ISFET source separation: Foundations and techniques

Ion-sensitive field-effect transistors (ISFET) are solid-state electronic devices for chemical sensing. They are sensitive to the concentration of a particular ion in the solution to be tested, but they can also be strongly affected by specific interfering ions found in the solution. They should therefore only be employed where possible interferences are negligible and this limits their range of operation. However, since ISFETs behave as non-linear mixers of main ion activities and interferences, blind source separation (BSS) techniques and related methods such as independent component analysis (ICA) are suitable for attempting to separate the original main ion activity and the interferences from the mixed response. In this paper, we review the most important groundwork and techniques that can be employed when using ISFET arrays in this way to sense particular ions. Several experiments based on linear independent component analysis (ICA) in a 2 NH4-ISFET array demonstrate the usefulness of employing BSS for dealing with the separation of ion activities in ISFET responses and their later reconstruction in operating regions where interferences notably affect the response, and how this can cancel the interference effect in the ISFET response.