Download Review for Wilson and Hu: Chemical Reviews, Enzyme

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Sensors and Instruments
Spring 2001
Review Questions pertaining to the reading of the article written by Wilson and Hu: Chemical Reviews,
Enzyme-based biosensors for in vivo measurements.
1) What was the key innovation in the development of enzyme-based biosensors?
2) What is meant by "reagentless" with respect to enzymatic biosensors? Why is this important for in
vivo applications?
3) Consider the following reactions.
Sred + Eox  Sox + Ered
(1)
S'ox + Ered  S'red + Eox
(2)
If Eq 1 needs to become rate limiting (sensitive to Sred only), what must be done to S'ox in step 2.
What methods are available for reducing S'ox?
4) Only four enzyme-based biosensors are commercially available. What do these sensors measure?
What are their commercial applications?
5) Why are oxidoreductase enzymes preferred over dehydrogenase enzymes for electrode sensors?
6) Heller’s group has spent a significant effort in designing redox mediator electrodes. These electrodes
have a reduced potential of 350 mV vs. SCE for measuring the products of the glucose oxidase reaction as
compared to 650 mV vs. SCE for Wilson's sensor. What would be a logical reason for dropping the
oxidation potential for measuring byproducts of the glucose oxidase reaction?
7) What is the appeal for making mediator-based enzyme sensors? In what cases would these be useful as
sensing devices?
8) The requirements for a successful biosensor are quite strict. For each of the following requirements,
give a definition. Give examples of what the limitations are for in vivo enzyme-based biosensors for each
requirement.
a)
b)
c)
d)
e)
sensitivity
selectivity
detection limit
stability
response time
9) What type of electrode is Pt/Ir (polarizing or nonpolarizing)?
10) What is the Clarke error grid analysis? What is plotted on the x-axis and y-axis for the Clarke error
grid analysis? If a manufactured sensor repeatedly gave a Clarke error grid current response
corresponding to the regions in "section D and E" on the left side of the graph, what types of experimental
factors might cause this response? How could these be corrected for in a sensor?
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If the sensor repeatedly gave responses in D and E on the right side of the graph, what factors might cause
these errors? How would these errors be corrected in subsequent designs?
11) What are the assumptions with respect to calibration when a fingerstick approach is used to calibrate
the sensor?
12) What are the assumptions for a one-point calibration?
13) If glutamate has a concentration of 2 µM in brain tissue and ascorbic acid has a concentration of 40
µM, what would the selectivity ratio have to be for ascorbate/glutamate to have a response that is 0.1%
that for the above-mentioned concentrations?
14) If in brain tissue glutamate has a concentration of 2 µM and dopamine has a concentration of 40 nM
and the current response was 1:1 for glutamate:dopamine, what would the percent interference be for this
system for a glutamate electrode? Rather than adding additional coating layers on the electrode, what
type of method or design could be used to continuously background subtract current during the
measurement process.
15) What enzyme is available to remove ascorbate interference? Why is their some reluctance with
respect to using this enzyme in the design of in vivo enzyme-based biosensors?
16) Wilson and Hu state that a difficulty with using microdialysis sampling for in vivo measurements is
the poor temporal and spatial resolution. What are the typical dimensions for a microdialysis probe (in
your notes)?
Robert Kennedy's group at Florida has been actively involved in developing rapid analytical techniques
coupled to microdialysis probes. They currently achieve 10-s response and measurement times for
glutamate and other brain amino acids (Anal. Chem., 72 (4), 865 -871, 2000.) How do these response
times compare to different sensor responses used for making measurements in the brain?
17) What are the advantages to obtaining samples from tissues with microdialysis probes?
18) Patients in intensive care units with severe head trauma often have oxygen and pressure probes
inserted into healthy and damaged areas of their brain. The ratios and concentrations of glucose, lactate,
pyruvate, glycerol, and glutamate are important indicators for outcome of the damaged tissue and can
help clinicians tailor therapy. Oxidoreductase enzymes are available for all the above-mentioned
analytes. How could a sensor be devised for monitoring all these analytes simultaneously and what
would be the design limitations for such a sensor?
19) If the clinicians wanted to obtain brain concentrations in healthy and damaged tissue (damaged tissue
may have decreased blood flow, etc.) of drugs given such as morphine and the above-mentioned analytes,
what would be the best method to perform these analyses?
20) When a biosensor is implanted in vivo, what are the major time domains that are important for
biosensor function? (A biocompatibility question)