Green synthesis of BaSO4@SiO2 core–shell and BaSO4@ZIF-8 nanocomposite and assessment of their cytotoxicity effects and photocatalytic performance

FTIR

FTIR spectroscopy was employed to elucidate the chemical bands generated during the synthesis of BaSO₄@SiO₂ Fig. 3a and BaSO₄@ZIF-8 Fig. 3b within the spectral range of 400–4000 cm⁻¹. As delineated in the extant literature, four principal vibrational modes are associated with the sulfate, which includes one non-degenerate mode (ν₁), one doubly degenerate mode (ν₂), and two triply degenerate modes (ν₃ and ν₄). The frequency band detected at 3428 cm⁻¹ is ascribed to the asymmetric stretching vibrations of water molecules. The stretching and bending vibrational bonds about sulfur/oxygen were recorded at 2063 cm⁻¹²⁷. The band recognized at 1630 cm⁻¹ corresponds to the stretching vibration of ν₃ ²⁸. In the FTIR spectrum, the bands situated within the range of 1074–1200 cm⁻¹, in addition to the shoulder at 984 cm⁻¹, are indicative of symmetric stretching vibrations^27,29. Lastly, the bands detected at 609 and 642 cm⁻¹ are associated with out-of-plane bending vibrations^30,31. Figure 3a showed absorption bands at 3428, 2063, 1630, 1200, 1114, 1074, 984, 642, and 609 cm⁻¹. BaSO₄@SiO₂ core–shell doesn’t have significant changes in structure³². Figure 3b showed absorption bands at 3426, 2067, 1633, 1201, 1114,1074, 983, 642, and 609 cm⁻¹ for BaSO₄ and bands at 3417, 3135, 2929, 2451, 1587, 1459, 1423, 1310, 1138, 995, 759, 694, and 420 cm⁻¹ were related to ZIF-8 in BaSO₄@ZIF-8 ³³. In the case of pure ZIF-8, the spectral peaks observed at 2929 and 3135 cm⁻¹ are indicative of the aliphatic and aromatic C-H stretching vibrations associated with the imidazole ring, respectively. The in-plane and out-of-plane bending vibrations of the imidazole ring manifest at frequencies ranging from 694 to 759 cm⁻¹ and from 995 to 1138 cm⁻¹, respectively. Furthermore, the spectral peak located at 420 cm⁻¹ is representative of the Zn-N bond^34,35,36.

FTIR spectrum of BaSO₄@SiO₂ (a) and BaSO₄@ZIF-8 (b).

UV–Vis/PL spectra

Figure 4a,b shows the UV–Vis spectra of BaSO₄@SiO₂ and BaSO₄@ZIF-8 in the 200–800 nm range. The maximum absorption for the BaSO₄@SiO₂ core shell was identified at 290 nm and for BaSO₄@ZIF-8, it was detected at 280 nm which is correlated with the UV area. Using the Tauc equation (Eq. 1), the energy of the compounds was measured from the UV–Vis data. The calculated energy values for BaSO₄@SiO₂ and BaSO₄@ZIF-8 are reported to be 5.5 eV and 5.04 eV respectively, as shown in Fig. 4c,d³⁷.

Note: “α” is a symbol of the absorption coefficient, “A” represents a constant value, “hν” denotes the energy of the incident photon, and “n” assumes a value of either 2 or 0.5 contingent upon the nature of the transition being classified as direct or indirect. Also, PL Spectroscopy is used to investigate the optical properties of synthesized nanomaterials. Figure 4e,f shows the PL spectra of BaSO₄@SiO₂ and BaSO₄@ZIF-8. The maximum emission was detected at 571 nm for the BaSO₄@SiO₂ core–shell and 551 nm for the BaSO₄@ZIF-8 composite.

UV–Vis of BaSO₄@SiO₂ (a)/BaSO₄@ZIF-8 (b), the Band gap of BaSO₄@SiO₂ (c)/BaSO₄@ZIF-8 (d), and PL spectra of BaSO₄@SiO₂ (e)/BaSO₄@ZIF-8 (f).

