Reference Section
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77
Chapter 5
Electrochemistry
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Using the MultiClamp 700A for electrochemistry.
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See also electrochemistry application notes under ‘Technical Support’ at
http://
www.moldev.com/support
Electrochemistry, with the meaning intended here, is the use of an electrochemical
sensor to record signals that reflect the presence of electro-active chemicals in
biological tissue. For biological applications, the sensor is typically a carbon-fiber
microelectrode. Examples of electro-active biological chemicals are dopamine and
norepinephrine. The MultiClamp 700A, like the Axopatch 200B, can be used to
measure the electrical signals generated by the presence of these chemicals.
To make electrochemical measurements, a voltage is typically applied to the sensor.
This results in the oxidation or reduction of the electro-active species in solution near
the tip of the sensor. The current that is derived from the measurement is a complex
combination of chemical kinetics and molecular diffusion that is relatively specific for
different chemical classes of compounds. In short, the technique generates a chemical
fingerprint for each compound of interest. Furthermore, the current that is derived
from the oxidation (or reduction) of these compounds is directly proportional to the
concentration.
Two methods are used for making electrochemical measurements, cyclic voltammetry
and amperometry.
Cyclic voltammetry typically involves applying an episodic voltage ramp to the sensor
while the resultant current is measured under voltage clamp. The potential at which
dopamine (and other catechol-containing species such as epinephrine and
norepinephrine) oxidizes is approximately 0.15 V
cf.
the silver/silver chloride
reference potential. In order to accurately measure the voltammetric response of
dopamine in solution, the sensor is poised at a reducing potential between
measurements and ramped to more oxidizing potentials to generate the electrochemical
fingerprint. In a typical experiment, the ramp may last 100 ms; this, then, is the
resolution of the measurement. Cyclic voltammetry is most often used to make
relatively slow, volume-averaged measurements of the concentrations of electro-active
compounds.
