Abstract:
Easily fabricated random network carbon nanotube field-effect transistors (CNT-FETs) have
benefitted from improved separation techniques to deliver CNTs with current formulations
providing at least 99% semiconducting tube content. Amongst the most promising
applications of this device platform are electronic biosensors, where the network conduction
is affected through tethered probes such as aptamers which act as molecular scale
electrostatic gates. However, the prevailing assumption that these biosensor devices would be
optimized if metallic tubes were entirely eliminated has not been examined. Here, we show
that metallic-semiconducting junctions in aptasensors are sensing hotspots and that their
impact on sensing is heightened by the CNT network’s proximity to percolation. First, we use
a biased conducting AFM tip to gate a CNT-FET at the nanoscale and demonstrate that the
strongest device response occurs when gating at metallic-semiconducting junctions. Second,
we resolve the target sensitivity of an aptasensor as a function of tube density and show
heightened sensitivity at densities close to the percolation threshold. We find the strongest
sensing response where the 1% of metallic tubes generate a high density of metallic semiconducting junctions but cannot form a percolated metallic path across the network.
These findings highlight the critical role of metallic tubes in CNT-FET biosensor devices and
demonstrate that network composition is an important variable to boost the performance of
electronic biosensors.
