Spherical gold UMEs

Figure 6.3.7.1 Optical (a-c) and scanning electron (d) microscope images of spherical gold UMEs self-assembled at the very tip of micropipettes. The horizontal scale bar in (d) represents 1 pm. Electrode diameters (a) 26 pm, (b) 8 pm, (c) 3 pm, and (d) 0.9 pm. (Reprinted with permission from...

Figure 63.7.2 Schematic representation of the self-assembled spherical gold UME fabrication...

Figure 63.1.7 Cyclic voltammetry of ferrocenedimethanol at submicrometer-sized spherical gold UMEs produced by spark-induced melting of the tip of etched gold microwires. The cyclic voltammograms (a and b) were recorded, respectively, at the UMEs shown in the SEM pictures (a and b) of Figure 6.3.7.4. From the value of the plateau currents, the UME diameters were estimated to be of (a) 250 nm and (b) 150 nm. The scan rate was of 20 mV sec . The forward and backward traces of each voltammogram are perfectly supeiimposable at this scan rate. The open circles correspond to the fit of the whole voltammogram using the equation given in the text and the following...

The early applications of fast CV mainly focused on the measurement of the peak-potential separation, AEp, for the reduction or oxidation process of aromatic compounds, to obtain the pertinent standard heterogeneous rate constant k° from the relationship given in Table 2 [22]. The largest k° values of about 4 cm s were found for the reduction of aromatic hydrocarbons such as anthracene at a gold electrode in acetonitrile. The peak-potential separation increased from the 58 mV expected for a reversible process at low v to about 100 mV on going to v values of 10 kV s . This also shows that there is no real need for employing extremely large sweep rates in the determination of k° for the majority of compounds. Rather, it is important to ensure that the measurements at the lower sweep rates are not hampered by the contribution from spherical diffusion if a too small UME is used. 

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. 

Successive steps for the fabrication of gold spherical UMEs from these preformed structures are summarized graphically in Figure 6.3.7.5. 

Previously reported studies of gold spherical UMEs prepared by self-assembly of gold nanoparticles (17) show similar behavior (see Section 6.3.7). 

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