Antibacterial properties of spinel ferrite nanoparticlesformatex.info/microbiology4/vol1/239-250.pdf · Antibacterial properties of spinel ferrite nanoparticles ... The third part

Antibacterial properties of spinel ferrite nanoparticles

Noppakun Sanpo1, Cuie Wen1, Christopher C. Berndt1,2 and James Wang1 1Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia

2Stony Brook University, Materials Science and Engineering, Stony Brook, USA 11794

This book chapter is organized into four major parts. Firstly, it will cover an introduction of using spinel ferrite nanoparticles in biomedical applications, including the antibacterial properties. The second part will provide an overview of the structure and magnetism of spinel ferrites. The third part will focus on the preparation of cobalt ferrite and transition metal substituted cobalt ferrite nanoparticles by sol-gel methods. The last part will highlight the influence of transition metal concentration on the physical properties, and antibacterial property of transition metal substituted cobalt ferrite nanoparticles. A discussion on the mechanisms for the antibacterial properties will be presented.

Keywords Spinel ferrite; Antibacterial properties; Sol-gel, Transition metals; Nanoparticles

1. Introduction

Nanotechnology incorporates the understanding and control of matter at dimensions of 1–100 nm, where unique physical phenomena enable novel applications. The physical and chemical properties of nanomaterials are governed largely by their size and shape or morphology. Therefore, materials scientists are focusing on developing simple and effective methods for fabricating nanomaterials with controlled size and morphology and, hence, with a view to tailoring their properties [1]. A variation in particle size can also modulate the physical property attributes without altering composition. As the particle size decreases below 100 nm, a large fraction of the constituting atoms are found on the surface of the nanocrystals; thus inducing significant changes in the magnetic structure and properties of the materials at nanoscale as compared to their bulk counterparts. Specially, the domain wall structure encountered in, for example, bulk crystalline ferrites is replaced by a single domain structure for each particle. These nano-micro-changes lead to new phenomena such as superparamagnetism, enhanced anisotropy, and spin canting [2]. Spinel ferrite nanoparticles are of interest because of their well-known unique optical, electronic and magnetic properties. These nanoparticles have high permeability, good saturation magnetization, and no preferred direction of magnetization. They are magnetically “soft”, being easily magnetized and demagnetized, and electrically insulating. For these reasons, ferrites have been used as magnetic materials as well as refractory materials and catalysts [3]. However, using such magnetic nanoparticles for biological and medical purposes remains a challenge due to lack of research in this field. Buteicǎ et al. [4] prepared magnetic, core-shelled Fe3O4 nanoparticles to improve colloidal dispersion and to control particle sizes. It was observed that, for the same time interval, the inhibition zone diameters for cephalosporins were higher than those for the cephalosporin-nanofluid. An advantage of this technology was that the nanofluid acted only as a carrier for the antibiotic. In addition, small magnetic nanoparticles allowed delivery of an antibiotic when targeting certain organs such as brain and kidney. Sun et al. [5] developed a thermal decomposition method that used a mixture of an iron salt, 1,2-hexadecanediol, oleic acid, oleylamine, and biphenyl ether to obtain Fe3O4 nanoparticles. The Fe3O4 nanoparticles were then coated with silver to improve bacterial activity. The distinctive properties of magnetic nanoparticles required for biomedical applications require precise control of particle size, shape, dispersion and any external factors that influence these properties. In principle, it is necessary to stabilize the magnetic nanoparticle dispersion in the aqueous environment. Thus, coating the magnetic nanoparticles with a polymer shell, including organic (e.g., polyethylene glycol, dextran, chitosan, polyethyleneimine, and phospholipids) or inorganic (e.g., silica) materials, leads to highly dispersed and high quality nanoparticles with good biocompatibility [6].

Recently, Sanpo et al. [7-9] suggested a new approach to enhance the antibacterial property of ferrite nanoparticles for biomedical applications. They found that substitution of spinel ferrite with a transition metal can improve the antibacterial capability of nanoparticles. However, the biomedical application of these newly-synthesized nanoparticles requires further study concerning their biocompatibility and antibacterial properties. Techniques reported for synthesizing spinel ferrite nanoparticles of chemical formula, MFe2O4 (where M = Co, Mg, Mn, Zn, etc.) [10] include solid state reaction, microemulsion, combustion, the redox process, chemical co-precipitation, the hydrothermal method, and microwave synthesis. However, sol-gel techniques offer enhanced control over homogeneity, elemental composition and powder morphology. In addition, uniform nano-sized metal clusters can be achieved, which are crucial for enhancing the properties of the nanoparticles. These advantages favor the sol-gel route over other conventional preparation methods of ceramic oxide composites [11]. Therefore, the objectives of this book chapter are threefold. The first objective is to develop novel multifunctional magnetic iron-based nanoparticles that also exhibit antibacterial properties to fulfil the requirements of a drug delivery

Microbial pathogens and strategies for combating them: science, technology and education (A. Méndez-Vilas, Ed.)

