While classical sensors convert physical quantities or chemical reactions into electrical signals, biosensors are concerned with the conversion of biological quantities into electrical signals.
A biosensor consists of the bioreceptor, the transducer, a downstream preamplifier and the detector. The bioreceptor is the sensing element that responds to a wide variety of biological systems. For example, to oxidants, glucose, cholesterol, fatty acids, enzymes and amino acids, which cause chemical reactions, known as redox reactions, in the bioreceptor. The reactions, which are reflected in the change of physicochemical properties, are converted into an electrical voltage in the transducer and amplified to a downstream preamplifier and processed to a measured value in the detector.
Biosensors can be piezoelectric-based, optical-based, or electrochemical-based. Piezoelectric sensors are vibration sensors used to measure enzymes and antibodies. The quartz crystals used are coated with enzymes, which changes the quartz mass and its vibration frequency. Optical sensors use fluorescence. Biosensors are used to measure the oxygen concentration in liquids. The oxygen concentration is shown by the light absorption and the change of fluorescence. And electrochemical biosensors are about metabolism. These measurements are accompanied by reactions in which released ions are measured.