To turn bare ISFET dies into working pH sensor systems, you need to add the following yourself:
Packaging of the chip (mounting on a substrate or in a probe, electrical contracting and sealing the chip exposing only the ISFET gate to the liquid)
External Reference Electrode
Readout and control electronics
Bare ISFET chips are usually interesting for larger volumes and customers that have the above expertise and are willing to bear the associated development costs. This development requires in-depth knowledge about the ISFET principles.