A nanoparticle is a particle of matter with at least one dimension between 1 and 100 nanometers. This small size gives these particles unique physical and chemical properties, causing them to behave differently than their larger, bulk counterparts. While the intentional design of nanoparticles is a modern field, these particles also exist throughout the natural world. The study and application of these materials, known as nanotechnology, leverages their distinct behaviors for a wide range of uses.
Understanding the Nanoscale
A nanometer is one-billionth of a meter, a scale that is difficult to comprehend without comparison. To put it into perspective, a single sheet of paper is approximately 100,000 nanometers thick. A strand of human hair has a diameter of around 80,000 nanometers, and a red blood cell is about 7,000 nanometers wide. Many nanoparticles are so small that they are smaller than the wavelength of visible light, making them invisible to the naked eye and even to many conventional microscopes.
Natural and Engineered Nanoparticles
Nanoparticles are categorized as either naturally occurring or intentionally engineered. Natural nanoparticles are common in the environment, formed through various geological and biological processes. Examples include particles in volcanic ash, soot from forest fires, and certain proteins within living organisms, such as the casein micelles in milk. Viruses are another common example of a biological nanoparticle, typically ranging from 20 to 400 nanometers in size.
Engineered nanoparticles are synthesized in a laboratory to perform a specific function. Scientists can design these particles with precise control over their size, shape, and surface properties. Common types include metal-based nanoparticles, such as those made from gold or silver, which have applications in diagnostics and antimicrobial coatings. Carbon-based nanoparticles, like carbon nanotubes, are known for their strength and conductivity. Lipid-based nanoparticles are often used in medicine to encapsulate and deliver drugs.
Distinct Properties at the Nanoscale
Nanoparticles exhibit different behaviors from bulk materials due to two main phenomena: a high surface-area-to-volume ratio and quantum effects. As a particle gets smaller, a greater proportion of its atoms are located on the surface compared to its interior. This increased surface area makes nanoparticles more chemically reactive. An analogy is how granulated sugar dissolves in water much faster than a solid sugar cube; the greater surface exposure of the granules allows for quicker interaction with the water molecules.
At this scale, the principles of quantum mechanics begin to take over from classical physics. A material’s electronic and optical properties can change based on its size. For example, bulk gold is yellow and chemically inert. However, gold nanoparticles can appear as different colors, such as red or blue, depending on their exact size. They also become highly effective catalysts, capable of speeding up chemical reactions.
Common Applications of Nanoparticles
The unique properties of engineered nanoparticles are harnessed for a wide array of practical uses across various fields. In medicine, nanoparticles are used for targeted drug delivery, where they can transport therapeutic agents directly to diseased cells, and as contrast agents to improve the clarity of medical imaging techniques.
In consumer goods, nanoparticles are common in sunscreens. Tiny particles of titanium dioxide and zinc oxide are highly effective at blocking ultraviolet (UV) radiation while appearing transparent on the skin, an improvement over older formulations that left a white residue. Other applications include creating stain-resistant coatings for fabrics and anti-scratch surfaces for eyeglasses.
In electronics, quantum dots—a type of semiconductor nanoparticle—are used in advanced television displays (QLED) to produce more vibrant and precise colors. They are also being explored to enhance the efficiency of solar cells.