Biosensor

Biosensors are sensors that are equipped with biological components. These are applied in the biotechnology instrumentation. The term was coined in 1977 by Karl Cammann; Since 1997 there is a definition of IUPAC for it.

Structure and principle

Biosensors based on the direct spatial coupling of an immobilized biologically active system with a signal converter ( transducer ) and an electronic amplifier. For the detection of the substances to be determined use biosensors biological systems on different high level of integration. Such biological systems, may be, for example, antibodies, enzymes, organelles, or microorganisms. The immobilized biological system of the biosensor to interact with the analyte. This results in physico-chemical changes such as changes in film thickness, refractive index, light absorption, or electrical charge. These changes can be determined by means of the transducer, such as optoelectrical sensors, amperometric and potentiometric electrode or specific field effect transistors ( chemically sensitive field-effect transistor). After measurement of the initial state of the system must be restored.

The measurement of an analyte using a biosensor, therefore, takes place in three steps. Initially, the specific detection of the analyte is carried out by the biological system of the biosensor. Then is to convert the physico-chemical changes which result from the interaction of the analyte with the receptor, rather than to an electrical signal. This signal is then processed and amplified. Signal converting electronics and can be combined, for example, in CMOS-based micro-sensor systems. Its selectivity and sensitivity relates a biosensor used in the biological system.

Types of biosensors

• By amperometry: when amperometry is measured in a measuring chamber with two electrodes at constant voltage, the current flow. It is suitable for metabolites that can be readily oxidized or reduced. Often, mediators are used, which are redox couples that interfere indirectly from the oxidation of the substrate itself and are used for electron transfer. If, for example, a substrate to be determined of FAD, the coenzyme oxidase is the most oxidized, wherein the FAD is reduced to FADH so FADH is then oxidized by the oxidized form of the mediator back to FAD. The resulting reduced form of the mediator is oxidized anodically again. About shots of current-voltage curves can make statements to the redox behavior and the actual concentration of the substrate. As mediators, for example, Used hydroquinone or derivatives of ferrocene. The advantage of mediators is that you can specify a much lower voltage, thus avoiding undesirable side reactions. Amperometric biosensors for example, used to determine glucose, cholesterol, fatty acids and L-amino acids with the corresponding enzymes as oxidases.

• By potentiometry: The potentiometry is used for ionic reaction products. The quantitative determination of these ions takes place on the basis of their electric potential at a measuring electrode, which is covered for the determination of a substrate with a suitable enzyme. In hydrolases, such as Urease, the change of the pH value or the change of ammonium ions and bicarbonate ions is determined. As a measuring electrode often ion-sensitive field effect transistors ( ISFET) or metal oxide - coated electrode acid (MOSFET ) may be used. As the reference electrode is an electrode used the same Typps, without assignment to an enzyme. The potentiometric method is used to determine, for example, Urea, creatinine and amino acids.

• With ion-selective electrodes: If this is an enzyme, so they work on the same principle as described for the potentiometry.

Applications

The first measuring system, which may be referred to as a biosensor, as defined above, was developed by Clark and Lyons 1962. Described a measuring system which enables the determination of blood glucose during and after surgery. This biosensor consisted either of an oxygen electrode of Clark or a pH electrode as a transducer, in which the enzyme glucose oxidase was applied between two membranes. The glucose concentration could be determined as a change in pH or a change in oxygen concentration due to the oxidation of the glucose under the catalytic action of the enzyme glucose oxidase.

With this structure, the biological material is enclosed between two membranes or the biological system is applied to a membrane and is directly connected to the surface of the transducer. The applications of biosensors in the analysis of water and wastewater can be divided in biosensors for the determination of individual components, biosensors for the determination of toxicity and mutagenicity, as well as in biosensors for the determination of biochemical oxygen demand ( BOD ).

Biosensors for the determination of proteins were realized with silicon field effect sensors ( so-called ChemFETs ). They enable the label-free analysis of proteins in the field of protein analysis by in situ processes, since they detect the protein binding on the intrinsic amount of charge of the protein by means of field effect.

The bacterial content of bathing waters or wastewaters by means of a biosensor can be determined. On a vibrating membrane, antibodies are attached to certain types of bacteria. Swimming the corresponding bacteria on the sensor over, they attach themselves to the antibody, thus slowing the vibrations of the membrane. Below the vibrations a certain value, an alarm is triggered.

The penicillin concentration in a bioreactor in which fungal strains cultivated, can be determined with a biosensor. The biological component of the sensor used for this purpose is in this case the enzyme penicillin acylase dar. This cleaving enzyme is applied to a membrane, which rests a pH electrode. Now takes the penicillin concentration in the medium to, the enzyme cleaves always greater amounts of an acid, phenylacetic acid, from. Wherein the pH at the electrode is changed. One can now include the pH, the concentration of penicillin so.

The biosensors including surface plasmon resonance spectroscopy. Here, the binding of materials by plasmon detection is measured.

A new development for monitoring of food based on nanosensors. The fluorescence of nanoparticles that are located in an agarose nutrient medium changes significantly when changing the pH due to bacterial metabolism in the food. Here, two fluorescent dyes embedded in the nanoparticles. The first is a water-repellent fluorescein dye. It glows green when excited by a light emitting diode and is sensitive to a change in pH. The second, a dye with pH - independent red fluorescence, used as an internal reference.

A novel pH sensor can be pH changes in living cells traced over longer periods. The principle is based on a combination of fluorescent nanocrystals with moving oligonucleotides that fold or stretch as a function of the ambient pH. So that the distance between the energy donor nanocrystal is pH dependent change with a green fluorescent dye and a FRET acceptor, which is composed of a red fluorescent dye. A FRET energy transfer and thus to light the red fluorescent dye, it happens when the distance is small. Observed is the ratio between green and red fluorescence with a fluorescence microscope.

Swell

• R. D. Schmid, U. Bilitewski: biosensors, chemistry in our time, 26 Jahrg 1992, No. 4, pp. 163-173, ISSN 0009-2851
• Brian R. Eggins: Chemical Sensors and Biosensors. Analytical Techniques in the Sciences. 2nd Edition, Wiley, 2002, ISBN 0-471-89914-3.
• Perpeet, M., Glass, . S., Gronewold, T., Kiwitz, A., Malavé, A., Stoyanov, I., Tewes, M., Quandt, E. SAW sensor system for marker- free molecular interaction analysis. Anal. Lett. 39 (8): 1747-1757 (2006).