Abstract
The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) make them good candidates for high sensitivity biosensors. When a charged molecule binds to such a sensor surface, it alters the carrier density in the sensor, resulting in changes in its DC conductance. However, in an ionic solution a charged surface also attracts counter-ions from the solution, forming an electrical double layer (EDL). This EDL effectively screens off the charge, and in physiologically relevant conditions ~100 millimolar (mM), the characteristic charge screening length (Debye length) is less than a nanometer (nm). Thus, in high ionic strength solutions, charge based (DC) detection is fundamentally impeded. We overcome charge screening effects by detecting molecular dipoles rather than charges at high frequency, by operating carbon nanotube field effect transistors as high frequency mixers. At high frequencies, the AC drive force can no longer overcome the solution drag and the ions in solution do not have sufficient time to form the EDL. Further, frequency mixing technique allows us to operate at frequencies high enough to overcome ionic screening, and yet detect the sensing signals at lower frequencies. Also, the high transconductance of SWNT transistors provides an internal gain for the sensing signal, which obviates the need for external signal amplifier. Here, we describe the protocol to (a) fabricate SWNT transistors, (b) functionalize biomolecules to the nanotube, (c) design and stamp a poly-dimethylsiloxane (PDMS) micro-fluidic chamber onto the device, and (d) carry out high frequency sensing in different ionic strength solutions.