Spectroscopic and antibacterial activities of cobalt and nickel nanoparticles: a comparative analysis

Abstract

This study aimed to compare the spectroscopy, morphological, electrocatalytic properties, and antibacterial activities of cobalt nanoparticles (CoNPs) with nickel nanoparticles (NiNPs). Cobalt nanoparticles and NiNPs were prepared via a chemical reduction approach and characterized utilizing transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD) techniques. The result from XRD and TEM analysis revealed that the synthesized nanoparticles exhibit face-centered cubic with smooth spherical shape, having average particles size of 12 nm (NiNPs) and 18 nm (CoNPs). The electrochemical properties of the nanoparticles were examined via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. The CV results showed that GCE-Ni (35.6 μA) has a higher current response compared to GCE-Co (10.5 μA). The EIS analysis revealed that GCE-Ni (1.39 KΩ) has faster electron transport capability compared to GCE-Co (2.99 KΩ) as indicated in their R_ct values. The power density of the synthesized nanoparticles was obtained from their "knee" frequency (f°) values, with GCE-Ni (3.16 Hz) having higher f° values compared to GCE-Co (2.00 Hz). The antibacterial activity of the nanoparticles was evaluated against multidrug-resistant Escherichia coli O157, Escherichia coli O177, Salmonella enterica, Staphylococcus aureus, and Vibrio cholerae. The result from the antibacterial study revealed that at low concentrations both CoNPs and NiNPs have significant antibacterial activities against E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae. NiNPs showed better antibacterial activities at low concentrations of 61.5, 61.5, 125, 61.5, and 125 µg/mL compared to CoNPs with minimum inhibitory concentrations of 125, 125, 250, 61.5, and 125 µg/mL against E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae, respectively. These promising antibacterial activities emphasize the potential of CoNPs and NiNPs as effective antibacterial agents, which could aid in the development of novel antibacterial medicines.

Introduction

The high mortality and morbidity resulting from disease outbreak cases worldwide is associated with the increasing incidence of antibiotic resistance among pathogens (Hofer 2019). As a result, the quest to synthesize metal nanoparticles as an alternative approach to combat pathogens becomes necessary. Metal-based nanoparticles with novel antibacterial properties are potentially promising areas for overcoming the limitations of antibiotics and resistant spread (Breijyeh et al. 2020; Abolarinwa et al. 2022; Ajose et al. 2022). Nanoparticles and nanomaterials are gaining prominence in the field of medicine owing to their diminutive size and large surface area-to-volume ratio (Rashidi Ranjbar et al. 2020; Saidin et al. 2021). Metal nanoparticles have drawn increasing attention for their possible antibacterial activities owing to their distinct physicochemical properties (Abolarinwa et al. 2022). Transition metal nanoparticles, for example, have been widely researched for their prospective applications in electronics, catalysis, and biomedical sciences (Dutta et al. 2020). Nickel nanoparticles and CoNPs have gained special interest due to their remarkable catalytic and magnetic capabilities (Tian et al. 2016).

Synthesis methods are critical in identifying nanoparticle physicochemical characteristics. Nanoparticle characterization is crucial in comprehending their chemical and physical properties, which can influence their biological activities. Cobalt nanoparticles are renowned for their high melting point, oxidation resistance, and magnetic properties. They are frequently utilized in energy storage devices, biological imaging, catalysis, and magnetic data storage, such as hard disk drives (Barakat et al. 2009). On the other hand, NiNPs possess high conductivity with outstanding mechanical properties, making them suitable for catalytic, sensor, and electronic applications. They are also used in magnetic data storage and manufacture of rechargeable batteries, such as nickel–cadmium and nickel-metal hydride batteries (Calandra 2009). In biomedical applications, both CoNPs and NiNPs have shown potential for use in drug delivery, imaging, and cancer therapy (Gupta et al. 2007).