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system so that the antibiotic concentration could be minimized. A potential delivery system could be based on a ferro magnetic fluid. The second objective is to synthesize transition metal-substituted spinel ferrite nanoparticles (X0.5Y0.5Fe2O4 with X = Co; Y = Cu, Zn, Mn, and Ni) by the sol-gel process using citric acid (CA) as the chelating agent. The final objective is to investigate the effect of chelating agents and substitution of these transition metals on the physical and antibacterial properties of the synthesized transition metal-substituted spinel ferrite nanoparticles. The flow chart of this research study is presented in Fig. 1.

Fig. 1 The flow chart of preparation and characterization of spinel ferrite.

2. Ferrites

Ferrites are chemical compounds with the formula of AB2O4, where A and B represent various metal cations; usually including iron. Ferrites are considered a class of spinels that consist of cubic closed pack oxides with A cations occupying 1/8th of the octahedral voids and B cations occupying half of the octahedral voids. For the inverse spinel structure, half the B cations occupy tetrahedral sites and both the A and B cations occupy the octahedral sites. Meanwhile, divalent, trivalent and quadrivalent cations can occupy the A and B sites and they include Mg, Zn, Fe, Mn, Al, Cr, Ti and Si. Ferrites are usually non-conductive ferromagnetic ceramic compounds derived from iron oxides as well as oxides of other metals. Like most other ceramics, ferrites are hard and brittle. Ferrites are widely used in high-frequency applications because an AC field does not induce undesirable eddy currents in an insulating material. Based on the magnetic properties, ferrites are often classified as soft and hard ferrites as shown in Fig. 2.

Fig. 2 Hysteresis loop for (a) soft and (b) hard ferrite.


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2.1. Soft ferrites

Soft ferrites are characterized by a small value of coercivity so they cause low hysteresis loss at high frequency owing to which they are widely used in electromagnetic cores of transformers, switching circuits in computers and RF inductors; e.g., lithium ferrite, nickel ferrite and manganese-zinc ferrite.

2.2. Hard ferrites

Hard ferrites are characterized by a large value of retentivity and coercivity after magnetization so they find applications as permanent magnets in radios; e.g., barium and strontium ferrite. Maximum magnetic field strength is about 0.35 T and magnetic field strength is about 30 to 160 kA/m. Ferrites are ferromagnetic materials that are typically oxides of mixed transition metals involving the iron. For example, MnFe2O4 and Mg-Zn ferrite are described as Mn1-xZnxFe2O4. They are usually insulating in nature, like most other ceramics, hard and brittle. In terms of the magnetic properties, ferrites are often classified as "soft" and "hard" which refers to their low or high coercivity of their magnetism, respectively. These ceramic materials are used in applications such as magnetic components in microelectronics. Most ferrite particles used in synthesizing magnetic fluids exhibit a spinel structure. This structure consists of a cubic closed packed case of oxygen ions with the metallic ions occupying the tetrahedral A and octahedral B interstitial sites.

2.3. Unit cell structure of spinel ferrite

The unit cell of spinel ferrites consists of 32 oxygen, 16 trivalent iron, and 8 divalent metal irons. The most important feature of the unit cell is that its array of oxygen ions creates two kinds of interstices, which can be filled by the metal ions. These interstices are referred to as tetrahedral or A sites and octahedral or B sites. Figure 3 shows the unit cell of spinel structure. There are two groups of four cubes (octants). The ionic positions are different in two octants sharing a face or a corner and the same in two octants sharing an edge. Thus, to give a complete picture, it is necessary only to show the positions of the ions in two adjacent octants. Note that each octant contains a metal ion in the centre (small violet sphere) surrounded by the tetrahedral of oxygen ions: this ion is said to occupy an A site. The right hand octant shows four metal ions (small red spheres) each surrounded by an octahedran (one of which is shown) formed by six oxygen ions. Such ions are said to occupy B sites. In a unit cell, there are 64 A sites, 8 of which are occupied, and 32 B sites, 16 of which are occupied. The divalent metal ions commonly used in ferrites can be classified into those preferring B sites (Co, Fe, Ni) and those preferring A sites (Mn and Zn). In the normal spinel structure the 8 divalent metal ions go into the A sites and the 16 trivalent iron ions have preference for B sites. They will displace eight of the trivalent iron ions which go over into the A sites. This results in an inverted spinel. As two ionic species are then distributed over the octahedral sites, randomness may be present, contributing to the line width of the materials. These are, however, limiting cases [12].

