Effect of Annealing Temperature on the Structural and Physicochemical Properties of Green-Synthesized Iron Oxide Nanoparticles from Ocimum sanctum.”

Green synthesis represents an environmentally responsible and sustainable strategy for producing nanoparticles, offering a clear advantage over conventional techniques that typically rely on toxic reagents and substantial energy input [1]. Using plant extracts such as those derived from Ocimum sanctum provides a particularly effective route for the fabrication of iron oxide nanoparticles due to the plant’s abundance of bioactive compounds [2]. This process minimizes environmental damage while simultaneously improving the biological compatibility and useful applications of the produced nanoparticles [3]. The thermal treatment applied during annealing plays a decisive role in modifying the structural and physicochemical characteristics of these nanoparticles, influencing parameters such as crystallinity, phase composition, and morphology [4]. As an eco-friendly methodology, green synthesis is increasingly valued in nanoparticle research for its ability to utilize naturally derived reducing and stabilizing agents in place of synthetic chemicals, thereby lowering production costs, minimizing hazardous waste, and reducing environmental impact [1].

In this context, O. sanctum leaf extract offers substantial benefits because it contains a diverse array of phytochemicals—including eugenol, flavonoids, tannins, alkaloids, terpenoids, and phenolic compounds that effectively facilitate reduction and stabilization processes during nanoparticle formation [2, 5, 6]. These naturally occurring metabolites enable the synthesis of iron oxide nanoparticles with enhanced purity, better crystallinity, and more controlled particle dimensions, ultimately improving their structural integrity and physicochemical performance [2]. Annealing plays an important role in determining the structural evolution of nanoparticles, exerting a strong influence on their crystallinity and phase behaviour [4]. In the case of iron oxide nanoparticles, thermal treatment promotes the growth of well-defined crystalline structures, drives phase transitions, and helps reduce lattice imperfections. This process is particularly important for inducing the transformation of maghemite (γ-Fe₂O₃) into the more thermodynamically stable hematite (α-Fe₂O₃), thereby enhancing structural order and altering magnetic characteristics [4]. Such modifications are essential for fine-tuning the properties of iron oxide nanoparticles to meet the requirements of targeted technological and biomedical applications [7, 8]. Previous research has demonstrated that plant-mediated synthesis of Fe₂O₃ nanoparticles provides an eco-friendly and sustainable alternative to traditional chemical methods [9].

Plant extracts enriched with phytochemicals such as flavonoids, polyphenols, and other bioactive compounds act simultaneously as reducing and stabilizing agents, thereby influencing nanoparticle dimensions, morphology, and overall stability [10]. Numerous studies have successfully produced Fe₂O₃ nanoparticles with enhanced catalytic and biological activities using plant sources, including Citrus limetta, Hibiscus, and Phyllanthus niruri [11, 9, 12]. Annealing temperature plays a decisive role in further modifying these green-synthesized nanoparticles, as thermal treatment strongly affects their crystalline structure, particle size, and phase composition [4, 7, 13, 14]. Typically, increasing the annealing temperature improves crystallinity and increases crystallite size, reflecting the reorganization and growth of crystalline domains. This heating process also drives phase transformations, particularly the conversion of magnetite or maghemite (γ-Fe₂O₃) into the more stable hematite (α-Fe₂O₃), which subsequently alters their optical absorption and magnetic response [15, 7, 13]. Additionally, higher annealing temperatures help refine nanoparticle morphology by promoting uniformity and reducing structural defects [4, 7]. Studies consistently show that heating at 500–900 °C facilitates the transition from γ-Fe₂O₃ to α-Fe₂O₃, ultimately yielding nanoparticles with superior crystallinity and improved magnetic properties [7].

A clear research gap exists in the current literature regarding the relationship between Ocimum sanctum–mediated green synthesis of iron oxide nanoparticles and the role of post-synthesis thermal treatment. Although many studies have explored the biosynthesis of iron oxide nanoparticles using O. sanctum and the independent effects of annealing on nanoparticle behaviour, comprehensive investigations linking these two aspects remain limited. Most available reports emphasize synthesis and preliminary characterization but lack detailed evaluations of how different annealing temperatures modify the structural, morphological, and physicochemical features of nanoparticles produced through this plant extract. In particular, systematic studies are needed to understand how controlled thermal treatment influences critical transitions—such as the transformation from maghemite (γ-Fe₂O₃) to hematite (α-Fe₂O₃)—and other key material properties when O. sanctum is employed as the reducing and stabilizing agent.

