Biomedical Engineering Reference
In-Depth Information
fabricated, or created by controlled deformation of a thin metal
sheet to create a break-junction. The addition of a third electrode,
not in direct contact with the trapped molecule, acts as a tunable
gate
, because the voltage difference tunes the molecular electronic
levels as well as the occupation of these with electrons, which in
turn modulates the conductivity of the molecule at a given bias
voltage between the two contacting electrodes. Such field-effect
transistor devices are typically regarded as molecular electronics
that are strictly speaking not electrochemical (defining electro-
chemistry as the translation between electronic and ionic current)
and thus beyond our scope. However, when the junction device is
immersed in an electrolyte, a reference electrode can be used as
liquid gate
, creating a molecular electrochemical transistor. Such a
configuration can be applied to probe the redox-properties of a
single molecule.
148-156
Trapping a single molecule between two electrodes can be
conveniently and repeatedly achieved by scanning probe micros-
copy (STM and AFM) on a flat macroscopic electrode with a di-
lute sub-monolayer of adsorbed target redox molecules. For elec-
trochemical
in-situ
STM and AFM, the conducting probe tip, to-
gether with the substrate, reference and auxiliary electrodes are
immersed in electrolyte and connected to a bipotentiostat. To sup-
press diffusive background current, all but the very end of the tip
needs to be insulated, thereby creating a nanoscopic electrode with
all the fabrication and characterization consequences discussed
above. Electrochemical STM and electrochemical AFM can thus
be regarded as enhanced nanoscopic equivalents of SECM, by
which adsorbed molecules can be individually addressed, the di-
rect tunneling junction or diffusive mediator-bridged gap can be
modulated, and the surface topography can be simultaneously
probed.
Following early breakthroughs of
in-situ
STM in water
157
in
1986, EC-STM under potentiostatic control
158
in 1988, and tunnel-
ing spectroscopy
159,160
in vacuum in 1993, Tao was the first to
demonstrate resonant enhancement of the
in-situ
tunneling current
under potentiostatic control with added reference and auxiliary
electrodes.
161
By co-adsorbing Fe(III)-protoporphyrin IX and pro-
toporphyrin IX without iron on graphite, he was able to directly
verify resonant tunneling due to the redox-active Fe(III). Starting
from high potential, he observed an increasing apparent height of
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