Ž . Sensors and Actuators B 56 1999 6–14 Production of a microelectrode for intracellular potential measurements based on a PtrIr needle insulated with amorphous hydrogenated carbon M. Schwank a, ) , U. Muller a , R. Hauert a , R. Rossi b , M. Volkert b , E. Wintermantel c ¨ a ¨ ( ) Swiss Federal Laboratories for Materials Testing and Research EMPA , Uberlandstrasse 129, 8600 Dubendorf, Switzerland ¨ b Institute of Veterinary Physiology, UniÕersity of Zurich, CH-8057 Zurich, Switzerland ¨ ¨ c Chair of Biocompatible Materials Science and Engineering, ETH Zurich, Wagistrasse 23, 8952 Schlieren, Zurich, Switzerland ¨ ¨ Received 19 May 1998; received in revised form 29 October 1998; accepted 30 October 1998 Abstract A new microelectrode is presented, based on an electrochemically etched PtrIr needle with a high aspect ratio and a radius of Ž . curvature smaller than 1 mm. The needle is electrically insulated by a thin 15 to 20 nm insulation film made of hydrogenated amorphous Ž . 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 Ž . 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 Ž. 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 Ž. 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 Ž 2 . graphitic-like state increased content of sp 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. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Microelectrode; Reactive ion etching; STM; Lithography; Transmembrane potential 1. Introduction Electrophysiological studies have provided significant wx insight into the nature of biological cells 1 . The measure- ments of transmembrane potentials of biological cells un- der various conditions lead to a basic model for the origin of the electrical potentials observed. According to this model, the negative potential observed in the interior of a Ž . cell relative to the extracellular media can be explained w x by the constant field equation 2,3 which describes active and passive ion transports across the membrane. First measurements of transmembrane potential were wx performed in 1949 by Ling and Gerard 4 . They used micropipettes made from hand-drawn glass. Nowadays, these micropipettes are fabricated from borosilicate or aluminium silicate glass capillary tubing, which is heated wx and drawn to a fine tip with a micropipette puller 5. Micropipettes used for the measurement of membrane potentials in cells with a diameter smaller than 10 mm ) Corresponding author. Tel.: q41-1-823-4076; Fax: q41-1-821-6244; E-mail: mike.schwank@empa.ch should have a tip diameter of less than 2 mm in order to minimize cell membrane damage and the dissipation of ionic gradients, leading to membrane depolarization. These micropipettes are generally filled with 3 M KCl electrolyte which has a low electrical resistivity, and the comparable diffusional mobilities of the K q - and Cl y -ions which mini- mizes the liquid junction potential at the end of the mi- cropipette. The required small tip diameter initially re- Ž . sulted in mechanical problems choking or breaking and difficulties in filling the pipette with the electrolyte solu- tion, avoiding the formation of air bubbles. In this work, we report the development of a new microelectrode, designed for transmembrane potential measurements of biological cells. These microelectrodes are based on sharp PtrIr needles which are electrically Ž . insulated with the exception of the very end with a thin Ž . film 15 to 20 nm of amorphous hydrogenated carbon Ž . a-C:H . In comparison to the glass micropipette technique, the most important advantages of this new type of microelec- Ž. trode are: a their possible lateral resolution of less than Ž. 100 nm and b the increased mechanical properties of the 0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0925-4005 99 00026-X