Applications of Small Molecule-Carbon Nanotube Interactions
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The interactions of single-walled carbon nanotubes (SWNTs) with small molecules are critical to harnessing their remarkable electronic properties. Due to quenching effects, properties such as near infrared fluorescence are only seen in individually dispersed SWNTs. Individually dispersing SWNTs in aqueous solution using surfactants such as sodium dodecylbenzene sulfate has allowed SWNTs to be studied spectroscopically but only at low concentration. I demonstrate that by combining small molecules additives such as sucrose, trehalose, and glycerine into the surfactant dispersion process, the efficiency of the process is greatly increased. Utilizing the additive sucrose allows for the production of individually dispersed SWNT solutions at a concentration 100 times greater (up to 3.3 g/L) than the highest concentration reported previously. Spectroscopic studies suggest the small-molecule additives do not interact electronically with the surfactant-encapsulated SWNTs in solution but instead increase
the solution's viscosity, slowing down Brownian motion of molecules in solution. In solution, SWNTs move slower than surfactant molecules due to the large size difference between the two molecules. After sonication induced cavitation breaks apart a SWNT bundle, the rebundling process is slowed allowing the surfactant molecules more time to stabilize the individualized nanotubes. When dried, the nanotubes from these solutions retain their near-infrared fluorescence, indicating that the nanotubes do not become highly bundled upon solvent evaporation. While the small-molecules do not induce spectral shift in the SWNT solution spectra, in dried films these additives cause the fluorescence of individually dispersed SWNTs to red-shift by nearly 30 nm.
Wax based inkjet printing is a popular method to create paper microfluidic devices. The wax is first printed on paper and then reflowed. During reflow, the paper is heated to the wax's melting point causing the wax to wick into the paper and create a hydrophobic barrier. Wax-reflowing is an inexpensive method to create paper microfluidic devices but gives poor resolution due to a high wicking rate in the lateral direction of the paper. To demonstrate how small-molecules such as trehalose can interact with structures other than SWNTs, I studied the transport kinetics of molten wax through paper. When paper is saturated with trehalose, the wax diffuses more slowly in the lateral direction due to blockage of the transport mechanism through cellulose fibers. Transport through larger fiber-created pores is still available but is omnidirectional due to the structure of paper. This mechanism was used to create high-resolution paper microfluidic devices.