Optimization of Photocurable Polyurethane Membrane Composition for Ammonium Ion Sensor Andrey Bratov, Nataliya Abramova, Javier Mufloz, and Carlos Dominguez Centre Nacional de Microelectrànica, Universitat Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Spain Salvador Alegret and Jordi Batroli Department of Chemistry, Universitat Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Spain ABSTRACT The composition of ammonium-ion sensitive membrane based on a photocurable aliphatic urethane diacrylate oligomer has been optimized. Membranes were prepared with exposure to TJV and studied in a traditional ion-selective electrode configuration with liquid inner contact. The optimum composition is found to be: urethane diacrylate (45 to 50%), hexandiol diacrylate (10%), photoinitiator (0.8 to 1.2%), plasticizer (bis(2-ethylhexyl) sebacate or di-5-nonyladi- pate (35 to 40%), nonactin (2%), and KTpC1PB (0.5%). The resulting ion-sensors show a sensitivity of 55.5 mV/decade, a range of linear response from 1 to 4.5 iO mol/liter of NH4C1, a limit of detection around 10' mol/liter, and good selec- tivity in the presence of potassium ions. Introduction of 7% PVC into the membrane formulation gives a possibility to enhance the sensitivity up to 58 mV/decade but prevents application of photolithography for membrane patterning in case of ion sensitive field effect transistor-based sensors. The developed polymer composition was used to prepare ISFET- based ammonium sensors with the membrane deposited and structured on a wafer level. Ammonium ion-selective sensors based on a poly(vinyl chloride) (PVC) polymer membrane impregnated with nonactin as an ionophore find wide applications in elec- troanalytical practice.1 They are also widely used as a base sensing element in biosensors for urea determination2-' with an enzyme containing layer deposited over ion-sensi- tive membrane. To exploit the advantages offered by mod- em technologies in a fabrication of solid-state chemical sensors based on ion-sensitive field effect transistors (ISFETs)4'6 and planar analogues of coated wire electrodes, in which a polymer membrane is in direct contact with a metal,7 new requirements are imposed on a polymer matrix for ion sensor membranes. The first is the adhesion of a polymer to a solid substrate (insulator or metal). PVC, commonly used as a polymer matrix for ion sensors, has poor adhesion to a solid support. It is possible to enhance the adhesion properties of the PVC by chemical modifica- tion,8'9 utilization of photocross-linkable," or high molec- ular weight plasticizers.1' Several other alternate polymer matrices for solid-state ion sensors like silicon rubber,6 polyurethane,' and polyimide" with better adhesion to a solid support were tested. The second requirement is the compatibility of an ion- sensitive membrane deposition process with microelec- tronic technology. From this point of view photocurable polymers7" offer the most attractive advantages, because they can be deposited by screen-printing or by spin-coat- ing and patterned using a standard photolithography process. Among possible photocurable polymers that can be applied as ion-selective membrane matrices acrylate and methacrylate-based compositions'4-'8 and poly- styrene'9 were studied. Earlier we have reported" on ammonium sensitive ISFET with photocurable polymer ion-selective membrane based on urethane acrylate. In this work, a conventional ion-selective electrode configu- ration with liquid inner contact was used to optimize the membrane composition and to obtain more detailed infor- mation on electrochethical properties of the membrane. Experimental Reagents.—Nonactin, bis(2-ethylhexyl)sebacate (DOS), bis(2-ethylhexyl)phthalate (DOP), di-5-nonyladipate (DNA), tetraundecylbenzhydroh3,3 ' ,4,4 '-tetracarboxylate (ETH 2112), and potassium tetrakis(p-chlorophenyl) borate (KTpC1PB), poly(vinyl chloride) PVC high molecu- lar weight were purchased from Fluka. Aliphatic urethane diacrylate oligomer Ebecryl 270 (Mw 1500) and hexanedi- ol diacrylate (HDDA) were donated by IJCB Chemicals, Photoinitiator 2,2 '-dimethoxyphenylacetophenone (Irga- cure 651) was from Ciba-Geigy. All other chemicals were of an analytical-reagent grade. Standard solutions were prepared with deionized water. Preparation of ion-selective membranes and evaluation of chemical response—First, the main polymer composi- lion was prepared by mixing together the aliphatic ure- thane diacrylate oligomer, reactive diluent HDDA, and photoinitiator Irgacure 651 in an 81:17:2 weight/weight ratio. Then 0.3 g of the main polymer composition was dis- solved in 0.2 ml of tetrahydrofuran and to this solution plasticizer, nonactin, and potassium tetrakis(p-chloro- phenyl) borate were added. The mixture was thoroughly stirred in an ultrasonic bath until homogeneous and then left for several hours to evaporate the solvent. Ion selective membranes 8 mm in diameter and 0.3 to 0.4 mm thick were formed by pipetting several drops of the membrane cock- tail into a special cylindrical holder. This layer was ex- posed to ultraviolet (UV) light using standard mask align- er equipment with irradiance of 22 mW cm' at the wavelength of 365 nm. The typical exposure time was 2 mm. After rinsing with ethanol, the membranes were glued to the end of an acrylate plastic tube (8 mm od, 6 mm id) using the main polymer composition without a plasti- cizer and with exposure to UV for 10 s. The membranes were characterized in conventional ion- selective electrode (ISE) configuration with inner liquid contact. At least three electrodes were prepared to charac- terize one of the different membrane compositions. The internal reference was a Ag/AgC1 wire with a 0.1 mol/liter NH4C1 as an internal solution. The potentiometric response of the electrodes was measured relative to a dou- ble-junction Ag/AgC1 reference electrode (Orion 90-02). The outer filling solution of this reference was a 0.1 mol/liter solution of lithium acetate. Calibration curves were obtained in the concentration range 10' to 10' mol/liter in pure solutions of NH4C1 by adding a known amount of prepared stock solutions under stirring. The effect of pH buffering of the solution on electrode response was tested in the background solutions based on tris(hydroxymethyl)aminomethane_hydrochoic acid (TRIS-HC1, pH 7.2) or urotropine-HC1 (pH 7.0) pH buffers. The activities of the ions were calculated using the Debye- Huckel approximation. A Crison micropH 2002 digital potentiometer with 0.1 mV accuracy provided potential measurements. All the measurements were carried out at 25 1°C in a thermostated laboratory room. Between the measurements the electrodes were stored in a 10-2 mol/ liter solution of NH4C1. J. Electrochem. Soc., Vol. 144, No.2, February 1997 The Electrochemical Society, Inc. 617 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 54.167.105.44 Downloaded on 2017-04-04 to IP