Nanoelectrochemistry: Fundamentals and Applications in Biology and Medicine - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 30

Figure 5. (a) Current measured through the top and bottom
electrodes of a thin-layer cell during redox cycling. The fluc-
tuations are not due to instrumental noise, as can be seen
from the fact that the two independently measured signals are
perfectly anticorrelated. This signal instead arises from equi-
librium statistical fluctuations in the (small) number of mole-
cules present in the device at any given instant. Adapted with
permission from Ref. 122, Copyright (2009) American
Chemical Society . (b) The average current scales linearly
with concentration or, equivalently, with average number of
molecules in the device, ¢ N ². Instead, the noise amplitude
scales with ¢ N ² 1/2 , as predicted by Poisson statistics. Note that
the signal-to-noise ratio therefore decreases with decreasing
¢ N ², placing a fundamental limit on the performance of this
type of sensor. Adapted with permission from Ref. 123,
Copyright (2006) American Chemical Society.