Metal oxides, membrane-mediated

Fig 21. Reaction scheme for the membrane-mediated crystal growth of metal oxides in phospholipid vesicles. 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

Fig. 2.8. Factors controlling the production of free radicals in cells and tissues (Rice-Gvans, 1990a). Free radicals may be generated in cells and tissues through increased radical input mediated by the disruption of internal processes or by external influences, or as a consequence of decreased protective capacity. Increased radical input may arise through excessive leukocyte activation, disrupted mitochondrial electron transport or altered arachidonic acid metabolism. Delocalization or redistribution of transition metal ion complexes may also induce oxidative stress, for example, microbleeding in the brain, in the eye, in the rheumatoid joint. In addition, reduced activities or levels of protectant enzymes, destruction or suppressed production of nucleotide coenzymes, reduced levels of antioxidants, abnormal glutathione metabolism, or leakage of antioxidants through damaged membranes, can all contribute to oxidative stress.

The excellent electron-transfer mediator properties of nanoparticles find special use in the different oxidation [126] and reduction [143,144] reactions catalyzed by noble metal colloids. Recently, Ung et al. [145] showed how Ag particles coated with a thin layer of silica act as redox catalysts, and how the control of the rate of the catalyzed hydrogen evolution reaction was possible by tuning the silica shell thickness. It was concluded that the shell acts as a size-selective membrane, which can be used to alter the chemical yields for competing catalytic reactions. This kind of tailoring of the catalyst properties opens up very interesting prospects in future catalyst planning. 

Sohd electrodes modified with various compounds have been used to improve DNA oxidation response. Siontorou and coworkers [264] obtained peaks A° and of degraded DNA at GCE modified with self-assembled bdayer hpid membrane. Using GCE modified with Nafion-ruthenimn oxide pyrochlore, enhancement of oxidation peaks of both peaks, G° and A°, was achieved [145]. Thorp s group investigated DNA oxidation response at GC and indium-tin oxide (ITO) electrodes modified with self-assembled dicarboxylate monolayers [265], and with nitrocellulose and nylon membranes [266]. In these experiments, DNA was attached to the electrode either covalently or via adsorption forces in the modifier layer bare ITO surface did not adsorb DNA. Oxidation of DNA was mediated by a redox metal chelate [Ru(bipy)3], which shuttled electrons to the electrode surface from DNA in solution or attached at the modifier film [265, 266]. Electro-catalytic oxidation of DNA was observed also when a redox mediator was immobilized on the electrode surface, for example, on ITO modified with electropolymerized poly[Ru(bipy)3] film [146]. 

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