Here is your free sample essay on Bio- Sensor

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Living cells and organisms carry out their functions in a specific and ‘time-bound manner. A biochemical reaction occurs, when there is a need for it to occur, otherwise not, as though it is pre-programmed prior to its occurrence.

The cells and organisms do so to fulfill their own requirement and to adjust their internal environment in response to the external environment, they live in. Alternately speaking, they have a sense of time and place of occurrence. They know where they are and what and how much they have to perform at a specific point of time.

This is possible due to continuous feedbacks (sensing) from a class of bio-molecules, which occur in and around their environment. These molecules are unable to give a direct quantitative feed-back such that a cell knows how much to perform in response to the bio-molecule in question. It is based on the concept of an integrated sensing mechanism (device), known as a sensor.

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A sensor is an analytical physical device that senses the presence of a substance in its vicinity and measures its concentration.

The concentration of the substance is transducer into a directly proportional electrical signal, which can be read in a digital recorder. A biological sensor or bio-sensor is based on the above principle.

It essentially consists of an enzyme (biocatalyst) / an antibody / an entire cell / a tissue and a transducer, which transforms a biological or a biochemical signal into a quantifiable electrical signal. The electrical signal is read in a digital recorder like that of a sensor. This digital read-out speaks about the concentration of a molecule (substrate)

Outline of a simple bio-sensor

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The outline of a simple bio-sensor is presented in. It consists of the following components:

1. A substrate.

2. A membrane separating the substrate from the enzyme / antibody / microbial cell.

3. An immobilized enzyme / antibody / microbial cell.

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4. A membrane separating the product from the transducer.

5. A transducer and an amplifier.

6. A microchip.

7. Data processor.

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8. Digital reading device.

The substance, whose quantity is to be measured (substrate), is separated from the sensing agent (an immobilized enzyme / antibody / a whole microbial cell) by a thin membrane. The substrate diffuses through the membrane and reacts with the sensing agent forming a product.

The product then diffuses through another membrane into a transducer-amplifier, which transforms and amplifies the biological / biochemical signal into an electrical signal. The electrical signal data is then processed by a microchip of a computer and presented in a readable form to an output device (digital reading device).

A simple bio-sensor

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A large number of the diabetics are dependent on insulin injection for keeping their blood-glucose at a constant level. The injected insulin is directly proportional to the excess of blood glucose above normal.

Therefore, a very precise quantity of insulin has to be injected. This raises a need to measure the blood glucose level in a diabetic’s blood in a rapid, precise and continuous manner such that a right quantity of insulin will be injected at a right moment.

This gave an impetus for designing a simple bio-sensor based on an enzyme (sensing agent), glucose oxidize. This enzyme acts on the substrate (excess blood glucose). Glucose reacts with molecular oxygen to form glycolic acid and hydrogen peroxide. This reaction is catalyzed by the enzyme, glucose oxidize.

Glucose oxidases

Glucose + Oz ————————- Gluconic acid + H2Oz

In the above mentioned biochemical reaction, the substrate (glucose) disappears and the product (Gluconic acid) appears in a reciprocal relation to each other.

The rate of consumption of oxygen is also proportional to the quantity of glucose present. Any one of these three parameters may be considered as an index of the initial concentration of the substrate (glucose). Thus, the rate of the reaction is measured by any one of the three following methods:

1. The rate of consumption of 02 can be measured by its reduction at a platinum cathode vs. a standard calomel electrode.

2. The rate of production of H,0, can be measured by its oxidation at a platinum anode vs. a standard calomel electrode a thin membrane.

The substrate diffuses through the membrane and reacts with the sensing agent forming a product. The product then diffuses through another membrane into a transducer-amplifier, which transforms and amplifies the biological / biochemical signal into an electrical signal.

The electrical signal data is then processed by a microchip of a computer and presented in a readable form to an output device (digital reading device).

A simple bio-sensor

A large number of the diabetics are dependent on insulin injection for keeping their blood-glucose at a constant level. The injected insulin is directly proportional to the excess of blood glucose above normal! Therefore, a very precise quantity of insulin has to be injected.

This raises a need to measure the blood glucose level in a diabetic’s blood in a rapid, precise and continuous manner such that a right quantity of insulin will be injected at a right moment.

