Ultramicroelectrode fabrication

Mauzeroll J, Hueske EA, Bard AJ (2003) Scanning electrochemical microscopy. 48. Hg/Pt hemispherical ultramicroelectrodes fabrication and characterization. Anal Chem 75 3880-3889... 

Kounaves, S.P., Deng, W., Hallock, P.R., Kovacs, G.T.A. and Storment, C.W. (1994) Iridium-based ultramicroelectrode array fabricated by microlithography. Anal. Chem., 66, 418-423. 

Nolan, M.A., Tan, S.H., Kounaves, S.P. (1997). Fabrication and characterization of a solid state reference electrode for electroanalysis of natural waters with ultramicroelectrodes. Anal. Chem. 69 1244-7. 

As it was mentioned earlier, a much larger range of micro/nanofabrication tools are utilized in the design and fabrication of micro/nanotransducers than applied in the fabrication of microfluidic systems. Standard photolithography as described above is only one of the many techniques used. Three examples will be described here the fabrication of freestanding transducers, UV-photolithography with lift-off technique for preparation of interdigitated ultramicroelectrode arrays (IDUAs), and more traditional techniques for microtransducer preparations. 

Fig. 6.6. Inter digitated ultramicroelectrode arrays (IDUAs). (a) schematic, (b) optical micrographs with 1000 x magnification. IDUAs were fabricated using standard photolithography and lift-off techniques on silicon and were made out of gold.

Fig. 6.7. Photolithography and hft-off process used for the fabrication of gold interdigitated ultramicroelectrode arrays on silicon or glass substrates.

The versatility of this mode of operation has made it extremely powerful for fabrication of microstructures. In the feedback mode an ultramicroelectrode is held close above a substrate in a solution containing one form of electroactive species, either reduced or oxidized, that serves as a mediator (Fig. 1). The latter is usually used both as a means of controlling the distance between the UME and the surface and to drive the microelectrochemical process on the surface. This poses a number of requirements that must be taken into account when configuring the system. The basic limitation stems from the requirement that the electrochemical reaction be confined only to the surface. This means that the electroactive species generated at the UME will react with the surface or with other species attached to it. In addition, it is preferable in most cases that the redox couple used should exhibit chemical and electrochemical reversibility, so that it is effectively regenerated on the surface. The regeneration of the redox couple on the surface is required for controlling the UME-substrate distance. Finally, the thermodynamics and kinetics of the electrochemical process on the surface will dictate the choice of the redox couple introduced. 

Exploration of electrochemistry in unconventional media. Electrochemical research has traditionally focused on measurements at electrodes fabricated from conductors immersed in solutions containing electrolytes. However, interfacial processes between other phases need to receive further attention, and they can be probed with electrochemical techniques. Electrochemistry can play a unique role in exploring chemistry under extreme conditions. The movement of charges in frozen electrolytes, poorly conducting liquids, and supercritical fluids can be experimentally measured with ultramicroelectrodes. Opportunities exist to study previously inaccessible redox processes in these media. Electrochemistry in environments of restricted diffusion... 

Zhu, M. Jiang, Z. Jing, W. Fabrication of polypyrrole-glucose oxidase biosensor based on multilayered interdigitated ultramicroelectrode array with containing trenches. Sens. Actuators B Chem. 2005, 2, 382-389. 

The fabrication and the use of nanometer-sized ultramicroelectrodes as small as I nm (so called nanodes) is described in [37]. The most important procedures in fabrication of nanodes are... 

In the last decade special attention has been paid to this type of electrode arrangement. Two eventualities appeared in electroanalytical practice. The first one is represented by assemblies of casually or regularly arranged micro- or ultramicroelectrodes. The second eventuality is realized by a set of microbands that have the length of macroscopic dimension (usually several mm of magnitude, cf. Fig. 10). These types are not microelectrodes in the proper sense of this term. They are fabricated usually by microlithographic techniques. 

Nranura S, Nozaki K, Okazaki S (1991) Fabrication and evaluation of a shielded ultramicroelectrode for submicrosecond electroanalytical chemistry. Anal Chem 63 2665-2668. doi 10.1021/ac00022a022... 

This section discusses the fabrication of microelectrodes with diameters of a few micrometers to tens of nanometers using a laser-pulled technique. The methods discussed focus on the fabrication of platinum (Pt) ultramicroelectrodes (UMEs) sealed in... 

Conical-shaped ultramicroelectrodes (UMEs) are of special interest in connection with the imaging of surfaces, in kinetic studies, in probing thin films, and in probing minute environments, such as single cells. The most common fabrication procedure is by the etching of platinum wire or carbon fibers followed by coating with an insulating material except at the apex of the electrode. In this section, the fabrication of both blunt and sharp conical electrodes are discussed. 

Amicro-liquid/liquid (p-L/L) interface is usually formed either at the tip of a pulled glass micropipet or within a small hole made in a thin membrane (8, 91, 92). Unlike solid ultramicroelectrodes (UMEs), such pipets or holes are easily made. Pipets as small as a few nanometer radii have recently been fabricated using a laser puller (93). Dual-pipets (or d-pipets) can also be prepared by pulling 0-glass tubing (94-96). Another simple way of producing a p-L/L interface in a microcavity is to chemically dissolve microwire encapsulated within a glass tube (97). 

Scanning electrochemical microscopy (SECM), a member of the scanning probe microscopies (SPM), uses an ultramicroelectrode as the probe to characterize the localized electrochemical properties of the surfaces (1-3). Although the lateral resolution of SECM is inferior to that of STM or AFM because it is difficult to fabricate a small probe microelectrode with atom-size radius, SECM has abilities of detecting chemical reactions. In addition, it can induce chemical reactions in an extremely small volume. Using the unique properties of SECM, a single molecule (4) or a radical with a short lifetime (5) was recently detected. 

A boron-doped diamond array of notability is shown in Figure 6.4, which depicts the schematic plots of the fabrication of integrated all-diamond ultramicroelectrode arrays (UMEAs). As shown in Figure 6.4a, an insulating diamond (iD) film with a thickness of a... 

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