Cobalt nanoparticles and NiNPs have been found to possess antibacterial properties, which make them attractive for use in medical and industrial applications. Research studies have shown that these nanoparticles can effectively inhibit the growth and proliferation of bacteria of several types, encompassing Gram-positive and Gram-negative ones (Fazal et al. 2022; Harish et al. 2022; Sarian et al. 2022; Hassan Afandy et al. 2023). The antibacterial properties of these nanoparticles are thought to be due to their ability to generate reactive oxygen species (ROS) that can damage bacterial cell membranes and inhibit bacterial growth (Saravanan et al. 2021). Additionally, the small size of the nanoparticles allows them to penetrate bacterial cells more easily, leading to increased antibacterial activity (Sun et al. 2021). Furthermore, nanoparticles with smaller particle sizes have shown good antibacterial activity (Azam et al. 2012). The investigation of the antibacterial efficacy of nanoparticles was extensively researched using human pathogenic microorganisms such as E. coli (Al-Nabulsi et al. 2020; Xiao et al. 2021) and S. aureus (Attallah et al. 2022; Shaaban et al. 2023). Moreover, these microbes seem highly sensitive to transition metal nanoparticles (Yang et al. 2021; Jeevanandam et al. 2022).

Several metal oxide nanoparticles have been reported for their potential antibacterial activities against Gram-negative and Gram-positive bacteria, consisting of Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. These metal oxide nanoparticles include Al₂O₃ (Jawad et al. 2021; Karimi et al. 2021), CuO, Cu₂O, ZnO (Asamoah et al. 2020; Karuppannan et al. 2021; Han et al. 2022; Khosravi et al. 2022), CaO (Abbas and Aadim 2022), MnO₂, MgO (Ogunyemi et al. 2020; Amrulloh et al. 2021), TiO₂ (Wang et al. 2020; Zhang et al. 2021), Fe₃O₄ (Azizabadi et al. 2021; Kamali et al. 2022), Ag₂O (Rashidi Ranjbar et al. 2020; Dharmaraj et al. 2021; Mahlambi and Moloto 2022), NiO (Kannan et al. 2020; Christy et al. 2021), and SiO₂ (Attallah et al. 2022; Tran et al. 2023). The antibacterial activity lies in its physical and chemical properties, which include morphology, chemical composition, particle size, surface characteristics, solubility, crystallinity, aggregation, and crystal phase. The activity is also affected by the experimental conditions, nanoparticle concentration as well as type and number of bacteria. The pores within the bacterial external cellular membranes exhibit dimensions on the nanometer scale, while the size of bacterial cells is in the micrometer range. Since nanoparticles can be smaller than bacterial pores, they can permeate the cell membrane in a novel way (Abolarinwa et al. 2022). Preparing metal nanoparticles that are stable enough to effectively inhibit bacterial growth while in a nutrient medium is a serious issue.

Going through the literature, minimal information is available on the antibacterial properties of CoNPs and NiNPs. Realizing the potential antibacterial applications of these transition metal nanoparticles, we thereby synthesized CoNPs and NiNPs using a chemical reduction approach, examined their electrochemical properties via CV and EIS techniques, and then verified their antibacterial activities against Escherichia coli O157, Escherichia coli O177, Salmonella enterica, Staphylococcus aureus, and Vibrio cholerae. Additionally, the antibacterial activities of these metal nanoparticles were compared, as these properties are essential for their prospective usage in a variety of biomedical applications.

Materials, reagents, and instruments

Cobalt acetate tetrahydrate, potassium hexacyanoferrate IV (K₄(Fe(CN)₆), sodium borohydride, N,N-dimethyl formamide (DMF), ethylene glycol, sodium dihydrogen phosphate (NaH₂PO₄) potassium hexacyanoferrate III (K₃(Fe(CN)₆), disodium hydrogen phosphate (Na₂HPO₄) were procured from Sigma-Aldrich, a reputable supplier based in Darmstadt, Germany. Ethanol, hydrochloric acid (HCl), hydrazine hydrate, sulfuric acid, sodium hydroxide (NaOH), and nickel chloride were sourced from Glassworld Chemicals, a company based in Johannesburg, South Africa. Every reagent was of high analytical grade.

