Innovative Approaches to Antibacterial Agents: Synthesis and Characterization of Nickel Oxide Nanoparticles Using Plant Extracts

1. Introduction

Antibacterial agents play a vital role in the fight against microbial infections. With the increasing emergence of antibiotic - resistant bacteria, there is an urgent need to develop new and effective antibacterial agents. Nickel oxide nanoparticles (NiO NPs) have shown great potential in this regard. In recent years, the use of plant extracts in the synthesis of nanoparticles has gained significant attention. This innovative approach combines the unique properties of NiO NPs with the natural components present in plants.

2. Synthesis of NiO NPs Using Plant Extracts

2.1 Plant Extract Preparation

The first step in the synthesis of NiO NPs using plant extracts is the preparation of the plant extract. Different plants can be used for this purpose. For example, plants rich in phenolic compounds, flavonoids, and alkaloids are often preferred. The selected plant parts, such as leaves or stems, are thoroughly washed to remove any dirt or impurities. Then, they are dried under shade to preserve the bioactive compounds. After drying, the plant material is ground into a fine powder.
The powdered plant material is then soaked in a suitable solvent, usually water or ethanol. The ratio of plant material to solvent can vary depending on the plant and the desired concentration of the extract. This mixture is then heated at a specific temperature for a certain period of time. For instance, it may be heated at 60 - 80 °C for 2 - 4 hours. After heating, the mixture is filtered to obtain the plant extract, which is rich in bioactive compounds that will play a crucial role in the synthesis of NiO NPs.

2.2 Nickel Source

A nickel source is required for the synthesis of NiO NPs. Commonly used nickel sources include nickel nitrate (Ni(NO₃)₂), nickel chloride (NiCl₂), etc. These nickel salts are highly soluble in water, which makes them suitable for the reaction. The concentration of the nickel source also affects the synthesis process. For example, a higher concentration of nickel nitrate may lead to a faster reaction rate, but it may also result in larger particle sizes if not properly controlled.

2.3 Synthesis Reaction

The plant extract and the nickel source are then mixed together. The bioactive compounds in the plant extract act as reducing agents and capping agents. The reducing agents in the plant extract reduce the nickel ions (Ni²⁺) to nickel atoms (Ni⁰). This reduction reaction can be represented as follows:
Ni²⁺ + 2e⁻ → Ni⁰
The capping agents in the plant extract prevent the aggregation of the newly formed NiO NPs. The reaction mixture is usually stirred continuously at a specific speed, for example, 300 - 500 rpm. The reaction is carried out at a particular temperature, often in the range of 25 - 80 °C. The duration of the reaction can vary from a few hours to a day, depending on the reaction conditions.

3. Characterization of Synthesized NiO NPs

3.1 X - ray Diffraction (XRD)

X - ray diffraction is a powerful technique used to determine the crystal structure of the synthesized NiO NPs. The XRD pattern of NiO NPs shows characteristic peaks corresponding to the cubic crystal structure of NiO. These peaks can be indexed to the (111), (200), (220), etc. planes of NiO. By analyzing the position, intensity, and width of these peaks, information about the crystal size, lattice strain, and phase purity of the NiO NPs can be obtained. For example, the Scherrer equation can be used to calculate the average crystal size of the NiO NPs from the width of the diffraction peaks.

3.2 Scanning Electron Microscopy (SEM)

Scanning electron microscopy is used to study the morphology and surface characteristics of the NiO NPs. SEM images provide a detailed view of the shape, size, and distribution of the NPs. The NiO NPs synthesized using plant extracts may exhibit different morphologies such as spherical, rod - like, or irregular shapes. The size of the NPs can be measured directly from the SEM images. Moreover, SEM can also reveal any surface defects or coatings on the NiO NPs. For example, if there are any bioactive compounds from the plant extract attached to the surface of the NiO NPs, they may be visible in the SEM images.

3.3 Transmission Electron Microscopy (TEM)

Transmission electron microscopy offers a more detailed view of the internal structure of the NiO NPs. TEM can provide information about the crystallinity of the NPs at a very high resolution. It can also show the presence of any lattice fringes, which are characteristic of the crystal structure. In addition, TEM can be used to study the size distribution of the NPs more accurately compared to SEM. The NPs are usually dispersed in a suitable solvent and then placed on a carbon - coated grid for TEM analysis.

3.4 Fourier Transform Infrared Spectroscopy (FTIR)

Fourier transform infrared spectroscopy is used to identify the functional groups present on the surface of the NiO NPs. The FTIR spectrum of the NiO NPs synthesized using plant extracts may show peaks corresponding to the functional groups of the bioactive compounds from the plant extract. For example, if there are phenolic groups in the plant extract, characteristic peaks in the FTIR spectrum may be observed in the region of 1200 - 1500 cm⁻¹. These peaks can indicate the presence of capping agents or any interactions between the NiO NPs and the plant - derived compounds.

4. Antibacterial Activity of NiO NPs

4.1 Tested Pathogens

The antibacterial activity of the synthesized NiO NPs is tested against a variety of pathogens. These include both gram - positive and gram - negative bacteria. For example, common gram - positive bacteria such as Staphylococcus aureus and Streptococcus pyogenes are often tested. Gram - negative bacteria like Escherichia coli and Pseudomonas aeruginosa are also included in the antibacterial assays. The choice of these pathogens is based on their clinical significance and prevalence in causing infections.

