Zinc oxide Nanoparticles, Biosynthesis, characterization and their potent photocatalytic degradation, and antioxidant activities

2.1 Preparation of seed extract and green-synthesis of ZnO‑NPs

The seeds of Moringa oleifera were removed from the pods and crushed using a mechanical blender. A total of 10 g of seeds were powdered using a pestle and mortar and stirred with 100 mL double distilled water. It was placed on a heating mantle and stirred manually for 25 min at 80 ± 2 °C. The final aqueous extract was cooled and it was filtered (Whatman filter No: 1). The filtrate was collected and used as a reducing and capping agent for the green synthesis of ZnO‑NPs. A total of 5 gm of zinc nitrate hexahydrate [Zn (NO₃)₂·6H₂O] (Sigma-Aldrich, USA) was mixed with aqueous extract for 2 hrs and the development of brownish-yellow was monitored. The sample was maintained on a heating mantle at 70 ± 2 °C for 6 hrs to produce white ZnO-NPs. The mixture was centrifuged for 15 min at 8000 rpm and the sediment sample was retained. It was calcined for 2 hrs at 400 ± 2 °C and a white crystalline powder was obtained. It was stored in an amber color bottle for further analysis (Azizi et al., 2014).

2.2 Characterization

UV–visible spectrophotometer analysis was performed to analyze the absorption spectra range of the sample at 200–800 nm (Victoria, Australia). X-ray diffraction analysis (XRD) was used to determine the particle distribution, particle size and shape heterogeneity of ZnO-NPs. The white crystalline ZnO-NPs were used for characterization studies. The powder sample tested using X’pert PRO PAN alytical diffract meter. FT-IR analysis was performed to analyze the functional group of ZnO-NPs range from 4000 to 400 cm⁻¹. Transmission electron microscopy (TEM) analysis was used to study the morphology of ZnO-NPs (JEOL model JEM-1010, Tokyo, Japan). X-ray spectroscopy (EDX) analysis was performed to determine elements using Shimadzu DX-700HS machine. The zeta potential was calculated using the Zetasizer nano array, HT Laser, ZEN 3600 (Malvern Instruments, UK) with a dispersion angle and a temperature of 90 °C and 25 °C, respectively, using samples diluted with de-ionized distilled water at different concentrations.

2.3 Catalytic activity

In our study, photocatalytic activity of ZnO-NPs was analyzed against an azo dye, methylene blue. Experiment was performed under UV light (k360 nm). The initial methylene blue concentration was 50 ppm and 0.02 % ZnO-NPs was incorporated in a 100 mL beaker, under UV irradiation at 35 ± 1 °C. It was stirred continuously to achieve homogeneity of the sample. The absorbance of the sample was measured at 660 nm using a UV–vis spectrophotometer.

2.4 Antioxidant activity

Hydrogen peroxide (H₂O₂) free radical scavenging activity of the nanoparticles was performed as described previously (Pick and Mizel, 1981). The green synthesized ZnO-NPs was tested at various concentrations and the final reaction volume was 1 mL. To the ZnO-NPs, 0.6 mL (50 mM) H₂O₂ stock solution was added and incubated for 5 min at 28 ± 2 °C. Phosphate-buffered saline alone was considered as the blank. After 5 min incubation, hydrogen peroxide scavenging power (%) was determined using the following formula: $$\%{\text{H}}_2{\text{O}}_2\text{free}\text{radicals}=1-\text{As}/\text{Ac}\times 100.$$

where Ac = Control absorbance; As = test absorbance.

3.1 UV–visible spectrophotometer analysis

In Fig. 1a, pure ZnO-NPs as a white powder is shown, which reveals the development of nanoparticles in an early stage. The green synthesized nanoparticles were white crystalline nature and used for the determination of absorption spectra. In our study, a major peak was detected at 375 nm, which indicated the presence of ZnO-NPs (Fig. 1b).

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Fig. 1

A) pure ZnO-NPs as a white powder, b) UV–visible spectrum of ZnO-NPs synthesized using seed extract of M. oleifera.