Various techniques were employed to analyze the morphology and structure of the nanoparticles that were produced. Energy diffraction X-ray (EDX) analysis was performed utilizing a JEOL JSM-6610 LV equipment from Dearborn, USA. The UV–visible spectrophotometry analysis was conducted with a Cary 300 series UV–vis spectrometer sourced from Agilent Technology, based in Darmstadt, Germany. X-ray diffraction (XRD) analysis was performed utilizing instrumentation provided by Bruker Company, headquartered in Karlsruhe, Germany. The transmission electron microscopy (TEM) analysis was conducted employing a JEOL2100 instrument that was equipped with a LaB 6 electron gun manufactured by JEOL Ltd, based in Tokyo, Japan.

An electrochemical investigation involving cyclic voltammetry (CV) and impedance spectroscopy (EIS) was performed using an Autolab Potentiostat PGSTAT 302, manufactured by Eco Chemie in Utrecht, the Netherlands. The experimental setup was controlled by GPES software (version 4.9) and NOVA 2.1.3 software with FRA 32 module, which allowed for a frequency range spanning from 100 kHz to 0.1 Hz. Distilled water was used while carrying out all the experiments.

Cobalt and nickel nanoparticles synthesis

Cobalt nanoparticles were chemically synthesized via the dissolution of 2.0 g of cobalt acetate tetrahydrate in a solution comprising 2 mL of water and 8 mL of ethanol, followed by a 15-min stirring period. Next, under continuous agitation, a 5.0 mL volume of a sodium borohydride solution with a concentration of 1.08 M was gradually introduced into the amalgamation at a rate of 2 drops per second. This process continued until the formation of a greyish-black precipitate occurred. The greyish-black particles were gently collected using a magnet from the mixture and subjected to multiple washes with distilled water and ethanol. Subsequently, the particles were dried at room temperature for 24 h (Cruz et al. 2019a).

The production of NiNPs was accomplished by introducing 8 mL of a 1 M sodium hydroxide (NaOH) solution into an 800 mL ethylene glycol comprising a combination of 2 g nickel chloride and 10 mL hydrazine hydrate. The reaction mixture within a sealed container underwent continuous agitation for a duration of 45 min at a temperature of 70 °C. The black particles obtained were subjected to a meticulous cleaning process using ethanol (100%). Subsequently, the particles were sealed under vacuum conditions at a temperature of 27 °C for a duration of 24 h (Wu et al. 2012).

Glassy carbon electrode (GCE) modification procedure

The bare GCE undergoes a preliminary electrode treatment before being modified with the prepared nanoparticles. This was done by rubbing its surface on a Sic Emery pad soaked with aluminum oxide paste. Afterward, enough distilled water was used to rinse the GCE to eliminate any leftover aluminum oxide particles. To achieve thorough cleaning, the GCE was ultrasonicated for about 5 min in a container containing distilled water, followed by another 5 min in methanol. Each of the nanoparticle pastes was applied to the cleaned GCE surface via a drop-casting method. The cleaned GCE was modified with about 5 µL of the prepared CoNPs and NiNPs using drop-dry method, for about 5 min at 50 °C in the oven. The coated electrodes were designated as GCE-Co and GCE-Ni.

Antimicrobial assay of the nickel and cobalt nanoparticles

The bacterial isolates

Five pathogenic bacteria (Escherichia coli O157, Escherichia coli O177, Salmonella enterica, Staphylococcus aureus, and Vibrio cholerae) were obtained from the Antimicrobial Resistance and Phage Biocontrol Laboratory, Department of Microbiology, North-West University (Mafikeng Campus), South Africa. The bacteria had been confirmed to be multi-drug resistant, and details on the bacteria source, virulence, and multi-drug resistance profiles are described in Table 1.

Table 1 Virulence and antibiotics resistance profile of the bacteria sample

Preparation of bacteria culture

Before NPs antimicrobial test, the isolates were revived by cultured in nutrient broth and subjected to incubation at a temperature of 37 °C for a duration of 24 h. The absorbance of the bacterial cultures was standardized using a spectrophotometer (MB-580 model, Shenzhen Huisong Technology Development Co., Ltd., Shenzhen, China) to achieve a standardized value of 0.5 on the MacFarland Standard scale.

