Facile synthesis of Rh/Ti3+-TiO2 nanocomposites and its photodisinfection properties on Staphylococcus aureus under visible-NIR excitation

doi:10.21203/rs.3.rs-25790/v1

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Facile synthesis of Rh/Ti3+-TiO2 nanocomposites and its photodisinfection properties on Staphylococcus aureus under visible-NIR excitation

https://doi.org/10.21203/rs.3.rs-25790/v1

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In this study, we synthesized a series of rhodium-modified and Ti³⁺ self-doped TiO₂ (Rh/Ti³⁺-TiO₂) nanocomposites via the one-pot method. We prepared samples of Rh/Ti³⁺-TiO₂, which were analyzed using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Electron Spin Resonance (ESR), and Uv-vis-NIR analysis. We found that the ability of TiO₂ to absorb near-infrared and visible light was significantly improved by the Rh/Ti³⁺-TiO₂ nanocomposites, due to Ti³⁺ doping as well as modification of Rh. The disinfection properties of these materials were tested using Staphylococcus aureus under visible light and NIR light excitations. The synthesized photocatalyst was found to exhibit significantly enhanced photocatalytic inactivation of S. aureus under both visible and NIR light irradiation, as compared to pure TiO₂. This was particularly true with respect to the 5% Rh/Ti³⁺-TiO₂ sample. Our results suggest that the Rh/Ti³⁺-TiO₂ composites could extend the range of optical response range of pure nano TiO₂ materials to the Vis -NIR region.

Ceramics

Rh/Ti3+-TiO2

Near-infrared light

Nanocomposites

Self-doped

Antibacterial activity

Photodisinfection technology based semiconductors has attracted attention of many researchers as a novel potential bacterial inaction technology in food safety area [1]. Among various studied, TiO₂ was one of the most attractive photocatalyst owing to its properties of high photocatalytic efficiency, high chemical and physical stability, and low cost [2–5]. Previous reports have demonstrated that the TiO₂ photocatalyst can be used for food microorganism disinfection under UV or visible light irradiation [6–9]. Jing and Hung reported that the Escherichia coli O157: H7 cell can be inactivated with TiO₂ nanoparticle embedded cellulose acetate films under UV-A light illumination [1]. Muranyi et al. found that the Kocuria rhizophil could be reduced 3.3 orders of magnitude on titanium dioxide coated glass slide after 4 h of UV-A light exposure [8]. Xu et al. studied the antibacterial effect of the graphene oxide and chitosan biopolymer loaded TiO₂, and found that the synthesized nanocomposites exhibited high antibacterial activity against Aspergillus niger and Bacillus subtilis [9]. The photocatalytic antimicrobial performance of a TiO₂ nanocomposite with low-density polyethylene (LDPE) film and its fresh-keeping test for fresh pear were studied by Li et al [10].

However, the low quantum yields and poor efficiency of visible-light use are the two primary challenges for practical applications of pristine TiO₂ [11,12]. A number of methods have been used to overcome these challenges, such as doping with metal or non-metal elements, grouping with other semiconductors, and grouping with plasmonic metal [2,13,14]. Recently, studies on Ti³⁺ self-doped TiO₂ have been carried out due to the ability of this process to overcome the above disadvantages [15–17]. The addition of oxygen vacancies (Ov) or Ti³⁺ to the altered TiO₂ grid greatly improves the photocatalytic ability within the band of visible light [18,19]. Ti³⁺, when self-doped, also increases the efficiency of separation of the photogenerated charge carriers [20]. The Rh³⁺-modified TiO₂ exhibited much higher photoactivity than TiO₂ modified by either Cu²⁺ or Fe³⁺ [21].

In this study, we synthesized rhodium-modified and Ti³⁺ self-doped TiO₂ (Rh/Ti³⁺-TiO₂) nanocomposites using a facile, solvothermal method. In the Vis-NIR area, we found a significantly rate of absorption in the synthesized Rh/Ti³⁺-TiO₂ nanocomposite samples after analyzing the structure of their crystals using TEM, XRD, and ESR methods. We also confirmed the Ti³⁺ by XPS and ESR analysis. Antibacterial activity was tested using foodborne pathogenic bacteria of Staphylococcus aureus under both visible light and near-infrared light irradiation. Our disinfection results demonstrated that S. aureus could be disinfected via illumination with Vis-NIR light in the Rh/Ti³⁺-TiO₂ nanocomposite.

