Nanoparticles

Both Old-fashioned and Modern

Nanoparticles are defined as very small particles with diameters between 1-100 nm. At this scale, these nanoparticles generally behave differently from their bulk materials. The application of nanoparticles can be traced back to more than 3,000 years ago, even though we can confirm that people did not realized the existence of nanoparticles. For instance, the craftsmen in Italy had made a type of red glass from 1,200 B.C. to 1,000 B.C. The red color was originated from the Cu nanoparticles. The dichroic Lycurgus Cup from Roma in the 4th-century B.C. is due to the incorporated silver-gold nanoparticles. Another example is the copper nanoparticles inside the glaze of ceramics from 9th century of Mesopotamia that give rise to different colors from different viewing angles.

In 1857, Faraday was the first to explain that the special properties of nanoparticles should originate from their tiny size. After 1940, the development of electron microscopy made it possible for scientist to better characterize nanoparticles and study the relationship between size and property. The progress in the synthetic methodology in 1990 guarantees the large-scale synthesis of nanoparticles with specific chemical composition, specification, and function.

Today, nanoparticles have widely been used in our lives, for example, the UV-blocking nanoparticles in some sunscreens, the penetration-enhancement nanoparticles in some cosmetic products, the nanoparticles with whitening effect in some toothpaste, and the nanoparticle inhibiting bacteria and odor in some cloth. However, concerns still remain because we only have limited knowledge of its environmental and the health side effects. Some animal experiments suggested that TiO2 nanoparticles might increase the cancer risk when they entered animals. Fortunately, the safety issues are becoming more and more acknowledged by researchers, international organizations, and consumer product manufactures. We believe that a complete set of laws and regulations will be created to regulate the production and the application of nanoparticles, so that we can enjoy the benefits of nanoparticles without the potential hazards to environmental and human health.

Top diagram: a nanoparticle covered with surfactant molecules. Surfactant molecules can prevent the aggregation of nanoparticles.

Nanoparticles. Nanoparticles are generally particles with sizes between 1 and 100 nm. In such scales particles usually display quite different properties to their bulk counterparts, for example, color change or melting point decrease. The application of nanoparticles by human being can be traced back to 3,000 years ago. The above image illustrates the Lycurgus Cup collected in British Museum. The Lycurgus Cups was made in the 4th century B.C. of the ancient Roma era. Under the regular illumination (up left image), the cup shows a green color. However, if we placed the light source inside the cup, it instead displays a red color (up right image). A scientific description is that the reflection of white light on its surface is green while the transmission light is red. Based on modern analytical methods, scientists find that the dichroism is originated from the Ag-Au alloy nanoparticles with a size distribution of 50 – 100 nm. The Lycurgus Cup fully reflects the craftsmanship of ancient Roma. But scientists also suggest that it is impossible to precisely control the property of nanoparticles based on the technology at that time. This is the reason why only a few Lycurgus Cups or similar relics are preserved.

Faraday’s discovery in the 1850s’ pioneered the modern scientific research on the nanoparticles. He synthesized Au nanoparticles that turned the solution into red. Now we understand that the color of Au nanoparticles changes with the size. For example, 20 nm and 100 nm Au nanoparticles will show red and pink colors, respectively.

Nanoparticle self-assembly. In the past 20-30 years, the progress in the synthesis of nanoparticles made it possible to precisely control their size, property, and morphology. Some common morphology of nanoparticles is listed in the left side of this page. A current hot topic in the research of nanoparticles is to build functional materials based on the self-assembly of nanoparticle building-blocks. Scientists also discovered that particles with different sizes can self-assemble into nanocrystals with a similar structure as NaCl (also known as colloidal crystals). Such self-assembly is much more complicated for non-spherical particles. Octahedral and octapod nanoparticle self-assembly structures are show here. The above image is a type of colloidal crystal structure reported by a research group in 2015. This structure was formed by gluing spherical and cubic Au nanoparticles together by DNA. For this structure, both the diameter of spheres and the side length of cubes are 46 nm. The self-assembly of these nanoparticles can form 1-2 µm colloidal crystals. [Figure reference: Lu F. et al. Nature Comm. 6, 6912 (2015)]

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