Density gradient ultracentrifugation and stability of SWNT–peptide conjugates

Density gradient ultracentrifugation was used for preparation of biocompatible single-wall carbon nanotube (SWNT)–peptide conjugates. Aggregate size selected SWNT suspensions were analyzed by absorption and photoluminescence spectroscopy to determine stability and structure of conjugates. The results suggest that conjugates with densities over 1.19 g cm - 3 consist of SWNT aggregates. More buoyant fractions with ρ ≈ 1.18 g cm - 3 contain mostly individual SWNTs. Dilution experiments indicate that equilibration between SWNT-bound and free peptide in solution is approached within a few minutes making excess peptide in solution necessary to prevent peptide–SWNT conjugate aggregation. This has significant implications for the use of SWNT–peptide conjugates in biochemical applications.