Ion-selective micropipettes

Nicholson C., Bruggencate G.T., Steinberg R., and Stockle H. 1977 Calcium modulation in brain extracellular microenvironment demonstrated with ion-selective micropipette. Proc Natl Acad Sci U S A 74, 1287-1290. 

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-... 

This section deals with the fabrication of potentiometric probes and their use in SECM studies. Potentiometric probes (see Chapter 7) can detect many non-electroactive species not accessible to amperometric techniques. They are highly selective and have found widespread application in clinical chemistry, in environmental studies and the food industry. A general review of potentiometric probe fabrication has been presented previously, and several publications have demonstrated the utility of potentiometric probes in SECM studies (55). This section will provide the reader with a highlight of potentiometric probe fabrication techniques taken from the literature. The section will also include a discussion of the basic concepts, fabrication steps, necessary equipment, and characterization of ion-selective micropipettes applied in SECM studies. 

Typically, ion-selective micropipettes can be used in a concentration range of 10 to 10 M and have a detection limit of about 10 M. As they can drift easily it is importance... 

The response time of an electrode describes the ion-selective electrode s sensitivity to ion activity changes. The response time is a sensitive parameter defined as the time taken by the experimental setup to reach a chosen percent of the final cell voltage following a given change in the activity of the primary ion. This definition is specific to a given experimental setup as the time response of the micropipette will vary with the type of micropipette used, the particular experimental conditions, the electronics, and the method used to cause the primary ion activity change. In terms of macro ion-selective electrode, common response times would be on the order of seconds (56, p. 70) while for ion-selective micropipettes, response times on the order of 10 ms have been reported (60). The response time of the electrode is of particular interest when close proximity measurements are made. 

The SECM potentiometric mode makes use of ion-selective micropipettes. These devices increase the range of detectable species by SECM but are technically very challenging to make and must be cahbrated before and after experiments. The electronic requirements to acquire good data are very specific and must include a means to adequately evaluate the tip-substrate distance. Potentiometric detection remains underexploited in SECM studies and has great promise in terms of biological applications. 

As described below, potentiometric and amperometric methods have been used to stndy ion channels. The K+ transport across gramicidin channels imbedded in a horizontally snp-ported BLM was stndied potentiometrically, for example. In this stndy, a selective micropipette was nsed to acqnire approach cnrves and measure the transport of K+ across the gramicidin channel. The ion-selective micropipette electrodes consisted of silanized pulled borosihcate capillaries (i.d. 0.7-20 pm) fiUed with a solution of 10 mM valino-mycin and 10 mM ETH 500 in dichloroethane. Characterization of the electrodes was accomphshed via the steady-state tip cnrrent for K+ (0.05-0.3 mM). The tips were then used in the SECM feedback and generation/collection modes to stndy K+ transfer through gramicidin channels imbedded in a horizontal BLM of glycerol monooleate. 

In the early days of ion-selective micropipette measurements, the cation exchanger, potassium tetra(p-chloro)phenylborate commercialized by Corning (K exchanger, type 477317), was very popular for measuring potassium activity. Its limited selectivity allowed only certain applications however, this electrically charged material advantageously provided low electrode resistance. The... 

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]. 

Ion selective micropipettes have high resistance, and short lifetime. They are fragile tools. Regardless of these disadvantages, they are used in potentiometric SECM. Potassium micropipettes based on valinomycin or on BME 44 irmophores, nonactin based ammonium ion selective pipettes [51], or zinc [52] selective ones were proved well applicable for SECM imaging or line scanning over different targets. 

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