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 mu m. The needle is electrically insulated by a thin (15 to 20 nm) insulat ion 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 spec ially designed scanning tunneling microscope (STM) working in a high pressu re oxygen atmosphere. The: reduction of the total resistance after the loca l plasma treatment was proved by measuring the transition resistance betwee n the needle and a 0.1 M NaCl solution. It is supposed that the two process es 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:N film into a more graphitic-like state (increase d content of sp(2) bondings) by a thermal process;The functioning of this n ew type of electrode was tested by measuring the transmembrane potential of mouse liver cells in vitro. (C) 1999 Elsevier Science S.A. All rights rese rved.