Based on what can be called as the polymer counterpart of semiconductor (material whose conductivity is intermediate between that of a metal and an insulator), the sensors have been developed by depositing a thin film (having a thickness in the range of ten millionth of a metre) of the polymer on an appropriate substrate.

These polymeric thin film sensors could be used in mining industry, food processing units and environmental monitoring, SCK Misra, who along with Subhas Chander and colleagues at the National Physical Laboratory (NPL) has developed the sensors in a three-year project funded by the Department of Electronics (DoE).

Conducting polymers are made by doping ordinary plastics with elements like boron, aluminium, iron, copper and nickel. The sensors were successfully tested todetect a score of microbes such as escherichia coli, pseudomonas, salmonella, yeast, staphylococcus aureus and rhizobiaceae which are known to rot food.

"We have also tested the sensors to spot trace amount of carbon monoxide in underground mines with success," he said.

The NPL team is currently optimising the sensors to detect other gases like hydrochloric acid, hydrocyanic acid, ammonia, nitrogen oxides and sulphur dioxide.

The sensitivity of these detectors depends upon the concentration of the gases and microbes, as well as on the exposure time. The low-cost, fast-response sensors could be deployed in food processing units, which suffered from the problem of microbial contamination that could lead to serious health hazards for online monitoring, co-developer Subhas Chander said.

It could be used in underground alarm systems in mines when levels of toxic gases exceed safety limits, he said. The NPL team, however, agrees that it does not know the exact working mechanism of these sensors. "It is anew area and science still does not completely understand the way it works," Misra said.

"But we tested these sensors repeatedly and it works well," he added.

According to his theory, the sensors probably work on the principle of charge transfer between them and the subject to be sensed, as microbial cell walls are known to contain electrical charges.

Though multinational companies including Hoechst, Ciba-Geigy and Bayer were manufacturing such sensors, Indian companies were not yet geared up to exploit these sensors commercially, Misra added.