs [19, 21, 36]. A continuous broad absorption band occurred from 350 to 450 nm ensure the involvement of phytochemicals with Fe2+ for the synthesis of γ-Fe2O3 NPs. Almost, similar surface plasmon resonance (SPR) spectra of γ-Fe2O3 NPs in UV–visible were obtained using the previously reported methods [20, 22].

XRD Analysis

The successful synthesis of γ-Fe2O3 NPs and its nature and phase composition with the smallest size were confirmed by XRD analysis. Figure 2 shows the typical XRD pattern of as-synthesized single phase γ-Fe2O3 (no reflections of impurity) and confirms its high crystalline nature. The XRD peaks, which appeared at 2 theta (2θ) value of 30.20, 35.54, 43.21, 53.72, 57.20 and 62.79° representing (220), (311), (400), (422), (551) and (440) crystallographic planes respectively [39]. The observed Bragg peaks of FCF extract mediated γ-Fe2O3 NPs are in agreement with pure phase cubic lattice maghemite γ-Fe2O3 with ICSD No: 98-024-7034. It should be noted that the standard XRD patterns of γ-Fe2O3 and magnetite (Fe3O4) are alike. The characteristic peaks of γ-Fe2O3 at 30.20° and 43.21° confirm the formation of γ-Fe2O3 NPs and also rules out the fabrication of Fe3O4 nanoparticles in sample [40]. Sharp and strong peaks represent a good degree of organization in the particles packing, evidencing the monodispersity (uniform shapes) of the nanoparticles whereas, broad peaks indicate smaller size nanoparticles. The average particle size of the sample was measured by the width of the most prominent XRD peak (311) by using the Debye–Scherrer’s equation [21]. The calculated particle size is 3.62 nm and no other peaks relating to other impurities were observed, indicating the single crystalline phase of γ-Fe2O3 NPs.

Fig. 2
figure 2

XRD diffractograms of the iron − FCF extract complex γ-Fe2O3 NPs

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FTIR and TGA Analysis

As a powerful tool, FTIR spectra of the synthesized γ-Fe2O3 NPs were recorded by scanning the sample in the range of 400 to 4000 cm−1 to analyze the possible biomolecules responsible for the biosynthesis. Figure 3 indicated the surface chemical structure of γ-Fe2O3 NPs. The main characteristic peaks of nanoparticles were observed at 441, 477, and 565 cm−1 corresponds to Fe–O band vibration (400–700 cm−1), confirming the formation of γ-Fe2O3 [39, 41]. The other peaks at 1096, 1355, 1410, 1644, 2793, 2882, 2948 and 3412 cm−1 occurred due to the functional groups present in FCF extract. The FTIR band at 1096 cm−1 corresponds to C–O–C stretching vibration of alcohol and phenolic groups [21]. The C–O and C–H vibration show a weak band around 1355 and 1410 cm−1. The medium vibrational peak at 1644 cm−1 denote the O–H bending and C=O vibration of carbonyl groups [42]. The peaks at 2882 and 2948 cm−1 have been attributed to symmetric and unsymmetric C–H stretches in aliphatic compounds [43]. As prepared γ-Fe2O3 NPs also exhibit a broad peak near 3412 cm−1, which indicates the presence of hydrogen bonds involved in O–H and adsorbed water molecules [42]. Overall, it is seen that there is a presence of carbonyl, alcohols and phenolic group stuck on the γ-Fe2O3 surface even though the FCF extract are washed repeatedly and cannot be removed during the drying process.

