Preparation of La₂Sn₂O₇, Mg₂SnO₄, and MgSn(OH)₆ and their antiviral/antibacterial activities

Abstract

In order to develop inorganic antibacterial and antiviral materials that function in the dark, we synthesized La₂Sn₂O₇, Mg₂SnO₄, and MgSn(OH)₆, which are compounds of La₂O₃ and MgO, which have solid basicity, and SnO₂, which has the Mars-van Krevelen (MvK) mechanism. After obtaining each sample as a single-phase white powder through hydrothermal or coprecipitation method, their antibacterial and antiviral activities were evaluated with reference to ISO procedures in the dark for bacteria and viruses with different characteristics. The dependence of activity on the evaluation method suggested that, except for Mg₂SnO₄, the proximity or contact of the viruses or bacteria to the sample surface played an important role in activity. Comparison of the activity of each sample with those of the simple oxides of constituent elements, La₂O₃, MgO, and SnO₂, clarified that pH, solid basicity, phosphate affinity, and the MvK mechanism contribute to antibacterial and antiviral activity. The extent of these contributions varied depending on the sample. The study results revealed that La₂Sn₂O₇ not only exhibits high antibacterial and antiviral activity against bacteria and viruses in the dark; it also has the ability to decompose organic dyes under UV irradiation. This material might be used as a newly developed environmental purification material providing continuous antibacterial and antiviral effects day and night, able to clean surfaces during UV light exposure.

Graphical Abstract

Introduction

Improving infection prevention techniques is an important issue for human society’s preparation for recurrence of a pandemic like that of COVID-19. One mode of infection prevention is antibacterial and antiviral materials. Inorganic antibacterial and antiviral materials that are generally effective against various bacteria and viruses have attracted particular attention. Bacteria and viruses are known to be less likely to develop resistance against such materials. Conventional inorganic antibacterial and antiviral materials include precious metals such as Ag and Cu [1–5], photocatalysts [6, 7], and lime [8, 9]. Nevertheless, precious metals have associated shortcomings including cost, changing coloration over time, and degradation of activity. Moreover, photocatalysts present issues such as limited use environments (necessity of light). Lime presents issues such as alkalization of soil and groundwater. Demand is therefore rising for the development of new inorganic antibacterial and antiviral materials able to overcome these difficulties.

Recently, we reported that LaMnO₃, LaCoO₃, and Y₂Sn₂O₇, which are composite oxides made of inexpensive elements, exhibit antibacterial and antiviral activities and organic substance decomposition activity in the dark [10, 11]. These studies revealed that the antibacterial and antiviral activities of these materials are attributable to the negative charge adsorption ability related to the solid basicity of La–O bonds and Y–O bonds, and to the decomposition and denaturation of organic substances by the Mars – van Krevelen (MvK) mechanism of Mn, Co, and Sn [10, 11]. Based on these results, we focused on La₂Sn₂O₇, Mg₂SnO₄ and MgSn(OH)₆ that are complex oxides of La₂O₃ or MgO, which are solid basic oxides [12], and SnO₂, which is reported to exhibit the MvK mechanism [11, 13]. Several reports have described the antibacterial activity [14–19] and antiviral activity [20] of La₂O₃. Also, MgO and its composite oxides have been studied actively for their antibacterial activity for a long time [21, 22]. Some reports have described their antiviral activity [9, 23]. Other reports have described the antibacterial activity of SnO₂ depending on its particle size and morphology [24–26]. Nevertheless, no report has described a study that has quantitatively evaluated the antibacterial and antiviral activities of La₂Sn₂O₇, Mg₂SnO₄, and MgSn(OH)₆.

Reportedly, La₂Sn₂O₇, alone or in combination with g-C₃N₄, decomposes organic dyes in water through photocatalytic reactions [27–29]. Similarly, Mg₂SnO₄ and MgSn(OH)₆ decompose organic dyes through photocatalytic reactions [30–32]. Therefore, confirming the antibacterial and antiviral activities of these materials in the dark might be eventually provide continuous antibacterial and antiviral effects day and night, and might facilitate surface cleaning using light irradiation. Based on the background presented above, we synthesized these composite oxides for this study, evaluated their antibacterial and antiviral activities using methods based on ISO standards, and investigated the mechanisms underlying their activity expression.

