Biosensor Principles

The term “biosensor” is short for “biological sensor” and is a device made up of a transducer and a biological element that may be an enzyme, an antibody, or a nucleic acid. The biological element or bioelement interacts with the analyte being tested and the biological response is converted into an electrical signal by the transducer. Every biosensor has a biological component that acts as the sensor and an electronic component that detects and transmits the signal.

Types of biosensors

Biosensors can be grouped according to the type of biological element and transducer they contain. They may also be named according to how the biosensing takes place.

The types of biological elements include:

• Enzymes
• Antibodies (also called immunosensors)
• Micro-organisms
• Biological tissue
• Organelles

Types of biosensing

The different ways that biosensing may occur are described below:

• If the bioelement binds to the analyte, the sensor is referred to as an affinity sensor.
• If the bioelement and the analyte give rise to a chemical change that can be used to measure the concentration of a substrate, the sensor is called a metabolic sensor.
• If the biological element combines with the analyte and does not change it chemically but converts it to an auxiliary substrate, the biosensor is called a catalytic sensor.

Types of sensing elements

Enzymes

An enzyme is a protein that has a high selectivity for a particular substrate, which it binds to,  bringing about a catalytic change. Enzymes are commercially available in highly purified states and are therefore useful in the mass production of enzyme sensors. Enzymes can be fixed onto the surface of a transducer through adsorption, covalent attachment, and entrapment in a gel or an electrochemically generated polymer.

Antibodies or immunosensors

Antibodies are produced by B-lymphocytes in response to antigenic stimuli such as foreign invaders or microbes. When used as biosensors in immunoassays, antibodies are immobilized on the surface of a transducer through covalent attachment by conjugation of amino, carboxyl, aldehyde or sulfhydryl groups. Antibodies are sensitive to changes in pH, ionic strength, chemical inhibitors and temperature. Immune sensors usually employ optical, fluorescence or acoustic transducers.
 

Microorganisms

Microbes may be used to detect the consumption of oxygen or carbon dioxide in an environment using electrochemical techniques. Microbe biosensors have the advantage of being cheaper than enzymes or antibodies and are more stable. However they may be less selective than enzymes or antibodies.

Other bioelements

Organelles, nucleic acids and biological tissues have been researched as biosensors.

Types of transducer

Electrochemical transducers

These are useful in electrochemical, amperometric and potentiometric signals. These electrodes are commonly made of platinum, gold, silver, stainless steel, or carbon-based inert materials.

Amperometric transducers, detect changes in current that occur due to oxidation or reduction. The current reflects the reaction that takes place between the analyte and the bioelement.

Potentiometric transducers can measure the charge accumulation (potential) of an electrochemical cell. The transducer is usually made up of an ion-selective electrode and a reference electrode.

Optical transducers

Fluorescence is commonly used in signal transduction, especially when using enzymes and antibodies. Fibre optic probes consist of at least two fibres. One is connected to a light source of a given wavelength range and produces the excitation wave. The other is linked to the photodiode that detects the change in optical density at a selected wavelength. Plasmon resonance transducers measure alterations in the refractive index at and close to the sensing element’s surface.

Acoustic transducers

These are devices in which mechanical acoustic waves act as the transduction system. The membrane contains chemically interactive materials in contact with a piezoelectric material. The devices vary according to the wave guiding process used. Usually, bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices are used.

Calorimetric transduction

These measure the heat from the biochemical reaction between the sensing element and the analyte.