Thorn-like TiO2 nanoarrays with broad spectrum antimicrobial activity through physical puncture and photocatalytic action

Abstract

To overcome the conventional limitation of TiO₂ disinfection being ineffective under light-free conditions, TiO₂ nanowire films (TNWs) were prepared and applied to bacterial disinfection under dark and UV illumination. TNW exhibited much higher antibacterial efficiencies against Escherichia coli (E. coli) under dark and UV illumination conditions compared to TiO₂ nanoparticle film (TNP) which was almost inactive in the dark, highlighting the additional contribution of the physical interaction between bacterial membrane and NWs. Such a physical contact-based antibacterial activity was related to the NW geometry such as diameter, length, and density. The combined role of physical puncture and photocatalytic action in the mechanism underlying higher bactericidal effect of TNW was systematically examined by TEM, SEM, FTIR, XPS, and potassium ion release analyses. Moreover, TNW revealed antimicrobial activities in a broad spectrum of microorganisms including Staphylococcus aureus and MS2 bacteriophage, antibiofilm properties, and good material stability. Overall, we expect that the free-standing and antimicrobial TNW is a promising agent for water disinfection and biomedical applications in the dark and/or UV illumination.

Figure 1

(a) Cross-sectional SEM images of TNWs grown for 5, 8, 12, and 24 h. (b) NW lengths and diameters were plotted as a function of growth time. (c) XRD patterns of TNWs. (d) FFT pattern and HRTEM image of TNWs grown for 8 h.

Figure 2

(a) Inactivation of E. coli by TNP and TNWs with different lengths (3.5, 5.0, 7.5, and 12.0 μm) upon a 30 min exposure determined by plate count method. Different letters above the bars represent significant differences (ANOVA, P < 0.05). (b) Flow cytometry analysis of E. coli after treatment with TNP and TNW/5 μm. The left and right panels in (b) display the dot plot and the percentage of damaged cells (indicated by dashed line in dot plot). **P < 0.01, Student’s t tests.

Figure 3

Photocatalytic decomposition of (a) 4-CP, (b) formate, and (c) DCA on TNP and TNW/5 μm. Experimental conditions: [4-CP]₀ = [DCA]₀ = [formate]₀ = 50 μM, pH_i = 3.0, and λ > 320 nm. (d) Time profiles for the photocatalytic production of p-hydroxybenzoic acid (p-HBA) from the oxidation of benzoic acid on TNW/5 μm and TNP under successive dark and UV illumination conditions. Experimental conditions: [BA]₀ = 10 mM, pH_i = 3, and λ > 320 nm.

Figure 4

(a) ESEM images of E. coli after a 30 min treatment with (a-1) TNW/5 μm in the dark, (a-2) TNW/5 μm under UV irradiation, (a-3) TNP in the dark, and (a -4) TNP under UV irradiation. The yellow arrows in the images represent the cell directly penetrated by NWs. Some of the bacteria were flattened and aggregated each other, indicating severe damage (yellow dash lines). (b) FT-IR difference spectra between samples and E. coli control (trace 1, TNW/5 μm under UV irradiation – E. coli; trace 2, TNW/5 μm in the dark – E. coli; trace 3, TNP in the dark – E. coli). (c) Molecular composition (weight percent) of the surface of E. coli cells before and after treatment deduced from XPS data. ▩, hydrocarbon compounds; □, polysaccharides; ▨, peptides. (d) Inactivation efficiencies toward E. coli with TNW/5 μm in the presence of various scavengers. Different letters above the bars represent significant differences (ANOVA, P < 0.05). Experimental conditions were [Cr(VI)]₀ = 0.05 mM, [SOD]₀ = 70 mg/L, [oxalate]₀ = 0.5 mM, [Fe(II)-EDTA]₀ = 0.1 mM, and [isopropanol]₀ = 0.3 M. (e) Extracellular ROS production.

Figure 5

Reusability test of TNW/5 μm up to 5 cycles for the disinfection of E. coli. The same letter indicates no significant difference at the 95% confidence level (ANOVA).

Figure 6

Inactivation of E. coli and S. aureus as single and mixed cultures and disruption of E. coli biofilm in the presence of (a) TNW/5 μm and (b) TNP with/without UV light for 30 min. Biofilm amount is presented as percentages of the control (the amount of Ti foil in the dark). **P < 0.01, ***P < 0.001, ****P < 0.0001, Student’s t tests.