6.3.7 - Gold spherical microelectrodes

Abstract

The fabrication technique of spherical microelectrodes is based on the fact that, in the presence of dithiol linkers, gold nanoparticles self-assemble to form an electrically conductive gold-like material. By confining the dithiol linker molecules inside the micrometer-sized tip of a glass micropipette immersed into a solution of gold nanoparticles, the dithiol–nanoparticle encounter could be restricted to the very tip of the micropipette, thus limiting the growth of the self-assembled material to this region. This technique was observed to result in the formation of perfectly spherical structures at the micropipette tip. To use the self-assembled spheres as ultramicroelectrodes (UMEs), electrical contact has to be established between the sphere and the macroscopic end of the supporting micropipette. Although it has been demonstrated that the nanoparticles maintain their individualities within the self-assembled spheres, the electrochemical behavior of the self-assembled UMEs is similar to that of UMEs made of bulk gold. Submicrometer-sized spherical electrodes may be produced by spark-induced melting. The steps involved in this process include fabrication of submicrometer-sized spherical gold electrodes; sharpening the tip of a gold microwire by electrochemical etching; spark-induced melting of the tip of the etched gold microwire; insulation of the gold microwire, selective exposure of the spherical end of the microwire; and electrochemical characterization of spherical gold UMEs.