Biofilm Inhibition Against Staphylococcus aureus and Alizarin Red Dye-Removing Capability of Plant-Based Green Synthesis of Lanthanum Oxide (La₂O₃NPs) Nanoparticles

Abstract

The unique properties of lanthanum oxide nanoparticles (La₂O₃NPs) make them highly suitable for various environmental applications. This study explores the plant-based synthesis of La₂O₃NPs using Drypetes sepiaria as a reducing agent. The synthesized La₂O₃NPs were characterized through a range of spectroscopic and microscopic techniques. Scanning electron microscopy (SEM) revealed that the La₂O₃NPs have an uneven surface and a stony appearance. A morphological analysis indicated that the nanoparticles range in size from 20 to 50 nm. The appreciable band gap energy values were concluded as 5.5 eV. The crystal structure and elemental composition were determined using X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The results from the microplate assay method demonstrated enhanced anti-biofilm properties, and photocatalytic tests showed significant dye-degradation capabilities. The degradation efficiency and zone inhibition values of the La₂O₃NPs were found to be 90.12% and 39.18%, respectively.

1. Introduction

Biomolecules existing in plant materials having capability of act as both reducing and stabilizing agents, serving as an alternate to chemical reducing agents for converting metal ions into their respective nanoparticles in a single-step process known as the green synthesis method [1,2]. The biogenic reduction methodology of a metal ion is typically rapid, even at room temperature, and quite effortless [3,4]. The eco-friendly reducing agents used include various water- and alcohol-soluble plant metabolites and co-enzymes. In addition to the extracts from a wide variety of species, live plants have also been successfully used in nanoparticle synthesis, resulting in nanoparticles with a reduced size range [5,6,7]. Nanoparticles have gained considerable technical attention, as they act as bridges between bulk materials and molecular structures [4,8,9]. In recent years, nanotechnology has developed with multifunctional applications across a variety of engineering fields [10,11]. The majority of the research has mainly focused on the synthetic aspect of nanoparticles, as they can be easily prepared and utilized [12,13]. Although various physical and chemical methods have been conventionally used for the nanoparticles synthesis, recent research has shifted toward the advancement of environmental friendly synthesis approaches [6,14,15,16]. Recently, phyto-nanotechnology has emerged as a novel approach for single-step synthesis, offering a non-toxic, straightforward, robust, and inexpensive method [17,18]. This technique utilizes plant-derived nanoparticles by employing readily available plant materials and capitalizing on the non-toxic nature of plants, making it well-suited to meet the high demand of nanoparticles in biomedical and environmental applications [18,19,20]. Notably, various parts of plants, beyond just leaves, have been employed in the synthesis of metal nanoparticles (MNPs). Plant extracts containing flavonoids, alkaloids, polyphenols, terpenoids, heterocyclic compounds, and polysaccharides play significant roles in the synthesis of MNPs [21,22]. These compounds act as reducing agents for the conversion of metal ions and also serve as stabilizing, impregnating, and fabricating agents during synthesis methods [23,24]. Nickolas et al. reported the synthesis of silver nanoparticles using origanum vulgare, where the NaOH concentration played a dominant role in the synthesis of AgNPs [25]. Lanthanum oxide nanoparticles (La₂O₃NPs) are among the most optically active oxides of all other rare earth oxides. The wide band gap energy is typically greater than 4 eV [26,27,28]. Bhusankar et al. reported rod-shaped lanthanum oxide nanoparticles as efficient bifunctional electrocatalysts for hydrogen and oxygen evolution reactions. As a result, rod-shaped lanthanum oxide exhibits good electrocatalytic activity [29]. La-based materials are receiving increasing attention for their magnetic, optical, electrical, and therapeutic properties [30,31].

La₂O₃NPs have been synthesized in various compositions, including hydroxides, fluoride, carbonate, phosphates, borate, and oxides [32,33,34]. Normally, hydrothermal, chemical precipitation methods have been used to prepare La₂O₃NPs. To minimize the interaction of harmful substances, eco-friendly green synthesis methods have been employed [6,35,36]. This article mainly focuses on the application part of bio fabricated nanomaterials as effective anti-biofilm agents and photocatalytic applications. Biofilms are accumulations of bacterial cells; they cause infections and are problematic for components and environments. These infectious cells, along with their extracellular polymeric substances, adhere to both living and non-living surfaces, posing serious problems.

