Green synthesis of SiO2 nanoparticles from Rhus coriaria L. extract: Comparison with chemically synthesized SiO2 nanoparticles.

The usage of the green synthesis method to produce nanoparticles (NPs) has received great acceptance among the scientific community in recent years. This, perhaps, is owing to its eco-friendliness and the utilization of non-toxic materials during the synthesizing process. The green synthesis approach also supplies a reducing and a capping agent, which increases the stability of the NPs through the available phytochemicals in the plant extractions. The present study describes a green synthesis method to produce nano-silica (SiO2) NPs utilizing Rhus coriaria L. extract and sodium metasilicate (Na2SiO3.5H2O) under reflux conditions. Sodium hydroxide (NaOH) is added to the mixture to control the pH of the solution. Then, the obtained NPs have been compared with the chemically synthesized SiO2 NPs. The structure, thermal, and morphological properties of the SiO2 NPs, both green synthesized and chemically synthesized, were characterized using Fourier-transform infrared spectroscopy (FTIR), Ultraviolet-Visible Spectroscopy (UV-Vis), X-ray diffraction (XRD), and Field Emission Scanning Electron Microscopy (FESEM). Also, the elemental compassion distribution was studied by energy-dispersive X-ray spectroscopy (EDX). In addition, the zeta potential, dynamic light scatter (DLS), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) was used to study the stability, thermal properties, and surface area of the SiO2 NPs. The overall results revealed that the green synthesis of SiO2 NPs outperforms chemically synthesized SiO2 NPs. This is expected since the green synthesis method provides higher stability, enhanced thermal properties, and a high surface area through the available phytochemicals in the Rhus coriaria L. extract.

1. Introduction

Researchers have recently focused on nanotechnology due to the extraordinary growth and development in this field, with the expectation that it will improve the current standard of living in society [1]. In addition, nanoparticles (NPs) have attracted the interest of researchers worldwide owing to their unique qualities, including shape, size, and distribution, which can be utilized in many applications [2]. In recent times, silica (SiO2) NPs are widely paid great attention in several applications as a result of their simplicity, stability, low toxicity, superior biocompatibility, and the ability to be functionalized with a variety of polymers and molecules, in addition to controllable particle sizes [3, 4]. The development of the SiO2 NPs in different applications, such as technology, building industry, agriculture, food industry, medical and consumer products, have a great impact on altering and improving the industries [5]. In addition, SiO2 NPs have been intensively used in the foundation of the advanced building industry, especially in concrete and cement-based materials, to increase compressive strength, the main goal of civil engineers, and thus reduce cement consumption greenhouse gas emissions [6]. Two distinct reaction processes can occur during the hydration of cement in the presence of nano-silica [7]. When nano-silica is applied to cement, H2SiO2 particles will be formed and engage with the existing Ca2+ to generate additional calcium–silicate–hydrate particles (C–S–H). Such particles are distributed in the water among the cement grains and perform as seeds to generate a much more compacted C–S–H phase [8]. The C–S–H phase production is no longer restricted to the grain surface alone but also occurs in the pore space [9]. As a result of the production of a high number of seeds, early cement hydration is accelerated [10]. shows, apart from the effective factor on the size and shape of SiO2 NPs, the uniformity of dispersion and separation of particles, i.e., dis-agglomeration. These are important features and significantly affect the above-mentioned chemical reactions.

In general, as shown in , three distinct strategies are available for the synthesis of NPs: chemical, physical, and biological [11]. The physical techniques require highly sophisticated machines with high pressure and high temperatures, some of which are complicated and costly [12]. Chemical procedures negatively impact the employment of chemical materials as reducing agents, which raises many worries regarding employees and the environment [13].

Recently, biological methods or green processes have been introduced as a novel method for synthesizing NPs vital, easy, and economical [14]. This method is a safe and environmentally friendly method for nanomaterial production, which is considered an alternative to conventional physical and chemical techniques [15]. This method has opened a new gate in front of the researchers to produce metal, metal oxide, and semiconductor NPs in a one-pot method. In fact, green synthesis methods of NPs using plant extraction, fungi, algae, and microbes have numerous advantages, especially for biomedical applications [16]. Among all green methods, plants have noteworthy quantities due to a huge number of phytochemicals such as flavonoids, polyphenols, glycosides, terpenoids, and proteins [17]. The most significant difference between green synthesis and other techniques for producing NPs is that, first, the NPs are reduced, capped, and stabilized by the phytochemicals present in the plant extractions [18, 19]. Second, these phytochemicals are antioxidants, ecologically friendly, and can be obtained easily from plant extracts [20]. Third, after NP preparation, some residual functional groups are tied with the NPs and cap the NPs. This makes the NPs more responsive than the synthesized NPs by the other methods [21]. According to the chemical approach, it is not ecologically friendly. A number of the reducing agents are harmful, which means that they have destructive effects on human life and the environment and are prohibitively expensive [22, 23]. summarizes the approach for green synthesis of SiO2 NPs as provided by researchers. Except for the last one, they used green synthesis in the first stage to produce sodium silicate (Na2SiO3), and chemical synthesis in the second (final) stage to produce SiO2 NPs. The novelty of this study is that it used green synthesis to manufacture SiO2 NPs from their first product (Na2SiO3), completing their approach entirely using green synthesis. This investigation is our ongoing study in the green synthesis NPs area [24-35].

