Paper-Based Capacitive Solid-State Reference Electrode with Carbon Nanotubes and Adsorbed Heavy-Metal Ions

We herein fabricated a capacitive solid-state reference electrode by loading a paper strip with carbon nanotubes (CNTs) to increase double-layer capacitance and adsorbing heavy-metal ions thereon to charge the electrical double layer and minimize the effects of other ions on the interfacial potential difference. CNTs were loaded by immersing the paper strip into a CNT solution followed by drying. By repeating these steps up to 20 times and adsorbing Cu 2+ , we stabilized the electrode potential and minimized the influence of other ions and electroactive compounds. The potential of the best-performing CNT electrode in the presence of different electrolytes and electroactive compounds ranged from 243 to 249 mV, exhibiting a variation of ≤6 mV over 60 min, whereas gold, carbon paste, and copper electrodes exhibited potential variations of tens of mV or larger. The influence of pH was negligible, although that of buffer components was not negligible in some cases. The CNT reference electrode was tested by potentiometric ion sensing and cyclic voltammetry and shown to be a viable alternative to a macroscopic Ag/AgCl electrode.