Trends Sci. 2026; 23(6): 12525

Review on Recent Advances in Metal-substitution Modify Cobalt Ferrite Nanoparticles as Antibacterial Applications

Retna Arilasita^1,3, Nurdiyantoro Putra Prasetya³,

Utari¹, Suharno², Riyatun¹ and Budi Purnama^1,*

¹Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret, 

Surakarta 57126, Indonesia

²Department of Physics Education, Faculty of Teacher Training and Education, Universitas Sebelas Maret,

Surakarta 57126, Indonesia

³Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Tanjungpura,

Pontianak 78124, Indonesia

(^*Corresponding author’s e-mail: [email protected])

Received: 4 November 2025, Revised: 13 December 2025, Accepted: 20 December 2025, Published: 30 January 2026

Abstract

In recent decades, the modification of magnetic nanoparticles-cobalt ferrite (NPM-cobalt ferrite) for antibacterial agents has been widely studied. These NPM-cobalt ferrites have unique characteristics, making them a viable option for the development of antibacterial agents. This paper presents a thorough analysis of the metal used to modify of cobalt ferrite in order to enhance antibacterial activity. Furthermore, this review delves into the antibacterial processes of cobalt ferrite, which are contingent upon critical characteristics including particle size, surface area, and ion replacement. The key findings of this review highlight the antibacterial performance of NPM-metal-cobalt ferrite based on the zone of inhibition (ZOI) and the standard percentages. However, it is important to carry out further investigate to optimize its activity. One of which is utilizing other metal elements that have not been utilized by looking at important parameter aspects in the results reported in this paper. This review contributes to the understanding and potential utilization of NPM-cobalt ferrite as an approach that is suitable for antibacterial agents.

Keywords: Antibacterial, Cobalt ferrite, Metal-cobalt ferrite, Nanoparticles, ZOI, Surface area, Structural properties

Introduction

In recent years, several bacterially-induced diseases have resurfaced as a major global health concern, generating significant morbidity and mortality. The emergence of bacterial strains that are resistant to several drugs has presented new difficulties for the healthcare sector, necessitating the urgent development of novel and potent antibacterial treatment. This issue has been tackled using a number of approaches, including the application of nanotechnology. In this context, nanoparticles have attracted great attention due to their unique properties at the nano order scale, which allow them to interact more effectively with bacterial cells compared to bulk materials [1,2].

Several nanoparticles have been explored as antibacterial materials, one of which is a promising candidate is cobalt ferrite (CFO) nanoparticles. The combination of unique structural, magnetic, and chemical properties, along with a simple fabrication procedure, makes it excellent for application as antibacterial materials. In general, cobalt ferrite is a magnetic material with an inverse spinel structure and a number of advantageous characteristics. Its nano-scale size, less than 100 nm, causes a significant increase in the surface area to volume ratio, allowing for more effective contact with bacterial membranes [3]. Since there is a more noteworthy ratio of surface area to volume, nanoparticles (NPs) of smaller sizes are more reactive. Therefore, the microbial application resulting from the synthesis of the cobalt ferrite must have a nanometer unit size. The surface shape of nanoparticles also acts as an antimicrobial, and an open bond is helpful for binding oxygen so that microbes will die [4]. Its elemental properties make it non-toxic and biodegradable [5]. Furthermore, their magnetic effects can be directed and utilized to penetrate and destroy biofilms. Functionalization of magnetic nanoparticles is crucial for efficient capture of the target [6].

Cobalt ferrite nanoparticles exhibit strong antibacterial properties, making them effective against a wide range of bacterial pathogens. There are 3 main mechanisms in antibacterial activity, namely adsorption, diffusion, and antibiofilm [7]. The adsorption mechanism utilizes nanoscale size with a large surface area to inhibit bacterial growth. Treatments that can change the size of a material include annealing temperature and metal ion substitution (metal type or concentration) [8-10]. Nevertheless, several studies have shown that the size of nanoparticles is not the main determinant of antibacterial activity. In such cases, other mechanisms, such as diffusion, play a role. These metal ions' interactions can result in a Fenton reaction, which both inactivates the enzyme proteins and serves as a precursor to the generation of ROS. Consequently, metal ion substitution is one potential way to increase the antibacterial activity of cobalt ferrite nanoparticles.

Metal substitution into the cobalt ferrite lattice can affect its structural, morphological, and magnetic properties. According to Kumar et al. [11] when the concentration of aluminum increases, aluminum-doped cobalt ferrite can decrease the crystallite size from 36.87 to 10.54 nm. Ion Sm³⁺ substitution causes the distribution of cations where some tetrahedral Fe³⁺ ions move towards the octahedral (B) site due to the replacement of Fe³⁺ ions with Sm³⁺ ions [12]. This affects the magnetic properties of the nanoparticles. Additional research has demonstrated that substituting different ions for rare earth metals can result in cation rearrangement and grain size changes, both of which have a major impact on cobalt ferrite’s coercivity [13]. This analysis demonstrates that a key factor in metal substitution is the development of characteristics that permit usage as antibacterial materials.

This paper will review several research papers on magnetic nanoparticles-cobalt ferrite (NPM-CFO) applied as antimicrobial materials, whether pure NPM-CFO or substituted with other metals. It will also describe the synthesis of NPM-CFO, which produces tiny particle sizes, as needed as an antibacterial. Important factors influencing the effectiveness of biomedical applications of NPM-cobalt ferrite are biocompatibility and surface functionality. Particle size is one of the benchmarks for the success of the NPM-CFO synthesis process. Magnetic properties are sometimes not reported by researchers, even though these magnetic properties are the advantages of NPM-CFO materials. Antibacterial activity will be reviewed based on the percentage comparison of the area of ​​the inhibition zone with the standard used.

Characteristic of NPM-cobalt ferrite

NPM-cobalt ferrite is one of the most commonly investigated magnetic spinel ferrites exhibiting such unique properties as hard magnetic material [14], high coercive field [12,15], high magnetostrictive [16], high magneto-crystalline anisotropy [7], chemical stability [18], moderate saturation magnetization [19,20], mechanical hardness [21], wear resistance and electrical insulation [22].

