Filled Glass Micropipette Electrodes

One method for producing a well-insulated microelectrode is to start with a glass capillary tube and bond it to an internal metallic wire. Several techniques exist. A low-melting-point metal can be used to fill the tube, or a wire of the same diameter as the internal diameter of the tube can be passed through the glass capillary, which is then heated to produce bonding. One must be careful to select glasses and metals which have nearly the same temperature coefficients of expansion. Three basic techniques exist as described below. 

Low-Melting-Point Glass and High-Melting-Point Meta  

High-Melting-Point Glass and Low-Melting-Point Metal or Meta Alloy 

An alternative technique is to use high-melting-point glass capillaries and low-melting-point metals such as indium (Dowben and Rose, 1953 Gesteland et al, 1959) or metal alloys such as silver solder (Svaetichin, 1951 Gray and Svaetichin, 1951). 

The basic technique is as follows Coarse (10-30/x) capillary tubes are drawn out. The metal to be used is then melted into the tube lumen. The metal-filled tubes are then heated and drawn down to 1 /x or less (see Section 4.3 for pulling techniques). The electrode tips may then be plated, and they are usually platinized. Platinizing technique was discussed in Chapter 2. 

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Fluid-filled glass micropipettes are inherently more noisy than their metal counterparts. In addition to feTRA/ noise associated with the metal-electrolyte connector in the stem, we must deal with ionic flow. Ion flow occurs between the electrolyte in the lumen and the external electrolyte of the medium in which the electrode is placed. Ion movement depends upon tip size, ion concentrations in the two electrolytes, current in the electrode, pressure differentials, and other factors. It is a complex problem and not amenable to easy theoretical treatment. In general, the smaller the tip diameter, the lower the noise figure for glass electrodes. Little ionic flow occurs in pipettes with tips < 1 /x diameter. 

A much smaller micropipette silver electrode was prepared for chloride measurements by Kerkut and Meech (37). They introduced a concentrated ammoniac silver nitrate solution into borosilicate glass micropipettes and kept the micropipettes for 10 hours in 20% formaldehyde solution. The formaldehyde diffusing into the narrow opening reduced the silver and formed a metallic silver plug inside the tip of the micropipette. Silver chloride-coated silver wire was used as inner reference electrode in the silver nitrate internal filling solution. 

Glass micropipettes with tip sizes down to nanometers and with different steepness are broadscale used tools in Ufe science micromanipulations. From micropipettes, as it was shown by Walker [49], submicron sized ion selective microelectrodes can be prepared. For these the internal waU of the tip is silanized and hydrophobic cocktail plug that contains selective ionophore is introduced. As final step introduction of the internal filling solution and silver chloride coated silver wire serving for internal reference electrode follows. 

In electrophysiology ion-selective microelectrodes are employed. These electrodes, resembling micropipettes (see Fig. 3.8), consist of glass capillaries drawn out to a point with a diameter of several micrometres, hydrophobized and filled with an ion-exchanger solution, forming the membrane in the ion-selective microelectrode. 

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