Fig. 3 The unit cell structure of spinel ferrite.

2.4. Substitution in spinels

The unit cell structure of substituted ferrites is more complex. In these materials, some of the ferric iron ions (Fe3+) are replaced by trivalent ions of another metal. The effect on the magnetization depends on the site preferred by the substitution; e.g., Al prefers octahedral coordination and therefore reduces the magnetization behaviour. It is not easy to predict the ion distribution.


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2.5. Spinel structures

The oxygen ions in the spinel structure form an fcc lattice and the A+2 valence and B+3 valence ions occupy tetrahedral and octahedral interstitial sites, depending on the spinel type.

2.5.1. Normal spinel structure

In the unit cell of spinel structure, there are 8 MO.Fe2O3 molecules. In this structure the eight M+2 ions occupy eight tetrahedral sites and the 16 Fe+3 ions occupy 16 octahedral sites [13]. In other words, in this structure, the non-magnetic ions occupy the A sites and consequently there is no AB interaction. The negative BB interaction now becomes dominant and the trivalent iron ions align themselves in an anti-parallel fashion; thereby producing zero net magnetization.

2.5.2. Inverse spinel structure

In the inverse spinel structure there are 8 M+2 ions that occupy 8 octahedral sites and the 16 Fe+3 ions are divided into 8 octahedral sites and 8 tetrahedral sites as shown in table 1 [13]. Table 1 Metal ion arrangements in spinel ferrite unit cell with composition (MO.Fe2O3).

Types of interstitial site Number available Number occupied Normal spinel Inverse spinel

Octahedral 64 8 8 M+2 8 Fe+3 Tetrahedral 32 16 16 Fe+3 8 Fe+3 8 M+2

3. Sol-gel technology

Sol-gel processing, which is based on chemical engineering methods, is a technique to manufacture ceramic powders, especially oxides. It has been down-selected for the synthesis of spinel ferrite nanoparticles due to relative simplicity in combination with novelty concerning the intended application. The term sol refers to the initial solution of the chemical components for the final powder; whereas the term gel describes the final consolidation stage that forms the ceramic product. Sol-gel procedures have been successful in the preparation of bulk metal oxides, e.g., ceramics, glasses, films and fibers [14] and, therefore, they have been applied for nanoparticle synthesis. However, sol-gel chemistry is not favoured for the preparation of oxide nanoparticles and, thus, represents an opportunity for the size- and shape-controlled synthesis of nanoparticles. Such morphological attributes are important since they have the potential to enhance the efficacy of nano-based biomaterials. A synthesis protocol developed for a bulk metal oxide could not be directly adapted to its corresponding counterpart on the nanoscale. The reasons for this observation are manifold. Aqueous sol-gel chemistry is complex, on the one hand due to the high reactivity of the metal oxide precursors towards water and the double role of water as ligand and solvent. On the other hand there are a large number of reaction parameters that must be controlled (e.g., hydrolysis and condensation rate of the metal oxide precursors, pH, temperature, method of mixing, rate of oxidation, the nature and concentration of anions) to provide good reproducibility of the synthesis protocol [15]. The aqueous sol-gel process can be defined as the conversion of a precursor solution into an inorganic solid via inorganic polymerization reactions induced by water. In general, the precursor or starting compound is either (i) an inorganic (i.e., no carbon), (ii) a metal salt (chloride, nitrate, sulfate) or (iii) a metal organic compound such as an alkoxide. Metal alkoxides are the most widely used precursors because they react readily with water and are known for many metals. Some alkoxides, which are widely used in industry, are commercially available at low cost (e.g., Si, Ti, Al, Zr), whereas others are rarely available or costly (e.g., Mn, Fe, Co, Ni, Cu, Y, Nb, Ta). The sol-gel process consists of the following 5 prime steps (Fig. 4).

