An Introductory Tutorial on Biosensor Technology

HTML conversion from BASIC programs by L. Tescione 12/90 and Andrew Gregor 12/ 10,1991

Needed Vocabulary: -----------------

Analyte - molecule or substance one is interested in detecting

Reagent - biological component of a biosensor that detects the analyte

Optode - optical biosensor

Transducer - a device that converts energy from one form into another e.g., telephone companies use transducers to convert sound energy into electrical energy to be carried long- distance through telephone lines and then another transducer at the receiving end to convert the electrical energy back into sound A biosensor is a sensing device that consists of a biological component coupled to a transducer that converts biochemical activity into, most commonly, electrical energy.

USE FOR BIOSENSORS:

monitoring processes for better control in the biotechnology industry in both R&D product development and quality control in the plant.

At the present time monitoring of a bioprocess is limited to the measurement of pH, temperature, and dissolved oxygen. A biosensor can be used to monitor the presence of product, either biomass, enzyme, or antibody, or of a by-product of the process as an indirect measure of process conditions. Biosensors could make monitoring and control in the biotech industry accurate and reproducible. Specific fields of interest:

  1. the bioprocess industry
  2. the medical field
  3. environmental detection of pollutants or toxins

All biosensors must perform two tasks: molecular recognition and signal transduction

MOLECULAR RECOGNITION:

The biological component could be an enzyme, antibody, cell membrane receptors, tissue slices, or microbial cells that interact with a specific analyte. The use of enzymes and antibodies is the most popular because they are simpler. Tissue slices and microbial cells are more complicated to use because they must be kept alive; nutrients must be supplied and wastes removed.

SIGNAL TRANSDUCTION:

The transducer can measure electrical activity (by either measuring voltage or current), light (optical), sound, mass (piezo- electric), or temperature.

In this tutorial we will concentrate on electrical transducers for application in the bioprocess industry. We will only consider biosensors that use enzymes or antibodies as their biological component. In the operation of a bioreactor it is always necessary to monitor the properties of the media in which the cells are living. Typically the situation involves an organism that will not survive or will not produce a product if the media is not maintained at the correct physical parameters. Measuring media properties such as temperature and pH is relatively easy. Measuring the concentrations of nutrients, metabolic wastes, and products is not easy, but like the measuring of temperature and pH they are of great importance to maintenance of the biological environment that exists in a bioreactor. One possibility for monitoring media properties such as concentration is the use of biosensers.

REVIEW:

A biosensor consists of an immobilized biological system such as a cell colony, an enzyme,or an antibody; and a measuring device. In the presence of a certain molecule the biological system changes the environment. The measuring device sensitive to this change sends a signal. This signal can be converted into the measurement parameter. Often the biological system is an actual cell. The key thing to remember is that it is an actual organism that detects the concentration change of the molecule in the media. This organism could be the same one as the one in the media or it could be different. In either case it must be kept separate from the media. This can be done with a membrane that is permeable to the molecule that is being measured but impermeable to the cells and most other macromolecules in the reactor media.

The change that biosensor organism produces is usually a result of its use of the measured molecule in its metabolic processes. The biosensor organism might metabolize molecule A and release molecule B into the system. A B-electrode would be affected by this. In another common variation the biosensor organism metabolizes molecule A by using molecule B which is sensed. A B-electrode would detect the change in the concentration of B.

What are the advantages of biosensors over other measurement schemes?

  1. They can measure nonpolar molecules that do not repond to most measurement devices.
  2. They are as specific as the immobilized system used in them.
  3. They allow rapid continuous control.
There are also disadvantages to be dealt with.
  • Heat sterilization is not possible as this would denature the biological part of the biosensor.
  • The membrane that separates the reactor media from the immobilized cells of the sensor can become fouled by deposits.
  • The cells in the biosensor can become intoxicated by other molecules that are capable of diffusing through the membrane
  • Changes in the reactor broth (i.e., pH) can put chemical and mechanical stress on the biosensor that might eventually impair it.
    BIOSENSING WITH IMMOBILIZED CELLS GENERAL CONSTRUCTION AND MECHANISM:

    Construction ------------ 1. The cells are retained over some type of device that will detect a change in their culture fluid. The cells 'mediate' between the analyte and sensor.

    2. A membrane covers the cell compartment. The previous screens contained general information. There are biosensors for several hundred substrates documented in the literature today. As a result there are many variations on the design. Now to understand better how biosensors work we will look at a specfic example.

    Although not the best biosensor for glucose, we will demonstrate how it can be done with immobilzed cells. 

    These are labels for a drawing by the BASIC program:
1. Transducer
2. Recorder
3. Lead
Electrode
4. Platinum
Electrode
5. Immobilized
Cell
Layer
6. Teflon
Membrane
Glucose Biosensor Using Immobilized Cells
Broth
    This is the end of a glucose electrode immersed in reactor media. The g's are glucose molecules in the media. The m's are cells and other large materials. The o's represent oxygen molecules inside the biosensor itself. Imagine the glucose and the other molecules moving randomly due to diffusion and eddy currents.they contact the teflon membrane of the biosensor. Let's follow some of these molecules and see what happens when First let's look at a microbial cell. This cell cannot penetrate the membrane. Small molecules that penetrate but don't react have no effect. Now let's see what happens to glucose.

    Glucose can penetrate and react, but oxygen is needed.

    This oxygen comes from within the biosensor.

    The cell metabolizes the glucose with the oxygen.

    Now imagine many molecules diffusing back and forth through the membrane at once.