Silver, colloidal crystallization

Tran [391,392] has reported ng detection of dyes on filter paper by SERS. Silver colloidal hydrosols stabilised by filter supports enhance the Raman scattering of adsorbed dyes. Typical detection limits are 500 pg (crystal violet), 7 ng (l,l,9-trimethyl-2,2 -cyanine perchlorate), 15 ng (3,3 -diethylthiacarbocyanine chloride) and 240 ng (methyl red) using a 3 mW He-Ne laser. 

The colloidal metallic silver formed in this way is opaque (it would appear black in reflected or transmitted light). The halogen atoms combine with an organic substrate mixed with the silver halide crystals to produce bromides. 

The silver halide crystals can be formed as microscopic grains suspended in a protective colloid, usually gelatin. This dispersion, the photographic emulsion, can be coated on a suitable support to obtain photographic films, plates, and papers. 

Silver iodide particles in aqueous suspension are in equilibrium with a saturated solution of which the solubility product, aAg+ai, is about 10 16 at room temperature. With excess 1 ions, the silver iodide particles are negatively charged and with sufficient excess Ag+ ions, they are positively charged. The zero point of charge is not at pAg 8 but is displaced to pAg 5.5 (pi 10.5), because the smaller and more mobile Ag+ ions are held less strongly than-the 1 ions in the silver iodide crystal lattice. The silver and iodide ions are referred to as potential-determining ions, since their concentrations determine the electric potential at the particle surface. Silver iodide sols have been used extensively for testing electric double layer and colloid stability theories. 

M in concentration. This is in the range required for single-molecule detection. These sensitivity levels have been obtained on colloidal clusters at near-infrared excitation. Figure 10.3 is a schematic representation of a single-molecule experiment performed in a gold or silver colloidal solution. The analyte is provided as a solution at concentrations smaller than 10-11 M, Table 10.1 lists the anti-Stokes/Stokes intensity ratios for crystal violet (CY) at 1174 cm-1 using 830-nm near-infrared radiation well away from the resonance absorption of CY with a power of 106 W/cm2 [34]. CV is attached to various colloidal clusters as indicated in the table. Raman cross sections of 10-16 cm2/molecule or an enhancement factor of 1014 can be inferred from the data. 

Lofton, C. and Sigmund, W. (2005). Mechanisms controlling crystal habits of gold and silver colloids. Adv. Funct. Mater. 15 1197-1208. 

The size-selective precipitation (SPP) was predominantly developed by Pileni [50c]. One example (SPP) is monodisperse silver particles (2.3 nm, 0= 15%), which are precipitated from a polydisperse silver colloid solution in hexane by the addition of pyridine in three iterative steps. Recently, Schmid [52a] has reported the two-dimensional crystallization of truly monodisperse AU55 clusters. Chromatographic separation methods have thus far proven unsuccessful because the colloid decomposed after the colloidal protecting shell had been stripped off [42a]. The size-selective ultracentrifuge separation of Pt colloids has been developed by Colfen [52b]. Although this elegant separation method gives truly monodisperse metal... 

In kinetic studies of the formation of silver bromide, Meehan and Miller found that formation of colloidal material was complete within 6 msec or less. They further concluded that fast flocculation, in which every collision of particles results in an agglomeration, occurs during the mixing process and for a few seconds thereafter. Berry and Skillman and Berriman determined that the total number of silver halide crystals remained essentially unchanged after the first minute of doublejet precipitation. 

Figure 10.7 shows a time series of SER spectra of CV adsorbed at the silver colloid. Because of the low dye concentration, the monitored scattering volume contains, on average, 0.6 dye molecule and about 100 Ag clusters. Therefore, it is urihkely that more than one dye molecule is present on a single particle. The spectra show typical Raman bands of crystal violet, for example at 915, 1174,... 

Z. Chen, P. Zhan, Z. L. Wang, J. H. Zhang, W. Y. Zhang, N. B. Ming, C. T. Chan, and P. Sheng, Two- and three-dimensional ordered structures of hollow silver spheres prepared by colloidal crystal templating, Adv. Mater., 16, 417-422 (2004). 

The width of the plasmon absorption band of silver particles has often been discussed in the literature, most of the observations having been made on particles in glasses. In the case of aqueous silver colloids, the height and the width of the band strongly depend on the kind of preparation. This is due to the adsorption of substances which change the electronics of the nanoparticles (see below) and to defects in the crystal lattice of the particles. Another property that has not yet been considered is the outer shape of the particles In the two preparations described in the experiments of Figures 1... 

Mechanisms controlling crystal habits of gold and silver colloids. Advanced Functional Materials, 15,1197. 

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