Green synthesis of titanium dioxide nanoparticles using green tea (Camellia sinensis) extract: Characteristics and applications

2.1 Chemicals

The chemical synthesis of materials in this study includes the following: Folin Ciocalteu’s phenol reagent (2,2-diphenyl-1-picrylhydrazyl [DPPH], C₁₈H₁₂N₅O₆) and titanium(iv) isopropoxide (C₁₂H₂₈O₄Ti, 99%), which were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA).

Sodium hydroxide (NaOH, 99%), Potassium acetate (CH₃COOK, 99%), and ethanol (C₂H₅OH, 99.7%) were obtained from Xilong Chemical Co., Ltd. (Shantou, China).

Cosmetic chemicals, including non-ionic surfactants, emollients, moisturizers, and preservatives, were purchased from Nguyen Ba Trading Production Co., Ltd, Tan Binh District, HCMC, Vietnam.

2.2 Plant materials

Green tea is harvested in Lam Dong province, Vietnam, and then transported to the laboratory. The materials are pre-treated to remove dirt, damaged leaves, and cleaned with running water to remove any remaining chemicals from the surface. Tea leaves are completely dried to avoid the growth of bacteria and mold using specialized drying equipment at a temperature of 50°C for an appropriate time.

2.3 Extraction process of the green tea plant

After drying, tea leaves can be ground into powder or left whole and then stored in airtight containers to avoid exposure to air and moisture. Take 1 g of ground green tea leaf powder and add 100 mL of solvent (water, ethanol, or water:ethanol in a 1:1 ratio). This solution is then stirred at 300 rpm for 90 min at a temperature of 50°C. The solution is then filtered to recover the extract for use in the synthesis of TiO₂ nanoparticles.

2.4 Green synthesis of TiO₂ nanoparticles using green tea leaf extract

The synthesis of TiO₂ nanoparticles using 20 mL of green tea leaf extract and 20 mL of titanium tetraisopropoxide (0.1 mol·L⁻¹) is based on the research of Pushpamalini et al. [18]. The ingredients were stirred well using a magnetic stirrer maintained at 60°C for 5 h. The solution was then cooled to reach room temperature (30°C). The resulting suspension was separated by centrifugation at 6,000 rpm for 15 min. After the reaction process, continue washing with ethanol solvent to remove impurities. The product is then dried at 120°C overnight. Subsequently, the sample was calcined in a JeioTech furnace at 600°C for 3 h. The TiO₂ nanoparticles were cooled to room temperature, and their structural characteristics were analyzed.

2.5 Characterization of TiO₂ nanoparticles

Characterization of TiO₂ nanoparticle structure was performed through various analytical methods to determine important properties such as particle size, surface morphology, crystal structure, and chemical properties. Details of the surface structure, size, and morphology of the nanoparticles were analyzed using a scanning electron microscope (SEM) S4800 (JEOL, Japan). The D8 Advanced equipment (Bruker, Germany) with Cu Kα radiation (λ = 1.5418 Å) and a 2θ range from 10° to 70° was used for X-ray diffraction (XRD) analysis to determine the crystal structure and lattice parameters. Fourier transform infrared spectroscopy was conducted on a Nicolet 6700 (Thermo Fisher Scientific, USA) to determine functional groups and chemical bonds in TiO₂. The N₂ adsorption–desorption isotherm method based on the Brunauer–Emmett–Teller (BET) theory was used to determine the specific surface area and pore size distribution of nanoparticles, performed on Micromeritics 2020 equipment (USA). A UV–Vis spectrophotometer (Metash UV-5100, China) was used to analyze the phytochemical composition of green tea extract.

2.6 Antibacterial activity

Antibacterial activity was evaluated using the agar disk diffusion method with two bacterial standards, Gram-negative (E. coli ATCC 8739) and Gram-positive (Staphylococcus aureus ATCC 6538). Bacteria were activated in Mueller Hinton medium for 24 h with a bacterial density of approximately 10⁶–10⁷ CFU·mL⁻¹ [19]. Different concentrations of the sample solution were pipetted into 8 mm diameter wells on agar plates spread with test bacteria. Chloramphenicol (1 mg·mL⁻¹) was used as a positive control and solvent as a negative control. After 24 h, results were recorded by image and diameter of the sterile zone. The minimum inhibitory concentration (MIC) value was determined as the lowest concentration of TiO₂ particles that inhibited bacterial growth (no change in color of the resazurin reagent in the well) [20].