(i) Preparation of a homogeneous solution either by dissolution of metal organic precursors in an organic solvent that is miscible with water, or by dissolution of inorganic salts in water.

(ii) Conversion of the homogeneous solution into a sol by treatment with a suitable reagent that is generally water with or without an acid base.

(iii) Aging of the solution. (iv) Shaping of the gel. (v) Thermal treatment or sintering of the final product.


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Fig. 4 The production steps for a sol-gel production route. The first step in a sol-gel reaction is the formation of an inorganic polymer by hydrolysis and condensation reactions; i.e., the transformation of the molecular precursor into a highly crosslinked solid. Hydrolysis leads to a sol, which is described as a dispersion of colloidal particles in a liquid. Further condensation results in a gel; that is, an interconnected, rigid and porous inorganic network enclosing a continuous liquid phase. This transformation is called the sol-gel transition. The gel may be dried under two alternative routes. In the first method, an aerogel is produced on removal of the pore liquid under hypercritical conditions. The second method produces an xerogel when the gel is dried under ambient conditions and in this case pore shrinkage occurs. An attractive feature of the sol-gel process is the opportunity to shape the material into desirable morphologies such as monoliths, films, fibers, and monosized powders, and subsequently to convert this shape into a ceramic by heat treatment. The sol-gel conversion of metal alkoxides involves two reaction types: hydrolysis and condensation (Fig. 5).

Fig. 5 Main reactions in the sol-gel process using metal alkoxides. Equation 1 represents hydrolysis, while condensation involves oxolation (Eq. 2) and alkoxolation (Eq. 3).

The major challenge of sol-gel methods, especially for those methods based on the hydrolysis and condensation of molecular precursors, is control over the reaction rates. For most transition metal oxide precursors, these reactions are too fast, resulting in loss of morphological and structural control over the final oxide material. Moreover, the metal alkoxides exhibit a range of reactivities that make it difficult to control the composition and the homogeneity of complex multimetal oxides formed via the sol-gel process. The reactivity of the precursors can be altered by employing organic additives such as carboxylic acids, β–diketones or functional alcohols, which act as chelating ligands and modify the reactivity of the precursors [16]. An alternative strategy involves the gradual release of water by chemical or physical processes; thus allowing control over the local water concentration and the hydrolysis of the metal oxide precursors.

4. Approach and Methodology

Spinel ferrite and transition metal-substituted spinel ferrite nanoparticles were prepared via a sol-gel route using citric acid (CA) as a chelating agent. The influence of transition metal-substituted spinel ferrite nanoparticles concentration on the microstructure, and antibacterial properties of these nanoparticles against E. coli and S. aureus were investigated.

4.1. Fabrication of spinel ferrite and transition metal-substituted spinel ferrite nanoparticles

The chelating agent was prepared by dissolving the necessary powders in distilled water (5%, w/v) at 70 °C. The chelating agent solutions were maintained at 70 °C for 5 h or until the solution became clear. Metal precursors were dissolved into the chelating agent solution under magnetic stirring. The sol-gel reaction was continued for 3 h and the


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temperature increased to 80 °C for 10 h or until the gel dried into the form of a powder. Finally, all samples were sintered at 800 °C for 4 h and then ground to a powder as presented in Fig. 6. The morphology of the nanoparticles was observed using a field emission scanning electron microscope (FeSEM ZEISS SUPRA 40 VP). The antibacterial properties of nanoparticles were carried using both qualitative and quantitative analysis test.

Fig. 6 The preparation of spinel ferrite and transition metals-substituted spinel ferrite nanoparticles

4.2. Antibacterial property of spinel ferrite nanoparticles

The antimicrobial activity of transition metal-substituted cobalt ferrite nanoparticles was tested against gram negative bacteria, Escherichia coli (E. coli), and gram positive bacteria, Staphylococcus aureus (S. aureus). The SEM images of E. coli and S. aureus are presented in Fig. 7. The bacteriological test series were carried out according to the modified ASTM E2180-07: ‘Standard Test Method for Determining the Activity of Incorporated Antimicrobial Agent(s) in Polymeric or Hydrophilic Materials’. All tests were performed on solid agar plates with different transition metal-substituted cobalt ferrite nanoparticles. E. coli and S. aureus were grown aerobically at 37 °C overnight with shaking (200 rpm) in an ordinary broth medium. The optical density (OD) of the overnight culture (OVN culture) was measured at 600 nm by UV spectrometry and diluted with lysogeny broth (LB) to achieve an OD of 0.1. The diluted OVN culture was then seeded into the 20 ml LB and incubated at 37 °C on the shaker for 2 h to obtain an OD of 0.3. At OD = 0.3, the diluted OVN culture and transition metal-substituted cobalt ferrite nanoparticles were further incubated for 24 h and 100 μl of these mixtures were spread on an agar plate using a L shaped spreader.

