Green Synthesis

Nanoparticles are materials with sizes typically ranging from 1 to 100 nanometers, and due to their small dimensions they exhibit optical, electronic, magnetic and catalytic properties distinct from those of conventional materials. These unique properties have made nanoparticles attractive for potential applications in numerous fields including medicine, environmental science, energy, agriculture and textiles. Traditional nanoparticle synthesis methods often require high temperatures, high pressures, toxic solvents and reducing agents, which can lead to the generation of harmful environmental waste. This has driven researchers to develop more sustainable and environmentally friendly synthesis approaches. Green synthesis has emerged as a prominent methodology in this context, enabling nanoparticle production based on principles of non-toxic chemicals, renewable resources and low energy consumption. In particular, green synthesis methods using plant extracts, microorganisms and biomolecules have attracted significant interest due to their environmental compatibility and ability to produce biologically compatible nanoparticles.

Green synthesis is an approach in chemistry that embraces sustainability principles and is grounded in several key principles for nanoparticle production. These principles focus on optimizing reaction conditions, minimizing the use of toxic substances and reducing environmental impact. Unlike traditional methods, the chemicals used in green synthesis are typically biodegradable and derived from renewable sources. This helps reduce the carbon footprint of the synthesis process and minimizes waste generation. For example, the use of natural reducing agents such as plant extracts replaces harmful reductants like sodium borohydride, offering a safer synthetic route. Additionally, reactions carried out at room temperature or under mild heating significantly reduce energy consumption, making the process more economically and environmentally efficient.

Nanoparticle synthesis using plant extracts is one of the most popular and effective approaches in green synthesis. Plants synthesize a variety of bioactive compounds as a result of their metabolic activities, including flavonoids, alkaloids, terpenoids, phenolic compounds, proteins, enzymes and vitamins. These compounds possess the ability to reduce metal ions into nanoparticles and stabilize the resulting nanoparticles. The synthesis process is generally straightforward: plant extract is added to an aqueous solution of a metal salt, and the mixture is stirred or heated for a specified period to facilitate the reaction. The complex composition of plant extracts allows them to function simultaneously as both reducing and stabilizing agents, eliminating the need for additional chemicals. Using this method, a wide range of metal and metal oxide nanoparticles such as silver, gold, zinc oxide, copper oxide and titanium dioxide have been successfully synthesized. For instance, the synthesis of silver nanoparticles using Azadirachta indica (neem) leaf extract has been reported as a rapid and high-yield method, with the resulting nanoparticles demonstrating antimicrobial properties.

^{Schematic representation of typical green synthesis of inorganic nanoparticles using plant-based extracts.} (Villagrán et al., 2024)

Microorganisms, particularly bacteria, fungi, yeasts and algae, have the potential to convert metal ions into nanoparticles through intracellular or extracellular mechanisms. Although not as widely adopted as plant-assisted synthesis, these biological methods offer flexibility in producing nanoparticles with diverse morphologies and surface properties. Microorganisms utilize various enzymes, such as nitrate reductase, and metabolites to reduce metal ions. Bacterial nanoparticle synthesis occurs via two main mechanisms: intracellular and extracellular. In intracellular synthesis, metal ions are taken up into the cell and reduced within the cytoplasm to form nanoparticles. In extracellular synthesis, enzymes or other biomolecules are secreted outside the cell and reduce metal ions in the external environment. Fungi are often preferred microorganisms due to their high metal tolerance and greater potential for biomass production. Extracellular synthesis of gold and silver nanoparticles using the fungus Fusarium oxysporum has demonstrated the production of highly crystalline and stable nanoparticles.

Isolated biomolecules such as amino acids, proteins, DNA and carbohydrates can also serve as both reducing and stabilizing agents in nanoparticle synthesis. This approach allows for greater control over the synthesis process and provides increased flexibility for surface modification of the resulting nanoparticles. For example, various proteins and peptides have been shown to reduce metal ions and prevent nanoparticle aggregation. Proteins such as albumin, casein and gelatin have been used to synthesize biocompatible nanoparticles for drug delivery systems and biomedical applications. DNA-based synthesis, on the other hand, leverages DNA’s unique self-assembly and recognition properties to create complex nanoparticle structures and networks. These methods enable the development of customized nanoparticles for targeted drug delivery, imaging and diagnostic applications, particularly in biomedicine.

Nanoparticles produced via green synthesis methods have a broad range of applications, with particular interest in the biomedical field. They have been shown to be effective as antimicrobial agents against various bacteria and fungi. Numerous studies have explored the potential use of green-synthesized silver and gold nanoparticles in combating infections and promoting wound healing. In cancer therapy, they are being investigated as drug delivery systems or for their direct anticancer effects. For example, selenium nanoparticles synthesized using plant extracts have been reported to exhibit cytotoxic effects on cancer cells.

In environmental science, green-synthesized nanoparticles are used in water purification and environmental remediation applications. Their photocatalytic and adsorbent properties are harnessed for the removal of heavy metals, degradation of organic pollutants and inactivation of microorganisms. In agriculture, they show potential as nano-fertilizers and nano-pesticides to promote plant growth and combat diseases. In the energy sector, they are employed to enhance efficiency in solar cells, fuel cells and catalytic conversion processes.