In this study, a certain amount of nano-silica was produced by the green method using Rhus coriaria L. extract. At the same time, a certain amount of nano-silica was chemically synthesized using Sol-gel method [44]. For both samples, numerous characterization techniques, including Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), Zeta potential, field emission scanning electron microscopy (FESEM), Transmission Electron Microscope analysis (TEM), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) analysis, and dynamic light scattering (DLS) were utilized. Consequently, it was found that the green synthesized silica NPs, in general, possess superior properties over the chemically synthesized silica NPs.

2. Experimental procedure

2.1. Materials and methods

Sodium metasilicate (Na2SiO3.5H2O) Molecular weight 212.14 g/mol, and purity >98.8 percent was used for synthesizing nanosilica using the green synthesis method. Sodium hydroxide (NaOH) with a Molecular weight of 40 g/mol was used to control the pH value of the growth solution. A sealed bottle of commercial nanosilica with 99% purity has been used in this study for comparison. All these materials and salts were ordered from Sigma-Aldrich Company. In addition, Rhus coriaria L. seeds were provided from Akre city, located in the Dohuk Governorate in the Kurdistan Region of Iraq. The latitude and longitude coordinates of Akre city are 36.741390 and 43.893333. The hot plate temperature was set at 80°C during the SiO2 nanoparticles synthesis. The chemically synthesized SiO2 NPs were prepared using Verma and Bhattacharya’s procedure [44].

2.2. Characterization

To study the structure of SiO2 nanoparticles, X-ray diffraction (XRD) machine was utilized by using PANanalytical (CuK = 1.54 nm). The scanning value was 1° per minute in the scope of 10° up to 80°. To confirm the formation of SiO2 NPs, the UV-Vis technique was performed using a double beam spectrophotometer (Super Aquarius Spectrophotometer-1000). Field emission scanning electron microscopy (FESEM Quanta 450) was used to study the morphology and grain dispersion. The elemental composition of the produced nanostructures was determined using dispersive X-ray spectroscopy (EDX), installed inside FESEM. The functional groups from leaf extracts attached to the SiO2 NPs were studied by attached to the SiO2 NPs using a Fourier transform infrared (Perkin Elmer FTIR) spectrophotometer in the acquisition range of 400–4000 cm−1. Dynamic light scattering was used to determine the size of SiO2 NPs. The DSC curve was performed using a differential scanning calorimetry (DSC) type (TA Instruments USA, DSC Q10) in the temperature range of 50–600°C. Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer-Pyris1 analyzer (USA).

2.3. Preparing of Rhus coriaria L. extract

Rhus coriaria L. seeds were collected and dried in a shadow area at 25°C, then crushed using a grinder machine. Then, 50 g of crushed Rhus coriaria L. was heated by magnetic stirring in 100 ml of double-distilled water for 40 minutes. The final mixture was filtered through the filter paper, and the pure extract was stored in the refrigerator for further use.

2.4. Silica nanoparticles (SiO2 NPs) synthesis using the green method

20 mL Na2SiO3 was added dropwise to 100 mL of freshly generated Rhus coriaria L. extract while the mixture was being stirred at 60°C. The procedure was carried out under reflux conditions for 12 hours at pH 9 until the SiO2 NPs were synthesized. The pH value of the solution was adjusted using 0.1 M NaOH, which was dissolved well in 50 mL of double-distilled water. Therefore, the mixture was filtered, and the obtained precipitate was washed carefully with double distilled water three times and methanol to remove all contamination. Centrifugation at 7000 rpm for 25 minutes separated the residues from the mixture, then heated at 550°C for 45 minutes in a furnace to eliminate any contaminants and organic elements around the SiO2 NPs. Then as indicated in , the residue was dried in an oven and kept for further characterization and application activities.

