Chiral Sorting of Carbon Nanotubes Using Tripeptides

There are many potential applications of carbon nanotubes (CNTs) in flexible electronics, biosensors, fuel storage materials and charge storage devices. CNTs can be categorized as either metallic or semiconducting depending on their chirality, which in turn is influenced by the electronic properties based on the delocalization of the fourth electron in the sp2 hybridized orbital. Before using CNTs in potential large-scale applications, it is required that they be sorted out and separated based on their chiral vectors into metallic or semiconducting to be used for their respective purposes in electronics and bioengineering. Other methods such as electrophoresis, ultracentrifugation and chromatography have been used in the past to separate CNTs, but have been found to be expensive and lacking efficiency. We propose a study with nine different tripeptides to sort through a mixture of multi-walled (50-90 nm diameter) and single-walled (0.78 average dimeter) CNTs. We analyzed the interactions of the tripeptides having glycine as the repeated amino acid at the center of each tripeptide and the positive, negative, polar and non-polar flanking residues. It is found that tripeptides with threonine as one of the flanking residues have a high affinity for metallic CNTs, whereas those with an uncharged polar group or negatively charged group have a high affinity for semiconducting CNTs. Based on our prior simulation data on the different scenarios with different tripeptides and CNTs, we understood the initial interactions between the tripeptides and different chirality CNTs. However, this has never been experimented in a laboratory before. In order to analyze our results, we used a UV/vis spectrophotometer and circular dichroism spectroscopy to show the chirality and hence the nature of the sorted CNTs. In future, we plan on implementing the sorted metallic multi-walled and single-walled CNTs to fabricate a scaffold that can be electrically stimulated for bioengineering applications of cell culture and communication. The importance of having CNTs as the substrate is to be able to accurately measure the electrical stimuli of cells. CNTs are more suitable due to their sensitivity, greater surface area to volume ratio, and excellent electrical conductivity allowing for a wider range of applications and fabrication from electrodes for neural recording to substrates for stimulating biological cells.