Fig. 3
figure 3

FTIR spectrum of as-synthesized γ-Fe2O3 NPs

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The TG analysis of as-synthesized γ-Fe2O3 NPs supports the FTIR results. The TGA curve in Fig. 4a indicated a substantial loss in the weight of FCF-γ-Fe2O3 NPs between 27 and 500 °C. The decomposition of the FCF organic layer was completed at 400 °C. An initial reduction of weight at 27–137.5 °C was observed due to the evaporation of physisorbed/chemisorbed water and other volatile contents present in FCF γ-Fe2O3 NPs [44]. A weight loss of 4.18% was noticed from 100 to 300 °C which might be due to the decomposition of biomolecules surrounding the FCF γ-Fe2O3 NPs. Additional information was obtained from the first derivative curve analysis that indicated the temperature (Tmax) at which the degradation rate reached its maximum. The endothermic peak of γ-Fe2O3 NPs were found to have a Tmax of 278.5 °C, which clearly indicates that as prepared γ-Fe2O3 NPs contain 92% metal and 4.1% phytochemical coating [20]. From all these results, it is attributable to organic molecule of FCF extract layering on the nanoparticle surface. Figure 4b shows the magnetic separation of as-synthesized γ-Fe2O3 NPs using an external magnetic field.

Fig. 4
figure 4

a TG–DTA curve for the FCF extract capped γ-Fe2O3 NPs and b magnetic separation of as-synthesized γ-Fe2O3 NPs using an external magnetic field

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TEM, SAED and DLS Study

Morphology and hydrodynamic diameter of nanoparticles, which were dispersed in water were studied using TEM and DLS. TEM and SAED images of as-synthesized γ-Fe2O3 NPs are shown in Fig. 5. According to TEM images (Fig. 5a), fabricated nanoparticles are spherical/dot in shape and size in the range of 4–6 nm. Most of the nanoparticles are embedded in organic matrix, stuck each other and become agglomerated. It is due to interparticle interaction and their size increased in the range of 10 to 50 nm. Figure 5b showed the presence of an organic layer coverage around the γ-Fe2O3 NPs and a lower contrast of particles is more embedded in the organic matrix [45]. Figure 5c indicate the SAED analysis. The spotty fring’s of micrograph denote the nanocrystalline structure and well pronounced circular diffraction ring confirms the spherical shape of as-prepared γ-Fe2O3 NPs [24]. Furthermore, the strong spotty ring has been indexed as (311) and clearly indicate the enhanced crystallinity of the nanoparticles in comparison with the other (hkl) planes in the synthesized γ-Fe2O3 NPs, which is evident in the XRD pattern based on their intensity [24]. The DLS method is a measurement based on the Brownian motion of particles and used to determine the size of nanoparticles in colloidal suspension at nanosize scale. In this method, light is sent to the particles and intensity fluctuations in the scattered light are measured [28]. The average particle size of well sonicated and diluted γ-Fe2O3 NPs are 6.7 ± 1.7 nm with narrow distribution (Fig. 6). The calculated polydispersity index (PDI) value is 0.064 (< 0.1), therefore its particle distribution will be monodispersive. The size calculated from TEM and DLS gives higher values than those observed by XRD. Finally, all characterization data were well supported the nature and morphology γ-Fe2O3 NPs along with the slight difference between the size observed in XRD, TEM and DLS is likely due to the amorphous organic layer and adsorbed species which surround the particles.

Fig. 5
figure 5

TEM images (a, b) and SAED pattern (c, b1) of biosynthesized γ-Fe2O3 NPs

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Fig. 6
figure 6

DLS pattern for ultrasonicated-diluted γ-Fe2O3 NPs

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Suggested Mechanism for the Biosynthesis of γ-Fe 2 O 3 NPs

The FCF extract is rich in high levels of flavonoids, coumarins, chlorogenic acid, organic acids and anthocyanin [29,30,31]. For the one-pot biosynthesis of γ-Fe2O3 NPs via co-precipitation, Fe(OH)2 is starting material and it was prepared by adding 0.1 M NaOH solution to FeSO4 solution and pH is adjusted to 9–10. We believe that Fe(OH)2 oxidized and precipitated as Fe(OH)3 in the presence of oxygen. It was observed that, when FCFE is added to Fe(OH)3 solution, they form an instantaneous brownish-black complex because of the electrostatic interaction between the negatively charged phytochemicals (RO−, ArO−, COO−) and positively charged Fe3+ ions. Upon continuous heating at 78–80 °C with continous stirring for 2 h and drying at 90 °C for 12 h, the self-assembled complex of γ-Fe2O3 NPs formed.