Fig. 7 The SEM images of (a) E. coli and (b) S. aureus.


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The inoculated plates were incubated at 37 °C and the number of colonies on the petri plates was counted after 24 h. The colony forming units (CFUs) were calculated by multiplying the number of colonies by the dilution factor. The survival percentage, which is defined by formula [17], was used to evaluate the antimicrobial effect of particles.

Survival % = Colony number of treated bacteria

Colony number of control bacteria × 100 (4)

For the modified Kirby-Bauer method [18], an equal amount of transition metal-substituted cobalt ferrite nanoparticles obtained from CA were coated on filter papers. All samples were placed on the E. coli growth LB agar plate and incubated overnight at 37 °C. The zone of inhibition was measured and the experimental results were discussed. Prior to each bacterial attachment experiment, a fresh bacterial suspension of OD = 0.2 was prepared from E. coli and S. aureus cells grown in lysogeny broth (LB) at room temperature (~22 °C) for 24 h. All nanoparticles were attached to the glass slides using double sided tape. A 3–5 mL portion of bacterial suspension was poured into sterile Petri dishes where glass slides (one glass slide per Petri dish) were immersed and left to incubate for 12 h at room temperature (~22 °C). After incubation all of the slides were washed with deionized water and left to air dry. This experimental approach allowed bacterial attachment to be performed under identical conditions for each microscope slide. An FeSEM–ZEISS SUPRA 40VP was used to obtain high-resolution images of the bacterial cells.

5. Experimental results

5.1. Microstructure of spinel ferrite nanoparticles

SEM micrographs investigated the grain structure of the nanoparticles and assisted in understanding the development of the grain sizes. Figure 8 shows the SEM images of the synthesized cobalt ferrite and transition metal-substituted cobalt ferrite nanoparticles. The CoFe2O4 nanoparticles, Fig. 8a, exhibit an angular morphology. Meanwhile, the

Fig. 8 SEM images of synthesized transition metal-substituted cobalt ferrite nanoparticles: (a) CoFe2O4, (b) Co0.5Cu0.5Fe2O4, (c) Co0.5Zn0.5Fe2O4, (d) Co0.5Mn0.5Fe2O4 and (e) Co0.5Ni0.5Fe2O4 [9].


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nanoparticles synthesized with transition metal doping in Fig. 8b-e present particles with irregular shapes and agglomeration where some particles form large clusters. The average sizes of 30 grains, measured by ImageJ manufactured from NIH Image Company, for CoFe2O4, Co0.5Cu0.5Fe2O4, Co0.5Zn0.5Fe2O4, Co0.5Mn0.5Fe2O4, and Co0.5Ni0.5Fe2O4 are 43.0, 40.1, 44.2, 40.3, and 41.0 nm, respectively.

5.2. Antibacterial activities of nanoparticles

5.2.1. Contact biocidal studies

The contact biocidal property of all ferrite nanoparticles obtained was investigated using a modified Kirby-Bauer technique. Filter papers are partially covered with and without ferrite nanoparticles and placed on a lawn of E. coli in an agar plate. The contact antibacterial property can be measured by the clear zone of inhibition around the filter papers after 24 h incubation, Fig. 9. The diameter of the zone of inhibition for the copper-substituted cobalt ferrite nanoparticles (Fig. 9c) is the largest, followed by zinc-substituted cobalt ferrite (Fig. 9d), nickel-substituted cobalt ferrite (Fig. 9f), pure cobalt ferrite (Fig. 9b), and manganese-substituted cobalt ferrite (Fig. 9e), in decreasing order. The results agree well with the quantitative bacterial tests that indicated the copper-substituted cobalt ferrite nanoparticles have the most effective antibacterial property against E. coli (Fig. 12) among all of the nanoparticles investigated in this study.

Fig. 9 Image of E. coli incubated for 24 h at 37 °C together with filter paper: (a) without cobalt ferrite nanoparticles, (b) CoFe2O4, (c) Co0.5Cu0.5Fe2O4, (d) Co0.5Zn0.5Fe2O4, (e) Co0.5Mn0.5Fe2O4 and (f) Co0.5Ni0.5Fe2O4 [9].

