Antioxidant and photocatalytic properties of zinc oxide nanoparticles phyto - fabricated using the aqueous leaf extract of Sida acuta

: Nanoparticles have gained considerable atten tion during the present millennium due to its unique properties and usage of same in all the scienti ﬁ c ﬁ elds. The present study was aimed to phyto - fabricate zinc oxide nanoparticles ( ZnO NPs ) from Sida acuta and eval uate its antioxidant and photocatalytic activity against the dye victoria blue ( VB ) . The phyto - fabricated ZnO NPs when subjected for physico - chemical characterization showed an absorbance peak at 373nm and was spherical in nature. Strong and well - distinguished sharp peaks were noticed in X - ray di ﬀ raction analysis with an average size of ∼ 32.82 nm calculated through Scherrer ’ s formula. The size was also authenticated through dynamic light scattering analysis and transmission electron microscopy. The Fourier trans form - infrared spectroscopy con ﬁ rmed that the phyto - con stituents of the plant extract served as capping/stabilizing agents during the synthesis of ZnO NPs. The atomic force microscopy studies on morphology and geometrics of the synthesized particles indicated that particles were mono dispersed with colour di ﬀ erence. In addition, the surface area of ZnO NPs measured by Braunauer – Emmett – Teller experimental studies for adsorption isotherms was found to be 7.364 m 2 · g − 1 . The antioxidant e ﬃ cacy of the phyto fabricated ZnO NPs o ﬀ ered concentration - dependent anti oxidant activity with an IC 50 value of 0.74 mg · mL − 1 . Further, the VB ( 9 mM ) dye degradation studies using the phyto fabricated ZnO NPs ( 0.75 g · L − 1 ) resulted in dye degradation of 93% at 40 min in natural sunlight. Further, the reuse and recycling of the photocatalyst for dye degradation o ﬀ ered 70.25% dye degradation ability within 40 min exposure to sunlight at the ﬁ fth cycle of reusability thereby indicating e ﬀ ective dye degradation ability of the phyto - fabricated ZnO NPs from the aqueous leaf extract of S. acuta .


Introduction
Zinc oxide nanoparticles (ZnO NPs) are one of the most commonly used metal NPs and have been shown to have excellent and important biological properties, including antimicrobial, antioxidant, antidiabetic, anticancer, antiinflammatory, and photocatalytic as well as drug delivery system [1][2][3][4]. The plant-mediated synthesis of NPs has many advantages over conventional physico-chemical approaches and wide range of biological applications [5][6][7]. Plants have a wide range of genetic variations in biomolecules and metabolites such as proteins, carbohydrates, vitamins, phenols, flavonoids, sterols, alkaloids, and intermediates [8,9]. These plant metabolites have several functional groups such as hydroxyl (OH), carbonyl (C]O), and amine (NH 2 ) that react with metal ions to reduce their size to the nanoscale. These molecules aid in the reduction of metal ions into NPs and act as capping for NPs, which is critical for their stability and biocompatibility [4].
The higher toxicity of plant-mediated NPs such as copper (Cu), gold (Au), silver (Ag), and other metal oxides to animals and humans is a significant restriction for their usage in the medical fields [10]. However, ZnO is currently affirmed as safe, which outperforms many other metal oxides in terms of biocompatibility and selectivity, and has a wide range of applications in the antimicrobial, antiviral, biomedical, and environmental fields [1]. Because of their low toxicity and size-dependent properties, ZnO NPs have a greater potential for treating infectious diseases in humans and animals [2]. In the literature, it has been reported that the synthesis of ZnO NPs has been performed using different parts of plant extract, including leaves, stem, bark, flower, fruits, seeds, etc. [4]. Recently various reports have also demonstrated several biological activities of ZnO NPs [3,4] and suggested that the medical industry could benefit greatly from plant-mediated ZnO-NP synthesis.
ZnO is one of the best likely used materials for performing photocatalytic task, as a substitute to the extensively used, comparatively expensive titanium dioxide (TiO 2 ). Even though researchers recognized comparable photocatalytic mechanisms with both ZnO and TiO 2 , they showed that ZnO was found to be a better photocatalyst in degrading the herbicide triclopyr, pesticide carbetamide, pulp mill bleaching wastewater, and phenol, 2-phenylphenol [11]. The capacity of some NPs to speed up a certain reaction in combination with light is generally employed for photocatalysis and this effect is used for self-cleaning surfaces. Photocatalytic activity has been broadly established as one of the most advanced oxidation processes for pollutant degradation. Many oxide semiconductors, like ZnO, SnO 2 , have been proven as proficient photocatalysts for degradation of dyes and organic pollutants [2,12,13].
Sida acuta Burm. f., is a weed plant that belongs to the family Malvaceae with numerous medicinal potencies. Because of the wide range of phytochemicals, the plant exhibits a variety of medicinal properties, including antimicrobial, antioxidant, cytotoxic, and other properties [14].
Traditionally, the plant is often believed to treat diseases such as fever, headache, and infectious diseases. It is distinct from other species of the genus due to its acute leaf apex shape and different pair of each stipule. Recently, our research team has phyto-fabricated ZnO NPs from leaf extract of Sida rhombifolia Linn. to explore their antibacterial, antioxidant, and genotoxic properties [15]. With this knowledge, the phyto-fabrication of ZnO NPs was also prepared from leaf extract of S. acuta to study their antioxidant and photocatalytic properties.

