Gold spherical microelectrodes

Laboratoire d Electrochimie Moleculaire, University Paris 7-UMR CNRS 7591, France 

1 Spherical microelectrodes self-assembly of gold nanoparticles (a) Fabrication technique 

Recording of such well-defined voltammetric signals, especially in nonaqueous solvent, together with the fact that this ideal behavior was maintained even at the highest scan rate explored (10 V sec 0, shows that the self-assembled spherical UMEs are not porous and are tightly sealed to the supporting glass capillary surface. 

Self-assembled spherical electrodes have been successfully used as SECM amperomet-ric probes both in positive and negative feedback modes (la,b) (Chapter 12). A very close tip-substrate separation could be attained due to the great smoothness of the self-assembled structures. 

The self-assembled spherical electrodes have a fabrication success rate over 70%. In some instances, the presence of a golden conducting film adjacent to the sphere was seen on the ontside wall of the micropipette tip. This thin film could be removed by 

---

Demaille, C. Gold spherical microelectrodes. In Handbook of Electrochemistry, C. G. Zoski (ed.), Elsevier, Amsterdam, the Netherlands, 2006, pp. 230-235. 

Electrochemical Behavior of Self-Assembled Spherical Gold Microelectrodes. The electrochemical behavior of these spherical microelectrodes... 

This fabrication technique was also successfully applied to combined AFM-SECM probes (7, 9). Following a reported technique (10), the gold microwire was bent to a right angle -1 mm away from one of its ends while its other end was flattened between stainless-steel plates. The flattened part of the microwire serves as a flexible AFM cantilever arm while its short extremity is converted into a spherical microelectrode following the method described above. The AFM-SECM experiments using these combined probes, confirmed the ideal behavior of the spherical gold microelectrodes formed by the spark-method. 

Figure 2.21 shows the dependence of dimensionless net peak currents of ferrocene and ferricyanide on the sphericity parameter (note that A0p = AT], andy = p)-The SWV experiments were performed at three different gold inlaid disk electrodes (ro = 30, 12.5 and 5 pm) and the freqnencies were changed over the range from 20 to 2000 Hz [26]. For ferrocene the relationship between AT], and p is linear A Fp = 0.88 + 0.74p. This indicates that the electrode reaction of ferrocene is elec-trochemically reversible regardless of the frequency and the electrode radius over the range examined. For ferricyanide the dependence of AT], on p appears in sequences. Each seqnence corresponds to a particular value of the parameter The results obtained with the same freqnency, but at different microelectrodes, are cormected with thin, broken lines. The difference in the responses of these... 

Figure 17-27 Gold microelectrode with spherical tip. [From J. Abbou. C. Demaille. M. Druet, and J. MoirouK "Fabrication of Submicromeler-Sized Gold Electrodes of Controlled Geometry for Scanning Electrochemical-Atomic Force Microscopy," Anal. Chem. 2002, 74,6355.]...

FIG. 5 Schematic diagram for the preparation of the self-assembled spherical gold microelectrodes. (Reprinted with permission from Ref. 20. Copyright 1997 American Chemical Society.)... 

FIG. 7 Cyclic voltammogram of a self-assembled spherical gold microelectrode, (a) In a 7.5 mM aqueous solution of hexaamineruthenium(III) chloride and 1 M KC1 electrolyte. Electrode diameters, from top to bottom, 10, 5, 4, 3, 1.7 /rm. Scan rate 0.1 V/s. (b) In a 6.5 mM solution of tetracyanoquinodimethane in acetonitrile. Supporting electrolyte tetrabutylammonium tetrafluoroborate (0.1 M). Electrode diameter 6 /am. Scan rate 0.1 V/s. (Reprinted with permission from Ref. 20. Copyright 1997 American Chemical Society.)... 

FIG. 8 Cyclic voltammogram of a 0.35 /xm diameter self-assembled spherical gold microelectrode in a 6.5 mM aqueous solution of hexaamineruthenium(III) chloride and 1 M KC1 electrolyte. Scan rate is 0.1 V/s. 

Spherical UMEs can be made for gold (16), but are difficult to realize for other materials. Hemispherical UMEs can be achieved by plating mercury onto a microelectrode disk. In these two cases the critical dimension is the radius of curvature, normally symbolized by ro- The geometry of these two types is simpler to treat than that of the disk, but in many respects behavior at a disk is similar to that at a spherical or hemispherical UME with the same tq. 

As illustrated in Fig. 3, a single-polystyrene sphere was first deposited on a microelectrode and then gold was electrodeposited around the sphere. A blocking layer was then adsorbed at the upper planar gold surface before sphere removal. This blocking layer confines the electrochemical reactions to the inside of the nanocavity. After sphere removal, a spherical cap recess is obtained, the dimensions of which are controlled by the templating sphere size and the thickness of gold electrodeposited. 

The method can be extended to spherical walls (sphere piarticle). A sphere is equipped with an inside channel, bent through 90°, in which a gold thread of 1 mm diameter was introduced, cut flush with the surface. A rigid tube serves as support. The microelectrode can be directed relative to the average direction of the liquid by rotating the support (figure 2). 

FIGURE 3.3 Optical (A-C) and SEM (D) pictures of the self-assembled spherical gold microelectrodes. The horizontal bar at the lower corner of (D) represents 1 pm. Electrode diameter (A) 26, (B) 8, (C) 3, and (D) 0.9 pm. (Reprinted with permission from Demaille, C., Brust, M., Tsionsky, M., and Bard, A.J., Fabrication and characterization of self-assembled spherical gold ultramicroelectrodes, Anal. Chem., 69, 2323-2328, 1997. Copyright 1997 American Chemical Society.)... 

---