Biomedical Engineering Reference
In-Depth Information
1.4
1.2
1
0.8
0.6
0.4
0.2
0
80
100
120
0
20
40
60
Concentration (nM)
+100 nM
+50 nM
+25 nM
1 nA
100 s
Time (s)
FIGURE 13.30
Current response due to the addition of small concentrations of dopamine. Thin arrows
represent additions of 25 nM of dopamine to solution, medium arrows 50 nM additions, and thick arrows
100 nM additions of dopamine. The inset shows the change in current when a defi ned concentration of
dopamine is added.
small amplitude potential pulses. The current is measured before each pulse and is subtracted from
the current value at the end of the pulse. The recorded current is, therefore, in differential form,
which is plotted against a voltage range. Different chemical species oxidize at slightly different
voltages, resulting in current peaks centered at a characteristic voltage value for each molecule and
providing more specifi c detection than amperometry. Furthermore, DPV keeps the residual current
(due to the charging of double-layer capacitance) low, increasing the sensitivity. An accumulation
step can be used with DPV, preconcentrating the analyte at the electrode surface prior to applying
oxidizing potential, which also results in an improvement in sensitivity. Unfortunately, the increased
sensitivity is accomplished at the expense of time resolution, as the voltage scan requires about
80 s,
45
while the preconcentration step takes another 100 s
47
to achieve the sensitivity required for
measuring dopamine levels as low as 5 nM. This constraint makes real-time experiments measur-
ing dopamine release due to external stimulation very diffi cult. Therefore, electrodes with a faster
time response (due to decreased size as discussed above) can be particularly suitable for scan-
ning voltammetric techniques such as DPV or differential pulse stripping voltammetry (DPSV,
or DPV with the added accumulation step). In Figure 13.31, the nanoelectrode DPSV scan of
2.5 nM dopamine in PBS solution is shown with the background current digitally subtracted.
The accumulation time was reduced to only 5 s, but the dopamine peak is still clearly visible.
This shows that nanoelectrodes with improved time response can have an important role to
play in speeding up the voltage scan times that are used with the high-sensitivity voltammetric
techniques.
