SWCNTs and Biomolecules

Introduction

Single walled carbon nanotubes (SWCNTs) have been envisioned as one of the future nanostructured materials that will be used in a number of applications such as reinforcement of composites, electrical conducting polymer composites and electronic devices, such as biosensors. Thus the interaction between SWCNTs and biomolecules are expected to be of significant concern, not the least because humans will eventually be exposed to them in one way or another. When SWCNTs are produced in large quantities for commercial applications they grow in bundles, i.e. when grown they readily form aggregates due to strong attractive (Van der Waals) forces, which has complicated the understanding and use of their intrinsic individual character. The discovery that SWCNTs solubilise in water solutions (containing molecules that have a hydrophobic and a hydrophilic part, so called amphoteric molecules) have given rise to a large interest in the interaction between SWCNTs and amphoteric molecules (surfactants, DNAs and proteins). One complication when investigating the solubilisation of SWCNTs has been the quantitative evaluation of the solubilisation efficiency (which is a measure of the strength of the interaction). The molecular vibration of molecules is unique to each molecule and depends on the environment. Thus by probing how the vibrational fingerprint (spectra) changes with the environment one may conclude how SWCNTs debundle during solubilisation. In this project the spectra have been obtained using Raman spectroscopy, where certain spectral features of SWCNTs have been investigated with respect to acidity in the water (pH) and solubilisation (i.e. debundling). In a recent work we show that DNAs may be unusually efficient in solubilising SWCNTs, i.e. the interaction between DNAs and SWCNTs is under certain conditions very strong (Project funded by NANOMAT, NFR).

Debundling of SWCNTs investigated by Raman

SWCNTs exhibit large attractive (van der Vaals) forces between each other and form readily bundles if there is no compensating repulsive force (Fig. below show bundled SWCNTs after growth). Thus, for application point of view and for the understanding of their individual properties it is required to separate them from each other. Investigations of individual SWCNTs.

One way is to mix SWCNTs in an aqueous solution containing amphiphile molecules, which under vigorous mixing (often asisted by ultra-sonication) form individual tubes wrapped by the molecules that are electrostatically charge thus providing repulsive forces and stabilising the dispersion..Note that in order to maintain long term stability the binding between the tube and the amphiphile molecule (e.g. surfactant, DNA, peptide, etc) most be stronger than the tube-tube binding.Therefore, a large bulk of scientific work has dealt with finding efficient detergent molecules. However, since there has not been a reliable and established method to determine the debundling state of SWCNTs it has been difficult to compare in a quantitative way the debundling state of dispersions and more interesting the solubilising efficiency of different molecules.

Consequently, establishing methods to determine the debundling state has been of major importance. Therefore, co-workers and I have investigated the debundling process using Raman spectroscopy and shown that the G´-Raman band is sensitive to the debudling state. Below the G´-spectra of surfactant solubilised SWCNT dispersions sonicated using different energy densities is shown (SDS solubilised SWCNTs). As is seen the G´ band is described by two components with linewidths that decrease with increasing sonication. Recently, we have observed that single stranded DNA solubilised SWCNTs exhibit the smallest linewidth, which suggest that DNA binds strongly to SWCNTs.