Synthesis and characterization of zinc/iron oxide composite nanoparticles and their antibacterial properties

https://doi.org/10.1016/j.colsurfa.2010.10.015Get rights and content

Abstract

Inorganic metal oxides may serve as effective disinfectants, due to their relatively non-toxic profile, chemical stability and efficient antibacterial activity. Among metal oxide nanoparticles, zinc oxide demonstrates significant bacterial growth inhibition on a broad spectrum of bacteria, mainly by catalysis of reactive oxygen species (ROS) formation from water and oxygen. Aqueous suspensions of ZnO nanoparticles (ZnO nanofluids) are the preferred formulation for using the antibacterial agent in liquid phases and for the incorporation of the nanoparticles in different commercial products. However, ZnO nanoparticles in aqueous media tend to aggregate into large flocculates, due to their hydrophobic nature, and thus do not interact with microorganisms effectively. In this study, zinc oxide was combined with iron oxide to produce magnetic composite nanoparticles with improved colloidal aqueous stability, together with adequate antibacterial activity. For this purpose, the Zn/Fe oxide composite nanoparticles were synthesized by basic hydrolysis of Fe2+ and Zn2+ ions in aqueous continuous phase containing gelatin. The obtained composite nanoparticles were composed of iron oxide, zinc oxide and zinc ferrite phases. The effect of the weight ratio [Zn]/[Fe] of the composite nanoparticles on their properties (composition, size, magnetic behavior and colloidal stability) was elucidated. The antibacterial activity of these nanoparticles was tested against Staphylococcus aureus and Escherichia coli and was found to be dependent on the weight ratio [Zn]/[Fe], i.e., the higher the ratio, the higher the antibacterial activity. In addition, the activity against Staphylococcus aureus was significantly higher than that observed against Escherichia coli.

Research highlights

▶ Zn/Fe oxide composite nanoparticles were synthesized by growth of Zn/Fe oxide films on gelatin/Zn/Fe oxide nuclei. ▶ The [Zn]/[Fe] ratios within the particle affected particle diameter and magnetic properties. ▶ Colloidal stability was achieved while maintaining adequate antibacterial activity.

Introduction

Antibacterial agents are of great importance in several industries, e.g., water disinfection, textiles, packaging, construction, medicine and food [1], [2]. The organic compounds traditionally used for disinfection pose several disadvantages, including toxicity to the human body and sensitivity to high temperatures and pressures that are present in many industrial processes [1], [2], [3]. For these reasons, the interest in inorganic disinfectants such as metal oxides is increasing [1], [3], [4], [5]. These inorganic compounds present strong antibacterial activity at low concentrations [6]. They are also much more stable in extreme conditions [3], [4], considered as non-toxic, and some of them even contain mineral elements essential to the human body [7], [8], [9].

Among metal oxide powders, ZnO demonstrates significant growth inhibition of a broad spectrum of bacteria [10], [11]. The suggested mechanism for the antibacterial activity of ZnO is based mainly on catalysis of formation of reactive oxygen species (ROS) from water and oxygen [1], [3], [5], [12], [13], that disrupt the integrity of the bacterial membrane, although additional mechanisms have also been suggested [6], [10], [14], [15], [16], [17], [18], [19]. Since the catalysis of radical formation occurs on the particle surface [5], [20], particles with larger surface area demonstrate stronger antibacterial activity. Therefore, as the size of the ZnO particles decreases their antibacterial activity increases [1], [10], [12], [21].

Aqueous suspensions of ZnO nanoparticles (ZnO nanofluids) are the preferred formulation for using the antibacterial agent in liquid phases and for the incorporation of these nanoparticles in various commercial products such as building materials or water desalination systems [3], [5], [10]. The dispersion of metal oxide nanoparticles in physiological solutions is also important for biological in vitro and in vivo studies [22]. However, ZnO nanoparticles in aqueous media agglomerate into flocculates ranging from several hundred nanometers to several microns and thus do not interact with microorganisms effectively [12], [22], [23]. Several research groups have applied different methods, e.g., ultrasonication, milling and use of stabilizing agents [3], [16], [22], in order to avoid nanoparticle aggregation, but this appears to have been only partially successful.

By contrast, ferrofluid (suspensions of magnetic iron oxide nanoparticles such as Fe3O4) have good colloidal stability, and may be kept as an aqueous suspension without agglomeration for long periods of time. However, they are not known to have significant antibacterial properties, in spite of the recent finding that Fe3O4 magnetic nanoparticles possess catalytic activity toward the reduction of H2O2 [24]. The iron oxide nanoparticles act as Fenton's reagent (Fe2+/Fe3+ in solutions) that reacts with hydrogen peroxide to produce hydroxyls and peroxide radicals. One may suggest that this radical formation acts synergistically with the ZnO nanoparticles, to enhance their antibacterial effect.

In our laboratory, we have recently developed a new method for the preparation of stable non-toxic iron oxide magnetic nanoparticles of a narrow size distribution dispersed in an aqueous continuous phase [25], [26], [27]. These nanoparticles are prepared by nucleation and then growth of thin magnetic iron oxide layers on gelatin/iron oxide nuclei. Combining the ZnO nanoparticles with magnetic iron oxide nanoparticles may improve the colloidal stability of the ZnO nanofluid, and provide easier handling of the nanoparticles for concentrating and cleaning the nanofluids from excess reagents by a magnetic column [25], [26], [27]. In this study, several zinc/iron oxide composite nanoparticles were synthesized by basic hydrolysis of Fe2+ and Zn2+ ions of different weight ratios on gelatin/Zn/Fe nuclei. The stability of the different Zn/Fe oxide nanofluids and their antibacterial activity were also investigated.

Section snippets

Materials

Gelatin (from porcine skin), ferric chloride tetrahydrate, zinc chloride and sodium nitrate were all purchased from Sigma–Aldrich (Rehovot, Israel). Water was purified by passing deionized water through an Elgastat Spectrum reverse osmosis system (Elga Ltd., High Wycombe, UK).

Preparation of the Fe3O4, the ZnO and the Zn/Fe oxide composite nanoparticles

Magnetic iron oxide (Fe3O4) nanoparticles were prepared as described previously [25], [26]. Briefly, 240 μl FeCl2·4H2O solution (10 mmol/5 ml 0.01 N HCl) were added to 80 ml aqueous solution containing 240 mg porcine gelatin,

Results and discussion

Iron oxide nanoparticles of 17.3 ± 4.6 nm diameter were synthesized by nucleation, followed by controlled growth of iron oxide thin films onto gelatin/iron oxide nuclei, as shown in Fig. 1. The nucleation step is based on the complexation of Fe2+ ions with chelating sites of the gelatin (probably primary amines and/or carboxylates), followed by partial oxidation with sodium nitrate (up to approximately 50%) of the chelated Fe2+ to Fe3+, so that the water-soluble gelatin contains both Fe2+ and Fe3+

Conclusions

The present study demonstrates the unique synthesis of iron oxide, zinc oxide and Zn/Fe oxide composite nanoparticles of sizes ranging between 3 and 17 nm, by nucleation followed by controlled growth of Zn/Fe oxide films onto gelatin/Zn/Fe oxide nuclei. The iron oxide nanoparticles are composed of magnetite. The composite nanoparticles with the [Zn]/[Fe] weight ratios of 1:9, 3:7 and 1:1 are composed of magnetite and zinc ferrite (ZnO phase was not detected). The composite nanoparticles with the

Acknowledgements

This study was partially supported by a Minerva Grant (Microscale & Nanoscale Particles and Films). The authors also thank to Dr. Judith Grinblat for her help in the HRTEM analysis.

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