Experimental

Sample preparation and characterization

Synthesis of La₂Sn₂O₇ was conducted based on an earlier study [33]. After 10.825 g (2.50 × 10⁻²mol) of La(NO₃)₃·6H₂O (99.9% purity; Fujifilm Wako Pure Chemical Corp., Osaka, Japan) and 7.385 g (2.49 × 10⁻²mol) of Na₂SnO₃·3H₂O (90% purity; Kishida Chemical Co., Ltd., Osaka, Japan) were weighed and dissolved in 50 mL of ion-exchanged water, 2.0 mL of 30% (mass/mass) (3.18 × 10⁻²mol) NH₃ aqueous solution (Fujifilm Wako Pure Chemical Corp., Osaka, Japan) was added. The mixture was transferred to a Teflon container and was subjected to hydrothermal treatment at 200 °C for 24 h. After that hydrothermal treatment, the mixture was centrifuged at 5000 rpm to remove the supernatant. The precipitate was washed by repeating the process of washing with 75 mL of distilled water and centrifuging three times. After the washed precipitate was dried at 80 °C for 24 h, it was calcined at 600 °C for 2 h to obtain a sample powder.

On the other hand, Mg₂SnO₄ and MgSn(OH)₆ were synthesized according to the process condition obtained from preliminary investigation of the synthesis route. For Mg₂SnO₄, 4.066 g (1.96 × 10⁻²mol) of MgCl₂·6H₂O (98.0% purity; Fujifilm Wako Pure Chemical Corp., Osaka, Japan) and 3.506 g (9.80 × 10⁻³mol) of SnCl₄·5H₂O (98.0% purity; Fujifilm Wako Pure Chemical Corp., Osaka, Japan) were weighed and dissolved in 75 mL of ion-exchanged water. After stirring for 30 min, an aqueous NaOH solution obtained by dissolving 4.000 g (9.30 × 10⁻²mol) of NaOH (93.0% purity; Fujifilm Wako Pure Chemical Corp., Osaka, Japan) in 50 mL of ion-exchanged water was added and transferred to a Teflon container. After hydrothermal treatment at 200 °C for 24 h, the supernatant was separated by centrifugation at 10,000 rpm. The precipitate was washed by repeating the process of washing with 75 mL of distilled water and centrifuging three times. The washed precipitate was dried at 80 °C for 24 h. After drying, it was calcined at 900 °C for 12 h to obtain a sample.

Using coprecipitation method, we synthesized MgSn(OH)₆. Then 3.220 g (9.00 × 10⁻³mol) of SnCl₄·5H₂O and 1.867 g (9.00 × 10⁻³mol) of MgCl₂·6H₂O were dissolved in 50 mL of ion-exchanged water. An aqueous solution of NaOH obtained by dissolving it (2.230 g, 5.18 × 10⁻²mol) in 50 mL of ion-exchanged water was added to the solution to obtain a precipitate. This solution was then centrifuged at 10,000 rpm. The supernatant was removed. The precipitate was washed under the same conditions as those used for Mg₂SnO₄, and then dried at 80 °C for 24 h to obtain a sample.

For comparison, we also evaluated the simple oxides of the constituent elements: La₂O₃ (Fujifilm Wako Pure Chemical Corp., Osaka, Japan), MgO (Fujifilm Wako Pure Chemical Corp., Osaka, Japan), and SnO₂ (Kanto Chemical Co. Inc., Tokyo, Japan). Each sample was synthesized only once, and the same sample was used throughout all experiments to ensure consistency.

X-ray diffraction (XRD, XRD-6100; Shimadzu Corp., Kyoto, Japan) was used to identify the crystalline phase. The specific surface area of the sample powder was measured using nitrogen gas adsorption and the Brunauer–Emmett–Teller method (B.E.T., Belsorp-vac II; MicrotracBEL Corp., Osaka, Japan). The sample powder size and morphology were observed using a field-emission scanning electron microscope (FE-SEM, JSM7500 F; JEOL Ltd., Tokyo, Japan). The ultraviolet–visible spectra of the sample powder were obtained using a UV–visible light spectrophotometer (UV–Vis, V-660; Jasco Corp., Tokyo, Japan). The surface composition was measured using X-ray photoelectron spectroscopy (XPS, PHI 1600; Ulvac-Phi Inc., Kanagawa, Japan) with Al-Kα ray. The bulk composition of samples was measured using an inductive coupled plasma optical emission spectrometer (ICP-OES, 5100 VDV ICP-OES; Agilent Technologies Japan Ltd., Tokyo, Japan).

Antiviral and antibacterial activity evaluation

For antibacterial testing, Staphylococcus aureus (S. aureus, NBRC 12732) was used as the gram-positive bacterium. Escherichia coli (E. coli, NBRC 3972) was used as the gram-negative bacterium. For antiviral testing, bacteriophage Qβ (Qβ, NBRC 20012, host E. coli, NBRC 106373) was used as the non-enveloped virus. Bacteriophage Φ6 (Φ6, NBRC 105899, host Pseudomonas syringae, NBRC 14084) was used as the enveloped virus. These two viruses were selected for this study because they are listed in the ISO standard (Qβ), are highly safe, and are typical viruses with a relatively large amount of data. For the test, a 2.5 × 2.5 cm glass plate was used with 450 µg of powdered sample supported on it. Antibacterial and antiviral activities were evaluated according to film adhesion method procedures presented in ISO17094 and ISO18071. In addition, tests using the dissolved ion contact method were conducted to evaluate the antibacterial and antiviral activities of ions dissolved from the powder sample. For this test, the amounts of ions dissolved in the film adhesion method were adjusted to be equivalent to that of the virus liquid or of the bacteria liquid. Each activity was measured three times. The amounts of dissolved ions were investigated using ICP. The test procedure flow is presented in Fig. SI-1. These test procedures and conditions are the same as those used for earlier studies [20, 34, 35].