5.2.2. Bacterial attachment studies

The potential for adhesion between a bacterial cell and a substratum surface is governed by several factors, including the physico-chemical properties of the bacterium, the physico-chemical properties of the substratum and the environmental conditions under which the attachment takes place [19]. With all other parameters being equal, any differences in the degree of attachment of a given bacterium on two different substrata under the same conditions will be due to the different surface properties of the materials. While a substantial amount of research has been conducted to investigate the effects of substratum surface chemistry on bacterial adhesion, the recent trend has been focused on the


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role that surface topography plays in the bacterial attachment process [20]. Typical SEM images, Fig. 10, indicated that the morphology of the S. aureus attached to the glass surface exhibited features similar to cells attached to the transition metal-substituted cobalt ferrite nanoparticles. The number of the attached cells on the glass surfaces decreased compared to transition metal-substituted cobalt ferrite nanoparticles. The number of the attached cells on the nanoparticles surface decreased in the order of Co0.5Zn0.5Fe2O4 < Co0.5Cu0.5Fe2O4 < Co0.5Mn0.5Fe2O4 < CoFe2O4 < Co0.5Ni0.5Fe2O4. This could be attributed to the difference in chemical composition, surface architecture, and roughness of the nanoparticles, as previously discussed. SEM images representing the attached E. coli on synthesized transition metal-substituted cobalt ferrite nanoparticles are presented in Fig. 11. Both the morphology and number of bacteria attached to the glass surface were significantly different from those attached to the synthesized transition metal-substituted cobalt ferrite nanoparticles. Biofilms were also noticed on the surface of transition metal-substituted cobalt ferrite nanoparticles. Biofilms are the most common mode of bacterial growth in nature and are highly resistant to antibiotics. This form of microbial growth has been studied in a wide range of scientific disciplines including biomedicine, water engineering and evolutionary biology [21]. The intensity of biofilms covered on the nanoparticles surface decreased in the order of Co0.5Cu0.5Fe2O4 < Co0.5Zn0.5Fe2O4 < Co0.5Mn0.5Fe2O4 < CoFe2O4 < Co0.5Ni0.5Fe2O4.

Fig. 10 Typical SEM images representing the attached S. aureus on synthesized transition metal-substituted cobalt ferrite nanoparticles: (a) without cobalt ferrite nanoparticles, (b) CoFe2O4, (c) Co0.5Cu0.5Fe2O4, (d) Co0.5Zn0.5Fe2O4, (e) Co0.5Mn0.5Fe2O4 and (f) Co0.5Ni0.5Fe2O4 after 12 h incubation [9].


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Fig. 11 Typical SEM images representing the attached E. coli on synthesized transition metal-substituted cobalt ferrite nanoparticles: (a) without cobalt ferrite nanoparticles, (b) CoFe2O4, (c) Co0.5Cu0.5Fe2O4, (d) Co0.5Zn0.5Fe2O4, (e) Co0.5Mn0.5Fe2O4 and (f) Co0.5Ni0.5Fe2O4 after 12 h incubation [9].

5.2.3. Bacterial quantitative studies

The antibacterial activities of the transition metal-substituted cobalt ferrite nanoparticles against E. coli and S. aureus are shown in Fig. 12. All tests were repeated ten times for statistical studies after culture incubation at 37 °C overnight. The concentration of cobalt ferrite nanoparticles was fixed at 1 g/L. Compared to the control (the sample of bacteria solution without adding nanoparticles), cobalt ferrite and transition metal-substituted cobalt ferrite nanoparticles inhibit the growth of both E. coli and S. aureus, and the E. coli killing rate of all ferrite composite nanoparticles is higher than the S. aureus killing rate. The antibacterial properties of manganese and nickel-substituted cobalt ferrite nanoparticles are lower than the pure cobalt ferrite nanoparticles. However, their antibacterial abilities became more dominant when copper and zinc were substituted into cobalt ferrite nanoparticles. There are several possible mechanisms for the antibacterial action of zinc and copper ceramic nanoparticles. Studies suggested that when E. coli is treated with copper nanoparticles, changes take place in its cell membrane morphology. These nanoparticles adhere to the bacterial cell wall and penetrate through the cell membrane as presented in Fig. 13 [22]. Copper ions cause destruction of the bacterial cell wall, degradation and lysis of the cytoplasma; leading to cell death. Moreover, high concentrations of copper nanoparticles demonstrate complete cytotoxicity against E. coli [23]. Nanoparticles have a large surface area, thus their bactericidal efficacy is enhanced compared to large sized particles. Hence, nanoparticles are believed to impart cytotoxicity to microorganisms. Copper nanoparticles exhibit a large surface-to-volume ratio, which enhances their bioactivity and makes them effective bactericidal agents [24]. For the zinc nanoparticles system, studies showed that zinc binds to the membranes of microorganisms, similar to mammalian cells, prolonging the lag phase of the growth cycle and increasing the generation time of the organisms so that it takes each organism more time to complete cell division [25].