The structural formula of CoFe₂O₄ is . The round and square brackets belong to tetrahedral and octahedral lattice sites, respectively [13]. Based on its distribution, cobalt ferrite belongs to the inverse spinel group where the divalent ion is in the octahedral site and the trivalent ion is distributed equally in the tetrahedral and octahedral sites [23]. The magnetic properties of NPM-CFO are considerably influenced by such factors as sample preparation and the synthesis method used [24,25], cobalt content [26], crystallite (particle) size [27], and cation distribution [28-32]. The synthesis method also has a considerable effect on cation distribution inside the lattice of cobalt ferrite [33,34].

Cobalt and iron oxides are desirable magnetic materials due to their specific properties and low-cost production. NPM-cobalt ferrite is a small-sized biocompatible device that can be assembled and lead to the advancement of cellular intake. The appropriate staying time and low dose in the patient body are interesting for in vivo applications of NPM-cobalt ferrite [35].

The ratio of surface area and volume causes the difference in surface atomic spins compared to the bulk ions due to the difference in the oxidation state. Spinel ferrites within the nano range have been investigated widely due to narrow size distribution and smaller particles possessing a larger surface area. The typical morphological characteristics of cobalt ferrite at the nanoscale are represented in Figure 1. Figure 1 clearly shows that for NPM-cobalt ferrite, the TEM profile shows the presence of fine nano-grains. Then, the HR-TEM results show adjacent lines indicating the lattice parameter (a). It’s hard work to achieve different properties for this material as morphology, size, and change of chemical composition. The effect of vacancies and the distribution of cations on properties like catalytic reaction is evident.

Figure 1 Typical images of TEM and HR-TEM the NPM-cobalt ferrite.

Synthesis of NPM-cobalt ferrite

NPM-cobalt ferrite has been synthesized using various preparation techniques, such as microwave [36], thermal decomposition [37], solvothermal [38,39], sol-gel processing [40] and (auto) combustion [41], hydrothermal [42,43], co-precipitation [44-50], reverse co-precipitation [51], organic precursor decomposition [52], double sintering, citrate complex, mechanochemical synthesis, mechanical milling, sonochemical [38], microemulsion [53], solid state reaction [54], solid state combustion [55], and modified pechini [56].

The wet reaction material CFO includes a method with hydrothermal, microemulsion, sol-gel formation, and co-precipitation has been a popular method. At hydrothermal, the metallic nitrates are combined and added to distilled water, followed by treatment in autoclave and deposition of NPM-cobalt ferrite. At microemulsion, NPM-cobalt ferrite is formed in an emulsion using an appropriate surfactant, sol well prepared by polymerization or hydrolysis reactions via adding suitable reagents in the precursor solution, and the gelation process is conducted through polymers addition or sol condensation to gel [35]. In the sol-gel process, a hard porous gel is produced after a sol is created through hydrolysis and polymerization events. After that, the solvent and residuals are removed from the gel’s pores by aging, drying, and annealing to produce the finished product. In the co-precipitation route, 2 solutions containing a precursor chemical are mixed, and then parameters like mixing rate, pH or temperature are changed to cause the precipitation of nanoparticles. The co-precipitation technique is reported to have advantages because it has been quite involved as a unique technique to prevent the agglomeration of nanoparticles (NPs) [57].

Recently, the use of natural materials to make environmentally friendly nanoparticles using green synthesis methods has been widely developed. Some natural ingredients that have been used are lemon juice [56], rosemary extract [58], aloe vera [59], Swertia chirata extract [60], torajabin [61], sesame seed extract [62], Salix alba (white willow) [63], hibiscus extract [64], rooibos (Aspalathus linearis) tea [65], Zingiber officinale and Elettaria cardamom seed extracts [66], and Tamarindus indica fruit extract [67].

In addition to employing diverse techniques, there are other ways for investigation in the synthesis of NPM-CFO, including the employment of different fuels [68], variations in synthesis temperature, pH [69], molarity [70], chemical composition, and other factors. However, other post-synthesis treatments such as annealing temperature [71,72], annealing time [73], microwave treatment [74], and gamma irradiation [75] can also improve the performance of NPM-CFO. On the other hand, the preparation method is essential for studying antibacterial activity. Although the preparation method doesn’t directly affect antibacterial performance, it does support the structural, morphological, and magnetic properties of the material, thus impacting its long-term antibacterial performance.

The antibacterial mechanism of NPM-cobalt ferrite

Most known that antibacterial activities of NPM-cobalt ferrite mainly depend on the reactive oxygen species (ROS) produced. It also depends on the size, morphology, surface area, increase in oxygen vacancies, chemical molecule diffusion ability, and the discharge of metal ions. The mechanisms of antibacterial activity in nanoparticles are adsorption, diffusion and antibiofilm [7].

Adsorption is the nanoparticle mechanism that adheres to the bacterial cell surface. The mechanism of transporting electrons and proteins is disrupted when bacterial cell walls absorb nanoparticles, which have a size comparable to that of a nanoscale particle. This causes membrane depolarization [76]. In the end, this mechanism may cause the death of bacterial cells.

The process of nanoparticles moving across a medium or bacterial environment is called diffusion. By this method, bacteria that are more widely distributed in the environment - such as those found in fluids or biofilms - can be reached by the nanoparticles. Smaller nanoparticles can diffuse more rapidly and effectively, opening up new possibilities for interactions with bacterial cells. Fenton reactions can arise from the release of metal ions by nanoparticles. Reactive oxygen radicals (ROS), which can damage cell membranes, break down DNA and proteins, and interfere with metabolic activities, are created via the Fenton reaction [7,77]. Additionally, the antibacterial mechanism of M-substituted NPM-cobalt ferrite is illustrated in Figure 2. First, the substituted metal ions are gradually released from the spinel lattice and interact with bacterial membrane components, disrupting the structural stability of the membrane. Second, the release of ions from the spinel ferrite enhances the surface redox reaction, leading to increased ROS generation. Finally, the combined effect of the cobalt ferrite properties and ROS attack then disrupts the membrane integrity, causing leakage of intracellular contents and ultimately resulting in bacterial inactivation.

Figure 2 Antibacterial mechanism of metal-substituted NPM-cobalt ferrite.

Moreover, antibiofilm functions by preventing or eliminating the development of biofilm, or protective layers. Antibiotics and other antibacterial drugs frequently cause biofilms to become extremely resistant. In order to prevent bacterial adherence, breach and harm the protective biofilm matrix, and prevent bacterial communication, nanoparticles adhere to the surface of bacteria [78].