The possible mechanism for the synthesis of γ-Fe2O3 is proposed in Eqs. (2)–(6) [24, 36, 37, 46]:

$$ {\text{Fe}}^{2+} + 2{\text{H}}_{2} {\text{O}}({\text{pH}} > 9) \to {\text{Fe}}({\text{OH}})_{2}{\downarrow} + 2{\text{H}}^{ + } $$
(2)
$$ {\text{Fe}}({\text{OH}})_{2} + [{\text{O}}] \to {\text{Fe}}({\text{OH}})_{3}{\downarrow}$$
(3)
$$ {\text{Fe}}({\text{OH}})_{3}{\downarrow} + {\text{polyphenols}}/{\text{phytochemicals}} \to {\text{Fe}}{-}{\text{phytochemicals}}\,{\text{complex}} $$
(4)
$$ {\text{Fe}}{-}{\text{phytochemicals complex}} + {\text{Heat}}(78{-}80\,^\circ {\text{C}}) \to {\text{FeO}} + {\text{FeOOH}},\alpha - {\text{Fe}}_{2} {\text{O}}_{3} {\text{NPs}}{\downarrow} + {\text{Fe}}_{3} {\text{O}}_{4} {\text{NPs}}{\downarrow} $$
(5)
$$ {\text{Fe}}_{3} {\text{O}}_{4} + {\text{Heat}}( > 90\,^\circ {\text{C}}) \to \gamma{\text{-Fe}}_{2} {\text{O}}_{3} {\text{NPs}} \downarrow $$
(6)

Free Radical Scavenging Activity

Antioxidant applicability of as-synthesized γ-Fe2O3 NPs was demonstrated using DPPH as model reagent. Antioxidant activity by as-synthesized γ-Fe2O3 NPs is shown in Fig. 7. The result clearly depicted that scavenging activity of the nanoparticles was increased in a dose-dependent manner on DPPH radical with the significant inhibitory rate of 1.18, 2.79, 11.48, 14.54 and 17.01% when the quantity of nanoparticles increases from 20, 40, 60, 80, and 100 µg, respectively. The antioxidant activity of nanoparticles can be attributed to their outer organic layer of nanoparticles, composed of various phyto-constituents of FCF extract which enable them to trap reactive oxygen species (hydroxyl radicals, superoxide anions, singlet oxygen, hydrogen peroxide, etc.) on its spherical surface [36, 37, 47]. This result shows the substantial importance of FCF extract coated-γ-Fe2O3NPs as antioxidant and favors the development of various iron oxide-based nanoparticles in therapeutic products.

Fig. 7
figure 7

The DPPH scavenging activity of as-synthesized γ-Fe2O3 NPs

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Catalytic Activity of γ-Fe2O3 NPs

