ON-WAFER FABRICATED FREE-CHLORINE SENSOR WITH PPB DETECTION LIMIT FORDRINKING-WATER MONITORING

The fabrication and characterization of a planar silicon-based sensor for free chlorine, present in the form of hypochlorous acid, is presen ted. The sensor uses a three-electrode electrochemical cell, with thin -film platinum working and counter electrodes, and a thin-film partial ly chloridized silver reference electrode. The working electrode is co vered with a photopolymerized polyHEMA hydrogel membrane, which serves as a diffusion-limiting membrane. All electrodes are contacted by pol ysilicon films isolated with silicon nitride, to obtain a precise defi nition of the electrode area and to make the encapsulation less critic al. The optimal polarization potential for hypochlorous acid detection at which there is no interference from the reduction of dissolved oxy gen is +350 mV versus SCE. Calibration curves show an excellent linear ity (R2 = 0.9996, n = 21) and a sensitivity of approximately 10 nA/(mg chlorine/l). The detection limit (SIN=3) is approximately 1 mug chlor ine/l (1 ppb).