Engineering, such as food [37], water [38,39], dairy, textiles, concrete [40] constructions, metals like steel, iron, etc. [41,42], water purification units, and marine production are susceptible to biofilm formation. Although numerous traditional methods have been implemented to control these biofilms, most provide only short-term solutions. This highlights the need for advanced research to discover anti-biofilm agents. Many studies have reported the degradation of toxic dyes using various metal particles, such as Au, Ag, Co, Cu, Pd, and Pt [43,44,45]. In this context, plant-based synthesized La₂O₃NPs, with their unique properties and high efficiency, were explored as catalysts for the degradation of Alizarine red dye. Typically, photo-catalytic processes involve the generation of free radicals under light irradiation. However, sunlight penetration is often insufficient in heavily colored wastewater, such as dye-laden industrial effluents, which limits the effectiveness of photo-catalytic methods [46,47]. Alizarin Red dye, also known as 3,4-dihydroxy-9,10-dioxo-9,10-dihydroanthracene-2-sulfonic acid, is a water-soluble sodium salt of Alizarin sulfonic acid. It belongs to the category of azo dyes and finds widespread use in the dyeing of wool and nylon. In this context, biosynthesized La₂O₃NPs were utilized to investigate degradation, offering a promising solution for treating wastewater from the textile industry.

This study introduces the new plant-based synthesis of La₂O₃NPs using Drypetes sepiaria as a reducing agent, providing an eco-friendly alternative to conventional synthetic methods. Furthermore, the research highlights the unique structural and functional properties of the synthesized La₂O₃NPs, including their distinct stony morphology, a well-defined energy band gap of 3.2 eV, and excellent photocatalytic and biofilm activity. These findings demonstrate the dual potential of La₂O₃NPs for environmental applications, offering an innovative approach to addressing biofilm-related challenges and dye pollution simultaneously.

2. Materials and Methods

2.1. Materials

Lanthanum chloride (99.5%), sodium hydroxide (98%), and ethanol (99%) were procured from SRL Chemicals, Mumbai, India. Sodium acetate (99.5%) and crystal violet were obtained from Nice Chemicals (P) Ltd., Ernakulam, India, while Alizarin Red (ARS) dye was sourced from Loba Chemie (P) Ltd., Mumbai, India.

2.2. Qualitative Preliminary Phytochemical Screening

The powdered leaves of Drypetes sepiaria (DS) were collected from Pachaimalai hills, located near Tiruchirappalli district at 119° N, 78°210 E of Tiruchirappalli, Tamil Nadu, India. Initially, the leaves were thoroughly washed several times with running tap water and then dried under shade. Approximately, 100 g of the powdered leaves was soaked in 500 mL of ethanol using the cold percolation method [48]. The extract was filtered at regular intervals, and the crude extract was collected through distillation. The extract was preserved in an amber bottle and refrigerated. This prepared extract was utilized for phytochemical screening.

2.3. Synthesis of Lanthanum Oxide Nanoparticles Using Eco-Friendly Leaf Extract as a Reducing Agent

La₂O₃NPs were synthesized using a green synthetic process with a Drypetes sepiaria (DS) leaf extract. A stoichiometric amount of analytical-grade lanthanum chloride (0.1 M, 100 mL) was added 100 mL of the plant extract, and the reaction mixture was stirred continuously for 1 h [49]. Subsequently, 0.1 M NaOH was added to the reaction mixture to adjust the pH to above 12.5. A magnetic stirrer was used to maintain the reaction mixture at 70 °C for 3 h. During the reaction, a color change was observed, indicating the formation NPs. After the completion of the reaction, the resulting La₂O₃NPs were dried in a hot-air oven at 60 °C for 24 h. After drying, the NPs were calcined at 600 °C for 4 h. The resulting La₂O₃NPs were characterized using various spectroscopic and microscopic techniques. Their biofilm-inhibition activity and dye-degradation efficiency for waste water were subsequently evaluated [50].

