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Review
. 2020 Jan 9;10(1):124.
doi: 10.3390/nano10010124.

Visible-Light Active Titanium Dioxide Nanomaterials with Bactericidal Properties

Chengzhu Liao  1 , Yuchao Li  2 , Sie Chin Tjong  3
Affiliations
Review

Visible-Light Active Titanium Dioxide Nanomaterials with Bactericidal Properties

Chengzhu Liao et al. Nanomaterials (Basel). .

Abstract

This article provides an overview of current research into the development, synthesis, photocatalytic bacterial activity, biocompatibility and cytotoxic properties of various visible-light active titanium dioxide (TiO2) nanoparticles (NPs) and their nanocomposites. To achieve antibacterial inactivation under visible light, TiO2 NPs are doped with metal and non-metal elements, modified with carbonaceous nanomaterials, and coupled with other metal oxide semiconductors. Transition metals introduce a localized d-electron state just below the conduction band of TiO2 NPs, thereby narrowing the bandgap and causing a red shift of the optical absorption edge into the visible region. Silver nanoparticles of doped TiO2 NPs experience surface plasmon resonance under visible light excitation, leading to the injection of hot electrons into the conduction band of TiO2 NPs to generate reactive oxygen species (ROS) for bacterial killing. The modification of TiO2 NPs with carbon nanotubes and graphene sheets also achieve the efficient creation of ROS under visible light irradiation. Furthermore, titanium-based alloy implants in orthopedics with enhanced antibacterial activity and biocompatibility can be achieved by forming a surface layer of Ag-doped titania nanotubes. By incorporating TiO2 NPs and Cu-doped TiO2 NPs into chitosan or the textile matrix, the resulting polymer nanocomposites exhibit excellent antimicrobial properties that can have applications as fruit/food wrapping films, self-cleaning fabrics, medical scaffolds and wound dressings. Considering the possible use of visible-light active TiO2 nanomaterials for various applications, their toxicity impact on the environment and public health is also addressed.

