Before deposition of the PVC membranes, a surface
treatment was performed: the ISFET insulator surface was
treated with HMDS hexamethyldisilazane in order to Ž .
functionalize surface sites hydroxyl groups . This treat- Ž .
ment allows to neutralize the acid base properties of these
sites and thus to obtain a blocking insulatorrPVC interface.
Moreover, the hydrophobic methyl groups at the
grafted surface insures a good adhesion of the PVC membrane.
This type of treatment can avoid the use of an
intermediate hydrogel layer as it was recommended by
Bergveld et al. 19 in order to eliminate the effect of CO w x 2
diffusion on the surface charge of the insulator.
Next, the plasticized PVC membranes were formed by
depositing 0.5 ml of each of the previous solutions on the
surface of one ISFET for differential measurement and Ž .
the solvent was evaporated under nitrogen flow. Then, the
sensor was soaked during some hours in a 0.1 M NH Cl 4
solution in order to obtain conditioning of the NHq
4 –PVC
membrane.