Micropipette ion-selective microelectrodes

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. 

Ion-selective microelectrodes [18, 70,71, 164] are chiefly used for measurement of ion activities in individual cells and in intracellular liquid. They were developed from micropipettes, which are miniature liquid bridges used for measurement of cell membrane potentials [94]. Micropipettes and ion-selective microelectrodes are made using commercial drawing devices. Ion-selective... 

The micropipettes described above can be made into ion-selective microelectrodes if the tip is closed by means of an ion-selective membrane (Figure IB). The membrane may be in the form of an ion-sensitive glass, for example, for the measurement of H and Na" ", or more conveniently a liquid ion exchanger (LIX), which is available for the measurement of Na +, K+, H+, Li +, Mg +, Ca +, and Cl (see Table 1). The input amplifier for the ion-selective micropipette should have an input resistance two orders of magnitude larger than the impedance of the electrode itself, i.e., This can be ob-... 

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. 

FIG. 1 Schematic cross sections of selected microelectrode configurations, (a) Nomenclature for parts of microelectrode, (b) Na+-sensitive microelectrode (22), (c) recessed-tip Na+-sensitive microelectrode (27), (d) liquid ion-exchanger micropipette electrode (38), (e) coated wire electrode (16), (f) flow-through ISE (e.g., NOVA 6, Boehringer ISE 2020), (g) micro-capillary glass electrode of tubular shape (e.g., Radelkis OP-266), (h) planar sensor fabricated by microelectronic technology (93), (i) ISFET sensor (94). 

Several of the procedures described in the previous sections can be advantageously carried out with double barrel tips. Such a probe consists of two capillaries (see Sec. V.B), one of which acts as the potentiometric sensor, while the other is used to determine the tip-substrate distance. For example (79), a gallium microdisk was combined with an ion-selective (K+) potentiometric probe to image K+ activity near the aperture of a capillary (see Fig. 7). Similarly (77), a double barrel tip with one channel as an open Ag/ AgCl micropipette for solution resistance measurement and the other channel as an ion-selective neutral carrier-based microelectrode for potentiometric measurements was successfully used to image concentration distributions for NH4 (Fig. 8) and Zn2+ (Fig. 9). While dual-channel tips facilitate the approach of the substrate and permit a direct determination of the absolute tip-substrate distance, their difficult fabrication severely limits their use. Reference 80 compares the above methods. 

In corrosion studies, ISEs have been used to map the local concentration of ions over corroding metals in the liquid phase with high resolution and selectivity. For example, H+-, Mg -, and Zn +-selective microelectrodes were nsed in combined applications of the scanning ISE technique (SIET, akin to potentiometric SECM) and of the SVET over corroding alloys. The SIET image showed the local ion activities, while the SVET mapped the local ionic current density. In all cases, their ISE tips were made from single barreled micropipettes with cocktail of ionophores. 

Keywords Ion selective micropipettes Metabolic rate Reactive oxygen species Respiratory activity Scanning Electrochemical Microscopy Surviving rate Transport through biomembranes Voltammetric ultra-microelectrodes... 

The small size electrodes with perspective applicability as measuring tip in SECM can be listed in three categories. These are glass microelectrodes, metal based microelectrodes, and ion selective micropipettes. 

Life scientists were the pioneers in preparing and using glass microelectrodes [37 3]. In practice of SECM, however, application of glass electrodes is scarce. Metal based microelectrodes and ion selective micropipettes became much more popular. Miniaturized version of silver electrodes could be prepared and used for measuring chloride or silver ion concentrations [44-46]. 

Selective amperometric microelectrodes are also obtained from a microinterface between two immiscible electrolyte solutions (ITIES) formed at the tip of a glass micropipette. These micro-ITIES behave as disk UME and allow the quantitative amperometric detection of different ions based on ion-transfer voltammetry. A K+-selective micro-ITIES successfully imaged the transport of K+ across a 50 pm diameter pore by SECM. ... 

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