To investigate the mechanisms of antibacterial and antiviral activity expression, we performed enzyme protein inactivation tests, dye adsorption tests, and phosphate adsorption tests. The enzyme protein used was catalase (Fujifilm Wako Pure Chemical Corp., Osaka, Japan). The test procedures are displayed in Fig. SI-2. Cationic dye methylene blue (Fujifilm Wako Pure Chemical Corp., Osaka, Japan) and anionic dye eosin Y (Fujifilm Wako Pure Chemical Corp., Osaka, Japan) were used for dye adsorption tests. For phosphate adsorption tests, we used a neutral phosphate pH standard solution (Fujifilm Wako Pure Chemical Corp., Osaka, Japan) diluted 1000-fold. These tests were conducted using the same procedures and conditions as those used for earlier studies [10, 11, 36]. The enzyme protein inactivation tests were conducted three times. However, due to the high reproducibility of the adsorption test, it was performed only once. Details of procedures are described in Supporting Information. In addition to the series of tests, we performed a cytotoxicity test using MDCK cells. This is because MDCK cells are easy to culture, highly sensitive to drugs, yield reproducible results, and are animal-derived cells expected to have a complementary relationship with human cells. We have consistently used MDCK cells in our previous studies [10, 11, 20, 34, 37]. Tests were performed in triplicate for each sample using a method described in earlier reports [20, 34] (Fig. SI-3). The test was performed four times.

Measurement of changes in organic dye concentration under light irradiation

Concentration changes of organic dyes under UV light irradiation were evaluated for the three synthesized composite oxides using methylene blue (Fujifilm Wako Pure Chemical Corp., Osaka, Japan) and methyl orange (Fujifilm Wako Pure Chemical Corp., Osaka, Japan) based on procedures used for earlier studies [37, 38]. The spectrum of the irradiated UV light is shown in Fig. SI-4. The concentration changes of the dye solutions adjusted initially to 7.5 mg/L were measured from the absorbance change measured using UV–vis. The methylene blue solution was measured in the wavelength range of 550–750 nm [39]. The methyl orange solution was measured for wavelengths of 350–600 nm [40]. Details of procedures are presented in Supporting Information.

Results and discussion

Characters of synthesized samples

The synthesized sample powders were white. Their XRD diffraction patterns are depicted in Fig. 1. They were identified as La₂Sn₂O₇ (JCPDS card #13-81), Mg₂SnO₄ (JCPDS card #24-723), and MgSn(OH)₆ (JCPDS card #9-27), confirming that they were all single-phase powders. The oxides used for comparison were also confirmed as single-phase powders (Fig. SI-5).

Fig. 1.

XRD patterns of prepared samples

The specific surface areas of the synthesized composite oxides and of the simple oxides of the reagents measured were 27 m²/g, 23 m²/g, 81 m²/g, 4.7 m²/g, 29 m²/g, and 39 m²/g, respectively, for La₂Sn₂O₇, Mg₂SnO₄, MgSn(OH)₆, La₂O₃, MgO, and SnO₂. The specific surface area of La₂O₃ was less than those of the other samples, whereas that of MgSn(OH)₆ was larger than those of the others. The high specific surface area of MgSn(OH)₆ is attributable to the fact that it had not been subjected to heat treatment. The specific surface areas of the samples other than La₂O₃ and MgSn(OH)₆ were almost identical: 23–39 m²/g. SEM images of the synthesized samples are presented in Fig. 2. Spherical particles of La₂Sn₂O₇ and Mg₂SnO₄ were approximately 100 nm. The MgSn(OH)₆ were spherical particles of approx. 30 nm. SEM images of the simple oxides are presented for comparison in Fig. SI-6.

Fig. 2.

SEM images of prepared samples: a La₂Sn₂O₇, b Mg₂SnO₄, and c MgSn(OH)₆

The elemental molar ratios of the synthesized composite oxides found using ICP and XPS were the following: La:Sn = 51:49 (ICP), 42:58 (XPS) for La₂Sn₂O₇; Mg:Sn = 69:31 (ICP), 50:50 (XPS) for Mg₂SnO₄; and Mg:Sn = 52:48 (ICP), 32:68 (XPS) for MgSn(OH)₆. Results obtained using ICP reflect the bulk composition ratio, which is roughly the same as the stoichiometric composition (set composition). XPS revealed the composition ratio near the particle surface. In all samples, Sn was in excess compared to the stoichiometric composition. This excess is expected to be attributable to the differences in the degrees of supersaturation of Mg, La, and Sn during the hydrothermal treatment and neutralization coprecipitation process. This trend was particularly noticeable in the Mg-Sn samples.

