Antibacterial properties of F-doped ZnO visible light photocatalyst

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Highlights

  • F doped ZnO nano-powders were obtained by a modified sol–gel method.

  • These materials were found to be effective against S. aureus and E. coli.

  • Enhanced visible light photocatalytic and antimicrobial properties were obtained.

  • The toxic effect of ZnO on bacteria can be due to the release of zinc cations.

  • Production of reactive oxidation species influences bacterial viability.

Abstract

Nanocrystalline ZnO photocatalysts were prepared by a sol–gel method and modified with fluorine to improve their photocatalytic anti-bacterial activity in visible light. Pathogenic bacteria such as Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) were employed to evaluate the antimicrobial properties of synthesized materials. The interaction with biological systems was assessed by analysis of the antibacterial properties of bacteria suspended in 2% (w/w) powder solutions. The F-doping was found to be effective against S. aureus (99.99% antibacterial activity) and E. coli (99.87% antibacterial activity) when irradiated with visible light. Production of reactive oxygen species is one of the major factors that negatively impact bacterial growth. In addition, the nanosize of the ZnO particles can also be toxic to microorganisms. The small size and high surface-to-volume ratio of the ZnO nanoparticles are believed to play a role in enhancing antimicrobial activity.

Introduction

Environmental pollution has become one of the major problems in developing and developed countries around the world. Traditionally sedimentation, coagulation or methods based on adsorption, have been used in treatment of industrial wastes. However, due to a number of drawbacks these methods often do not meet requirements of stringent quality standards [1], [2], [3]. As a result, in recent years continuous efforts have been made to develop innovative technologies in order to remediate polluted environment. Among them, photocatalysis serves as an attractive tool due to its efficiency and relatively low cost [4]. Currently the most widely used and studied photocatalyst is titanium dioxide (TiO2). However, several studies have reported that zinc oxide (ZnO) can also serve as a suitable alternative, mainly due to its low cost and the ability to absorb a larger fraction of the solar spectrum than TiO2 [5], [6], [7].

Zinc oxide (ZnO), a wide band gap semiconductor (3.37 eV), has a vast range of applications which include but are not limited to photocatalysts [8], varistors [9], [10], pharmaceutical products [11], and lasers [12]. As a photocatalyst, in the presence of UV radiation, ZnO facilitates production of reactive oxygen species (ROS) on its surface [13]. During photocatalysis, the initial reaction starts with UV irradiation of the semiconductor. Light with photonic energy greater than the band gap of a semiconductor metal oxide (e.g., ZnO), can excite an electron from the filled valance band (VB) to the empty conduction (CB) band. This excitation of electrons from VB to CB results in the formation of an excited electron (e, CB)—positive hole (h+, VB) pair. The e CB can reduce oxygen from the surrounding environment to form superoxide radicals (radical dotO2) and then, with the aid of hole (h+ VB), singlet oxygen (1O2) can be formed. The h+ VB can also react with water to produce hydroxyl radicals (radical dotOH), hydrogen peroxide (H2O2), or protonated superoxide radical (radical dotHO2). Hydrogen peroxide can further react with radical dotOH radicals to form radical dotHO2 [14], [15]. The reactive oxygen species (ROS) such as radical dotOH, 1O2, radical dotO2, radical dotO2H, H2O2 can cause decomposition of bacteria through various actions [16], [17]. In biological systems, antimicrobial action of ROS is achieved by lipid peroxidation of the cell membrane and subsequent inactivation of the microorganisms [18]. As the antimicrobial activity of ZnO has also been observed in the absence of light, it is thus assumed that production of ROS is not the only mechanism of its activity and other factors such as the impact of Zn2+ ions could play an important role [16].

It is known that by modifying the band gap, the photocatalytic properties of ZnO can be changed and tailored to a specific application. For example doping with low concentration of fluorine can significantly improve its photocatalytic activity in visible light, increase the mobility of ROS carriers and hence enhance the antimicrobial activity of ZnO in environments void of UV radiation [18]. Although the antimicrobial property of ZnO has already gained significant attention [19], [20], [21], its photocatalytic activity in visible light still requires improvement. The aim of this investigation was therefore to synthesize fluorine-doped nanoparticulate ZnO powder via a sol–gel route and to explore the effect of this process on ZnO with respect to its antimicrobial activity against selected pathogens under visible light conditions.

Section snippets

Preparation of nanopowders

All reagents for synthesis were purchased from Sigma–Aldrich. A sol–gel method was used for the synthesis of ZnO and fluorinated (F-doped) ZnO with varying molar ratio of trifluoroacetic acid (TFA) was used as the precursor. In a typical experiment, zinc acetate dihydrate (10.9 g) was mixed with absolute ethanol (300 ml) at the temperature of 75–78 °C (mixture A). Simultaneously, oxalic acid dihydrate (12.6 g) was dissolved in ethanol (200 ml) at room temperature (mixture B). For the fluorination to

Results and discussion

An ethanolic solution of zinc acetate was added to an ethanolic solution mixture of oxalic acid and trifluroacetic acid, which produced a thick semi-gel. Subsequent drying at 80 °C in an oven produced a dried xerogel, which was then calcined at 500 °C to produce a free flowing nanopowder. There have been three batches of powders prepared in order to evaluate the reproducibility of the ZnO synthesis.

XRD was employed to determine the phase composition and the crystalline size of synthesized ZnO

Conclusions

Pure and F-doped zinc oxide nanopowders were successfully synthesized by a simple sol–gel method. XPS analysis showed that fluorine doping of the ZnO structure was successful and effective. Doping of the ZnO structure enhanced the biological performance of the material, which was indicated by the improved antimicrobial properties in comparison with undoped material. Among the obtained nanopowders, ZnO:TFA 1:1 was the most effective against pathogens: S. aureus (log Red = 4.62, which is equivalent

Acknowledgments

The authors wish to acknowledge financial support under the US  Ireland R&D Partnership programme from the Science Foundation Ireland (SFI-grant number 10/US/I1822(T)) and U. S. National Science Foundation-CBET (Award 1033317). The authors also thank CREST, Focas DIT (Dr. Muhammad Morshed and Dr. Brendan Duffy), Dublin City University (School of Biotechnology) and School of Science, IT Sligo (Saoirse Dervin) for the laboratory facilities and analytical assistance. D.D. Dionysiou also

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