2.7 Application orientation for foaming face wash products

For the preparation of foaming facial cleanser products, the process typically involves carefully combining the oil and water phases to create a stable and effective product based on the formula provided in Table 1. The oil phase preparation includes weighing emollients, emulsifiers, thickeners, preservatives, and oil-soluble active ingredients, heating the mixture to 70–75°C until all ingredients are melted and homogenous. The water phase preparation involves mixing water-soluble ingredients such as humectants, surfactants, chelating agents, and preservatives, heating the mixture to the same temperature as the oil phase, and stirring continuously until all ingredients are dissolved.

2.8 Physicochemical properties of raw materials and cosmetic products

Parameters for evaluating the physical and chemical properties and biological activity of raw materials and cosmetic products are shown in Table 2.

2.9 Analyzing SPF value

The SPF value is analyzed based on the level of skin protection from UVB rays in the absorption range from 290 to 320 nm (UVB range). The sample was prepared at a concentration of 0.2 g and titrated to 100 mL with ethanol solvent. Based on the publication by Mansur et al. [23], the absorbance results were used to determine the SPF value according to the formula:

where EE (λ) × 1 is the erythemal effect of radiation with wavelength λ, Abs (λ) is the value of spectral absorbance at wavelength λ, and CF is the correction factor [10]. The values of EE × λ are constant.

2.10 Photocatalytic activity

Tetracycline is an antibiotic used to evaluate the photocatalytic ability of TiO₂ in aqueous medium. The light source used is a 40 W LED lamp. At room temperature, the reaction system is carried out with a solution mixture including a catalyst sample (5 mg) and tetracycline (20 ppm) stirred in a 250 mL beaker. About 1 mL of the solution mixture is taken out at a time point, including stirring in dark and light conditions. The sample after being taken out is centrifuged at 6,000 rpm for 10 min to completely remove solids. Then, the concentration of the solution is determined using a UV–Vis spectrophotometer, at the maximum absorption wavelength λ = 357 nm.

3.1 Physicochemical of green tea extract

To date, conventional organic solvents such as hexane, acetone, and chloroform remain the most commonly used solvents for extracting bioactive compounds from plant materials. However, some organic solvents have distinct disadvantages, such as high cost, residue levels, and toxicity [24]. Recently, the concepts of ecological sustainability and a green economy have sparked interest in solvents like water or ethanol as potential substitutes for organic solvents.

Green tea extraction can be performed using various solvents to obtain desired chemical compounds. Common and effective solvents for extracting polyphenols and flavonoids and determining antioxidant activity are listed in Table 3. The extraction efficiency is presented in Table 4, showing the highest TFC, total polyphenols content (TPC), and DPPH content when extracted with ethanol solvent as 12.425 ± 0.941, 351.45 ± 7.546, and 109.757 ± 0.184, respectively, compared to water solvent and a water–alcohol solvent mixture.

Green tea is rich in polyphenols that can reduce metal ions to form metal nanoparticles. Polyphenols can bind to the surfaces of nanoparticles, stabilize them, and prevent agglomeration. Phenolic groups in polyphenols form strong interactions with metal atoms on the nanoparticle surfaces, helping maintain their size and stability, making them durable, and preventing agglomeration of the resulting nanoparticles. Polyphenols in green tea extract can interact with the surface of TiO₂ nanoparticles, preventing agglomeration through electrostatic and/or steric stabilization mechanisms. This is due to the chelating (binding) ability of polyphenols with metal ions, as well as hydrogen interactions between the hydroxyl group of polyphenols and the TiO₂ surface [25,26].

