# Synthesis of Mesoporous Silica and Graphene-Based FeO and ZnO Nanocomposites for Nutritional Biofortification and Sustained the Productivity of Rice (Oryza sativa L.)

Authors: Shubham A. Durgude; Shri Ram; Rajeew Kumar; Shiv Vendra Singh; Virendra Singh; Anil G. Durgude; Biswajit Pramanick; Sagar Maitra; Ahmed Gaber; Akbar Hossain
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

Considering nutrient delivery and micronutrient use efficiency problems, mesoporous nanosilica (mNs) and reduced graphene oxide (rGO)-based iron and zinc nanocomposites were formulated. Prepared nanocomposites were characterized for FTIR spectroscopy, XRD, FE-SEM, HR-TEM, and AAS to examine surface functional groups, morphology, and structural composition. XRD spectrum confirmation with SAED image of nanosilica and graphene oxide nanocomposites confirms the polycrystalline and crystalline nature with 30–70-nm crystal size. The SEM revealed that the modified surface of mesoporous nanosilica and reduced graphene oxide are well-distributed clusters and are composed of targeted micronutrients. The impact of nano Fe and Zn foliar application was evaluated on rice grain fortification, productivity, and micronutrient use efficiency. The iron and zinc uptake at 60 days after sowing (DAT) and at harvest was significantly increased with foliar application of mNs and rGO-based Zn at 30 ppm + Fe at 5 ppm nanocomposites as well as led to nutrient fortification by increasing grain uptake and content, with the application of 30 ppm zinc and 5 ppm iron through mNs resulted in an improvement of the rice grain yield by 53% over conventional fertilization. Besides significant increment in grain yield, foliar application of mNs and rGO-based nanocomposites (Zn at 30 ppm + Fe at 5 ppm) increased the Zn and Fe use efficiency by 527 and 380%, respectively, over conventional micronutrient fertilization (ZnSO4 and FeSO4).


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## Body

## 1. Introduction

Many Asian countries including India are deficient in micronutrients due to calcareous soil characteristics, high pH, salt pressure, persistent droughts, high water bicarbonate levels, and imbalanced fertilizer usage [1–3]. Surveys in soil and plant testing demonstrated that about 49% of Indian soils are conceivably deficient in Zn, Fe, Mn, Cu, B, and Mo by 12, 5, 3, 33, and 11%, respectively [2–4]. The adverse effects of inadequate micronutrients in plants include low harvest yield and efficiency and impure plant morphology. Iron plays an essential role in chlorophylls and some proteins synthesis. Recent data show that zinc plays an important role in RNA and DNA structural stabilization, maintenance of DNA synthesizing enzyme’s activity, and regulation of RNA degrading enzyme’s activity. Zinc has its function in regulating gene expression linked with proteins in seed grain as well as the formation of protein and nitrogen assimilation process [5]. Increasing the use and uptake efficiency of such micronutrients may serve as the feasible solution for sustainability in plant nutrition. The micronutrient fertilizer use efficiency (MUE) in crops is <5% due to the lack of coordination between the release of the nutrient availability and the crop requirement [6]. Similarly, the iron and zinc use efficiency of traditional micronutrient fertilizers is also <5% [7]. Moreover, soil-applied micronutrient fertilizer’s utilization rate remains low due to rainfall, adsorption on soil complexes [8], precipitation with colloids and organic matter, and low solubility [9] which further reduce the bioavailability of micronutrients to plants.Rice (Oryza sativa L.) is the world’s important cereal contributing digestible energy of 45% and total protein of 30% in the human diet [10–12] and essentially feeding livestock resources. Rice is a staple nourishment for the greater part of the total population in more than 100 nations. Despite the fact that there are more than 110,000 developed varieties of rice, fluctuation in nutritional quality after postharvest processing is of great concern. Rice is not only a good source of calories but also a decent source of magnesium, phosphorus, manganese, selenium, zinc, iron, folic acid, thiamin, and niacin. However, rice is typically low in bioavailable Iron (Fe) and Zinc (Zn), contributing to deficiencies in these micronutrients in developing countries where rice and wheat are consumed as a staple food. Inadequate retention of micronutrient fertilizers may cause significant loss of yield in various plants and forages, while micronutrient deficit in edible parts can lead to persistent weaknesses and several diseases in domestic animals and humans. Alarming scarcity of essential micronutrients like iron and zinc is primarily responsible for “hidden hunger” and ultimately for the disorders in dietary intake, immune, skeletal, and reproductive systems due to zinc deficiency and anemia due to iron deficiency. Hence, zinc (Zn) and iron (Fe) nutrition is not only important of the dietary component but also became a yield-limiting factor as well.The well-known features of nanotechnology and nanocomposites can help to enhance micronutrient use efficiency by increasing the uptake and partitioning [8]. Scientists throughout the world are constantly trying to prepare smart fertilizers sources through nanotechnology which can improve productivity by enhancing seed germination, shoot and root growth, abiotic stress tolerance, photosynthesis, and photosynthate partitioning as well as crop quality (nutrient composition) [13–15]. The mechanism of enhancing seed germination may include the upregulation of aquaporin genes and the enhancement of α-amylase production, while shoot and root growth could be explained as the expression of cell division genes and water channel genes [16]. Scientists are exploring the various dimensions of nanotechnology such as mitigation of environmental pollution [17], antibacterial nanoagents [18], nanofertilizers [19], and nanomaterials for genetic engineering [20]. Mesoporous silica products are recognized as an advantageous competitor that can overcome problems and have controllable and viable effects. Specifically, the mesoporous nanosilica, which has positive compound properties such as thermal stability and biocompatibility, is commonly used as a supply agent. The new mesoporous geometry of silica facilitates strong loading of the target substances to the relevant site [21]. Reduced graphene oxide is randomly functioned due to its interesting compound properties, in particular, the large surface areas and high-water solubility and a solitary, dense, and two-dimensional atom reinforced, honeycomb cross-section [22]. The monolayer graphene has wonderful physicochemical qualities because of its remarkable two-dimensional form and geometry. These outstanding characteristics provide essentially boundless opportunities for various applications in multiple areas such as electronics, energy storage and processing, agriculture, biotechnology, and nanocomposites [23].Hence, biodegradable natural molecules such as chitosan, kaolinite, mesoporous silica, reduced graphene, and polymers are being used to develop customized micronutrient nanofertilizers [24]. Traditional soil application of micronutrient fertilizers has low utilization efficiency as compared to foliar-applied nanofertilizers having the advantage of quick absorption as well as being cost-effective and posing a minimum adverse effect on soil health and environment as well as achieving biofortification [25]. With all the above facts, an experiment was laid out with the objectives to produce and characterize micronutrient nanofertilizers using mesoporous silica and reduced graphene materials and to evaluate uptake, productivity, and micronutrient use efficiency in rice crops.


## 2. Materials and Methods

### 2.1. Experimental Details

The present experiment for engrafting of mesoporous nanosilica (mNs) and reduced graphene oxide-based iron and zinc nanocomposites was carried out at the Nanotechnology Unit, College of Basic Sciences and Humanities, G.B. Pant University of Agriculture and Technology, Uttarakhand. Iron and zinc nanocomposites were synthesized by the reference method outlined by [26–28]. Synthesized micronutrient nanocomposites were analyzed for Fourier transform infrared (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscope (FE-TEM), and atomic absorption spectrometry (AAS) analysis to evaluate their structural and morphological composition. After detailed characterization, derived iron and zinc nanocomposites using mesoporous nanosilica and reduced graphene oxides were further included in the pot experiment. The impact of foliar-applied nanoparticles on the uptake and yield of the rice crop was evaluated.


### 2.2. Synthesis Iron and Zinc Nanocomposites

#### 2.2.1. Mesoporous Silica-Based Fe and Zn Nanocomposites

The nano-iron and mesoporous nanosilica was added in 50 mL FeCl3 solution (0.45 M) stirred at 800 rpm for one hour. The 20 mL of sodium borohydride solution (0.25 M) was added drop by drop. After complete mixing, the solution was filtered, and the filtrate was washed twice with water and ethanol until the transparent filtrate was obtained. The precipitate on the filter was dried in the oven at 60°C for 2 h to obtained powder. Similarly, zinc oxide nanoparticles were loaded onto mesoporous nanosilica through a wet impregnation method. The 50 mL of zinc nitrate (100 mmoL) solution and zinc oxide nanoparticles material composite matter (MCM-41) were mixed at 30°C with stirring for 24 h. ZnO-MCM was obtained after being filtered. The precipitated material was washed twice with water and ethanol until the transparent filtrate was obtained and it is dried. Finally, calcination was followed at 550°C for 6 h in a muffle furnace. In the end, ZnO-MCM was obtained.


