Batch microfabrication and testing of a novel silicon-base miniaturized reference electrode with an ion-exchanging nanochannel array for nitrite determination

Abstract

The reference electrode (RE) provides a stable potential for electrochemical detection; therefore, the RE plays an important role in environmental monitoring. In this paper, a novel batch of microfabricated silicon-base miniaturized Ag/AgCl RE was reported. A specially designed mini-tank for saturated KCl solution storage and a nanochannel array for ion-exchange were fabricated on a 4 inch (100) silicon wafer using a two-step KOH anisotropic etching process. An Ag/AgCl electrode was fabricated on a 4 inch Pyrex 7740 glass substrate. Finally, the finished silicon and glass substrates were anode bonded to form the entire system. By comparing with a conventional solid-state Ag/AgCl RE in electrochemical microsensors, a pre-packaged saturated KCl solution in the mini-tank provided a stable working environment for the Ag/AgCl electrode to ensure a constant reference potential. Compared with a routine glass-structured RE and by replacing the ion-exchange membrane with a nanochannel array, the miniaturized RE achieved a longer lifetime. The size of the finished miniaturized RE electrode was 11 mm × 14 mm. The reference potential variation was only 0.1 mV under continuous testing for 3000 s. The standard deviation in the reference potential was only 1.314 mV in different Na₂SO₄ buffer concentrations ranging from 3 mM to 30 mM. To verify the practicality of the novel silicon-base miniaturized RE, the fabricated RE was applied to measure the amount of nitrite in a water sample and achieved a better linearity of R² = 0.998. This miniaturized RE showed better reference potential stability and consistency because of the batch fabrication technique. This novel strategy for the design and manufacture of the miniaturized RE shows a bright future in the wide use of electrochemical sensors in online monitoring of water pollutants.