Download Biosensor unit – BME 140 Worksheet for nanopipette

Biosensor unit – BME 140
Worksheet for nanopipette paper #1
Current Rectification at Quartz Nanopipet Electrodes
Wei, C.; Bard, A.J.; Feldberg, S.W. Analytical Chemistry 1997, 69, 4627-4633
The following questions should be answered and turned in together with your article summary
according to the guidelines
http://courses.soe.ucsc.edu/courses/bme140/Fall11/01/pages/homework
Be aware you are not questioned on the final section “Model for Asymmetric Tip Currents,” which is
quite complicated. You should be able to answer most of the questions below from careful reading of
the rest of the paper.
1. A circuit diagram of the nanopipette electrode.
a) Why is an electrolyte solution required to complete the circuit between the two electrodes?
b) Write the chemical reaction taking place at the auxiliary electrode.
c) On the diagram above, indicate with an arrow the direction of electrons flowing through the
electrochemical cell.
d) What is the reason that solution resistance (Rsol) can be said to dominate electrical resistance in this
system, even though there are multiple electrochemical processes taking place?
e) In chemical terms, we say reduction occurs at the cathode in an electrochemical cell. Where is the
cathode in the diagram above? What is being reduced?
2. Voltage-current behavior in a nanopipette electrode.
a) What is a diffuse double layer? Why does it form on charged and not neutral surfaces in solution?
b) For a quartz (SiO2) surface in a solution of KCl at neutral pH, what species will make up the double
layer?
c) Consider the Diagram below for a micropipette with tip diameter of 1000 nm. The relative size of the
electrical double layer is drawn (not to scale). Given the current vs. voltage plot in Fig. 3 of the paper, do
you think that cations exiting the tip have significant interaction with the double-layer? Explain.
d) Draw your own diagram based on the setup above, but with a nanopipette o f tip diameter 10 nm.
Show the relative size of the double-layer, assuming an electrolyte concentration of 0.01 M KCl.
e) Considering your drawing above, why would the size of the double layer affect ion current flow in the
nanopipette?
f) The exclusion of certain ions at the nanopipette tip is called permselectivity. Is permselectivity alone
enough to explain the asymmetric current-voltage (I-V) curves shown in Figure 2 of the paper? Why or
why not?