Use of photopolymerizable membranes based on polyacrylamide hydrogels for enzymatic microsensor construction

The construction of enzymatic microsensors based on semiconductor technology and photo-curable membranes is reported in this work. A planar three-microelectrode cell and a pH-sensitive Ion Selective Field-Effect Transistor (ISFET) with Si3N4 membrane have been used as transducers for glucose and urea measurements, respectively. Polyacrylamide has been employed as matrix material for the immobilization by entrapment of the enzyme. This hydrogel permits the membrane deposition and patterning by photolithographic techniques which are compatible with semiconductor technology. It also provides reproducible and thin membranes due to the possible automatization of the whole process. In order to improve the long-term stability of the sensor, the introduction of glutaraldehyde in the membrane for inter-enzyme cross-linking has been tested. Both methods of membrane preparation, with the cross-linker and without it, have been compared observing an increase of the long-term stability of around two times for glucose and urea sensors. The response characteristics of glucose and urea sensors using several membrane preparations have been studied under different measurement conditions. A sensitivity of 45-67 nA mM-1 in a linear range 4 x 10-3-1 mM depending on the membrane thickness and enzyme concentration has been observed for the amperometric glucose sensor. A sensitivity from 50 to 58 mV per decade in a concentration range 0.2-8 mM urea depending on the membrane preparation has been obtained for the urea enzymatic Field-Effect Transistor (ENFET) sensor.