Plant-Mediated Green Synthesis, Characterization And Applications Of Cr2O3 And Mgo Nanoparticles

Alpinia officinarum, a medicinal rhizomatous herb of the Zingiberaceae family, contrasts significantly with inorganic materials such as chromium and magnesium oxide in origin, composition, and applications.[5] While A. officinarum is rich in diverse bioactive phytochemicals including flavonoids, diarylheptanoids, and phenylpropanoids conferring antioxidant, antimicrobial, and reproductive health benefits, chromium is a transition metal primarily valued for its physicochemical properties such as high hardness, corrosion resistance, and multiple oxidation states (+2, +3, +6), enabling its extensive use in metallurgy, electroplating, and pigments.[4,3,6] In contrast, magnesium oxide (MgO), a thermally stable inorganic compound with a cubic crystal structure, exhibits strong basicity, high melting point, and notable biocompatibility, making it suitable for applications in refractory materials, medicine, and nanotechnology. Importantly, both A. officinarum and MgO share emerging relevance in green nanotechnology,[1,2] where plant-derived phytochemicals and MgO nanoparticles demonstrate significant antimicrobial and anticancer activities. Additionally, A. officinarum shows protective effects against oxidative stress and chemotherapy-induced toxicity, whereas chromium compounds, particularly in higher oxidation states, may exhibit toxicological concerns despite their industrial importance. Overall, this comparison highlights the distinction between a bioactive medicinal plant and inorganic materials while emphasizing their converging roles in advanced biomedical and nanotechnological applications.[7]

MATERIALS & METHODS

a. Collection of plants

The Alpinia officinarum plant was collected from the Municipal Corporation Nursery in Thoothukudi during the month of July. The leaves were separated from the plant, thoroughly washed with double distilled water to remove surface impurities and dried in the dark at room temperature for approximately 15 days.

b. Preparation of Leaf Extract

The leaves were smashed into a coarse powder using a blender. The leaf powder was stored in a dry, airtight container away from direct sunlight. About 5 g of dry powder was mixed with 100 mL of double distilled water and kept in a magnetic stirrer for 45 minutes. The herbal extraction was cooled to 37^oC before being filtered using Whatman filter paper. The purified leaf extract was stored at 4°C for further nanoparticle (NP) synthesis.

c. Synthesis of Cr₂O₃ Nanoparticles

A clear solution containing 40 mL of plant extract and 0.2 M chromium chloride salt solution was placed in a 250 mL beaker. Three drops of NaOH were added and the mixture was magnetically stirred for 1 hour at 100°C. The formation of nanoparticles (NPs) was monitored by observing the color change of the solution. After allowing the solution to cool, the formed Cr₂O₃ nanoparticles were collected and dried in an oven at 80°C. The dried nanoparticles were then collected for further characterization and applications. The synthesized Cr₂O₃ nanoparticles prepared using Alpinia officinarum leaf extract were designated as throughout the study.

d. Synthesis of MgO Nanocomposite

To 40 mL of Alpinia officinarum leaf extract, 10 mL of 0.1 M Mg(NO₃)₂ solution was added and the mixture was stirred continuously for 45 minutes. Subsequently, 6.0 mL of 0.2 M NaOH was added dropwise to the mixture to form a visible precipitate. After the addition of NaOH, a color change from dark yellow to light yellow was observed. The mixture was then aged overnight. After aging, the solution was kept in an oven at 150°C for 4 hours until it dried and formed a powder. The obtained powder was then annealed at 500°C for 2 hours and collected for further characterization and applications.

RESULTS AND DISCUSSION

a. UV -Visible analysis

The UV–Visible absorption spectra of chromium oxide (Cr₂O₃) and magnesium oxide (MgO) nanoparticles reveal comparable optical characteristics, with both materials exhibiting strong absorption bands in the ultraviolet region at approximately 266 nm and ~270 nm, respectively, attributed to intrinsic electronic transitions. In both cases, the absorbance markedly decreases beyond 400 nm, indicating negligible interaction within the visible region and suggesting their potential applicability in UV-based optical systems. Notably, Cr₂O₃ nanoparticles exhibit an additional absorption band around 353 nm, reflecting more complex electronic transitions, whereas MgO nanoparticles display a relatively simple spectral profile with a single dominant absorption peak. These observations confirm the successful synthesis of nanoscale materials, with MgO nanoparticles (10–40 nm) demonstrating a sharper and more defined absorption feature compared to the broader spectral response observed for Cr₂O₃ nanoparticles [8,14].

