Homework Problems 3
In this section, we derived expressions for I and Eout (Eq. (15) for the amperometric sensor, and Eqs. (23) for the potentiometric sensor, respectively) for case when [S] << KM. Derive equivalent expressions for both type sensors under 'kinetics control'. What would be the effect of temperature and enzyme loading for the sensor ouputs?
(Hint) Under 'kinetics control', the reaction is the slow step and [S] in the enzyme layer is the same as [S]o . Therefore, Eq. (8) will become:
The same modification has to be made with Eq. (11). The solution procedure will be simpler because the right hand side of the equation is a constant.
Problem Set Biosensor Part
1. Between amperometric sensor and potentiometric sensor, which one tends to have mass transfer limitation? Why?
2. Enzyme-based glucose sensor has two membranes - one outer and one inner. What are the roles of each membranes?
3. What is the advantage of using optical fiber as the transducer in making biosensors over the amperometry.
4. In class, we discussed amperometry, potentiometry, optical method, thermistors, and piezo crystal for use as a base transducer for making biosensors. Illustrate other possible forms of transducers and discuss their advantages and disadvantages.
5. What is the advantages and disadvantages of using biomolecules such as enzymes and antibodies for sensor applications?
6. Calculate the steady state current output from the DO sensor that we have made in the lecture when it is placed in air saturated water. Assume that we used 25 µm thick Teflon FEP membrane. Use the data given on pp 2-27.
7. Suppose we use two 25 m Teflon membranes to cover the cathode. (a) What will be the output current under the same condition as in Problem 1? (b) Comment on the response time. Will this sensor become slower? By how much? Take diffusivity of oxygen in water as 2x10-5 cm2/s.
8. Suppose you place the DO sensor in a stagnant liquid which has a magnetic stirrer in it. Qualitatively show the sensor output current as you increase the stirring rate.
9. Flow dependency of DO sensor is a big problem in actual measurements. The flow dependency can be reduced by placing a silicone membrane over the Teflon membrane. Explain why this reduces the flow sensitivity.
10. Suppose we measure DO concentration in (a) dense aerobic culture, and (b) low cell density aerobic culture. In which case the accuracy will be better. Explain why. What can you do to improve the accuracy?
11. For the glucose sensor that we discussed in class, design an experiment to obtain Km value. State what kind of measurements are necessary under what conditions.
12. How can one make a diffusion-controlled biosensor?
13. What would be the flow dependency (i.e., the effect of agitation of the measurement medium) of the biosensor output for (a) a kinetically controlled biosensor (b) a diffusion-controlled biosensor
14. How can you determine whether a biosensor is diff usion-controlled or kinetics-controlled without the information on Vmax , d, KM, and Ds.
15. State your criterion on selecting the outer and the inner membrane for a glucose sensor intended for implantation in a human body. Consider long term stability, biocompatibility, and fast response time.
16. What is the disadvantage of using antibodies instead of enzymes for sensing? Suggest possible solutions.
17. What is the effect of enzyme loading in terms of sensor performance?
18. Discuss the effect of temperature on 'kinetics limited' biosensor and 'diffusion limited' biosensor.
19. Discuss the effect of response time on 'kinetics limited' biosensor and 'diffusion limited' biosensor.