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Fig. 12 Antibacterial activities against E. coli and S. aureus of synthesized transition metal-substituted cobalt ferrite nanoparticles: (a) without cobalt ferrite nanoparticles, (b) CoFe2O4, (c) Co0.5Cu0.5Fe2O4, (d) Co0.5Zn0.5Fe2O4, (e) Co0.5Mn0.5Fe2O4 and (f) Co0.5Ni0.5Fe2O4 [9].

Another proposition maintained that the main chemical species contributing to the occurrence of the antibacterial activity were assumed to be active oxides; for example, hydrogen peroxide (H2O2) and super-oxide (O2

–), generated from the surface of the zinc ceramics [26]. These active oxides readily penetrate the cell wall of bacteria and cause cell destruction. The penetration rate of active oxides through the bacteria cell wall plays an important role in the killing rate of zinc ceramic nanoparticles against bacteria. Furthermore, the structure and chemical composition of the cell walls are quite different between E. coli and S. aureus. The E. coli cell wall consists of lipid A, lipopolysaccharide and peptidoglycan; whereas the cell wall of S. aureus consists mainly of peptidoglycan. The results indicate that active oxides generated from transition metal-substituted cobalt ferrite have more capability to penetrate the cell wall and decrease the cell division of E. coli rather than S. aureus. However, the precise mechanism of interaction between bacteria (E. coli and S. aureus) and transition metal-substituted cobalt ferrite nanoparticles needs to be investigated further.

Fig. 13 The mechanisms for the antibacterial action of spinel ferrite nanoparticles.

6. Conclusions

A sol-gel technique for synthesizing spinel ferrite nanoparticles that employs citric acid as a chelating agent was developed. Transition metal-substituted cobalt ferrite nanoparticles formed a cubic spinel structure and exhibited irregular morphology with a crystallite size in the range of 40–50 nm. The substitution of zinc and copper in cobalt ferrite nanoparticles significantly improved antibacterial activity against E. coli and S. aureus. Copper-substituted cobalt ferrite nanoparticles exhibited the most effective contact biocidal property among all of the nanoparticles.


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The antibacterial activity of transition metal-substituted cobalt ferrite nanoparticles against E. coli was higher than S. aureus. Zinc and copper-substituted cobalt ferrite nanoparticles have high potential to be used in drug delivery systems as well as in other biomedical and biotechnology applications.

Acknowledgements The authors acknowledge financial support for this research through the Australia-India Strategic Research Fund (AISRF) ST060048.

References [1] Jiang K, Li K, Peng C, Zhu Y. Effect of multi-additives on the microstructure and magnetic properties of high permeability Mn-

Zn ferrite. Journal of Alloys and Compounds. 2012;541:472-6. [2] Sohn BH, Cohen RE. Processible Optically Transparent Block Copolymer Films Containing Superparamagnetic Iron Oxide

Nanoclusters. Chemistry of Materials. 1997;9:264-9. [3] Mathew T, Malwadkar S, Shivanand, Pai, Sharanappa N, Sebastian CP. Oxidative dehydrogenation of ethylbenzene over

Cu1-xCoxFe2O4 catalyst system: Influence of acid-base property. Catalysis Letters. 2003;91:217-24. [4] Buteicǎ AS, Mihaiescu DE, Grumezescu AM, Vasile BŞ, Popescu A, Mihaiescu OM. The anti-bacterial activity of magnetic

nanofluid: Fe3O4/oleic acid/cephalosporins core/shell/adsorption-shell proved on S. Aureus and E. Coli and possible applications as drug delivery systems. Digest Journal of Nanomaterials and Biostructures. 2010;5:927-32.