4.2 Antibacterial Assay Methods

There are several methods to evaluate the antibacterial activity of NiO NPs. One of the commonly used methods is the disk diffusion method. In this method, a filter paper disk impregnated with a known concentration of NiO NPs is placed on an agar plate seeded with the test pathogen. After incubation for a specific period of time, usually 18 - 24 hours at a suitable temperature (e.g., 37 °C), the zone of inhibition around the disk is measured. A larger zone of inhibition indicates a stronger antibacterial activity.
Another method is the broth dilution method. In this method, different concentrations of NiO NPs are added to a liquid broth containing the test pathogen. After incubation, the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) are determined. The MIC is the lowest concentration of NiO NPs that inhibits the visible growth of the pathogen, while the MBC is the lowest concentration that kills the pathogen completely.

4.3 Mechanisms of Antibacterial Action

The antibacterial action of NiO NPs can be attributed to several mechanisms. One mechanism is the generation of reactive oxygen species (ROS). NiO NPs can produce ROS such as superoxide anions (O₂⁻), hydroxyl radicals (·OH), etc. These ROS can cause oxidative damage to the bacterial cell membrane, proteins, and DNA. Another mechanism is the interaction of NiO NPs with the bacterial cell membrane. The NPs may disrupt the integrity of the cell membrane, leading to leakage of intracellular components and ultimately cell death. Additionally, the presence of bioactive compounds from the plant extract on the surface of the NiO NPs may also contribute to the antibacterial activity. These compounds may have their own antibacterial properties or may enhance the activity of the NiO NPs.

5. Conclusion

In conclusion, the synthesis of nickel oxide nanoparticles using plant extracts is an innovative approach in the development of antibacterial agents. The plant - mediated synthesis method offers several advantages, such as being environmentally friendly, cost - effective, and potentially producing nanoparticles with enhanced antibacterial properties. The characterization of the synthesized NiO NPs using techniques like X - ray diffraction, scanning electron microscopy, etc., provides valuable information about their physical and chemical properties. The antibacterial activity of these NPs against various pathogens shows their potential as a new class of antibacterial agents. However, further research is still needed to fully understand the mechanisms of action, optimize the synthesis process, and evaluate the in - vivo efficacy and safety of these NiO NPs.

FAQ:

What are the advantages of using plant extracts in the synthesis of nickel oxide nanoparticles?

Using plant extracts in the synthesis of nickel oxide nanoparticles offers several advantages. Firstly, plant extracts are a natural source, which makes the synthesis process more environmentally friendly compared to some chemical - based synthesis methods. Secondly, plants contain a variety of bioactive compounds such as flavonoids, tannins, and alkaloids. These compounds can act as reducing agents and stabilizers during the synthesis of nanoparticles, which helps in controlling the size and shape of the nanoparticles. Additionally, the natural components of plants may endow the synthesized nickel oxide nanoparticles with unique biological properties, enhancing their potential applications, for example, in antibacterial activity.

How does the plant - mediated reaction work in the synthesis of NiO NPs?

In the plant - mediated synthesis of NiO NPs, the bioactive compounds present in the plant extract play a crucial role. These compounds can reduce the nickel ions (Ni²⁺) present in the precursor solution to nickel atoms. The reduction process is a key step in the formation of nanoparticles. At the same time, the plant - derived molecules can also adsorb onto the surface of the newly formed nickel nanoparticles, preventing their aggregation and thus acting as stabilizers. The overall plant - mediated reaction is a complex process that is influenced by factors such as the type of plant extract, its concentration, reaction temperature, and time.

What can X - ray diffraction tell us about the synthesized NiO NPs?

X - ray diffraction (XRD) is a powerful technique for characterizing the synthesized NiO NPs. It can provide information about the crystal structure of the nanoparticles. From the XRD pattern, we can determine the lattice parameters, crystal phase, and crystallinity of the NiO NPs. For example, in the case of NiO, a characteristic set of diffraction peaks can be observed, which corresponds to the cubic crystal structure of NiO. The intensity and position of these peaks can give insights into the quality and purity of the synthesized nanoparticles. If there are any impurities or deviations from the ideal crystal structure, they can be detected through changes in the XRD pattern.

How effective are the synthesized NiO NPs against different types of bacteria?

The antibacterial effectiveness of the synthesized NiO NPs against different types of bacteria can vary. In general, they have shown significant antibacterial activity against a range of pathogens. The mechanism of their antibacterial action is complex and may involve multiple factors. One possible mechanism is the generation of reactive oxygen species (ROS) by the NiO NPs. These ROS can damage the bacterial cell membrane, proteins, and DNA, leading to cell death. The antibacterial activity also depends on factors such as the size, shape, and surface properties of the nanoparticles. Smaller nanoparticles may have a higher surface - to - volume ratio, which can enhance their interaction with bacteria and thus increase their antibacterial efficacy. Additionally, the presence of plant - derived components on the surface of the NPs may also contribute to their antibacterial activity.

Can the synthesis method of NiO NPs using plant extracts be scaled up for industrial production?

Scaling up the synthesis method of NiO NPs using plant extracts for industrial production has both challenges and possibilities. On one hand, there are some challenges. For example, ensuring a consistent supply of high - quality plant extracts can be difficult as it depends on factors such as the season, growth conditions of the plants, and extraction methods. Also, the reproducibility of the synthesis process on a large scale needs to be carefully addressed. On the other hand, there are also possibilities. If these challenges can be overcome, the plant - extract - based synthesis method has the potential for industrial - scale production. It is more environmentally friendly compared to some traditional chemical synthesis methods, which may be more appealing in the current context of increasing environmental awareness. Moreover, the natural origin of the plant - based synthesis may also lead to unique properties of the NPs that could have commercial value in the antibacterial agent market.

Related literature

• Synthesis of Nickel Oxide Nanoparticles Using Green Chemistry Approaches"
• "Plant - Mediated Synthesis of Metal Oxide Nanoparticles: Properties and Applications"
• "Antibacterial Nanoparticles: A Review of Synthesis and Mechanisms of Action"