#### 2.2.2. Reduced Graphene Oxide-Based Fe and Zn Nanocomposite

The 0.04 g powder of reduced graphene oxide (rGO) and iron nanoparticles was added in 100 mL aqueous solution (FeCl2.4H2O and FeCl3.6H2O), and the solution was subjected to ultrasonication for 30 minutes. The potassium hydroxide in ethanol (1 M) was added drop by drop into the mixture during continuous stirring carried out by a thermostatic magnetic stirrer for one hour at 100°C temperature. The black precipitate (rGO–FeO) was obtained by centrifugation and washed twice with water and ethanol followed by vacuum drying of GO–FeO nanocomposite at 45°C temperature. The rGO powder was dissolved into ethanol and kept for 2 h for ultrasonication, and it was denoted as rGO solution. The zinc nanoparticles (according to 1 : 1 ratio) were dissolved into the mixture of 10.0 mL methanol and 90.0 mL chloroform, and it was kept for 2 h for ultrasonication, and this was denoted as ZnO(II) solution. The rGO solution and the ZnO(II) solutions were mixed and kept for 24 h under constant stirring. The collected precipitate was obtained by centrifugation and washed thrice with methanol. The product was dried in deep freeze overnight, and this was denoted as rGO-ZnO nanocomposite.


### 2.3. Characterization of Nanocomposites

Prepared nanocomposites were characterized for FTIR, XRD, FE-SEM, and HR-TEM at the Indian Institute of Technology, Bombay (India). The FTIR analysis was done to investigate the surface adsorption of functional groups on nanoparticles with FTIR analyzer (Perkin Elmer, C94012). The XRD giving the information about the crystalline structure, nature of the phase, lattice parameters, and grain size (TableS1) was done using Bruker D8 Advance instrument. The FE-SEM was done to capture the microstructure image of synthesized nanocomposites using Carl Zeiss SUPRA 55 operating at 10 kv. High-resolution TEM that allows the direct imaging of the atomic structure of a sample was carried out by using HR-TEM analyzer made by JEOL Pvt. Ltd. (JEM-2100Plus). Knowing the atomic concentration of nanoparticles was done with the help of an AAS analyzer (PinAAcle 500) at Mahatma Phule Krishi Vidyapeeth, Rahuri, Maharashtra (India).


### 2.4. Crop Response to Foliar Application of Nanocomposites

After the detailed characterization of mesoporous nanosilica (mNs) and reduced graphene oxide-based iron and zinc nanocomposites, a pot experiment was executed to evaluate their response in transplanted rice. The experiment was carried out with eight treatments of nanocomposites containing iron and zinc in a complete randomized design (CRD) replicated thrice. Treatments taken into consideration wereT1: control; T2: ZnSO4 at 0.5% + FeSO4 at 0.5% (application of conventional micronutrients); T3: mNs-Zn at 10 ppm + mNs-Fe at 1 ppm; T4: mNs-Zn at 20 ppm + mNs-Fe at 3 ppm; T5: mNs-Zn at 30 ppm + mNs-Fe at 5 ppm; T6: rGO-Zn at 10 ppm + rGO-Fe at 1 ppm; T7: rGO-Zn at 20 ppm + rGO-Fe at 3 ppm; and T8: rGO-Zn at 30 ppm + rGO-Fe at 5 ppm. Under each treatment, recommended dose of NPK was kept common, while commercial and nanocomposites of zinc and iron micronutrients were applied at 30 and 45 DAT. The plant samples were analyzed for iron and zinc at different stages of rice growth and at harvest (straw and grain) was determined by AAS following the method described by [29] and uptake was calculated on a dry matter basis. The grain yield was reported per hill basis, and nutrient use efficiency was calculated using the formula given below [30]:
(1)NutrientUseEfficiency=Grainyieldoftreatedplot−grainyieldofuntreatedplotAmountofnutrientapplied


### 2.5. Statistical Analysis

The data were collected from the experiment, while the investigation was processed with Windows Excel, and statistical analysis was carried out with software SPSS v23.0. The significant difference between treatments was examined by analysis of variance (ANOVA) at 5% level of significance. The correlation between micronutrient uptake at 60 DAT and harvest has been presented with Pearson correlation.


## 2.1. Experimental Details

The present experiment for engrafting of mesoporous nanosilica (mNs) and reduced graphene oxide-based iron and zinc nanocomposites was carried out at the Nanotechnology Unit, College of Basic Sciences and Humanities, G.B. Pant University of Agriculture and Technology, Uttarakhand. Iron and zinc nanocomposites were synthesized by the reference method outlined by [26–28]. Synthesized micronutrient nanocomposites were analyzed for Fourier transform infrared (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscope (FE-TEM), and atomic absorption spectrometry (AAS) analysis to evaluate their structural and morphological composition. After detailed characterization, derived iron and zinc nanocomposites using mesoporous nanosilica and reduced graphene oxides were further included in the pot experiment. The impact of foliar-applied nanoparticles on the uptake and yield of the rice crop was evaluated.


## 2.2. Synthesis Iron and Zinc Nanocomposites

### 2.2.1. Mesoporous Silica-Based Fe and Zn Nanocomposites

The nano-iron and mesoporous nanosilica was added in 50 mL FeCl3 solution (0.45 M) stirred at 800 rpm for one hour. The 20 mL of sodium borohydride solution (0.25 M) was added drop by drop. After complete mixing, the solution was filtered, and the filtrate was washed twice with water and ethanol until the transparent filtrate was obtained. The precipitate on the filter was dried in the oven at 60°C for 2 h to obtained powder. Similarly, zinc oxide nanoparticles were loaded onto mesoporous nanosilica through a wet impregnation method. The 50 mL of zinc nitrate (100 mmoL) solution and zinc oxide nanoparticles material composite matter (MCM-41) were mixed at 30°C with stirring for 24 h. ZnO-MCM was obtained after being filtered. The precipitated material was washed twice with water and ethanol until the transparent filtrate was obtained and it is dried. Finally, calcination was followed at 550°C for 6 h in a muffle furnace. In the end, ZnO-MCM was obtained.


### 2.2.2. Reduced Graphene Oxide-Based Fe and Zn Nanocomposite

The 0.04 g powder of reduced graphene oxide (rGO) and iron nanoparticles was added in 100 mL aqueous solution (FeCl2.4H2O and FeCl3.6H2O), and the solution was subjected to ultrasonication for 30 minutes. The potassium hydroxide in ethanol (1 M) was added drop by drop into the mixture during continuous stirring carried out by a thermostatic magnetic stirrer for one hour at 100°C temperature. The black precipitate (rGO–FeO) was obtained by centrifugation and washed twice with water and ethanol followed by vacuum drying of GO–FeO nanocomposite at 45°C temperature. The rGO powder was dissolved into ethanol and kept for 2 h for ultrasonication, and it was denoted as rGO solution. The zinc nanoparticles (according to 1 : 1 ratio) were dissolved into the mixture of 10.0 mL methanol and 90.0 mL chloroform, and it was kept for 2 h for ultrasonication, and this was denoted as ZnO(II) solution. The rGO solution and the ZnO(II) solutions were mixed and kept for 24 h under constant stirring. The collected precipitate was obtained by centrifugation and washed thrice with methanol. The product was dried in deep freeze overnight, and this was denoted as rGO-ZnO nanocomposite.


## 2.2.1. Mesoporous Silica-Based Fe and Zn Nanocomposites

The nano-iron and mesoporous nanosilica was added in 50 mL FeCl3 solution (0.45 M) stirred at 800 rpm for one hour. The 20 mL of sodium borohydride solution (0.25 M) was added drop by drop. After complete mixing, the solution was filtered, and the filtrate was washed twice with water and ethanol until the transparent filtrate was obtained. The precipitate on the filter was dried in the oven at 60°C for 2 h to obtained powder. Similarly, zinc oxide nanoparticles were loaded onto mesoporous nanosilica through a wet impregnation method. The 50 mL of zinc nitrate (100 mmoL) solution and zinc oxide nanoparticles material composite matter (MCM-41) were mixed at 30°C with stirring for 24 h. ZnO-MCM was obtained after being filtered. The precipitated material was washed twice with water and ethanol until the transparent filtrate was obtained and it is dried. Finally, calcination was followed at 550°C for 6 h in a muffle furnace. In the end, ZnO-MCM was obtained.


