Microelectrodes metal

Scanning electrochemical microscopy seeks to overcome the lack of sensitivity and selectivity of the probe tip in STM and AFM to the substrate identity and chemical composition. It does this by using both tip and substrate as independent working electrodes in an electrochemical cell, which therefore also includes auxiliary and reference electrodes. The tip is a metal microelectrode with only the tip active (usually a metal wire in a glass sheath). At large distances from the substrate, in an electrolyte solution containing an electroactive species the mass-transport-limited current is therefore... 

In this chapter, several methods for the fabrication of different types of amperometric tips suitable for SECM are described. We have also suggested some methods for microelectrode fabrication, which have not yet been tested for SECM but may provide alternative ways for its tip preparation. Section II.A describes the techniques for the preparation of various metallic microelectrodes, including Pt, Ir-Pt, Au, Hg, and W. The manufacture of carbon microelectrodes is presented in Section II.B. Most of these tips are encapsulated in or supported with glass capillaries. Other coating materials and techniques are treated in Section II.C. 

The first references to the use of microelectrodes appear in the biochemical literature and relate not to microvoltammetric electrodes, as will largely be considered here, but rather to very small ion-sensitive electrodes (particularly pH electrodes) capable of making measurements inside a single biological cell. Biochemists have also used metal microelectrodes to provide direct electrical stimulation of nerves, for example in the design of auditory protheses [1]. Indeed, the two types of electrode have often been used in combination, i.e. an ion-sensitive electrode is used to detect changes caused by the electrical stimulation of a nerve. Applications of both these types of... 

McIntyre, C.C. and Grill, W.M. Finite element analysis of the current-density and electric field generated by metal microelectrodes. Ann. Biomed. Eng. 2001 29 ITI-l iS. 

Musallam, S. Bak, M.J. Troyk, P.R. Anderson, R.A.(2007). A floating metal microelectrode array for chronic implantation. Journal of Neuroscience Methods, 160,122-127. 

In terms of the mechanics of assembly, metal microelectrodes are the simplest to produce. They are simply metal wires or needles which have very small tips. All of the electrode, save the tip, is insulated with a suitable material as shown in Figure 4.2. The only mechanical problem associated... 

Suitable metals for metallic microelectrodes are stainless steel, tungsten (tungsten carbide), and hard platinum (platinum-iridium alloy). The basic technique for electropointing is illustrated in Figure 4.3. A simple plating system is established with the electrode to be pointed as the anode. For stainless steel electrodes, Grundfest et al. (1950) have recommended the following electrolyte ... 

Figure 4.5. Circuit for tip-plating a glass-metal microelectrode.

Metallic microelectrodes have the advantage over glass micropipettes that they produce lower amounts of electrical noise. Gesteland et al. (1959) have shown that noise in metal microelectrodes is approximately that for a pure resistance that is. 

For a typical metal microelectrode, the rms (root-mean-square) noise voltage for the audio band of frequencies is of the order of 60 /iV for a 10-MQ electrode at room temperature. 

In connection with associated electronic circuitry, fluid-filled microelectrodes behave as low-pass filters, while metal microelectrodes act as high-pass filters (Gesteland et a/., 1959). The reasons for this are simply that glass electrodes exhibit high shunt capacitance and series resistance (Figure 4.10) while electrode polarization impedance associated with metal microelectrodes produces a frequency-dependent RC combination whose series impedance decreases as frequency increases. 

Generally, metal microelectrodes are useful for stimulation, or for recording high-frequency processes. Glass electrodes are useful in recording from cells in which membrane processes are of interest. Unfortunately, there is no general-purpose microelectrode. Preamplifiers will be discussed in subsequent chapters, as other aspects of microelectrode use are considered. 

Figure 7.7. Basic equivalent circuit for metal microelectrode.

When metal microelectrodes are used, Figure 7.7 indicates an approximate input circuit configuration. is nearly zero and is omitted. C, the electrode polarization capacitance, now becomes important. When direct coupling to the amplifier is used, only Cp is present. For ac coupling, the series capacitance is given by... 

We now see the reason for the general rule that glass microelectrodes are used for slowly varying processes (intercellular recording) and metal microelectrodes are used for rapid processes (neuron records). 

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