Reagents

TiCl₄ (99.5%), RhCl₃·3H₂O, (38.5–42.5%), and NaOH (98%), were purchased from Aladdin Industrial Corporation (Shanghai, PR China), absolute ethanol (99.8%) was purchased from Sinopharm. Chemical Reagent Co., Ltd. (Shanghai, PR China), and used without further purification. Nutrient agar was BR grade and purchased from Beijing Aoboxing Biotechnology Co., Ltd. (Beijing, PR China). Deionized water was prepared from a lab ultra-pure water purifier.

Fabrication and characterizations of Rh/Ti³⁺-TiO₂ Nanocomposite

We synthesized the Rh/Ti³⁺-TiO₂ samples using a one-pot solvothermal reaction, which is briefly described below. First, we dissolved 6 mmol titanium tetrachloride and 0.06 mmol rhodium chloride hydrate (1% molar ratio of titanium tetrachloride) in 30 mL of ethanol, which resulted in a mixture. Then we added 30 mL of NaOH ethanol solution in order to get a fixed molar ratio of NaOH to TiCl₄ and RhCl₃ of 4:1 and 3:1, at room temperature. During this process, we observed a precipitate while the reaction occurred, for 30 min under vigorous agitation. Lastly, we placed the mix into a 100 mL Teflon-lined autoclave bottle where it was stored for 4 h at 180 ℃. The mix was then allowed to cool to room temperature. Following the solvothermal reaction, we washed the precipitate three times with distilled water and ethanol, after which they were dried at 80 ℃ for 12 h. After the precipitate dried, it was crushed into a fine powder and marked as 1% Rh/Ti³⁺-TiO₂. We prepared samples with different Rh molar ratios of 3%, 5%, and 7% Rh according to this method, while the TiO₂ without Rh was also prepared for use as a reference.

We analyzed the structure of the crystal using powder X-ray diffraction (XRD) with Cu Kα radiation on a D8 Advance X-ray Diffractometer (Bruker, Germany). The scan rate was 0.5°/min, while the scan range of 2θ was 20° to 80°. Using a JEM-2100 transmission electron microscope (JEOL, Japan), we examined images from the transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). We analyzed Ti³⁺ using Electron Spin Resonance (ESR) spectroscopy on a JES-FA200 (JEOL, Japan) electron spin resonance instrument at 110 K. We studied traces of Ti, Rh, and O in the Rh/Ti³⁺-TiO₂ sample via X-ray photoelectron spectroscopy (XPS), and performed XPS measurements with an ESCALAB250 X-ray photoelectron spectrometer (Thermo Fisher Scientific Inc., MA) with an Al K anode (1,486.6 eV photon energy, 300 W). The optical absorbance was calculated from measurements of diffuse reflectance, which we got from a U-3600 UV-VIS-NIR spectrophotometer (Shimadzu, Japan). BaSO₄ was the control.

Cell Culture and Viability Assay

Wild type Staphylococcus aureus subsp. Aureus (ATCC 6538) cells were kindly provided by Dr. Juan Du, School of Food and Bioengineering, Zhengzhou University of Light Industry. S. aureus cells (1% v/v final) were inoculated into Nutrient agar media (the agar without added for the liquid media). The cells were incubated on a BS-1E rotary shaker (Jintan Xinhang Instrument Factory) under 150 rpm and 37 ^oC for 12 h. After overnight culture, cells were harvested by centrifugation at 6000 rpm for 5 min at room temperature, washed twice with a phosphate buffer solution (PBS, pH 7.0), and suspend with the same volume of the Nutrient agar media in PBS (ca. 10⁹ CFU/ml). All solid/liquid materials had been autoclaved for 30 min at 121 ^oC before use.

A fixed concentration of 1 mg photocatalyst/mL S. aureus suspension was used in the experiments. 10 mg photocatalyst with 9.9 mL buffer solution was first injected into a sterile 60 mm × 15 mm Petri dish and was dispersed ultrasonically for 10 min. Then, 0.1 mL S. aureus suspensions (ca. 10⁹ CFU/mL) was added into the Petri dish, so that the initial S. aureus concentration used in the photoctalytic disinfection experiments was ca. 10⁷ CFU/mL.