## 2.2.2. Reduced Graphene Oxide-Based Fe and Zn Nanocomposite

The 0.04 g powder of reduced graphene oxide (rGO) and iron nanoparticles was added in 100 mL aqueous solution (FeCl2.4H2O and FeCl3.6H2O), and the solution was subjected to ultrasonication for 30 minutes. The potassium hydroxide in ethanol (1 M) was added drop by drop into the mixture during continuous stirring carried out by a thermostatic magnetic stirrer for one hour at 100°C temperature. The black precipitate (rGO–FeO) was obtained by centrifugation and washed twice with water and ethanol followed by vacuum drying of GO–FeO nanocomposite at 45°C temperature. The rGO powder was dissolved into ethanol and kept for 2 h for ultrasonication, and it was denoted as rGO solution. The zinc nanoparticles (according to 1 : 1 ratio) were dissolved into the mixture of 10.0 mL methanol and 90.0 mL chloroform, and it was kept for 2 h for ultrasonication, and this was denoted as ZnO(II) solution. The rGO solution and the ZnO(II) solutions were mixed and kept for 24 h under constant stirring. The collected precipitate was obtained by centrifugation and washed thrice with methanol. The product was dried in deep freeze overnight, and this was denoted as rGO-ZnO nanocomposite.


## 2.3. Characterization of Nanocomposites

Prepared nanocomposites were characterized for FTIR, XRD, FE-SEM, and HR-TEM at the Indian Institute of Technology, Bombay (India). The FTIR analysis was done to investigate the surface adsorption of functional groups on nanoparticles with FTIR analyzer (Perkin Elmer, C94012). The XRD giving the information about the crystalline structure, nature of the phase, lattice parameters, and grain size (TableS1) was done using Bruker D8 Advance instrument. The FE-SEM was done to capture the microstructure image of synthesized nanocomposites using Carl Zeiss SUPRA 55 operating at 10 kv. High-resolution TEM that allows the direct imaging of the atomic structure of a sample was carried out by using HR-TEM analyzer made by JEOL Pvt. Ltd. (JEM-2100Plus). Knowing the atomic concentration of nanoparticles was done with the help of an AAS analyzer (PinAAcle 500) at Mahatma Phule Krishi Vidyapeeth, Rahuri, Maharashtra (India).


## 2.4. Crop Response to Foliar Application of Nanocomposites

After the detailed characterization of mesoporous nanosilica (mNs) and reduced graphene oxide-based iron and zinc nanocomposites, a pot experiment was executed to evaluate their response in transplanted rice. The experiment was carried out with eight treatments of nanocomposites containing iron and zinc in a complete randomized design (CRD) replicated thrice. Treatments taken into consideration wereT1: control; T2: ZnSO4 at 0.5% + FeSO4 at 0.5% (application of conventional micronutrients); T3: mNs-Zn at 10 ppm + mNs-Fe at 1 ppm; T4: mNs-Zn at 20 ppm + mNs-Fe at 3 ppm; T5: mNs-Zn at 30 ppm + mNs-Fe at 5 ppm; T6: rGO-Zn at 10 ppm + rGO-Fe at 1 ppm; T7: rGO-Zn at 20 ppm + rGO-Fe at 3 ppm; and T8: rGO-Zn at 30 ppm + rGO-Fe at 5 ppm. Under each treatment, recommended dose of NPK was kept common, while commercial and nanocomposites of zinc and iron micronutrients were applied at 30 and 45 DAT. The plant samples were analyzed for iron and zinc at different stages of rice growth and at harvest (straw and grain) was determined by AAS following the method described by [29] and uptake was calculated on a dry matter basis. The grain yield was reported per hill basis, and nutrient use efficiency was calculated using the formula given below [30]:
(1)NutrientUseEfficiency=Grainyieldoftreatedplot−grainyieldofuntreatedplotAmountofnutrientapplied


## 2.5. Statistical Analysis

The data were collected from the experiment, while the investigation was processed with Windows Excel, and statistical analysis was carried out with software SPSS v23.0. The significant difference between treatments was examined by analysis of variance (ANOVA) at 5% level of significance. The correlation between micronutrient uptake at 60 DAT and harvest has been presented with Pearson correlation.


## 3. Results

### 3.1. Characterization of Nanocomposites

#### 3.1.1. Fourier Transform Infrared Spectroscopy

The Fourier transforms infrared (FTIR) spectra of silica-iron oxide, silica-zinc oxide, graphene-iron oxide, and graphene-zinc oxide nanoparticles demonstrated successful modification of the silica and graphene surface with iron oxide and zinc oxide nanoparticles. The FTIR spectra (FigureS1) exhibited symmetric stretching vibration and bending vibration of Si-O-Si at 1,150, 870, and 650 cm−1 which are particular groups of silica nanoparticles. A generally sharp peak was noticed for Si-C at 970.95 cm−1. The presence of this peak can additionally affirm holding of silicon and dynamic carbons because of the calcinations of mNs. The FTIR range of rGO nanoparticles displayed the band of oxygen-containing bunches 3410 cm−1 appointed to O-H groups. Vibrating bands at around 1444 and 1080 cm−1 confirm C=O extending. The vibrations of Fe–O assimilation bonds were reported at 660 and 3354 cm–1. Spectral peaks at 652 cm−1, 665 cm−1, and 990 cm−1 are ascribed to Fe-O bond, while 625 cm−1 and 657 cm−1 demonstrates Fe-O obligations of magnetite nanoparticles. In Figure S1, various ingestion tops from 1000 to 4000 cm–1 were compared to the carboxylate and hydroxyl pollutants in materials, and an expansive band at 3500 cm–1 is relegated to the O-H extending method of the hydroxyl group. Peaks somewhere in the range of 2850 and 3000 cm–1 are because of C-H extending the vibration of the alkane group. The peak saw at 1635 and 1390 cm–1 is because of the asymmetric and balanced stretching of the zinc carboxylate individually.


#### 3.1.2. X-Ray Diffraction

The X-ray diffraction (XRD) patterns of mNs–FeO, mNs–ZnO, rGO–FeO, and rGO–ZnO nanocomposites are portrayed in Figures1(a), 1(b), 1(c), and 1(d), respectively. The peak at 21.80° and 21.93° confirms the presence of silica, and the polycrystalline nature is confirmed with the corresponding SAED-TEM images (Figures 1(a) and 1(b)).Figure 1
XRD spectrum of (a) mNs–FeO, (b) mNs–ZnO, (c) rGO–FeO, and (d) rGO–ZnO with selected area electron diffraction (SAED) micrographs.
(a)(b)(c)(d)The presence of both iron and zinc on mesoporous nanosilica and graphene oxide nanocomposites was also ensured with AAS findings as well in the oxidative state of FeO(II) and ZnO(II). The crystal size of 28.7 mm and 58.31 nm was observed in the case of silica present in the respective nanocomposite. The phase and structural analysis of graphene oxide-based nanocomposites were done, and the characteristic diffraction peak was found with reduced graphene oxide at 12.64° and 11.86° for nanocomposites of rGO–FeO and rGO–ZnO, respectively. The crystalline nature of both nanocomposites was confirmed with respective SAED TEM images (Figures1(c) and 1(d)). The crystal size of 24.72 nm (d spacing 2.48 A0) and 26.38 nm (d spacing 7.49 A0) is observed in the case of rGO–FeO and rGO–ZnO nanocomposites, respectively.The prominent peaks at 30.29°, 35.67°, 43.34°, 53.99°, 57.21°, and 62.93° corresponding to the crystal planes of 220, 311, 400, 422, 511, and 440 of crystalline iron oxide nanoparticles were observed (Figure2(a)), and this confirmed the presence of iron oxide in mNs–FeO nanocomposite with crystal size of 26 nm (d spacing 2.51 A0). The intense peaks at 26.61°, 29.97°, 35.14°, 42.76°, 54.03°, 56.34°, and 62.42° (Figure 1(c)) with an average crystal size of 11.58 nm (d spacing 2.55 A0) were observed confirming the existence of nanoparticles of iron oxide in rGO–FeO nanocomposite. The characteristics peaks of Zinc observed in mNs-ZnO nanocomposite (Figure 2(b)) at 31.67°, 34.19°, 36.19°, 47.51°, 56.41°, 62.81°, and 67.90° gave conformity of Zn+2 intercalation. The peaks are clear evidence of the presence of ZnO(II) nanoparticles in the same nanocomposite with a crystal size of 30.08 nm.Figure 2
FE SEM micrographs of radied nanocomposites (a) mNs, (b) mNs-FeO, (c) mNs-ZnO, (d) rGO, (e) rGO–FeO, and (f) rGO–ZnO.
(a)(b)(c)(d)(e)(f)


