Nano Titanium Dioxide

June 28, 2012, 12:26 pm
Content Cover Image

Nanowire of gallium nitride coated with nanoclusters of titanium dioxide. Source: NIST.

This article was written by an EoE student intern and published under the general guidelines of the EoE Student Science Communication Project.

Introduction

Titanium dioxide is a white solid chemical substance used as a pigment in paints, paper, ink, plastic. Also, it is used in cosmetics and sunscreens. It is used to increase the whiteness or opacity of many products.

The nanochemical form of titanium dioxide (TiO2)—nanotitanium dioxide—measures from 1 to 100 nanometers wide, depending on the formulation.

It occurs in nature as rutile, anatase, or brookite crystals. When used as a sunscreen nano titanium dioxide absorbs and scatters light  (it absorbs ultraviolet B (UV B) light from 290 – 320 nm and scatters UV A from 320 – 400 nm). Rutile is the most common form found in nature, while anatase is more photocatalytic or photoreactive than rutile (i.e., it becomes more chemically reactive—with greater potential to react with such chemicals as oxygen—forming potentially toxic products). Like other nanoparticles, nanotitanium dioxide exhibits unique properties due to its small size and its greatly increased surface area. One characteristic of nanotitanium dioxide that makes it useful for sunscreens is that unlike its larger bulk form, it scatters very little visible light—appearing transparent on the skin rather than opaque white.  

Toxicity

Like other nanosized materials, nanotitanium dioxide has more surface area than its larger counterparts, and as a result it tends to be more reactive. Additionally, as noted above it is photocatalytic in the presence of wavelengths under 400 nm (UV light).  Anatase, the more photoreactive form, for example, is proposed for use in water treatment while the rutile form currently is preferred for use in sunscreen, because of its higher stability and reduced photoreactivity. The TiO2 particles in sunscreen are coated with silica, alumina, or other compounds to increase photostability.

Because of its photoreactivity, that can lead to the production of reactive oxygen species (ROS) in the presence of sunlight or the equivalent, nanotitanium dioxide is toxic to certain bacterial species. While this suggests a promising technique for water treatment or other antimicrobial treatment—it raises some concern about potential adverse effects of nanotitanium on microbial species in the environment.

Although little is known about potential human health effects, these particles are thought to pose little to no toxicity to humans (with the exception of potential inhalation through occupation exposures). To date, TiO2 used in sunscreen has not been found to pose a danger to healthy skin. However, the use of photocatalytic forms in sunscreen, particularly without additional chemical suppressants, creates the potential for formation of ROS that can induce oxidative stress or cytotoxicity in cells.  However, most studies indicate that in healthy intact skin, sunscreen penetrates no further than the outer, dead layer of cells. There are some indications that small amounts of nanoTiO2 can penetrate broken or damaged (e.g., sunburned) skin. 

Recent studies in mice indicate that oral exposures to nanotitanium dioxide can cause genotoxicity either through ROS generation or an inflammatory response. This raises concerns about the use of nano TiO2 in consumer products that may be ingested.

Environmental Exposure and Effects

As a result of nanotitanium dioxide’s growing use in sunscreens and cosmetics, there is some concern about the potential impacts of these particles in the environment and in sewage treatment plants as they are washed down the drain. Although the effects of these particles on the environment are still largely unknown, there is the possibility that photostable TiO2 could block sunlight in aquatic environments, inhibiting algae growth; or, that antimicrobial activity of TiO2 might interfere with sewage treatment processes or other aquatic environmental functions. Some studies have shown that nano-TiO2 particles can attach externally to individual algal cells leading scientists to speculate that exposures could pose a risk to species that fertilize their gametes externally.

Nanotitanium dioxide particles have been found to bioaccumulate in fish, although the uptake mechanism is not clear. Fish and invetebrates exposed to pure photoreactive nanoTiO2 experience a range of such sublethal effects as reproductive difficulty, pathological changes in gills and intestines, and behavioral changes. However, some scientists have suggested that in more natural settings nanotitanium dioxide particles will aggregate together or bind onto organic matter in aquatic environments, reducing their toxicity.

Currently, information on the ecological toxicity of nanotitanium dioxide is limited.

Further Reading

Glossary

Citation

Bowers, J. (2012). Nano Titanium Dioxide. Retrieved from http://www.eoearth.org/view/article/154762

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