Doped pencil leads for drawing modified electrodes on paper-based electrochemical devices

A simple strategy is here described for assembling graphite-based pencil leads doped with electrode modifiers suitable for drawing electrochemical devices on porous materials such as paper in a very reproducible and easy way. They were prepared by mixing controlled amounts of the chosen modifier with different quantities of carbon powder (conductive material), sodium bentonite (binding agent) and sodium silicate (hardening agent). After pressurisation, these mixtures were extruded at room temperature from a suitable die in thin rods which were then inserted in commercial lead holders to facilitate their use for drawing electrodes on paper. Lead composition (80% graphite powder; 8% sodium bentonite; 12% potassium silicate) and their fabrication procedure were optimised by drawing working electrodes for paper-based electrochemical cells with leads prepared with different contents of their components and evaluating their performance by voltammetric measurements conducted on hexacyanoferrate(II). Two prototype species (decamethylferrocene and cobalt(II) phtalocyanine, chosen as model compounds displaying a reversible redox process and versatile electrocatalytic properties, respectively) were assayed for doping leads. A quite satisfactorily reversible electrochemical behaviour was observed for decamethylferrocene incorporated into graphite-based working electrodes drawn on paper-based cells (RSD% for peak height and peak potential of 4.1 and 2.4, respectively) and a good electrocatalytical activity towards cysteine and hydrogen peroxide was displayed by graphite-based working electrodes modified by cobalt(II) phtalocyanine.