Nanoparticles preparation for antibacterial test

Before the NPs antimicrobial test, the NPs were heated for aim of sterilization in an oven for 2 h. Thereafter, NPs suspension (1000 µg/mL) was prepared by adding 1000 µg of each NPs to 1 mL of an organic solvent, dimethyl sulfoxide (DMSO). The mixture was sonicated for few minutes to form a solution.

Antibacterial activity of nickel and cobalt nanoparticles

The bacteria's susceptibility to the NPs was examined via disc diffusion technique. First, freshly prepared Mueller–Hinton Agar (MHA) was inoculated with a standardized bacteria culture. Thereafter, sterilized paper disc soaked in NPs solution was placed aseptically on inoculated MHA and incubated at a temperature of 37 °C for a duration of 24 h. The paper disc containing DMSO was adopted as control. The zones of inhibition were observed and measured in millimeter. The experiments were performed in triplicate.

Minimum inhibitory concentration of nickel and cobalt nanoparticles

The minimum inhibitory concentration (MIC) of the NPs was evaluated utilizing 96-microplate broth dilution method. Firstly, all the microtiter plate wells marked for the experiments were filled with 100 µL of sterile double-strength nutrient broth. Exactly, 100 µL of the NPs (1000 µg/mL) solution was added to the first well and dilution was carried out to obtain various concentrations (31.25, 62.5, 125, 250, and 500 µg /mL) of the nanoparticles. Thereafter, a 10 µL of the standardized bacteria was aseptically dispensed to each well and incubated at 37 °C. Immediately (at 0 h) and after 24 h, the optical density (OD) of the suspension in the microtiter plates was measured using a microplate reader at 630 nm. The DMSO was used as solvent control. The experiments were performed in triplicate. The MIC of NPs was taken as the lowest concentration of the NPs where its OD at 24 h was equal to OD at 0 h.

Minimum bactericidal concentration of nickel and cobalt nanoparticles

The minimum bactericidal concentration (MBC) of the NPs against the bacterial pathogens was determined from MIC experiment. Immediately the MIC was determined, and the wells that contained suspension of MIC and above concentration were marked. Exactly 100 µl of the suspension from each marked well was withdrawn and mixed with nutrient agar at 45 °C and pour into Petri dish plate. Thereafter, the plates underwent incubation for 24 h at 37 °C and observed for colony formation. The MBC was taken as the minimum concentration of NPs at which no visible colony was observed.

Results and discussion

Energy-dispersive X-ray analysis

Energy-dispersive X-ray is a useful technique for analyzing the elemental composition of nanoparticles, as it is non-destructive and can provide information about the composition of the sample at the nanoscale. The elemental composition of CoNPs and NiNPs is given by their EDX spectra depicted in Fig. 1. The existence of a small percentage of carbon, sodium, and oxygen in Fig. 1a (CoNPs spectrum) is intruded impurities from the precursors, ditto to Fig. 1b (NiNPs spectrum) where the carbon and oxygen were also impurities from the precursors.

Fig. 1

EDX spectra of a CoNPs and b NiNPs

X-ray diffraction analysis

The crystalline structures of the synthesized CoNPs and NiNPs were examined via XRD. As shown in the XRD diffractograms, both CoNPs and NiNPs (Fig. 2) exhibit three major diffraction peaks each. Cobalt nanoparticles have its diffraction peaks at 2θ of 44.7° (1 1 1), 52.0° (2 0 0), and 76.3° (2 2 0), which correspond to the face-centered cubic (FCC) crystal structure of pure CoNPs. The diffraction peaks observed exhibit a resemblance to those documented in the literature for CoNPs (Khusnuriyalova et al. 2021; Shi et al. 2021; Cruz et al. 2019b), thereby confirming the accomplished synthesis of pure CoNPs in the FCC structure. The diffraction peaks of NiNPs can be observed at 2θ which is equal to 44.5° (1 1 1), 51.9° (2 0 0), and 76.5° (2 2 0), indicating the presence of an FCC crystal structure in the pure NiNPs.