Keywords: Escherichia coli; Staphylococcus aureus; antibacterial activity; doping; nanomaterial; photocatalyst; reactive oxygen species; silver nanoparticle; titania; visible light.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The possible mechanisms of antibacterial activities exhibited by different metal nanoparticles (NPs) and photocatalytic semiconductors. The activation of the photocatalytic semiconductor by visible light is depicted on the left-hand side of the figure. Reactive oxygen species created by various semiconductors destruct bacterial cell components, as indicated by red arrows. Ag, Cu, and Au nanoparticles also generate reactive oxygen species (ROS) for bacterial killing. The green arrow represents targets of Ag. Reproduced with permission from [22]. Copyright Frontiers, 2018.
Figure 2
Figure 2
Connecting the chains of distorted TiO6 octahedra by sharing edges and corners in different ways to form rutile, brookite and anatase polymorphs. Titanium atoms are blue; oxygen atoms are red. Reproduced with permission from [42]. Copyright Nature Publishing Group, 2017.
Figure 3
Figure 3
The charge transfer processes between excited electrons from the valence band of TiO2 with (a) Mn2+ ions of Mn-doped TiO2, and (b) Fe3+ ions of Fe-doped TiO2. CB and VB are the conduction and valence bands of TiO2, respectively. Reproduced with permission from [92,95], respectively. Copyright Elsevier, 2017; Copyright American Chemical Society, 2013.
Figure 4
Figure 4
Ultraviolet-visible diffuse reflectance spectra of TiO2 and its nanocomposites. Reproduced with permission from [126]. Copyright Elsevier, 2019.
Figure 5
Figure 5
The creation of reactive oxygen species in Ag/TiO2 nanomaterials due to the localized surface plasmon resonance (LSPR) effect of AgNPs under visible light. After excitation, LSPR decays into hot electrons and holes through Landau damping, creating highly energetic charge carriers. On the other hand, AgNPs serve as an excellent electron accumulator for TiO2 under UV irradiation. Reproduced with permission from [138]. Copyright MDPI, 2019.
Figure 6
Figure 6
Schematics displaying the (a) roles of graphene layers of graphene/TiO2 composite in photocatalysis, and (b) charge transfer mechanism under ultraviolet or visible light irradiation. Reproduced with permission from [145,153], respectively. Copyright MDPI, 2018 and 2017.
Figure 7
Figure 7
UV–vis diffuse reflectance spectra of (a) anatase TiO2 and (b) nanostructured rGO/TiO2. Inset: plot of transformed KM function [F(R).hv]1/2 vs. hv for bandgap determination of anatase TiO2 and rGO/TiO2; R is reflectance and hv is photon energy. Reproduced with permission from [146]. Copyright Springer, 2013.
Figure 8
Figure 8
Electronic band structure of titania due to non-metal doping. CB and VB represent conduction band and valence band, respectively. Reproduced with permission from [160]. Copyright Elsevier, 2013.
Figure 9
Figure 9
Transmission electron micrographs of solvothermally synthesized TiO2 with nanospheres (NP1–NP2) and nanorods (NP3–NP4) morphologies. Reproduced with permission from [209]. Copyright Elsevier, 2017.
Figure 10
Figure 10
(a) Field-emission scanning electron image and (b) transmission electron micrograph of the solvothermally synthesized rGO/TiO2 nanocomposite. (c,d) Enlarged images of high-resolution transmission electron micrographs showing the lattice fringes of TiO2 and a clean interface between TiO2 and rGO. Reproduced with permission from [146]. Copyright Springer, 2013.
Figure 11
Figure 11
Schematic illustration displaying (a) the set-up for anodization and (b) frmation of compact titania layer on the Ti substrate in electrolytes without fluoride, and self-organized titania nanotube arrays in electrolytes with fluoride. Reproduced with permission from [222]. Copyright Elsevier, 2007.
Figure 12
Figure 12
Schematic showing the formation of titania nanotube (TNT) arrays: (a) an initial development of a compact oxide layer on the surface of Ti, (b) small pits formation due to the etching of oxide by F− ions, (c) local growth of nanopores into well-aligned TNT arrays, and (d) the shape and wall thickness of a nanotube. Reproduced with permission from [223]. Copyright MDPI, 2019.
Figure 13
Figure 13
(a) Top-view scanning electron micrographs of TNT arrays prepared by anodizing Ti in a mixed ethylene glycol/NH4F and water solution under applied voltages of 5, 15 and 20 V for 5 h. Reproduced with permission from [225]. Copyright IOP Publishing, 2014. (b) Cross-sectional SEM image (inset) and top view of TNTs fabricated by anodizing Ti in a mixed ethylene glycol/NH4F and water solution at 30 V for 1 h. Reproduced with permission from [223]. Copyright MDPI, 2019.
Figure 14
Figure 14
(A) TEM image of a single, Ag-decorated TiO2 nanotube with a diameter of 100 nm. The white arrow indicates the growth direction of a nanotube. (B) High-magnification image of the selected are, as marked by a dashed square in (A). (C) Enlarged view of a single AgNP, and (D) the corresponding EDS spectrum of AgNP and TiO2 nanotubes. Reproduced with permission from [226]. Copyright Public Library of Science, 2013.
Figure 15
Figure 15
Scanning electron micrographs of (a) electrospun titania nanofibers before heating, and (b) after heating in 100% argon atmosphere at 900 °C. (c) UV-visible spectra of as-spun titania nanofibers without calcination, and with calcination at 900 °C in 100% air, 50% air–50% argon, 25% air–75% argon and 100% argon. Reproduced with permission from [233]. Copyright Elsevier, 2016.
Figure 16
Figure 16
Inactivation of (a) E. coli and (b) S. aureus by Ni-doped TiO2 NPs as a function of time. Reproduced with permission from [248]. Copyright Elsevier, 2014.
Figure 17
Figure 17
Photocatalytic inactivation of (a) E. coli and (b) S. aureus with 0.5% Cu/TiO2 calcined at 650 °C, pure anatase and rutile specimens. N/No is the reduction in the concentration of the bacteria. Reproduced with permission from [109]. Copyright MDPI, 2018.
Figure 18
Figure 18
Photocatalytic bactericidal efficacy of 0.5% Cu/TiO2, pure anatase and rutile with E. coli and S. aureus upon visible light irradiation for 30 min. Reproduced with permission from [109]. Copyright MDPI, 2018.
