Production of a microelectrode for intracellular potential measurements based on a Pt/Ir needle insulated with amorphous hydrogenated carbon

A new microelectrode is presented, based on an electrochemically etched Pt/Ir needle with a high aspect ratio and a radius of curvature smaller than 1 μm. The needle is electrically insulated by a thin (15 to 20 nm) insulation film made of hydrogenated amorphous carbon (a-C:H). In order to use the needle as a microelectrode, the very end is made conductive again through a local oxygen plasma. The localization of the plasma is achieved in a specially designed scanning tunneling microscope (STM) working in a high pressure oxygen atmosphere. The reduction of the total resistance after the local plasma treatment was proved by measuring the transition resistance between the needle and a 0.1 M NaCl solution. It is supposed that the two processes responsible for the decrease of the resistance are: (a) the reduction of the thickness of the a-C:H insulation by reactive oxygen ion etching, (b) transformation of the a-C:H film into a more graphitic-like state (increased content of sp² bondings) by a thermal process. The functioning of this new type of electrode was tested by measuring the transmembrane potential of mouse liver cells in vitro.