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3.2 XRD analysis

The green synthesized ZnO-NPs were subjected for XRD analysis to determine crystalline structure. The XRD pattern of ZnO-NPs was described in Fig. 2. The number of Bragg reflections for ZnO-NPs using aqueous extract of M. oleifera matter appears at 2θ = 31.80°, 34.45°, 36.28°, 47.59°, 56.65°, 62.94°, 66.46, 68.00°, 69.09, 72.57 and 77.0648° which correspond to the refection from (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), to (0 0 4) respectively. The present finding showed good agreement with previous results revealed hexagonal wurtzite structure of ZnO-NPs (Swee-Yong et al., 2012).

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Fig. 2

XRD pattern of ZnO-NPs synthesized using aqueous seed extract of M. oleifera.

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3.3 TEM image

The shape and size of the green synthesized ZnO-NPs were analyzed using TEM analysis and the result was described in Fig. 3. Fig. 3 clearly shows the spherical structure of NPs and the size ranging between 25 and 30 nm. Most of the analyzed particles were almost uniform in size. The mean value of the green synthesized ZnO-NPs was 28 nm. The present finding revealed the potential of seed extract as reducing and capping agent. The spherical and hexagonal structures of NPs were in accordance with previous results reported by Liu et al., (2017) and Malviya (2011).

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Fig. 3

TEM of ZnO-NPs.

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3.4 EDX analysis

EDX analysis was used for the determination of elemental composition of biosynthesized ZnO-NPs and the result was depicted in Fig. 4. In our study, the presence of oxygen and zinc components was determined in EDX spectrum. The ZnO-NPs element characterization revealed 86.79 % zinc, 10.48 % oxygen and 2.73 carbon 2.73 for weight %67.27 % zinc, 25.81 % oxygen and 6.92 % carbon.

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Fig. 4

EDX analysis of ZnO-NPs.

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3.5 Zeta-potential

Zeta-potential analysis was performed to determine the stability and surface charge of the particles of interest. The zeta potential of ZnO-NPs revealed the presence of a major peak at −38 mV revealed good stability of the particle (Fig. 5). The present finding clearly revealed that the capping agent from the plant source is circled with ZnO-NPs. Naturally, the selected plant seed contains rich of flavonoids and proteins and these phytochemical components involved in the reduction of metal ions and good stabilization of ZnO-NPs. Our findings were highly consistent with (Kaur et al., 2022).

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Fig. 5

Zeta potential of ZnO-NPs.

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3.6 FTIR analysis

FTIR analysis was carried out for the ZnO-NPs detection of biomolecules responsible for biosynthesized ZnO-NPs. FTIR analysis is useful to determine the available functional group from the phytochemical components implicated in the stabilization and reduction of ZnO-NPs. The comparative FTIR spectrum of M. oleifera seed extract and green synthesized NPs revealed the functional group of seed extract (Fig. 6). M. oleifera seed extract showed various peaks at 3420, 2923, 2852, 1800, 1740, 1715, 1685, and 1492 cm⁻¹.The absorption band at 3420 cm⁻¹ (strong O—H stretching) established fatty acids, protein, and carbohydrate molecules. At this region N—H stretching was observed because of increased protein content from the seed (Stuart et al., 2004). Two peaks detected at 2852 cm⁻¹, 2923 cm⁻¹ and this wave number indicated symmetric and asymmetric stretching related to C—H bonding. The intensity of these bonding was high, which revealed the presence of lipid similar with protein content (Krilov et al., 2009).

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Fig. 6

FTIR spectrum of a) M. oleifera seed extract, and b) biosynthesized ZnO-NPs.

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In our study various overlapping bands were detected from 1750 to 1630 cm⁻¹. This mainly due to the presence of C⚌O stretching. The carbonyl group (C⚌O) detected in the FTIR analysis related to the protein part and fatty acid content of the aqueous seed extract. The small absorption bands at 1740 and 1715 cm⁻¹ region revealed carbonyl groups and the band at 1658 cm⁻¹ revealed carbonyl amides.

