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Characterisation of copper oxide nanoparticles for antimicrobial applications

https://doi.org/10.1016/j.ijantimicag.2008.12.004Get rights and content

Abstract

Copper oxide (CuO) nanoparticles were characterised and investigated with respect to potential antimicrobial applications. It was found that nanoscaled CuO, generated by thermal plasma technology, contains traces of pure Cu and Cu2O nanoparticles. Transmission electron microscopy (TEM) demonstrated particle sizes in the range 20–95 nm. TEM energy dispersive spectroscopy gave the ratio of copper to oxygen elements as 54.18% to 45.26%. The mean surface area was determined as 15.69 m2/g by Brunau–Emmet–Teller (BET) analysis. CuO nanoparticles in suspension showed activity against a range of bacterial pathogens, including meticillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli, with minimum bactericidal concentrations (MBCs) ranging from 100 μg/mL to 5000 μg/mL. The ability of CuO nanoparticles to reduce bacterial populations to zero was enhanced in the presence of sub-MBC concentrations of silver nanoparticles. Studies of CuO nanoparticles incorporated into polymers suggest release of ions may be required for optimum killing.

Introduction

As particles are reduced from a micrometre to a nanometre size, the resultant properties can change dramatically. For example, electrical conductivity, hardness, active surface area, chemical reactivity and biological activity are all known to be altered. The bactericidal effectiveness of metal nanoparticles has been suggested to be due to both their size and high surface-to-volume ratio. Such characteristics should allow them to interact closely with bacterial membranes, rather than the effect being solely due to the release of metal ions [1]. In theory, metal nanoparticles could be combined with polymers or coated onto surfaces, which may then have a variety of potential antimicrobial applications. The antimicrobial properties of both silver [2] and copper nanoparticles [3] have been previously reported, and both of these have been coated onto or incorporated into various materials [4].

Copper oxide (CuO)/copper (II) oxide/cupric oxide is a semiconducting compound with a monoclinic structure. CuO has attracted particular attention because it is the simplest member of the family of copper compounds and exhibits a range of potentially useful physical properties such as high temperature superconductivity, electron correlation effects and spin dynamics [5], [6]. As an important p-type semiconductor, CuO has found many diverse applications such as in gas sensors, catalysis, batteries, high-temperature superconductors, solar energy conversion and field emission emitters. In the energy-saving area, energy transferring fluids filled with nano CuO particles can improve fluid viscosity and enhance thermal conductivity [7]. CuO crystal structures possess a narrow band gap, giving useful photocatalytic or photovoltaic properties as well as photoconductive functionalities [8].

Limited information on the possible antimicrobial activity of nano CuO is available. CuO is cheaper than silver, easily mixed with polymers and relatively stable in terms of both chemical and physical properties. Highly ionic nanoparticulate metal oxides, such as CuO, may be particularly valuable antimicrobial agents as they can be prepared with extremely high surface areas and unusual crystal morphologies [9]. The aims of this study were to characterise physically and chemically nano CuO and to investigate this compound with respect to its potential antimicrobial applications.

Section snippets

Nanomaterial preparation

Nanoparticles of CuO, Cu, Cu2O, zinc oxide (ZnO) and silver (Ag) were prepared by Intrinsiq Materials Ltd. (Farnborough, UK) using thermal plasma (Tesima™) technology as shown in Fig. 1. This process allows the continuous gas phase production of bulk nanopowders.

Transmission electron microscopy (TEM) and TEM energy dispersive X-ray spectroscopy (EDS) analysis

To determine particle size, morphology and composition, CuO nanoparticles were visualised using a high-resolution transmission electron microscope (JEOL 2010; JEOL Ltd.) and subjected to TEM-EDS for high-speed elemental analysis.

Surface area determination using Brunau–Emmet–Teller (BET) method analysis

The BET

TEM-EDS and TEM analysis

TEM-EDS produced the trace spectrum of nano CuO as shown in Fig. 2A. The weight compositions for copper (Cu) and oxygen (O) were 80.65% and 16.96%, respectively. The atomic compositions were then calculated as 54.18% and 45.26%, respectively. A small proportion of impurities, such as pure Cu and Cu2O nanoparticles, were detected owing to interactions with air after plasma processing. TEM analysis demonstrated that nano CuO particles exhibited an approximate equi-axes shape with no sharp edges

Discussion

Identification of the precise elemental composition, particle size range and surface morphology of nanoparticulate CuO is a prerequisite to a full understanding of its potential application capabilities. TEM-EDS analysis demonstrated that the atomic composition of the Cu and O elements was 54.18% and 45.26%, respectively. The mean ratio of Cu and O was therefore 54.18:45.26 and an accurate compound formula based on the atomic ratio of Cu and O can thus be given as Cu1.2O or CuO0.84. Therefore,

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