1 Introduction

The growing concern with the pollution of the aquatic ecosystem makes it essential to develop ecologically appropriate and economically viable technologies for wastewater treatment, especially with emerging organic pollutants [1]. Dyes are chemical compounds that color materials and surfaces [2]. They are present in several industries, such as plastics, papers, leather, and textiles, to provide a specific coloring to the final product [3]. Among synthetic dyes, Rhodamine B (RhB) is an important laser dye with excellent photophysical properties such as long-wavelength absorption and emission, high fluorescence quantum yield, and large extinction coefficient, and it is classified as highly dangerous properties [4]. Colored wastewater is a major problem due to the diversity of compounds (with different functional groups) and the high biological stability of industrial dyes. Thus, conventional treatments such as physical–chemical present low removal for these organic pollutants [5]. Thus, it is necessary to use effective techniques to correct the treatment of dyes wastewater, such as the advanced oxidative processes (AOPs), including heterogeneous photocatalysis [6,7,8]. AOPs are based on the production of highly oxidizing radicals (•OH) under ultraviolet (UV) or visible irradiation, resulting mainly in the complete mineralization of the persistent organic pollutants in CO2, H2O, and inorganic ions [9, 10].

Heterogeneous photocatalysis is a process that uses the semiconductor (denominated catalyst) induced under irradiation, promoting the hydroxyl radical generation by redox reaction [11]. Moreover, the high efficiency of heterogeneous photocatalysis is based on the redox reactions between the organic pollutant molecules adsorbed onto the catalytic surface and the hydroxyl radicals, avoiding possible recombination of the electron/hole pairs [12].

However, some semiconductors have specific limitations to the application under visible irradiation, such as titanium dioxide commercial [13], for example, due to the high band gap energy. Thus, it is necessary to research alternative and promising materials associated with nanotechnology, such as nanostructured systems [14, 15].

Nanostructured systems have specific textural, morphological, and structural properties that allow interactions with biomolecules, making them essential for applications at the biotechnological level, such as green metallic nanoparticles [16], and supported nanocatalysts for dye removal [17]. Green synthesis represents the processes between different metabolites or biomolecules, acting as reducing agents, with a primary precursor, being non-toxic, biodegradable, and low-biological [18, 19]. Among the eco-friendly metallic nanoparticles, TiO2-NPs have been used in biomedical sciences [20], technological sciences [21], and agricultural sciences [22] due to the properties of non-toxicity, high specific surface area, and biocompatibility.

Thus, this work aims to biosynthesize and characterize titanium dioxide nanoparticles (TiO2-NPs) from Aloe vera extract as a reducing agent and to evaluate the antimicrobial activity (S. aureus and P. aeruginosa), cytotoxicity (293 T, MDBK, and HaCat cell lines), and photocatalytic activity for the RhB dye removal under visible irradiation. The novelty of this works is the green synthesis of TiO2-NPs with the presence of the photoactive phase (anatase), without the need for a heat treatment step. Furthermore, it demands the achievement of the Sustainable Development Goals (SDGs), specifically goals 6 (Potable water) and 14 (Life in water) correlating with topics of nanotechnology and toxicity.

2 Materials and methods

2.1 Aloe vera extract (AvE) and TiO2-NP green synthesis

Aloe vera leaves (Aloe arborescens) were collected in Santa Maria (29° 41′ 29″ S, 53° 48′ 3″ W) and dried at 25 ± 2 °C for 24 h (Forced Air Lab Oven Cubic Foot 39.4 L) with a relative humidity of around 60% and a heating rate of 4 °C min−1. After, the dry material was grounded in a knife mill (Willye TE-650) and sieved (#106 nm). Thus, 30 g of ground leaves were mixed with distilled water (30 min/250 rpm/25 ± 2 °C) [23]. TiO2-NPs were synthesized by the green synthesis method [24]. For the bioreduction and nucleation steps, 130 mL of titanium isopropoxide (0.25 mol L−1, C12H28O4Ti, Sigma-Aldrich®, 97%) and 150 mL of AvEt were mixed (90 min/250 rpm/25 ± 2 °C). After, for the stabilization step, TiO2-NPs were dried (80 °C/720 min) (Fig. 1).