This study aims to provide a systematic assessment of how varying annealing temperatures influence the structural, morphological, optical, magnetic, and overall physicochemical characteristics of iron oxide nanoparticles synthesized through a green route. It specifically addresses the existing gap in the literature by delivering a detailed examination of the relationship between thermal treatment conditions and the resulting properties of Fe₂O₃ nanoparticles produced using Ocimum sanctum leaf extract. The objective is to clarify how controlled annealing drives structural transformations and property modifications in these nanoparticles, ultimately generating valuable insights for precisely tailoring their features to meet the requirements of targeted technological and biological applications.

2. MATERIALS AND METHODS

2.1 Plant Material Collection:

Fresh and healthy leaves of Ocimum sanctum (commonly known as Krishna tulsi) were collected from the botanical garden of Yashwantrao Chavan Institute of Science, Satara, and were identified by the Department of Botany. Further authentication with the help of the herbarium. The leaves were thoroughly washed several times with tap water, followed by distilled water to remove dust and surface contaminants. The cleaned leaves were shade-dried at room temperature until completely moisture-free.

2.2 Reagents and chemicals:

The chemicals used in this study were of analytical grade and used without further purification. Ferrous sulfate heptahydrate (FeSO₄·7H₂O), NaOH, and H₂SO₄ for pH adjustment were purchased from Bio Treasure India Scientific Centre. Double-distilled water was used throughout the synthesis process.

2.3 Ocimum sanctum leaf extract preparation:

The aqueous extract of Ocimum sanctum leaves was prepared using the hot infusion method to extract bioactive phytochemicals responsible for the reduction and stabilization of iron oxide nanoparticles. Fresh green leaves of Ocimum sanctum (approx. 20 grams) were thoroughly washed with tap water, followed by distilled water to remove dust and surface impurities. The cleaned leaves were then chopped into small pieces and boiled in 100 mL of distilled water at 70–80°C for 15–20 minutes. During heating, the mixture was gently stirred to ensure maximum extraction of active compounds into the solution. After boiling, the extract was allowed to cool to room temperature. It was then filtered using Whatman No. 1 filter paper to remove leaf residues and obtain a clear filtrate. The resulting light brown to greenish extract was collected in a sterile container and stored at 4°C. The extract was used within 24-48 hours to ensure the stability of the phytochemicals during the nanoparticle synthesis process.

2.4 Green Synthesis of Iron Oxide Nanoparticles:

The green synthesis of iron oxide (Fe₂O₃) nanoparticles was carried out using the aqueous leaf extract of Ocimum sanctum as both a reducing and stabilizing agent. This eco-friendly method eliminates the need for hazardous chemicals typically used in conventional synthesis. A 0.1 M solution of ferrous sulfate heptahydrate (FeSO₄·7H₂O) was prepared by dissolving the appropriate amount in double-distilled water, adding H₂SO₄ dropwise to prevent oxidation of Fe ions. Iron oxide nanoparticles were synthesized by mixing 100 mL of 0.1 M (FeSO₄·7H₂O) with a 50 mL solution of freshly prepared Ocimum sanctum leaf extract, which was added in a 1:2 volume ratio under constant stirring at room temperature.  The pH of the solution was kept at 8 using .0 M NaOH solution reaction mixture, which was maintained stirred at 60–70°C for 50-60 minutes to facilitate the reduction of Fe²⁺ ions and the formation of iron oxide nanoparticles, and NaOH (1.0 M stock) was added for pH adjustment. The gradual colour change from yellowish-brown to dark brown (black) indicated the formation of iron oxide nanoparticles. The reaction mixture was allowed to cool, and the precipitate was collected by centrifugation at 7500 rpm for 15 minutes. The pellet was washed several times with distilled water and ethanol to remove any unbound phytochemicals or impurities, and dried at 80 °C for 12 h. The dried powder was stored for subsequent annealing.

2.5 Annealing of synthesized nanoparticles:

After drying, the green-synthesized iron oxide (Fe₂O₃) nanoparticles were subjected to controlled annealing to investigate the effect of thermal treatment on their structural and morphological properties. Approximately 500 mg of dried Iron oxide nanoparticles were placed into high-purity ceramic crucibles and annealed in a programmable muffle furnace at four different temperatures: 300°C, 500°C, and 700°C for 3 hours each. The primary purpose of annealing was to improve crystallinity, remove residual organic compounds (such as plant metabolites), and promote phase transformation. After annealing, the furnace was allowed to cool gradually to room temperature. The samples were labeled according to their annealing temperature and stored in airtight containers for further characterization.

