Since the beginning, detecting chemical compounds has been an important tool for the growth of biological and chemical sciences. The evolution of new-age techniques such as NMR, HPLC, and GLC with enhanced capabilities with respect to reproducibility, precision, and sample handling has shown enormous potential for a sustained progress in analysis. Within the last decade, immense interest has seemed to have developed in biological activity. This applied research led to the new concept of biosensors, devices or arrangements that are used to detect and measure wide array of compounds based on molecular interaction or modification brought by biomolecules with catalytic abilities.


Biosensors, a recent product of biotechnology, have resulted in deeper research of biological and biocatalysis phenomena. Additionally, new discoveries in the fields of optical transduction, electronics, and electrochemistry have contributed significantly to the future of biosensors. The commercial success of a biosensor depends on its wide range of applications on current analytical techniques. The important component of a biosensor is a biological system or molecule; whether in a purified form (nucleic acid, enzyme, receptor, and antibody) or in a complex structure form (tissue, whole cell, and organelle). Predominantly, biosensors are conducted in aqueous media following the success of an enzyme electrode.

Systems Based on Bioreactions

Gases are analyzed by enzymatic methods and have been practiced for several years now. The procedure is similar to procedures that are used for any soluble compound, namely after gas analyte solubilization in aqueous buffer containing the transformation enzyme. This step is followed by the transduction stage, which is mostly potentiometric or amperometric, to quantitate the analyte. The value in the solution is extrapolated to the gas-phase concentration using the co-efficient of gas-liquid partition. The biggest advantage one would derive is with systems in which the bioactive ingredient comes in direct contact with the gaseous phase thereby a measurable response is detected or amplified within a short time.

Bioreaction System- Formaldehyde Vapors and Ethanol Analysis

This approach proved successful to formaldehyde vapors and ethanol analysis and in simple devices that contain dehydrated alcohol oxidase. Using this technique, it is now possible to establish the blood alcohol content (BAC) related directly to the alcohol content in breath. Market products include disposable tubes that are filled with a powerful oxidizing agent like the (NH4)2Cr2O7 in sulfuric acid impregnated on a silica gel that changes color when exposed to the ethanol present in breath. The system had an enzymatic powder, which is packed in a glass tube and maintained in place by wool plugs. The powder was prepared using a solution that contained alcohol oxidase, 2, 6 dichloroindophenol (DCIP), and peroxidase and blended with microcrystalline cellulose powder (example, Avicel) and stirred for a short duration.

The resulting paste was air-dried till the water content reached the optimal level (10-60 percent) and the final air-dried powder was stored at 5. Celsius. The aqueous solution was used to determine the enzymatic activities of the solid biocatalyst. Kinetic studies in the aqueous solution showed that the reaction rate was time linear and was dependent on the initial ethanol concentration and independent of the color indicator concentration.

The color change time control for a fixed ethanol concentration was achieved by the manipulation of enzyme to DCIP ratio. This principle was tested to determine ethanol vapors as well.

The gas-phase concentration corresponding to BAC of 1 g 1-1 was calculated as 10 μg 1-1 and the system showed sensitivity even at low concentrations with a 2.5 minutes response time with the water content being adjusted to 35 percent in the biocatalytic powder.

Taking advantage of the important fact that the enzyme alcohol oxidase has the potential to oxidize formaldehyde, this technique was used to describe the formaldehyde presence in an air stream.

Based on this system, biocatalytic or the direct enzymatic conversions of vapor or gases represent an approach that has been fully explored with high impact in gas-phase sensors.

The advantages of directly converting gaseous compounds include:
(1) recovery of biocatalysts from the reactor is easy;
(2) efficient substrate transfer rate to the catalytic entity; and
(3) product recovery is facilitated by adsorption or fractional condensation.

The development of biosensor primarily depends on the understanding and progress of the phenomena involving enzymatic reactions in an anhydrous media especially for gas-phase biocatalysis.

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