3. Results and discussion

3.1. Characterization of green synthesized (GS) and chemically synthesized (CS) SiO2 NPs

The morphology, structure, optical, toughness, and thermal properties of both types of SiO2 NPs have been investigated using several characterization techniques.

3.1.1. FTIR Spectroscopy

Infrared spectroscopy has been extensively used to identify the functional group’s modification and bonding properties of nanomaterials. The FTIR spectrum analysis might give valuable information about the structure, chemical composition, and purity of NPs. It can also identify and classify probable biomolecules that can be reliable for capping, leading to proficient stabilization of the NPs. The samples containing the NPs are entirely dried and ground with KBr pellets and investigated. Scanning is confirmed to acquire a covered signal-to-noise ratio for suitable consequences. illustrates a typical FTIR spectrum of colloidal silica nanoparticles reported for both GS-SiO2 NPs and CS-SiO2 NPs. According to , two dominant peaks were observed at around 1053 cm-1 and 786 cm-1, which are related to the asymmetric stretching vibration of the siloxane bond (Si-O-Si) and the symmetrical vibration of the Si-O bond [4, 45]. While from , it is clear that the bands nearby 3468 cm-1 is associated with the Si-OH, which is usually characteristic of the OH group corresponding to the available phytochemical in Rhus coriaria L. extract [37]. In addition, the band at 1637 cm–1 is caused by the scissor bending vibration of water molecular H2O. Similarly, the peaks at 1097 and 802 cm-1 correspond to the asymmetric stretching of the Si-O-Si and Si-O band bending vibrations, respectively. Moreover, the band at 463 cm-1 was connected with the bending vibration of O-Si-O modes [46].

FTIR spectra of (a) CS-SiO2 NPs and (b) GS-SiO2 NPs.

3.1.2. X-ray diffraction

To study the crystalline structure of both chemical approach and green synthesized of SiO2 NPs, the X-ray diffraction (XRD) analysis was carried out as shown in . For both, the commercial SiO2 NPs () and the green synthesized SiO2 NPs (), a broad peak centered at 2θ = 23° (hkl = 100) is observed, which can be assigned to the SiO2 NPs and no other diffraction peaks can be detected [4]. illustrates this point. That the X-ray diffractograms in both instances confirmed the amorphous nature of the SiO2 NPs [47]. Also, in both cases, the wide peaks display a whole amorphous structure. Nevertheless, the green synthesized SiO2 NPs show relatively lower noise than the chemically synthesized SiO2 NPs. It can be stated that the peaks were matched with the reference JCPDS file No. 89–0510 for SiO2, and it discloses no impurities peak for SiO2 [48].

X-ray diffraction spectra of (a) CS-SiO2 NPs and (b) GS-SiO2 NPs.

The particle size (D) of the CS-SiO2 NPs and GS-SiO2 NPs was calculated using Debye-Scherer equation [49].

Where λ is the wavelength of the utilized X-ray (λ = 1.5406 Å), β is the full width at half maximum (FWHM) of the most intense peak, and θ is the Braggs angle. From the previous equation, the SiO2 NPs sizes were 18 nm and 23 nm for the CS-SiO2 NPs and the GS-SiO2 NPs, respectively. The absence of sharp peaks shows that there is no crystalline structure of the SiO2 NPs. The GS-SiO2 NPs produced by Rhus coriaria L. extract is an outstanding process, and the yield is close to 100 percent. This is a very encouraging result, and it was expected since plant extracts were utilized instead of the toxic chemical. Also, the phytochemicals present in the plant extracts acted as a reduction, capping, and stabilizing agent of the GS-SiO2 NPs.

3.1.3. FESEM analysis

The surface morphology of the CS-SiO2 NPs and the GS-SiO2 NPs was primarily determined by FESEM, and the results are shown in . It can be seen that the majority of the SiO2 NPs in both samples are on the nanoscale and possess a spherical shape. The main difference between the CS-SiO2 NPs and the GS-SiO2 NPs is that the green synthesized method produces polydisperse NPs, while in the chemically synthesized sample, most of the NPs are monodisperse NPs with different particle sizes. The formation of tiny particles as a result of a rise in the concentration of the OH group causes a decrease in the rate of condensation and hydrolysis process, which in turn inhibits the formation of bigger particles. Furthermore, the SiO2 nanoparticles are somewhat agglomerated, which is characteristic of the green synthesis method. This is attributed to the fact that nanoparticles have a larger surface area and long-lasting affinity throughout the dehydrating process, which causes agglomeration of the particles [22, 23]. It can be noted that the phytochemicals inside plant extraction have a significant effect on the stability of NPs and their agglomeration. As a result, the SiO2 NPs adhere to one another and spontaneously produce asymmetrical clusters [24, 26]. The process for fabricating the SiO2 NPs is related to a number of growth factors, including the growth time, the concentration of plant extract, the pH value of the solution, the temperature of the solution, and the concentration of salt. Consequently, calibration of these growth components is critical to accomplishing the desired shape and size of nanoparticles to allow for their maximal modification and request.