2.4. Microplate Assay for Biofilm Inhibition

To assess the effectiveness of the medication in preventing biofilm formation, we used 96-well flat-bottom polystyrene plates for the MTP assay. Each well was filled with 180 µL of BHI broth, followed by the addition of 10 µL of an overnight culture of pathogenic bacteria as an inoculant. Additionally, 10 µL of the TR-FEND sample, prepared from stock solutions with concentrations ranging from 500 to 31.25 µg/mL, was added. The mixture was then incubated for 24 h at 37 °C. After incubation, the well contents were aspirated and washed with 0.2 mL of phosphate-buffered saline (PBS, pH 7.2) to remove free-floating bacteria. Sessile bacteria were fixed using 2% sodium acetate and stained with 0.1% w/v crystal violet. Excess stain was rinsed off with deionized water. The percentage of biofilm inhibition was calculated using the below-mentioned formula (Equation (1)) [51].

% Biofilm inhibition = [(Control OD − Test OD/Control OD)] × 100

(1)

2.5. Photocatalytic Dye Degradation

The photocatalytic efficiency of the La₂O₃NPs was tested based on the degradation of Alizarin Red (ARS) under visible light irradiation. Initially, 50 mL of Alizarin Red (10 mg/L) solution was taken in a beaker, and 30 mg of La₂O₃NP photocatalysts was added. The degradation of Alizarin Red dye was studied, through the elimination of the Alizarin Red dye. The photocatalytic behavior of the synthetic sample was determined using the Alizarin Red dye under UV or solar radiation. A 150 W Xe (Xenon) lamp was used as the light source, with a 10 cm gap between the UV source and the photo-reaction vessel. The suspensions were magnetically agitated for 30 min in the dark before being exposed to radiation. Next, under typical ambient conditions, UV irradiation was applied to the photoreaction vessel. In the photo-removal experiment, the dye of choice was used with the sample; 3 mL of the suspension was extracted at regular intervals of time to remove the catalyst particles using a centrifugal process, and then, UV-Vis absorption spectrometer was used. Using the equation below, the photo-removal efficiency percentage was calculated. Following that, five consecutive cycles of recycling experiments (0, 60, 120, 180, 240, and 300 min) were conducted to assess the durability of the photo-catalytic sample. Prior to being used again in the subsequent trial, the NPs were dried, centrifuged, and cleaned in ethanol and deionized water. The degradation efficiency was calculated using the following formula (Equation (2)) [52].

% Photo-removal efficiency = C₀ − C/C₀ × 100

(2)

where C₀ is the initial concentration of dye and C is the concentration of dye after photo-irradiation (final).

2.6. Characterization

To determine the absorbance, the synthesized La₂O₃NPs were exposed to a UV visible spectrometer (Perkin Elmer, Lambda 365). The morphological property was studied using SEM analysis, which clearly described the exact size of the synthesized La₂O₃NPs (JEOL Model JSM—6390LV). The functional group analysis was obtained from the FT-IR spectrometer (Thermo Nicolet, Avatar 370, Waltham, MA, USA). The crystalline structure of the sample was determined from the Bruker AXS Advance powder X-ray method. The elements present, along with their respective percentages, in the synthesized biofabricated La₂O₃NPs were explained using an elemental analysis (EDAX).

3. Results

The active phyto compounds present in the alcoholic leaf extract of Drypetes sepiaria (DS) are illustrated in

Table 1. Qualitative preliminary phytochemical screening of ethanolic extract of Drypetes sepiaria (DS).

Phyto-Constituents	Inference	Phyto-Constituents	Inference
Carbohydrates	+	Anthraquinone Glycosides	+
Reducing Sugar	+	Saponin Glycosides	+
Hexose Sugar	+	Cyanogenic Glycosides	−
Non-Reducing Sugar (Starch)	+	Alkaloids	+
Proteins	−	Tannins	+
Amino Acids	−	Phenolic Compounds	−
Tyrosine	−	Flavonoids	−
Steroids	+	Terpenoids	+
Glycosides	+	Saponins	+

+ Present, − absent.