Antiviral and antibacterial activities

The antiviral activity test results are displayed in Fig. 3. The graph shows the value of -log(N⁄N₀), where N₀ represents the number of virus plaques at 0 h; N denotes that after 4 h. All error bars in the figures in this study represent standard deviations. In this graph, the number of viruses decreases by 1/10 when the vertical axis increases by one scale. Higher vertical axis values are associated with higher antiviral activity. The test results for SnO₂ using the film adhesion method were referred from our previous study [11]. Photographs of actual virus plaques are depicted in Fig. SI-7. For Qβ, all samples except MgSn(OH)₆ and SnO₂ exhibited high antiviral activity. Particularly, the antiviral activity of La₂Sn₂O₇ exceeded that of La₂O₃, despite being combined with almost inactive SnO₂. For Qβ, the antiviral activity found using the film adhesion method exceeded that found using the dissolved ion contact method overall, suggesting that the approach or direct contact of the virus to the sample surface, rather than the dissolved ions, is important for antiviral activity.

Fig. 3.

Antiviral activities of the samples against a Qβ and b Φ6

On the other hand, in the activity test against Φ6, antiviral activity was confirmed by the film adhesion method for samples other than SnO₂. For Φ6, MgSn(OH)₆, which possessed no activity for Qβ, exhibited antiviral activity. Its activity was equivalent to that of MgO despite being combined with inactive SnO₂. As with the test results for Qβ, La₂Sn₂O₇ exhibited activity greater than that of La₂O₃. Test results for Φ6 confirmed that the antiviral activity achieved using the film adhesion method also exceeded that obtained using the dissolved ion contact method, except for SnO₂.

One plausible reason why SnO₂ alone exhibited activity against Φ6 in the dissolved ion contact method is that SnO₂ adsorbed some components of the 1/500 NB medium during preparation of the ion leakage solution, which might have destabilized Φ6. In fact, Φ6 is more sensitive than Qβ. It is deactivated even by slight fluctuation of environmental conditions. Because SnO₂ showed almost no ion elution and because no activity was observed with the film contact method, the possibility of an effect of adsorption of the medium was inferred. To evaluate the effect, we investigated the change in the number of viruses when contacted with distilled water containing no NB medium. Results confirmed a certain degree of inactivation of Φ6 (Fig. SI-8). Because the meat extract component of the NB medium is not clear, clarifying what caused the inactivation is difficult. However, inactivation of SnO₂ by the dissolved ion contact method is likely to depend on the experiment conditions rather than on ions derived from the material. Although evaluation using a different test method is required, this is left as a topic for future study.

Results of the antibacterial testing are presented in Fig. 4. The graph shows the value of -log(N⁄N₀), where N₀ stands for the number of colonies at 0 h; N denotes the number after 4 h. A higher value on the vertical axis represents higher antibacterial activity. Results of the test using the film adhesion method for SnO₂ were taken from our earlier study [11]. Photographs of actual colonies are displayed in Fig. SI-9. La₂Sn₂O₇, La₂O₃, and MgO exhibited antibacterial activity against E. coli. The antibacterial activities of samples containing La was particularly high. Furthermore, La₂Sn₂O₇, despite being combined with inactive SnO₂, had higher antibacterial activity than that shown by La₂O₃. Similarly, samples containing La showed high antibacterial activity against S. aureus. The activities of La₂O₃ and La₂Sn₂O₇ against S. aureus were almost identical. Regarding antibacterial tests, the film adhesion method showed higher activity than the dissolved ion contact method, confirming that some approach or direct contact with the powder surface is important for activity. The antibacterial and antiviral activity of La₂Sn₂O₇, which had high antibacterial and antiviral activity throughout, was equal to or greater than that of Y₂Sn₂O₇, a rare earth Sn-based oxide, evaluated by a similar method in our previous study [11].

Fig. 4.