FESEM images of (a) CS-SiO2 NPs and (b) GS-SiO2 NPs.

3.1.4. Transmission Electron Microscope (TEM) analysis

displays the TEM analysis of the obtained CS-SiO2 NPs and the GS-SiO2 NPs. The outcomes confirmed that the SiO2 NPs possess a spherical shape in both cases. The average size of the CS-SiO2 NPs is around 30 nm, while the average size of the GS-SiO2 NPs is around 55 nm and the NPs were amorphous in nature. It can be stated that only TEM and DLS analysis provide the real size of the synthesized NPs. This outcome agrees in high precision with the DLS results discussed in the next section. The results of this study agree with the results obtained by Yadav and Fulekar [50].

TEM images of (a) CS-SiO2 NPs and (b) GS-SiO2 NPs.

3.1.5. Dynamic light scattering (DLS)

DLS is used to calculate the consistent size of the SiO2 NPs distributed in the water medium. The size distribution profile of the chemically and green synthesized SiO2 NPs using the DLS technique is shown in . This study found that the average diameter for chemically and greenly synthesized SiO2 NPs was 32 nm and 60 nm. DLS measurements () showed that the hydrodynamic diameter of the SiO2 NPs stayed nearly unchanged during the measurement for one hour. The green synthesis method generally provides smaller NPs than the chemical methods [46].

DLS for (a) CS-SiO2 NPs and (b) GS-SiO2 NPs.

Additionally, the CS-SiO2 NPs remained monodisperse throughout the DLS test. The statement that some engineered NPs, such as SiO2, tend to combine and agglomerate speedily to form bigger particles, with sizes beyond the nanoscale, might bring together an extra level of difficulty to describe particle performance in the aqueous medium [51]. Nevertheless, this larger agglomeration will normally resolve swiftly by using an ultrasonic bath. Several researchers have achieved comparable results [52].

3.1.6. Zeta potential

Zeta potential is one of the most significant factors that explain the rheological performance of ceramic suspensions. The zeta potential is a measurement of the real electric field around the surface of the nanoparticle [53]. When a nanoparticle is charged totally, the amount of the charge is calculated by the ion concentration versus different charges close to the surface of the NPs. The greater the level of zeta potential, the stronger the surface repulsion/attraction. For the determination of the Zeta potential, it is necessary to consider the mobility of nanoparticles under the influence of an applied electromagnetic field. This work demonstrated that the capping molecules present on the surface of the green produced SiO2 NPs are mostly made of negative charge families and that they are also accountable for the stability of the NPs in the growing solution, as shown before [54].

The Zeta potential gives information about the stability of the particles. shows the zeta potential analysis for both the CS-SiO2 NPs and the GS-SiO2 NPs. It can be regarded that the values of zeta potential for CS-SiO2 NPs and the GS-SiO2 NPs are -38 mV and -45 mV, respectively. This is a good indicator that GS-SiO2 NPs provide stable NPs. The variation of the zeta potential value between the CS-SiO2 NPs and the GS-SiO2 NPs is most likely due to the agloromation that has been shown in the SEM analysis. To a great extent, agglomeration is associated with the green synthesis method. This is more likely due to the insufficient number of capping agents around the NPs in some plant extracts [55]. Conventional methods for synthesizing NPs involve high-priced chemical and physical procedures that frequently utilize hazardous ingredients, most of which are carcinogenic. These issues arise mostly from utilizing organic solvents that work as reducing, capping, and stabilizing agents that prevent the NPs from agglomerating. Consequently, the presence of these toxic creation agents during the synthesizing process limits the application of the NPs in environmental and biomedical applications [56]. Accordingly at present, researchers are focusing on a harmless, consistent, inexpensive, and environmentally-friendly method to prepare NPs.

Zeta potential for (a) CS-SiO2 NPs and (b) GS-SiO2 NPs.