.

3.1. Gas Chromatography-Mass Spectrum Study (GC-MS)

The mass spectrum was compared and identified with the database of the National Institute of Standard Technology (NIST). The results indicated twenty peaks, showing the presence of twenty chemical constituents with the retention time (RT), molecular formula, molecular weight (MW), and peak area (%) (Figure 1). The structures of the compounds were given (Figure 2). The prevailing compounds were 1,1,3-triethoxy-propane (35.88%), (2e, 6e)-1,1-dideutero-3,7,11-trimethyl-2,6,10-dodecatrien-1-ol (14.38%), alpha-tocopheryl acetate (10.72%), 1-hexadecanol (6.18%), and ethyl ester hexadecanoic acid (5.99%) (

Table 2. Chemical components identified in the ethanolic extract of the leaves of Drypetes sepiaria (DS) by GC-MS.

RT	Name of the Compound	Molecular Formula	Molecular Weight	Peak Area %
5.811	1,1-diethoxy-3-methyl- butane	C9H20O2	160	1.06
5.843	1,1-diethoxy- pentane	C9H20O2	160	1.36
6.025	3,3-diethoxy-2-butanone	C8H16O3	160	1.00
7.840	1,1,3-triethoxy- propane	C9H20O3	176	35.82
16.590	2,5-dimethyl-5-hexen-3-ol	C8H16O	128	1.22
18.739	5-cyclohexyl- undecane	C17H34	238	1.50
19.758	diundecyl ester 1,2-benzenedicarboxylic acid	C30H50O4	474	1.32
19.889	ethyl ester hexadecanoic acid	C18H36O2	284	5.99
20.851	1-hexadecanol	C16H34O	242	6.18
21.075	1-cyclopentylethyl ester pentanoic acid	C12H22O2	198	1.01
21.194	3,7-dimethyl-6-octen-1-ol	C10H20O	156	3.58
21.778	1,10-decanediol	C10H22O2	174	1.73
23.164	Hexyl(hexyloxy)methanethioate	C13H26O2S	246	1.19
23.824	cis-3-hexenyl-.alpha.-methylbutyrate	C11H20O2	184	1.65
25.867	tetrahydro-2h-pyran-2-one	C8H14O2	142	2.50
26.158	2-(2-propenyl)-1,4-butanediol	C7H14O2	130	2.45
26.322	alpha-tocopheryl acetate	C31H52O3	472	10.72
26.648	(2e,6e)-1,1-dideutero-3,7,11-trimethyl-2,6,10-dodecatrien-1-ol	C15H24D2O	224	14.38
28.284	10-methyl-6-trimethylsilyl-5(z),9-undecadien-1-otms	C18H38OSi2	326	1.76
28.757	dinonyl ester 1,2-benzenedicarboxylic acid	C26H42O4	418	3.57

).

A particle size analysis using Dynamic Light Scattering (DLS) indicated that the synthesized particles were approximately 60 nm in size. The presence of La₂O₃NPs was confirmed via UV spectroscopy (Figure 3), which showed a symmetric single-band absorption peak at 230.9 nm (λ max) that steadily increased in intensity over time, with no observed shift.

The diffuse reflectance and energy spectra of bio-fabricated La₂O₃NPs synthesized using Drypetes sepiaria (DS) showed major peaks at 230 nm, as depicted in Figure 3a. Additionally, a few weak peaks were observed, possibly due to external factors, such as surface traps, shape defects, or impurities. The direct band gap energy values were concluded to be 5.5 eV for biosynthesized La₂O₃NPs and was given by Wood and Tauc’s relation [53]. The appreciable band gap energy value was clearly due to the existence of smaller-particle-size nanoparticles of biosynthesized La₂O₃NPs.

The SEM investigation (Figure 4) indicated that as the temperature increased, the grain size increased, leading to crystal-like grain growth with a grain size of 1 µm. The surface morphology was found to be a spherical shape with an uneven, porous surface, suggesting the possibility of the rapid release of byproducts during the progression of the reaction.