3.2 Characterization of TiO₂ material from green tea extract

The XRD pattern of the synthesized TiO₂ clearly shows the crystal structure of the nanoparticles (Figure 1). Sharp diffraction peaks are observed at 2θ values of 25.47°, 27.60°, 36.24°, 37.95°, 38.72°, 48.23°, 54.08°, 55.25°, 62.88°, and 69.05°. The diffraction peaks were compared with standard reference samples such as rutile Ti (JCPDS 88-1175) and anatase Ti (JCPDS 78-2486). The anatase phase shows the most intense diffraction peak corresponding to the Miller index (hkl) value of (101), along with other peaks responsible for anatase such as (004), (200), (105), (211), (204), and (116). The rutile phase is represented by a peak at approximately 27.60° (110), along with peaks of small intensity such as (101), 41.20° (111), 54.30° (211), and 56.60° (220). The Scherrer equation was used to estimate the average nanoparticle size of 21.04 nm (Table 5):

where K is a constant (usually ∼0.9), λ is the X-ray wavelength, β is the full width at half maximum (FWHM) of the peak, and θ is the Bragg angle.

Figure 1

XRD pattern of ZnO.

Similarly, nanoparticles with different crystal sizes depending on the reducing agent from plant extracts such as neem extract (A. indica) are about 10–50 nm [27], orange peel extract (C. reticulata) is 24 nm [28], and jasmine extract has an average crystal size of 31–42 nm [29]. Calcination at 600°C can cause the conversion of anatase to rutile; high calcination temperatures often increase the crystallinity of TiO₂, resulting in sharper and more well-defined peaks in the XRD patterns. At 600°C, TiO₂ crystals grow larger, reducing the width of the diffraction peaks.

For TiO₂ synthesized using green tea extract, FT-IR can help determine the presence of organic compounds from green tea extract and the formation of TiO₂ before and after heating at 600°C (Figure 2). In the TiO₂ sample before calcination, the spectrum was dominated by peaks related to organic functional groups from green tea extract and hydroxyl groups [30]. O–H stretching vibrations at broad bands around 3,200–3,600 cm⁻¹ indicate the presence of hydroxyl groups (from green tea extract and adsorbed water). C–H bonds in organic molecules from green tea extract are shown through peaks at 2,800–3,000 cm⁻¹. Peaks around 1,600–1,650 cm⁻¹ (aromatic ring C═C) and 1,000–1,300 cm⁻¹ (C–O stretching) reflect the organic components in green tea extract [31]. The removal of organic compounds is evident from the disappearance of the peaks associated with C–H, C═C, and C–O vibrations after heating to 600°C [32]. The peaks of Ti–O–Ti (493 cm⁻¹) and Ti–O (614 cm⁻¹) become more obvious, and the prolonged absorption band of Ti–O at peak 763 cm⁻¹ shows the anatase phase of the structure of TiO₂. The TiO₂ structure was determined through green synthesis [33]. The presence of the Ti–O–Ti bond proves the interaction of biological molecules with the TTIP precursor by the phytochemical constituents polyphenol, flavonoids, alkaloids, tannins, and terpenoids.

Figure 2

FT-IR diagram of TiO₂.

SEM analysis provides detailed images of the surface morphology and size of TiO₂ nanoparticles. When TiO₂ nanoparticles are heated at 600°C, their morphology and structure can change significantly. TiO₂ nanoparticles synthesized through green methods typically appear as small, uniformly distributed particles. There may be some degree of agglomeration, where the nanoparticles clump together. The surface may appear relatively smooth with some organic residue from the green synthetic agents. After calcination at 600°C, there is a decrease in the degree of agglomeration as some smaller particles combine into larger particles, indicating increased crystallinity. There is less agglomeration compared to the unburnt sample. These changes are indicative of thermal processes occurring during firing, such as sintering and phase transformation. When analyzing TiO₂ nanoparticles, energy dispersive X-ray (EDX) can provide valuable information on the presence and proportions of Ti and O elements and can detect any impurities or residues from the synthesis such as carbon in Figure 3. The sample before calcination in Figure 3 shows characteristic peaks of Ti commonly observed at about 4.51 and 4.93 keV. The O peak typically appears at about 0.52 keV, and there are small peaks for carbon (C at about 0.28 keV), indicating the presence of organic residues from the green synthesis. After calcination, the EDX spectrum still shows prominent peaks for Ti (64.23%) and O (35.77%), but there is no presence of the C peak, which indicates that these residues have been removed during the firing process.