Plant collections and phyto-fabrication of ZnO NPs
The healthy leaves of S. acuta were collected from Mysore, Karnataka, India. The fresh collected leaf material was cleansed to get rid of dust and debris with running water followed by sterile double distilled water (SDW) and blot dried. In 100 mL of SDW, 10 g of leaf material was grounded using a blender and agitated for 2 h using a rotary shaker at 100 rpm for uniform mixture. Later the solution was filtered using Whatman No. 1 filter paper which served as a source of plant extract. For the NP synthesis of ZnO, zinc nitrate hexahydrate -2 g was introduced to 20 mL of the above extract and boiled at around 60-80°C, which results in nanopaste. Further the nanopaste was placed in a ceramic crucible and calcinated inside a furnace for 2 h at 300°C. The final product was ZnO NPs in powder form and used for further experimentation [15].

Characterization of phyto-fabricated ZnO NPs
The UV-visible (UV-Vis) analysis of the phyto-fabricated ZnO NPs was carried out on spectrophotometer (DU730, Beckman Coulter, Krefeld, Germany). X-ray diffraction (XRD) wide-angle patterns were assessed by RIGAKU, Ultima-III Series, Japan. Fourier-transformed infrared (FT-IR) spectra were attained by Thermo Scientific Nicolet, 6700 Analytical FT-IR Spectrometers (FLS-1000). The morphology of the phyto-fabricated ZnO was studied by FE-SEM, HITACHI (Noran System 7, USA) along with detection of metals through energy dispersive X-ray (EDX) analysis (Zeiss Supra 55VP, Japan). To determine the size of the ZnO NPs high-resolution-transmission electron microscopy (HR-TEM) examination was performed using Jeol/JEM-2100 (Japan) instrument. Particle size analyser (Microtrac, USA) was designated to evaluate the phyto-fabricated ZnO for particle size distribution (PDS). The synthesized particles were also subjected to atomic force microscopy (AFM 100, Italy) for the surface morphology and particle size variation. The Braunauer-Emmett-Teller (BET) aids in the efficient measuring of the specific surface area, pore size, pore volume, and their distribution using BEL:2 SORP, Italy.

Antioxidant activity of phyto-fabricated ZnO NPs
The radical scavenging activity (RSA) of phyto-fabricated ZnO NPs was performed by 2,2-diphenyl-1-picrylhydrazyl (DPPH) method [16]. In brief, to 3.5 mL of 0.1 mM DPPH containing different concentrations of NPs (0-1 mg·mL −1 ) were subjected to sonication before incubation at 37 ± 2°C for 30 min under dark conditions. The absorbance of the incubated samples was read at 517 nm in a spectrophotometer and percent RSA was determined: Radical scavenging activity % Absorbance of control -Absorbance of test sample Absorbance of control 100 (1)

Photocatalytic activity
The photocatalytic degradation of victoria blue (VB) dye using phyto-fabricated ZnO NPs was evaluated under solar irradiation of 430-770 THz. Approximately, 0.75 g·L −1 of catalyst was allowed to react with VB of 9 mM concentration initially in a flask. The experiment was conducted upon various pH concentrations from 2 to 7. The flasks were placed upon the continuous magnetic stirrer under the solar radiation [17]. The level of photocatalyst dye degradation efficiency was evaluated via characteristic absorption peak of 550 nm [18]. A pseudo-first-order kinetic model was used to describe the relationship of adsorption and photo-degradation between solid-liquid systems. The experimental values were fitted with the rate equation: where C t is the absorbance of VB at t time, C 0 is the absorbance of VB after dark adsorption, t is the irradiation time, and k(p) is the pragmatic kinetic constant. The pseudo-first-order rate constant, kp (min −1 ), was calculated from the slope of ln(C t /C 0 ) versus irradiation time (t).