3.2. FT-IR

The FTIR spectra of the green synthesis of the plant extract (PE) and La₂O₃NPs are shown in Figure 5. In the FTIR spectrum of PE, the downshift of the O-H frequency from 3600 to 3434.12 cm⁻¹ denoted hydrogen bonding. The peaks at 2928.19 cm⁻¹, 2862.41 cm⁻¹ represent the presence of –C-H- bonding. The peak observed at 1586.16 cm⁻¹ indicates the presents of an NO₂ stretch. The peaks at 1627.43 cm⁻¹ and 1640.88 cm⁻¹ indicate the presents of alkene bonding. The peak at 1414.90 cm⁻¹ indicates the presents of a CH₂ bend, and 1336.90 cm⁻¹ indicates the presents of the -CH₃ bend. The observed peaks at 1022.15 cm⁻¹ and 1049.20 cm⁻¹ indicate the presence of C-F bonds. Additionally, peaks at 864.62 cm⁻¹, 713.06 cm⁻¹, and 703 cm⁻¹ indicate the presents of carbon and halogen bonding. The same characteristic peaks appeared in the plant extract stabilized La₂O₃NPs. Furthermore, an additional peak at 514.86 cm⁻¹ appeared due to the metal–oxygen bond, thus confirming that the La2O3NPs were stabilized by the plant extract [54].

3.3. XRD Analysis and EDAX Analysis

XRD analysis plays a vital role in determining the microstructure property of different phases of materials (Figure 6). For La₂O₃NPs, the observed 2θ peaks were at 25°, 30°, 40°,44°, and 57°, which corresponded to plans of (100), (101), (102), (110), and (103), respectively. The peaks obtained were compared with Joint Committee for Powder Diffraction studies (JCPDS) card No: 05–0602, which shows that the synthesized La₂O₃NPs have a hexagonal structure.

Equations (1) and (2) represent the Debye–Scherer equation and Bragg’s law, which were used to calculate the particle size [55].

Debye-Scherer equation (D) = 0.9λ/β cosθ

(3)

Bragg’s Law (nλ) = 2dsinθ

(4)

In this context, λ denotes the wavelength of X-rays (0.154 nm), β represents angular or line broadening at full width at half maximum (FWHM), θ stands for the diffraction angle, D denotes the particle diameter in nanometers, and d refers to planar spacing. The elements and their respective percentages, as shown in the table indexed in Figure 7, are depicted in the EDAX analysis. The EDAX analysis reveals that the sample contains a reasonable number of C, O, and La atoms.

The HRTEM analysis was used to examine the surface of biosynthesized La₂O₃NPs at different magnifications (Figure 8). The HRTEM images provide detailed morphological and structural information regarding the La₂O₃NPs. Figure 8a shows a higher magnification image, where individual particle sizes are more distinguishable. The particles exhibit a relatively uniform size distribution, with dimensions ranging between 28 nm to 32 nm, as indicated by the measurements. However, the overall morphology suggests that the particles are slight aggregates of varying sizes. This is consistent with the expected size range obtained from the synthesis method, further confirming the nano-scale nature of the particles.

Figure 8b provides a closer look at the crystalline structure of the particles, where the lattice fringes are clearly visible. The interplanar spacing (d-spacing) of 0.248 nm, as marked in the image, corresponds to the characteristic crystallographic planes of the synthesized material, confirming the crystalline nature of the nanoparticles. Figure 8c shows the selected area electron diffraction (SAED) pattern, which consists of a series of concentric rings. This pattern indicates that the nanoparticles possess a polycrystalline structure. The well-defined diffraction spots further confirm the crystallinity observed in the high-resolution TEM image (Figure 8b), supporting the notion that the particles are crystalline in nature with some degree of an ordered structure. Further, they provide a comprehensive understanding of the particle size distribution, a particle size histogram was constructed, as shown in Figure 8d. The histogram indicates a relatively narrow size distribution, with most particles falling within the range of 30 ± 2 nm. This consistent size range suggests that the synthesis process using Drypetes sepiaria as a reducing agent facilitated controlled and uniform nanoparticle formation. The particle sizes obtained through the plant-based green synthesis method result in La₂O₃NPs with a uniform size distribution, crystalline structure, and favorable morphology, making them well-suited for photocatalytic and anti-biofilm applications.

