Lastly, we investigated multiplexed detection of different
viruses at the single particle level by modifying nanowire device
surfaces in an array with antibody receptors specific either for
influenza A (nanowire 1) or adenovirus (nanowire 2). Simultaneous
conductance measurements obtained when adenovirus,
influenza A, and a mixture of both viruses are delivered to the
devices (Fig. 5C) show several key points. Introduction of
adenovirus, which is negatively charged at the pH of the experiment
(18), to the device array yields positive conductance
changes for nanowire 2 with an on time of 16 6 s, similar to
the selective bindingunbinding in Fig. 3 for a comparable
density of surface receptors. The magnitude of the conductance
change for binding of single adenovirus particles differs from
that of influenza A viruses because of differences in the surface
charge densities for the two viruses. Shorter, 0.4-s duration
positive conductance changes are also observed for nanowire 1.
These changes are characteristic of a charged virus diffusing past
and rapidly sampling the nanowire element (see above) and can
be readily distinguished from specific binding to the antibody
receptors. On the other hand, addition of influenza A yields
negative conductance changes for nanowire 1, with a binding
unbinding behavior similar to that in Fig. 3 under comparable
conditions. Nanowire 2 also exhibits short duration negative
conductance changes, which likely correspond to diffusion of
influenza A viral particles past the nanowire device, although
these are also readily distinguished from specific binding events
by the temporal response. Significantly, delivery of a mixture of
both viruses demonstrates unambiguously that selective binding
unbinding responses for influenza A and adenovirus can be
detected in parallel by nanowire 1 and nanowire 2, respectively,
at the single virus level.