Antibacterial activity of the samples against a E. coli and b S. aureus

Table 1 presents the ion concentrations in the ion leakage solution used in the pH change and dissolved ion contact method. Both MgO and Mg₂SnO₄ had large amounts of Mg eluted. Their pH values were high: 11 and 9.9, respectively. Reportedly, pH of 9 or higher contributes to the inactivation of Qβ at room temperature (25 °C) [41]. Similarly, pH of 10 or higher contributes to the inactivation of Φ6 [42]. It is also generally known that high pH also contributes to the denaturation of proteins [43]. These samples exhibited antiviral activity very likely because of pH change. However, S. aureus is difficult to kill even at a high pH of approximately 11 [44]. Reportedly, E. coli is killed only to a slight degree at pH lower than 11 [44, 45]. Referring to pH change effects found from an earlier study [44], E. coli apparently has poorer pH resistance than S. aureus does. In fact, antibacterial test results indicate that both MgO and Mg₂SnO₄ samples exhibited low activity against S. aureus. Only MgO, which changed to pH 11, showed slight antibacterial activity against E. coli. In other samples, the amount of ion elution was small. The change in pH was slight. Therefore, the effect of pH is expected to be minor to these bacteria. The reason why activity because of pH was not observed in the dissolved ion contact method might be that the pH decreased because of mixing with the virus liquid, or that the locally high pH near the powder contributed to the activity. Measuring the pH of a liquid containing a virus is difficult because of biohazard countermeasures. It is also difficult to evaluate pH only near the surface. This subject remains as a challenge for future studies.

Table. 1.

pH and ion elution amounts of samples

	pH (4 h)	La (µM)	Mg (µM)	Sn (µM)	I (µM)
La2Sn2O7	5.8	6.1	—	N. D.	—
Mg2SnO4	9.9	—	321	N. D.	—
MgSn(OH)6	8.1	—	16.7	N. D.	—
La2O3	7.3	0	—	—	—
MgO	11	—	580	—	—
SnO2	6.0	—	—	15.2	—
Control	6.0	—	—	—	—

Results of the dye and phosphate ion adsorption tests are depicted in Fig. 5. Results of the dye adsorption tests conducted to evaluate the contributions of electrostatic interactions to adsorption confirmed that La₂Sn₂O₇, La₂O₃, and MgO can adsorb the anionic dye eosin Y well. Reportedly, the viruses and bacteria used for the tests all have a negative charge, ranging from near neutral to near basic [46–49]. Materials that adsorb eosin Y well are useful for adsorbing various bacteria and viruses. However, Mg-Sn-based samples and SnO₂ were able to adsorb the cationic dye methylene blue well. These materials are expected to repel bacteria and viruses electrostatically, making them unfavorable for adsorption to the surface. It was confirmed that MgO is able to adsorb eosin Y well and to contribute to negative charge adsorption, although this effect was not observed in Mg-Sn-based samples. XPS analysis revealed that the Mg-Sn-based samples had an excess of Sn on the surface compared to the stoichiometric composition. Probably, this is true because the negative charge from the excess Sn on the surface cancels out the positive charge from Mg. MgSn(OH)₆ and SnO₂, which showed positive charge adsorption, were found to have no antiviral activity against Qβ. It was inferred that the change in pH contributed to the activity of Mg₂SnO₄, counteracting the effect of repulsion caused by electrostatic interactions.

Fig. 5.

Results of dye and phosphate ion adsorption tests

When looking at the results of activity for Φ6, all samples other than SnO₂ exhibit activity. Because it is possible that adsorption mechanisms other than electrostatic interactions might contribute to the activity, we conducted a phosphate adsorption test. The results of the phosphate adsorption test confirmed that samples containing Mg and La adsorbed phosphate well. Reportedly, MgO and La₂O₃ have high affinity for phosphate [50–55]. The phosphate adsorption test results suggest that this effect was maintained even by forming composite oxides. The envelopes of enveloped viruses such as Φ6 and the cell membrane of bacteria are made of phospholipids [56–58] and contain phosphate. The affinity with phosphate on the surface of bacteria and enveloped viruses is highly likely to contribute to the adsorption of samples containing Mg and La, which have affinity for phosphate. Even if adsorption by electrostatic interaction does not occur for Φ6, the surface of which is covered with a lipid membrane containing phosphate, a certain degree of adsorption to the surface is expected to be possible if some affinity for phosphate exists. Therefore, it is expected that MgSn(OH)₆, which showed no negative charge adsorption to the surface because of electrostatic interaction, exhibited activity for Φ6, which can be adsorbed to the surface because of phosphate affinity. In addition, the surface of Qβ is covered with a protein capsid with little phosphate on the surface. Therefore, it is possible that phosphate affinity did not contribute to the antiviral effect. Specific examination of the results of the phosphate adsorption test for La₂O₃ and MgO showed La₂O₃ to have higher phosphate adsorption, reflecting higher phosphate affinity than MgO. Reportedly, La³⁺ and La₂O₃ put the bacteria in a state of phosphate starvation by depriving bacteria of phosphate and forming LaPO₃ [17]. It is expected that the inclusion of La, which has higher phosphate affinity than that of Mg, should be advantageous for weakening bacteria through phosphate starvation. This phenomenon is expected to be the reason why samples containing La had higher antibacterial properties than samples containing Mg.

