Kinetics and mechanism of sodium (Z)-3-(5-chloro-2-((5-chloro-3-(3-sulfonatopropyl) benzothiazol-2(3H)-ylidene)methyl)benzo-thiazol-3-ium-3-yl)propane-1-sulfonate adsorption on gold nanoparticle colloids of different properties and size

The borate (C6 and C9,5) and citrate (C17 and C30) capped gold nanoparticles of different average core sizes and properties, were characterized by transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS) and zeta potential measurements, in the absence and presence of cyanine dye, sodium (Z)-3-(5-chloro-2-((5-chloro-3-(3-sulfonatopropyl) benzothiazol-2(3H)- ylidene)methyl)benzothiazol-3-ium-3-yl)propane-1-sulfonate (TC) in water conditions, and the obtained results confirmed that TC was adsorbed on the surface of these nanoparticles. The measurements of fluorescence of the particle–dye assembly, corrected for inner filter effects, clearly indicated that the fluorescence of TC was quenched by gold nanoparticles on the concentration dependent manner. This process was found to be quantitatively related to the surface coverage of the Au nanoparticle by TC dye molecules. Significant increase of quenching efficiency was noticed when nanoparticle size increased, keeping the concentration of nanoparticles of different size constant. The experimental results of fluorescence quenching were compared with those obtained by theoretical calculations, in which we assumed that the maximum quenching of TC dye fluorescence was restricted to full monolayer coverage of TC on the nanoparticle surface. In case of C17 and C30 nanoparticles, the experimentally obtained values for nanoparticle concentration needed to completely quench TC dye fluorescence were lower than the calculated ones, while higher experimental values in comparision with calculated ones of quenched TC dye at constant nanoparticle concentration were obtained. These findings leaded to the conclusion that the most probable orientation of TC dye molecules on the surface of these nanoparticle was slanted. On the other hand, in case of C6 and C9,5 nanoparticles, the obtained calculated values for TC concentration needed to cover the nanoparticle surface and be completely quenched, for all three possible orientations of TC dye, were lower in comparision with the experimental values. The experimentally obtained value of C9,5 concentration needed to completely quench TC dye fluorescence most closely corresponds to vertical TC dye orientation along the short side, while the experimentally obtained results in case of C6 nanoparticles were much lower than theoretically calculated values for a