Furthermore, lipoteichoic and trichoic acids are present in the peptidoglycan layer of Gram-positive bacteria. Gram-negative bacteria have an outer membrane made of protein, phospholipids, and lipopolysaccharides, as well as a thin coating of peptidoglycan. This leads to the conclusion that the resistance of the outer membrane envelope to reactive oxygen species produced on the photocatalytic surface as well as cell shape determine the rate of bacterial photoinactivation [79,80]. The all mechanism will produce clear zone of inhabitation (ZOI) as the result of antibacterial activity. Finally, the comparison of nanoparticle’s area ZOI by standard is introduced for antibacterial performance as follows:

NPM-CFO as antibacterial agent

Several papers have reported that NPM-cobalt ferrite has antibacterial properties. Below are various categories of NPM-cobalt ferrite compositions.

Aftab et al. [81] used solvothermal methods to manufacture a number of materials, including cobalt ferrite. X-ray diffraction study revealed that the

crystallites’ size was 5 nm, which is less than that of the other materials examined. The agar-well diffusion method was used to test its antibacterial efficacy against gram-positive S. aureus and gram-negative E. coli. Consequently, it was discovered that the nanomaterials’ antibacterial activity depended on concentration. As concentration increased, the S. aureus bacterial zones of inhibition were found to be 0 and 11 mm, respectively. On the other hand, E. coli had a better inhibitory zone, measuring 4 and 12 mm with increasing concentrations. They came to the conclusion that there is a lot of promise for biomedical uses for nanoparticles as antibacterial agents.

Kaur et al. [81] fabricated magnetic heterocomposite of graphene supported CoFe₂O₄/BiVO₄ and explored the photocatalytic and antibacterial activities. The hydrothermal process was used in the fabrication of this material. Antimicrobial activity of CoFe₂O₄/BiVO₄ against S. aureus and E. coli was found to be good. It was discovered that as the concentration of E. Coli bacteria increased from 25 to 100 µg/mL, the inhibition zone increased from 11 to 20.5. In contrast, S. aureus bacteria’s inhibitory zone very little changed as concentration increased.

Next, Shahriarinour et al. [83] also reported the antibacterial properties of magnetic cobalt ferrite nanoparticles. This material is synthesized using a one-step hydrothermal method of Fe³⁺ to Fe²⁺ with palm pollen suspension as a substrate and reducing agent. The particle size of date palm pollen loaded with cobalt ferrite magnetic nanoparticles (CFMNP-DPP) is around 19 nm, which is in line with the estimated particle size from XRD data using the Scherrer method (20 nm). The magnetic intensity values were measured at 5 and 21 emu/g for cobalt ferrite nanoparticles which contained date palm pollen and those that did not. The coercive field of cobalt ferrite with date palm pollen, however, is higher than it is in the absence of date palm pollen. The antibacterial activity was studied using Escherichia coli, Klebsiella pneumonia, and Staphylococcus aureus bacteria with varying concentrations (10, 20, 30, 40, 50, 60 and 70 ppm). As a result, the zone of inhibition for all bacteria increased with increasing concentrations. The zone of inhibition is 6 - 10 mm for E. coli, 9 - 12 mm for K. pneumonia, and 7 - 11 mm for S. aureus. They concluded that the antibacterial properties of nanoparticles on K. pneumonia and S. aureus were higher than on E. coli. Differences in the antibacterial effects of nanoparticles against bacterial strains can be attributed to the nature of the amino acid residues used in their synthesis and differences in the structural composition of each bacterial culture.

El-khawaga et al. [84] promising photocatalytic and antimicrobial activity of novel capsaicin-coated cobalt ferrite nanocatalyst. Cobalt ferrite nanoparticles were prepared by the co-precipitation method and then surface modified with capsaicin. This material is confirmed to have grain sizes ranging between 15 and 25 nm for cobalt ferrite, while capsaicin-coated cobalt ferrite has a grain size ranging between 25 and 35 nm. Antimicrobial activity was tested on Gram-positive (S. aureus) and Gram-negative (E. coli) by the disk diffusion method. The inhibition zones for cobalt ferrite nanoparticles are 7.5 and 9.7 mm for S. aureus and E. coli. Meanwhile, the capsaicin-coated cobalt ferrite material has a larger inhibition zone, namely 23.5 and 17 mm for S. aureus and E. coli. Nanocomposites are more active against S. aureus than E. coli. This is influenced by the composition of the bacteria themselves; unlike Gram-positive bacteria, which combine a very compact form of peptidoglycan, the cell wall of Gram-negative bacteria consists of a layer of lipids, lipopolysaccharide, and peptidoglycan.

Manjunatha et al. [85] reported the effect of calcination temperature on the structural, antibacterial, radiation-shielding, and magnetic properties of cubic spinel cobalt ferrite. They claim that for the first time, cobalt ferrite nanoparticles were synthesized by the combustion method using BigThyme leaf extract as a reducing agent. The effect of the calcination temperature used was as follows: 200, 400, 600, and 800 °C. As a result, the crystal size increased from 12 - 79 nm with increasing calcination temperature. On the other hand, increasing the calcination temperature shows a soft ferromagnetic behavior with a high M_s value (70.68 emu/g) and high coercive field (2.2 kOe). The antibacterial activity of the synthesized nanoparticles against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) pathogens was determined using the well diffusion method. Cobalt ferrite shows good sensitivity to gram-positive and gram-negative bacteria. However, the as-formed sample has a smaller zone of inhibition compared to calcined cobalt ferrite (800 °C). In S. aureus bacteria, there are inhibition zones of 52 and 38 mm, respectively, for the as-formed and 800 °C samples. Meanwhile, E. coli bacteria have a smaller inhibition zone than S. aureus bacteria at 25 and 19 mm, respectively, for as-formed and 800 °C sample.