The FTIR analysis of the green NPs synthesized using aqueous seed extract revealed specific changes in related peaks, indicating the presence of capping agent or bonded to the NPs surface. In the seed extract the major peaks were detected appeared at 3420, 2923, 2852, and 1800 cm⁻¹ appear with no change in ZnO-NPs, while other peaks shifted from 1740, 1715, and 1685 cm⁻¹ to 1760, 1710, and 1681 cm⁻¹ respectively, were ascribed to N—H binding. A shift of peaks was observed between these two FTIR spectrum revealed the interaction of various functional groups of the phenols and flavonoids with the green synthesized ZnO-NPs. The available functional group from the seed extract donated electrons and these electrons reduced zinc ions (Zn²⁺) to Zn⁺¹ and finally reduced as zinc NPs. The seed extract contains negative functional group and working as a stabilizing agent. The appearance of new peaks at 2212, 2230, 878, 700, 546, and 482 cm⁻¹ revealed that ZnO-NPs participated C⚌C, and C—H bending, and the peak at 482 cm⁻¹ revealed bending vibration (Selim et al., 2020). An intense and sharp band was observed at 546 cm − 1, revealing the presence of Zn-O vibrations and a peak at 878 cm⁻¹ showed the existence of the weak vibration (Stan et al., 2016). In general, the meal oxides show very strong absorption bands with the wave number < 1000 cm⁻¹ (Kaviyarasu et al., 2016, Ngom et al., 2016; Bhuyan et al., 2016). In ZnO-NPs, intrinsic absorption peak was observed at 481 cm⁻¹, which revealed the presence of stretching mode and OH group also determined at higher wave numbers (Nethravathi et al., 2015).

3.7 Dye degradation under UV radiation

Photocatalytic technology is a critical method for eliminating dyes in sewage water. In our study, azo dye degradation was observed by analyzing the color shift. After 6 h incubation of MB with green synthesized ZnO-NPs, the color of dye changed to light blue from the initial dark blue coloration after UV exposure (Fig. 7a). After 12 h of incubation, the light blue color changed into light green color. After 24 h, degradation was achieved completely and the final solution was colorless. It is very clear that the absorption peak reduced considerably after 24 hrs incubation revealed UV-mediated photocatalytic reaction (Fig. 7b). The green synthesized ZnO-NP effectively reduced the color intensity, revealed the dye degradation efficacy. The dye degradation kinetics as a function of incubation time was depicted in Fig. 7c. The present study revealed maximum photocatalytic activity (71 %) after 24 hrs incubation. Nava et al., (2017) have been biosynthesized ZnO-NPs using Camellia sinensis extract and used for the degradation of MB and achieved about 80 % degradation within two hours. Likewise, 80 % degradation was achieved within 120 min using silver nanoparticles (Al-Zaban et al., 2021). In photocatalytic reaction, under UV irradiation condition, electron shift was observed. In general, oxidizing species can oxidize various organic pollutants. The photocatalytic activity of nanoparticles and the final products was mainly based on the number of defects (Ngoepe et al., 2018; Liang et al., 2020).

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Fig. 7

Catalytic activity of ZnO-NPs synthesized using seed extract of M. oleifera under UV radiation.

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3.8 Antioxidant properties

The antioxidant properties of green synthesized ZnO-NPs were described Fig. 8. ZnO-NPs showed H₂O₂ reducing power activity and were dose dependent. The IC50 value of ZnO-NPs was 62 µg/mL, whereas the standard ascorbic acid showed decreased IC50 value (43 µg/mL). The antioxidant activity was high in ZnO-NPs than seed extract which revealed the presence of metallic structure. Medicinal plants contain various phenolic compounds and these compounds contributed antioxidant property (Arvanag et al., 2019). The green synthesized ZnO-NPs from the extracts of Ocimum basilicum, Rosmarinus officinalis and Allium sativum showed strong antioxidant activity (Parashant et al., 2015).

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Fig. 8

Antioxidant activity of green synthesized ZnO-NPs at various concentrations. Hydrogen peroxide reducing power was analyzed and compared with standard (ascorbic acid).

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