Development of a photopolymerisable membrane for calcium ion sensors: Application to soil drainage waters

Abstract

The fabrication of ion-sensitive electrochemical sensors for calcium is described in the present work. The membrane uses a photocurable polymer based on aliphatic diacrylated polyurethane. The use of photocurable polymers as the support matrix instead of PVC simplifies the preparation and the casting of the developed ion-sensitive membranes. Additionally, these polymers are compatible with the photolithographic fabrication techniques used in the microelectronics industry. This aspect permits the patterning of membranes on wafer level for semiconductor based sensors thus the automatization of the whole sensor fabrication. These polymer matrices show also better adhesion to silanised semiconductor surfaces, such as the gate surfaces of ion selective field effect transistors (ISFETs). Membranes sensitive to calcium ions were optimised according the type of plasticizer and the polymer/plasticizer ratio. Ion selective electrodes (ISEs) with a solid internal reference and ISFETs were used to evaluate the membrane. The resulting sensors were functional for periods of more than 8 months and the resulting sensitivities were quasi-Nernstian (26–27 mV/dec) in a range of 5×10 −6–8×10 −2 M. These sensors were used to measure calcium activity in water samples extracted from agricultural soils. The results were compared with those yielded by standard methods, finding a good correlation between the two analytical procedures.