Figure 3

SEM (a) and (b) and EDX (d) and (c) images of TiO₂ nanoparticles. “a” and “c” at initial condition and “b” and “d” at 600°C.

The BET surface area is used to determine the surface area, pore size distribution, and porosity of TiO₂ materials. When TiO₂ is synthesized via green methods, Figure 4 isotherms can provide detailed information on how the synthesis process affects the nanoparticle properties. According to the IUPAC classification of physical absorption isotherms, TiO₂ nanoparticles correspond to type IV isotherms. This type of isotherm often shows a hysteresis loop, characteristic of capillary condensation occurring in mesopores. The significant reduction in the BET surface area from 691.24 to 18.33 m²·g⁻¹ upon calcination at 600°C can be attributed to the removal of organic samples used in the synthesis [34]. Green tea extract acts as a template, creating a highly porous structure with small mesopores or micropores, resulting in a high BET surface area (Table 6). Firing at 600°C decomposes and removes the green tea extract. Heat treatment causes sintering, leading to the collapse of the pores and the formation of larger pores. The change from many small pores to fewer large pores significantly reduces the overall surface area. The pore size confirms significant structural changes in the TiO₂ nanoparticles before and after calcination, increasing from 4.1 to 19.5 nm. These changes reflect the typical effects of calcination of a TiO₂ material sample, where the organic components are removed, and the inorganic framework undergoes sintering.

Figure 4

N₂ adsorption–desorption isotherm (a) and (c) curve and pore size distribution (b) and (d) of TiO₂: “a” and “c” at initial condition and “b” and “d” at 600°C.

3.3 Evaluation of the antibacterial ability of TiO₂ nanoparticles

TiO₂ nanoparticles were used to determine the antibacterial activity against two bacterial strains, E. coli and S. aureus. The results of determining the antibacterial activity of TiO₂ are presented in Figure 5. TiO₂ nanoparticles did not show significant antibacterial activity against E. coli and S. aureus at the concentrations tested by the disk diffusion method. No inhibition zones were observed around the wells impregnated with TiO₂ nanoparticles. The absence of an inhibition zone for the two bacterial strains indicates that the concentration of TiO₂ nanoparticles used in this study was not effective against E. coli and S. aureus in the disk diffusion assay. Another study by Nguyen et al. tested antibacterial activity and showed that pure TiO₂ nanoparticles did not show any inhibitory effect on E. coli and S. aureus bacteria [35].

Figure 5

Antibacterial activity of TiO₂ nanoparticles by the agar diffusion method: (a) Escherichia coli and (b) Staphylococcus aureus.

Therefore, determining the MIC is an important assessment to determine the lowest concentration of TiO₂ nanoparticles required to inhibit the visible growth of E. coli and S. aureus after 24 h of incubation with TiO₂ (Table 7 and Figure 6). The MIC values of TiO₂ against E. coli were 7.5 mg·mL⁻¹. This indicated that a concentration of 7.5 mg·mL⁻¹ TiO₂ was sufficient to inhibit the visible growth of E. coli after 24 h of incubation. Similarly, a concentration of 11.25 mg·mL⁻¹ could inhibit the growth of S. aureus. The MIC values indicate that E. coli is more susceptible to inhibition by TiO₂ than S. aureus, as it requires a lower concentration of TiO₂ to inhibit its growth. Based on the results of Table 7, it shows that S. aureus bacteria are less sensitive than E. coli bacteria. The higher MIC value for S. aureus indicates that this bacterium is more resistant to the effects of TiO₂, possibly due to a thicker peptidoglycan layer, which may provide additional protection against invasion by TiO₂ nanoparticles [36].

Figure 6

Broth dilution method of the 96-well microplate on tested bacterial strains.

