Green Synthesis of Nickel Nanoparticles: A Review of Plant-Mediated Approaches

1. Introduction

In recent years, the synthesis of nanoparticles has gained significant attention due to their unique physical and chemical properties. Among various nanoparticles, nickel nanoparticles (NiNPs) have shown great potential in diverse applications. Traditional methods of synthesizing NiNPs often involve the use of toxic chemicals and high - energy processes, which pose environmental and health risks. As a result, there has been a growing interest in developing green synthesis methods, especially those mediated by plants.

Plant - mediated synthesis of NiNPs offers several advantages. It is a more environmentally friendly approach as it utilizes natural plant extracts, which are generally biocompatible and biodegradable. Moreover, plants can act as both reducing and capping agents during the synthesis process, leading to the formation of stable NiNPs with controlled size and shape. This review aims to comprehensively discuss the plant - mediated green synthesis of NiNPs, including the factors influencing the synthesis, characterization techniques, and their applications.

2. Factors Influencing Plant - Mediated Green Synthesis of NiNPs

2.1 Plant Type

Different plant species have been explored for the synthesis of NiNPs. For example, medicinal plants such as Aloe vera and Ocimum sanctum have been found to be effective in NiNP synthesis. The chemical composition of plants, which varies from species to species, plays a crucial role. Plants contain a variety of secondary metabolites such as phenolic compounds, flavonoids, and alkaloids. These metabolites can act as reducing agents for nickel ions, converting them into nanoparticles.

For instance, the phenolic compounds present in plants have the ability to donate electrons, which is essential for the reduction of nickel ions. Some plants may also have a higher content of certain metabolites, which can lead to a more efficient synthesis of NiNPs. Additionally, the structure and morphology of plant tissues can also influence the synthesis process. The surface area and porosity of plant materials can affect the adsorption and reaction of nickel ions, ultimately determining the size and shape of the synthesized NiNPs.

2.2 Extraction Method

The method used to extract the active components from plants is another important factor. There are several extraction techniques available, including solvent extraction, microwave - assisted extraction, and ultrasonic extraction.

• Solvent extraction: This is a common method where plant materials are soaked in a suitable solvent such as ethanol or water. The choice of solvent can affect the extraction efficiency as different solvents have different affinities for the plant metabolites. For example, ethanol is often used as it can dissolve a wide range of phenolic compounds and flavonoids. However, the extraction time and temperature also need to be optimized. Longer extraction times may lead to the degradation of some active components, while higher temperatures may cause the evaporation of the solvent and loss of some volatile metabolites.
• Microwave - assisted extraction: This technique uses microwave energy to accelerate the extraction process. Microwaves can penetrate plant tissues and cause rapid heating, which enhances the mass transfer of metabolites from the plant matrix to the solvent. Compared to traditional solvent extraction, microwave - assisted extraction can significantly reduce the extraction time. However, it requires careful control of the microwave power and irradiation time to avoid overheating and degradation of the plant extracts.
• Ultrasonic extraction: Ultrasonic waves are applied to the plant - solvent mixture in this method. The ultrasonic cavitation effect can break the cell walls of plants more effectively, releasing the intracellular metabolites into the solvent. This method is also relatively fast and can improve the extraction yield. However, similar to the other methods, the parameters such as ultrasonic frequency and extraction time need to be optimized to obtain the best results.

2.3 Reaction Conditions

The reaction conditions during the synthesis of NiNPs also have a significant impact. These include parameters such as pH, temperature, and reaction time.

• pH: The pH of the reaction medium can affect the redox potential of the plant metabolites and the solubility of nickel ions. In general, a slightly acidic to neutral pH range is often favorable for the synthesis of NiNPs. At a very low pH, the plant metabolites may be protonated, reducing their ability to act as reducing agents. On the other hand, at a high pH, the formation of nickel hydroxides may compete with the formation of NiNPs.
• Temperature: Temperature affects the reaction rate. Higher temperatures can accelerate the reduction of nickel ions by plant metabolites. However, too high a temperature may also cause the aggregation of nanoparticles or the degradation of the plant extracts. Therefore, an optimal temperature needs to be determined for each plant - mediated synthesis system.
• Reaction time: The reaction time determines the extent of the reduction reaction. Longer reaction times may lead to the complete conversion of nickel ions into nanoparticles. However, if the reaction time is too long, it may also result in the over - growth or aggregation of nanoparticles.

