Platinum-gold sites

No simple salts of 2,1 -benzisothiazole have been described in the literature, other than those of amino-substituted derivatives,101 in which the site of protonation is probably the cyclic nitrogen atom, at least in the 3-substituted compounds. The parent compound does, however, form a simple crystalline nitrate 114 which is not very stable. A number of complex salts with platinum, gold, lead, and mercury salts and with picric acid have been described.96,97,116 The picrate is highly crystalline and stable, and is useful for the isolation and characterization of 2,1-benzisothiazole. Many substituted 2,1-benzisothiazoles form similarly useful picrates.106... 

The models incorporate two microscopic parameters, the site density and the critical nucleus size. A fit of experimental current transients to the models allows conclusions, for example, concerning the effect of additives on nucleation rate. Fabricus et al. found by analysis of current transients that thiourea increases the nucleation density of copper deposited on glassy carbon at low concentration, but decreases it at higher concentration [112], Schmidt et al. found that Gold nucleation on pyrolytic graphite is limited by the availability of nucleation sites [113], Nucleation density and rate were found to depend on applied potential as was the critical nucleus size. Depending on concentration, critical nuclei as small as one atom have been estimated from current transient measurements. Michailova et al. found a critical nucleus of 11 atoms for copper nucleation on platinum [114], These numbers are typical, and they are comparable to the thermodynamic critical radii [86],... 

In contrast to the mediator-modified electrodes, Degani et al. modified glucose oxidase itself by means of covalently bound ferrocene [4]. After modifying enzymes with ferrocene carboxylic acid, they observed direct electron transfer from the active site of the enzyme to a gold or platinum... 

A direct evidence of the way of tetrahedral anion adsorption at three-fold sites and the degree of hydratation is not available at present. However, a strong indication of such adsorption of sulphates is found in voltammetry on gold (14) and in our data for platinum surfaces (12). A pronounced difference between the sulphate and perchlorate adsorption effects is... 

The immunoreplica technique (14) is used when it is necessary to detect antigenic sites on the plasma membrane of cultured cells. The cells are cultured on coverslips, and are fixed as described above depending on the antibody in question, and immunolabeled in situ as described in Section 3.1.1.2., steps 3-9. After immunolabeling (Section 3.1.1.2., step 9), they are further fixed with 1% osmium tetroxide and are dehydrated in a graded series of ethanol (70, 90, 100%), critically point-dried, and replicated with a layer of carbon and platinum, The replicas are cleaned with sodium hypochlorite and chronic acid before examination with the transmission electron microscope. Large areas of the replicated plasma membrane remain intact for observation. Colloidal gold probes are probably the only probes of sufficient density that can be detected on these surfaces. 

One approach to describe the kinetics of such systems involves the use of various resistances to reaction. If we consider an irreversible gas-phase reaction A — B that occurs in the presence of a solid catalyst pellet, we can postulate seven different steps required to accomplish the chemical transformation. First, we have to move the reactant A from the bulk gas to the surface of the catalyst particle. Solid catalyst particles are often manufactured out of aluminas or other similar materials that have large internal surface areas where the active metal sites (gold, platinum, palladium, etc.) are located. The porosity of the catalyst typically means that the interior of a pellet contains much more surface area for reaction than what is found only on the exterior of the pellet itself. Hence, the gaseous reactant A must diffuse from the surface through the pores of the catalyst pellet. At some point, the gaseous reactant reaches an active site, where it must be adsorbed onto the surface. The chemical transformation of reactant into product occurs on this active site. The product B must desorb from the active site back to the gas phase. The product B must diffuse from inside the catalyst pore back to the surface. Finally, the product molecule must be moved from the surface to the bulk gas fluid. 

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