The present invention provides a method of differentiating metallic carbon nanotubes from semiconducting carbon nanotubes. The method comprising providing a nanotube dispersion, wherein the nanotube dispersion comprises a plurality of carbon nanotubes, osmium tetroxide, or ruthenium tetroxide, and a solvent; and irradiating the nanotube dispersion with ultraviolet light, wherein the metallic carbon nanotubes are osmylated, or ruthenylated, thereby differentiating the metallic carbon nanotubes from the semiconducting carbon nanotubes.