3. Characterization Techniques for Plant - Mediated NiNPs

3.1 Spectroscopic Techniques

Ultraviolet - visible (UV - Vis) spectroscopy is one of the most commonly used spectroscopic techniques for characterizing NiNPs. NiNPs exhibit characteristic absorption peaks in the UV - Vis region due to their surface plasmon resonance. The position and intensity of these peaks can provide information about the size, shape, and concentration of the nanoparticles. For example, a blue shift in the absorption peak may indicate a decrease in the size of the nanoparticles.

Fourier - transform infrared (FT - IR) spectroscopy is used to study the chemical bonding in NiNPs. It can identify the functional groups present on the surface of the nanoparticles. The plant metabolites that act as capping agents can be detected by FT - IR spectroscopy. For instance, the presence of characteristic peaks corresponding to phenolic or flavonoid groups can suggest the involvement of these compounds in the capping of NiNPs.

3.2 Microscopic Techniques

Transmission electron microscopy (TEM) is a powerful tool for visualizing the size, shape, and morphology of NiNPs at the nanoscale. TEM can provide high - resolution images of individual nanoparticles, allowing for the determination of their diameter, aspect ratio, and crystal structure. It can also reveal the presence of any aggregation or surface defects on the nanoparticles.

Scanning electron microscopy (SEM) is used to study the surface topography of NiNPs. SEM can provide information about the overall shape and size distribution of the nanoparticles. It can also be used in combination with energy - dispersive X - ray spectroscopy (EDS) to analyze the elemental composition of the nanoparticles, confirming the presence of nickel.

3.3 X - ray Diffraction (XRD) Technique

XRD is used to determine the crystal structure of NiNPs. The diffraction pattern obtained from XRD analysis can be compared with the standard diffraction patterns of nickel to identify the phase of the synthesized nanoparticles. XRD can also provide information about the crystallite size and lattice strain of the NiNPs. This information is important for understanding the physical and chemical properties of the nanoparticles.

4. Applications of Plant - Mediated NiNPs

4.1 Antibacterial Activity

Plant - mediated NiNPs have shown promising antibacterial activity against a variety of pathogenic bacteria. The antibacterial mechanism may involve multiple factors. One possible mechanism is the interaction between the NiNPs and the bacterial cell membrane. NiNPs can adhere to the cell membrane and disrupt its integrity, leading to the leakage of intracellular components and ultimately cell death.

Another mechanism may be related to the generation of reactive oxygen species (ROS) by NiNPs. ROS can cause oxidative damage to bacterial cells, including damage to DNA, proteins, and lipids. The antibacterial activity of plant - mediated NiNPs has been tested against both gram - positive and gram - negative bacteria. For example, studies have shown that NiNPs synthesized using plant extracts can effectively inhibit the growth of Escherichia coli (gram - negative) and Staphylococcus aureus (gram - positive).

4.2 Electrochemistry

In electrochemistry, plant - mediated NiNPs can be used as electrode materials. Their high surface area and unique electronic properties make them suitable for applications such as electrocatalysis and energy storage.

In electrocatalysis, NiNPs can catalyze various electrochemical reactions. For example, they can be used in the oxygen evolution reaction (OER) in water electrolysis. The plant - mediated synthesis method can produce NiNPs with well - controlled properties, which can improve their electrocatalytic performance. In energy storage, NiNPs can be used in batteries or supercapacitors. The small size and high surface area of NiNPs can enhance the charge - storage capacity and improve the cycling stability of these energy storage devices.

4.3 Optics

The optical properties of plant - mediated NiNPs, such as their absorption and scattering properties, make them useful in optics. They can be used in optical sensors and photonic devices.