Fig. 1
figure 1

Schematic representation of the TiO2-NPs from AvE

Full size image

2.2 Characterization techniques

X-ray diffraction (XRD) was used to verify the TiO2-NPs crystallinity or amorphism using a Bruker diffractometer (model D2 Advance) with λCu-α = 0.15418 nm ranging from 10°–70°, 30 kV (acceleration voltage) and 30 mA (applied current), where Debye–Scherer equation was used to determine the particle size of TiO2-NPs, according to Eq. (1) [25]:

$$d=\frac{0.9\lambda }{\beta \mathrm{cos}\left(\theta \right)}$$
(1)

where λ = 0.15418 nm, β is the FWHM (full width at half maximum), and θ (º) is the Bragg diffraction angle.

Field emission gun scanning electron microscopy (FEG-SEM) was used to determine the morphological characteristic in a MIRA3 (TESCAN, Czech Republic) with 15 kV acceleration and 25 mm working distance with 400 and 5700 × magnification. The size of the TiO2-NPs was measured using ImageJ software (NIH, USA), where 50 random points were selected and used to calculate the mean. Malvern-Zetasizer® model nanoZS (ZEN3600) was used to measure the surface charge value by zeta potential using closed capillary cells (DTS 1060). The specific surface area (SBET) and pore size distribution (Vp and Dp) were determined in the ASAP 2020 Plus Micromeritics equipment using the BET/BJH method [26]. To identify the elements, energy-dispersive X-ray spectroscopy (EDX) was used in a Phenom Pro X microscope (Thermo Fisher Scientific) with 4000 × magnification at 15 kV and full backscattered electron. High performance liquid chromatography (HPLC) equipped with gradient elution capability, ultraviolet spectrophotometer and photodiode array as detector and an autosampler was used to the determination of Aloe vera extract composition. Data processing system used was the LabSolutions. A C18 reverse column (3.9 × 150 mm, 4 µL). Gradient elution consisted of two mobile phases (a) water (99.7%) and formic acid (0.3%) and (b) methanol (99.7%) and formic acid (0.03%). The detection wavelength was 280 nm and the flow rate was 1.0 mL min−1. Each injection volume was 20 µL, and the column temperature was maintained at ambient conditions (25 ± 2 °C) [27].

2.3 Antimicrobial activity

MIC was carried out against S. aureus (ATCC 25923) and P. aeruginosa (ATCC 27853) by microdilution method [28] in triplicate with TiO2-NP solution. one hundred microliters of Mueller Hinton broth (MH, Sigma-Aldrich®) was mixed with TiO2-NPs (1:1 v/v), followed by a series of dilutions (500–0.98 µg mL−1). Bacterial inoculum (1 × 108 CFU mL−1) was added and incubated (24 h/37 ± 2 °C). After, TTC (5% w/w) was added and reincubated (2 h/37 ± 2 °C). MHB only was used as negative control and MHB with bacterial inoculum was used as positive control.

2.4 Cell cultivation

293 T (embryonic kidney human, ATCC CRL-3216), MDBK (kidney bovine, ATCC CCL-22), and HaCat (human keratinocyte, ATCC PCS-200-011TN™) cell lines from the Cell Bank (Rio de Janeiro, Brazil) were used to determine the safety profile of the TiO2-NPs. Cells were cultured using Dulbecco’s modified Eagle medium (DMEM, Sigma-Aldrich®) with 10% fetal bovine serum (FBS) (Sigma-Aldrich®) and 1% penicillin–streptomycin-neomycin (PSN) antibiotic mixture [29]. Cells were kept in a 5% CO2 incubator at 37 ± 2 °C with controlled humidity. These cells were seeded in 96-well plates (1–300 µg mL−1) during 24 h of incubation. One hundred millimoles per liter of hydrogen peroxide (H2O2) was used as a positive control (PC) for cell viability and ROS generations, while for the NO generation sodium nitrite (NaNO2, 1 μg mL−1) was used. Negative control (NC) was the cells in the culture.

