Biomedical Engineering Reference
In-Depth Information
signals of amplitude 0.5-1 pA
32
that corresponded to the redox
cycling of one or few FcTMA
+
molecules. Slow fluctuations in
current and the shape of responses were also seen. In the absence
of any clear picture of the exact cell geometry, it was proposed that
the wax shroud had cracked in places leading to microscopic crev-
ices in which the molecules could get trapped and hence not con-
tribute to the current until they egressed back into the cell. Another
possible cause was identified as being the slow drift in the tip-
substrate spacing due to temperature fluctuations.
An approach similar to the above was used by Sun and
Mirkin
121
where they prepared a glass encased disc-type platinum
electrode (5 to 150 nm radius) which was etched to create a re-
cessed zeptoliter-sized cavity. This cavity was filled with an aque-
ous solution containing redox species and the etched electrode was
dipped in a pool of mercury (Hg) to create a TLC geometry. With
redox cycling, steady-state voltammograms of the trapped mole-
cules were obtained which were assigned to a few and even single
molecules. By repeating the experiments, different steady-state
current values were obtained, which was ascribed to different
number of molecules being trapped. It remains unclear to what
extent the geometry of the device varied between experiments.
Since the volume of the TLC was closed off, there was no way of
observing statistical fluctuations in the TLC cavity.
In both the approaches mentioned above, the measured signal
is a consequence of multiple electron-transfer events as the mole-
cule redox cycles between the two electrodes. The measured cur-
rent thus results from a large number of electron transfer events,
and the advantage of probing an individual molecule as opposed to
an ensemble measurement is thus lost. This situation can be partly
ameliorated by employing a detection configuration that is a hy-
brid of electrochemical and optical methods, as discussed in Sec-
tion IV.1, although this invariably requires the immobilization of
target molecules.
Another ingenious strategy to detect single nanoparticles re-
lies on amplification through electrocatalytic reactions that occur
only on the metal nanoparticle in question but not on the underly-
ing electrode.
170
Ordinarily, the nanoparticle charging events
would transfer only one or a few electrons, yielding a current that
would be essentially indistinguishable from the background noise.
However, if there is a species present in solution whose reduction
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