In optical sensors, NiNPs can be used to detect various analytes based on the change in their optical properties. For example, the interaction between NiNPs and a target analyte may cause a shift in the absorption or fluorescence spectra, which can be detected and used to quantify the analyte. In photonic devices, NiNPs can be incorporated into materials to modify their optical properties, such as refractive index and light scattering, for applications in areas such as light - emitting diodes (LEDs) and optical waveguides.

5. Future Outlook

Despite the significant progress in plant - mediated NiNP synthesis, there are still several challenges and opportunities for future development.

5.1 Optimization of Synthesis

There is a need for further optimization of the synthesis process. This includes the selection of more suitable plant species, improvement of extraction methods, and fine - tuning of reaction conditions. By optimizing the synthesis process, it is possible to obtain NiNPs with more uniform size, shape, and better - controlled properties.

5.2 Scale - up and Commercialization

For the practical applications of plant - mediated NiNPs, scale - up production is essential. Currently, most of the research is carried out at the laboratory scale. To achieve commercialization, it is necessary to develop cost - effective and scalable production methods. This may involve the development of continuous - flow synthesis processes and the use of large - scale plant cultivation for raw material supply.

5.3 Toxicity and Environmental Impact

Although plant - mediated synthesis is considered a green approach, the toxicity of NiNPs themselves needs to be further investigated. Understanding the potential toxicity of NiNPs towards different organisms, including humans, is crucial for their safe use. Additionally, the environmental impact of the entire synthesis process, from plant cultivation to nanoparticle production, should be evaluated to ensure its sustainability.

In conclusion, the plant - mediated green synthesis of NiNPs is a promising area of research with great potential for various applications. By addressing the challenges and opportunities in future development, it is expected that plant - mediated NiNPs will play an increasingly important role in the fields of medicine, energy, and environmental protection.

FAQ:

What are the main factors influencing plant - mediated green synthesis of NiNPs?

The main factors include plant type, extraction method, and reaction conditions. Different plant species may have different components that can interact with nickel ions to form nanoparticles. The extraction method affects the availability of the reducing and capping agents from the plant. Reaction conditions such as temperature, pH, and reaction time also play crucial roles in the synthesis process.

What are the common characterization techniques for plant - mediated NiNPs?

Common characterization techniques include X - ray diffraction (XRD) for determining the crystal structure, Transmission Electron Microscopy (TEM) to observe the size and shape of the nanoparticles, Fourier - transform infrared spectroscopy (FT - IR) to identify the functional groups involved in the synthesis and capping of the nanoparticles, and UV - Vis spectroscopy which can be used to monitor the formation of NiNPs based on their characteristic absorption.

How do plant - mediated NiNPs show antibacterial activity?

The antibacterial activity of plant - mediated NiNPs can be attributed to several factors. The small size of the nanoparticles allows them to interact with the bacterial cell membrane, causing disruption. They may also release nickel ions which can interfere with the normal metabolic processes inside the bacteria. Additionally, the surface properties of the NiNPs, which may be modified by the plant - derived components during synthesis, can play a role in their antibacterial action.

What applications do plant - mediated NiNPs have in electrochemistry?

In electrochemistry, plant - mediated NiNPs can be used as catalysts. Their high surface area to volume ratio provides more active sites for electrochemical reactions. They can be used in fuel cells, for example, to enhance the efficiency of reactions such as the oxygen reduction reaction. Also, they can be incorporated into electrodes to improve their electrochemical performance.

What are the challenges for the commercialization of plant - mediated NiNP synthesis?

Some of the challenges include scalability of the synthesis process. Ensuring consistent quality and properties of the NiNPs on a large - scale production can be difficult. There may also be regulatory issues as nanoparticles need to meet certain safety and environmental standards. Cost - effectiveness is another factor, as the plant - mediated synthesis should be competitive with other traditional methods of nanoparticle synthesis.

Related literature

• Green Synthesis of Nickel Nanoparticles Using Plant Extracts: A Sustainable Approach"
• "Plant - Mediated Synthesis of Nickel Nanoparticles: Properties and Potential Applications"
• "Advances in Green Synthesis of Nickel Nanoparticles via Plant - Based Routes"

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