Antimicrobial properties of highly efficient photocatalytic TiO2 nanotubes

https://doi.org/10.1016/j.apcatb.2015.03.029Get rights and content

Highlights

  • TiO2 nanotubes (diameter 20 nm; thickness of 5 nm) were synthesised.

  • Titania nanotubes contain partially crystalline/amorphous titania.

  • High anti-bacterial activity (E. coli (97.53%); S. aureus (99.94%)) was observed.

  • Results were compared with commercial Evonik Degussa P-25 TiO2.

  • Surface hydroxyl groups are responsible for the high bacterial inactivation.

Abstract

A rapid chlorine-based electrochemical anodization method resulted in the production of free-standing bundles of titania (TiO2) nanotubes with high-aspect ratio (up to 100 μm long and about 20 nm in diameter). XRD and Raman spectroscopy revealed the presence of partially crystalline amorphous titania nanostructures modified with surface hydroxyl groups. Photocatalytic antimicrobial properties of these nanotubes have been investigated using Escherichia coli and Staphylococcus aureus and compared with a commercial reference sample, Evonic-Degussa P25. Titania nanotubes were found to be highly efficient in inactivating both E. coli (97.53%) and S. aureus (99.94%) in under 24 h of UV irradiation. On the other hand, commercial Evonik Degussa P-25 titania nanoparticles and control samples did not reveal antimicrobial properties for the same amount of time under either light or dark conditions. These results indicate that along with material properties, the high-aspect ratio nanotube architecture, surface hydroxyls, physicochemical properties of TiO2 nanotubes as well as experimental conditions of the biological investigations play a significant role in the antibacterial activity.

Introduction

Titanium dioxide is an extensively studied semiconductor due to its great potential in the photocatalytic degradation of organic pollutants and bacteria [1], [2], [3], [4], [5], [6]. Recently, there has been a great influx of interest in using nanostructured titania for enhancing the efficiency of photocatalytic applications. In this regard, the nanotube architecture is one of the most promising morphologies for antimicrobial applications because of desirable features such as high-aspect ratio, enhanced active surface area and improved light harvesting and trapping. While titania nanotubes have been produced in the past either using a hydrothermal method or using a fluoride-based electrochemical method, a novel and efficient route for the synthesis of very long, high-aspect ratio nanotubes (up to 100 μm long and about 20 nm in diameter) using a simple chlorine-based electrochemical method has been recently demonstrated [7], [8], [9]. This procedure results in the production of free-standing bundles of nanotubes with surface hydroxyl groups. Such high-aspect ratio, surface hydroxyl rich and highly porous particles can be directly used for photocatalytic antimicrobial applications.

The photocatalytic process involves the formation of electron (eCB) and hole (h+VB) pairs upon the irradiation of light that exceeds the band gap energy (3.2 eV) of TiO2. Positive holes (h+VB) become trapped by water molecules in the atmosphere. The water molecule is oxidized by h+VB producing H+ and radical dotOH radicals, which are extremely powerful oxidants [1], [2]. The hydroxyl radicals then oxidize organic molecules from the surrounding environment to final products such as CO2, mineral acids and H2O. Electrons in the conduction band (eCB) can be rapidly trapped by atmospheric oxygen. The oxygen can further be reduced by eCB to form superoxide (O2−radical dot) radicals that will further combine with H+, to form peroxide radicals (radical dotOOH) and hydrogen peroxide, H2O2. These reactive oxidation species produced during the irradiation process are able to oxidize the majority of volatile organic compounds (VOC) and organic matters such as bacteria until complete mineralization . The objective of the current investigation was to understand the antimicrobial properties of the hydroxyl group modified titania nanotubes which are produced by rapid breakdown anodization.

Section snippets

Synthesis and characterization of titania nanotube powders

Powders of amorphous titanium dioxide (titania) nanotubes have been synthesized by DC (V = 16 V) rapid breakdown anodization [7], [8], [9] of 0.89 mm thick titanium foil (Alfa Aesar, 99.7% metal basis) for several hours in an aqueous solution containing 0.1 M ammonium chloride (Alfa Aesar, purity >99.5%). Bundles of nanotubes were continuously released in the solution from corrosion sites on the titanium foil surface, forming a white precipitate which was recovered, washed repeatedly with water and

Results and discussion

SEM imaging (Fig. 2) revealed that each microscopic powder grain is in fact a bundle of highly ordered ultra-high aspect ratio titania nanotubes, with average outer diameters around 20 nm and lengths of the order of tens of microns. The grain size is widely distributed from sub-micron all the way to several tens of microns, usually the grains having an elongated shape in the direction parallel to the nanotubes that are forming them. As reported in a previous work [9], the nanotubes are tightly

Conclusions

The anti-microbial properties of the TiO2 nanotubes prepared by a rapid breakdown anodization process were studied and compared with Evonic-Degussa P25. TiO2 Nanotubes showed excellent antibacterial properties against E. coli (97.53%) and S. aureus (99.94%) under 24 h of UV irradiation, while commercial and control samples did not show any antimicrobial properties. It has been noted that the morphology, surface properties and physicochemical properties of TiO2 nanotubes, as well as experimental

Acknowledgement

One of the authors S.C. Pillai wishes to acknowledge financial support under the US-Ireland R&D Partnership Initiative from the Science Foundation Ireland (SFI-grant number 10/US/I1822 (T)).

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