2.6 Characterization of nanoparticles.

The green-synthesized iron oxide nanoparticles (IONPs) annealed at different temperatures were characterized by using UV-visible spectroscopy, X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM-EDX) to evaluate their structural, morphological, optical, and elemental properties

3. RESULTS AND DISCUSSION

3.1 UV-Visible Spectroscopy Analysis:

The UV–Visible absorption spectra of green-synthesized iron oxide nanoparticles (Fe₂O₃ NPs) using Ocimum sanctum leaf extract, along with samples annealed at 300°C, 500°C, and 700°C, are presented in Fig. 1. All samples exhibit strong absorption in the ultraviolet region (200–300 nm), which is characteristic of iron oxide nanoparticles and is attributed to ligand-to-metal charge transfer transitions between O²⁻ and Fe³⁺ ions. The unannealed sample (O.S) shows the highest absorbance intensity (~0.75 at 200 nm), which can be attributed to the presence of phytochemicals such as flavonoids, phenolics, and proteins from Ocimum sanctum acting as capping and stabilizing agents. These biomolecules contribute to enhanced optical density and broader absorption features. With increasing annealing temperature, absorbance intensity gradually decreases, particularly in the visible region (400–800 nm). This reduction indicates the thermal decomposition of organic capping agents and the removal of residual impurities, leading to improved purity and structural ordering of the nanoparticles. The absorption curves also become smoother with increasing temperature, suggesting reduced surface defects and enhanced crystallinity. A slight shoulder observed in the 250–300 nm region is associated with surface interactions and residual phytochemicals, which diminish upon annealing. Overall, the UV–Vis analysis confirms the successful synthesis of iron oxide nanoparticles. These observations confirm that annealing significantly influences the optical behaviour of Fe₂O₃ nanoparticles.

 

 

Fig. 1. UV–Visible absorption spectra of green-synthesized iron oxide nanoparticles prepared using Ocimum sanctum leaf extract at different annealing temperatures. (O.S – Ocimum sanctum extract, 300 °C, 500 °C, 700 °C)

 

 

Fig. 2. Tauc plots αhν)2

versus photon energy hν

for Fe₂O₃ nanoparticles synthesized using Ocimum sanctum leaf extract and annealed at different temperatures (300°C, 500°C, and 700°C).

 

The optical band gap of Fe₂O₃ nanoparticles was determined using the Tauc plot shown in Figure 2 αhν)2

vs. hν

, showing a decrease from 3.16 eV (O.S) to 3.06 eV (300°C), 2.96 eV (500°C), and 2.76 eV (700°C). This reduction with increasing annealing temperature is attributed to crystallite growth, reduced quantum confinement, improved crystallinity, and the formation of defect states such as oxygen vacancies. The higher band gap values compared to bulk α-Fe₂O₃ (~2.1 eV) confirm the nanoscale nature of the samples, while the decreasing trend indicates a shift toward bulk-like behaviour. The results agree with reported literature (2.2–3.1 eV range). The estimated error (±0.02–0.05 eV) mainly arises from linear region selection in the Tauc plot, instrumental limitations, and the use of absorbance instead of absorption coefficient, with minor effects from scattering and baseline correction.

 

XRD:

The X-ray diffraction (XRD) patterns of green-synthesized iron oxide nanoparticles annealed at 300,500 and 700 °C (Figure 3; Table 1) reveal a clear temperature- dependent evolution in phase composition, crystallinity, and crystallite size, consistent with the standard JCPDS card No. 00-033-0664 corresponding to rhombohedral α-Fe₂O₃ (hematite). At 300 °C, the diffraction pattern exhibits broad and low-intensity peaks at 2θ ≈ 18.2° (100), 23.5° (110), and 29.8° (200), which are associated with iron oxalate hydrate and intermediate phases, indicating incomplete crystallization. Weak reflections observed at ~35.6° (211), 43.2° (220), and 57.1° (311) suggest the initial formation of the Fe₃O₄ phase. The broadness of these peaks reflects poor crystallinity and smaller crystallite size due to structural disorder, which is consistent with previous reports on low- temperature green-synthesized iron oxide nanoparticles [16] [17]. At 500 °C, the diffraction peaks become sharper and more intense, indicating improved crystallinity and phase development. Prominent peaks at 2θ ≈ 30.1° (220), 35.5° (311), 43.1° (400), 53.5° (422), 57.0° (511), and 62.6° (440) correspond to the characteristic planes of cubic spinel Fe₃O_4, confirming the formation of a well-defined magnetite phase. The reduced peak broadening and increased intensity indicate enhanced crystallite growth and better structural ordering. Such improvements in crystallinity with increasing annealing temperature have been widely reported in green synthesis studies, where thermal treatments facilitate decomposition of organic residues and promote stabilization [9]. Upon further annealing at 700 °C, a complete phase to  α-Fe₂O₃ (hematite)  is observed, as evidenced by the appearance of strong and sharp diffraction peaks at 2θ ≈ 24.1° (012), 33.2° (104), 35.6° (110), 40.9° (113), 49.5° (024), 54.1° (116), 57.6° (018), and 62.5° (214), which are in excellent agreement with JCPDS card NO. . 00-033-0664. The high intensity and sharpness of these peaks confirm the formation of a highly crystalline and phase-pure hematite structure. However, the significant narrowing of peaks at this temperature indicates excessive grain growth and particle agglomeration. Which is also consistent with recent findings, where higher annealing temperature leads to increased crystallite size (~40–70 nm) due to thermal coalescence [18]. Overall, the crystallite size increases progressively with annealing temperature, confirming thermally induced grain growth and reduction in lattice strain. While the 700 °C sample exhibits crystallinity, the associated increase in particle size may limit surface-dependent properties such as antibacterial activity. In contrast, the 500 °C sample offers an optimal balance between crystallinity, phase purity, and nanoscale crystallite size, making it more suitable for applications requiring high surface reactivity. This trend is in strong agreement with recent literature, where moderate annealing temperatures are reported to yield nanoparticles with superior functional performance due to their favourable size and properties [16] [9]

 

d

c

b

a

 

Fig. 5 (a–d) SEM micrographs of green-synthesized iron oxide nanoparticles annealed at 300 °C at different magnifications, showing irregular agglomerated morphology with nanoscale particle domains of approximately 53–58 nm.

 

e

g

f

h

 

Fig. 5(e–h). SEM micrographs of green-synthesized iron oxide nanoparticles annealed at 500 °C, showing homogeneous granular morphology with particle sizes in the range of approximately 264 nm.

 

j

i

k

l

 

Fig. 5 (i–l). SEM micrographs of green-synthesized iron oxide nanoparticles annealed at 700 °C, showing fused plate-like aggregated morphology with an average particle/agglomerate size of approximately 278 nm, indicating significant thermal sintering and particle coalescence.

 

 

Fig. 6. Energy-Dispersive X-ray Spectroscopy (EDX) spectrum of the sample treated at 300 °C, illustrating its elemental composition and purity.

 

 

Fig. 7. Energy-Dispersive X-ray Spectroscopy (EDX) spectrum of the sample treated at 500 °C, illustrating its elemental composition and purity.

 

 

Fig. 8. Energy-Dispersive X-ray Spectroscopy (EDX) spectrum of the sample treated at 700 °C, illustrating its elemental composition and purity.

The elemental composition of the green-synthesized iron oxide nanoparticles annealed at 300, 500, and 700 °C was analysed by energy-dispersive X-ray spectroscopy (EDX), and the corresponding spectra are shown in Figures 6, 7, and 8. All samples exhibit prominent characteristic peaks corresponding to Fe and O, confirming the successful formation of iron oxide nanoparticles. The sample annealed at 300 °C shows the presence of C, O, and Fe, with carbon accounting for approximately 40.44 wt%, indicating the retention of significant residual organic phytochemical content from the Ocimum sanctum leaf extract due to incomplete thermal decomposition at a lower annealing temperature. Upon annealing at 500 °C, the carbon peak disappears completely, and only Fe and O signals are observed, with Fe and O contents of approximately 61.80 wt% and 38.20 wt%, respectively, confirming effective removal of organic residues and formation of relatively pure iron oxide nanoparticles. Further annealing at 700 °C results in an increase in Fe content to approximately 64.25 wt% with corresponding oxygen content of 35.75 wt%, indicating improved phase purity and densification of the oxide structure due to enhanced thermal crystallization. The gradual elimination of carbon impurities and increase in Fe content with rising annealing temperature demonstrate that thermal treatment significantly improves the compositional purity of the synthesized nanoparticles. Overall, the EDX results corroborate the FTIR, XRD, and SEM analyses, confirming progressive purification and structural refinement of iron oxide nanoparticles with increasing annealing temperature.