3.1.7. Differential scanning calorimetry (DSC) analysis

The morphology analysis of the SEM images shows that the GS-SiO2 NPs possess agglomeration. More importantly, the agglomeration can be affected by the temperature. Thus, the thermal behavior of SiO2 NPs is investigated by DSC analysis at temperatures varying from 25 to 1000°C as shown in . The slow heating rate of 10°C/min was chosen to observe similar behavior for both chemically and green-synthesized SiO2 NPs. From , it can be regarded that the DSC curve of CS-SiO2 NPs clearly shows one endothermic peak, centered at 115°C, and one exothermic peak, centered at 537°C. While the DSC curve of GS-SiO2 NPs shows one endothermic peak, centered at 190°C, and two exothermic peaks, centered at 55°C and 522°C, respectively. The endothermic peak temperatures of 115°C and 190°C, which were began at 100°C, for both the CS-SiO2 and the GS-SiO2 NPs may be due to dehydration, which is the simultaneous removal of physisorbed surface moisture of SiO2 NPs. This is a very good indicator that the GS-SiO2 NPs are stronger absorbers of the surrounding moisture. This obvious endothermic peak shift is more likely due to the dihydroxylation due to the hydroxyl groups (OH) condensation in GS-SiO2 NPs. Kim et al. [57] stated that the right reduction of SiO2 NPs occurs at temperatures ranging from 150°C to 550°C. As a result, it can be decided that this substantial weight reduction is related to removing water and the decomposition of some of the functionalizing agents [58]. This decomposition, accompanied by an exothermic peak, occurs between 400 and 550°C. Similar results have been found by other authors [59], stating that above 600°C, the inaccessible hydroxyl groups are gradually eliminated, resulting in a little mass loss. Another significant trend in the DSC curve is that the GS-SiO2 NPs possess more heat capacity than the CS-SiO2 NPs. This can be reflected directly on the heat flow rate in both cases. Again, this is an excellent indicator that the GS-SiO2 NPs are better than the CS-SiO2 NPs, especially in solar cells, electronic devices, and engineering applications [60].

DSC analysis for (a) CS-SiO2 NPs and (b) GS-SiO2 NPs.

3.1.8. Thermogravimetric analysis (TGA)

DSC and TGA are two of the most extensively utilized thermal analysis techniques in the characterization of crystalline and amorphous materials. However, TGA is a complementary technique to DSC analysis. Briefly, the TGA technique monitors the weight of the sample during the heating or cooling process. However, DSC determines the amount of energy absorbed or released by a sample throughout the cooling or heating procedure. The thermal stability of both CS-SiO2 and GS-SiO2 NPs as a function of temperature has been investigated using the TGA technique. shows the TGA state of both CS-SiO2 and GS-SiO2 NPs. Through comparing their TGA curves, it can be seen that the GS-SiO2 NPs are more thermally stable than the CS-SiO2 NPs. It loses only 7% of its weight at ~400°C. The CS-SiO2 NPs lose ~ 90% of their weight at this temperature. This is no surprise since the DSC analysis reinforces these consequences. This remarkable weight loss in the CS-SiO2 NPs could be attributed to the decomposition of the attached organic groups [61]. On the other hand, indicates that both CS-SiO2 and GS-SiO2 NPs lose about 4% of their weight due to water molecules’ evaporation. The significant weight loss occurred between 200–450°C, and 200–750°C for CS-SiO2 and GS-SiO2 NPs, respectively. Again, this is a very good indicator that the capping and stabilizing agents around the GS-SiO2 NPs are strong enough to resist high temperatures [62]. While the major weight loss for CS-SiO2 NPs occurs at a temperature ~400°C, which is ~92%. Consequently, this study showed that GS-SiO2 NPs could be utilized in high-temperature devices and applications [60].

TGA analysis for (a) CS-SiO2 NPs and (b) GS-SiO2 NPs.

4. Conclusions

This study demonstrated that spherical, nano-sized, thermally stable SiO2 NPs could be synthesized using a green method. This method is eco-friendly, one-pot, cheap, productive, and leads traditional synthesizing methods. Rhus coriaria extract, as a reducing, capping, and stabilizing agent, used with sodium metasilicate (Na2SiO3.5H2O), as a precursor, under reflux conditions and pH 9. Several The shape, purity, crystal structure, stability, thermal, and optical characteristics of biosynthesized SiO2 were investigated using characterization methods. Considering all of these aspects, the green synthesized SiO2 NPs were compared with the chemically synthesized SiO2 NPs.

FTIR analysis for CS-SiO2 NPs demonstrated two dominant peaks at roughly 1053 cm-1 and 786 cm-1, which are associated with the asymmetric stretching vibration of the Si-O bond. While FTIR for GS-SiO2 NPs revealed that the peak at 3468 cm-1 is related to the Si-OH, theat the peak at 3468 cm-1 is related to the Si-OH, which normally characteristics of the OH group match the accessible phytochemical in Rhus coriaria.