A 300 W xenon lamp (HSX-F300, Beijing NBET Technology Co. Ltd., Beijing, China) was used for photocatalytic inactivation experiments, and the light with wavelengths below 400 nm and above 700 nm was blocked by glass filters for the disinfection under visible light. In addition, the light with wavelengths below 800 nm and above 1100 nm was blocked by other glass filters for the disinfection under NIR light. A cooling water circulating device was used for keep the temperature under NIR light irradiation (see schematics S1). The light intensity striking the cells was at ca. 30 mW/cm², as measured by a FZ-A optical Radiometer (Photoelectric Instrument Factory of Beijing Normal University, Beijing, China).

At regular time intervals, 0.1 mL of the cell suspensions was withdrawn in sequence. Following the appropriate dilution in PBS buffer solution (pH 7), aliquots of 0.1 mL were spread onto an agar medium plate, and incubated at 37 ^oC for 24 h. Then, the number of viable cells in terms of colony-forming units was counted. The comparison experiments in the dark with the photocatalyst and under light illumination only without photocatalyst were also carried out under otherwise identical. All analyses were conducted in triplicate.

The XRD patterns of Rh/Ti³⁺-TiO₂ nanocomposite (Fig. 1) indicate that these TiO₂ samples are in the anatase phase. Figure 2 displays the TEM image of the Rh/Ti³⁺-TiO₂ nanocomposite. The XRD diffraction peak of Rh is clearly visible in the XRD patterns, while the signal strengthened as the atomic ratio of Rh/Ti increased [22]. Figure 2 displays the TEM image of the Rh/Ti³⁺-TiO₂ nanocomposite. Figure 2(a) displays nanosized particles with non-uniform shapes, with the average particle size being ~ 5 to 10 nm. Figure 2(b) displays the high-resolution TEM (HRTEM) image. The d-spacing was set at ~ 0.35 nm. This aligns with the (101) plane at TiO₂. A group of lattice planes is easily identified on one nanocrystallite, with d-spacing at ~ 0.22 nm, corresponding to the (111) plane of Rh [23]. These lattice planes are clearly visible, which concurs with the results of our XRD analysis.

Using XPS, we examined both the surface components as well as the chemical valence state of the 5% Rh/Ti³⁺-TiO₂ nanocomposite (Fig. 3). After analyzing the XPS spectrum we found traces of Ti, Rh, and O (Fig. 3a). The Ti 2p XPS spectra of the Rh/Ti³⁺-TiO₂ nanocomposite was used to determine binding energies at 464.7, 463.9, 458.7, and 458.1 eV, which align, respectively, with Ti⁴⁺ 2p_1/2, Ti³⁺ 2p_1/2, Ti⁴⁺ 2p_3/2, and Ti³⁺ 2p_3/2 [15], (Fig. 3b). ESR is particularly useful for detecting the existence of Ti³⁺, due to its high sensitivity to species containing unpaired electrons. The presence of Ti³⁺ in the 5% Rh/Ti³⁺-TiO₂ nanocomposite was also identified using low-temperature ESR. A sharp and steep signal at g = 1.999 indicates the existence of Ti³⁺ [24] in the Rh/Ti³⁺-TiO₂ nanocomposite (Fig. 4). In Fig. 3c, the binding energies of Rh 3d_3/2 at 312.2 eV and Rh 3d_5/2 at 307.5 eV can be attributed to the Rh⁰ [23] valence states (Fig. 2c). The weak peak binding energies of Rh 3d_3/2 at 314.0 eV and Rh3d_5/2 at 309.0 are attributed to the Rh³⁺ valence states. There are a number of factors that influence the incidence of Rh³⁺: partial oxidization during treatment, high surface activity, and a small particle size. The binding energy of 530.0 eV can be attributed to the O lattice of TiO₂, while the peak at 532.0 eV is due to the O lattice of Ti^3+, or chemisorbed hydroxyl groups (Fig. 3d).