Fig. 2

XRD spectra of a CoNPs and b NiNPs

The diffraction peaks observed exhibit a resemblance to the ones documented in the literature for NiNPs (Wu et al. 2012, 2010; Eluri and Paul 2012; Balogun and Fayemi 2023), thereby confirming the successful formation of FCC pure NiNPs. Applying Scherrer’s equation (Monshi et al. 2012), 12 nm and 18 nm were obtained as average crystallite size (L) for NiNPs and CoNPs, respectively, as given in Eq. 1.

$$L = \frac{0.89\lambda }{{B \cos \theta }}$$

(1)

In this context, θ denotes the Bragg angle of the X-ray diffraction, λ designates the wavelength (0.15418 nm), and B stands for full-width half-maximum.

Transmission electron microscopy

The prepared nanoparticles' sizes and shapes were identified utilizing transmission electron microscopy. Figure 3a, b displays the TEM images of CoNPs and NiNPs, respectively. The TEM image of CoNPs as depicted in Fig. 3a, displays a spherical particle shape with an irregular particle morphology. The average particle size is measured to be 17.4 nm, which closely aligns with the value calculated using Scherrer's equation. The NiNPs, conversely, show a spherical particle configuration with a uniform particle morphology, possessing an average particle size of 11.7 nm, akin to the outcome derived from the utilization of Scherrer's equation for the NiNPs.

Fig. 3

TEM images of a CoNPs and b NiNPs

Ultraviolet–visible analysis

The UV–visible spectra for CoNPs and NiNPs are illustrated in Figure 4. Cobalt nanoparticles exhibit an absorption peak at 291 nm and abroad absorption peak from 385 to 510 nm (Figure 4a, b). The peaks can be assigned to the O²⁻→ Co²⁺ and O²⁻ → Co³⁺ charge transfer process (Adekunle et al. 2020). Similarly, NiNPs have an absorption peak at 293 nm. The absorption peaks that were observed bear similarities to those reported in the literature concerning NiNPs and CoNPs (Bathla and Pal 2018; Moumen et al. 2019; Balogun and Fayemi 2024).

Fig. 4

UV–visible spectra of a CoNPs and b NiNPs

Interestingly, the absorption peaks of the two nanoparticles are close to each other.

Electrochemical studies

Cyclic voltammetry

All electrodes were electrochemically characterized using cyclic voltammetry CV to evaluate the nanoparticles' electron transport behaviors. As seen in the cyclic voltammograms (Fig. 5), all of the electrodes exhibited a pair of redox peaks in 5 mM K₃/K₄(Fe(CN)₆ produced in a solution of 0.1 M PBS at a pH of 7. The electrodes' peak separations (Ep) values for GCE, GCE-Co, and GCE-Ni were (0.39 V), (0.50 V), and (0.18 V), respectively, as seen in Table 2. The obtained Ep values are more than the predicted 0.059 V for quick one-electron transportation, demonstrating a quasi-reversible mechanism at the electrode interfaces (Haque et al. 2013; Zangeneh Kamali et al. 2014). All electrode's Ipa/Ipc values are nearly equal to one, as shown further in Table 2, which indicates reversibility at all the electrodes. As depicted in Fig. 5, GCE-Ni (35.6 μA) has a higher current response compared to GCE-Co (10.5 μA). The anodic peak current (Ipa) obtained for the GCE-Ni is thrice compared to GCE-Co. This implies that among the electrodes, the GCE-Ni has the best current response. The electrochemical properties of these electrodes were further assessed via EIS analysis.

Fig. 5

Cyclic voltammograms of the electrodes in 5 mM K₃/K₄(Fe(CN)₆ produced in 0.1 M of PBS at 25 mV s⁻¹

Table 2 Cyclic voltammetry data obtained for the electrodes

Scan rate study

The scan rate study focused on analyzing the impact of altering the scan rate (ranging from 25 to 300 mV s⁻¹) on the electrochemical phenomena occurring at the surface of the electrode. The experiment utilized a solution containing 5 mM of K₃/K₄(Fe(CN)₆ in a PBS solution of 0.1 M. As depicted in Fig. 6a, b, increasing in the scan rate exhibits a direct correlation an increase in the peak currents. The relationship between the square root of the scan rate and the peak current displayed in Fig. 6x and y, gives a linearity, with the following regression Eqs. (2–5):

$${\text{For}}\;{\text{CoNPs}}:I_{{{\text{pa}}}} = 0.0001v^{1/2} 8.0928E - 6\;\quad \left( {R^{2} = \, 0.9856} \right)$$