According to the XRD measurements, both GS and CS SiO2 NPs had the same peaks at 2θ = 23°, which confirmed the JCDPS reference. Although the intensity of the GS-SiO2 NPs was higher than the CS-SiO2 NPs, this was due to phytochemical presence in green synthesis.

The morphological studies revealed that the CS-SiO2 NPs possess monodisperse NPs while the GS-SiO2 NPs show polydisperse NPs. This is more likely due to the fact that in the green synthesis method, different phytochemicals are participating in reducing capping and stabilizing the process. Besides, the FESEM images revealed that CS and GS SiO2 NPs are spherical in shape with a minimum degree of agglomeration, and the NP sizes were between 45~60 nm and 10~15 nm, respectively.

The effect of this agglomeration on the zeta potential analysis was not shown. In other words, although the particles are agglomerated, GS-SiO2 NPs provide good NPs stability.

The DSC analysis of CS-SiO2 NPs displays that there is one endothermic peak, centered at 115°C and one exothermic peak, centered at 537°C. Whereas the DSC analysis of GS-SiO2 NPs shows one endothermic peak, centered at 190°C and two exothermic peaks, centered at 55°C and 522°C, respectively.

The TGA analysis shows that the GS-SiO2 NPs are more thermally stable than the CS-SiO2 NPs. It loses only 7% of its weight at ~400°C. The CS-SiO2 NPs lose 90% of their weight at this temperature. The overall results revealed that the green synthesis of SiO2 nanoparticles was better than chemically synthesized SiO2 in terms of particle stability and thermal properties.

2 Mar 2022

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Reviewer #1: In this manuscript, the authors reported that green synthesis of SiO2 nanoparticles from rhus coriaria l. extract: comparison with chemically synthesized SiO2 nanoparticles. The authors claimed that the overall results revealed that the green synthesis of SiO2 NPs outperforms chemically synthesized SiO2 NPs. This is expected since the green synthesis method provides higher stability, enhanced thermal properties, and high surface area through the available phytochemicals in the Rhus Coriaria L. extract. Although the approach is interesting, the present paper contains several weak points, originating from the fact that the lack of novelty and poor broader impact. In fact, the authors should emphasize on their research novelty compared to other’s work for their green synthesis of SiO2 nanoparticles from rhus coriaria l. extract. The recent literature review on green synthesis of SiO2 nanoparticles from rhus coriaria l. extract or derived from other green resources in term of their physical and chemical properties should be summarized in a table as benchmarking purpose and discussed in detail with your research findings. In my opinion, technical significance and novelty of the work is still lacking. Furthermore, the details studies on reflux conditions such as reaction time and pH value for green synthesis of SiO2 nanoparticles should be determined and reported. The detailed HRETM and XPS analysis can be included to further explain their lattice fridge, chemical state and electronic state of the elements that exist within green synthesis of SiO2 nanoparticles. In addition, the authors are suggested to conduct the PL/EIS spectroscopy analysis to provide information such as charge carrier trapping, immigration, and transfer within the green synthesis of SiO2 nanoparticles and chemically synthesized SiO2 nanoparticles. In this case the potential application for those green synthesis of SiO2 nanoparticles and chemically synthesized SiO2 nanoparticles can be expected and further determined. The English language of this manuscript is suggested to be carefully polished. There are many grammatical errors within manuscript In my opinion, the manuscript contains few experimental results is at a moderate scientific level, rendering this in its present form inappropriate for publication in PLOS ONE.

Reviewer #2: The authors have reported the results of Green Synthesis of SiO2 Nanoparticles from Rhus Coriaria L. Extract. The manuscript may be accepted after addressing the following queries.

1. There is no need of such basic information like Figure 1 and 2 to be given in introduction.

2. The authors claims the plant extract acting as reducing agent but it is not really acting as reducing agent, beacuse there is no reduction of metal precursor?

3. The mechanism may be proposed and authors may take help from the following papers and their citations should be given in manuscript

https://onlinelibrary.wiley.com/doi/abs/10.1002/jccs.202100548

https://www.tandfonline.com/doi/abs/10.1080/24701556.2021.1978491

4. Conclusion part should be more quantitative, the results should be compared between green synthesis and chemical synthesis comparing XRD pattern, morphology and size etc.

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Reviewer #1: No

Reviewer #2: Yes: Muhammad Akhyar Farrukh

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19 Apr 2022

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1:

In this manuscript, the authors reported that green synthesis of SiO2 nanoparticles from Rhus coriaria L. extract: comparison with chemically synthesized SiO2 nanoparticles.

