Analytica Chimica Acta 408 (2000) 57–64 Ion-selective field effect transistor (ISFET)-based calcium ion sensor with photocured polyurethane membrane suitable for ionised calcium determination in milk Andrey Bratov ∗ , Natalia Abramova, Carlos Dom´ ınguez, Antonio Baldi Centre Nacional de Microelectrónica, Campus UAB, 08193 Bellaterra, Barcelona, Spain Received 1 July 1999; received in revised form 15 November 1999; accepted 17 November 1999 Abstract Ca 2+ -ion-sensitive ISFETs with photocured polyurethane-based polymer membranes with alkylphosphate exchanger (DOPPCa) and three neutral carrier ionophores (ETH 1001, ETH 129 and ETH 5234) and dioctylsebacate as plasticizer have been studied in pure CaCl 2 solutions and in background solutions containing Na + ,K + , Mg 2+ ions in concentrations normally found in milk. In case of DOPPCa and ETH 1001 the influence of Mg 2+ ions on the slope of the response curve is reported. ISFETs with membranes containing ETH 129 are shown to have high selectivity, more than 6 weeks life-time in constant contact with a solution and high precision of ionised calcium determination. Experimental results on determination of calcium ions in a whole milk are presented. ©2000 Elsevier Science B.V. All rights reserved. Keywords: ISFET; Photocured polymer membrane; Ca-ion sensor; Milk 1. Introduction Calcium ion-selective sensors based on a poly(vinyl chloride) (PVC) polymer membranes impregnated with different ionophores find wide applications in electroanalytical practice [1]. They are widely used in medical and biological investigations as well as in the food industry. An important application is the control of calcium (Ca 2+ ) ion concentration in milk and milk products especially in cheese production during the processes of coagulation [2,3]. To exploit the advantages offered by modern tech- nologies in the fabrication of solid-state chemical sensors based on ISFETs [4,5] new requirements are ∗ Corresponding author. Fax: +34-93-580-1496. E-mail address: andrey@cnm.es (A. Bratov). imposed on a polymer matrix for ion sensor mem- branes. They are the adhesion of a polymer to a solid substrate and the compatibility of an ion-sensitive membrane deposition process with microelectronic technology. From this point of view photocurable polymers [6–9] offer attractive advantages over tradi- tional PVC membranes, because they can be deposited by screen-printing or by spin-coating and patterned using standard photolithography processes. Also, the polymer curing process occurs very fast in comparison with other methods of polymer ion-selective mem- brane formation. Acrylate and methacrylate-based compositions are possible photocurable polymers that can be applied as ion-selective membrane matrices [10,11]. Among different polymers studied as a pos- sible polymer matrix for ion sensors polyurethane [12–16] showed most promising properties. Earlier 0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII:S0003-2670(99)00871-5