Gold between colloidal particles

Figure 6-13. Transmission electron micrograph of 36 nm gold colloid particles stabilized with P(m-C6H4S03Na)3. The mean distance between colloid particle surfaces corresponds to two layers of the ligand. (Reproduced by permission from ref. [175].)...

A transition step between the reaction system just considered (silver ions, p-phenylenediamine, silver sol) and actual physical development of a fixed photographic plate or film is supplied by the investigations of Arens (27, 28). He used a series of silver, gold, and silver sulfide sols which had been coated on glass plates, using gelatin as the binding material. In this way he obtained plates in which colloidal particles... 

The color of the colloidal solutions of gold depends on the size of colloidal particles (clusters). Several methods have been used for the preparation of such clusters (for a review see [578]). Since the size of clusters may change from one to several hundred angstroms, their electronic structure may vary between that of single atoms and the solid metal. 

Important consequences result from the increase of the redox potential of metal clusters with their nuclearity. Indeed, independently of the metal, the smaller clusters are more sensitive to oxidation and can undergo corrosion even by mild oxidizing agents. Moreover, size-dependent redox properties explain the catalytic efficiency of colloidal particles during electron transfer processes. Their redox potentials control their role as electron relays the required potential being intermediate between the thresholds of the potentials of the donor (more negative) and of the acceptor (more positive). Catalytic properties of the nanoparticles are thus size-dependent. Haruta and co-workers reported that gold nanoparticles smaller than 5 nm have potential applications in catalysis as they are very active in... 

A streaming current detector based on a completely different principle than the above instruments is presented in [299], The dispersion is in a narrow space between a vertical cylindrical vessel and a coaxial piston, which moves back and forth along the axis. The potential between two gold electrodes on the wall of the cylinder at different heights is measured, and its zero value is identified with the IEP. The apparatus own response corresponds to the electrokinetic behavior of the piston and cell materials. In the presence of a colloid, the piston and the cell are assumed to be covered with colloidal particles. The above design has been utilized in some commercial instruments ... 

Since colloidal particles of silver and gold also give large SERS effects, it is useful to consider these as disconnected microelectrodes. Controlled variations in the potential at the interface between silver sol particles and electrolyte solution have been achieved by adding the Eu3+/Eu2+ redox system and varying the concentration ratio [22]. From measurements of SER spectra of pyridine adsorbed on the silver sol, the potential-dependent effects have been found to be identical with those given by bulk silver electrodes. These experiments also demonstrated that the intensity ratio of the pyridine Raman bands at ca. 1010 and 1040 cm-1 may be used as a simple but effective measure of sin-face potential for colloidal metals. 

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