CONCLUSION

The present study successfully demonstrates the green synthesis of iron oxide (Fe₂O₃) nanoparticles using Ocimum sanctum leaf extract and highlights the significant influence of annealing temperature on their physicochemical properties. UV- Visible analysis confirmed nanoparticle formation, showing strong absorption in the UV region and a systematic decrease in optical band gap from 3.16 eV to 2.76 eV with increasing annealing temperature, indicating enhanced crystallinity, reduced quantum confinement, and particle growth. XRD results confirmed the formation of phase-pure rhombohedral α-Fe₂O₃ with improved crystallinity at higher temperatures, while FTIR analysis revealed the presence of Fe–O bonds and phytochemical functional groups that progressively diminished upon annealing due to thermal decomposition. SEM studies showed a clear morphological transformation from highly agglomerated and poorly defined nanoparticles at 300 °C to more uniform granular structures at 500 °C, followed by excessive particles fusion and coalescence at 700 °C. EDX analysis confirmed the elemental composition and demonstrated a gradual reduction in carbon impurities as temperature increased, indicating improved compositional purity. Among the studied conditions, 500 °C was identified as the optimal annealing temperature, providing the best balance between crystallinity, nanoscale morphology, and retention of surface functional groups, which are crucial for surface-dependent application. Overall, this study established that controlled annealing of green-synthesized Fe₂O₃ nanoparticles is an effective strategy for tuning their structural and optical properties, making them promising candidate for applications in photocatalysis, antibacterial activity, and other advanced functional materials, while also offering a sustainable and eco-friendly approach to nanomaterial synthesis.

Acknowledgements

Author Ms. Shubhangi R. Pawar would like to acknowledge the Mahatma Joyita Phule Research Fellowship (MJPRF-2022), Government of Maharashtra, India. for providing financial assistance.  The author also acknowledges the Department of Zoology, Yashavantrao Chavan Institute of Science, Satara, Maharashtra, India, for providing laboratory facilities and the Department of Zoology, Shivaji University, Kolhapur, for continuous support and encouragement throughout the work.

Funding

This work was supported by [Mahatma Jyotiba Phule Research Fellowship (MJPRF-2022), Government of Maharashtra, India]. Grant numbers [Fellowship2022_30]. Author Ms. Shubhangi R. Pawar has received research support from the Mahatma Jyotiba Phule Research Fellowship (MJPRF-2022), Government of Maharashtra, India.

Ethics declarations:  Not applicable.

