Ultrathin, long-term stable, solid-state reference electrode enabled by enhanced interfacial adhesion and conformal coating of AgCl

Continuous biochemical monitoring with a flexible electrochemical sensor offers a new wearable electronic system that can measure real-time voltage and current signals. The signal quality is determined by long-term stability of a silver/silver chloride reference electrode (Ag/AgCl RE). However, it is very challenging for any solid-state electrode to have a long-term stable operation. Even though new membrane technologies have improved the voltage stability, the existing thin film Ag/AgCl REs have limitations of insufficient film adhesion and structural instability. Here, this paper introduces an ultrathin, all-solid-state RE that demonstrates a long-term functional stability for more than two weeks via enhanced interfacial adhesion and conformal coating of AgCl. An optimization of chlorination factors allows a highly uniform, 800 nm-thick RE surface. The voltage response of the sensor in a saline solution shows a change of 0.09 mV/h for 18 days with a quasi-stable behavior, capturing the potential as an amperometric biosensor. Moreover, the characterization with an enzymatic working electrode verifies that two-electrode system using the thin-film RE has a sensitivity (S = 606 nA·mmol^−1. cm⁻²), compatible to that with a commercial RE (S = 532 nA·mmol^−1. cm⁻²). Collectively, this work provides a comprehensive study of materials and surface functionalization of all-solid-state REs for thin-film biosensors, which will pave the way for long-term usable wearable biosystems.