2.5 Cell viability

To determine the cell viability (24 h) was carried out the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide MTT test [30,31]. Thus, 20 μL of the TiO2-NPs was mixed with MTT solution MTT (5 mg mL−1) and incubated (4 h/37 ± 2 °C/5% CO2). The solution was carefully removed and the formazan crystals dissolved in 200 µL of the DMSO. Cell growth inhibition was detected using a microplate reader (Biochrom® Anthos) at λ = 570 nm.

2.6 Semi-quantification of reactive oxygen species (ROS)

ROS generation was determined by the DCFH- DA (2,7-dichlorofluorescein diacetate) [32] with TiO2-NPs extracts (1–300 μg mL−1). DCFH-DA solution (1 mmol L−1) was diluted in ethanol (1:10 v/v). Ten microliters of the solution was mixed with Tris HCl (65 µL) and treated cells (50 µL). After, the solution was incubated (1 h/37 ± 2 °C/5% CO2), and the fluorescence intensity was determined at 520 nm of emission and 480 nm of excitation using a microplate reader (Biochrom® Anthos).

2.7 Indirect determination of nitric oxide generation

Griess solution (0.1% N-(1-naphthyl)ethylenediamine dihydrochloride, and 1% sulfanilamide in 5% phosphoric acid) was used to detect the presence of nitrites (NO2−) in the sample, which is a NO metabolite, according to the literature [33]. Thus, 100 µL of the TiO2-NP extracts (1–300 μg mL−1) was added to a 96-well plate with Griess solution (100 µL) and incubated (30 min/37 ± 2 °C/5% CO2). After, an ELISA reader (photometer) was used to determine the intensity of the formed color (λ = 540 nm).

2.8 Photocatalytic activity

RhB dye was used as the target molecule in contact with TiO2-NPs for 60 min (without radiation) and 180 min under visible irradiation, where aliquots were collected at predetermined times (0, 5, 15, 30, 45, 60, 75, 90, 120, 150, and 180 min). All samples were filtered (ϕ = 0.45 μm) and diluted (1:10 v/v). To determine the photodegradation (%R) of RhB dye, a UV–vis spectrophotometer (Varian Cary 100) was used at λ = 553 nm.

2.9 Photodegradation kinetic

The Langmuir–Hinshelwood model (L–H) was used for the kinetic study of experimental data, according to Eqs. (2) and (3) [34, 35]:

$$(-{r}_{i})=-\frac{d{C}_{i}}{dt}=\frac{{k}_{s}.K.{C}_{i}}{1+K.{C}_{i}}$$
(2)
$${C}_{i}={C}_{io}.{e}^{-k.t}$$
(3)

where \((-{r}_{i})\) is the reaction rate (mol·min−1·L−1), \(K\) is the adsorption constant, \({k}_{s}\) is the apparent constant of reaction, \({C}_{io}\) is the initial RhB dye concentration, \({C}_{i}\) is the RhB dye concentration, and k is the apparent rate of the pseudo-first-order reaction (min−1).

2.9.1 CCRD

CCRD 23 was used to determine the ideal condition of the heterogeneous photocatalysis process using pH, RhB concentration (mg L−1), and TiO2-NPs concentration (g L−1) as independent variables, and as the response variable, the percentage of photodegradation (Table 1).

Table 1 CCRD 23 experimental design for the photocatalytic tests
Full size table

2.10 TiO2-NPs recycling

After the first cycle, the RhB solution was centrifuged (5000 rpm/10 min), and TiO2-NPs were separated and reintroduced into the reactor using the ideal condition by CCRD. Therefore, the procedure was repeated five times, and the percentage of the degradation and the apparent rate of the pseudo-first-order reaction were calculated.

