Types of electrodes for cell-electrode coupling
what kind of mechanisms can they use?
Metal microelectrode arrays
–> capacitive, pseudo-capacitive and Faradaic charge injection mechanisms
Fundamental mechanisms for electrode - liquid contacts
- Faradaic stimulation
metal electrodes can be used in a combination of capacitive and Faradaic stimulation
Faradaic current increses the charge transfer from the electrode to the electrolyte
–> take care not to cause irreversible electrochemical reactions
Safe electrochemical window (SECW)
faradaic reactions may include: gas evolution, pH value changes, metal dissolution
- SECW to avoid irreversible and harmful electrochemical reactions
- substantial to charge storage capacity
Linear Sweep Voltammetry (LSV)
working principle
- fixed potential range
- scan voltage from lower limit to upper limit with certain voltage scan rate
- current response plotted as a function of voltage
factors influencing the characteristics of the recorded LSV
- rate of electron transfer reactions
- chemical reactivity of the electroactive species
- voltage scan rate
Why is there a peak in LSV?
- scan begins where no current flows.
- voltage is swept to the right (to more reductive values) –> current begins to flow
peak: at some point the diffusion layer has grown sufficiently above the electrode so that the flux of reactant to the electrode is not fast enough to satisfy that required by the Nernst equation
Influence of the scan rate (LSV)
- scan rate influences current resonse
higher scan rate –> higher currents
lower scan rate –> longer recording time –> size of diffusion layer above the elctrode surface will be different
Scan rate and peak position
position of current peak lies at same voltage only for electrode reactions with rapid electron transfer kinetics –> “reversible electron transfer reactions”
slow electron transfer processes shift the peak to the right (higher voltages)
Cyclic Voltammetry (CV) working principle
voltage is swept between two values at a fixed rate
1) forward sweep = LSV
2) backward sweep: gradually move back voltage –> converting electrolysis product (Fe++) back to reactant (Fe+++)
CV: characteristics
- voltage separation between current peaks is ideally: 95/n mV
- position of peak voltage don’t depend on voltage scan rate (reversible reactions)
- ratio of peak currents is equal to 1
- peak currents are proportional to sqrt(scan rate)
CV: non-reversible reactions
- peak current no longer function of sqrt(scan rate)
- current after backward sweep not equal to 0 anymore
CV: Oxidation and Reduction
From low to high potentials –> Oxidation (forward sweep)
From high to low potentials –> Reduction (backward sweep)
Pseudo Capacitor
can store electrical energy faradaically by electrochemical reactions at the electrode-electrolyte interface
- accomplished through electrosorption, redox reactions & intercalation processes
Supercapacitor
formed out of electric double layer capacitor
&
pseudo-capacitor (part of an electrochemical capacitor) –> higher capacitance
Charge Storage Capacity (CSC)
predictor of how much charge an electrode can inject during stimulation
Charge injection capacity
how is it defined?
better measure for the behavior of a microelectrode during stimulation
- defined as maximum charge injection density
CSC characterisation with CV
cathodic CSC of electrode: area below 0
- maximum CSC depends on electrode diameter
High pass characteristics
depend strongly on choice of OP
- pseudo-capacity of microelectrode with the virtual leak resistor of OP form high pass
General cell-contact experimental configuration
- cellular membrane has to be divided into free membrane and attached membrane areas
Contact Geometry
plays decisive role in the recorded signal amplitude
Position of cell to electrode
influences recorded signal shapes
ISFET as voltage sensor
–> sensitivity to pH value
shifts to left and right caused by varying pH-value in the electrolyte
Important parameters in cell-sensor coupling
- ratio of free and attached membrane
- conductivity and height of cleft
Point Contact Model
Simplifications
- capacitive current through Gate is negligible
- potential at point contact is small compared to membrane voltage
- ion concentration changes in the cleft with respect to bulk solution can be neglected