Metal nanoparticles exhibit excellent catalytic properties owing to the high rate of surface adsorption and high surface area to volume ratio [38, 48]. This hypothesis were subjected to testing the catalytic efficacy of γ-Fe2O3 NPs, for the reduction of 4-NP to 4-AP as model reaction with NaBH4 at λmax = 400 nm. The kinetics of the reaction were spectrophotometrically studied at 20 µg and 100 µg of biogenic γ-Fe2O3NPs and control of the reaction were performed without taking nanoparticle in the reaction mixture, is presented in Fig. 8a. It shows that after the induction time = 30 min, there is a good linear relation between ln(A0/At) versus time (t) and followed the first-order kinetics. The rates constant were directly calculated from the slope of these plots. The first-order rate constant (k) is highest for FCFE-γ-Fe2O3NPs 2 (k2 = 49.975 × 10–3 min−1, R2 = 0.995) containing 100 µg of γ-Fe2O3 NPs whereas, k for FCFE-γ-Fe2O3 NPs 1 was decreased more than 6 times with a decreasing quantity of γ-Fe2O3 NPs to 20 µg. The observed k1 and correlation coefficients (R2) for FCFE-γ-Fe2O3NPs 1 were 7.4359 × 10–3 min−1 and 0.995. Similarly, kcontrol and R2 for control reaction were 0.9444 × 10–3 min−1 and 0.918. Such diminishing catalytic rates with decrease in nanoparticle quantity can be attributed to a decrease in number of reaction sites, surface area of particles and retards the diffusion of 4-NP to the nanoparticle surface [49]. To understand the mechanism of reduction by 20 and 100 µg of γ-Fe2O3 NPs, we plotted UV–vis spectrum from 220–550 nm, shown in Fig. 8 (b, c). Addition of alkali/OH− to the reaction medium causes deprotonation of 4-NP resulting in the formation of intermediate nitrphenolate ions. The 4-NP shows an absorbance peak at 317 nm, which red shifts to 400 nm due to the formation of 4-nitrophenolate ion. Subsequently added NaBH4 reduces nitrophenolate ion to form 4-AP. In addition of γ-Fe2O3 NPs there is a rapid decrease in the intensity of the absorption peak at 400 nm while there is a concomitant appearance of a new peak at 298 nm (Fig. 8b, c) indicating the formation of reduction product, 4-AP. The decrease in absorbance of 4-NP during the initial minutes of reaction was very slow when the amount of γ-Fe2O3NPs were 20 µg but its reaction becomes faster when 100 µg γ-Fe2O3NPs was added. Strong nucleophile, BH4− acts as electron donor in all reactions. The occurrence of reaction was easily observed with the naked eye owing to the fading of 4-NP yellow colour along with the progress of the reaction [50]. A comparative chart representing the catalytic efficiency of different nanoparticles for the reduction of 4-NP in the presence of NaBH4 as well as present study are summarized in Table 1.

Fig. 8
figure 8

a Plots of ln(A0/At) for different concentrations of γ-Fe2O3 NPs toward the catalytic reduction of 4-NP by NaBH4; b, c time-dependent UV–Vis spectra of the reduction of 4-NP in the presence of 20 µg and 100 µg γ-Fe2O3 nanoparticles

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Table 1 A comparative chart representing the catalytic efficiency of different nanoparticles for the reduction of 4-NP in the presence of NaBH4 as well as present study
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The probable mechanistic pathway for the hydrogenation of 4-NP explained as mention in Fig. 9. Though the reduction of 4-NP to 4-AP using aqueous NaBH4 is thermodynamically favorable (E0 for 4-NP/ 4-AP = − 0.76 V and H3BO3/BH4− = − 1.33 V versus NHE), but the presence of the kinetic barrier due to the large potential difference between donor and acceptor molecules decreases the feasibility of this reaction [51]. To overcome the kinetic energy barrier, BH4− ions and 4-nitrophenolate ions co-adsorb on γ-Fe2O3 NPs surface. During the catalytic reaction, γ-Fe2O3 NPs relay electrons from donor BH4− ions by forming Fe–hydride complex, to the acceptor 4-nitrophenolate. Eventually, 4-AP forms and desorbs from the catalyst. The aqueous medium provides the required amount of H+ ion for complete reduction of 4-NP to 4-AP [52].

Fig. 9
figure 9

Suggested mechanism for the γ-Fe2O3 NPs mediated catalytic reduction of 4-NP using NaBH4

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Conclusions

In conclusion, we have successfully developed a simple, economical and environment-friendly route to prepare ultrafine γ-Fe2O3NPs using an extract of Ficus carica fruits. As-prepared γ-Fe2O3 NPs were extensively characterized through various spectroscopic and microscopic techniques to interpret its purity, composition, nature and morphology. XRD data showed the presence of pure and single γ-phase of Fe2O3 NPs. FTIR results ascertained the specific bands for Fe–O and TGA confirmed the presence of organic coating around the surface of γ-Fe2O3 NPs. The resulting nanoparticles are spherical and have average sizes ≈ 4–6 nm confirmed by TEM and DLS. Furthermore, γ-Fe2O3NPs showed considerable antioxidant activity against DPPH and catalytic activity for the reduction of 4-Nitrophenol. Such desired advantages make these nanoparticles suitable for bioengineering purposes and catalysis.