  Molecular motor gliding motility assays based on myosin/actin or
kinesin/microtubules are of interest for nanotechnology applications ranging
from cargo-trafficking in lab-on-a-chip devices to novel biocomputation
strategies. Prototype systems are typically monitored by expensive and bulky
fluorescence microscopy systems and the development of integrated, direct
electric detection of single filaments would strongly benefit applications and
scale-up. We present estimates for the viability of such a detector by
calculating the electrostatic potential change generated at a carbon nanotube
transistor by a motile actin filament or microtubule under realistic gliding
assay conditions. We combine this with detection limits based on previous
state-of-the-art experiments using carbon nanotube transistors to detect
catalysis by a bound lysozyme molecule and melting of a bound short-strand DNA
molecule. Our results show that detection should be possible for both actin and
microtubules using existing low ionic strength buffers given good device
design, e.g., by raising the transistor slightly above the guiding channel
floor. We perform studies as a function of buffer ionic strength, height of the
transistor above the guiding channel floor, presence/absence of the casein
surface passivation layer for microtubule assays and the linear charge density
of the actin filaments/microtubules. We show that detection of microtubules is
a more likely prospect given their smaller height of travel above the surface,
higher negative charge density and the casein passivation, and may possibly be
achieved with the nanoscale transistor sitting directly on the guiding channel
floor.