2.11 Statistic analysis

To determine the ideal condition, the Statistical 10 software (StatSof, Tulsa, USA) was used through surface response analysis and ANOVA (p < 0.05). GraphPad Prism and Tukey’s post hoc test were used for all the biological tests with *p < 0.05, **p < 0.01, and ***p < 0.001.

3 Results and discussion

3.1 Characterization of the AvE and TiO2-NPs

Table 2 shows the AvE chromatogram by HPLC, where it was possible to detect a series of bioactive compounds such as the following: (a) phenolic compounds—ellagic acid, epigallocatechin gallate, and catechin in the concentration of 99.79, 0.16, and 38.80 mg L−1, respectively; (b) flavonoids—naringin, myricetin, quercetin, and kaempferol in the concentration of 97.39, 103.05, 1.96, and 108.29 mg L−1, respectively. The identified compounds play different roles in preventing and treating pathologies [36]. It is noteworthy that the presence of these functional biomolecules from Aloe vera extract is responsible for the active reduction step of metal ions (Ti+4 → Ti0), due to the presence of a series of functional groups (e.g., –C–O–C, – C–O–, –C = C–, and –C = O–), derived from heterocyclic compounds. Then, the metal ions aggregate and form metal nanoparticles (nucleation step), where the biocompounds form a stabilizing layer around the nanoparticles, preventing them from aggregating [37, 38].

Table 2 Metabolite identification by HPLC from AvE
Full size table

Figure 2 shows the XRD diffraction with characteristic peaks at 25.20° (101), 37.71° (004), 47.89° (200), 53.71° (105), 54.98° (211), and 62.58° (204) with a = 3.755 Å and c = 9.5114 Å, confirming the anatase phase of TiO2-NPs according to the JCPDS file 21–1272 31 [39] and 32 nm of the particle size. In addition, the peak at 36.0º (101) was assigned to the rutile phase [40]. Among the polymorphic phases of TiO2-NPs, the most active phase photocatalytic is anatase, with a high surface area, slower recombination, and greater electron mobility [41], indicating that it was possible to synthesize a titanium dioxide nanostructured with the predominance of the active phase, without the need for thermal treatment.

Fig. 2
figure 2

XRD diffractogram of the TiO2-NPs from AvE

Full size image

About the textural and structural properties, TiO2-NPs showed SBET of 118 m2 g–1, Dp of 9.2 nm, and Vp of the 0.2 cm3 g−1, indicating a mesoporous characteristic, high specific surface area, and considerable porosity [42, 43], and a negative charge surface (− 4.90 ± 0.30 mV) compatible with RhB cationic dye.

Figure 3a shows the adsorption/desorption isotherm of the TiO2-NPs, which was characterized for type V with H1 hysteresis (uniform spheres with a form of cylinders and open ends) [44, 45], while the Fig. 3b illustrates the pore size distribution curve.

Fig. 3
figure 3

a N2 adsorption/desorption isotherm and b the distribution of the pore volume of the TiO2-NPs from AvE

Full size image

Figure 4a shows the FEG-SEM micrographs where it was possible to visualize a heterogeneous surface with small agglomerations of TiO2-NPs and irregular particle sizes, which indicates an interconnection between the pores [46] with a particle diameter around 287 ± 115 nm (Fig. 4b). Moreover, it is possible to verify a spherical morphology of the nanoparticles with considerable porosity, favoring the interparticle diffusion of RhB molecules into the active site, increasing the amount of hydroxyl radical generated and directly affecting the photocatalytic activity [47].

Fig. 4
figure 4

a FEG-SEM micrograph and b average particle size of the TiO2-NPs from AvE

Full size image

To investigate the elemental composition of the TiO2-NPs, SEM–EDX was carried out according to Fig. 5, where there was a predominance of oxygen (62.42%) and titanium (30.63%), resulting in the formation of titanium dioxide nanoparticles from the reduction metallic precursor (Ti+4), with a heterogeneous morphology of approximately small nanocrystals, giving a large surface area and promoting the formation of clusters [48].