Azizi-Lalabadi et al. have shown that the thick peptidoglycan layer in the cell wall of the Gram-positive bacteria S. aureus will create more resistance than other bacteria. Gram-negative bacteria only have a thin peptidoglycan layer, which easily interacts with the charge of TiO₂ nanoparticles [37]. The publication also shows that the negative charge on the cell surface of Gram-negative bacteria is higher than the negative charge on the cell surface of Gram-positive bacteria [38].

3.4 Photocatalytic activity of TiO₂ nanoparticles

The efficiency of tetracycline removal in the photochemical reaction by TiO₂ catalyst is shown through the light absorption intensity in the UV–Vis spectrum in Figure 7. As the irradiation time increases, the intensity of the characteristic peaks of tetracycline at 357 nm wavelength decreases abruptly after 240 min of irradiation, with a removal efficiency of about 73%. However, there is a shift in the maximum wavelength to the left at about 180 and 240 min at the end of the photochemical stage. The longer the reaction time, the higher the decomposition rate of tetracycline. This is because there is more time for photochemical reactions to take place, which helps to break down the structure of tetracycline into smaller decomposition products. When the reaction time is too long, the decomposition efficiency can reach a saturation level, meaning that there is no significant increase in the decomposition rate. This may be because all tetracycline molecules have been degraded or the degradation products have become saturated.

Figure 7

Effect of catalytic time (a) and adsorption spectrum of tetracycline (b).

The point of zero charge (pH_pzc) is an important parameter in understanding the surface properties and photocatalytic activity of TiO₂. For TiO₂ with a pH_pzc of 5.8, the surface charge characteristics at different pH values will affect its interaction with tetracycline antibiotics and its overall photocatalytic performance. Tetracycline is an amphoteric molecule, meaning it can act as both an acid and a base. Its ionization state depends on the pH of the solution. In general, tetracycline has different protonation states under acidic, neutral, and alkaline conditions, which affect its interaction with the TiO₂ surface. Based on Figure 8b, the TiO₂ surface is positively charged when pH < 5.8; TiO₂ can adsorb negative charges (anions) due to electrostatic attraction. In contrast, solutions with pH > 5.8 have a negatively charged TiO₂ surface, which is favorable for the adsorption of cationic species. The effective pH for the photodegradation of tetracycline by TiO₂ (pH_pzc 5.8) is usually slightly higher than pH_pzc, ranging from 6 to 8 (Figure 8a). This allows for the efficient adsorption of positively charged tetracycline molecules and the generation of reactive oxidant species.

Figure 8

Effect of solution pH (a) and pH_pzc value (b) of TiO₂ nanoparticles.

In this study, increasing the dosage from 3 to 5 mg resulted in an increase in the tetracycline degradation efficiency from 48% to 80% after 240 min of illumination, respectively. The increase in decolorization efficiency with increasing amount of catalyst in the reaction is due to the increase in available surface area or active sites of the catalyst; increasing the amount of catalyst in the reaction will generate more ˙OH radicals. However, when the amount of photocatalyst was increased further (7 and 9 mg), the degradation efficiency tended to decrease due to the increased opacity of the suspension, leading to increased adsorption of tetracycline onto the material surface. Therefore, the light transmission efficiency to the material will be reduced, and the surface part of the photocatalyst will no longer be able to absorb light.

The tetracycline concentration significantly affects the photocatalytic degradation process using TiO₂ (Figure 9b). At a tetracycline concentration of 20 mg·L⁻¹ after 240 min, the degradation efficiency was approximately 76%. At higher concentrations, the degradation efficiency decreased. This was due to increased competition for available reactive oxygen species and active sites on the TiO₂ surface. The reaction rate slowed as the TiO₂ surface became saturated with tetracycline molecules. In addition, intermediate products formed during the degradation process may adsorb onto the TiO₂ surface, further inhibiting the reaction.

Figure 9

Effect of catalyst dosage (a) and tetracycline antibiotic concentration (b).

