244 CHEMICAL SENSORS. VOLUME 5: ELECTROCHEMICAL AND OPTICAL SENSORS 10.12. SODIUM ION, Na + SENSOR Chudy et al. (2001) used the ionophore 4-tert-butylcalix[4]arene-tetraacetic acid tetraehyl ester. Before membrane deposition, poly(2-hydroxyethyl methacrylate), polyHEMA, was conditioned over 3 h in an aqueous solution. The membrane solution was prepared by dissolving the membrane components (100 mg): ionophore (2 mg), lipophilic salt potassium tetrakis [3,5-bis(trifluoromethyl)phenyl] borate (KTFPB) (0.72 mg), Siloprene K 1000 (64.1 mg), and cross-linking agent K 11 (0.6 mg), in 0.25 ml of freshly distilled THF. Then 10 l of the membrane solution was dropped on the gate oxide covered with a polyHEMA layer, and the membrane solvent was allowed to evaporate. The slope of the sensor for Na + ion was >50 mV/decade, and the detection limit was 1.58 × 10 -5 M. The reponse time was <60 s. 11. ISFET-BASED SENSORS FOR NEGATIVE IONS 11.1. CHLORIDE ISFET Bratove et al. (2004) studied Cl - -sensitive ISFETs with photocured polyurethane-based polymer mem- branes with six different ionophores (ETH 9033, ETH 9009, MnTPPCl, organotin compounds, and the traditional ion exchanger TDMACl). PVC or silicon resin was used to form the membranes in cases