Bacteria and viruses might maintain their infectivity and proliferation ability when adsorbed only by electrostatic interactions or phosphate affinity. For the film adhesion method, viral or bacterial suspensions were collected with sample plates into SCDLP medium to stop their antiviral or antibacterial activity. Therefore, when viruses or bacteria did not lose their infectivity and when proliferation ability on the sample plates remains, no antiviral or antibacterial activity would be confirmed. For samples in which antibacterial and antiviral activity were confirmed, reactions on the surface after adsorption are expected to contribute to the activity. We investigated the inactivation of proteins, which would be a part of such reactions. Viruses use proteins to invade cells [57]. Bacteria use proteins to transfer substances between the outside environment and the bacteria themselves [43, 58]. Therefore, protein inactivation will cause them to lose their infectivity and proliferation ability. To examine the effects of protein inactivation on bacteria and viruses, we evaluated inactivation of the enzyme protein catalase.

Catalase inactivation test results are presented in Fig. 6. All oxides were confirmed as inactivating catalase. Particularly, La₂Sn₂O₇ exhibited an inactivation rate higher than that of La₂O₃. Also, MgSn(OH)₆ possessed an inactivation rate equivalent to that of MgO, even though it was combined with SnO₂, which has a low inactivation rate. These results confirmed that all samples have the potential to inactivate virus and bacterial proteins. In addition to the high pH of MgO and Mg₂SnO₄, acid–base reactions might contribute to catalase inactivation. Because the acid–base reaction with E. coli might be related to the antibacterial activity of MgO [59], it is possible that MgO and La₂O₃, which have similar solid basicity, also contribute to protein inactivation through acid–base reactions. Additionally, organic substance oxidation by the MvK mechanism [11] might contribute to inactivation of SnO₂.

Fig. 6.

Results of catalase inactivation test

Next, an ethanol impregnation test was conducted to evaluate effects of organic substances oxidation by the MvK mechanism. First, after 0.10 g of the sample powder was mixed into 10 mL of ethanol (Fujifilm Wako Pure Chemical Corp., Osaka, Japan) in the dark, it was shaken at 110 rpm at room temperature for 6 h. The powder was then separated by suction filtration and dried at room temperature for 24 h. We conducted XPS analyses of the dried powder to evaluate the presence or absence of oxidation–reduction reactions based on the peak shift in the Sn3d orbital. Figure 7 presents the XPS spectra obtained before and after the ethanol impregnation test. When Sn⁴⁺ is reduced to Sn²⁺ because of the oxidation of organic substances by the MvK mechanism, the XPS spectrum shifts to the lower energy side [11, 60]. After ethanol impregnation, the peaks shifted to the lower energy side by 0.35 eV for La₂Sn₂O₇, 0.15 eV for MgSn(OH)₆, and 0.028 eV for SnO₂, suggesting that the oxidation of organic substances by the MvK mechanism might contribute to protein inactivation in these samples. The electrode potential of C₂H₅OH + 3H₂O / CO₂ + 12H⁺ + 12e^- is 0.084 V vs. SHE [61]. This value is lower than that of 0.844 V vs. SHE by SnO₃^2- + 6H⁺ + 2e^- / Sn²⁺ + 3H₂O [62] and E₀ = 0.15 V vs. SHE by Sn⁴⁺ + 2e^- / Sn²⁺. Therefore, ethanol oxidation by Sn(IV) is feasible.

Fig. 7.

Sn3d spectra of XPS before and after ethanol exposure: a La₂Sn₂O₇, b Mg₂SnO₄, c MgSn(OH)₆, and d SnO₂

No peak shift to the lower energy side was observed for Mg₂SnO₄. Of these oxides, only Mg₂SnO₄ elutes a large amount of ions, which might be associated with inhibition of oxidation. However, additional details must be accumulated. In addition, even samples that have a certain oxidizing power by the MvK mechanism are unlikely to be active if they are not adsorbed to the surface. In fact, although results suggest that the MvK mechanism has been obtained for SnO₂, no antibacterial or antiviral activity has been shown against any virus or bacterium. This finding suggests that adsorption to the oxide surface is necessary for the oxidation and subsequent inactivation of proteins. Reportedly, catalase is inactivated by modification of lysine, histidine, and arginine residues [63]. It is therefore possible that some modification is occurring at these sites in the protein, but the precise site at which these substances act in bacteria and viruses remains a topic for future research.