Figure 5 shows the light absorbance of Rh/Ti³⁺-TiO₂ nanocomposite. Pure TiO₂ powder (without adding Rh) was also tested as a control. The Rh/Ti³⁺-TiO₂ nanocomposite showed a clear shift of absorbance in the visible and NIR light range (1100 nm > λ > 400 nm). We also observed that as the atomic ratio of Rh increased, the visible-NIR light absorbance capacity of the Rh/Ti³⁺-TiO₂ nanocomposite was also increased. The photographs in Fig. 5 are digital photos of the synthesized TiO₂ and Rh/Ti³⁺-TiO₂ nanocomposite materials. We observed the darkest color of the 7% Rh/Ti³⁺-TiO₂ nanocomposite, which coincides with the absorbance spectrum of visible light.

Figure 6a and 6b demonstrate the survival ratio of S. aureus when treated with Rh/Ti³⁺-TiO₂ nanocomposites, with different levels of Rh under visible and NIR light illumination (λ = 400–700 nm and λ = 800–1100 nm) when compared to S. aureus treated under different conditions (such as visible light or NIR light illumination only, TiO₂ nanoparticles under visible light illumination, and 5% Rh/Ti³⁺-TiO₂ nanocomposite in the dark). The survival rate of S. aureus was 26.3% under visible light and 68.2% under NIR light, which may be induced by photodamage of the cell. When treated with TiO₂ nanoparticles for 4 h, the survival ratio of S. aureus was approximately 20% under visible light and 62.9% under NIR light. Therefore, the TiO₂ nanoparticles demonstrated lower activity for S. aureus. Rh/Ti³⁺-TiO₂ nanocomposites, however, demonstrated an increased ability for photocatalytic disinfection of S. aureus under both visible light and NIR light. After 4 h of treatment with 1%, 3%, 5%, and 7% Rh/Ti³⁺-TiO₂ nanocomposites, the survival ratio of S. aureus was approximately 4.7%, 0.56%, 0.0049%, and 0.056%, respectively. An Rh content of 5% displayed the strongest disinfection rate, while the lower disinfection rate of the 7% Rh/Ti³⁺-TiO₂ nanocomposite could be due to the increased amount of Rh. Without visible light illumination, the survival ratio of S. aureus remained constant at 50% after 4 h of treatment with 5% Rh/Ti³⁺-TiO₂ nanocomposite, which could be explained by mental rhodium nanoparticles accepting the electron of the cell membrane. In Fig. 6b and Figure S1, the survival ratio of S. aureus was approximately 6.3% under treatment with the 5% Rh/Ti³⁺-TiO₂ nanocomposite and NIR light illumination.

In conclusion, we successfully synthesized a metallic, rhodium-modified, and Ti³⁺ self-doped TiO₂ nanocomposite photocatalyst using a facile solvothermal method. Our results confirmed that the Ti³⁺ ion and metallic rhodium were both present in the synthesized sample. This indicates that, when illuminated with both visible and NIR light, the synthesized nanocomposite demonstrates enhanced antibacterial activity as compared to pure TiO₂ nanoparticles.

Acknowledgments

We are grateful for the National Natural Science Foundation of China (No. 21571160), the National Natural Science Foundation of China-Henan Talents Fostering Joint Funds (No. U1504311), and the Key Research Projects of the Science and Technology Department of Henan Province (No. 182102210153 and 182102210619) for funding.

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Download PDF

Journal Publication

published 01 Oct, 2020

Read the published version in Materials Letters →

Version 1

posted

You are reading this latest preprint version

Facile synthesis of Rh/Ti3+-TiO2 nanocomposites and its photodisinfection properties on Staphylococcus aureus under visible-NIR excitation

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Methods

Reagents

Fabrication and characterizations of Rh/Ti³⁺-TiO₂ Nanocomposite

Cell Culture and Viability Assay

3. Results And Discussion

4. Conclusions

Declarations

Acknowledgments

References

Supplementary Files

Status:

Journal Publication

Version 1

Facile synthesis of Rh/Ti3+-TiO2 nanocomposites and its photodisinfection properties on Staphylococcus aureus under visible-NIR excitation

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Methods

Reagents

Fabrication and characterizations of Rh/Ti3+-TiO2 Nanocomposite

Cell Culture and Viability Assay

3. Results And Discussion

4. Conclusions

Declarations

Acknowledgments

References

Supplementary Files

Status:

Journal Publication

Version 1

Fabrication and characterizations of Rh/Ti³⁺-TiO₂ Nanocomposite