The authors claimed that the overall results revealed that the green synthesis of SiO2 NPs outperforms chemically synthesized SiO2 NPs. This is expected since the green synthesis method provides higher stability, enhanced thermal properties, and high surface area through the available phytochemicals in the Rhus coriaria L. extract. Although the approach is interesting, the present paper contains several weak points, originating from the fact that the lack of novelty and poor broader impact. In fact, the authors should emphasize on their research novelty compared to other’s work for their green synthesis of SiO2 nanoparticles from Rhus coriaria L. extract. The recent literature review on green synthesis of SiO2 nanoparticles from Rhus coriaria L. extract or derived from other green resources in term of their physical and chemical properties should be summarized in a table as benchmarking purpose and discussed in detail with your research findings.

Table 1: Production Nanosilica using green synthesis in literature

Primary stage Final stage

Green SiO2 source Chemical

reactant Primary

product Chemical

reactant plant extract Final product Ref

Plant species Part Plant species Part NPs Size (nm) Shape

Bamboo Leaf NaOH Na2SiO3 + H2SO4 - - Silica various Spherical [34]

Bamboo Leaf NaOH Na2SiO3 + H2SO4 - - Silica 70-90 Spherical [35]

Olive Leaf NaOH Na2SiO3 + H2SO4 - - Silica 30-40 Spherical [36]

Sugarcane Leaf NaOH Na2SiO3 + H2SO4 - - Silica 29.13 Hexagonal [37]

Rice Leaf NaOH Na2SiO3 + H2SO4 - - Silica 39.47 Spherical [37]

Weed Leaf NaOH Na2SiO3 + H2SO4 - - Silica 30.56 Spherical [37]

Sugarcane bagasse Stem, leaf NaOH Na2SiO3 + H2SO4 - - Silica 30 Spherical [38]

Rice Husk NaOH Na2SiO3 + H2SO4 - - Silica various Spherical [39]

Pine Cone NaOH Na2SiO3 + H2SO4 - - Silica 20 Spherical [40]

In my opinion, technical significance and novelty of the work is still lacking.

The novelty of this study can be summarized here that spherical, nano-sized, thermally stable SiO2 NPs can be obtained using a green method. This method is eco-friendly, one-pot, cheap, mass productive, and leads traditional synthesizing methods. Table 1, summarizes the approach for green synthesis of SiO2 NPs as provided by researchers. They used green synthesis in the first stage to produce sodium silicate (NaSiO4), and chemical synthesis in the second (final) stage to produce SiO2 NPs. The other novelty of this study is that it used green synthesis to manufacture SiO2 NPs from their first product (NaSiO4), completing their approach entirely using green synthesis.

Furthermore, the details studies on reflux conditions such as reaction time and pH value for green synthesis of SiO2 nanoparticles should be determined and reported.

These have been mentioned in the manuscript already.

The detailed HRETM and XPS analysis can be included to further explain their lattice fridge, chemical state and electronic state of the elements that exist within green synthesis of SiO2 nanoparticles.

1.1.2 Transmission Electron Microscope (TEM) analysis

Figure 7 displays the TEM analysis of the obtained CS-SiO2 NPs and the GS-SiO2 NPs. The outcomes confirmed that the SiO2 NPs possess a spherical shape in both cases. The average size of the CS-SiO2 NPs is around 30 nm, while the average size of the GS-SiO2 NPs is around 55 nm and the NPs were amorphous in nature. It can be stated that, only TEM and DLS analysis provide the real size of the synthesized NPs. This outcome agrees in high precision with the DLS results which will be discussed in the next section. This study's results agree with the results obtained by Yadav and Fulekar [1].

[1] Yadav, Virendra Kumar, and M. H. Fulekar. "Green synthesis and characterization of amorphous silica nanoparticles from fly ash." Materials Today: Proceedings 18 (2019): 4351-4359.

Figure 7: TEM images of (a) CS-SiO2 NPs and (b) GS-SiO2 NPs

In addition, the authors are suggested to conduct the PL/EIS spectroscopy analysis to provide information such as charge carrier trapping, immigration, and transfer within the green synthesis of SiO2 nanoparticles and chemically synthesized SiO2 nanoparticles. In this case the potential application for those green synthesis of SiO2 nanoparticles and chemically synthesized SiO2 nanoparticles can be expected and further determined.

PL/EIS spectroscopy analysis is not available in our country.

The English language of this manuscript is suggested to be carefully polished. There are many grammatical errors within manuscript In my opinion, the manuscript contains few experimental results is at a moderate scientific level, rendering this in its present form inappropriate for publication in PLOS ONE.

Overall grammar of the manuscript has been checked.

Reviewer #2:

The authors have reported the results of Green Synthesis of SiO2 Nanoparticles from Rhus Coriaria L. Extract. The manuscript may be accepted after addressing the following queries.

