Generation collection experiments

In a typical generation-collection experiment, two barrels of the 0-pipette are filled with water [10,11]. If one of the barrels ( generator ) contains a cation, it can be transferred to the outer organic solvent by biasing this pipette at a sufficiently positive potential (Eg). A significant fraction of ejected cations reaches the negatively biased second pipette ( collector ) and gets transferred back into the aqueous phase [Fig. 5(a)]. When only one... 

Figure 5. Generation/collection experiment with poly(I)-coated microelectrodes in CH3CN/O.I M [n-Bu4N]PFg at 10 mV/s. The lower cyclic voltammograms are for the generator electrode as its potential is swept between -0.2 V and -0.9 V vs. Ag+/Ag while the potential of the collector electrodes is held at 0.0 V vs. Ag+/Ag.

The generation-collection experiments have a much higher sensitivity, which was brought into an analogous formula by Hor-rocks and Wittstock [55]. 

A video microscope is not required for an SECM instrument, but it is very useful and is a highly recommended addition. A video microscope is preferred over a normal optical microscope because it allows the probe to be continuously observed while operating the instrument. A video record of the experiment is also available. The video microscope aids in positioning the probe in generation/collection experiments, where the lack of a feedback response makes accurate distance control difficult. In addition, video microscopy helps in positioning the probe near features of interest on the substrate. 

Three approaches to kinetic analysis were proposed (1) steady-state measurements in a feedback mode, (2) generation/collection experiments, and (3) analysis of the chronoamperometric SECM response. Unlike the feedback mode, the generation/collection measurements included simultaneous analysis of both IT-L and Is-L curves or the use of the collection efficiency parameter (IS/IT when the tip is a generator and the substrate is a collector). The chronoamperometric measurements were found to be less reliable (5), so only steady-state theory will be discussed here. 

GC mode experiments have been applied to the study of immobilized oxi-doreductases such as diaphorase and the glucose oxidase/glucose system (15,21), which has also been investigated by feedback with ferrocenyl mediators (10). In the feedback experiment, the catalytic cycle of the enzyme is dependent on ferrocenium species generated at the tip, as described in Sec. I.C. A generation-collection experiment utilizes a bulk solution concentration of the oxidized mediator, and the tip is poised at a potential sufficiently positive to detect ferrocene, or another reaction product such as H202, present near the interface (Fig. 6). The enzymatic reaction therefore occurs over the whole specimen, wherever there is active enzyme and a supply of substrate. In the feedback experiment, the tip current includes the flux of mediator from bulk solution as well as the flux of reduced mediator due to... 

An interesting phenomenon in a generation-collection experiment involving a UME array is that the current at the generator can be enhanced by the collector through a mechanism called feedback. Without active collection, all of the R produced at the generator... 

Generation-collection experiments can be carried out in UME arrays aside from the double band. An obvious extension is to use a triple band so that the middle electrode serves as a generator and the two flanking electrodes work in parallel as collectors. A more elaborate approach involves an interdigitated array, which is an extensive series of parallel bands, the alternate members of which are connected in parallel. One of the sets serves as the generator and the other as the collector. 

Generation/collection experiments, analogous to those at the RRDE, can be carried out with pairs of microband electrodes. In this case, the relevant time parameter is that required for the electrogenerated species to diffuse across the gap, d, separating the microbands... 

Lee C, Kwak JY, Anson FC (1991) Application of scanning electrochemical microscopy to generation/ collection experiments with high collection efficiency. Anal Chem 63(14) 1501-1504. doi 10.1021/ac00014a030... 

The tip signal in a generation-collection experiment with an enzymatic reaction occurring at the specimen surface depends on the enzyme reaction kinetics and the rate of mass transport of the products to the tip. Quantitative kinetic investigations of immobilized enzymes can be made by GC experiments when this mass transport rate is well defined. In principle, a macroscopic specimen... 

Instead of detecting them amperometrically, chemical species diffusing from the surface in generation-collection experiments can be detected potentiometrically with an ion-selective electrode (Fig. 18B). Potentiomet-ric SECM has been used successfully to image dissolution across ionic crystal surfaces, such as Na+ dissolution from NaCl surfaces. Potentiometric detection has the advantage that the concentration of diffusing species is not perturbed, i.e., molecules or ions are not electro-chemically oxidized or reduced to some other charge state. 

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