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About 4 gms of single walled carbon nanotubes have the same surface area as a football field.
Image Credit: NASA Ames Center for Nanotehnology

Nanoparticles (sized between 1 and 100 nanometers) are considered a bridge between bulk materials and atomic or molecular structures.  A bulk material has constant physical properties regardless of its size, but often this is not the case at the nano scale. 

One of the important characteristics of nanoparticles is their large ratio of surface area to volume (compared to bulk materials). This property can be illustrated by the following thought experiment: consider a cube of edge W=1 meter, and cut it into two pieces, thereby exposing additional 'faces' of the material. That is, new visible or usable areas are added while the total volume remains the same. Repeat this exercise until all particles reach a size of approximately one nanometer. The result is a group of small particles with enormous surface surface area which occupy the same volume we started with. There are nanomaterials which exhibit a surface area equivalent to a football field for just a few grams of weight.

Large surface area makes some nanoparticles highly soluble in liquids. This property found applications in paints, pigments, medicine pills, and cosmetics. A 30 nanometer iron particle has 5% of its atoms on the surface while the remaining reside inside. However, a 3 nm particle has 50% of its atoms on the surface. The atoms on the surface, which are not bonded on one side, are far more active than the atoms residing inside (which are bonded all around, thus they are "satisfied"). An increase in surface area therefore leads to an increase in reactivity.

Model of gold particles.
Image Credit: Mickael Havel and Alia Sabur, Drexel University

Particle size can also affect the color of the material, with applications in the production of inks, for example for glossy magazines where color schemes are of great interest. Large surface areas are also attractive for a process called adsorption. Adsorption occurs when a gas or liquid solution accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms. Large surface areas increase adsorption. This process is useful in applications such as catalytic converters, water purification devices, waste recovery technologies, and manufacturing of deodorants, perfumes, varnishes and adhesives.

This image illustrates how surface area increases as the size is reduced.  Image Credit: NASA Ames Center for Nanotechnology

Once nanoparticles are produced, it is desirable that they maintain their small dimensions. However, in some cases, agglomeration of small particles into a bigger lump has been observed. When this happens, most of the advantages of using nanoparticles are lost.  Scientists are looking into preventing agglomeration - for example by using surface coatings that prevent the union of particles.