Did the colour change?

Workings of the litmus paper mimicked to check for pesticides

Published: Tuesday 15 December 2009

DownToEarth litmus paper is used to tell whether a solution is acidic or basic. The paper is dipped in the solution and a colour change either to blue or to red does the trick. Similar, easy-to-use paper strips have been developed for checking the presence of pesticides in food and water.

Tests for detecting pesticides are expensive, time consuming and require complex equipment. The paper strip can produce the same result within five minutes. Researchers from the department of chemistry and chemical biology, McMaster University in Canada described the working of these dips in the November 1 issue of Analytical Chemistry.

The test strip is one cm wide and 10 cm long. Acetyl-cholinesterase and a colour producing chemical indophenyl acetate (ipa) are deposited on the paper in two different zones: sensing and substrate zones. Acetyl-cholinesterase is an enzyme responsible for proper nerve impulse transmission.The organophosphate pesticides work by inhibiting the enzyme. The colour of the substrate (ipa) is yellow. In the presence of pesticides its colour changes to bluish-purple.

To perform the test, the researchers dipped the strip in the substance which could have the pesticide so that it reached the sensing zone. Then the strip was removed and the opposite end, containing the susbtrate zone, was dipped in purified water. This caused the ipa to dissolve in the water and move towards the sensing zone. The change of the substrates colour to bluish-purple was easily visible. In the absence of pesticides, the colour of the ipa remains yellow.

Another advantage is that the strip can detect very small amounts of pesticides. They can be used by the food industry and in under-developed regions where performing the conventional tests would be difficult in the absence of electricity.

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