The specific surface area of the La₂O₃NPs was obtained using the Brunauer–Emmett–Teller (BET) analysis, as shown in Figure 9, based on the N₂ adsorption/desorption isotherm [56]. The observed shape of the isotherm suggests Type IV behavior according to the IUPAC classification, which is characteristic of mesoporous materials with well-defined pores. The specific surface area of the La₂O₃NPs was found to be 49.24 m²/g. Furthermore, the pore volume and pore diameter of the La₂O₃NPs were determined to be 0.0817 cm³/g and 6.64 nm, respectively. Additionally, the average pore size distribution was calculated using the BJH (Barrett–Joyner–Halenda) method, which is a procedure for determining pore size distribution based on the Kelvin equation and DH methods. The average pore size distribution of the La₂O₃NPs was found to be 4.10 nm. The obtained pore size distribution results reveal that the La₂O₃NPs exhibit a broad range of pore diameters, with a peak in the mesoporous range at around 4.10 nm. Moreover, the green-synthesized La₂O₃NPs provide more active sites for light absorption and catalysis, thereby enhancing the photocatalytic degradation of organic dyes, such as ARS.

3.4. Biofilm-Inhibition Activities

The percentages of biofilm inhibition were calculated and tabulated. The OD value at 600 nm with various concentrations of synthesized bio-fabricated nanoparticles is represented in

Table 3. OD value at 600 nm; control mean OD value: 1.556.

Tested Sample Concentration (μg/mL)	OD Value at 600 nm (in Triplicate)
Control	1.577	1.532	1.561
500 µg/mL	0.978	0.874	0.987
250 µg/mL	1.032	1.036	1.060
125 µg/mL	1.198	1.335	1.315
62.5 µg/mL	1.363	1.372	1.335
31.25 µg/mL	1.495	1.486	1.494

, which shows the inhibition of bio-film formation against Staphylococcus aureus, with a clear inhibition zone caused by the antibacterial properties of the synthesized La₂O₃NPs, as observed in the in vitro results (Figure 10a,b).

The optical density (OD) values were also observed, showing that the OD values increase with a decrease in the concentration of the samples [31]. The percentage of inhibition was higher at the higher concentration of 500 µg/mL and decreased with a lower concentration (

Table 4. Percentage of inhibition.

Tested Sample Concentration (μg/mL)	Percentage of Inhibition (in Triplicate)			Mean Value (%)
Control	100	100	100	100
500 µg/mL	37.14	43.83	36.56	39.18
250 µg/mL	33.67	33.41	31.87	32.99
125 µg/mL	23.00	14.20	15.48	17.56
62.5 µg/mL	12.40	11.82	14.20	12.81
31.25 µg/mL	3.92	4.49	3.98	4.13

). The synthesized bio-fabricated La₂O₃NPs demonstrated enhanced biofilm-inhibition capability.

3.5. Photocatalytic Dye-Degradation Activities

During the photodegradation process, color changes in the dye solution were observed at different time intervals, as shown in Figure 11. Figure 12a illustrates the photodegradation performance of biosynthesized La₂O₃NPs for Alizarin Red (ARS) dye under visible light illustration. The concentration ration ratio C/C₀ of ARS steadily decreased over time, indicating a progressive degradation process. The concentration ratio (C/C₀) of ARS steadily decreased over time, indicating a progressive degradation process. A degradation efficiency of 90.12% was achieved within 5 h, demonstrating the effective photocatalytic activity of the La₂O₃ NPs. Figure 12b further confirms the kinetics of the degradation process by plotting the −ln(C/C₀) vs. as a function of time. The linearity of the data suggests that the degradation follows pseudo-first-order kinetics, as evidenced by the excellent fit, with an R² value close to unity. Further, the rate constant values were found to be 6.19 × 10⁻², indicating a rapid reaction rate under the given experimental conditions. These results highlight the potential of La₂O₃NPs as effective photocatalysts for the degradation of organic pollutants like ARS, which is attributed to their enhanced light absorption and active surface properties. The linear kinetic trend and high-rate constant underline the nanoparticles’ promising role in environmental remediation applications.

