As project leader for the Particles, Tubes and Colloids project at NIST I seek to develop metrologies to characterize the properties, both fundamental and distribution, of dispersed nanoparticles, polymers, and colloids using solution phase methods. These metrologies include both advanced methods for using existing characterization methods, and development of new methods for previously unmet measurement needs.
A major focus of my project are single-wall carbon nanotube (SWCNT) populations. Both as a pristine material, and more commonly as dispersions in a liquid media, SWCNTs are an exciting nanomaterial class with significant potential for use in a wide range of applications. A long standing factor hindering development of SWCNTs for most applications has been a general uncertainty in the fundamental properties and metrics of these materials. In general, even defining the purity of a SWCNT sample has been a substantial challenge, and one that has delayed more rapid development of commerce and applications. A significant factor in this challenge is that SWCNTs are actually a class of materials, comprised of many different chemical species, with each species defined by the exact manner in which the hexagonal carbon lattice comprising the SWCNT is (euphemistically) rolled up to generate the tube structure. Each SWCNT species, like any other chemical, has its own unique diameter, chemical and physical properties. Depending on the species, SWCNTs can be metallic, semi-metallic or semi-conducting, and can have optical band gaps in different parts of the visible and infrared spectrum.
In my project, we pursue an iterative strategy based on separation of pure(r) populations to develop the basis of measurement science for SWCNTs and to measure fundamental nanotube properties. To produce purified fractions we use and develop dispersed-phase separation techniques (i.e., on nanotubes individualized in liquids) taken or extrapolated from the biosciences, colloids, and polymers fields. We have developed techniques in chromatographic, field-flow based, ultracentrifugation based, and utilizing multiphase separating polymer solutions to accomplish a range of technical capabilities. These include the ability to separate SWCNTs by their species to the single species level, by length with high resolution, and even by enantiomeric handedness (i.e., whether the lattice is rolled up with a left or right-handed twist). By producing better enriched samples with resolved parameters, we enable better (and cleaner) measurements of the fundamental properties of the SWCNTs, which in turn can drive improvement in the purification/separation science. Additionally, these improvements enable us to develop meaningful documentary standards by which SWCNT properties should be measured, and to produce reference materials with certified property values.
A key characterization method I use is analytical ultracentrifugation (AUC). With this technique it is possible to measure size distributions, shape distributions, binding interactions, and materials properties such as effective particle densities, all with the particles in their native environment. Recent efforts in AUC have demonstrated the power of small changes in the chemical structure of dispersant molecules adsorbed at the nanotube interface to make significant differences to the amount of molecules that adsorb, which then governs how the nanotubes interact with their environment. By careful experimentation, we can link even the effects of single-point structural modifications to the dispersing molecules to the resulting properties of the nanoparticle-dispersant complex.
Other recent efforts include characterization and controlled preparation of chemicals ingested into the interior, "endohedral," volume of SWCNTs. By controlling what is inside the nanotube, we can affect the optical and physical properties of the nanotubes in useful ways. This work builds upon our separation science strengths and builds upon our long standing interest in differences between empty and water-filled nanotubes.
Potential Post Doc Project Areas
1) Protein and Surfactant Corona Measurement on Dispersed Carbon Nanotubes
The shell of protein or surfactant molecules that surround a dispersed nanotube, whether in biological fluids such as serum, or non-biological dispersion dominates many of the interactions of the nanotube with its environment. Measuring the structure and nature of the dispersing molecules will allow for predictions of properties such as the zeta potential, and will provide important information for addressing the effects and potential accumulation sites of dispersed nanotubes in the human body and in the environment. Using nanotube samples with known length, purity and chirality we can measure the structure and nature of the surrounding shell of adsorbed molecules and their effects on useful properties.
2) Purified Nanotubes for Intrinsic Property Measurement
Although we have made significant advances in separation of specific SWCNT species, continued advancement of our separation technologies is still desirable. In particular we are advancing aqueous two-polymer phase extraction (ATPE) to enable extraction of yet larger diameter SWCNT species and each specific enantiomer of previously extracted populations. Using these samples we can advance characterization of fundamental optical process in nanomaterials and enable applications requiring unique materials properties.
3) Characterization of Complex Colloids via Analytical Ultracentrifugation
Processing of nanoparticles and colloids in the dispersed phase requires measurement methods that can adequately describe distributions of particles in complex fluid environments. Possible ideas for projects include characterization of biomolecules and colloids in deep eutectic solvents, characterization of extended and/or structured polymers in organic solvents, characterization of interfacial coatings and interparticle interactions between single-wall carbon nanotubes in complex environments such as depleting polymers, and development of high temperature instrument capabilities.