Zinc oxide nanoparticles as antimicrobial agent

Zinc oxide nanoparticles have been known for their antimicrobial properties for some time.1 Also, the activity of magnesium oxide and copper in this field is already described.2,3 CANdot Series D, consisting of Cu and Mg doped zinc oxide, combine these effects in one nanoparticle system. The particles show growth inhibiting properties against various micro organisms, e.g. Staphylococcus aureus, Staphylococcus epidermidis or Malassezia furfur. This makes them a promising tool for applications in the field of cosmetics, such as preservatives or anti-dandruff agents, just to name two. Other potential fields could be the application of CANdot Series D in polymer composites or paints, to create antibacterial surfaces. Regarding EU cosmetics regulations, particles with a size over 100 nm were developed.

Microorganisms are omnipresent in everyday life. It is said that one human who consists of ten trillion cells, harbours 100 trillion bacteria. They are present in the mouth, on the skin and in the gastrointestinal tract. Most of the bacteria are, of course, harmless or in the case of the flora of the intestine, essential for us to survive. Problems arise from bacteria who reach, by contamination or injuries, the wrong parts of the body, or pathogenic microorganisms, which cause severe illnesses. Therefore, the battle of humans against harmful microorganism is as long as the history of humanity itself. Due to remarkable progress in the field of hygiene, the scientific exploration of microorganisms and the development of antibiotics, the dangers of bacterial infections have been significantly reduced over the last hundred years. Nevertheless, bacteria still remain a severe problem because of their astonishing ability to develop resistances against practically all new antibiotics. One strategy, of course, is to reduce the amount of microorganism in our surroundings, which is achieved by numerous biocides. Obviously, this leads to new problems. Because of their chemical nature, most biocides come with unwanted side effects such as toxicity to humans and the environment. This leads to a strong need for antimicrobial products which are effective against microorganisms, but whose effectiveness relies upon other mechanisms than the chemical effect of antibiotics or biocides. One alternative way to reduce microorganisms in places, where they cause negative effects is to address them in a physical way by creating an environment or a surface where their ability to survive or proliferate is reduced. One system that pursues this strategy is zinc oxide based nanoparticles. Regarding the antimicrobial properties of zinc or magnesium oxide, two different theories exist. On the chemical side, some works refer to the formation of peroxides on the surface of the material. Peroxides and other reactive oxygen species (ROS) can cause stress to microorganisms which can kill them or affect their ability to proliferate.1,2 On the physical side, theories mention the direct adhesion of particles to the microbial cell membrane, due to electrostatic effects. This leads to a damage of the membrane, followed by an increased uptake of copper or zinc ions, released from the nanoparticles. Copper and zinc ions are known to influence the metabolism of the organisms, which also leads to damage or death of the cells, which, in the case of copper, results from the interference of the ions with the organism’s respiratory chain.3 In copper doped zinc oxide nanoparticles, these benefits are combined. Also, the high surface to volume ratio means that there are far more effective atoms at the surface of the nanoparticles than in bulk material. This leads to increased contact of the microorganisms to the substrate. In personal care, many problems originate from the presence of unwanted microorganisms. For example cosmetic products provide a nutrient medium for organisms like Klebsiella pneumonia, Pseudomnonas bacteriae or Aspergillus niger. Although it can be considered a solvable problem, to keep the production process free from contamination sources, contamination during the use of the customer is almost unavoidable. On the one hand, this results in a lower shelf life of the products, and on the other hand it also bears the risk of serious bacterial infections to the consumer if microorganisms, which may be rather harmless on the skin surface, manage to penetrate the skin through small lesions or wounds. Another strategy aims directly at microorganisms, which cause classical problems addressed by the cosmetic industry. In this field, CANdot Series D could become a helpful active ingredient. For example, the inconvenient problem of dandruff is a well known issue to many people. Dandruff generation results from the rapid release of skin cells from the scalp, in heavier cases accompanied by itching and an uncomfortable skin feeling. Responsible for dandruff problems are, among other factors, different species of the microorganism Malassezia, a form of yeast fungus which disturbs the skin flora of the scalp. First tests where Malassezia furfur was incubated in liquid culture with magnesium doped zinc oxide nanoparticles, show a good activity of the particles against this form of yeast. The second example for a widespread problem, which involves microorganisms is acne. The bacteria, which play a role in the formation of this ailment, can also be addressed by the particles introduced in this article.

Materials and methods