duration are 24  1 nS and 20  4 s

duration are 24  1 nS and 20  4 s, and 20  3 nS and 15 
7 s, respectively. The similarity in responses is consistent with
good reproducibility in the nanowires electronic properties (14)
and a uniform density of antibody receptors on their surfaces,
which determine the response to and duration of binding of a
single virus, respectively. Second, an excess of free antibody
added to the viral solution (monoclonal antihemagglutinin for
influenza A was added to a standard solution containing 100
virons per
l to yield an antibody concentration of 10
g
ml)
eliminates the well defined conductance changes consistent with
blocking sites on the viruses that are recognized by the same
antibodies attached to the nanowire surfaces. Lower antibody
concentrations produced partial reduction of the nanowire
response. Third, the discrete conductance changes are uncorrelated
for the two nanowire devices in the microfluidic channel
and are thus consistent with stochastic binding events at or near
the surfaces of the respective nanowires. Fourth, concentrationdependent
measurements show that the frequency of the discrete
conductance drops is directly proportional to the number
of virus particles in solution (Fig. 3A Inset and Fig. 7, which is
published as supporting information on the PNAS web site). The
observed frequencies also agree with estimates for a diffusion
limited process (17). Lastly, little or no purification of virus
samples is required in these measurements; that is, similar results
were obtained on samples purified by simple gel filtration or
diluted directly from allantoic fluid.