Chemical and Biological Sensing with Carbon Nanotubes in Solution

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Carbon nanotubes---individual rolled-up graphene sheets---have emerged as exciting tools for probing the biomolecular world. With diameters of about a nanometer, they are roughly the same size as DNA molecules or cell membranes. Nanotubes can be either metallic or semiconducting, and the electronic properties of either type rival the best materials known. The extreme sensitivity of semiconducting nanotubes to their environment, coupled with their small size and ability to operate in a variety of electrolyte solutions, gives us a versatile probe for studying biochemical systems. Although nanotubes have previously been used to electrically detect a variety of molecules and proteins in solution, the mechanisms behind this detection are not always well understood. In this thesis, we have endeavored to improve our understanding of the nanotube interaction with a variety of analytes in solution. We present experiments exploring the nanotube response to redox-active transition metal complexes, DNA molecules, charged microspheres, and living cells. In our experiments with redox-active complexes, we find that the nanotube is highly sensitive to the oxidation states of the molecules. We also show that this response is not related to the interaction of the molecules with the nanotube; rather, the nanotube acts as a tiny reference electrode and measures the changing electrostatic potential of the solution, which changes due to the properties of the molecules. This new result has important implications for the interpretation of other nanotube sensing experiments, and could also lead to novel nanoscale electrochemistry experiments. By studying the nanotube response to local electrostatic gating by DNA, microspheres, and cells, we discover that the proximity of the nanotube to the analyte is of critical importance to prevent changes in the electric field from being screened by ions in the solution. Because of this effect, we are unable to observe a consistent signal from the DNA or microspheres, but we explore possibilities for better immobilizing small objects near a nanotube device. In our experiments with living cells, we see that placing these cells on suspended nanotubes does cause a large electrical response. We discuss attempts to understand the origin of this signal, as well as future directions for this work.
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carbon nanotubes; sensing
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dissertation or thesis
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