What are some of the common misconceptions about nanoparticles?
A nanoparticle refers to an ultra-fine particle that measures between 1 and 100 nanometres in diameter. A nanometre is a billionth of a metre. The size of nanoparticles is so minute that they are undetectable to the eye.
One of the key concerns about nanoparticles is that their size allows for easy penetration into the skin. Many sunscreen products contain zinc oxide nanoparticles. There has therefore been a misconception that these nanoparticles can get into the bloodstream by crossing the skin barrier. Zinc oxide may be toxic to humans and cause a range of health problems if ingested and absorbed into the bloodstream. It is however considered safe for use as a topical application, even in nanoparticle size, as they are not able to penetrate the skin. A scientific review report by Australia’s Therapeutic Goods Administration found that the nanoparticles simply remain on the skin where applied and act as a protective barrier against the sun’s UV rays.
Another related misconception is that by blocking UV rays, these nanoparticles can contribute to vitamin D deficiency. This is not true. Tests have shown no difference in vitamin D levels between that those that apply sunscreen that contains zinc oxide nanoparticles and that those that do not. This means you get the benefit of sun protection while still being able to absorb the needed vitamin D needed for good health.
How Zinc Oxide is produced
Zinc oxide (ZnO) is an inorganic compound of zinc that can be synthesised in a large variety of ways. One of the most frequently used commercial processes involves the high-temperature oxidation of metallic zinc or zinc ores. Metallurgical processes are often used to synthesise zinc oxide. Zinc ore is heated into a vapour and upon reacting with oxygen in the air makes zinc oxide.
Zinc oxide can also be made through chemical techniques like the mechanochemical process involving dry milling. It is an effective and affordable way to create nanoparticles of zinc oxide on an industrial scale. Controlled precipitation is another chemical process that involves the application of reducing agents to zinc salt, with precipitation of the resulting solution to create a precursor of zinc oxide. Milling is later done to further remove impurities.
Solvothermal and hydrothermal reactions can also be used to produce high purity zinc oxide. These processes are simpler and energy-efficient. They involve the mixture of substrates that are gradually heated up and left to cool, forming crystal nuclei that grow. Zinc oxide can also be obtained through growing from the gas phase, synthesis from microwaves and sonochemical methods. There is ongoing research into new and existing ways to synthesise zinc oxide so this list may not be exhaustive.
Zinc oxide is also naturally present in the mineral zincite, which is rare in nature. Hence it is rarely sourced this way due to lack of deposits, though this mineral can be synthetically grown.
How zinc oxide nanocrystals prevent bacterial growth
Zinc oxide has been found to offer several therapeutic benefits in the field of biomedicine. One of these benefits lies in its antimicrobial and antibacterial abilities. A study was done of synthesised hedgehog-like zinc oxide structures and commercial zinc oxide particles in liquid environments. The results showed that the growth inhibition of bacteria could be achieved where there was continuous mixing of the particles in the liquid.
The effect was strongest with nanoparticles of zinc oxide against Gram-negative E. coli than Gram-positive S. aureus. SEM images showed that the hedgehog-like spiky structure of nanoparticles caused greater damage to the bacterial cells, more easily piercing the oval-shaped membranes of E. coli. The damage was more pronounced than when mixed with S. aureus whose structure is smaller with a more spherical shape. Similar membrane damage was also seen in another study of zinc oxide nanoparticle action on carbapenem-resistant A. baumannii bacteria.
Other research has also proven that zinc oxide microcrystals can efficiently absorb light from UV rays, which can then be used to penetrate bacterial cell walls through diffusion. SEM and TEM images show that zinc oxide nanoparticles can damage the cell membranes of harmful organisms and congregate in the cytoplasm to interact with biomolecules resulting in cell death. This antibacterial action that is catalysed by sunlight means that zinc oxide nanoparticles can be safely used in wound care and the treatment of microbial infections. It also indicates potential in its use as an antibacterial drug that may one day replace antibiotics in the treatment of resistant bacterial infections.
