  Single-molecule detection at a nanometric interface in a femtomolar solution,
can take weeks as the encounter rate between the diffusing molecule to be
detected and the transducing nano-device is negligibly small. On the other
hand, several experiments prove that macroscopic label-free sensors based on
field-effect-transistors (FET), engaging micrometric or millimetric detecting
interfaces are capable to assay a single-molecule in a large volume within few
minutes. The present work demonstrates why at least a single molecule out of a
few diffusing in a 100 ul volume has a very high probability to hit a large
capturing and detecting electronic interface. To this end, sensing data,
measured with an electrolyte-gated FET whose gate is functionalized with 1012
capturing anti-immunoglobulin G, are here provided along with a Brownian
diffusion-based modelling. The EG-FET assays solutions down to some tens of zM
in concentrations with volumes ranging from 25 ul to 1 ml in which the
functionalized gates are incubated for times ranging from 30 s to 20 min.
