Reaction rate imaging

This situation allows making two basic experiments scanning the UME at constant distance d provides an image that reflects the distribution of heterogeneous reaction rates on the sample (reaction rate imaging). Moving the UME vertically towards the sample allows a more detailed kinetic investigation of the reaction O+ue R at... 

Recently, Colley et al. [104] studied the distribution of electrochemical activity in microarray electrodes. The array contained 50-pm diameter boron-doped regions spaced 250 pm apart in an intrinsic diamond disk. Reaction rate imaging was done in the substrate generation/tip collection mode. The electroactive boron doped regions were biased at a suitable potential to reduce the mediator, [Ru(NH3)6]3+, and the product of the reduction reaction was collected at the tip. Two-dimensional scans over different regions (Fig. 21) revealed wide variations in local electroactivity. 

Since the SECM response is a function of the rate of the heterogeneous reaction at the substrate, it can be used to image the local chemical and electrochemical reactivity of surface features. A technique called reaction-rate imaging, which is unique to SECM, is particularly useful in imaging the areas on a surface where reactions occur. Membranes (5-7), leaves (8-10), polymers (11,12), surface films (13-17), and artificially patterned biological systems (7,18-20) have been imaged with SECM. 

Reaction rate imaging is unique to SECM and clearly illustrates its chemical imaging capability. By proper choice of solution components to control the tip reaction and the electrochemistry at the substrate/solution interface by varying the electrode potential, differential reaction rates at various surfaces can be probed. For example, the location of enzyme sites in a membrane or organelle, where a particular reaction is catalyzed, can be... 

FIGURE 6.10 (a) Topographic and (b) reaction-rate images of a defect-rich surface of a glassy carbon elec-... 

Macpherson, J. V., Unwin, P. R. Scanning electrochemical microscope induced dissolution Rate law and reaction rate imaging for the dissolution of the (010) face of potassium ferrocyanide trihydrate in non-stoichiometric aqueous solutions of the lattice ions. J. Phys. Chem. 1995, 99, 3338. 

A striking feature of the images is the nonunifonnity of the distribution of the adsorbed species. The reaction between O and CO takes place at the boundaries between the surface domains and it was possible to detennine reaction rates by measuring the change in length L of the boundaries of the O islands. The kinetics is represented by the rate equation... 

Fig. 4 shows the SEM images of SWNTs purified by the thermal oxidation and acid-treated. Fig. 4(a) shows a SEM image of the raw soot. In addition to the bundle of SWNTs, carbonaceous particles are shown in the figure. These stractural features mi t be causal by various in the arcing process because of an inhomogeneous distribution of catalysts in the anodes [7]. It can be seen that the appearance of SWNTs was curled and quite different fiom that of MWNTs. Fig. 4(b) shows a decrease of amorphous carbons after oxidation. The basic idea of the selective etching is that amorphous carbons can be etched away more easily than SWNTs due to the faster oxidation reaction rate [2]. Since the CNTs are etched away at the same time, the yield is usually low. The transition metals can be etched away by an add treatment. Fig. 4(c) shows the SEM image of the acid-treated sample, where the annealed sample was immersed in 10 % HCl. 

Another important catalytic reaction that has been most extensively studied is CO oxidation catalyzed by noble metals. In situ STM studies of CO oxidation have focused on measuring the kinetic parameters of this surface reaction. Similar to the above study of hydrogen oxidation, in situ STM studies of CO oxidation are often conducted as a titration experiment. Metal surfaces are precovered with oxygen atoms that are then removed by exposure to a constant CO pressure. In the titration experiment, the kinetics of surface reaction can be simplified and the reaction rate directly measured from STM images. 

Wittstock G, Burchardt M, Pust SE, Shen Y, Zhao C (2007) Scanning electrochemical microscopy for direct imaging of reaction rates. Angew Chem Int Ed 46 1584—1617... 

Fig. 9. Simultaneous plots of reaction rate (relative Vmax) and activation volume (AV() for the lactate dehydrogenase system as functions of salt (KSCN) concentration. This mirror image may be explained by the similarity between Eq. (134) and Eq. (136). (By courtesy of Prof. G. N. Somero, unpublished.)...

Very interestingly, the IPD is found to show practically no or very little dependence on the volume fraction and only depends on the reaction rate (Fig. 35). Here the IPD is calculated from the mean diameter taken from image analysis and the volume fraction, which has been determined directly from the porosity of these macroporous thermosets. Both values could be determined experimentally with high accuracy and no simplifications are needed for the calculations. 

Measurement of creatine kinase reaction rate in human brain using magnetization transfer image-selected in vivo spectroscopy (MT-ISIS) and a volume radiofre-... 

At equilibrium, depending on the temperature of the reaction, almost any concentration of the substances present can exist. If, at equilibrium in the reaction between sodium thiosulfate and silver ions, mostly silver ions and thiosulfate ions are present, we would not be successful in removing the silver ions to preserve a photo image. We need a system that shows us which substances are in excess at equilibrium, the reactants or products. It is possible to have equal concentrations of reactants and products at equilibrium, but this is usually not the case. At equilibrium, forward and reverse reaction rates are equal, not the amounts of reactants and products. At equilibrium, the rate of reactants making products equals the rate of products making reactants. This results in constant product and reactant concentrations. 

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