#### 3.1.3. Field Emission Scanning Electron Microscopy

The scanning electron microscopy (FE-SEM) micrographs revealed surface morphology of semispheroidal to cuboidal nanoparticles of silica with little aggregation (Figure2(a)). The aggregation induced by material dissolving in alcohol when performing FE-SEM the semispheroidal structure may be seen with disconnected nanomaterial particles. Nanosilica particles were porous providing an average space of 40-50 nm. The SEM micrograph revealed that the addition of FeO nanoparticles significantly altered the morphology of silica nanomaterial. Engrafting of FeO nanoparticles over silica is clearly visible (Figure 2(b)) (average diameter of iron oxide nanoparticles ranging from 30-40 nm). The ZnO(II) nanoparticles were engrafted onto pore opening surface adsorption sites of loading particles with the average grain size ranging from 30-50 nm (Figure 2(c)). The representative SEM picture of free-standing rGO nanosheets (Figure 2(d)) shows a crumpled and rippled structure caused by deformation during exfoliation and restacking processes. However, as seen in SEM micrographs, the rGO nanosheets are layer organized, irregular, and folding. They are caught up in each other’s webs. The single or few-layer rGO nanosheets had a lot of wrinkles. Most graphene sheets have an average thickness of 5 nm, so this kind of special microstructure of rGO could provide more space for engrafting of targeted elements. FE-SEM representations of rGO–FeO nanocomposites (Figure 2(e)) demonstrate that FeO(II) nanoparticles are evenly and densely anchored on the surface of rGO sheets; as a result, optimal anchoring and uniform dispersion of Fe+2O nanoparticles on rGO sheets resist agglomeration, resulting in a greater surface region. The FeO particles are closely bound onto the rGO sheets microspheres, as seen in the illustration of rGO-Fe nanocomposites (Figure 2(e)). The FeO(II) particles have a diameter of less than 100 nanometers. SEM analysis represents the surface morphology and particle size of the rGO-ZnO nanocomposites (Figure 2(f)) to depict the dispersion and agglomeration of ZnO(II) particles on graphene oxide as ZnO(II) particles. Intercalation of targeted nutrients (Fe2+ and Zn2+) was further confirmed by EDX spectra of loaded mesoporous silica and reduced graphene oxide (Figure S3). The elemental analysis (AAS, EDX) also helps to assure engrafting of Fe+2 and Zn+2 particles attached to the matrix.


#### 3.1.4. High-Resolution Transmission Electron Microscopy

A transmission electron microscopy (TEM) of silica nanoparticles shows that nanosilica powder is standardized and hardly agglomerated (Figure3(a)). The silica grains are spherical to cuboidal, with an average homogeneous particle size distribution of around 40-50 nm, whereas the addition of iron oxide nanoparticles to silica (Figure 3(b)) results in a tiny amount of aggregate spherical-like particles with a diameter of about 80 nm that are well bound to the surface of silica nanoparticles. The micrographs of (Figure 3(c)) depicted doped ZnO(II) nanoparticles with sizes ranging from 50 to 75 nanometers. The TEM picture clearly shows the existence of aggregated ZnO(II) nanoparticles (Figure 3(c)) and clearly shows the trapped ZnO nanoparticles within the silica matrix.Figure 3
HR TEM micrographs of radied nanocomposites (a) mNs, (b) mNs-FeO, (c) mNs-ZnO, (d) rGO, (e) rGO-FeO, and (f) rGO-ZnO.
(a)(b)(c)(d)(e)(f)The photographs show that silica–zinc oxide nanocomposite was successfully synthesized. The SAED approach was used to determine the crystallographic composition of the nanocomposites (SAED Figures1(a) and 1(b)). The TEM micrographs of rGO sheets (Figure 3(d)) display the graphene lattice fringes as well as additional detail regarding the rGO material’s interplanar distance of 7.00 nm. In the transmission electron microscopy micrographs of the rGO – FeO composites (Figure 3(e)), iron oxide nanoparticles with certain aggregation electrostatically self-assembled on crumpled as silk-like and reduced graphene oxide sheets. Iron oxide nanoparticles are tightly bound on the reduced graphene oxide layer (Figure 3(f)). The ZnO(II) nanorods with an average diameter of approximately 54-75 nm are distributed into the GO layers. The solvent employed for dissolving of nanocomposites may have altered the composite 3D structure; certain particles evident in the gathering around the silica matrix may be due to the same dissolution impact; grain size was determined using an updated image processor. The in situ hydrothermal reductions on the surface of graphene are responsible for the uniform distribution of ZnO(II) nanorods among the GO.


#### 3.1.5. Atomic Absorption Spectroscopy

The elemental composition of all the nanocomposites presented in the supplementary file (FigureS2) reveals that higher elemental composition in the acid diluted nanocomposites sample than the samples diluted in deionized water. The concentration of elements in question from the nanocomposites was taken for study with weak acid dilution to mimic rhizospheric conditions and to test maximum possible concentrations in the composites. In acid diluted nanocomposites, 21.3% and 27.6% of Fe and Zn were reported in the nanocomposites of mNs-FeO and mNs-ZnO, respectively. The test was considered confirmative for the elemental composition of engrafted nanocomposites and set point for further studies. However, rGO-FeO and rGO-ZnO reported 23.4% and 38.1% of Fe and Zn content, respectively. The test was considered confirmative for the elemental composition of engrafted nanocomposites and set point for further studies.


### 3.2. Crop Response to Foliar Applied Micronutrient Nanocomposites

#### 3.2.1. Plant Nutrient Uptake

Foliar nutrition through nanoparticles significantly increased micronutrients uptake at 60 DAT and harvest stage (Table1). The nutrient uptake further increased with the consecutive days after transplanting which might be due to the enhanced sorptivity of iron and zinc through foliage over control. The highest iron uptake was recorded at 30 DAT under treatment, T2 being at par with treatment, T5 and T8. Uptake of iron at 60 DAT recorded with T5 and T8 was increased by 76% and 29% over T1 and T2, respectively. However, zinc uptake was recorded with the treatment; T5 was 50% higher over T2, while Zn uptake under T8 was increased by 12.5% as compared to T2. Iron uptake in grain and straw of rice at harvest stage was recorded significantly higher with T5 being at par with T8, while the uptake was significantly higher over the rest of the treatments. Similarly, a significant increment in Zn uptake was recorded with T5 and T8 as compared to all the treatments. A similar trend was observed in straw Fe and Zn uptake as well. Zinc uptake in straw was significantly higher in treatment with mNs-based Zn at 30 ppm + Fe at 5 ppm.Table 1
Effect of varying levels of Zn and Fe through mNs and rGO-based nanocomposites on Fe and zinc uptake of rice at 30 and 60 DAT and harvest stage.
TreatmentIron uptake (mg hill-1)Zinc uptake (mg hill-1)30 days60 daysAt harvest30 days60 daysAt harvestGrainStrawGrainStrawT1Control0.210.550.840.140.020.046.580.50T2ZnSO4 at 0.5% + FeSO4 at 0.5%0.330.641.520.260.050.089.560.72T3mNs-Zn at10 ppm + mNs-Fe at1 ppm0.190.631.140.200.030.058.280.64T4mNs-Znat20 ppm + mNs-Fe at3ppm0.220.691.520.260.040.079.360.74T5mNs-Znat30 ppm + mNs-Fe at 5 ppm0.300.832.980.520.040.1211.980.98T6rGO-Zn at 10 ppm + rGO-Fe at 1 ppm0.240.561.10.200.030.067.160.56T7rGO-Zn at 20 ppm + rGO-Fe at 3 ppm0.260.611.840.300.040.079.320.72T8rGO-Zn at 30 ppm + rGO-Fe at 5 ppm0.240.832.340.400.030.0911.220.92LSD (p ≤0.05)0.050.090.400.140.010.021.680.12


#### 3.2.2. Yield and Nutrient Use Efficiency

The highest grain yield over all the treatments in rice was recorded with the treatment,T5 (30 ppm Zn+5 ppm Fe mNs), and the yield under T5 was 219% and 53% more as compared to T1 and T2 (conventional fertilizer), respectively. The grain yield recorded under the treatment T8 was found 70% and 17% more comparing the grain yield under T1 and T2, respectively (Figure 4).Figure 4
Grain and straw yield in rice as influenced by foliar application of mesoporous nanosilica and reduced graphene-based iron and zinc nanoparticles.The straw yield of rice was found to be the maximum in treatment of 30 ppm Zn+5 ppm Fe mNs via the foliar application, and this treatment resulted in 79% and 20% increment in straw yield over absolute control and conventional fertilizer, respectively. The increment in iron and zinc content and uptake at 60 DAT and at harvest stage possessed positively correlated with corresponding rice grain yield withr2 value of 0.78 and 0.88 and 0.83 and 0.89, respectively (Figure 5).Figure 5
Correlation between grain yield and iron and zinc uptake at 60 DAT and harvest stage (p=0.05, n=24).All foliar spray treatments of graded levels of nano-iron and zinc showed improved micronutrient fertilizer use efficiency over conventional fertilization (T2). The maximum fertilizer use efficiency (FUE) for Fe was recorded with T3, while the maximum FUE of Zn was found under T4 (Figure 6). Zn at 30 ppm + Fe at 5 ppm through both mNs and rGO-based nanoparticles showed lower FUE compared to another foliar application doses of nanoparticles. However, this treatment showed the highest positive correlation with rice grain yield. Besides the comparative lower FUE under Zn at 30 ppm + Fe at 5 ppm through mNs and rGO-based nanocomposites as compared to over another level of nanoparticle foliar application, it was still reported to increase FUE-Fe and FUE-Zn by 527 and 380%, respectively, over conventional micronutrient fertilization (ZnSO4 and FeSO4).Figure 6
Micronutrient use efficiency of foliar-applied iron and zinc nanocomposites [line above the bars represent standard error (n=21)].


