Analytical Ultracentrifugation of Carbon Nanotubes

  Sedimentation velocity (SV) experiments on dispersed nanoparticles are a powerful method for determining otherwise hard to evaluate properties of the nanoparticle such as the density and size of the adsorbed interfacial layer, or for a direct evaluation of the entire distribution of a property such as particle size. The ability to measure these properties in situ in either aqueous or non-aqueous media, and via multiple detection techniques, provides information not readily available through other methods such as electron microscopy. My research has focused in particular on utilizing the capabilities of sedimentation velocity methods for the characterization of single-wall carbon nanotubes (SWCNTs).

  Single-wall carbon nanotubes are a family of chemical species comprised entirely of a “rolled up” single layer hexagonal lattice of carbon. Depending on the diameter of the formed cylinder, and the vector of the hexagonal lattice relative to the tube axis, these nanoparticles can be metallic, semi-metallic or semi-conducting, each having unique optical, mechanical, thermal, and electrical properties. Many of the applications for this set of materials however depend on the properties of the material when dispersed in the liquid phase, or contain liquid phase processing steps. The key aspects of liquid phase dispersion and processing though hinge on controlling the properties of the interfacial layer between the nanotube and the solution to enable dispersion, selective fractionation and/or biocompatibility. Density contrast variation methods combined with sedimentation velocity experiments provide a high resolution methodology for evaluating this interface. I will present results detailing the use of such density contrast techniques on the characterization of the interfacial layer on specific SWCNT species that demonstrate the power of analytical ultracentrifugation to resolve this interfacial layer.

  Separately, a second critical factor for SWCNT use in dispersion form is the size distribution of the dispersed population; this distribution strongly affects the transport of the nanotubes in many environments as well as the achievable properties in end applications, and is thus a necessary parameter for quality control. Measuring the whole size distribution of a nanoparticle is however typically very challenging for non-monodisperse particles. In a second set of results, I will show that the distribution of sedimentation coefficients for SWCNTs can be used to calculate through hydrodynamic theory the size distribution of the population in the dispersion in quantitative agreement with other techniques such as atomic force microscopy (AFM).
What you will learn:

  Methodology for the characterization of an important class of nanomaterials via SV, particularly involving variations of density contrast experiments to elucidate different interfacial parameters.
Discussion of concerns, details, assumptions, and problems in utilizing SV for nanoparticles, including some considerations for overcoming these issues through experimental methodology.

Learning Objectives:

• Understand how sedimentation velocity (SV) experiments on dispersed nanoparticles are a powerful method for determining otherwise hard to evaluate properties of the nanoparticle such as the density and size of the adsorbed interfacial layer
• Understand how to utilize the capabilities of sedimentation velocity methods for the characterization of single-wall carbon nanotubes (SWCNTs)