In addition, parallel electrical and optical experiments carried
out on devices with a low density of specific antibodies (Fig. 4B)
show discrete conductance changes caused by the interaction of
single viruses with an average duration, 1.1 0.3 s. This average
on time is 20 times shorter than observed in Fig. 3, where an
intermediate antibody density was used. Careful analysis shows
that some of the conductance changes (e.g., event 2) have a time
scale of 1–1.5 s, although other events have a time scale of 0.4
0.1 s that is characteristic of diffusion of the virus past andor
rapid touching of the nanowire surface (see below). Interestingly,
further analysis of event 2 (Movie 3, which is published as
supporting information on the PNAS web site) shows that the
virus rapidly samples two nearby positions on the nanowire
surface before unbinding. Overall, these coverage-dependent
detection data are consistent with selective detection and the
ability to vary unbinding kinetics with the density of specific