The results presented above can be summarized. Although Mg₂SnO₄ has difficulty in adsorbing bacteria and viruses through electrostatic interactions, the large pH change is expected to contribute to the antiviral activity of proteins. Although Φ6 would be adsorbed by Mg₂SnO₄ because of its affinity for phosphate, no result has been obtained to suggest oxidation by the MvK mechanism. Therefore, protein inactivation by pH change is expected to be the most likely mechanism for virus inactivation. However, because bacteria have a certain resistance to alkaline pH, Mg₂SnO₄ showed no antibacterial activity. Cells have chaperones that have the ability to repair proteins denatured by pH, etc. [43]. Therefore, it is possible that this effect inhibited the inactivation of bacteria. The lack of contribution of oxidation by the MvK mechanism and the fact that bacterial adsorption depends only on phosphate affinity should also be regarded as factors leading to the lack of activity against bacteria.

Because the amount of ion leakage from MgSn(OH)₆ was small, leading to a small pH change, little activity because of pH was expected. Furthermore, MgSn(OH)₆ has phosphate affinity derived from Mg, but no negative charge adsorption caused by electrostatic interaction occurs. Therefore, Qβ, which has little phosphate on the surface, could not be adsorbed to the MgSn(OH)₆ surface, which leads to little activity. However, because Φ6 has phosphate, it was able to adsorb to the MgSn(OH)₆ surface. Results suggest that the MvK mechanism and resultant protein inactivation on MgSn(OH)₆ surface inactivated Φ6. For bacteria larger than Φ6, phosphate affinity alone is insufficient for adsorption. The effect of oxidation by the MvK mechanism cannot be fully exerted, which would have led to low antibacterial activity.

However, La₂Sn₂O₇ would easily adsorb various bacteria and viruses because it has both electrostatic interactions and phosphate affinity. In addition, organic substance oxidation by the MvK mechanism has been suggested, which leads to protein inactivation. Unlike MgSn(OH)₆, La₂SnO₄ can adsorb bacteria strongly because of both phosphate affinity and electrostatic mutual interactions. Therefore, the ability to exert the effect of oxidation fully by the MvK mechanism led to antibacterial activity. Furthermore, La₂O₃ is reported to destroy the genes of bacteria and viruses by itself [17, 64]. This effect might therefore contribute to activity.

Cytotoxicity test results are presented in Fig. 8. No decrease in viable cells was observed, confirming that no risk of cytotoxicity exists.

Fig. 8.

Results of cytotoxicity test

Measurement of changes in organic dye concentration under light irradiation

Changes in methyl orange concentrations for La₂Sn₂O₇, Mg₂SnO₄, and MgSn(OH)₆ caused by UV irradiation are displayed in Fig. 9a–c. Only La₂Sn₂O₇ shows a decrease in the methyl orange concentration caused by light irradiation. No change in concentration was confirmed for Mg₂SnO₄ or MgSn(OH)₆. The change in concentration progressed gradually over 8 h and did not occur in the dark, making it unlikely to be attributable to simple adsorption: it is more likely that decomposition is progressing. Earlier studies [27–29] have demonstrated that decomposition advances because of a photocatalytic reaction.

Fig. 9.

Concentration change of methyl orange under UV irradiation: a La₂Sn₂O₇, b Mg₂SnO₄, and c MgSn(OH)₆. Concentration change of methylene blue under UV irradiation: d La₂Sn₂O₇

The UV-Vis spectra of the synthesized samples are depicted in Fig. 10. All samples were confirmed to absorb the light in UV range. The electronic band dispersions of La₂Sn₂O₇ and MgSn(OH)₆ calculated by first-principles calculations are presented in Fig. 11. The first-principles calculations in this study were conducted using the Projector Augmented-Wave (PAW) method [65] as implemented in the VASP code [66, 67]. The exchange-correlation functional used was GGA-PBEsol [68]. The geometry was optimized using a plane wave basis set with an energy cutoff of 550 eV and a k-point mesh determined based on the convergence of the total energy. Note that Mg₂SnO₄ has a disordered structure in which the positions of Mg and Sn are random. It was not possible to apply first-principles calculations to this material, because a calculable structure could not be obtained.

Fig. 10.

UV–vis spectra of prepared samples: a La₂Sn₂O₇, b Mg₂SnO₄, and c MgSn(OH)₆

Fig. 11.

Electronic band structure of prepared samples: a La₂Sn₂O₇ and b MgSn(OH)₆

As a result of the calculation, the irreducible representation of the conduction band minimum (CBM) of La₂Sn₂O₇ is ${\mathrm{\Gamma }}_1^{+}$, and that of the valence band maximum (VBM) is ${\mathrm{\Gamma }}_4^{+}$, indicating that the direct transition between CBM and VBM is a forbidden transition. Therefore, it is possible that the direct allowed transition from the electronic state with the irreducible representation of ${\mathrm{\Gamma }}_4^{-}$ under the VBM to the CBM contributes to the light absorption. On the other hand, the irreducible representation of the CBM of MgSn(OH)₆ is ${\mathrm{\Gamma }}_1^{+}$, and that of the VBM is ${\mathrm{\Gamma }}_4^{-}$, the direct allowed transition between them would contribute to the light absorption. Based on the UV-Vis spectrum and the results of the band calculation, a Tauc plot was made (Fig. SI-10) to calculate the optical band gap. The optical band gap of La₂Sn₂O₇ was 3.9 eV, and that of MgSn(OH)₆ was 5.6 eV. The wavelengths corresponding to the band gaps of La₂Sn₂O₇ and MgSn(OH)₆ are 318 nm and 221 nm, respectively. Looking at the wavelength of the UV lamp used (Fig. SI-4), the wavelength range below 250 nm is not included. Therefore, MgSn(OH)₆ should not be excited by the UV lamp we used in this study.

