What’s at stake:
Bacteria and fungus are big problems, yet the antibiotic and antifungal drugs available to us are losing their effectiveness at an alarming rate. Nanoparticles are a promising alternative, alone or paired with drugs.
Bacteria: More than 2.8 million drug-resistant bacterial infections occur in the U.S. each year. Yet many drug companies have stopped research on new antibiotics altogether: that’s a 78 percent decline since 1990.
Fungus: Fungal infections affect both humans and crops. Crops are increasingly vulnerable to mold and mildew (types of fungi) as we see warmer, wetter weather. One study estimates that crops lost to fungi each year could feed eight percent of the global population. And the black fungus C. auris, which attacks human faces, is now resistant to all three classes of antifungals.
What’s a nanoparticle?
Nanoparticles are synthetic molecules so tiny that they are smaller than the wavelength of light. They typically contain metals such as zinc or silver. In the past, nanoparticles were only produced chemically. However, nanoparticles can be made from plants as well.
How are nanoparticles used?
Because of their minuscule size, nanoparticles can readily travel through cell walls and deliver compounds that can kill bacteria or fungi. They are used for many purposes and fight fungal and bacterial infections in crops, livestock, and people. They are often used to keep packaged foods from spoiling. There are currently over 500 clinical trials in progress on nanoparticles in medicine, including one on the use of silver nanoparticles against drug-resistant bacteria.
In many cases, nanoparticles work better than antibiotics or antifungals. While the typical antibiotic can kill just a handful of bacterial types, nanoparticles can kill hundreds. A 2020 study found that zinc oxide nanoparticles killed a fungus threatening coffee crops. The nanoparticles inhibited 96% of fungal growth, compared with about 89% using a fungicide.
How are nanoparticles made?
The fastest-growing research on nanoparticle production involves creating them from plants, known as “green nano.” There are also two types of chemical production: bottom-up and top-down. The nanoparticles used against bacteria and fungi are usually made with a bottom-up method, which means building new molecules by combining even tinier molecules and atoms. The top-down approach involves breaking down matter into smaller molecules. Top-down is typically used in electronics manufacture.
In bottom-up manufacturing, nanoparticles can be created as gases, droplets, or liquids.
Gas processes turn the components of nanoparticles into aerosols. The gases are heated, and the resulting compounds are collected on surfaces. There are various ways to heat the gases:
High-temperature flame reactors are often used to produce large quantities of titanium dioxide (found in toothpaste, sunblock, and other products)
Plasma reactors break electrons free from molecules
Laser reactors heat gases and break them down into new substances
Hot wall reactors heat gas under low pressure, so the gas cools quickly and condenses. This method can create nanoparticles containing nickel and iron.
Vacuum condensation vaporizes materials in a vacuum and condenses them on a heated surface using a chemical reaction.
Droplet processes use various methods, including centrifuges, compressed air, ultrasound, and vibration to create droplets. The droplets are then heated or react with other gases to produce a powder.
Liquid processes happen at lower temperatures than aerosol or droplet methods. The two most common types of liquid processes are precipitation and sol-gel.
Precipitation processes cause metal ions to react with salts, yielding metallic nanoparticles.
Sol-gel processes use gels to produce porous nanoparticles, ceramic oxides, and other polymers. One advantage of porous sol-gel nanoparticles is that other substances can be embedded in their holes. For example, nanoparticles can be combined with drugs. The nanoparticles “smuggle” the drugs into targeted tumor or pathogen cells, using their tiny size to gain entrance through the cell wall.
Green processes start with plants and are considered less toxic and more energy-efficient than the chemical processes listed above. They don’t require high temperatures, high pressures, or harsh chemicals.
Green nanoparticles are also often more effective against pathogens. For example, a 2012 study made zinc oxide nanoparticles from aloe plants and found that they were more effective against bacteria and fungi than their chemically based counterparts. Scientists have even synthesized zinc oxide nanoparticles from heated garlic extract. Plants including aloe vera, oats, alfalfa, tulsi, lemon, neem, coriander, mustard, and lemongrass have produced silver and gold nanoparticles.
Plants naturally contain numerous compounds that will react with metal salts, turning them into metal nanoparticles. Bacteria sometimes assist this process. Bacteria are easy to manipulate and are effective at producing silver nanoparticles with unusual shapes and sizes. Finding new nanoparticle structures creates fresh opportunities to fight pathogens or to deliver different drugs into the body.
While we still need to develop new antibiotics and antifungals, nanoparticles show great promise to fill the gap in our fight against pathogens.