TECHNOLOGY FOCUS - BIOLOGY - Preferred Components: The biological moiety associates with a side wall of the nanotube. The biological moiety is selected from a nucleic acid, an oligonucleotide, an amino acid, an enzyme, a protein, a chemically derivatized nucleic acid, a chemically derivatized oligonucleotide, a chemically derivatized amino acid, a chemically derivatized enzyme, a chemically derivatized protein, or their segment (preferably oligonucleotide or a chemically derivatized oligonucleotide, especially single-stranded DNA or their derivatives). The biological moiety is crosslinked by the metallic moiety. The metallic moiety is a DNA crosslinker.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The metallic moiety comprises a catalytic metal (preferably platinum). The metallic moiety is a metal salt or a metal complex. The metallic moiety and the biological moiety forms an adduct. The metallic moiety is a metal nanoparticle.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - The method for functionalizing a nanotube involves contacting a mixed population of nanotubes with at least one surface active component in a solvent to provide a dispersion, where the mixed population of nanotubes comprise a range of at least one of chirality, nanotube diameter, and electronic type; the surface active components debundle the nanotubes and associate with individual nanotubes; and individual nanotubes differing by at least one of chirality, nanotube diameter, and electronic type when associated with the binding moieties have different buoyant densities in the solvent; introducing the dispersion into a fluid medium comprising a density gradient; agitating the fluid medium to separate along the density gradient the nanotubes by at least one of chirality, nanotube diameter, and electronic type; allowing nanotubes differing by at least one of chirality, nanotube diameter, and electronic type to sediment into several separable fractions; collecting a separation fraction from the separable fractions; contacting the metallic moiety with a nanotube in the separation fraction; associating the biological moiety with a sidewall of the nanotube; and reducing the metallic moiety to provide a metal nanoparticle; or reducing the metallic moiety to provide a metallic nanoparticle bound to the biological moiety. Preferred Components: The nanotube is a single-walled carbon nanotube or a multi-walled carbon nanotube. EXAMPLE - A freshly prepared solution of potassium tetrachloroplatinate (10 mM) was mixed with DNA-encapsulated single-walled carbon nanotubes (SWNTs) to obtain platinum (Pt) concentration of 1 mM and SWNT loading of 5.8 mg/l to give a composite.