Biomedical Engineering Reference
In-Depth Information
a paradox: when decreasing the tip size to nanoscopic dimensions
(e.g., by using a conductive AFM tip) the finite interfacial reduc-
tion/oxidation rate of the shuttling mediator will not be able to
outcompete the lateral diffusion rate out of the gap, thus leading to
loss of resolution and sensitivity. Their solution is to tether the
mediator (ferrocene) via a short PEG polymer chain to a conduct-
ing AFM tip, such that the mediator will provide diffusive feed-
back but cannot escape. Tip-attached redox mediator (TARM)
AFM-SECM simultaneously provides high resolution topography
and current images in tapping mode. Because the lateral resolution
is governed by the length of the tether and not by the tip size and
lateral diffusion as in classical SECM, the local electrochemical
activity of the surface can be probed at a resolution comparable
with the AFM topographical image.
The methods discussed above strive to detect single redox
molecules by way of enhancing the number of passed electrons
across a junction (i.e., without relaxation to electrochemical equi-
librium), by coupling single-electron transfer reactions to multiple
photons via subsequent excitation/emission, or rapidly shuttling
electrons via a diffusing molecule. An alternative method is to
utilize the intrinsic chemical amplification provided by catalysis.
The large (10-100 pA) electrocatalytic water splitting or formation
current generated by a Pt or Pd particle is relatively easy to detect
above the background. When such particles are permanently ad-
sorbed to an electrode surface with a much lower catalytic activity,
the catalytic nanoparticle generates a localized diffusive reaction
layer in which the electrolyte composition (pH, concentrations of
hydrogen and oxygen) is distinctly altered. This can be detected
with SECM, or at higher resolution—with electrochemical STM.
An added advantage of STM is that a small part of the scanning tip
can be deposited on the surface to create a single catalytically ac-
tive nanoscopic feature, the shape and size of which can be instan-
taneously characterized. Potentiometric and amperometric electro-
chemical STM methods to detect the reaction layer around such
deposited catalytic nanoparticles have been pioneered by Stim-
ming and co-workers.
167, 168
Even more challenging is the application of enzyme molecules
as nanoscopic catalytic particles. Although the dimensions are
similar to metallic nanoparticles, enzyme molecules harbor only
one or few catalytic sites, while metallic nanoparticles support a
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