Microelectrodes
Goals
- To learn the difference between macro-scale linear diffusion and micro-scale
convergent diffusion. - To understand how diffusion affects the shape of a voltammogram.
- To understand why the time-scale of an experiment affects the shape of a voltammogram.
Experimental Apparatus
- Gamry Instruments Interface 1000T potentiostat
- Gamry Instruments Framework™ software package installed on a host computer
- Voltammetry cell (Gamry part number 990-00193)
- Platinum working electrode (Gamry part number 932-00003)
- Platinum counter electrode (Gamry part number 935-00056)
- Silver-silver chloride reference electrode (Gamry part number 930-00015)
- Platinum microelectrode (Gamry part number 932-00009)
- Polishing cloth (Gamry part number 935-00063)
Reagents and Chemicals
- Acetonitrile
- 0.1 M tetrabutylammonium hexafluorophosphate
- Solutions of 2, 5, 10, 15, and 20 mM ferrocene in 0.1 M tetrabutylammonium
hexafluorophosphate - 0.5 μm alumina polish (Gamry part number 935-00064)
Background
In this experiment we explore the effect of microelectrodes on the shapes of voltammograms
in cyclic voltammetry (CV). The working electrodes that you have used
so far are considered macroelectrodes. Microelectrodes, in contrast, are working
electrodes with a diameter typically less than 100 μm, and can be as small as a few
nanometers in diameter, but the most common ones are 5 to 10 μm in diameter.
The advantages that microelectrodes have over macroelectrodes, other than size, is
that they experience minimal solution-resistance effects, and have rapid response
times. In order to be the least invasive, microelectrodes are best in vivo. Resistance
reasons microelectrodes are a popular choice in highly resistive environments, both
in vitro and in vivo.
The shape of the voltammogram using a “slow” ( in Figure 7.1. The general shape of the voltammogram is sigmoidal (S-shaped), it is
flat on the left, and, as the species is reduced the current increases to a constant value.
This observed behavior varies from the CV of a macroelectrode in that the current
does not come to a peak and decrease again. The reason for this leveling-out
of the current is that there is enough material diffusing to the faces of the electrode
to keep the current constant. At a macroelectrode, on the other hand, the analyte is
exhausted at the face and not enough analyte is diffusing through solution, so the
current decreases.