Fig. 5
figure 5

a SEM micrography with 4000 × magnification and b EDX results of the TiO2-NPs from AvE

Full size image

3.2 Antimicrobial activity

The antibacterial potential was evaluated by MIC assay, indicating that there was no antimicrobial activity against the two pathogens respectively. TiO2-NPs have greater antimicrobial activity depending on their particle size, and the smaller the diameter of the nanomaterial, the greater the toxicity when exposed to microorganisms [49,50,51]. Moreover, the decomposition of the bacterial outer membrane by reactive oxygen species (ROS) is a bactericidal effect attributed to the TiO2-NPs [52, 53]. Thus, the textural properties of TiO2-NPs (SBET, Dp, and Vp), and the photoactive phase TiO2-NPs did not allow efficient contact with bacterial cells, limiting ROS generation, and the restricting mechanical resistance of the cell wall [54].

3.3 Cytotoxicity tests

Figure 6 shows the evaluation of cytotoxicity by the MMT test after 24 h, where it was possible to verify that in none of the treatments with TiO2-NPs, there was a significant decrease in cell viability, only in the positive control.

Fig. 6
figure 6

Evaluation of cytotoxicity by the MTT test after 24 h. Data were presented as mean *p < 0.05, **p < 0.01, and ***p < 0.001 versus negative control (NC)

Full size image

According to Fig. 6, TiO2-NP concentrations tested showed no significant decrease in cell viability, without toxicity and restricted proliferation, evidencing biocompatibility, and the possibility of expanding the application spectrum, as in biomaterials [55, 56]. Moreover, it is highlighted that TiO2-NP toxicity depends in concentration, exposure time, and degree of tolerance of the cell line tested [57]. Positive control helps to show that negative (untreated) samples are negative. The results of the controls must be different to validate the test. A positive control usually uses a substance that the test reagent will detect.

3.4 ROS generation

Figure 7 represents the evaluation of the ROS generation after 24 h, where all tested concentrations of TiO2-NPs did not produce the formation of free radical, except for the positive control, which significantly increased levels when compared to the NC.

Fig. 7
figure 7

Evaluation of ROS generation after 24 h. Data were presented as mean *p < 0.05, **p < 0.01, and ***p < 0.001 versus negative control (NC)

Full size image

According to Fig. 7, it was possible to notice that TiO2-NPs did not cause an increase in ROS generation, indicating that the green synthesis contributed to the production of nanoparticles free of chemical impurities, decreasing the ROS levels and being accepted in the medical field [58,59,60]. Moreover, the ROS generation using the TiO2- NPs occurs when the particle is subjected to UV irradiation. Due to the wider band gap, there is absorption and production of reactive species [61] which was not the focus of the study, since visible radiation was used.

Initially, the oxidation of H2DCF to DCF was thought to be specific for H2O2. However, recent evidence, has shown that other ROS, such as hydroxyl radicals, hydroperoxides, and peroxynitrite can oxidize H2DCF but are much less sensitive than H2O2 [62]. However, the most used assay in cells is H2O2 [63].

3.5 NO generation

Figure 8 shows the evaluation of nitric oxide generation after 24 h, where it was impossible to detect nitrite in the supernatants treated with TiO2-NPs.

Fig. 8
figure 8

Evaluation of NO generation after 24 h. Data were presented as mean *p < 0.05, **p < 0.01, and ***p < 0.001 versus negative control (NC). *NC: cells in culture medium; PC: 1 µg mL−1 of the NaNO2 and treatments (1; 10; 30, 100, and 300 µg mL.−1 of TiO2-NPs)

Full size image

According to Fig. 8, TiO2-NP treatments showed no changes in NO levels, due to the green synthesis process used from extracts acting as reducing agents and the presence of the richness of biomolecules [64, 65]. In the NO test, NO and its by-products, such as NO3− and NO2−, can be measured indirectly [66]. Sodium nitrite is often used with the Griess reagent to generate a standard curve [67] because low concentrations (0.1 μg mL−1) can be detected using this method [68].

