Abstract
The application of titanium oxide nanotubes for the removal of contaminants from freshwater is a rapidly growing scientific interest, especially when it comes to water conservation strategies. In this study we employed four different titanium oxide nanotube surfaces, prepared by a two-electrode anodic oxidation. Two of the surfaces were synthesised in aqueous media, while the other two surfaces were synthesised in ethylene glycol. One of the arrays synthesised in the organic medium was impregnated with silver nanoparticles, while the remaining surfaces were not. The chemical reactivity of the various surfaces was assessed using 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) as free electron sensitive probe molecules, in parallel with tannic acid degradation and copper ion reducing capacity. The potential antimicrobial activity of the surfaces was assessed against a panel of microorganisms composed of yeast, fungi, Gram-positive and Gram-negative bacteria. Field emission scanning electron microscopy revealed that surfaces produced in the aqueous medium had a smaller tube length and a smaller tube diameter. It was noted that one of the materials using sodium sulfate as the supporting electrolyte had the most irregular nanostructure morphology with tubes growing to the side rather than vertically. The structural variation of the surfaces directly reflected both the chemical and biological activity, with the nanotubes formed in ethylene glycol showing the fastest rates in the stabilization of DPPH and ABTS radicals, the fastest tannic acid decomposition under various pH conditions and the fastest metal reducing activity. Furthermore, the surface containing silver and its bare counterpart showed the most effective antimicrobial activity, removing approximately 82% of Gram-negative bacteria, 50% of Gram-positive bacteria, 70% of yeast and 40% of fungi, with Gram-negative bacteria being the most susceptible to these surfaces.
Highlights
In recent years, the application of titanium dioxide based photocatalytic systems has gained interest within the scientific community due to its promising implementation in the removal of pollutants and microorganisms from air and water systems [1,2]
Titanium dioxide exists in three crystalline forms, anatase, rutile and brookite, which differ in chemical stability and photocatalytic activity [4]
The nanotube arrays synthesised in ethylene glycol had the more ordered and regularly shaped tubes (Figure 3A,B)
Summary
The application of titanium dioxide based photocatalytic systems has gained interest within the scientific community due to its promising implementation in the removal of pollutants and microorganisms from air and water systems [1,2]. This is attributed to several reasons, including the high photocatalytic activity, excellent long-term physical and chemical stability, cost effectiveness, nontoxicity, and abundance of starting material. The point of excess charge in the conduction band causes the surface to become prone to oxidation, while the positive charge left behind inside the valence band causes the surface to become prone to reduction This redox duality of the TiO2 -NT surface enables the surface to act both as an oxidizing and reducing agent. The promotion of electrons to the conduction band is limited by the amount of energy required to promote an electron from the valence band to the conduction band
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