This paper introduces a new method to produce metal-coated nanowires using self-assembled bolamphiphile nanotubes as templates. Assembled structures of bis(N-
a-amido-glycylglycine)-1,7-heptane dicarboxylate, one of the bolaamphiphiles, display sensitivity to pH and the heptane bolaamphiphile grows to a crystalline tubule in an acidic solution (pH 6). Electroless Ni and Cu coatings on the bolamphiphile nanotubes were achieved in Ni and Cu baths with reducing agents. An interesting characteristic of bolaamphiphile nanotubes is that their non-hydrogen bonded amide groups can intercalate metal ions. These non-hydrogen bonded amide groups capture metal ions such as Pt, Pd, Cu, Co and Ni due to a stable four coordinate-planar coordinate formation as zwitterions between the amide groups and result in stable metallic coatings probed by x-ray and Raman studies. The difficulty in organic nanotube metallization is to create organic-inorganic junctions on the nanotube surfaces. But the heptane dicarboxylate nanotubes can overcome this difficulty through the intercalating of non-hydrogen bonded amide sites.
We also have demonstrated that this nanotube can be grown on Au surfaces via 4-mercaptobenzoic acid self-assembly monolayer (SAM). The SAM, with thiol groups at one end and carboxylic acid groups at the other end, serves as a junction between Au and the nanotubes because the thiol groups are attached firmly onto the Au surface while the carboxylic acid groups bind the carboxylic acid groups of the nanotubes via acid-acid dimer hydrogen bonds. The nanotubes did not grow on bare gold and self-assembly monolayer of alkyl-thiol / benzene-thiol, which can not bind the nanotubes via hydrogen bonds. These control experiments support the proposed scheme that the carboxylic acid groups of the SAM involve the connection between the SAM and nanotubes via hydrogen bonds. Although functional groups of the nanotube that bind the carboxylic acid groups of the SAM were not identified yet, molecular modeling suggests the carboxylic acid groups of nanotubes are more likely.
Therefore, the bolaamphiphile nanotubes have great potential to serve as excellent connections for nano-electrical circuits with metallic coatings. In addition to the metal coatings, we also succeed to coat the nanotubes with Au and Ag nanocrystals to fabricate nanocrystal wires. These nanocrystals were stabllized by carboxylic acid thiol. Control experiments showed that this complex formation is driven by hydrogen bonds between carboxylic acid groups of the carboxylic acid thiol and the amide groups of the nanotubes via hydrogen bonds. The size of the deposited nanocrystals may affect the electrical properties of nanotubes coated by the nanocrystals. Possible application to nano-cuvettes for surface enhanced Raman spectroscopy was also examined.
University of Central Florida, Department of Chemistry
Orlando, FL 32816 USA