3.6 CCRD

Figure 9 shows the Pareto graphic where it was possible to verify that the pH and [TiO2-NPs] showed a quadratic indirect effect on the percentage of RhB dye removal, because of the reduction in the number of active sites available for intermolecular diffusion of RhB molecules [69]. However, a high concentration of TiO2-NPs will decrease the degradation percentage, as visible radiation penetration into the aqueous medium will be reduced, making it an opaque system [70]. Regarding the pH effect, when the pH variation in higher or lower values provides the formation of the nanoparticles negative or positive surface charges, affecting the adsorption–desorption of the RhB molecules [71]. For acidic pH, low dye degradation using the TiO2-NPs was observed, due to the low electrostatic attraction, due to the repulsion between the target molecule and catalytic surface. However, the hydroxyl radicals are slowly absorbed, not having a high reaction with the dye under basic pH [72], favoring neutral pH for the reaction. Equation (4) shows the %R of the RhB depending on the pH and [TiO2-NPs], and Fig. 10 demonstrates the 3D surface response, indicating the ideal condition was [RhB] = 10 mg L−1, [TiO2-NPs] = 3.5 g L−1, and pH = 7.0 at 25 ± 2 °C, which showed the greatest degradation of 90% after 180 min under visible radiation.

Fig. 9
figure 9

Pareto chart using input variables and output parameters under visible radiation

Full size image
Fig. 10
figure 10

3D surface response for RhB photodegradation under visible radiation

Full size image
$$\%R=234.74-9.12 x p{H}^{2}-1.63 x [Ti{O}_{2}-NPs{]}^{2}$$
(4)

3.7 Photocatalytic activity and recycling

Figure 11 shows the photocatalytic activity of TiO2-NPs under visible radiation using the ideal condition ([RhB] = 10 mg L−1, [TiO2-NPs] = 3.5 g L−1 and pH = 7.0) after 180 min with 90% degradation. Moreover, it was possible to verify a pseudo-first-order kinetic model with an apparent rate of the pseudo-first-order reaction specific reaction (k) of 0.0146 min−1, according to the literature [73, 74].

Fig. 11
figure 11

Photocatalytic activity of the TiO2-NPs under visible radiation

Full size image

Figure 12 shows the TiO2-NPs recycling after five cycles with a decrease in RhB degradation (90 to 84.67%) and a decrease in the specific reaction rate (k) for k = 0.0146 min−1 for 0.0125 min−1, indicating the stability of the nanocatalyst.

Fig. 12
figure 12

Effect of the TiO2-NPs recycling for RhB degradation

Full size image

The general mechanism for heterogeneous photocatalysis using TiO2-NPs as a nanocatalyst has the following steps [75] (Fig. 13): (a) adsorption of RhB molecules onto the TiO2-NPs (Eq. 5); (b) excitation of the TiO2-NPs (Eq. 6); (c) load recombination (Eq. 7); (d) singlet oxygen formation (Eq. 8); (e) production of •OH radicals from O2 (Eq. 9), and (f) RhB degradation (Eq. 10). Thus, the efficiency of the photocatalytic process depends directly on the competition between electrons removed from the semiconductor surface and on the possible recombination of electron/vacancy pairs.