[5] Sun S, Zeng H, Robinson DB, Raoux S, Rice PM, Wang SX. Monodisperse MFe2O4 (M = Fe, Co, Mn) Nanoparticles. Journal of the American Chemical Society. 2004;126:273-9.

[6] Byrappa K, Ohara S, Adschiri T. Nanoparticles synthesis using supercritical fluid technology - towards biomedical applications. Advanced Drug Delivery Reviews. 2008;60:299-327.

[7] Sanpo N, Berndt CC, Wang J. Microstructural and antibacterial properties of zinc-substituted cobalt ferrite nanopowders synthesized by sol-gel methods. Journal of Applied Physics. 2012;112:084333.

[8] Sanpo N, Wang J, Berndt CC. Effect of zinc substitution on microstructure and antibacterial properties of cobalt ferrite nanopowders synthesized by sol-gel methods. Advanced Materials Research. 2012;436-9.

[9] Sanpo N, Berndt CC, Wen C, Wang J. Transition metal-substituted cobalt ferrite nanoparticles for biomedical applications. Acta Biomaterialia. 2013;9:5830-5837.

[10] Kim DK, Zhang Y, Voit W, Rao KV, Muhammed M. Synthesis and characterization of surfactant-coated superparamagnetic monodispersed iron oxide nanoparticles. Journal of Magnetism and Magnetic Materials. 2001;225:30-6.

[11] De G, Mattei G, Mazzoldi P, Sada C, Battaglin G, Quaranta A. Au−Cu Alloy Nanocluster Doped SiO2 Films by Sol−Gel Processing. Chemistry of Materials. 2000;12:2157-60.

[12] Degueldre C, Kuri G, Borca CN, Grolimund D. X-ray micro- fluorescence, diffraction and absorption spectroscopy for local structure investigation of a radioactive zinc ferrite deposit. Corrosion Science. 2009;51:1690-5.

[13] Charap SH. Magnetic viscosity in recording media. Journal of Applied Physics. 1988;63:2054-7. [14] Hench LL, West JK. The Sol-Gel process. Chemical Reviews 1990;90:33-72. [15] Livage J, Henry M, Sanchez C. Sol-gel chemistry of transition metal oxides. Progress in Solid State Chemistry. 1988;18:259-

341. [16] Hubert-Pfalzgraf LG. Some aspects of homo and heterometallic alkoxides based on functional alcohols. Coordination

Chemistry Reviews. 1998;178-180:967-97. [17] Lu Z, Li CM, Bao H, Qiao Y, Toh Y, Yang X. Mechanism of Antimicrobial Activity of CdTe Quantum Dots. Langmuir.

2008;24:5445-52. [18] Wikins TD, Holdeman LV, Abramson IJ, Moore WE. Standardized single-disc method for antibiotic susceptibility testing of

anaerobic bacteria. Antimicrobial Agents and Chemotherapy. 1972;1:451-9. [19] An YH, Friedman RJ. Concise review of mechanisms of bacterial adhesion to biomaterial surfaces. Journal of Biomedical

Materials Research. 1998;43:338-48. [20] Bos R, Van Der Mei HC, Busscher HJ. Physico-chemistry of initial microbial adhesive interactions - Its mechanisms and

methods for study. FEMS Microbiology Reviews. 1999;23:179-229. [21] Austin JW, Bergeron G. Development of bacterial biofilms in dairy processing lines. Journal of Dairy Research 1995;62:509-

19. [22] Hu C-H, Xia M-S. Adsorption and antibacterial effect of copper-exchanged montmorillonite on Escherichia coli K88. Applied

Clay Science. 2006;31:180-4. [23] Raffi M, Mehrwan S, Bhatti TM, Akhter JI, Hameed A, Yawar W. Investigations into the antibacterial behavior of copper

nanoparticles against Escherichia coli. Annals of Microbiology. 2010;60:75-80. [24] Stoimenov PK, Klinger RL, Marchin GL, Klabunde KJ. Metal oxide nanoparticles as bactericidal agents. Langmuir.

2002;18:6679-86. [25] Radke LL, Hahn BL, Wagner DK, Sohnle PG. Effect of Abscess Fluid Supernatants on the Kinetics of Candida albicans

Growth. Clinical Immunology and Immunopathology. 1994;73:344-9. [26] Yamamoto O, Sawai J. Preparation and characterization of novel activated carbons with antibacterial function. Bulletin of the

Chemical Society of Japan. 2001;74:1761-5.


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