Sulaiman et al. [82] reported the antibacterial activity of cobalt ferrite (CoFe₂O₄) nanoparticles against oral Enterococci. Cobalt ferrite is prepared using the sol-gel technique with sintering temperatures of 200, 400, and 600 °C. The crystal size from the calculation results was found to be 28.55 nm. The 600 °C sample has a coercive field value of 1,292.7 Oe and saturation magnetization of 91.6 emu/g. They reported that the magnetic moment of cobalt Co²⁺ induces a large anisotropy that allows the magnetic nanoparticles to enhance biocompatibility and bacterial activity. Antibacterial activity revealed that the inhibition zone for pure cobalt ferrite nanoparticles (at temperatures of 400 and 600 °C) did not appear. However, an inhibition zone appeared when added with chlorhexidine (CHX) at 8.1 and 8.5, respectively, for nanoparticle samples at temperatures of 600 + CHX and 400 + CHX. Increased antimicrobial activity of chlorhexidine due to the increased surface-to-volume ratio of nanoparticles enhances interactions with bacteria. They state that precise control over nanoparticle size, shape, dispersion, and conditions influencing these properties are key requirements where special properties of nanoparticles are required for medical applications.

Suba et al. [83] made a nanocomposite of Apocynaceae leaf waste activated carbon (ALW) in combination with CoFe₂O₄ by an auto-combustion method for antimicrobial activity. The crystallite size obtained was 10 nm. Evaluation of the magnetic properties of ALW/CoFe₂O₄ shows ferromagnetic behavior. The magnetization value obtained was 38.75 emu/g, which is lower than that of pure cobalt ferrite (94 emu/g). Meanwhile, the remanent magnetization and coercive field were obtained at 13.78 emu/g and 784.56 Oe. The antimicrobial activity of ALW/CoFe₂O₄ was tested by the well diffusion method. The synthesized nanocomposites were evaluated for their antimicrobial activity against Staphylococcus aureus, Escherichia coli, and Candida ablicans bacteria. As a result, there was an increase in the zone of inhibition for all bacteria with increasing concentrations (100, 200, 300, 400 and 500 mg/L). In E. coli bacteria, the inhibition zone is 10.5 - 17 mm, while in S. aureus and C. albicans bacteria, the inhibition zone is 11.83 - 17.5 and 11 - 17 mm. According to them, the antimicrobial potential of ALW/CoFe₂O₄ occurs through the electrostatic interaction of metal nanoparticles and microbial surfaces, which produces oxidative stress and DNA damage, which results in disruption of membrane function. Inactivation of bacterial enzymes by Co ions present in nanoclusters causes cell death.

Anusa et al. [84] investigated the structural comparison, optical properties, and antibacterial activity of cobalt ferrite with other materials. These nanoparticles were synthesized using the sol-gel method. The average crystallite size of cobalt ferrite nanoparticles has been calculated to be 35 nm, which is smaller than that of the reference material. Antibacterial activity was tested using E. coli and S. aureus bacteria. The inhibition zone for E. coli bacteria is 14 mm, while for S. aureus bacteria, there is no inhibition zone. Cobalt ferrite shows higher antibacterial activity on gram-negative bacteria than on gram-positive bacteria, which may be related to differences in cell wall structure.

Zachnowicz et al. [89] studied the structural, magnetic, cytotoxic, and antibacterial properties of polyrhodanine cobalt ferrite (PRHD@CoFe₂O₄) hybrid nanomaterials. Polymeric hybrid materials were synthesized via chemical oxidation polymerization. XRD analysis obtained a crystallite size of around 11 nm, which is in accordance with estimates using TEM analysis. Their analysis suggests changing the thickness of the PRHD layer can control the magnetic hardness and saturation magnetization of hybrid materials, which is attractive for biomedical applications. Antibacterial activity was tested using E. coli and S. aureus bacteria through in vitro assessment. Differences in composition ratios did not have a linear change in antibacterial activity. However, all samples showed prospective bactericidal properties against gram-positive and gram-negative bacteria. Polymeric shell thickness affected antimicrobial activity, reaching a critical value above which an increase in the amount of polymer did not influence activity. These results suggest that the investigated hybrid material may be a useful antimicrobial agent utilized for magnetic hyperthermia or magnetic carriers.

Sumathi et al. [85] synthesized NPM-cobalt ferrite using the co-precipitation method with a variety of precipitating agents, namely ammonium hydroxide, sodium hydroxide, and monoethanolamine. It was reported that different types of precipitating agents had an impact on the crystal size and antibacterial activity. The crystal sizes were found to range from 17 to 70 nm, obtained from the precipitating agent sodium hydroxide. Testing the antibacterial activity of the precipitating agent material, sodium hydroxide produced a higher antibacterial ratio (85%) against S. aureus than ammonium hydroxide (70%). The antibacterial mechanism is due to the hydrogen peroxide produced penetrating the cell wall and causing cell death. This penetration rate of hydrogen peroxide also plays a role in antibacterial activity. Thus, the antibacterial ratio depends on the various reagents used in the synthesis of NPM-cobalt ferrite.

Table 1 Percentage of antibacterial activity in NPM-cobalt ferrite.

NPM-Metal-CFO applied for antibacterial

Exploration of NPM-cobalt ferrite is increasingly varied, and the researchers report NPM- cobalt ferrite substituted with other elements (metal-CFO). The metals used are alkaline earth metals, transition metals, rare-earths and noble metals. Substitution can occur with Co (M_xCo_xFe₂O₄) or Fe (M_xCoFe_2−xO₄) depending on the cation number of each element which will affect the structural and magnetic properties of the nanoparticles. Several papers reveal the ZOI for pure CFO no zone [91-94], so that metal substitution in the CFO lattice becomes an interesting study to bring up or increase the bacterial inhibition zone. It is known that the release of metal ions allows to increase antibacterial activity.

Alkaline earth metals

Magnesium

There is very little substitution of cobalt ferrite with alkaline earth metals for antibacterial applications. Aisida et al. [95] synthesized Mg-substituted CFNPs using the sol-gel method.

They investigated the physical properties of antibacterial activity. The characterization results reveal that the crystal size of 40 nm is larger than that of pure cobalt ferrite at 31.3 nm. The particle size obtained is within the scope of the nanoparticle range for biomedical applications (100 nm magnification). By using the bacteria Bacillus subtilis, Klebsiella pneumonia, Escherichia coli, and Staphylococcus epidermidis, they looked at the susceptibility of the samples to pathogenic strains through the inhibition zone. A higher IZ was found in E. coli bacteria (16 mm) compared to the positive control. They assume this is because gram-negative and gram-positive organisms have varying cell wall and membrane processes. CFNPs-substituted Mg samples were considered to have more potential as antibacterial agents compared to pure cobalt ferrite and cobalt ferrite-substituted Mn, which they also tested.