Fig. 13
figure 13

Mechanism of RhB degradation by the photocatalysis process

Full size image
$${\mathrm{TiO}}_{2}-{\mathrm{NPs}}^{+}\left({\mathrm{O}}_{2}+{\mathrm{H}}_{2}\mathrm{O}+\mathrm{RhB}\right)\to {\mathrm{O}}_{2\left(\mathrm{Ads}\right)}+{\mathrm{H}}_{2}{\mathrm{O}}_{\left(\mathrm{Ads}\right)}{\mathrm{RhB}}_{\left(\mathrm{Ads}\right)}$$
(5)
$${\mathrm{TiO}}_{2}-\mathrm{NPs}+\mathrm{h\nu }\to {{\mathrm{e}}^{-}}_{\mathrm{CB}}+{{\mathrm{h}}^{+}}_{\mathrm{VB}}$$
(6)
$${{\mathrm{e}}^{-}}_{\mathrm{CB}}+{{\mathrm{h}}^{+}}_{\mathrm{VB}}\to \mathrm{Heat liberation}$$
(7)
$${\mathrm{O}}_{2\left(\mathrm{Ads}\right)}+{{\mathrm{e}}^{-}}_{\mathrm{CB}}\to {{\mathrm{O}}_{2}}^{\cdot -}$$
(8)
$${\mathrm{H}}_{2}{\mathrm{O}}_{2}+{{\mathrm{O}}_{2}}^{\cdot -}\to {\mathrm{O}}_{2}+{}^{\cdot }\mathrm{OH}$$
(9)
$$\mathrm{Rhb}+{}^{\cdot }\mathrm{OH}\to \mathrm{products}\left({\mathrm{CO}}_{2}+{\mathrm{H}}_{2}\mathrm{O}\right)$$
(10)

Different metallic nanoparticles have been reported in the literature to remove RhB dye using heterogeneous photocatalysts such as Ag@ZnO, TiO2@HNTs, AgNPs@BC, Fe@Bi-P-I, and AgBr@SnO2, indicating a versatility for the use of commercial semiconductors. However, aiming for sustainable development and green technology, it is necessary to search for new nanocatalysts from extracts or residual biomass using the green synthesis process, such as metallic nanoparticles. Thus, Table 3 shows some studies about RhB degradation using different nanocatalysts.

Table 3 Comparative studies for RhB removal by of heterogeneous photocatalysis with metallic nanoparticles
Full size table

According to Table 3, green synthesis has already been consolidated to obtain metallic nanoparticles. Thus, the advantages of this synthesis process compared to traditional processes (e.g., hydrothermal, coprecipitation, sol–gel) is its versatility and the easy access of biomolecules present in plant extracts to act as bioreducing agents, not requiring toxic reagents, meeting sustainable development.

4 Conclusion

TiO2-NPs were prepared from Aloe vera extract using the green synthesis process for application in the RhB removal by the heterogeneous photocatalysis process. The N2 porosimetry showed SBET = 118 m2 g−1, Vp = 0.2 cm3 g−1, and Dp = 9.2 nm, indicating a nanometric structure of the material with mesoporous characteristics and considerable porosity. XRD diffractogram showed characteristic peaks of the anatase active phase and d = 32 nm. FEG-SEM micrographs indicated a morphology of the nanoparticles with small clusters (about 32 cm) and a spherical structure. Furthermore, the zeta potential indicated a negative surface charge of − 4.90 mV, favoring the electrostatic interaction with the target molecule (RhB-cationic dye). The antimicrobial activity showed that the TiO2-NPs had no effective activity against the tested pathogens. About the heterogeneous photocatalysis process, when exposed to TiO2-NPs under visible radiation, 90% degradation was observed using the ideal conditions (pH = 10; [TiO2-NPs] = 3.5 g L−1; [RhB] = 10 mg L−1). The kinetic study indicated pseudo-first-order behavior with k = 0.0146 min−1. Regarding the in vitro safety of TiO2-NPs, there was no reduction in cell viability in the 293 T, MDBK, and HaCat cell lines. In the ROS generation, the concentrations (1–300 µg mL−1) used did not cause significant differences from the untreated control, showing that there is preliminary biocompatibility. Therefore, TiO2-NPs have potential application as nanocatalysts for the degradation of dye wastewater by the heterogeneous photocatalysis process.