1.1. What Is a Biosensor?

Biosensor = bioreceptor + transducer. A biosensor consists of two components: a bioreceptor and a trans-ducer. The bioreceptor is a biomolecule that recognizes the target analyte whereas the transducer converts the recognition event into a measurable signal. The uniqueness of a biosensor is that the two components are integrated into one single sensor (Fig. 1.1). This combination enables one to measure the target analyte without using reagents. For example, the glucose concentration in a blood sample can be measured directly by a biosensor (which is made specifically for glucose measurement) by simply dipping the sensor in the sample. This is in contrast to the conventional assay in which many steps are used and each step may require a reagent to treat the sample. The simplicity and the speed of measurement is the main advantages of a biosensor.

Enzyme is a Bioreceptor. When we eat food such as a hamburger and French fries, they are broken down into small molecules in our body via many reaction steps (these breakdown reactions are called catabolism). These small molecules are then used to make building blocks of our body such as proteins (these synthesis reactions are called anabolism). Each of these catabolism and anabolism reactions (the combination is called metabolism) are catalyzed by a specific enzyme. Therefore, an enzyme is capable of recognizing a specific target molecule (Fig. 1.2). This biorecognition capability of the enzyme is used in biosensors. Other biorecognizing molecules (= bioreceptors) include antibodies, nucleic acids, and receptors.

Immobilization of Bioreceptor. One major requirement for a biosensor is that the bioreceptor molecule has to be immobilized in the vicinity of the transducer. The immobilization is done either by physical entrapment or chemical attachment. Note that only minute quantities of bioreceptor molecules are needed, and they are used repeatedly for measurements.

Transducer. A transducer should be capable of converting the biorecognition event into a measurable signal (Fig. 1.3). Typically, this is done by measuring the change that occur in the bioreceptor reaction. For example, the enzyme glucose oxidase (used as a bioreceptor in a glucose biosensor) catalyzes the following reaction:

Glucose + O2 --------> Glucosnic acid + H2O2

Fig. 1.1. Biosensor configuration.

Fig. 1.2. Specificity of biosensor (TR: transducer).

To measure the glucose concentration, three different transducers can be used:

Note that an oxygen sensor is a transducer that converts oxygen concentration into electrical current. A pH sensor is a transducer that converts pH change into voltage change. Similarly, a peroxidase sensor is a transducer that converts peroxidase concentration into an electrical current.

Considerations in Biosensor Development. Once a target analyte has been identified, the major tasks in developing a biosensor involves:

1. Selection of a suitable bioreceptor molecule
2. Selection of a suitable immobilization method
3. Selection of a suitable transducer
4. Designing of biosensor considering measurement range, linearity, and minimization of interference
5. Packaging of biosensor

The item 1 requires knowledge in biochemistry and biology, the item 2 requires knowledge in chemistry, the item 3 requires knowledge in electrochemistry and physics, and the item 4 requires knowledge in kinetics and mass transfer. Once a biosensor has been designed, it has to be put into a package for convenience manufacturing and use. The current trend is miniaturization and mass production. Modern IC (integrated circuit) fabrication technology and micromachining technology are used increasingly in fabricating biosensors. Therefore, interdisciplinary cooperation is essential for a successful development of a biosensor.

Requirements for Sensors To be commercially successful, a biosensor has to meet the general requirements of commercial sensors (Table 1.2). These are:

1. Relevance of output signal to measurement environment
2. Accuracy and repeatability
3. Sensitivity and resolution
4. Dynamic range
5. Speed of response
6. Insensitivity to temperature (or temperature compensation)
7. Insensitive to electrical and other environmental interference
8. Amenable to testing and calibration
9.  Reliability and Self-Checking Capability
10. Physical robustness
11. Service requirements
12. Capital cost
13. Running costs and life
14. Acceptability by user
15. Product safety-sample host system must not be contaminated by sensor

Fig. 1.3. Three possible transducers for glucose measurement.

Table 1.1. Considerations for biosensor development

 Selection of a suitable bioreceptor molecule

Selection of a suitable immobilization method

Selection of a suitable transducer

Designing of biosensor considering measurement range,

linearity, and minimization of interference

Packaging of biosensor