Surface Analysis of Porous Carbon Microelectrodes with an Adsorbed Osmium Complex as a Mediator.

Abstract

Many different forms of carbon have been used for preparing chemically modified electrodes with electrontransfer mediators. In previous reports, we described porous carbon microelectrodes (PCMEs) developed as enzyme sensors.'-3 A platinum disk electrode packed with a porous carbon material supported in osmium complex ([Os(bpy)3]2+I3+) as a redox electron transfer mediator and enzyme. The PCME surface was modified by a Nafion solution. The osmium complex was adsorbed on the PCME dipped in the osmium complex+ 10 mM LiC104 (M=mol dm-3) solution. The osmium complex was electrostatically adsorbed onto the Nafion region on the porous carbon. The construction of the PCME surface with the Nafion and osmium complex is important for the performance of an enzyme sensor. It is of considerable interest to examine how the osmium complex interacts with a modified carbon surface, because the adsorption of this mediator is crucial to the operation and lifetime of an enzyme sensor.'-3 However, there have been few reports concerning surface analyses of PCME with an adsorbed osmium complex. In this paper we discuss the depth distribution of Nafion and an osmium complex on a porous carbon surface using X-ray photoelectron spectroscopy (XPS), and the determined amount of the osmium complex on the PCME by means of chronocoulometry. described.)-3 The platinum microelectrode (diameter, 20 μm) was etched in hot aqua regia (80° C,1 h) to create a cavity; this cavity was then packed with a porous carbon material. To load the osmium complex onto the surface, the PCME was first immersed in a 2.5% Nafion solution (Aldrich) for 24 h in order to impart a negative charge onto the surface. The PCME was dipped into a 0.1 mM [Os(bpy)3] (PF6)2+10 mM LiC104 solution for 24 h. [Os(bpy)3] (PF6)2 (bpy: 2,2'-bipyridine) was synthesized from osmium(III) trichloride (Aldrich).6 All of the other chemicals used were of analytical grade. The buffer solution used was a mixture of 0.2 M disodium hydrogenphosphate and 0.1 M citric acid for pH 8.0. All of the electrochemical experiments were performed using a Bioanalytical System Electrochemical Analyzer (BASl00B/W) with a three-electrode system. An Ag/ AgCI reference electrode and a Pt counter electrode were placed in the cell. The surface modification of sheets of porous carbon for use in XPS followed the same treatments as those mentioned above. The XPS spectra were acquired using a Shimadzu ESCA 1000A instrument and Mg-Ku radiation.