Statistical analysis
Every attempt of analysis was made in triplicates for accurate results using SPSS-Inc. 16.0 (ANOVA). Trivial effects of treatments were resolved by F values (p ≤ 0.05).
3 Results and discussion

Characterization of phyto-fabricated ZnO NPs
The UV-Vis spectral analysis of the phyto-fabricated ZnO NPs revealed a peak at 373 nm (Figure 1), which is in line with the results of the previous studies wherein a peak was observed between 315 and 400 nm that is characteristic of Zn [5,8].  Figure 2). The size of the ZnO NPs calculated using Scherrer's formula showed the average particle size of ∼32.82 nm. The strong and well-distinguished sharp peaks depicted particle morphology with well-outlined crystallinity [19,20]. The FT-IR spectrum of the phyto-fabricated ZnO NPs and plant extract was recorded in the 400-4,000 cm −1 range ( Figure 3 and Table A1 in Appendix). The spectral analysis helps in identifying the functional groups of the plant extract that are involved during reduction and stabilization of plant extracts during phyto-fabrication [8]. From the study it was noted that, absence of peaks around 3,250 cm −1 and the presence of same in plant extract indicated the particles were free from moisture. In addition, peak noticed at the 544 cm −1 in the phyto-fabricated ZnO NPs corresponds to Zn-O bond (metal oxide) as indicated by Yuvakkumar et al. [21] and Bhuyan et al. [22]. From the results of the FT-IR, it is noted that during the formation of ZnO NPs, zinc ions (Zn 2+ ) will cap with the available phyto-constituents of the plant extract to form a complex compound which undergoes direct decomposition at 300°C in static air atmosphere finally leading in the formation of ZnO NPs (Figure 4) which is in accordance with the previous studies [23,24]. In addition it has been reported that during the synthesis of the ZnO NPs the phyto-constituents (primary, secondary, or tertiary metabolites) act as capping, reducing, and stabilizing agents [25][26][27][28]. The FE-SEM results of the NPs showed that the particles were agglomerated ( Figure 5). Similarly, the quantitative elemental analysis using EDX analysis revealed the presence of zinc (53.88%),    oxygen (32.30%), and traces of carbon ( Figure 6). The TEM analysis of the phyto-fabricated ZnO NPs confirmed the nanorange in size (Figure 7a), while the selected area electron diffraction (SAED) is shown in Figure 7b, which confirmed the polycrystallinity nature of the particles. In addition, the XRD peaks (Miller indices) matched well with the SAED pattern of the particles and the estimated d-spacing value of the same was found to be 0.32 nm which confirmed clear separation of the inter-planar spacing (Figure 7c). Further, the dynamic light scattering (DLS) histogram obtained from the PDS demonstrated that the particles were of the average diameter of ∼44.5 nm (Figure 8) which was in agreement with the findings of Rouhi et al. [29].
The high-quality images obtained for the particles with AFM are vital for correcting ample background and slope [30,31]. The morphology and geometrics of the phyto-fabricated ZnO NPs were found to be monodispersed. Along with these findings, colour difference due to morphology and particle size variation was also presented by AFM analysis (Figure 9). It has been well documented that the inert nature via high surface area becomes a vital parameter in defining biological adaptability. A larger surface area draws the biological matter rapidly and provides biological systems a well-developed transport lanes and wider surface-active sites for reactant molecules and products. The BET experimental studies for adsorption isotherms were executed at 77.3 K with relative pressure up to P/P 0 ∼0.911. According to the hysteresis loop in the  relative pressure region of around 0.4-0.9, the nitrogen adsorption/desorption isotherms for the particles are depicted in Figure 8. The observed results demonstrate a type-IV isotherm curve with H-3 hysteresis loop characteristics of mesoporous structure, according to IUPAC classification [21,22]. The surface area of ZnO measured by BET was found to be 7.364 m 2 ·g −1 which was obtained through BET constant (C) (96.53) thereby indicating that the dye molecules may enter easily into the inner space of NPs and then disperse helping in the photocatalytic properties. Furthermore, the Barrette-Joyner-Halenda method was used to measure the pore size and pore volume distribution in addition to the idea of surface area and its mean pore size diameter is about 5.26 nm (Figure 10) [32].