1. There is no need of such basic information like Figure 1 and 2 to be given in introduction.

In fact, Figures 1 and 2 represent the main application area of SiO2 NPs, while Figure 2 is about the main methods for synthesizing NPs. These are general facts that’s why they placed in our introduction part.

2. The authors claims the plant extract acting as reducing agent but it is not really acting as reducing agent, because there is no reduction of metal precursor?

Since we utilized Sodium metasilicate, which contains both sodium and oxygen ions. The SiO2 NPs formation requires the reduction of these ions.

3. The mechanism may be proposed and authors may take help from the following papers and their citations should be given in manuscript

https://onlinelibrary.wiley.com/doi/abs/10.1002/jccs.202100548

https://www.tandfonline.com/doi/abs/10.1080/24701556.2021.1978491

They have been cited in the introduction as numbers 18,19 and highlighted in red color.

4. Conclusion part should be more quantitative, the results should be compared between green synthesis and chemical synthesis comparing XRD pattern, morphology and size etc.

1. According to the XRD measurements, both GS and CS SiO2 NPs had the same peaks at 2θ = 23°, which confirmed the JCDPS reference. Although the intensity of the GS-SiO2 NPs was higher than the CS-SiO2 NPs, this was due to phytochemical presence in green synthesis.

2. The morphological studies revealed that the CS-SiO2 NPs possess monodisperse NPs while the GS-SiO2 NPs show polydisperse NPs. This is more likely because in the green synthesis method, different phytochemicals are participating in reducing capping and stabilizing the process. Besides, the FESEM images revealed that CS and GS SiO2 NPs are spherical in shape with a minimum degree of agglomeration, and the NP sizes were between 45~60 nm and 10~15 nm, respectively.

Submitted filename: Reviewer comments and Response.docx

Click here for additional data file.

25 Apr 2022

Green Synthesis of SiO2 Nanoparticles from Rhus Coriaria L. Extract: Comparison with Chemically synthesized SiO2 Nanoparticles

PONE-D-22-02832R1

Dear Dr. Mohammed,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Comments to the Author

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Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: N/A

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Reviewer #1: Yes

Reviewer #2: No

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: In overall, this manuscript was technically well revised. This revised manuscript meets the criteria of PLOS ONE. Therefore, in my opinion, the revised manuscript can be accepted for publication.

Reviewer #2: The authors have revised the manuscript entitled Green Synthesis of SiO2 Nanoparticles from Rhus Coriaria L. Extract: Comparison with Chemically synthesized SiO2 Nanoparticles and may be accepted.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: No

Reviewer #2: Yes: Muhammad Akhyar Farrukh

28 Jul 2022

PONE-D-22-02832R1

Green Synthesis of SiO2 Nanoparticles from Rhus Coriaria L. Extract: Comparison with Chemically synthesized SiO2 Nanoparticles

Dear Dr. Mohammed:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

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on behalf of

Professor Dr. Amitava Mukherjee

Academic Editor

PLOS ONE

Table 1

Production nanosilica using green synthesis in literature.

Primary stage					Final stage
Green SiO2 source		Chemical reactant	Primary product		Chemical reactant	plant extract		Final product			Ref
Plant species	Part					Plant species	Part	NPs	Size (nm)	Shape
Bamboo	Leaf	NaOH	Na2SiO3	+	H2SO4	-	-	Silica	various	Spherical	[36]
Bamboo	Leaf	NaOH	Na2SiO3	+	H2SO4	-	-	Silica	70–90	Spherical	[37]
Olive	Leaf	NaOH	Na2SiO3	+	H2SO4	-	-	Silica	30–40	Spherical	[38]
Sugarcane	Leaf	NaOH	Na2SiO3	+	H2SO4	-	-	Silica	29.13	Hexagonal	[39]
Rice	Leaf	NaOH	Na2SiO3	+	H2SO4	-	-	Silica	39.47	Spherical	[39]
Weed	Leaf	NaOH	Na2SiO3	+	H2SO4	-	-	Silica	30.56	Spherical	[39]
Sugarcane bagasse	Stem, leaf	NaOH	Na2SiO3	+	H2SO4	-	-	Silica	30	Spherical	[40]
Rice	Husk	NaOH	Na2SiO3	+	H2SO4	-	-	Silica	various	Spherical	[41]
Pine	Cone	NaOH	Na2SiO3	+	H2SO4	-	-	Silica	20	Spherical	[42]
-	-	-	-		SiC₈H₂₀O₄	coconut	coir pith	Silica	18	Spherical	[43]