Biosensors

What Is a Biosensor?

A biosensor consists of two components: a bioreceptor and a transducer. The bioreceptor is a biomolecule that recognizes the target analyte, and 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 (Fig1). 

Fig 1. Biosensor Configuration. Biosensor = bioreceptor + transducer

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 made specifically for glucose measurement, by simply dipping the sensor in the sample. This is in contrast to the commonly performed assays, in which  many sample preparation steps are necessary and each step may require a reagent to treat the sample. The simplicity and the speed of measurements that require no specialized laboratory skills are the main advantages of a biosensor.

Enzyme is a Bioreceptor.

When we eat food such as hamburgers and french fries, it is 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 the 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 (Figure 2). This biorecognition capability of the enzyme is used in biosensors. Other biorecognizing molecules (= bioreceptors) include antibodies, nucleic acids, and receptors.

Fig 2. Specivity of biosensor. TR=Transducer

Immobilization of Bioreceptor

One major requirement for a biosensor is that the bioreceptor be immobilized in the vicinity of the transducer. The immobilization is done either by physical entrapment or chemical attachment. Chemical attachment often involves covalent bonding to transducer surface by suitable reagents. It is to be noted 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 (Figure 3). Typically, this is done by measuring the change that occurs in the bioreceptor reaction. For example, the enzyme glucose oxidase is used as a bioreceptor in a glucose biosensor that catalyzes the following reaction:

Fig 3. Three possible transducers for glucose measurement.

 To measure the glucose in aqueous solutions, three different transducers can be used:

1. An oxygen sensor that measures oxygen concentration, a result of glucose reaction 
2. A pH sensor that measures the acid (gluconic acid), a reaction product of glucose 
3. A peroxidase sensor that measures H2O2 concentration, a result of glucose reaction

Note that an oxygen sensor is atransducer 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.

Biosensor Characteristics

Biosensors are characterized by eight parameters. These are: 

1. Sensitivity is the response of the sensor to per unit change in analyte concentration.
2. Selectivity is the ability of the sensor to respond only to the target analyte. That is, lack of response to other interfering chemicals is the desired feature.
3. Range is the concentration range over which the sensitivity of the sensor is good. Sometimes this is called dynamic range or linearity.
4. Response time is the time required for the sensor to indicate 63% of its final response due to a step change in analyte concentration.
5. Reproducibility is the accuracy with which the sensor’s output can be obtained.
6. Detection limit is the lowest concentration of the analyte to which there is a measurable response.
7. Life time is the time period over which the sensor can be used without significant deterioration in performance characteristics.
8. Stability characterizes the change in its baseline or sensitivity over a fixed period of time. 

Considerations in Biosensor Development

Once a target analyte has been identified, the major tasks in developing a biosensor involve:

1. Selection of a suitable bioreceptor or a recognition molecule.
2. Selection of a suitable immobilization method.
3. Selection and design of a transducer that translates binding reaction into measurable signal.
4. Design of biosensor considering measurement range, linearity, and minimization of interference, and enhancement of sensitivity.
5. Packaging of the biosensor into a complete device.

The first item above requires knowledge in biochemistry and biology, the second and third require knowledge in chemistry, electrochemistry and physics, and the fourth requires knowledge of  kinetics and mass transfer. Once a biosensor has been designed, it must be packaged for convenient 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, as they reduce manufacturing costs. Therefore, an interdisciplinary research team, consisting of the various disciplines identified above, is essential for successful development of a biosensor.

Biosensor References:

Biosensors: Theory and Applic...

Principles of Bacterial Detection

Mathematical Modeling of Biosensors

Engineering Biosensors: Kinetics..

Biosensors (The Practical.....)

Electrochemical Sensors, Biosensors...