## 3.1. Characterization of Nanocomposites

### 3.1.1. Fourier Transform Infrared Spectroscopy

The Fourier transforms infrared (FTIR) spectra of silica-iron oxide, silica-zinc oxide, graphene-iron oxide, and graphene-zinc oxide nanoparticles demonstrated successful modification of the silica and graphene surface with iron oxide and zinc oxide nanoparticles. The FTIR spectra (FigureS1) exhibited symmetric stretching vibration and bending vibration of Si-O-Si at 1,150, 870, and 650 cm−1 which are particular groups of silica nanoparticles. A generally sharp peak was noticed for Si-C at 970.95 cm−1. The presence of this peak can additionally affirm holding of silicon and dynamic carbons because of the calcinations of mNs. The FTIR range of rGO nanoparticles displayed the band of oxygen-containing bunches 3410 cm−1 appointed to O-H groups. Vibrating bands at around 1444 and 1080 cm−1 confirm C=O extending. The vibrations of Fe–O assimilation bonds were reported at 660 and 3354 cm–1. Spectral peaks at 652 cm−1, 665 cm−1, and 990 cm−1 are ascribed to Fe-O bond, while 625 cm−1 and 657 cm−1 demonstrates Fe-O obligations of magnetite nanoparticles. In Figure S1, various ingestion tops from 1000 to 4000 cm–1 were compared to the carboxylate and hydroxyl pollutants in materials, and an expansive band at 3500 cm–1 is relegated to the O-H extending method of the hydroxyl group. Peaks somewhere in the range of 2850 and 3000 cm–1 are because of C-H extending the vibration of the alkane group. The peak saw at 1635 and 1390 cm–1 is because of the asymmetric and balanced stretching of the zinc carboxylate individually.


### 3.1.2. X-Ray Diffraction

The X-ray diffraction (XRD) patterns of mNs–FeO, mNs–ZnO, rGO–FeO, and rGO–ZnO nanocomposites are portrayed in Figures1(a), 1(b), 1(c), and 1(d), respectively. The peak at 21.80° and 21.93° confirms the presence of silica, and the polycrystalline nature is confirmed with the corresponding SAED-TEM images (Figures 1(a) and 1(b)).Figure 1
XRD spectrum of (a) mNs–FeO, (b) mNs–ZnO, (c) rGO–FeO, and (d) rGO–ZnO with selected area electron diffraction (SAED) micrographs.
(a)(b)(c)(d)The presence of both iron and zinc on mesoporous nanosilica and graphene oxide nanocomposites was also ensured with AAS findings as well in the oxidative state of FeO(II) and ZnO(II). The crystal size of 28.7 mm and 58.31 nm was observed in the case of silica present in the respective nanocomposite. The phase and structural analysis of graphene oxide-based nanocomposites were done, and the characteristic diffraction peak was found with reduced graphene oxide at 12.64° and 11.86° for nanocomposites of rGO–FeO and rGO–ZnO, respectively. The crystalline nature of both nanocomposites was confirmed with respective SAED TEM images (Figures1(c) and 1(d)). The crystal size of 24.72 nm (d spacing 2.48 A0) and 26.38 nm (d spacing 7.49 A0) is observed in the case of rGO–FeO and rGO–ZnO nanocomposites, respectively.The prominent peaks at 30.29°, 35.67°, 43.34°, 53.99°, 57.21°, and 62.93° corresponding to the crystal planes of 220, 311, 400, 422, 511, and 440 of crystalline iron oxide nanoparticles were observed (Figure2(a)), and this confirmed the presence of iron oxide in mNs–FeO nanocomposite with crystal size of 26 nm (d spacing 2.51 A0). The intense peaks at 26.61°, 29.97°, 35.14°, 42.76°, 54.03°, 56.34°, and 62.42° (Figure 1(c)) with an average crystal size of 11.58 nm (d spacing 2.55 A0) were observed confirming the existence of nanoparticles of iron oxide in rGO–FeO nanocomposite. The characteristics peaks of Zinc observed in mNs-ZnO nanocomposite (Figure 2(b)) at 31.67°, 34.19°, 36.19°, 47.51°, 56.41°, 62.81°, and 67.90° gave conformity of Zn+2 intercalation. The peaks are clear evidence of the presence of ZnO(II) nanoparticles in the same nanocomposite with a crystal size of 30.08 nm.Figure 2
FE SEM micrographs of radied nanocomposites (a) mNs, (b) mNs-FeO, (c) mNs-ZnO, (d) rGO, (e) rGO–FeO, and (f) rGO–ZnO.
(a)(b)(c)(d)(e)(f)


### 3.1.3. Field Emission Scanning Electron Microscopy

The scanning electron microscopy (FE-SEM) micrographs revealed surface morphology of semispheroidal to cuboidal nanoparticles of silica with little aggregation (Figure2(a)). The aggregation induced by material dissolving in alcohol when performing FE-SEM the semispheroidal structure may be seen with disconnected nanomaterial particles. Nanosilica particles were porous providing an average space of 40-50 nm. The SEM micrograph revealed that the addition of FeO nanoparticles significantly altered the morphology of silica nanomaterial. Engrafting of FeO nanoparticles over silica is clearly visible (Figure 2(b)) (average diameter of iron oxide nanoparticles ranging from 30-40 nm). The ZnO(II) nanoparticles were engrafted onto pore opening surface adsorption sites of loading particles with the average grain size ranging from 30-50 nm (Figure 2(c)). The representative SEM picture of free-standing rGO nanosheets (Figure 2(d)) shows a crumpled and rippled structure caused by deformation during exfoliation and restacking processes. However, as seen in SEM micrographs, the rGO nanosheets are layer organized, irregular, and folding. They are caught up in each other’s webs. The single or few-layer rGO nanosheets had a lot of wrinkles. Most graphene sheets have an average thickness of 5 nm, so this kind of special microstructure of rGO could provide more space for engrafting of targeted elements. FE-SEM representations of rGO–FeO nanocomposites (Figure 2(e)) demonstrate that FeO(II) nanoparticles are evenly and densely anchored on the surface of rGO sheets; as a result, optimal anchoring and uniform dispersion of Fe+2O nanoparticles on rGO sheets resist agglomeration, resulting in a greater surface region. The FeO particles are closely bound onto the rGO sheets microspheres, as seen in the illustration of rGO-Fe nanocomposites (Figure 2(e)). The FeO(II) particles have a diameter of less than 100 nanometers. SEM analysis represents the surface morphology and particle size of the rGO-ZnO nanocomposites (Figure 2(f)) to depict the dispersion and agglomeration of ZnO(II) particles on graphene oxide as ZnO(II) particles. Intercalation of targeted nutrients (Fe2+ and Zn2+) was further confirmed by EDX spectra of loaded mesoporous silica and reduced graphene oxide (Figure S3). The elemental analysis (AAS, EDX) also helps to assure engrafting of Fe+2 and Zn+2 particles attached to the matrix.


### 3.1.4. High-Resolution Transmission Electron Microscopy

A transmission electron microscopy (TEM) of silica nanoparticles shows that nanosilica powder is standardized and hardly agglomerated (Figure3(a)). The silica grains are spherical to cuboidal, with an average homogeneous particle size distribution of around 40-50 nm, whereas the addition of iron oxide nanoparticles to silica (Figure 3(b)) results in a tiny amount of aggregate spherical-like particles with a diameter of about 80 nm that are well bound to the surface of silica nanoparticles. The micrographs of (Figure 3(c)) depicted doped ZnO(II) nanoparticles with sizes ranging from 50 to 75 nanometers. The TEM picture clearly shows the existence of aggregated ZnO(II) nanoparticles (Figure 3(c)) and clearly shows the trapped ZnO nanoparticles within the silica matrix.Figure 3
HR TEM micrographs of radied nanocomposites (a) mNs, (b) mNs-FeO, (c) mNs-ZnO, (d) rGO, (e) rGO-FeO, and (f) rGO-ZnO.
(a)(b)(c)(d)(e)(f)The photographs show that silica–zinc oxide nanocomposite was successfully synthesized. The SAED approach was used to determine the crystallographic composition of the nanocomposites (SAED Figures1(a) and 1(b)). The TEM micrographs of rGO sheets (Figure 3(d)) display the graphene lattice fringes as well as additional detail regarding the rGO material’s interplanar distance of 7.00 nm. In the transmission electron microscopy micrographs of the rGO – FeO composites (Figure 3(e)), iron oxide nanoparticles with certain aggregation electrostatically self-assembled on crumpled as silk-like and reduced graphene oxide sheets. Iron oxide nanoparticles are tightly bound on the reduced graphene oxide layer (Figure 3(f)). The ZnO(II) nanorods with an average diameter of approximately 54-75 nm are distributed into the GO layers. The solvent employed for dissolving of nanocomposites may have altered the composite 3D structure; certain particles evident in the gathering around the silica matrix may be due to the same dissolution impact; grain size was determined using an updated image processor. The in situ hydrothermal reductions on the surface of graphene are responsible for the uniform distribution of ZnO(II) nanorods among the GO.