b. FT-IR Analysis

The FT-IR spectra of green-synthesized chromium oxide (Cr₂O₃) and magnesium oxide (MgO) nanoparticles, recorded in the range of 4000–400 cm⁻¹, exhibit several common features that confirm the role of biomolecules in nanoparticle formation and stabilization. In both spectra, a broad absorption band around ~3395–3396 cm⁻¹ corresponds to O–H stretching vibrations, while bands near ~2920 cm⁻¹ are attributed to aliphatic C–H stretching, indicating the presence of organic residues from the plant extract. Additional peaks observed in the regions ~1600–1700 cm⁻¹ and ~1000–1100 cm⁻¹ in both materials are associated with carbonyl (C=O), aromatic (C=C), and C–O/C–O–C functional groups, suggesting the involvement of phytochemicals as reducing and capping agents. However, a distinct band at ~552 cm⁻¹ in Cr₂O₃ nanoparticles corresponds to Cr–O–Cr stretching, confirming the formation of chromium oxide nanostructures, whereas MgO nanoparticles are characterized by metal–oxygen bonding in the lower wavenumber region without a similarly prominent isolated peak. Overall, the FT-IR analysis confirms the successful synthesis of both nanomaterials and highlights the presence of surface-bound biofunctional groups, with Cr₂O₃ showing more defined metal–oxygen vibrational features and MgO exhibiting a broader range of organic functional group interactions [9,15].

Figure 2: FT-IR Spectrum of a) Cr₂O₃ nanoparticles, b) MgO nanoparticles

c. X-RAY Diffraction Studies   The X-ray diffraction (XRD) patterns of the synthesized chromium oxide (Cr₂O₃) and magnesium oxide (MgO) nanoparticles confirm their crystalline nature and successful formation at the nanoscale. In both cases, well-defined and sharp diffraction peaks were observed, indicating high crystallinity and phase purity. The crystallite sizes, calculated using the Debye–Scherrer equation, with Cr₂O₃ exhibiting an average size of ~28 nm and MgO showing a smaller size of ~13.1 nm. However, distinct differences in crystal structure were evident: Cr₂O₃ nanoparticles displayed a rhombohedral phase with characteristic reflections corresponding to planes such as (012), (104), and (110), whereas MgO nanoparticles exhibited a cubic crystal structure with diffraction peaks indexed to planes including (111), (002), and (222). Additionally, the prominent

(104) peak in Cr₂O₃ suggests preferential orientation, while MgO demonstrates a polycrystalline nature with uniformly distributed peaks.

d. Field Emission Scanning Electron Microscopy (FESEM)

Field Emission Scanning Electron Microscopy (FE-SEM) analysis of the synthesized chromium oxide (Cr₂O₃) and magnesium oxide (MgO) nanoparticles reveals notable similarities in their surface morphology, with both materials exhibiting predominantly quasi-spherical particle shapes and significant agglomeration. In both cases, the particles form large aggregated clusters, with sizes ranging from approximately 500 nm to 10 μm and an average particle size of ~5.25 μm, indicating strong interparticle interactions during the synthesis process. However, MgO nanoparticles display a more distinct cauliflower-like clustered morphology with a relatively rough surface texture, as observed across multiple magnifications, whereas Cr₂O₃ nanoparticles exhibit a comparatively granular morphology with less pronounced structural complexity. These observations confirm that both nanomaterials are composed of aggregated nanoscale units forming microscale clusters, with MgO showing more defined surface structuring and Cr₂O₃ presenting a simpler granular appearance [11,17].

e. EDAX ANALYSIS

Energy Dispersive X-ray (EDAX) analysis of the synthesized chromium oxide (Cr₂O₃) and magnesium oxide (MgO) nanoparticles confirms that both materials are predominantly composed of their respective metal and oxygen elements, demonstrating successful formation and high purity. In both cases, the major elements (Cr and O for Cr₂O₃; Mg and O for MgO) exhibit significant weight and atomic percentages consistent with their expected stoichiometry, while minor trace elements such as sodium and other impurities are present in small amounts without contributing to secondary phase formation. Elemental mapping for both nanomaterials reveals a uniform distribution of the principal elements, indicating homogeneous composition and effective synthesis. However, Cr₂O₃ nanoparticles show relatively higher metal weight contribution compared to oxygen, whereas MgO nanoparticles display a more balanced elemental ratio between Mg and O. Additionally, MgO contains a slightly broader range of trace elements (Na, K, Ca) compared to Cr₂O₃ (Na, Cl). Overall, the EDAX results validate the successful synthesis of well-dispersed, high-purity nanostructures for both materials, with consistent elemental distribution and minimal impurity interference [12,19]

f. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) of the synthesized chromium oxide (Cr₂O₃) and magnesium oxide (MgO) nanoparticles demonstrates distinct thermal behaviors while confirming the stability of both materials at elevated temperatures. In both cases, an initial weight loss at lower temperatures is attributed to the removal of physically adsorbed moisture and surface-bound volatile species, followed by further weight reduction due to the decomposition of residual organic compounds originating from the synthesis process. However, Cr₂O₃ nanoparticles exhibit significantly higher thermal stability, with minimal total weight loss (less than ~5%) up to 1000 °C, indicating a robust and stable crystalline framework. In contrast, MgO nanoparticles display a more pronounced multi-step weight loss, particularly in the 250–500 °C range, associated with substantial decomposition of organic residues and intermediate degradation processes. Additionally, MgO shows continued weight reduction at higher temperatures, reflecting secondary decomposition and residue oxidation, whereas Cr₂O₃ undergoes only minor changes, suggesting structural stabilization. Overall, while both nanomaterials retain their inorganic frameworks at high temperatures, Cr₂O₃ demonstrates superior thermal stability compared to MgO, making it more suitable for demanding high-temperature applications [13,18]

Figure 6: TGA of the synthesized a) Cr₂O₃ nanoparticles, b) MgO nanoparticles

Table 1: Elemental composition of Cr₂O₃ and MgO nanoparticles

g. Antibacterial activity

The antibacterial potential of chromium oxide (Cr₂O₃) and magnesium oxide (MgO) nanoparticles was evaluated using the disc diffusion method against Gram-positive (Staphylococcus aureus, Bacillus subtilis, Bacillus cereus) and Gram-negative (Escherichia coli, Klebsiella pneumoniae) bacteria, with ciprofloxacin as a standard control. Both nanoparticles exhibited measurable zones of inhibition, confirming their antimicrobial activity, though with varying efficacy. Comparative analysis revealed that MgO nanoparticles displayed superior antibacterial activity relative to Cr₂O₃. MgO showed the highest inhibition against Bacillus cereus (15 mm) and Klebsiella pneumoniae (14 mm), whereas Cr₂O₃ demonstrated comparatively lower activity across all tested strains (Table 5.9). The enhanced activity of MgO is likely due to its higher surface reactivity, generation of reactive oxygen species (ROS), and stronger interaction with bacterial cell membranes. In contrast, the relatively inert oxide structure of Cr₂O₃ and lower ROS production may account for its reduced antibacterial performance. Despite both nanoparticles showing lower activity than ciprofloxacin, the presence of distinct inhibition zones confirms their antimicrobial potential. Overall, MgO nanoparticles exhibited more consistent and pronounced antibacterial effects compared to Cr₂O₃, highlighting their suitability for applications requiring antimicrobial functionality.

Figure 7: Antibacterial activity of Cr₂O₃ and MgO nano in a) E.coli b) Staphylococcus aureus c) Bacillus subtilis   d) Bacillus cereus e) Kleibsiella pneumonia

Table 2: Antibacterial activity of Cr₂O₃ and MgO nanoparticles

h. Anticorrosion Activity

The anticorrosion performance of chromium oxide (Cr₂O₃) and magnesium oxide (MgO) nanoparticles was systematically evaluated in acidic, basic, and neutral media. Both nanoparticle treatments significantly reduced the corrosion rate of the metal substrate relative to untreated samples, demonstrating their protective capability. In acidic medium, Cr₂O₃ and MgO exhibited anti-corrosion efficiencies of 40.85% and 22.23%, respectively, whereas in basic medium, the efficiencies were 11.62% for Cr₂O₃ and 60.00% for MgO. In neutral medium, the protective effect increased, with efficiencies of 41.30% for Cr₂O₃ and 73.31% for MgO. The observed reduction in corrosion is attributed to the formation of a stable barrier layer that limits direct contact between the corrosive environment and the metal surface. Notably, corrosion rates were lowest under neutral conditions, indicating greater stability of the nanoparticle coatings, while acidic and basic media induced comparatively higher corrosion due to their aggressive nature.

Comparative analysis reveals that MgO-treated samples consistently outperformed Cr₂O₃-treated samples across all media, likely due to enhanced surface coverage, higher surface reactivity, and the formation of a more uniform protective layer. Cr₂O₃ nanoparticles also contributed to corrosion resistance through passive oxide layer formation, although their effectiveness was lower under identical conditions. Overall, both Cr₂O₃ and MgO nanoparticle coatings improved the corrosion resistance of the metal substrate, with MgO demonstrating superior performance (Tables 5.10–5.12; Figures 5.20–5.22).

Table 3: Anti-correction activity of acidic medium

Figure 8 Anticorrosion Activity of Cr2O3 and MgO Nanoparticle in acidic medium

Table 4: Anti-correction activity of acidic medium

Figure 9 Anticorrosion Activity of Cr2O3 and MgO Nanoparticle in basic medium

Table 5 Anti-correction activity of Neutral

Figure 10 Anticorrosion Activity of Cr2O3 and MgO Nanoparticle in neutral