PXRD pattern

Figure 5 shows the PXRD spectrum, which confirms the type and crystalline size of BaSO₄@SiO₂ and BaSO₄@ZIF-8. According to the reported data for pure BaSO₄@SiO₂, the peak positions match well with the JCPDS card number # 96-900-0160 for BaSO₄ and # 96-901-0144 for SiO₂. Also, BaSO₄@ZIF-8 is compatible with the JCPDS card number 96-900-0160 for BaSO₄ and JCPDS # 96-411-9771 for ZIF-8. The patterns obtained from the XRD analysis for compounds BaSO_4, SiO_2, and ZIF-8 showed orthorhombic, monoclinic, and cubic phases and space groups Pnma, C12/c1, and Pn-3 m. As can be seen, PXRD patterns show high purity of the produced compounds. Additionally, the crystal sizes were received using the Scherrer formula (Eq. 2), and crystal sizes of BaSO₄@SiO₂ and BaSO₄@ZIF-8 were determined to be in the range of 35.6 and 28.4 nm.

Note: D represents the size (nm), \(\:\text{K}\) is quantified as 0.9, \(\:{\uplambda\:}\) is established at 0.154 nm, \(\:{\upbeta\:}\) denotes the line broadening measured at half the maximum intensity (FWHM), and \(\:{\uptheta\:}\) Signifies the angle.

PXRD of BaSO₄@SiO₂ (a) and BaSO₄@ZIF-8 (b).

FESEM/PSA/EDX/mapping

Figure 6a,b shows FESEM images of BaSO₄@SiO₂ and BaSO₄@ZIF-8. The morphological characteristics of the particles exhibit a spherical form in both instances. The analysis of the FESEM imagery unequivocally indicates that the NPs are uniformly dispersed, exhibiting no signs of aggregation. In addition, PSA analysis determined that BaSO₄@SiO₂ and BaSO4@ZIF-8 have dimensions of approximately 54 and 55 nm, respectively Fig. 6c,d. The EDX spectrum of BaSO₄@SiO₂ (a) and BaSO₄@ZIF-8 is shown in Fig. 7a,b. Results of EDX spectroscopy show that BaSO₄@SiO₂ includes elements of Oxygen, Sulfur, Barium, and Silica, and also, BaSO₄@ZIF-8 includes elements of Barium, Sulfur, Oxygen, Zinc, and Nitrogen. The atomic percentages for each compound are specified in the corresponding table. Also, Mapping images of synthesized nanomaterials showed that the elements were uniformly distributed, and the results are shown in Fig. 8a,b.

FESEM image (a)/PSA (c) of BaSO₄@SiO₂ and FESEM (b)/PSA (d) of BaSO₄@ZIF-8.

EDX analysis of BaSO₄@SiO₂ (a)/BaSO₄@ZIF-8 (b).

Mapping images of BaSO₄@SiO₂ (a)/BaSO₄@ZIF-8 (b).

TEM/PSA

The TEM and PSA images of BaSO₄@SiO₂ and BaSO₄@ZIF-8 are illustrated in Fig. 9a–d. Based on the observations derived from the TEM images, it can be concluded that the compounds exhibit a non-agglomerated structure and demonstrate a granular morphology attributed to their elevated surface energy and diminutive size. Furthermore, the average dimensions of BaSO₄@SiO₂ and BaSO₄@ZIF-8, as assessed utilizing Image J software, were approximately 181 nm and 51 nm, respectively, as depicted in Fig. 9c,d.

TEM image (a)/PSA (c) of BaSO₄@SiO₂ and TEM image (b)/PSA (d) of BaSO₄@ZIF-8.

BET

A standard method for determining the specific surface area of a sample is to use the BET theory for nitrogen adsorption and desorption isotherms^38,39. The obtained results from BET analysis for BaSO₄@ZIF-8 include the average pore size, pore volume, and surface area as 3.423 nm (34.229 Å), 0.312460 cm³/g, and 365.14 m²/g, respectively, and BET adsorption-desorption isotherm (a) and BJH pore-size distribution (b) analysis of BaSO₄@ZIF-8 are shown in Fig. 10a,b.

BET adsorption-desorption isotherm (a) and BJH pore-size distribution (b) of BaSO₄@ZIF-8.

