Micromachined Ammonia Gas-Sensing Electrode and its Application to a Micro Urea Sensor

Abstract

A micro urea sensor was fabricated using a micromachined Severinghaus-type ammonia gas-sensing electrode. The electrode detects a local pH change caused by the permeation of gaseous ammonia into the thin internal electrolyte layer in the sensitive area. An anodic iridium oxide film (AIROF) indicator electrode and a thin-film Ag/AgCl reference electrode were formed on a glass substrate by photolithography. The container for the electrolyte solution was formed by anisotropic etching of silicon. A through-hole was formed in one portion of the silicon container and a silicone rubber gas-permeable membrane was formed over the through-hole. Urease was immobilized on the gas-permeable membrane. Gaseous ammonia produced by the enzymatic reaction was detected by the ammonia gas-sensing electrode. Distinct response was observed, although the response was sluggish due to the low ammonia-permeability of the gas-permeable membrane. A linear relationship was obtained between the potential of the urea sensor and the logarithm of urea concentration between 5 and 50mM.