This gave an impetus for designing a simple bio-sensor based on an enzyme (sensing agent), glucose oxidases. This enzyme acts on the substrate (excess blood glucose). Glucose reacts with molecular oxygen to form Gluconic acid and hydrogen peroxide. This reaction is catalyzed by the enzyme, glucose oxidases.

Glucose oxidases

Glucose + Oz ————————- > Gluconic acid + H2Oz

In the above mentioned biochemical reaction, the substrate (glucose) disappears and the product (Gluconic acid) appears in a reciprocal relation to each other.

The rate of consumption of oxygen is also proportional to the quantity of glucose present. Any one of these three parameters may be considered as an index of the initial concentration of the substrate (glucose). Thus, the rate of the reaction is measured by any one of the three following methods:

1. The rate of consumption of 02 can be measured by its reduction at a platinum cathode vs a standard calomel electrode.

2. The rate of production of H202 can be measured by its oxidation at a platinum anode vs a standard calomel electrode.

3. The rate of production of Gluconic acid and the decrease in the pH can be measured using a pH electrode.

An electrode function as the transducer-amplifier device in the above mentioned bio-sensor. This bio-sensor may be permanently implanted under the skin of a diabetic, which will check the blood-glucose level from time to time.

This signal may be passed on to a mini pump filled with insulin, which may release a correct amount of insulin in response to the information provided by the bio-sensor.

This build up mimics a normal pancreas, which continually monitors the blood-glucose level and reacts promptly to the changes by decreasing or increasing the insulin output.

The same principle may be applied to the monitoring of chorionic gonadotropin concentration in the urine of pregnant women, creatinine concentration in the blood of persons suffering from heart attacks and urea concentration in the blood of patients suffering from kidney malfunctioning.

The bio-sensors have also found a place in the chemical manufacturing industries, which are involved in the manufacture of acids, alcohols and phenols.

It is used to monitor the levels of different types of chemical pollutants in the environment. In the two latter cases, whole cells, especially microbial cells are used instead of enzymes or antibodies. Bio-sensors have promising applications in the war front too. Bio-sensors may be designed, which may detect various chemical agents, used as chemical weapons.

Three levels of bio-sensors

Based on the relationship of the two components, the biocatalyst and transducer, the bio-sensors are now classified as: (l) first generation; (2) second generation; and (3) third generation.

In the first generation bio-sensor, as mentioned in the foregoing section, the biocatalyst and the transducer are easily separated and each may function in the absence of the other.

The two components act in a more intimate fashion in the second generation bio-sensors. The removal of one affects the functioning of the other. Here, the oxidation of glucose is not measured by the disappearance of glucose and appearance of Gluconic acid, but by the rate of flow of electrons to an electrode surface.

The redox reactions mentioned in l & 2 force the mediator [M (red)] to undergo oxidation at the electrode surface and release [M+ (ox)] and electrons. The M+ (ox) is used to keep the redox reactions going, while the electrons flow in the circuit to produce an electric current proportional to the rate of oxidation of glucose and hence to the glucose concentration.

This principle forms the basis of the functioning of the blood glucose meter used in measuring the blood-glucose level in diabetics. Ferrocene is used as the mediator. The third generation bio-sensors are at the research and trial level. It will be based on the principle that the enzyme will be directly reduced at the electrode surface skipping the use of a mediator.

Cell-based bio-sensors

Most of the present day bio-sensors are based on the use of enzymes / antibodies as the sensing agents. These are easy to produce, since the enzymes and antihodies are easily immobilized on inert supports. Recently, cell-based bio-sensors have been used, which use whole cells as sensing agents. However, these encounter a problem.

The cells require a harsh treatment for their immobilization on polyacrylamide / agarose gels. Despite this fact there are many advantages. In the multi-enzyme catalytic pathway of a substrate into a product, each of the enzymes need not be immobilized. Secondly, the cell is a self sufficient system, where no additive is necessary.

A cell-based bio-sensor has been constructed using the cell, Nocardia erythropolis, immobilized on polyacrylamide / agarose gel. The transducer-amplifier in this bio-sensor is an oxygen electrode, which measures the rate of molecular oxygen uptake by the substrate, cholesterol. It measures the cholesterol content of the sample, plasma. The biochemical reaction is presented below.

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