### 3.1.5. Atomic Absorption Spectroscopy

The elemental composition of all the nanocomposites presented in the supplementary file (FigureS2) reveals that higher elemental composition in the acid diluted nanocomposites sample than the samples diluted in deionized water. The concentration of elements in question from the nanocomposites was taken for study with weak acid dilution to mimic rhizospheric conditions and to test maximum possible concentrations in the composites. In acid diluted nanocomposites, 21.3% and 27.6% of Fe and Zn were reported in the nanocomposites of mNs-FeO and mNs-ZnO, respectively. The test was considered confirmative for the elemental composition of engrafted nanocomposites and set point for further studies. However, rGO-FeO and rGO-ZnO reported 23.4% and 38.1% of Fe and Zn content, respectively. The test was considered confirmative for the elemental composition of engrafted nanocomposites and set point for further studies.


## 3.1.1. Fourier Transform Infrared Spectroscopy

The Fourier transforms infrared (FTIR) spectra of silica-iron oxide, silica-zinc oxide, graphene-iron oxide, and graphene-zinc oxide nanoparticles demonstrated successful modification of the silica and graphene surface with iron oxide and zinc oxide nanoparticles. The FTIR spectra (FigureS1) exhibited symmetric stretching vibration and bending vibration of Si-O-Si at 1,150, 870, and 650 cm−1 which are particular groups of silica nanoparticles. A generally sharp peak was noticed for Si-C at 970.95 cm−1. The presence of this peak can additionally affirm holding of silicon and dynamic carbons because of the calcinations of mNs. The FTIR range of rGO nanoparticles displayed the band of oxygen-containing bunches 3410 cm−1 appointed to O-H groups. Vibrating bands at around 1444 and 1080 cm−1 confirm C=O extending. The vibrations of Fe–O assimilation bonds were reported at 660 and 3354 cm–1. Spectral peaks at 652 cm−1, 665 cm−1, and 990 cm−1 are ascribed to Fe-O bond, while 625 cm−1 and 657 cm−1 demonstrates Fe-O obligations of magnetite nanoparticles. In Figure S1, various ingestion tops from 1000 to 4000 cm–1 were compared to the carboxylate and hydroxyl pollutants in materials, and an expansive band at 3500 cm–1 is relegated to the O-H extending method of the hydroxyl group. Peaks somewhere in the range of 2850 and 3000 cm–1 are because of C-H extending the vibration of the alkane group. The peak saw at 1635 and 1390 cm–1 is because of the asymmetric and balanced stretching of the zinc carboxylate individually.


## 3.1.2. X-Ray Diffraction

The X-ray diffraction (XRD) patterns of mNs–FeO, mNs–ZnO, rGO–FeO, and rGO–ZnO nanocomposites are portrayed in Figures1(a), 1(b), 1(c), and 1(d), respectively. The peak at 21.80° and 21.93° confirms the presence of silica, and the polycrystalline nature is confirmed with the corresponding SAED-TEM images (Figures 1(a) and 1(b)).Figure 1
XRD spectrum of (a) mNs–FeO, (b) mNs–ZnO, (c) rGO–FeO, and (d) rGO–ZnO with selected area electron diffraction (SAED) micrographs.
(a)(b)(c)(d)The presence of both iron and zinc on mesoporous nanosilica and graphene oxide nanocomposites was also ensured with AAS findings as well in the oxidative state of FeO(II) and ZnO(II). The crystal size of 28.7 mm and 58.31 nm was observed in the case of silica present in the respective nanocomposite. The phase and structural analysis of graphene oxide-based nanocomposites were done, and the characteristic diffraction peak was found with reduced graphene oxide at 12.64° and 11.86° for nanocomposites of rGO–FeO and rGO–ZnO, respectively. The crystalline nature of both nanocomposites was confirmed with respective SAED TEM images (Figures1(c) and 1(d)). The crystal size of 24.72 nm (d spacing 2.48 A0) and 26.38 nm (d spacing 7.49 A0) is observed in the case of rGO–FeO and rGO–ZnO nanocomposites, respectively.The prominent peaks at 30.29°, 35.67°, 43.34°, 53.99°, 57.21°, and 62.93° corresponding to the crystal planes of 220, 311, 400, 422, 511, and 440 of crystalline iron oxide nanoparticles were observed (Figure2(a)), and this confirmed the presence of iron oxide in mNs–FeO nanocomposite with crystal size of 26 nm (d spacing 2.51 A0). The intense peaks at 26.61°, 29.97°, 35.14°, 42.76°, 54.03°, 56.34°, and 62.42° (Figure 1(c)) with an average crystal size of 11.58 nm (d spacing 2.55 A0) were observed confirming the existence of nanoparticles of iron oxide in rGO–FeO nanocomposite. The characteristics peaks of Zinc observed in mNs-ZnO nanocomposite (Figure 2(b)) at 31.67°, 34.19°, 36.19°, 47.51°, 56.41°, 62.81°, and 67.90° gave conformity of Zn+2 intercalation. The peaks are clear evidence of the presence of ZnO(II) nanoparticles in the same nanocomposite with a crystal size of 30.08 nm.Figure 2
FE SEM micrographs of radied nanocomposites (a) mNs, (b) mNs-FeO, (c) mNs-ZnO, (d) rGO, (e) rGO–FeO, and (f) rGO–ZnO.
(a)(b)(c)(d)(e)(f)


## 3.1.3. Field Emission Scanning Electron Microscopy

The scanning electron microscopy (FE-SEM) micrographs revealed surface morphology of semispheroidal to cuboidal nanoparticles of silica with little aggregation (Figure2(a)). The aggregation induced by material dissolving in alcohol when performing FE-SEM the semispheroidal structure may be seen with disconnected nanomaterial particles. Nanosilica particles were porous providing an average space of 40-50 nm. The SEM micrograph revealed that the addition of FeO nanoparticles significantly altered the morphology of silica nanomaterial. Engrafting of FeO nanoparticles over silica is clearly visible (Figure 2(b)) (average diameter of iron oxide nanoparticles ranging from 30-40 nm). The ZnO(II) nanoparticles were engrafted onto pore opening surface adsorption sites of loading particles with the average grain size ranging from 30-50 nm (Figure 2(c)). The representative SEM picture of free-standing rGO nanosheets (Figure 2(d)) shows a crumpled and rippled structure caused by deformation during exfoliation and restacking processes. However, as seen in SEM micrographs, the rGO nanosheets are layer organized, irregular, and folding. They are caught up in each other’s webs. The single or few-layer rGO nanosheets had a lot of wrinkles. Most graphene sheets have an average thickness of 5 nm, so this kind of special microstructure of rGO could provide more space for engrafting of targeted elements. FE-SEM representations of rGO–FeO nanocomposites (Figure 2(e)) demonstrate that FeO(II) nanoparticles are evenly and densely anchored on the surface of rGO sheets; as a result, optimal anchoring and uniform dispersion of Fe+2O nanoparticles on rGO sheets resist agglomeration, resulting in a greater surface region. The FeO particles are closely bound onto the rGO sheets microspheres, as seen in the illustration of rGO-Fe nanocomposites (Figure 2(e)). The FeO(II) particles have a diameter of less than 100 nanometers. SEM analysis represents the surface morphology and particle size of the rGO-ZnO nanocomposites (Figure 2(f)) to depict the dispersion and agglomeration of ZnO(II) particles on graphene oxide as ZnO(II) particles. Intercalation of targeted nutrients (Fe2+ and Zn2+) was further confirmed by EDX spectra of loaded mesoporous silica and reduced graphene oxide (Figure S3). The elemental analysis (AAS, EDX) also helps to assure engrafting of Fe+2 and Zn+2 particles attached to the matrix.


