Microelectrodes electrical connections

As described in the introduction, submicrometer disk electrodes are extremely useful to probe local chemical events at the surface of a variety of substrates. However, when an electrode is placed close to a surface, the diffusion layer may extend from the microelectrode to the surface. Under these conditions, the equations developed for semi-infinite linear diffusion are no longer appropriate because the boundary conditions are no longer correct [97]. If the substrate is an insulator, the measured current will be lower than under conditions of semi-infinite linear diffusion, because the microelectrode and substrate both block free diffusion to the electrode. This phenomena is referred to as shielding. On the other hand, if the substrate is a conductor, the current will be enhanced if the couple examined is chemically stable. For example, a species that is reduced at the microelectrode can be oxidized at the conductor and then return to the microelectrode, a process referred to as feedback. This will occur even if the conductor is not electrically connected to a potentiostat, because the potential of the conductor will be the same as that of the solution. Both shielding and feedback are sensitive to the diameter of the insulating material surrounding the microelectrode surface, because this will affect the size and shape of the diffusion layer. When these concepts are taken into account, the use of scanning electrochemical microscopy can provide quantitative results. For example, with the use of a 30-nm conical electrode, diffusion coefficients have been measured inside a polymer film that is itself only 200 nm thick [98]. 

Molecular electronics represents a pow-erfiil approach to the continued miniaturization of electronic circuits down to the lower nanometer scale. One significant challenge is the electrical connection of molecular devices by nanowires. In this regard, the ability of microelectrodes, to both image, for example, through SECM and fabricate, for example, through spatially controlled electrodeposition, micro- and nano-structures will continue to be invaluable. 

Whether metal or fluid-filled microelectrodes are used, the interface effect is still felt. In the case of metallic electrodes, the interface occurs at the active surface of the electrode. With fluid-filled micropipettes, the metal-electrolyte interface exists in the pipette stem where electrical connection is made to the recording or stimulating electronics. There is the additional complication of a two-electrolyte interface at the electrode tip, since the electrolyte filling the lumen is usually different from the external electrolyte. 

The electrical connection from a glass microelectrode to its accompanying electronics is made by a metal wire inserted in the stem end of the lumen and in contact with the filling electrolyte. Early literature frequently refers to tungsten wire for this purpose, but there seems to be no valid basis for using this metal. Apparently it was available and had been used in place of antimony in certain pH electrodes. Because of its metallurgical properties, tungsten is difficult to form, and it has undesirable electrical characteristics. 

Microelectrodes with typical dimensions in the micron- and submicron-range were fabricated on quartz as well as on oxidised silicon wafers by electron-beam-lithography (see Fig I A, B) The chips were mounted and bonded in standard ceramic carriers (LCC 90) The electrically connected chip could be inserted in a plastic ring system (Fig IC) and dnven from outside under sterile conditions (for more information see [5])... 

Mass transport phenomena in biological systems can be investigated with SECM if the species of interest can be detected either by an potenti-ometric or amperometric microelectrode. Theory and selected studies of localized mass transport are covered in Chapter 9 of this volume. SECM is particularly appropriate in these studies in view of the intimate connection of the imaging mechanism to mass transport effects. The investigation of oxygen and ion transport in various tissues and under a variety of driving forces (concentration gradient, electric field, convection) has been demon-... 

Figure 2 The principle of measurement by means of an ion-selective double-barreled microelectrode inside a cell. The cell is in a bath the solution of which is grounded via a reference electrode. Each barrel is connected via a chlorided silver wire (shown coiled) to amplifiers (triangles). The reference barrel of the double-barreled electrode directly records the intracellular electrical potential, the membrane potential (Em). The ion-selective barrel, indicated by the plug of ion exchanger in the tip, records the sum of the membrane potential and a potential , related to the chemical potential of the ion in question (of activity a) (see eqn [1]). / is obtained by electronic subtraction. The influence on from other ions (indicated by index j and the valencies Zy) can be obtained from calibration.

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