(2)

$$I_{{{\text{pc}}}} = - 6.5950E - 5v^{1/2} - 8.6212E - 6\quad \left( {R^{2} = \, 0.9356} \right)$$

(3)

$${\text{For}}\;{\text{NiNPs}}: I_{{{\text{pa}}}} = 2.7546E - 4v^{1/2} 3.5663E - 5\quad \left( {R^{2} = \, 0.9932} \right)$$

(4)

$$I_{{{\text{pc}}}} = - 2.256E - 4v^{1/2} - 4.4876E - 5\quad \left( {R^{2} = \, 0.9921} \right)$$

(5)

Fig. 6

Cyclic voltammogram of a GCE-Co and b GCE-Ni from 10 to 100 mVs⁻¹ and 25–300 mVs⁻¹, respectively, 5 mM of K₃/K₄(Fe(CN)₆ in a PBS solution of 0.1 M, and (x) and (y) linear plots of peak current vs scan rate’s square root

A diffusion-controlled process is therefore suggested for both GCE-Co and GCE-Ni based on the linear relationship existing between the peak currents and the scan rate's square root.

Stability study

Both GCE-Co and GCE-Ni were subjected to stability tests using CV with 50 mVs⁻¹ for 20 repetitive scans in 5 mM of K₃/K₄(Fe(CN)₆ in a PBS solution of 0.1 M. According to Fig. 7a, b, a small current drop of 7.4% and 4.5% was recorded for GCE-Co and GCE-Ni, respectively, implying a high stability of both electrodes. Going by this result, GCE-Ni (95.5%) is more stable than GCE-Co (92.6%).

Fig. 7

Cyclic voltammograms of a GCE-Co and b GCE-Ni with 20 scans at 25 mVs⁻¹ in 5 mM of K₃/K₄(Fe(CN)₆ made in a PBS solution of 0.1 M

Electrochemical impedance spectroscopy (EIS) study

An EIS investigation was conducted in 5 mM of K₃/K₄(Fe(CN)₆ made in a PBS solution of 0.1 M (pH 7) to thoroughly examine the electron transport behaviors of these electrodes. The operating parameters include 0.3 V, Ag/AgCl, 3 M KCl (saturated), and 100 kHz–0.1 Hz. Figure 8 shows the Nyquist plots of the electrodes’ EIS fitted data with an inset representing the equivalent circuit used to fit the EIS data. As seen in the electrochemical circuit, CPE designates constant phase element, W connotes Warburg impedance, R_ct represents charge transfer resistance, while R_S stands for solution resistance.

Fig. 8

Nyquist plots of the electrodes in 5 mM of K₃/K₄(Fe(CN)₆ made in a PBS solution of 0.1 M (pH 7), and inset represents the equivalent circuit employed to fit the obtained EIS data

The fitted EIS data and individual Chi-square (x²) values are provided in Table 3. The negative Chi-square values and the small % errors obtained, imply that the EIS data were accurately fitted. Table 3 presents the R_ct values of the electrodes as 1.39, 2.99, and 8.73 KΩ for GCE-Ni, GCE-Co, and GCE, respectively. It is observed that GCE-Ni exhibits the lowest R_ct, therefore having a faster ability to transport electrons than Co-GCE. The observed decrease in R_ct values of the treated electrodes indicates successful coating of the nanoparticles on the bare electrode. The EIS result exhibits a resemblance to the one obtained for CV except for CoNPs. The electrode n values (0.72–0.86) might also account for the ease at which ions diffuse between the solution and electrode.

Table 3 Overview of the EIS results

The determination of the "knee" frequency (f°) of the electrodes was conducted in order to get deeper insights into the electronic characteristics of the electrodes. This frequency plays a crucial role in determining the power density, which refers to the supercapacitor's charge and discharge capability. A supercapacitor with a greater f° has a quicker charge and discharge capacity, thus a better power density (Du and Pan 2006; Huang et al. 2007; Balogun and Fayemi 2022). The following f° values 3.16, 2.00, and 1.68 Hz were recorded for the GCE-Ni, GCE‐Co, and GCE, respectively. This result indicates that GCE‐Co has a lower f° value compared to GCE‐Ni. However, the two modified electrodes (GCE‐Co and GCE‐Ni) are far better than the uncoated electrodes. The large f° value of GCE‐Ni corroborates its high electron transport capability. In accordance with a study (Adekunle et al. 2013), the preponderance of commercially accessible supercapacitors, notably those designed for higher power applications, run at frequencies below 1 Hz.