In another study, Mg metal was also substituted into cobalt ferrite to see its biomedical applications [96]. These nanoparticles are reported to have a date phase structure with a crystal size of 30.56 nm, smaller than pure cobalt ferrite of 48.08 nm. Antibacterial activity was tested using gram negative bacteria (P. aeruginosa and E. coli) and gram positive (S. aureus) with an incubation mass of 24 h. They reported that Mg substituted into cobalt ferrite showed responses against all pathogens tested. The results are not much different from pure cobalt ferrite (4.5 - 5.6 mm) and slightly lower (0 - 6.5 mm) than the co-doping also studied in their report. Mg-CFO has a minimum effect on E. coli bacteria.

Table 2 Alkaline earth metals-CFO for antibacterial.

Transition metals

Titanium

Khalid et al. [97] have investigated titanium doped cobalt ferrite nanoparticles via the modified sol-gel method. Samples were prepared with the formula CoTi_xFe_2−xO4 where x = 0, 0.2, 0.4 and 0.6. They also produced CoTi_0.2Fe_1.8O₄/GO via a facile ultrasonication method. Antibacterial potential against P. aeruginosa and S. aureus was estimated from ZOI. The maximum ZOI was reported to be 52.3 mm in P. aeruginosa bacteria for nanocomposite material while the minimum ZOI of 12.4 mm was found in S. aureus bacteria for CoTi_0.6Fe_1.4O₄ nanoparticles. Titanium concentration has an inverse relationship with its antibacterial properties and is better against gram negative bacteria. This potential depends on the smaller size of the nanoparticles and the susceptibility due to the cell wall. In gram-negative bacteria, the peptidoglycan sheet is thinner which facilitates the diffusion of nanoparticles.

Chromium

Impact of Cr doping on the structural, surface morphology, magnetic properties and antibacterial activity of spinel nanoferrite (CoFe₂O₄) via sol-gel auto-combustion technique (SGACT) [97]. Samples are prepared with the formula Cr_xCo_1−xFe₂O₄ where x = 0, 0.025, 0.05, 0.1, 0.15 and 0.20. Nanoparticles have a single phase for samples up to x = 0.05, while for x = 0.10 - 0.20 several additional phases of Fe₂O₃ are visible. They stated that with increasing Cr concentration the crystal size has an anomalous trend (range 40.49 - 44.26) which may be caused by the replacement of the smaller Cr₂₊ ionic radius in place of Co₂₊. Magnetic properties measurements reveal that the saturation magnetization decreases with increasing Cr ion concentration (0, 0.1 and 0.2). Antibacterial activity was tested using gram-positive (S. aureus) and gram-negative (E. coli) bacteria. In E. coli bacteria the maximum ZOI (17 mm) was found when doping Cr = 0.2 and the minimum (14.5 mm) when doping Cr x 0.05. Meanwhile, the S. aureus bacteria doping Cr x = 0.025 and 0.1 did not show an inhibition zone.

Manganese

Sanpo et al. [99] reported a 50% metal substitution synthesis on NPM-cobalt ferrites by the sol-gel synthesis method route using citric acid as a chelating. The composition was (a) CoFe₂O₄, (b) Co_0.5Cu_0.5Fe₂O₄, (c) Co_0.5Zn_0.5Fe₂O₄, (d) Co_0.5Mn_0.5Fe₂O₄ and (e) Co_0.5Ni_0.5Fe₂O₄. From the XRD results it is concluded that transition metals were completely substituted into the cobalt ferrite lattice. The crystallite size obtained are 42.97 ± 0.5, 40.14 ± 0.4, 44.18 ± 0.6, 40.26 ± 0.2 and 41.01 ± 0.5 nm, respectively. In the fundamental study, the XRD pattern results of NPM-cobalt ferrite substituted by transition metals are shown in Figure 3(a). Furthermore, typical NPM-cobalt ferrite substituted with transition metal ions (Zn) is visualized using Vesta software (as shown in Figure 3(b)), which indicates that the cubic structure retains its crystal structure with the inclusion of Zn replacing Co in the cobalt ferrite structure. The antibacterial activity was obtained from (a) to (e) with the ZOI values respectively 42.97, 40.14, 44.18, 40.26 and 40.01 mm. The antibacterial properties of metal-cobalt ferrites are b > c > e > a > d. Therefore, it was concluded that based on the visualization of ZOI, the highest antibacterial property was (b) Cu-cobalt ferrite. In addition to the ZOI, typical SEM images representing the attached S. aureus and E. coli on synthesized transition metal-substituted cobalt ferrite nanoparticles are shown which look damaged by many dead bacteria.

Aisida et al. [95] have investigated the physical properties of cobalt ferrite nanoparticles doped with manganese for antibacterial activity. The crystal size of 23.7 nm is smaller than cobalt ferrite (31.3 nm) and other metal doping (40.9 nm). The prepared material showed the presence of an inhibitory zone which was tested on the 4 bacteria Bacillus subtilis, Klebsiella pneumonia, Escherichia coli, Staphylococcus epidermis. The highest zone of inhibition was found in Klebsiella penumoniae bacteria at 18 mm, followed by Bascillus subtilis (15 mm), E. coli (11 mm) and Staphylococcus epidermis (10 mm).

Figure 3 (a) XRD pattern of metal transitions substituted NPM-cobalt ferrite and (b) Vesta visualization of Zn-CoFe₂O₄.

Iron

Zelnaravicius et al. [100] have investigated the substitution of Fe (II) into cobalt ferrite ferrite (Co_xFe_1−xFe₂O₄, x = 0, 0.5 and 0.8) using the coprecipitation method. The cyst size varies from 3.1 - 3.8 nm. The smaller Fe doping concentration resulted in an increase in the antimicrobial potential to 93.1% - 86.3% for eukaryotic strains and to 96.4% - 42.7% for prokaryotic strains. This shows that pure cobalt ferrite has a higher antibacterial potential compared to the addition of Fe doping. This effect can be attributed to the determined higher positive Zeta potential values of pure cobalt ferrite than Fe(II)-doped ones in neutral and slightly acidic environments.

