Nanoparticle

A nanoparticle is a tiny particle of matter with dimensions typically between 1 and 100 nanometres (nm).

While the term is sometimes applied to larger particles up to 500 nm, or to tubes and fibers that are narrow in two directions, it describes a unique scale of existence. These particles are much smaller than microparticles, which range from 1 to 1000 micrometers (μm). They are also distinct from fine particles and coarse particles. At the smallest scale, metal particles smaller than 1 nm are usually called atom clusters. Nanoparticles are important because their tiny size creates physical and chemical properties that differ from the bulk material.

The unique behavior of nanoparticles is driven by their scale. Because they are so small, they are highly subject to Brownian motion, which is the random movement of particles in a fluid. This motion prevents them from settling or sedimenting like larger particles might. They are also much smaller than the wavelengths of visible light, which range from 400 to 700 nm. Because of this, nanoparticles cannot be seen with ordinary optical microscopes. Instead, scientists must use electron microscopes or laser microscopes to observe them.

Furthermore, dispersions of nanoparticles in transparent media can remain transparent. This happens because they do not scatter light in the same way that larger suspensions do. They can also pass easily through common ceramic filters, requiring special nanofiltration for separation.

One reason for these unique properties is the high surface-to-volume ratio. Since a typical atom has a diameter between 0.15 and 0.6 nm, a large fraction of a nanoparticle's material sits near its surface. This means the properties of the surface layer can dominate the behavior of the entire particle. This effect is even stronger when nanoparticles are dispersed in a medium made of a different composition. The interactions at the interface between the nanoparticle and the surrounding material become very significant. This transition between bulk materials and atomic structures allows for phenomena not seen at other scales.

Nanoparticles come in many different shapes, which is known as morphology. Common shapes include nanospheres, nanorods, nanochains, and nanoboxes. Some even resemble more complex structures like nanostars, nanoflowers, or nanoreefs.

The shape of a particle can be determined by its intrinsic crystal habit or by its environment. For example, coating additives can inhibit crystal growth on certain faces. Non-spherical nanoparticles, such as prisms or cubes, exhibit anisotropy. This means their physical and chemical properties change depending on their shape and size. In gold, silver, or platinum, these shapes can even change the colors seen in colloidal solutions.

Creating these particles involves a process called nucleation and growth. Nucleation is the foundation of synthesis and sets the stage for the particle's final size and shape. There are two main types: homogeneous nucleation, where nuclei form uniformly throughout a phase, and heterogeneous nucleation, where they form on surfaces or impurities. To control the final product, scientists often use the LaMer model. This model describes three steps: a rapid increase in monomer concentration, fast nucleation characterized by explosive growth, and finally, growth controlled by the diffusion of monomers. Controlling these steps allows for the creation of monodisperse products, which have a uniform size.

Humans have interacted with nanoparticles for much of history, even without knowing their scientific name. Artisans in prehistory used them in glassmaking and pottery. For instance, the Roman Lycurgus cup from the 4th century CE used nanoparticles to create dichroic glass. In the 9th century CE, Mesopotamian lusterware used silver and copper nanoparticles in its glaze.

In 1857, Michael Faraday provided the first scientific description of how metal nanoparticles interact with light. In the 20th century, researchers used the term "ultrafine particles" until the 1990s, when the National Nanotechnology Initiative helped make the term "nanoparticle" more common.

Today, nanoparticles are essential to many fields and natural processes. They occur naturally in interplanetary dust, which falls to Earth at rates of thousands of tons per year. They are also found in atmospheric dust and within many viruses.

In industry, they are key ingredients in paints, plastics, metals, and ceramics. Scientists use them in medicine, such as using liposomes as delivery systems for vaccines or anticancer drugs. By studying the transition between atoms and bulk matter, nanotechnology continues to expand our ability to build new materials.