Antibacterial study

Antimicrobial activities of nickel and cobalt nanoparticles

In this study, both NiNPs and CoNPs displayed antibacterial properties against the E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae tested (Fig. 9). Similar antibacterial activities have been reported by several researchers for NPs (Abbas and Aadim 2022; Ahghari et al. 2020; Bensy et al. 2022). Ahghari et al. (Ahghari et al. 2020) reported antibacterial activities of NiNPs against Staphylococcus aureus and Escherichia coli. Also, study by Abass et al. (Abass et al. 2021) indicates that NPs had bactericidal activity against multidrug-resistant bacteria.

Fig. 9

Antibacterial activities of CoNPs and NiNPs against the tested bacteria pathogens

Nanoparticles minimum inhibitory concentration (MIC)

The (MIC) of NPs against tested bacterial pathogens varies. It was observed that the NiNPs had better inhibitory activities against E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae, with MIC values of 61.5, 61.5, 125, 61.5, and 125 µg/mL compared to CoNPs MIC values of 125, 125, 250, 61.5, and 125 µg/mL, respectively. Both NiNPs and CoNPs show lower inhibitory concentration value (61.5 µg/mL) against V. cholerae (Table 4). In contrast, NiNPs and CoNPs show higher inhibitory concentration value (125 and 250 µg/mL) against S. enterica. The variation in antimicrobial activities of the NPs might be explained from several parameters including size, charge, chemical makeup, and agglomeration state of the NPs (Abolarinwa et al. 2022).

Table 4 The minimum inhibitory concentration of NiNPs and CoNPs against bacteria isolates

Minimum bactericidal concentration of the nanoparticles

In this study, NiNPs had better bactericidal activities against E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae tested compared to CoNPs. The MBC values of NiNPs against E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae were 500, 250, 250, 125, and 125 µg/mL, respectively (Fig. 10). In contrast, CoNPs had MBC values of 500, 500, 250, 250, and 250 µg/mL against E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae, respectively (Fig. 11).

Fig. 10

The minimum bactericidal concentration exhibited by NiNPs against bacteria isolates

Fig. 11

The minimum bactericidal concentration exhibited by CoNPs against bacteria isolates

Conclusion

In this study, we synthesized CoNPs and NiNPs via a chemical reduction approach and characterized them using FTIR, TEM, EDX, and XRD techniques. The result from XRD and TEM analysis revealed that the NiNPs was smaller (12 nm) in size compared to CoNPs (18 nm). The electrochemical properties of the nanoparticles were evaluated using CV and EIS analysis, demonstrating the superiority of GCE-Ni with a higher current response of 35.6 μA over to GCE-Co (10.5 μA). The EIS studies indicated that GCE-Ni (1.39 KΩ) has superior electron transport capability compared to GCE-Co (2.99 KΩ) due to its lower Rct values. Furthermore, the power density of the synthesized nanoparticles was determined based on their "knee" frequency (f°) values. The GCE-Ni (3.16 Hz) exhibited a greater f° value compared to GCE-Co (2.00 Hz). Interestingly, both NiNPs and CoNPs possessed significant antibacterial activities against E. coli O157, E. coli O177, S. enterica, S. aureus, and V. cholerae, possibly due to their size, electrical, and optical properties. Here, NiNPs displayed a higher antibacterial potential compared to CoNPs. Its smaller particle size and faster electron transport significantly influenced its performance among other properties by providing a larger specific surface area, an increased bacterial cell membrane permeability, and a higher antibacterial activity. Lastly, this research sheds light on the synthesis, characterization, and antibacterial properties of CoNPs and NiNPs, which can be used to generate novel antimicrobial drugs.