Biosensors

A biosensor comprises a biologically sensitive material immobilized in intimate contact with a suitable transducing system which converts the biochemical signal into a quantifiable and processible electrical signal (28,29). The biologically sensitive material is typically an enzyme, multienzyme system, antibody, membrane component, organelle, bacterial or other cell, or whole slices of mammalian or plant tissues. To produce an electrical signal, the biological interacting system is placed in close proximity to a suitable transducer (Fig. 4). When biological molecules and medicine, surface adsorption studies, gas detection, and the study of stochastic processes at various interfaces. The compatibility of the ion-selective membranes and semiconductor materials used in the construction of CHEMFETs is the major practical problem standing in the way of wide usage of these devices. By deliberate choice both the electrochemical element (the membrane) and the electronic preamplifier (FET chip) are exposed to a very hostile environment, the electrolyte solution. In such a situation, the requirements that are placed on the encapsulation materials are much more stringent than those required for ISEs. Once the encapsulation and membrane attachment problems are solved, design of sensor packages that will include data acquisition as well as data-processing elements will be feasible (16).

Using an operational amplifier, Perez-Olmos (19) improved the sensitivity of ISE for measuring potassium and calcium in wines. Knee and Srivastava (20) adapted an ISE to measure calcium in apple fruit tissue; calcium is a critical mineral affecting postharvest handling and quality. A fully automated battery-operated computerized field-based ISE method for monitoring fluoride in water was reported by Bond et al. (21).

Metal Oxide Gas Sensors

The most widely used sensors for combustible gases today are based on semiconducting metal oxides. Semiconducting gas sensors are usually based on the surface properties of the oxides of tin or zinc (Sn02 or ZnO). It is generally agreed that the surface conductivity of semiconductors can be markedly changed by the adsorption and subsequent reaction of gases with already-adsorbed atmospheric oxygen. This implies that for an «-type material (where electrical conduction is associated with electrons, as opposed to holes) such as Sn02 or ZnO, the concentration of electrons available for conduction can be changed by either oxidation or reduction processes. At elevated temperatures, atmospheric oxygen is adsorbed and it accepts electrons to become 02 , 0~, or 02~. If a reducing gas is then also adsorbed, it may either simply donate electrons and become a positively charged species; or it may react with oxygen, thus releasing bound electrons. In either case, electrons become available for conduction and the resistance of the surface layers decreases drastically. For an oxidizing gas, the converse mechanisms will operate and the resistance will rise. This conductivity versus gas concentration response is exploited in the metal oxide gas sensors.

An example of a possible food industry use of a gas sensor can be seen in the work of Mandenius and Mattiasson (22) for the on-line monitoring of ethanol during fermentation processes. A sensor (Fig. 3) similar to the one used by them is described by Watson (23). The sensor consisted of a small tubular ceramic former having an interdigitated metallization pattern on the outer surface, upon which was deposited the active materials, which was largely tin oxide plus catalytic dopants, notably palladium. A heater filament was put inside the tube, while the gold alloy sensor leads were bonded to annuli on the outer surface at the ends. These annuli were in fact part of the interdigitated

Lead

Lead

Tubular ceramic SnO former

Figure 3. Configuration of a semiconductor gas sensor. Source: Refs. 23 and 24.

Tubular ceramic SnO former

Figure 3. Configuration of a semiconductor gas sensor. Source: Refs. 23 and 24.

metallization deposited on the outer surface coated with the sintered, but porous, tin oxide mix. In the work of Mandenius and Mattiasson (22), ethanol, which is a reducing gas, was allowed to flow over the tin oxide coated surface of the sensor. The gas was absorbed onto the sensor surface and produced a marked decrease in the surface electrical resistance. By using a continuous dilution of the gas flow streams, the lower limit of detection of ethanol was extended by their work to operate within the concentration ranges of importance in biotechnological processes. A catalytic gas sensor device combined with a carrier gas flow control was recently developed into an integrated ethanol-sensing system (25). This permitted online monitoring of ethanol during the fermentation operation in a brewery. Gas detectors can be potentially made sensitive to other gases or volatiles present in biotechnological processes (for example, butanol, formate, acetate, and formaldehyde). For example, using metal oxide semiconductor gas sensors, it was possible to analyze wine vapors (26) or coffee aroma (27).

The advantages of this form of sensor include small size, convenient operation from low voltage power supplies, very high sensitivity in most applications and the need for only simple associated electronics. The disadvantages include a continuous power drain for sensor heating, sensitivity to ambient temperature and humidity and the presence of long-term drift. The first generation of gas sensors has suffered from poor selectivity, but many improvements are taking place, and the above example illustrates their potential on-line uses for food and agricultural processing.

Sleeping Sanctuary

Sleeping Sanctuary

Salvation For The Sleep Deprived The Ultimate Guide To Sleeping, Napping, Resting And  Restoring Your Energy. Of the many things that we do just instinctively and do not give much  of a thought to, sleep is probably the most prominent one. Most of us sleep only because we have to. We sleep because we cannot stay awake all 24 hours in the day.

Get My Free Ebook


Post a comment