The stability of TiO₂ after multiple uses is a crucial aspect to consider when assessing the viability of the catalyst for environmental remediation purposes, particularly in the breakdown of antibiotics like tetracycline. The decrease in photocatalytic efficiency of TiO₂ from 78.51% to 53.22% after four reuses indicates that various factors are impacting the reuse process and leading to a decline in catalyst performance (Figure 10). This decline is attributed to the build-up of degradation byproducts and impurities on the material’s surface, hindering light penetration to the catalyst. To address this issue, ethanol was employed to cleanse TiO₂ after each cycle and remove impurities. However, over the reuse cycles, the nano-TiO₂ particles tended to aggregate, diminishing the surface area and catalytic effectiveness. Additionally, some of the TiO₂ particles may transition from the anatase phase (high activity) to the rutile phase (lower activity) after four reuse cycles. Similarly, some publications on the photocatalytic activity of green synthesized TiO₂ particles have been made and presented in Table 8.

Figure 10

Recycling efficiency for green synthesis TiO₂ catalyst.

The photocatalytic mechanism of TiO₂ (titanium dioxide) with tetracycline antibiotic occurring on the surface of TiO₂ nanoparticles TiO₂ nanoparticles can absorb UV light with a wavelength of 357 nm (Figure 11). When exposed to UV light, electrons in the valence band of TiO₂ are excited to the conduction band, generating pairs of holes (h⁺) and electrons (e⁻). These holes and electrons can react with water and oxygen molecules on the TiO₂ surface to produce reactive species like hydroxyl (˙OH) and superoxide ( ˙ O 2 − ). These reactive species ˙OH and ˙ O 2 − are highly active and possess strong oxidizing properties. They will target and degrade the molecular structure of tetracycline, resulting in the breakdown into simpler by-products and eventually into CO₂ and H₂O.

Figure 11

Photocatalysis mechanism for TiO₂ nanoparticles.

The efficiency of electron transfer can be enhanced by ensuring that the electron donor and acceptor molecules have appropriate chemical structures. Electron-rich organic compounds, like polyphenols or flavonoids found in natural extracts, can serve as electron donors, reducing the activation energy and enhancing the separation of electron–hole pairs in TiO₂. Acceptor molecules with strong electron capture abilities, such as O₂ or potent oxidants, can aid in reducing electron–hole recombination.

3.5 Application orientation for foaming face wash products

Specifications of facial cleanser products containing TiO₂ may include factors such as pH, viscosity, and oxidation activity (Table 9). The pH of facial cleansers is usually in the range of 5–6.5. Research shows that products with a pH in the range of 5–6 are suitable for the natural pH of the skin, helping to maintain the acid mantle that protects the skin and prevents acne by inhibiting the growth of harmful bacteria. The viscosity of the product is in the range of 500–560 cP. The viscosity is adjusted relatively low to ensure the product has a moderate consistency for easy use in foam spray form and is easy to clean. Titanium dioxide is commonly used in skincare products, including facial cleansers, because of its various beneficial properties. TiO₂ is an effective physical blocker of UV rays, both UVA and UVB. When incorporated into facial cleansers, it provides a layer of protection against the harmful effects of UV radiation. This helps enhance the product’s SPF value, helping to protect the skin from the sun more effectively. TiO₂ is highly stable and does not easily decompose when exposed to light, heat, or air, ensuring the longevity and effectiveness of the facial cleanser. Table 10 shows the product’s effectiveness in protecting against UVB rays and the UVB-blocking ability of TiO₂. Formula F0, without TiO₂ nanoparticles, has the lowest SPF index of 8.02, while formulas F1–F5 have SPF values that change according to the content of TiO₂ nanoparticles, but the values do not show significant deviation. Additionally, the presence of plant extracts and active skincare ingredients contributes to protecting the skin from the harmful effects of free radicals caused by sunlight, which can help enhance the product’s SPF index due to its antioxidant and skin-protecting properties. Studies have consistently shown that TiO₂ nanoparticles do not penetrate beyond the outermost layer of the skin (stratum corneum) to reach skin cells. Based on a detailed analysis of studies, the SCCS has concluded that the use of nano-TiO₂ in cosmetic products, including facial cleansers, is safe for use on healthy, intact skin [16].