Antioxidant activity
The radical scavenging properties of the phyto-fabricated ZnO NPs evaluated through the DPPH method showed an increased RSA with an increase in the NPs' concentration. The NPs' half-maximal inhibitory concentration (IC 50 ) was found to be 0.74 mg·mL −1 (Figure 11). It was also noted that the aqueous plant extract did not offer any antioxidant activity at the above-said concentrations (Table A2), while the positive control ascorbic acid offered more than 75% inhibition at 50 µg·mL −1 . The antioxidant potentiality of the phyto-fabricated NPs may be attributed to the capping of phytoconstituents present in the plant extract during the synthesis process [11,33]. In addition, the results are in corroboration with the findings of many studies wherein the plant-mediated synthesis of ZnO NPs showed better antioxidant properties when evaluated through the DPPH method compared to their respective plant extracts [16] and are also known to be more effective than the chemically synthesized NPs [34].

Photocatalytic activity
The photocatalytic dye degradation efficiency was reflected through the experiments conducted by varying the concentrations of dye and catalysts [32]. Under visible light photocatalysis, the ZnO was relatively stable, whereas the VB dye deteriorated swiftly within 40 min. At 40 min in natural sunlight, the phyto-fabricated ZnO degraded VB up to 93% (0.75 g·L −1 ) according to this study. It was noted that with the increase in the concentration of the NPs the VB dye degradation ability increased (Figure 12a). In accordance, biosynthesized ZnO NPs have been found to be effective in degrading the dyes more efficiently [35,36]. The photo-   degradation reactions for VB exhibit pseudo-first-order kinetics and kinetics was well explained using the model [37]. The photocatalytic removal of VB follows the pragmatic rate constant for ZnO of 0.425 × 10 −2 s −1 (Figure 12b). The degradation of VB by the phyto-fabricated ZnO NPs exhibited relatively fast kinetics with a rate constant of k = 0.425 min −1 .

Recyclability of phyto-fabricated ZnO
In order to reduce the stress upon the raw material of phyto-fabricated ZnO photocatalyst, reuse and recycling of the photocatalyst were attempted. The nanocatalyst recovered after dye degradation was washed using deionized water and ethanol, followed by drying and reused for the next cycle. A total of five trials of dye degradation were experimented using the same photocatalyst from the first to the fifth cycle, the percentage VB dye degradation efficiency using phyto-fabricated ZnO NPs was found to be 81.36%, 77.98%, 75.99%, 73.01%, and 70.25%, respectively, within 40 min with sunlight influence ( Figure 13). The results of the study indicated remarkable photostability. As compared to the chemically synthesized nanomaterials, phyto-encapsulated ZnO NPs have the equivalent recyclable efficiency [38,39] and also the synthesized NPs being a plant extract makes it ecological and environmental friendly material. Due to the efficient recyclable property of the photocatalyst there will be fewer requirements for the production of new photocatalyst, and it is cost effective. Also the photocatalyst can be reused even for more than five cycles only with lesser degradation percentage. These give the photocatalyst the positive remarks.
In addition, Table 1 shows the importance of phytofabricated ZnO NPs and their antioxidant and photocatalytic applicability. The table represents the plant and its part used, size, morphology, and application of ZnO NPs and the results of the present study are also compared.

Conclusions
The present study is the first report on the phyto-fabrication of ZnO NPs from S. acuta. The phyto-fabricated NPs were of ∼32.82 nm calculated through Scherrer's formula. The UV-Vis spectra and FT-IR analysis of the particles showed a characteristic peaks at 373 nm and 544.12 cm −1 , which are correlated to Zn and metal oxide bond, respectively. The surface area of ZnO measured by BET was found to be 7.364 m 2 ·g −1 . The antioxidant studies revealed the phyto-fabricated particles possessed RSA which was concentration dependent with an IC 50 value of 0.74 mg·mL −1 . The dye degradation studies of the particles against the VB showed 93% degradation of the dye after 40 min exposure to natural sunlight. In addition, the reuse and recycling of the photocatalyst for dye degradation offered 70.25% degradation ability at the fifth cycle of reuse thereby signifying the dye degradation ability of the phyto-fabricated ZnO NPs.
Acknowledgment: The authors thank the Department of Studies in Environmental Science, Botany, and Biotechnology, University of Mysore, India for providing the necessary facility to carry out the research work.
Funding information: Authors state no funding involved.   Values are the means of four independent replicates. ± indicate standard errors. Means followed by the same letter(s) within the same column are not significantly (p ≤ 0.05) different according to Tukey's HSD.