Use of Biosensors to detect environmental pollutants

Biosensors are biophysical devices which can detect the presence of specific substances e.g. sugars, proteins, hormones, pollutants and a variety of toxins in the environment. They are also capable of measuring the quantities of these specific substances in the environment.

Technically a “Biosensor” is defined as “an analytical device containing an immobilized biological material (which could be an enzyme, or antibody, or nucleic acid, or hormone, or an organelle/whole cell), which can specifically interact with an analyte and produce physical, chemical or electrical signals that can be measured. An analyte is the compound (e.g. glucose, urea, drug, pesticide) whose concentration has to be measured. Biosensors basically involve the quantitative analysis of various substances by converting their biological actions into measurable signals. Generally the performance of the biosensors is mostly dependent on the specificity and sensitivity of the biological reaction, besides the stability of the enzyme.
A biosensor or an enzyme or an antibody is associated with microchip devices which is used for quantitative estimation of the substance. A biosensor equipment has the following components
a) a biological component - enzyme, cell etc,
b) a physical component-a device for measuring the quantity of this product, thus indirectly giving an estimate of the substrate e.g. transducer, amplifier etc.

The biosensors are being used in the area of medicine, industry etc. however their use in environmental monitoring is of great benefit. Special kits have been designed to identify the specific pollutants in the environment. E.g. special cost effective enzymatic tests are available which can detect pesticide contamination in water.

Principle of a biosensor

The biological material in use (e.g. an enzyme) is immobilized by conventional methods like physical or membrane entrapment, non-covalent or covalent binding. A contact is made between the immobilized biological material and the transducer. The analyte binds to the biological material to form a bound analyte which in turn produces the electronic response that can be measured. Sometimes the analyte is converted to a product which could be associated with the release of heat, gas (oxygen), electrons or hydrogen ions. The transducer then converts the product linked changes into electrical signals which can be amplified and measured.

A good example of a biosensor in frequent use is the glucose oxidase enzyme. The enzyme is immobilized on an electrode surface which acts as an electrocatalyst for oxidation of glucose. The biosensor gives reproducible electrical signal for glucose concentrations as low as 0.15 mM.

Another area where biosensors are being used is “Biomonitoring” or “biological monitoring”. Biomonitoring is defined as the measurement and assessment of work place agents or their metabolites either in tissues, secreta, excreta, or any combination of these systems in occupationally exposed human subjects. The “Biological effect monitoring” refers to the biological effects of these toxic agents in the workers exposed to these agents. A continuous evaluation of biological monitoring methods is done in order to assess the risk effectiveness of these tests against the various kinds of exposures to toxins. The use of genetic engineering to create organisms specifically designed for bio remediation also has great potential. The bacterium Deinococcus radiodurans , which is the most radioresistant organism known, has been modified to consume and digest toluene and ionic mercury from highly radioactive nuclear waste.

Some of the important biosensors used in environmental pollution monitoring are:

a) Gas biosensors- In order to detect gases such as sulphur dioxide, (SO2), methane, carbon dioxide etc, microbial biosensors have been developed. Thiobacillus-based biosensors can detect the pollutant SO2, whereas methane (CH4) can be detected by immobilized Methalomonas. A particular strain of Pseudomonas is used to monitor carbon dioxide levels.

b) Immunoassay biosensors- Immunoelectrodes as biosensors are used to detect low concentrations of pollutants. Pesticide specific antibodies can detect the presence of low concentrations of triazines, malathion and carbamates, by using immunoassay methods.

c) BOD biosensor- Biological oxygen demand (BOD) is widely used as a test to detect the levels of organic pollution. This requires five days of incubation but a BOD biosensor using the yeast Trichosporon cutaneum with oxygen probe takes only 15 minutes to detect organic pollution.

d) Miscellaneous biosensors- A graphite electrode with Cynobacterium and Synechococcus has been developed to measure the degree of electron transport inhibition during the photosynthesis due to certain pollutants e.g. herbicides. To detect phenol, phenol oxidase enzyme obtained from potatoes and mushrooms is used as a biosensor. Biosensors for the detection of polychlorinated biphenyls (PCBs) and chlorinated hydrocarbons and certain other organic compounds have also been developed. Biosensors employing acetylcholine esterase which can be obtained from bovine RBC can be used for the detection of organophosphorus compounds in water.