Cytotoxicity evaluation

The cytotoxicity of BaSO₄@SiO₂ core–shell and BaSO₄@ZIF-8 nanocomposite was investigated on the B16F0 cell line via the MTT method (Fig. 11). The B16-F0 cell line was purchased from the Pasteur Institute in Tehran, Iran. The B16-F0 cells were seeded in a 96-well plate and allowed to attach to the bottom of each well for 24 h. Next, various concentrations of nanomaterials (0–1000 µg/mL) were introduced into each well and incubated for 24 h at 37 °C. Subsequently, MTT (aq.) was administered to each well for incubation at a controlled temperature of 37 °C for 4 h. Following this incubation period, the DMEM culture medium was discarded, and 100 µL of DMSO was introduced to facilitate the dissolution of the formazan crystals. The optical density was subsequently quantified at a wavelength of 570 nm utilizing an ELISA. The determined results were expressed as cell viability, which indicated IC₅₀ values of 232.2 and 537.5 µg/mL for BaSO₄@SiO₂ and BaSO₄@ZIF-8 in order. Following the 24 h incubation period, the MTT results showed reduced cell viability compared to control wells for each sample, with a p-value ≤ 0.0001. These findings can be useful in confirming or rejecting the use of BaSO₄@SiO₂ and BaSO₄@ZIF-8 in medical applications. Among the mechanisms that can be mentioned for activating apoptosis of stem cells is the role of reactive oxygen species (ROS) in the destruction of cells by NPs. As we know, ROS is mainly obtained from mitochondria and NADPH oxidase. Since hydroxyl ion has very high activity and reactivity, it can attack DNA as a strong oxidant and convert it from double-stranded to single-stranded. The cytotoxic mechanism of NPs is shown in Fig. 12.

Cytotoxic study of BaSO₄@SiO₂ and BaSO₄@ZIF-8 on B16-F0 cell line with MTT assay.

Cytotoxic proposed mechanism of NPs.

Photocatalytic process

The photocatalytic process of the synthesized nanomaterial was surveyed through UV–Vis spectrometry. To execute this experimental procedure, a volume of 50 mL of MB solution with a molarity of 10⁻⁵ M was treated with 30 mg of each of the specified compounds, followed by a sonication process lasting for 15 min. The solutions were agitated for 40 min in a light-deprived environment to attain the state of adsorption and desorption equilibrium. Then, the solutions containing the catalyst were exposed to UV-A light, and their absorption was determined in 30 min intervals. The outcomes showed a decrease in the maximum absorbance (663 nm), which is due to the photocatalytic attributes of synthesized compounds. The percentage of deterioration for samples was calculated using Eq. (3) and the results were determined to be 96% and 98.5% for BaSO₄@SiO₂ (Fig. 13a) and BaSO₄@ZIF-8 (Fig. 13b) respectively after 120 min^15,40.

Note: “A” is initial absorbance and “A^*” is final absorbance.

The outcome of reaction kinetics was plotted as Ln (C′/C) versus time. The line graph derived from this examination shows pseudo-first-order kinetic for both compounds. Rate constant (\(\:{K}_{obs}\)) was calculated using Eq. (4) and the results were determined to be − 0.0211 and − 0.0205 min⁻¹ for BaSO₄@SiO₂ (Fig. 13c) and BaSO₄@ZIF-8 (Fig. 13d), respectively.

Note: \({K_{obs}}\)= Rate constant and the concentration at different times are indicated by C′ and the concentration at the initial time is indicated by C.

Excitation via UV-A occurs when the energy hν of the light source is equal to or greater than the band gap energy of the NPs. The schematic of this mechanism is shown in Fig. 14 and as can be seen, the electrons in the valence band (VB) are excited and then transferred to the conduction band (CB), which ultimately leads to the creation of holes (h⁺) through the VB and the creation of electrons (e⁻) in the CB. After that h⁺ and e⁻ are transferred to the catalyst surface, while the radicals \(^{\cdot}{\text{O}}_{2}^{-}\)and \(^{\cdot}{\text{OH}}\) go to the MB molecules through the O₂ and H₂O molecules in the environment and cause its destruction³⁷. A comparison of the photocatalytic activity of this work with recent papers was presented in Table 2.

Photocatalytic results of BaSO₄@SiO₂ (a) and BaSO₄@ZIF-8 (b) and kinetic studies of BaSO₄@SiO₂ (c) and BaSO₄@ZIF-8 (d).

Photocatalytic activity mechanism.