Conflict of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

REFERENCES

1. Parveen, K.; Banse, V.; Ledwani, L. Green Synthesis of Nanoparticles: Their Advantages and Disadvantages. 2016. https://doi.org/10.1063/1.4945168.
2. Balamurughan, M.G. & Sunderasan, Mohanraj & Kodhaiyolii, S. & Pugalenthi, V.. (2014). Ocimum sanctum leaf extract mediated green synthesis of iron oxide nanoparticles: Spectroscopic and microscopic studies. Journal of Chemical and Pharmaceutical Sciences. 2014. 201-204.
3. Abdullah, A.; Jiménez-Rosado, M.; Guerrero, A.; Romero, A. Effect of Calcination Temperature and Time on the Synthesis of Iron Oxide Nanoparticles: Green vs. Chemical Method. Materials 2023, 16 (5), 1798–1798. https://doi.org/10.3390/ma16051798.
4. Hassan, Y. M.; Zaid, H. M.; Guan, B. H.; Hamza, M. F.; Adam, A. A. Effect of Annealing Temperature on the Crystal and Morphological Sizes of Fe₂O₃/SiO₂ Nanocomposites. IOP Conference Series: Materials Science and Engineering 2021, 1092 (1), 012039. https://doi.org/10.1088/1757-899x/1092/1/012039.
5. Alagu, T. SYNTHESIS and IMPREGNATION of Fe2O3 NANOPARTICLES on CELLULOSE PAPER and SODIUM ALGINATE FILMS for the PRESERVATION of FRUIT and VEGETABLES. Journal of Microbiology, Biotechnology and Food Sciences 2020, 9 (6), 1166–1169. https://doi.org/10.15414/jmbfs.2020.9.6.1166-1169.
6. Thakur, M.; Poojary, S.; Swain, N. Green Synthesis of Iron Oxide Nanoparticles and Its Biomedical Applications. Nanotechnology in the Life Sciences 2021, 83–109. https://doi.org/10.1007/978-3-030-64410-9_5.
7. Chakraborty, A. R.; Tuz, F.; Alam, K.; Yousuf, S. B.; Hossain, K. S. Influence of Annealing Temperature on Fe₂O₃ Nanoparticles: Synthesis Optimization and Structural, Optical, Morphological, and Magnetic Properties Characterization for Advanced Technological Applications. Heliyon 2024, 10 (21), e40000–e40000. https://doi.org/10.1016/j.heliyon.2024.e40000
8. Federica Crippa; Rodriguez-Lorenzo, L.; Hua, X.; Goris, B.; Bals, S.; Garitaonandia, J. S.; Balog, S.; Burnand, D.; Hirt, A. M.; Laetitia Haeni; Lattuada, M.; Rothen-Rutishauser, B.; Alke Petri-Fink. Phase Transformation of Superparamagnetic Iron Oxide Nanoparticles via Thermal Annealing: Implications for Hyperthermia Applications. ACS Applied Nano Materials 2019, 2 (7), 4462–4470. https://doi.org/10.1021/acsanm.9b00823.
9. Khan, M. A.; Ahmed, M.; Abu-Hussien, S. H.; Zahid, M. U.; Alharbi, B. F. Green Synthesis of Iron Oxide Nanoparticles (Fe2O3-NPs) from Citrus Limetta Agrowaste for Biological and Photocatalytic Applications. Scientific Reports 2025, 15 (1). https://doi.org/10.1038/s41598-025-17750-3.
10. Singh, A. K. A Review on Plant Extract-Based Route for Synthesis of Cobalt Nanoparticles: Photocatalytic, Electrochemical Sensing and Antibacterial Applications. Current Research in Green and Sustainable Chemistry 2022, 5, 100270. https://doi.org/10.1016/j.crgsc.2022.100270.
11. N, G.; Michael, R. Green Synthesis of Iron Oxide Nanoparticles Using the Leaf Extract of Phyllanthus Niruri. Nanoscale Reports 2020, 3 (3). https://doi.org/10.26524/nr.3.18.
12. Buarki, F.; AbuHassan, H.; Al Hannan, F.; Henari, F. Z. Green Synthesis of Iron Oxide Nanoparticles Using Hibiscus Rosa Sinensis Flowers and Their Antibacterial Activity. Journal of Nanotechnology 2022, 2022, 1–6. https://doi.org/10.1155/2022/5474645.
13. Ibtihal Mimouni; Asmae Bouziani; Yassine Naciri; Mourad Boujnah; Alaoui, M.; Mohammed El Azzouzi. Effect of Heat Treatment on the Photocatalytic Activity of α-Fe2O3 Nanoparticles: Towards Diclofenac Elimination. Environmental Science and Pollution Research 2021, 29 (5), 7984–7996. https://doi.org/10.1007/s11356-021-16146-w.
14. Xu, X. N.; Wolfus, Y.; Shaulov, A.; Yeshurun, Y.; Felner, I.; Nowik, I.; Koltypin, Yu.; Gedanken, A. Annealing Study of Fe2O3 Nanoparticles: Magnetic Size Effects and Phase Transformations. Journal of Applied Physics 2002, 91 (7), 4611–4616. https://doi.org/10.1063/1.1457544.
15. Ndou, Nzumbululo & Rakgotho, Tessia & Nkuna, Mulisa & Ibrahima Zan, Doumbia & Mulaudzi-Masuku, Takalani & Ngece-Ajayi, Rachel. (2023). Green Synthesis of Iron Oxide (Hematite) Nanoparticles and Their Influence on Sorghum bicolor Growth under Drought Stress. Plants. 12. 1425. https://doi.org/10.3390/plants12071425
16. Aida, M.s & Alonizan, Norah & Zarrad, Boubaker & Hjiri, M.. (2023). Green synthesis of iron oxide nanoparticles using Hibiscus plant extract. Journal of Taibah University for Science. 17. https://doi.org/10.1080/16583655.2023.2221827
17. Jesus, Juliana & Regadas, Joana & Costa, Bárbara & Carvalho, João & Pádua, Ana & Henriques, Célia & Soares, Paula & Gavinho, Sílvia & Valente, Manuel & Graça, Manuel & Teixeira, S. (2024). Green Sol–Gel Synthesis of Iron Oxide Nanoparticles for Magnetic Hyperthermia Applications. Pharmaceutics. 16. 1578. https://doi.org/10.3390/pharmaceutics16121578
18. Edvinsson, T. Optical Quantum Confinement and Photocatalytic Properties in Two-, One- and Zero-Dimensional Nanostructures. Royal Society Open Science 2018, 5 (9), 180387. https://doi.org/10.1098/rsos.180387.