Although the ethanol oxidation test results suggest that MgSn(OH)₆ oxidized organic substances by the MvK mechanism, the decomposition test of the dye did not suggest oxidative decomposition. There are two plausible reasons for this difference. First, methyl orange, which is anionic, is difficult to adsorb onto the MgSn(OH)₆ surface, where negative charge adsorption is disadvantageous. Therefore, the frequency of contact with the surface decreased, which reduced the MvK mechanism contribution to the decomposition. Second, the oxidizing power of tin was insufficient to oxidize methyl orange. Specifically examining the test results obtained in the dark for La₂Sn₂O₇, which is assumed to be positively charged and to have a high frequency of contact with the surface, almost no decrease in the methyl orange concentration was observed. Therefore, it is highly likely that the oxidizing power of tin has difficulty decomposing methyl orange. As described before, the maximum standard electrode potential of Sn is E₀ = 0.844 V vs. SHE for SnO₃^2- + 6H⁺ +2e^- / Sn²⁺ + 3H₂O [62]. The redox potential for methyl orange oxidation varies depending on the reaction conditions and measurement method, but reportedly, a potential of approximately 0.955 V is necessary [69]. Although the band gap was not determined, the UV lamp wavelength might be also insufficient for excitation of Mg₂SnO₄.

Methylene blue decomposition tests were performed only on La₂Sn₂O₇, which was confirmed to decompose methyl orange. The results, shown in Fig. 9d, revealed that methylene blue was also slightly decomposed. Because methyl orange is anionic, it frequently comes into contact with the La₂Sn₂O₇ surface because of negative charge adsorption, whereas methylene blue is cationic. For that reason, it comes into contact with the La₂Sn₂O₇ surface less frequently, which might have reduced its activity. The change in dye concentration under light irradiation indicated that La₂Sn₂O₇ can be used as a new environmental purification material that has continuous antibacterial and antiviral effects day and night. It can clean surfaces by light irradiation.

Conclusion

For this study, single-phase powders of La₂Sn₂O₇, Mg₂SnO₄, and MgSn(OH)₆ were synthesized. Their antibacterial and antiviral activities in the dark and changes in dye concentration achieved under light irradiation were evaluated. La₂Sn₂O₇ was found to have high antibacterial and antiviral activity against bacteria and viruses of all types examined. The antibacterial and antiviral activities of La₂Sn₂O₇ are attributable to adsorption by electrostatic interactions and phosphate affinity, with subsequent oxidation by the MvK mechanism. Results also show that Mg₂SnO₄ exhibited antiviral activity against various viruses. Moreover, MgSn(OH)₆ showed activity against enveloped viruses. The antiviral activity of MgSn(OH)₆ is likely to be attributable to adsorption by phosphate affinity, with subsequent inactivation of proteins on the surface by the MvK mechanism. Mg₂SnO₄ exhibited antiviral activity by changing the pH to basic. However, in the Mg–Sn system, adsorption of bacteria to the surface depends only on phosphate affinity. The bacteria have a certain degree of resistance to pH changes. Therefore, sufficient antibacterial activity was not obtained. Additionally, it was confirmed that La₂Sn₂O₇ decomposed organic dyes when irradiated with ultraviolet light. Findings of this study revealed La₂Sn₂O₇ as a newly developed environmental purification material that exhibits not only antibacterial and antiviral activity but also photocatalytic activity.

Author contributions

RK was involved in data curation, methodology, investigation, validation, funding acquisition, and writing—an original draft. KS was involved in data curation, methodology, and investigation. YM was involved in software, investigation, data curation, methodology, validation, and writing—review & editing. TI and KK were involved in data curation, methodology, and validation. TN was involved in data curation, methodology, and investigation. HI was involved in data curation, methodology, investigation, validation, and writing—review & editing. AN was involved in conceptualization, supervision, data curation, project administration, funding acquisition, resources, and writing—review and editing.

Funding

This work was supported in part by a Grant-in-Aid for Scientific Research (23H01680) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. This work was also supported in part by JST BOOST, Japan Grant Nos. JPMJBS2417 and JPMJBS2430.

Data availability

The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary information

The online version contains supplementary material available at 10.1007/s10856-025-06921-3.