Figure 19
Figure 19
Viability of (a) E. coli and (b) S. aureus against the concentration of as-synthesized TiO2 NPs, annealed TiO2 NPs, and Ag-doped TiO2 NPs with 3% AgNPs (dash–dot curve) and 7% AgNPs (dot curve; blue). Reproduced with permission from [256]. Copyright Beilstein-Institut, 2013.
Figure 20
Figure 20
(a) Bacterial inactivation on neat TiO2–NT and Ag/TNTs photocatalysts exposed to solar-simulated light (50 mW/cm2, 310–800 nm). Error bars: standard deviation; n = 5. (b) Bacterial inactivation mechanism of Ag/TNTs, as described by Reaction (1–6). Reproduced with permission from [260]. Copyright Elsevier, 2018.
Figure 21
Figure 21
(a) Photographs showing the spread of S. aureus on commercially pure titanium (cpTi), titanium nanotubes (NTs) and Ag-doped TNTs samples. (b) Antibacterial efficacy of all samples immersed in PBS for 3 h and 7 d. The error bars are the standard deviation (n = 5); * denotes p < 0.05 compared with cp-Ti at a 3 h ion extraction time, # denotes p < 0.05 compared with cp-Ti at 7 days ion extraction time. Reproduced with permission from [261]. Copyright Wiley, 2014.
Figure 22
Figure 22
Survival ratio (C/Co) of S. aureus with neat TiO2, N-doped TiO2 NPs and Ni-doped TiO2 NPs under an 18 W visible light irradiation for different time periods. S. aureus without TiO2 in the dark is used as a control. Statistically significant at p < 0.05. Reproduced with permission from [161]. Copyright Springer, 2016.
Figure 23
Figure 23
Photocatalytic inactivation of S. aureus with N- and Ni-doped TiO2 NPs of different contents under visible light. Statistically significant at p < 0.05. Reproduced with permission from [161]. Copyright Springer, 2016.
Figure 24
Figure 24
Photocatalytic inactivation of E. coli with N-doped TiO2 NPs of different concentrations under an 18 W visible light. Statistically significant at p < 0.05. Reproduced with permission from [161]. Copyright Springer, 2016.
Figure 25
Figure 25
Photographs of E. coli colonies developed on agar plates treated with (A) control, (B) TiO2 and (C) N-doped TiO2 samples in the dark for 24 h. (a–c) are the images of E. coli colonies under visible light irradiation for 2 h. (a): control, (b): neat TiO2, and (c) N-doped TiO2. Reproduced with permission from [265]. Copyright Springer, 2013.
Figure 26
Figure 26
(a) Visible light excitation of N−F codoped TiO2 and subsequent filling of empty N state by electron transfer from either Ti3+ or oxygen vacancies (Ovac). Reproduced with permission from [268]. Copyright American Chemical Society, 2014. (b) Survival rate of E. coli treated with N-doped P25 (P25-N-HT), F-doped P25 (P25-F-HT) and (3) F-N codoped P25 (P25-F&N-HT) under simulated light illumination. Reproduced with permission from [270]. Copyright MDPI, 2017.
Figure 27
Figure 27
Inactivation of E. coli in the presence of rGO, TiO2 and (0.5–2.5 wt%) rGO/TiO2 samples in (A) dark contition and (B) under artificial solar light irradiation. Reproduced with permission from [101]. Copyright Elsevier, 2018.
Figure 28
Figure 28
MIC and MBEC values of rGO/1%Fe-N-doped TiO2 nanocomposites and commercial P25 TiO2 treated with S. aureus, E. coli, P. aeruginosa and fungal C. albicans under visible light at 37 °C for 24 h. Reproduced with permission from [277]. Copyright MDPI, 2017.
Figure 29
Figure 29
Survival of E. coli in CFU/mL treated with (a) Cu2O, (b) Cu2O/anatase TiO2, (c) Cu2O/P25, and (d) Cu2O/rutile TiO2 in solid curves with circle symbols; grey color (dark condition), violet (UV irradiation) and green (visible light). Umodified anatase TiO2, P25 and rutile TiO2 results are shown in dashed curves with diamond symbols in grey (dark condition), violet (UV irradiation) and green (visible light). Error bars in Cu2O: standard deviation determined from two or three independent measurements. Reproduced with permission from [177]. Copyright MDPI, 2018.
Figure 30
Figure 30
Photographs showing the preservation of red grapes wrapped with (a) plastic film, (b) chitosan film, and (c) chitosan-TiO2 film at 37 °C for six days. Reproduced with permission from [289]. Copyright Elsevier, 2017.
Figure 31
Figure 31
(a) Antibacterial activity of CS, CT and CS–CT samples against E. coli under visible light illumination. (b) Mechanism of antibacterial activity of CS/Cu-doped TiO2 nanocomposite. Reproduced with permission from [288]. Copyright Elsevier, 2011.
Figure 32
Figure 32
Bacterial reduction in percentage of (a) S. aureus and (b) K. pneumoniae on cotton fabrics with and without Mn-doped TiO2 NPs in the dark and under natural sunlight. NPs0, NPs10, NPs25 and NPs50 are cotton fabrics with zero, 10, 25 and 50 wt% Mn-doped TiO2 NPs; NPs50W is the NPS50 after 10 washing cycles. (c) Schematic of visible light induced the photocatalytic inactivation of bacteria and degradation of stain residues on contaminated fabric with Mn-doped TiO2 NPs. Reproduced with permission from [298]. Copyright American Chemical Society, 2018.
Figure 33
Figure 33
Biopersistence of titanium level in (a) liver, (b) lungs, (c) spleen and (d) kidneys after intravenous injection of 1 mg/kg TiO2 NPs in rats for 365 days. Grey and white bars are treated and control mice, respectively. Error bars represent the mean ± SD and n = 6. Statistical comparison is performed by two-way ANOVA, * p < 0.05; ** p < 0.01; *** p< 0.001. Reproduced with permission from [333]. Copyright BioMed Central, 2015.
Figure 34
Figure 34
(a) MTT, (b) NRU, and (c) lactase dehydrogenase leakage results of human liver cancer (HepG2) cells exposed to TiO2 NPs and Ag-doped TiO2 NPs of different concentrations. (d) ROS level of HepG2 cells exposed to pure TiO2 NPs and Ag-doped TiO2 NPs of 25, 50 and 100 µg/mL. Reproduced with permission from [318], Copyright Nature Publishing Group, 2017.
Figure 35
Figure 35
The viability of MC3T3-E1 murine osteoblasts obtained from the MTT assay. Error bars indicate the standard deviation (n = 5); # p < 0.05 compared with commercial pure Ti (cp-Ti). Reproduced with permission from [261]. Copyright Wiley, 2014.
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