Nickel

Naik et al. [101] reported the multifunctional properties (one of which is antibacterial) of microwave-assisted bioengineered nickel-doped cobalt ferrite nanoparticles. Synthesis of nickel-doped cobalt ferrite nanoparticles using an extract from the Andrographis paniculata plant. The size of the crystallite decreases (From 32 to 24 nm) with increasing concentration of nickel. Its antibacterial activity was tested against gram-positive (Staphylococcus aureus and Enterococcus faecalis) and gram-negative (Pseudomonas aeruginosa and Samonella typhi) bacteria using the agar-well diffusion technique. The nanoparticle concentration was varied from 25 to 100 µg/mL. The report’s findings demonstrated the antibacterial activity of nickel-cobalt ferrite nanoparticles against all strains and concentrations of bacteria. Both gram-positive and gram-negative bacteria have an expanded inhibitory zone in response to concentration increases. They discovered that the generation of reactive oxygen species (ROS) and the release of heavy metal ions are the primary factors influencing the produced nanomaterials’ antibacterial activity effectiveness.

Sanpo et al. [99] have investigated the substitution of transition metals Cu, Zn, Mn and Ni in NPM-CFO synthesized via the sol gel route. Nickel doping is done with the composition Co_0.5Ni_0.5Fe₂O₄ producing a crystal size of 40.01 nm, which is the smallest size among other transition metal doping. All transition metal doping on CFO showed antibacterial activity on E. coli and S. aureus bacteria, where the ZOI on E. coli was greater than on S. aureus. Cu and Mn metal doping on NPM-CFO showed better antibacterial properties.

Kiani et al. [96] produced metal-doped cobalt ferrite via the sol-gel technique for biological applications. XRD examination verified the structure of the nanoparticles is single-phase; this is also validated by EDX. There is a decrease in the lattice constant with increasing Ni content caused by the smaller Ni ion radius (0.63 Å) replacing the cobalt ion site (0.78 Å). The average crystallite size decreases from 48.08 (undoped sample) to 29.47 nm with increasing Ni content. FTIR analysis results reveal the presence of 2 typical peaks, which are spinel phases, in all samples. Meanwhile, the particle shape is generally spherical (cubic) with some agglomeration between the particles. The average particle size was found to be 29 - 49 nm, which is close to the XRD calculation. The antibacterial properties of nickel cobalt ferrite were assessed using the disc diffusion method for common pathogens, namely Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria. They revealed that nickel cobalt ferrite nanoparticles were active against S. aureus and P. aeruginosa bacteria with ZOI in the range of 4.5 - 5.6 mm. Meanwhile, no response was seen against E. coli bacteria.

Zinc

Zinc-substituted cobalt ferrite (Zn-CFO) nanoparticles with the formula Co_1-xZn_xFe₂O₄ with (x = 0, 0.3, 0.5, 0.7 and 1) have been synthesized via the sol-gel method by Sanpo et al. ⁹⁸. It is reported that the crystal size of Zn-CFO ranges from (43 - 45) nm. Increasing the Zn concentration in cobalt ferrite significantly increased the antibacterial activity against E. coli and S. aureus. Meanwhile, the antibacterial activity against E. coli is higher than S. aureus. The possible mechanism for antibacterial activity is based on the main chemical species. Active oxide easily penetrates bacterial cell walls and causes cell damage, so the penetration rate of active oxide plays an important role in the rate of killing bacteria. Therefore, the antibacterial activity against E. coli and S. aureus obtained different results because the chemical composition of the cell walls is very different. In LB-agar plate assays, Zn-CFO nanopowders showed surface-dependent antibacterial activity.

Maksoud et al. [103] conducted antibacterial studies with Zn-CFO nanoparticles prepared using the sol gel method. The crystal size was found to be 12.86 nm. Several pathogenic microbes were used to test antibacterial activity with maximum growth inhibition results against K. pneumoniae bacteria (ZOI 28.00 mm). The inhibition zone for other pathogenic microbe ranges from (18 - 27) mm.

Another antibacterial study with NPM Zn-CFO was also carried out by Naik et al. with varying concentrations (x = 0, 0.2, 0.4 and 0.6) via the combustion method [99]. The crystal size decreased from 21 to 12 nm as the Zn concentration increased. Antibacterial activity was tested on gram-positive (S. aureus) and gram-negative (S. typhi) bacteria. The antibacterial activity on S. typhi from Zn-CFO showed a high ZOI of 22 mm compared to pure CFO, namely 16 mm. In addition, increasing the mass concentration of nanoparticles (CFO and Zn-CFO) from 25 - 100 µg/mL was able to increase the ZOI value in both bacteria.

Iqbal et al. [105] reported the synthesis results of Zn-CFO with the composition Zn_0.5Co_0.5Fe₂O₄ which was prepared using the sol gel method. The average crystal size is 61.2 nm. NPM Zn-CFO has been proven to be able to inhibit the growth of E. coli and MRSA (Methicillin-resistant Staphylococcus aureus) bacteria. The zone of inhibition for MRSA (80%) is greater than for E. coli bacteria (69%). Meanwhile, increasing the incubation time (2, 4, 8, 21 and 24 h) resulted in a decrease in the ZOI growth of both bacteria.

Maksoud et al. [106] compared Zn, Cu, and Mn doping in cobalt ferrite (M_xCo_1−xFe₂O₄, x = 0, 0.25, 0.5, 0.75) for antibacterial applications prepared via the sol gel method. Zn-CFO nanoparticles have a smaller crystal size, namely from 12.04 - 11.73 nm compared to other doping. Meanwhile, Zn_0.75Co_0.25Fe₂O₄ has an M_s value of 68.42 emu/g (higher than other doping). Magnetic parameters are influenced by several factors such as homogeneity, porosity, density, morphology and arrangement of cations at lattice locations. The antibacterial test results showed that the composition Zn_0.75Co_0.25Fe₂O₄ had the best antibacterial activity in terms of the ZOI value against several bacteria tested. The largest ZOI was found in S. aureus bacteria (15.0 mm), followed by E. columbae bacteria (13.0 mm), and A. viridians (12.0 mm). The activity of Zn_0.75Co_0.25Fe₂O₄ may be related to the Debye temperature and rigidity of ferrite nanoparticles which increase antibacterial activity because bacteria diffuse into the holes of the synthesized ferrite nanoparticles.