## 3.1.4. High-Resolution Transmission Electron Microscopy

A transmission electron microscopy (TEM) of silica nanoparticles shows that nanosilica powder is standardized and hardly agglomerated (Figure3(a)). The silica grains are spherical to cuboidal, with an average homogeneous particle size distribution of around 40-50 nm, whereas the addition of iron oxide nanoparticles to silica (Figure 3(b)) results in a tiny amount of aggregate spherical-like particles with a diameter of about 80 nm that are well bound to the surface of silica nanoparticles. The micrographs of (Figure 3(c)) depicted doped ZnO(II) nanoparticles with sizes ranging from 50 to 75 nanometers. The TEM picture clearly shows the existence of aggregated ZnO(II) nanoparticles (Figure 3(c)) and clearly shows the trapped ZnO nanoparticles within the silica matrix.Figure 3
HR TEM micrographs of radied nanocomposites (a) mNs, (b) mNs-FeO, (c) mNs-ZnO, (d) rGO, (e) rGO-FeO, and (f) rGO-ZnO.
(a)(b)(c)(d)(e)(f)The photographs show that silica–zinc oxide nanocomposite was successfully synthesized. The SAED approach was used to determine the crystallographic composition of the nanocomposites (SAED Figures1(a) and 1(b)). The TEM micrographs of rGO sheets (Figure 3(d)) display the graphene lattice fringes as well as additional detail regarding the rGO material’s interplanar distance of 7.00 nm. In the transmission electron microscopy micrographs of the rGO – FeO composites (Figure 3(e)), iron oxide nanoparticles with certain aggregation electrostatically self-assembled on crumpled as silk-like and reduced graphene oxide sheets. Iron oxide nanoparticles are tightly bound on the reduced graphene oxide layer (Figure 3(f)). The ZnO(II) nanorods with an average diameter of approximately 54-75 nm are distributed into the GO layers. The solvent employed for dissolving of nanocomposites may have altered the composite 3D structure; certain particles evident in the gathering around the silica matrix may be due to the same dissolution impact; grain size was determined using an updated image processor. The in situ hydrothermal reductions on the surface of graphene are responsible for the uniform distribution of ZnO(II) nanorods among the GO.


## 3.1.5. Atomic Absorption Spectroscopy

The elemental composition of all the nanocomposites presented in the supplementary file (FigureS2) reveals that higher elemental composition in the acid diluted nanocomposites sample than the samples diluted in deionized water. The concentration of elements in question from the nanocomposites was taken for study with weak acid dilution to mimic rhizospheric conditions and to test maximum possible concentrations in the composites. In acid diluted nanocomposites, 21.3% and 27.6% of Fe and Zn were reported in the nanocomposites of mNs-FeO and mNs-ZnO, respectively. The test was considered confirmative for the elemental composition of engrafted nanocomposites and set point for further studies. However, rGO-FeO and rGO-ZnO reported 23.4% and 38.1% of Fe and Zn content, respectively. The test was considered confirmative for the elemental composition of engrafted nanocomposites and set point for further studies.


## 3.2. Crop Response to Foliar Applied Micronutrient Nanocomposites

### 3.2.1. Plant Nutrient Uptake

Foliar nutrition through nanoparticles significantly increased micronutrients uptake at 60 DAT and harvest stage (Table1). The nutrient uptake further increased with the consecutive days after transplanting which might be due to the enhanced sorptivity of iron and zinc through foliage over control. The highest iron uptake was recorded at 30 DAT under treatment, T2 being at par with treatment, T5 and T8. Uptake of iron at 60 DAT recorded with T5 and T8 was increased by 76% and 29% over T1 and T2, respectively. However, zinc uptake was recorded with the treatment; T5 was 50% higher over T2, while Zn uptake under T8 was increased by 12.5% as compared to T2. Iron uptake in grain and straw of rice at harvest stage was recorded significantly higher with T5 being at par with T8, while the uptake was significantly higher over the rest of the treatments. Similarly, a significant increment in Zn uptake was recorded with T5 and T8 as compared to all the treatments. A similar trend was observed in straw Fe and Zn uptake as well. Zinc uptake in straw was significantly higher in treatment with mNs-based Zn at 30 ppm + Fe at 5 ppm.Table 1
Effect of varying levels of Zn and Fe through mNs and rGO-based nanocomposites on Fe and zinc uptake of rice at 30 and 60 DAT and harvest stage.
TreatmentIron uptake (mg hill-1)Zinc uptake (mg hill-1)30 days60 daysAt harvest30 days60 daysAt harvestGrainStrawGrainStrawT1Control0.210.550.840.140.020.046.580.50T2ZnSO4 at 0.5% + FeSO4 at 0.5%0.330.641.520.260.050.089.560.72T3mNs-Zn at10 ppm + mNs-Fe at1 ppm0.190.631.140.200.030.058.280.64T4mNs-Znat20 ppm + mNs-Fe at3ppm0.220.691.520.260.040.079.360.74T5mNs-Znat30 ppm + mNs-Fe at 5 ppm0.300.832.980.520.040.1211.980.98T6rGO-Zn at 10 ppm + rGO-Fe at 1 ppm0.240.561.10.200.030.067.160.56T7rGO-Zn at 20 ppm + rGO-Fe at 3 ppm0.260.611.840.300.040.079.320.72T8rGO-Zn at 30 ppm + rGO-Fe at 5 ppm0.240.832.340.400.030.0911.220.92LSD (p ≤0.05)0.050.090.400.140.010.021.680.12


### 3.2.2. Yield and Nutrient Use Efficiency

The highest grain yield over all the treatments in rice was recorded with the treatment,T5 (30 ppm Zn+5 ppm Fe mNs), and the yield under T5 was 219% and 53% more as compared to T1 and T2 (conventional fertilizer), respectively. The grain yield recorded under the treatment T8 was found 70% and 17% more comparing the grain yield under T1 and T2, respectively (Figure 4).Figure 4
Grain and straw yield in rice as influenced by foliar application of mesoporous nanosilica and reduced graphene-based iron and zinc nanoparticles.The straw yield of rice was found to be the maximum in treatment of 30 ppm Zn+5 ppm Fe mNs via the foliar application, and this treatment resulted in 79% and 20% increment in straw yield over absolute control and conventional fertilizer, respectively. The increment in iron and zinc content and uptake at 60 DAT and at harvest stage possessed positively correlated with corresponding rice grain yield withr2 value of 0.78 and 0.88 and 0.83 and 0.89, respectively (Figure 5).Figure 5
Correlation between grain yield and iron and zinc uptake at 60 DAT and harvest stage (p=0.05, n=24).All foliar spray treatments of graded levels of nano-iron and zinc showed improved micronutrient fertilizer use efficiency over conventional fertilization (T2). The maximum fertilizer use efficiency (FUE) for Fe was recorded with T3, while the maximum FUE of Zn was found under T4 (Figure 6). Zn at 30 ppm + Fe at 5 ppm through both mNs and rGO-based nanoparticles showed lower FUE compared to another foliar application doses of nanoparticles. However, this treatment showed the highest positive correlation with rice grain yield. Besides the comparative lower FUE under Zn at 30 ppm + Fe at 5 ppm through mNs and rGO-based nanocomposites as compared to over another level of nanoparticle foliar application, it was still reported to increase FUE-Fe and FUE-Zn by 527 and 380%, respectively, over conventional micronutrient fertilization (ZnSO4 and FeSO4).Figure 6
Micronutrient use efficiency of foliar-applied iron and zinc nanocomposites [line above the bars represent standard error (n=21)].


