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bioelectrochemistry 2 - Coggle Diagram
bioelectrochemistry 2
FARADIC PROCESS
voltammetry
the working electrode potential is forced to follow a known potential-time function. So current vs voltage curve is registered
the electrochemical cell contain:
- working electrode (< area than 3): those at which the reaction take place
- reference electrode: maintain a constant potential irrespective of changes in current
- counter electrode: allow current to flow through the electrolyte but whose characteristics do
not influence the measured behavior
- solution with redox species
potential is applied between electrode 1 and 2, while current is measured between 1 and 3
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oxidation or reduction process cause a faradic current to flow in the electrochemical cell when a voltage is applied between the working and counter electrodes
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microelectrode
dimension comparable to or smaller than the diffusion layer thickness δ.
They are usually made of carbon fiber or noble metals (Pt), sealed inside a glass container
advantages:
- measures very small currents up to 10^-12 A
- the ohmic drop of potential is decreased
- capacitive currents are decreased
- scan of the applied potentil very rapidly
- higher velocity of the mass transport
- high signal-to-noise ratio
Microelectrode experiments are almost carried out in a still solution where diffusion is the only form of mass transport
Current density of the electrode:
1 term:
relative only in the firt part of the experiment, when the diffurion layer thickness remains thin compared to the dimensios of the microdisc. So linear diffusion is the mode of transport.
2 term:
results from hemispheric diffusion, which is predominant at long times and becomes the only form of transport once steady state is reached. Thus at steady state the current density is described only by the second term.
or i=AnFDCA in case the electrode geometry is different than a microdisk
exocytosis: intracellular vesicles fuse with the plasma membrane and the vesicle contents are released
electrochemical measurement of vesicular exocytosis can be carried out either on the cell apex by positioning a CF microelectrode gently against the cell surface. Vescicle secretion is recorded as current spike
another possibility is to seeding cells on conductive microelectrode substrate. vescicle secretion is recorded as current spike.
integration of the spikes gives the total charge transferred Q and with faradays law N=Q/nF we can calculate the number of moles released
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mass transport
migration:
the movement of charged substances in an
electrical gradient due to the application of an electric field on the charges particles.
the presence of the supporting electrolite reduce the effect of the electrical field inside the solution so migration is not really important
convection:
mass transport of electro active material to the electrode by gross physical movement of the solution (mixing, stirring)
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diffusion:
natural transport or movement of a
substance under the influence of a gradient of chemical potential. So substances move from regions of high concentration to regions of low concentration in order to minimize or eliminate concentration differences
If diffusion is the only form of mass transport, before the application of the potential the concentration of the oxidant species is uniform. Once the potential is applied, the concentration of the oxidant at the electrode surface becomes less than the bulk solution due to the elctrochemical conversion of oxidant to reductant. The flux of oxidant J is given by Fick's first law
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if we combine this equation with the expression for the current I=nFAJ, we obtein an expression for the time-dependent diffusion current
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the diffusion layer will not remain at a constant thickness. Instead, as time progresses so the diffusion layer grows in the the bulk solution
Fick's second low:
describe how the flow of material
on the electrode surface changes
as a function of time
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