A method for separating carbon nanotubes comprises: providing a mixture of carbon nanotubes; introducing an organic molecule having an end group capable of being chelated by a metal ion to the mixture of carbon nanotubes to covalently bond the organic molecule to at least one of the mixture of carbon nanotubes; and introducing a metal salt to the mixture of carbon nanotubes to chelate the end group of the organic molecule with the metal ion of the metal salt; and centrifuging the mixture of carbon nanotubes to cause the separation of the carbon nanotubes based on a density differential of the carbon nanotubes.