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Abstract
Biosensors utilizing carbon nanotube field-effect transistors have a tremendous potential
to serve as the basis for the next generation of diagnostic systems. While nanotubes
have been employed in the fabrication of multiple sensors, little attention has previously
been paid to how the nanotube density affects the biosensor performance. We conducted
a systematic study of the effect of density on the performance of nanotube biosensors
and discovered that this parameter is crucial to achieving consistently high performance.
We found that devices with lower density offer higher sensitivity in terms of both
detection limit and magnitude of response. The low density nanotube devices resulted
in a detection limit of 1 pM in an electrolyte buffer containing high levels of electrolytes
(ionic concentration ∼140 mM, matching the ionic strength of serum and plasma). Further
investigation suggested that the enhanced sensitivity arises from the semiconductor-like
behavior-strong gate dependence and lower capacitance-of the nanotube network at low
density. Finally, we used the density-optimized nanotube biosensors to detect the
nucleocapsid (N) protein of the SARS virus and demonstrated improved detection limits
under physiological conditions. Our results show that it is critical to carefully
tune the nanotube density in order to fabricate sensitive and reliable devices.