Abstract: A device useful in diagnostics in which a biochemical species is attached to the surface of a sensor, especially to the gate of a field effect transistor. In such a "BIOCHEMFET" it has been a problem to attach two or more biochemical species to the surface of the sensor, especially to the gates of a multi-gated FET. It has now been found possible to bond a group having a photoactivatable function covalently to the sensor surface, photo-expose the resultant modified surface selectively, e.g. through a mask, and bond the biochemical species, e.g. a hapten, antigen, antibody, lectin or enzyme, to the photoactivated function. In this way the biochemical species becomes attached in selected areas only. In view of the success of this technique realized under conditions of miniaturization, the invention makes possible "printed circuits for proteins".