The term “biosensor” refers to a wide variety of devices. The common element is that a biological component provides highly specific recognition of a certain target analyte, and this detection event is somehow transduced to give an easily detectable, quantifiable response, preferably one that can be easily converted to an electrical signal so that the result can be fed to an electronic device for signal processing, data storage, etc. Source
Using E. Coli as a host bacteria (Chassis), researchers added a recognition element and a reporter gene which, respectively, detected the presence of arsenic and expressed this condition by producing acids. These acids consequently caused a change in pH that could be easily read in the field. Varying speeds of the change in pH could be analyzed to estimate the concentration levels of arsenic in the tested water.