Table 3 NPM-cobalt ferrite doped transition metals for antibacterial.

Post-transition metals

Stannum (Tin)

Tin-doped cobalt ferrite nanoparticles were produced by Sobana et al. using the co-precipitation approach in order to study their magnetic and antibacterial characteristics [109]. An extract from the Alfafa plant was used in this synthesis (Lusan grass in north India). Consequently, the average crystallite size decreased from 39 to 22 nm with increasing Sn concentration (0, 0.2, 0.4, 0.6, 0.8 and 1.0). The production of an inverted spinel ferrite structure was also confirmed by Raman and FTIR. According to SEM and AFM studies, the surface had a uniform size distribution and was free of valleys. As the Sn concentration increased, the coercivity dropped from 960 to 470 G. They asserted that porosity, size distribution, magnetic particle shape, microstrain, magnetocrystallinity, and magnetic domain size are the primary parameters influencing coercivity. The antibacterial activity of undoped and Sn-doped cobalt ferrite was reviewed using Escherichia coli and Bacillus subtilis bacteria by the disc diffusion agar method. Compared to undoped cobalt ferrite, the zone of inhibition of Sn-doped cobalt ferrite was larger. They came to the conclusion that one of the main elements affecting antibacterial activity is particle size.

Bismuth

Kirankumar and Sumathi [107] used Bi ion doping and co-doping to investigate antibacterial activity. The nanoparticles were prepared using a combustion technique. The antibacterial test results showed that Bi_0.1CoFe_1.9O₄ nanoparticles were able to produce a greater ZOI than CFO, with a large ZOI for S. aureus bacteria (20 mm) and the ZOI for E. coli being 23 mm. Apart from that, they also added Cu ions to bismuth cobalt ferrite to form Cu_0.5Co_0.5Fe_1.9Bi_0.1O₄ which could increase the ZOI of 29 mm and 34 mm for the 2 bacteria tested. This confirms that metal substitution in CFO further increases antibacterial activity.

Another report regarding the antibacterial activity of Bi-CFO material was reported by Kalia et al. [108]. Cobalt ferrite was co-doped with bismuth and other metals of different concentrations via sol-gel combustion method. The crystallite sizes of these materials were ~54 nm for undoped and ~38 nm for doped Bi. Antibacterial activity was tested against bacillus and coccus bacteria using the disk diffusion method. There is an inhibition zone for both bacteria which shows antibacterial effectiveness. The Bi doped sample has an inhibition zone of 9 and 9.5 mm which is higher than the undoped sample.

Table 4 NPM-cobalt ferrite doped post-transition metal for antibacterial.

Rare-earth metals

Cerium

Ce substitution affects the structural, morphological, magnetic and antibacterial activity of cobalt ferrite nanoparticles prepared by the simple sol-gel combustion technique [110]. CoCe_xFe_2−xO₄ nanoparticles (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) have a nano-sized single-phase cubic structure. The crystal size was reported to decrease from 25.36 to 16.53 nm with increasing cerium content. Ce substitution reduces the saturation magnetization of cobalt ferrite due to the substitution of Ce³⁺ ion into the Fe³⁺ ion site in site B. Meanwhile, the antibacterial activity is more dominantly influenced by the higher Ce content. They claim that the important role in antibacterial activity is the particle size and surface area of the sample.

Neodymium & gadolinium

Rehman et al. [111] have synthesized CoNd_xFe2_-xO₄ (x = 0.0, 0.1, 0.15 and 0.2) using sonochemical method. The crystal size obtained was 9 - 16 nm. Antibacterial activity was tested on S. aureus, E. coli and C. albicans bacteria. The Nd-CFO material showed higher antibacterial activity. Associated with particle size, it shows that increasing the surface to volume ratio of nanoparticles results in better antibacterial interactions.

Velho-Pereira et al. [111] compared doping (Bi, Gd, Nd) in cobalt ferrite CFO synthesized by the sol gel autocombustion method. Nd-CFO has the smallest crystal size, namely 4 nm compared to other doping. The antibacterial activity test with an incubation period of 24 h revealed that Nd-CFO had the highest antibacterial potential (83.73%) for E. coli (gram negative) bacteria. Meanwhile, the antibacterial activity for the gram-positive bacteria S. epidermidis was also high at 84.88% (lower than Bi doping 89.53%). These results show the ability of Nd-CFO to fight gram-positive and gram-negative bacteria because the 2 cell walls have different compositions. Gram-positive cell walls contain teichoic acid and murein, whereas gram-negative cell walls have lipid A, an LPS layer and a lower proportion of murein.

Table 5 NPM-cobalt ferrite doped rare-earth metals for antibacterial.

Noble metals

Copper

Sanpo et al. [113] substituted Cu in NPM-CFO via the sol-gel route, Cu-CoFe₂O₄ with various concentrations, namely 0, 0.3, 0.5, 0.7, and 1. The addition of Cu⁺ ions reduced the average particle size from 41.52 to 37.54 nm. The lattice parameters also decrease with increasing Cu concentration, presumably because the ionic radius of Cu²⁺ is smaller than Co²⁺. The antibacterial test results showed that Cu-CoFe₂O₄ was able to inhibit the growth of E. coli and S. aureus bacteria. Increasing the concentration of Cu-CoFe₂O₄ increases the antibacterial effect on S. aureus while on E. coli bacteria it is consistent at around 45%. It was concluded that there are several factors that influence the level of bacterial killing such as crystal size, namely the surface to volume ratio, structure and chemical composition of the cell wall.

Prabagar et al. [114] investigated the effect of metal substitution (Cu, Zn, and Ag) in CFO. Synthesis was carried out using the sol gel auto combustion method with the composition Co_0.8Cu_0.2Fe₂O₄. The synthesis results show that the average crystal size of Cu-CFO is 18.89 nm, larger than pure CFO (13.50 nm). Magnetic studies show that the Ms, Mr and Hc values for Cu-CFO are smaller than for pure CFO. The antimicrobial activity of Cu-CFO is most effective compared to pure CFO and other doping, because it has higher antimicrobial activity against selected pathogenic microbes. On the other hand, increasing the concentration of nanoparticles can increase the ZOI value of all doping ions.

