Pencil leads doped with electrochemically deposited Ag and AgCl for drawing reference electrodes on paper-based electrochemical devices

We propose here a quick and easy procedure for preparing pencil leads doped with Ag and AgCl suitable for the reproducible and easy drawing on paper or other porous supports of carbon based Ag/AgCl conductive lines for the use as reference electrodes. They were prepared by careful electrodeposition of silver and silver chloride on carbon powder (conductive material) which was then mixed with sodium bentonite (binding agent) and sodium silicate (hardening agent). Extrusion at room temperature of these mixtures from a suitable die allowed thin rods to be obtained, which were then inserted in commercial lead holders to facilitate their use for drawing electrodes on paper. Their performance was evaluated by voltammetric measurements conducted by using hexacyanoferrate(II) and 1,2-hydroxybenzene as probe species known to display a reversible electrochemical behaviour. The results found pointed out that the potential exhibited by these drawn reference electrodes was fully reliable only when small but controlled amounts of chloride ions can be added to assayed samples, without interfering investigated processes. Conversely, a poorer reliability characterise these reference electrodes when chlorides cannot be added, in that they display potentials higher of about 100-150 mV than their theoretical value. Their reproducibility (± 0.02 V), controlled on a series of different PEDs wetted with the same sample, was however so narrow for voltammetric investigations as to make their use preferable to usual pseudo-reference electrodes whose potential is remarkably affected by the different components present in assayed samples.