## 3.2.1. Plant Nutrient Uptake

Foliar nutrition through nanoparticles significantly increased micronutrients uptake at 60 DAT and harvest stage (Table1). The nutrient uptake further increased with the consecutive days after transplanting which might be due to the enhanced sorptivity of iron and zinc through foliage over control. The highest iron uptake was recorded at 30 DAT under treatment, T2 being at par with treatment, T5 and T8. Uptake of iron at 60 DAT recorded with T5 and T8 was increased by 76% and 29% over T1 and T2, respectively. However, zinc uptake was recorded with the treatment; T5 was 50% higher over T2, while Zn uptake under T8 was increased by 12.5% as compared to T2. Iron uptake in grain and straw of rice at harvest stage was recorded significantly higher with T5 being at par with T8, while the uptake was significantly higher over the rest of the treatments. Similarly, a significant increment in Zn uptake was recorded with T5 and T8 as compared to all the treatments. A similar trend was observed in straw Fe and Zn uptake as well. Zinc uptake in straw was significantly higher in treatment with mNs-based Zn at 30 ppm + Fe at 5 ppm.Table 1
Effect of varying levels of Zn and Fe through mNs and rGO-based nanocomposites on Fe and zinc uptake of rice at 30 and 60 DAT and harvest stage.
TreatmentIron uptake (mg hill-1)Zinc uptake (mg hill-1)30 days60 daysAt harvest30 days60 daysAt harvestGrainStrawGrainStrawT1Control0.210.550.840.140.020.046.580.50T2ZnSO4 at 0.5% + FeSO4 at 0.5%0.330.641.520.260.050.089.560.72T3mNs-Zn at10 ppm + mNs-Fe at1 ppm0.190.631.140.200.030.058.280.64T4mNs-Znat20 ppm + mNs-Fe at3ppm0.220.691.520.260.040.079.360.74T5mNs-Znat30 ppm + mNs-Fe at 5 ppm0.300.832.980.520.040.1211.980.98T6rGO-Zn at 10 ppm + rGO-Fe at 1 ppm0.240.561.10.200.030.067.160.56T7rGO-Zn at 20 ppm + rGO-Fe at 3 ppm0.260.611.840.300.040.079.320.72T8rGO-Zn at 30 ppm + rGO-Fe at 5 ppm0.240.832.340.400.030.0911.220.92LSD (p ≤0.05)0.050.090.400.140.010.021.680.12


## 3.2.2. Yield and Nutrient Use Efficiency

The highest grain yield over all the treatments in rice was recorded with the treatment,T5 (30 ppm Zn+5 ppm Fe mNs), and the yield under T5 was 219% and 53% more as compared to T1 and T2 (conventional fertilizer), respectively. The grain yield recorded under the treatment T8 was found 70% and 17% more comparing the grain yield under T1 and T2, respectively (Figure 4).Figure 4
Grain and straw yield in rice as influenced by foliar application of mesoporous nanosilica and reduced graphene-based iron and zinc nanoparticles.The straw yield of rice was found to be the maximum in treatment of 30 ppm Zn+5 ppm Fe mNs via the foliar application, and this treatment resulted in 79% and 20% increment in straw yield over absolute control and conventional fertilizer, respectively. The increment in iron and zinc content and uptake at 60 DAT and at harvest stage possessed positively correlated with corresponding rice grain yield withr2 value of 0.78 and 0.88 and 0.83 and 0.89, respectively (Figure 5).Figure 5
Correlation between grain yield and iron and zinc uptake at 60 DAT and harvest stage (p=0.05, n=24).All foliar spray treatments of graded levels of nano-iron and zinc showed improved micronutrient fertilizer use efficiency over conventional fertilization (T2). The maximum fertilizer use efficiency (FUE) for Fe was recorded with T3, while the maximum FUE of Zn was found under T4 (Figure 6). Zn at 30 ppm + Fe at 5 ppm through both mNs and rGO-based nanoparticles showed lower FUE compared to another foliar application doses of nanoparticles. However, this treatment showed the highest positive correlation with rice grain yield. Besides the comparative lower FUE under Zn at 30 ppm + Fe at 5 ppm through mNs and rGO-based nanocomposites as compared to over another level of nanoparticle foliar application, it was still reported to increase FUE-Fe and FUE-Zn by 527 and 380%, respectively, over conventional micronutrient fertilization (ZnSO4 and FeSO4).Figure 6
Micronutrient use efficiency of foliar-applied iron and zinc nanocomposites [line above the bars represent standard error (n=21)].


## 4. Discussion

The FTIR spectra depicted surface modification of the silica and reduced graphene with iron oxide and zinc oxide nanoparticles giving the conformity of Fe band at 625 cm−1 and 657 cm−1. The sharp peak positioned at 680 cm is attributed to the ZnO(II) stretching bonds which agrees with the literature values of [31]. The XRD as well confirmed the polycrystalline nature of nanocomposites with the corresponding SAED images with the crystal size of 28.7 and 58.31 nm. Synthesis of nanocomposite was successful without any impurities as it does not contain peaks other than the materials in question [32]. The crystalline nature of both iron oxide nanomaterials is confirmed with SAED TEM images of respective nanocomposites by Rahman et al. [33] as well. The peaks are clear evidence of the presence of ZnO(II) nanoparticles in the same nanocomposite with a crystal size of 30.08 nm. The presence of crystalline ZnO(II) in rGO-ZnO nanocomposite was confirmed with the intense peaks at 26.47°, 31.67°, 34.43°, 47.53°, 56.41°, 62.71°, and 67.76° [34].The SEM micrographs depicted deposition of Fe and Zn nanoparticles over mesoporous silica and graphene oxide microspheres with gray worm-like structure appearing as a complex mixture [35]. The SEM Fe and Zn nanocomposites anchored on the surface of rGO sheets with uniform dispersion resulting in a greater surface region. Similarly, the TEM micrographs show standardized and hardly agglomerated homogeneous silica grains with spherical to cuboidal aggregate. The SAED approach was used to determine the crystallographic composition of graphene sheets (Figure 3(d)), and previous research of Huseynov et al. [36] also revealed the single series of hexagonal diffraction patterns on graphene sheets with sharp and transparent diffraction spots. The AAS analysis of all the nanocomposites confirmed the higher elemental composition in the acid dilution than deionized water giving the conformity of elemental composition of nanocomposites desired for soil application and impersonator rhizospheric conditions. As the soil pH modulates, the Fe and Zn availability and their availability are considerably high under slight acidic soils, and pH of rhizospheric soil in sub-Himalayan regions usually remains lower than bulk soils. The slowed rate of nitrification and uptake of nitrogen in the form of NH4+ under rice ecosystem as well may lead to rhizosphere acidification [37].The significant increment in micronutrients uptake with foliar application of nanoparticles was found on rice crops. This might be due to enhanced sorptivity of iron and zinc through foliage by applying nanofertilizers as compared to uptake of micronutrients in control plot [38] meeting the objectivity of national fortification. The increasing uptake of iron and zinc at the harvest stage was attributed to enhanced absorption as the nanoparticles can enter the plant system through stomata, lenticels, cuticle, and even through the physically damaged part of the plant leaves more efficiently as well as greater leaf penetration [39]. Kanjana et al. [12] also explained the phenomenon of entry of Fe nanoparticles in different ways and in a sophisticated manner which improved the uptake of iron in plant parts. Similarly, Lopez et al. [40] reported an increment in rice grain Fe and Zn content due to an increase in nutrients sorptivity and aerial penetration of a nanoformulation of iron and zinc. Many other studies also reported that Fe and Zn nanofertilizations resulted in enhanced plant-nutrient contents in different crops [41–44]. The increased grain Fe and Zn uptake observed under the present investigation with micronutrient nanofertilization led to biofortification with elevated grain nutritional compositions. The increased rice grain and straw yield under T5 can be due to nano-iron and zinc sprayed as foliar application caused higher absorption by leaf epidermis and remobilized into the rice through the phloem with the contribution of several Zn-regulating transporter proteins [45]. Secondly, the foliar Zn spray in this study showed more pronounced effects on grain yield, which was probably due to soil which was deficient in zinc. The findings were evident from the strong positive correlation of iron and zinc content and uptake with corresponding rice grain yield. At 60 DAT, photosynthetic rate, transpiration rate, leaf temperature, and chlorophyll content are known to be the highest [46]; hence, elevated micronutrient uptake, viz., iron and zinc at 60 DAT might have significantly influenced yield attributing characteristics and grain yield [47]. Nanoparticles itself shows the improved absorption in plant through aerial parts over ordinary particles. Nanosized formulation of micronutrients may improve absorption through the foliar assemblage of plant which eliminated soil absorption and fixation problems [48]. The results of the present investigation confirmed the finding of Liu et al. [49] reporting increased FUE as well as rice crop yield by 40% with nanoparticles application. The increases in both grain and straw yields might be attributed to the positive effect of nanosilica with applied iron and zinc in increasing the growth and yield components of rice [47] and ultimately increasing the fertilizer use efficiency.


## 5. Conclusion

The results of the present investigation show a great potential of compatible mesoporous nanosilica and reduced graphene to be used as a carrier for micronutrient (Fe and Zn) nanocomposites formulation. Nanocomposites compressing target micronutrient has the capability to reflect their effects in field crop in terms of uptake and yield. The surface morphology of silica and graphene nanoparticles allows it to serve as the nutrient carrier with ability to revolutionize the nutrient delivery system in agriculture. The application of both mesoporous nanosilica and reduced graphene oxide-based zinc and iron nanoparticles (Zn at 30 ppm and Fe at 5 ppm) has the perceptible potential to attain the Zn and Fe biofortification along with grain yield nutrient fertilizer use efficiencies. Besides the fact of marginally better results obtained with mesoporous nanosilica over reduced graphene oxide, depending upon the feasibility and applicability, any of them can be considered for nanoparticle formulation in order to attain the objectivities of nutritional fortification, increased productivity, and nutrient use efficiency.



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*Source: 5120307-2022-04-26.xml*