Argentum (Silver)

Satheskumar et al. [115] carried out Ag⁺ NPM-CFO ion doping with the chemical formula Ag_xCo_1-xFe₂O₄ (x = 0 and 0.2), which was synthesized using honey assisted combustion. The secondary phase was found in Ag-CoFe₂O₄ nanoparticles which was caused by changes in the Fe³⁺ and O^2– environment in the spinel structure. The particle size of Ag-CoFe₂O₄ is 28.4 nm compared to CoFe₂O₄, which is 25.2 nm. The Ag⁺ substitution also changes the magnetic properties of NPM-cobalt ferrites from the ferromagnetic order to the superparamagnetic order. This is thought to be caused by changes in particle size or anisotropic magnetocrystalline properties. The antibacterial activity of Ag-CoFe₂O₄ was better than CoFe₂O₄ against E. coli and C. ablicans. This better antibacterial activity is thought to be caused by the addition of Ag⁺ ions which strengthen the process of cell distortion and death of bacterial species.

Mahajan et al. [116] substituted Ag+ into NPM-CFO with the composition Ag_xCo_1-xFe₂O₄ (x = 0, 0.05, 0.01 and 0.02) using the sol gel auto-combustion method via chemical (citric acid) and green synthesis using tulsi extract seeds (Ocimum sactum) and garlic cloves (Allium sativum). These nanoparticles have a cubic structure (inverse spinel) with crystal sizes varying from 18 - 45 nm for different synthesis media. The magnetic characteristics of the Ms value were reported to be different, which was chemically mediated (49.95 emu/g), which used tulsi seeds extract (49.72 emu/g) and garlic cloves (28.89 emu/g). The Ag-CFO material was reported to be more effective as an antibacterial for gram-positive bacteria (L. manocytagenes) than for gram-negative bacteria (E. coli). The presence of Ag⁺ ions in NPM-CFO causes an increase in ZOI because it is thought that Ag⁺ ions induce the rate of oxygen oxidation on the surface of the species which will cause bacterial death.

In another study, Using the sol-gel process, Riyatun et al. [117] examined the actibacteria, magnetic properties, and microstructural features of silver-substituted cobalt ferrite nanoparticles. Ag concentrations of 0, 0.01, 0.02, 0.03, and 0.1 were employed. A concentration increase results in a 14 - 20.8 nm crystallite size increase. Ag⁺ concentrations less than 0.03 suggest the absence of impurities or second phases, but Ag₃O₄ and Fe₂O₃ are the forms of other phases that emerge at doping levels of 0.1. When non-magnetic Ag ions are substituted, the magnetic characteristics alter. The primary reason for the reduction in the coercive field is the substitution of non-magnetic Ag⁺ for magnetic Co²⁺. According to the findings of the study on antibacterial activity, every sample exhibited antibacterial activity against both gram-positive S. aureus and gram-negative E. coli bacteria. High Ag⁺ doping (10%) indicates weaker antibacterial ability. The best antibacterial activity was found at 3% Ag doping, with an inhibition zone of 10.59 and 10.55 mm for S. aureus and E. coli bacteria, respectively.

Next, the structural, magnetic, and antibacterial characteristics of silver-substituted cobalt ferrite nanoparticles produced via the co-precipitation technique were found to be temperature dependent upon annealing by Riyatun et al. [113]. The physical properties were altered by applying an annealing temperature treatment of 200, 300, 400, and 500 °C. The annealing temperature increased, the crystallite size grew from 19.78 to 24.11 nm, and the grain size increased from 54.75 to 61.39 nm, according to XRD examination. Due to an increase in the magnetocrystalline anisotropy constant, an elevated annealing temperature results in an increase in the saturation magnetization and coercive field, which rise from 31.80 to 50.60 emu/g and 651 to 1.077 Oe, respectively. Using the well diffusion method, the antibacterial characteristics have been investigated with S. aureus and E. coli. The most notable for Ag_0.02Co_0.98Fe₂O₄ annealed at 200 °C with the minor grain size materials on S. aureus and E. coli were 12.73 mm (mortality of 88%) and 12.43 mm (mortality of 80%), respectively.

Riyatun et al. [119] also investigated nanoparticle preparation procedures to tailor the physical, antibacterial, and photocatalytic properties of silver-substituted cobalt ferrite. Co-precipitation and sol-gel techniques were used to create this material, with the synthesis temperature being changed to 70, 80, and 90 °C. Using both the sol-gel and co-precipitation techniques, it was discovered that the crystal size increased with increasing synthesis temperature. The crystal size of the co-precipitation technique is lower, ranging from 16.71 to 19.89 nm. The magnetic properties of silver cobalt ferrite also change with different preparation procedures. Compared to the co-precipitation approach, the coercive field of samples prepared by the sol-gel method has a higher value. Conversely, fluctuations in synthesis temperature and the saturation magnetization of samples prepared by the sol-gel technique exhibit a random magnitude, making the trend unpredictable. As the synthesis temperature rises, samples undergo an increase in the saturation magnetization value, in contrast to the co-precipitation approach. Using both the sol gel and co-precipitation procedures, the examination of antibacterial efficacy against gram-positive bacteria S. aureus and gram-negative bacteria E. coli revealed an increase in the inhibitory zone with increasing synthesis temperature. They came to the conclusion that antibacterial activity improved with increasing synthesis temperature. The sample at a synthesis temperature of 90 °C has the least specific surface area, however for antibacterial purposes, the material with the largest surface area to volume ratio is needed. The fact that the saturation magnetization of the nanoparticle sample increases as the synthesis temperature increases supports the idea that there is another mechanism - magnetic interaction - that leads to this.

Kalia et al. [108] researched the antibacterial activity of Ag-doped cobalt ferrite material and several other doped metals. When compared to the undoped sample (~54 nm), the crystal size of Ag-doped cobalt ferrite nanoparticles is reduced (~47 nm). Antibacterial activity was studied using the disc diffusion method against Bacillus and Coccus bacteria. For the examined microorganisms, the Ag-doped cobalt ferrite sample's zone of inhibition (ZOI) measures 8.5 and 9 mm, respectively. It was found that these outcomes were higher than those of undoped samples.

Table 6 NPM-cobalt ferrite doped noble metals for antibacterial.