Phytochemical-Driven Green Synthesis of NiO Nanoparticles Using Ocimum basilicum and Urtica dioica: Correlating Phytochemical Surface Characteristics With Antimicrobial Activity

In the microscopic world of modern medicine, a quiet crisis is unfolding. Bacteria that once yielded to common antibiotics are learning to resist them, rendering many standard treatments ineffective. This growing resistance has forced scientists to look for new ways to fight infection, turning their attention to the nanoscale. Here, materials are engineered to be so small that they possess unique physical and chemical behaviors, often capable of piercing the defenses of stubborn microbes. Among these materials, nickel oxide nanoparticles have emerged as a promising candidate. They are tiny particles of a metal compound that naturally possess semiconducting properties, meaning they can interact with light and electricity in specific ways. When made small enough, their surface area expands dramatically, allowing them to interact more intensely with their surroundings. The challenge, however, has been how to create these particles without using harsh chemicals or generating toxic waste. Nature offers a solution: plants. For centuries, humans have known that plants contain a rich mix of organic compounds that can alter the state of metals. By using plant extracts, researchers can coax metal salts to transform into stable nanoparticles in a process that is cleaner and safer than traditional industrial methods.

A team of researchers in Turkey set out to explore how different plants influence the creation of these nickel oxide particles and, more importantly, how those differences affect the particles' ability to kill bacteria and fungi. They chose two very common plants: stinging nettle, known for its seeds, and sweet basil, known for its leaves and flowers. The goal was not just to make the nanoparticles, but to understand how the specific "coat" of plant chemicals left on the surface of each particle changed its behavior. In a laboratory setting, the team prepared extracts from the seeds of the stinging nettle and the leaves and flowers of the basil. They then mixed these liquid extracts with a solution containing nickel salts. As the mixtures reacted, the liquid changed color, signaling that the metal was transforming into solid nanoparticles. The plant chemicals acted as both the reducer, turning the metal salt into a solid, and the stabilizer, preventing the new particles from clumping together into useless lumps. The resulting powders were dried and ground, creating two distinct types of nanoparticles: one coated with chemicals from the nettle seeds and the other with chemicals from the basil.

To understand what they had created, the scientists examined the particles with powerful microscopes and light sensors. They found that both types of particles were indeed nickel oxide, arranged in a specific, orderly crystal structure. However, the two plants produced particles with different sizes and shapes. The particles made with basil extract were smaller on average, measuring about 5.14 nanometers in diameter, while those made with nettle seeds were larger, averaging 11.90 nanometers. The researchers also noted that the nettle-derived particles had a more porous, sponge-like surface, whereas the basil-derived ones were more compact. Crucially, the analysis showed that the surface of each particle was covered in a layer of organic molecules from the plant extract. These molecules included various acids and sugars that had bound themselves to the metal, effectively wrapping the particle in a protective and functional shell. The study confirmed that the size of the crystal inside the particle and the size of the particle itself were not always the same, a detail that matters because the outer shell plays a major role in how the particle interacts with the outside world.

The true test came when the researchers exposed these nanoparticles to a variety of harmful bacteria and fungi. They placed the particles on agar plates inoculated with microbes and measured how far the particles could stop the growth of the organisms. The results revealed a surprising twist: the smaller particles were not necessarily the better killers. The nanoparticles derived from stinging nettle seeds proved to be significantly more effective against certain bacteria, particularly Staphylococcus aureus and Enterobacter aerogenes. At a concentration of 1 milligram, the nettle particles created a clear zone of inhibition around them that reached 23.33 millimeters against S. aureus and 24.00 millimeters against E. aerogenes. In contrast, the basil-derived particles showed almost no activity against these specific bacteria, despite being smaller. The researchers found that the nettle particles were also highly effective against Escherichia coli and Klebsiella pneumoniae. When it came to fungi, both types of particles performed similarly, showing comparable ability to inhibit the growth of Candida albicans and Yarrowia lipolytica.

The study suggests that the difference in performance was not due to the size of the particles, but rather to what was on their surface. The nettle seed extract appeared to leave a coating that was richer in nickel and perhaps more effective at disrupting bacterial cell walls. The researchers propose a mechanism where the nanoparticles first stick to the surface of the bacteria, then trigger the production of harmful reactive oxygen species—essentially toxic chemicals that damage the cell membrane. Once the membrane is breached, nickel ions can enter the cell and disrupt its internal machinery, leading to the microbe's death. The data indicated that the nettle-derived particles might release more nickel ions or interact more aggressively with the bacterial surface than the basil-derived ones. This finding challenges the simple assumption that smaller particles are always more potent; instead, it highlights that the chemical identity of the plant used to make the particle is a critical factor in its biological activity.

Ultimately, this work demonstrates that the choice of plant source is a powerful tool in designing antimicrobial agents. It shows that the phytochemicals from stinging nettle seeds create a surface environment on nickel oxide nanoparticles that is far more lethal to certain bacteria than the surface created by basil. The study does not claim to have solved the problem of antibiotic resistance, but it provides a clear, evidence-based path forward. It suggests that by carefully selecting plant extracts, scientists can tailor the surface chemistry of nanoparticles to target specific pathogens more effectively. The research underscores that in the world of green synthesis, the plant is not just a tool for making the material; it is an active ingredient that defines how the material behaves in the biological world.