The reusability of La₂O₃NPs was evaluated over five consecutive cycles for the photodegradation of Alizarin Red, as depicted in Figure 13. The results demonstrate that the degradation efficiency remained consistently high, with only a slight decrease observed with repeated use. In the first cycle, the degradation efficiency was 90.12%, and even after five cycles, it remained at 84.24%, indicating the excellent stability and reusability of the catalyst. The reusable property was undergoing the practical applicability of La₂O₃NPs as a cost-effective and sustainable photocatalyst for the treatment of organic dyes like Alizarin Red in wastewater treatment. These findings reveal that La₂O₃NPs possess significant potential for long-term environmental applications with minimal performance degradation over repeated cycles. Further, the photocatalytic activity of the La₂O₃NP photocatalyst was compared with various types of catalysts and provided in

Table 5. Comparisons of photodegradation of ARS using various photocatalysts.

Samples	Quantity of ARS (mg/L)	Dosage of Photocatalyst (mg)	Photocatalytic Duration	Light Source	Degradation Efficiency (%)	References
MgO@ ZrO2@g-C3N4	25	50	60	OSRAM lamp 58 IM/W	92%	[57]
CuO NPs	40	100	120	Solar light	95.4	[58]
SnO2	3.5	5	120	Sun light	39	[59]
G-TiO2@Fe3O4/PS/Vis	100	250	60	Visible light	100	[60]
Ce3+/Ce4+/Bi2O3/Vis	20	1000	120	Visible light	78	[61]
SnO2-NP-AC	35	75	60	Sun light	99	[62]
Cd–ZnS	4	500	200	Sunlight	96.7	[63]
La2O3	10	50	300	150 W Xe	90.12	Present study

.

A photocatalytic mechanism of Alizarin Red (ARS) using La₂O₃NPs as a photocatalysts under sunlight irradiation is depicted in the Figure 14. As is known to all, the electrons are excited and transferred from the valence band (VB) to the conduction band (CB) with the illustration of light. The photogenerated charge carriers plays an crucial role in the photodegradation process. The electrons (e⁻) in the CB combined with dissolved oxygen (O₂) to generated superoxide radicals O₂•⁻. The superoxide radical further converted into hydroxyl radicals (OH^•) in in the presence of protons (H+), which are highly effective oxidizing agents. Instantaneously, the holes (h⁺) in the VB react with water (H₂O) to produce another hydroxyl radical. The combined form of reactive oxygen species helps to break down the toxic ARS molecule in non-toxic degraded products. This mechanism highlights the high photocatalytic efficiency of La₂O₃NPs, attributed to its ability to generate and separate charge carriers effectively under sunlight. The production of hydroxyl radicals as the primary oxidative species ensures the rapid degradation of ARS, showcasing the potential of La₂O₃NPs as a sustainable photocatalyst for environmental remediation applications.

4. Conclusions

This study demonstrates a novel green synthetic route for the synthesis of La₂O₃NPs using an alcoholic extract of Drypetes sepiaria (DS). The bio-synthesized La₂O₃NPs exhibit unique structural and functional properties, including a hexagonal crystalline structure, a significant band gap energy of 5.5 eV, and a porous spherical morphology with a stony-like structure. These characteristics, confirmed through UV, FT-IR, XRD, EDAX, and surface analysis, highlight the potential of Drypetes sepiaria as a sustainable resource for nanomaterial synthesis. The 90.12% of degradation efficiency was achieved within 5 h. The green synthesized La₂O₃NPs have good recycling efficiency, and the degradation efficiency remains constant up to the 5th cycles. Further, zone inhibition values (39.18%) demonstrate that the green synthesized La₂O₃ has good antibacterial efficiency against S. aureus. Furthermore, the nanoparticles show promising applications, evidenced by their efficient bio-scavenging activity and superior dye-absorption capabilities, making them a valuable candidate for environmental and biotechnological applications.