The stability of monodisperse silver colloids is strongly increased by gelatin even in the presence of compounds which usually cause coagulation. A concentration of 0.02% gelatin is sufficient to stabilize the colloids for several months but is not too high to prevent the adsorption of many organic compounds. The adsorption rate, however, di†ers strongly. Dyes such as oxazine 170 and crystal violet need several hours to substitute gelatin on the silver surface, whereas other compounds such as pseudoisocyanine (1,1º-diethyl-2,2º-cyanine) are adsorbed within a few minutes. With crystal violet the formation of H-aggregates is preferred, whereas with pseudoisocyanine a J- aggregate-like, almost planar, adsorption geometry is adopted. The adsorption rate can be monitored via the appearance of the hypsochromically or bathochromically shifted absorption band in the visible region and/or the increase in the SERS intensity. In the absence of salts the intensity of the SERS signal of the organic compounds, once the maximum is reached, remains unchanged for several weeks. Therefore, colloids stabilized with gelatin may be successfully applied for analytical purposes.
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