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Platinum nanoparticles are usually in the form of a suspension or colloid of submicrometre-size particles of platinum in a fluid, usually water. A colloid is technically defined as a stable dispersion of particles in a fluid medium (liquid or gas).
Platinum nanoparticles can be made with sizes between about 2 and 20 nanometres (nm), depending on reaction conditions. Trillions of platinum nanoparticles[clarification needed] are suspended in the colloidal solution of brownish-red or black color. Nanoparticles come in wide variety of shapes including spheres, rods, cubes,  and tetrahedra.
Due to the antioxidant properties of the platinum nanoparticles, they are the subject of substantial research, with potential applications in a wide variety of areas, including nanotechnology, catalysis, medicine, and the synthesis of novel materials with unique properties.
Platinum nanoparticles are made by reduction of hexachloroplatinate. A solution is rapidly stirred while a reducing agent such as hydrogen gas is added, causing platinum ions to be reduced to neutral platinum atoms. As more and more of these platinum atoms form, the solution quickly becomes supersaturated and platinum starts to precipitate in the form of sub-nanometre particles. As the reaction proceeds, platinum atoms are added to these nuclei, which grow in size. If the solution is stirred vigorously enough, the particles will be fairly uniform in size and shape. To prevent the particles from aggregating, some sort of stabilizing agent or stabilizer that sticks to the nanoparticle surface is usually added, such as sodium polyacrylate. They can be functionalized with various organic ligands to create organic-inorganic hybrid materials with advanced functionality.
Nanoparticles may present possible safety issues both medically and environmentally. Most of these issues usually arise due to the high surface-to-volume ratio, which can make the particles of some metals very reactive or catalytic. In particular, inhaled nanoparticles can pose health risks, and may cause inflammation and disease in the lung. They are able to pass through cell membranes in organisms and their interactions with biological systems are relatively unknown.
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