Individual single-walled carbon nanotubes (SWNTs) in aqueous suspension are visualized directly by fluorescence video microscopy. The fluorescent tagging is simple, biocompatible, and does not modify the SWNTs. The dynamics of individual SWNTs in water are observed and quantified for the first time. We measure the confined rotational diffusion coefficient and find it in reasonable agreement with predictions based on confined diffusion of dilute Brownian rods. We determine the critical concentration at which SWNTs in suspensions start interacting. By analyzing the fluctuating shape of SWNTs in the 3 to $5\mu \mathrm{m}$ range, we determine that their persistence length ranges between 32 and $174\mu \mathrm{m}$, in agreement with theoretical estimates; thus, commonly available SWNTs in liquids can be considered as rigid Brownian rods in the absence of imposed external fields or self-attractive forces.

• Received 13 December 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.246104