(329b) Unique Thermodynamic Co-Surfactant Equilibria of Single Walled Carbon Nanotubes for Fluorescent Biosensors

Single walled carbon nanotubes (SWCNTs) have been extensively used for sensing as their unique optical properties are extremely sensitive to interactions with analytes. The ability to selectively modulate optical spectra in response to different biomolecules has been key in development of SWCNT-based optical biosensors. Semiconducting SWCNTs are a mixture of different chiralities and separation of monochiral species has been a major hurdle limiting their use to fluorescent sensors. Various surfactants have been used to form stable suspensions of SWCNTs in different solvents. Surfactant structure around the nanotube plays an important role in stability and separation of (n,m) species. The presence of thermodynamically stable co-surfactants states enables selective coating of species in solution. Our group has previously utilized co-surfactant solution approach to elute (6,5) species at a very specific ratio of sodium dodecyl sulphate (SDS)/sodium deoxycholate (DOC). This idea can be further extended to selectively target other chiralities and coat them in the nanotube solution.

Here, we utilize this thermodynamic equilibrium for selectively coating a mixture of semiconducting SWCNT chiralities such that we can achieve sensing on specific chiralities without separating them into monochiral fractions. As we add SWCNTs with different co-surfactants solutions, the reorganization of the surfactant shell around nanotubes results into solvatochromatic shifts which can be observed with optical spectroscopy. Difference in surfactant binding strength can be used to protect certain chiralities against further functionalization using addition of controlled ratios of co-surfactant solutions to nanotubes. Unprotected chiralities in solution were functionalized with different ligands depending upon the analyte biomolecule to be detected. We investigated different experimental approaches to chirality specific functionalization as well as the role of covalent and non-covalent functionalization on the nanotube surface to achieve the best biosensing response. Protected chiralities provide an in-situ reference which helps in quantifying analyte based on quenching of functionalized chiralities. This has led to development of a sensitive fluorescent SWCNT-based biosensor whose response can be tuned with the analyte concentration.