Determination of the surface charge density of colloidal gold nanoparticles using second harmonic generation

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© 2015 American Chemical Society. Second harmonic generation is used to investigate the surface charge density of 50 nm colloidal gold nanoparticles in water. The gold nanoparticles are thiolated with mercaptosuccinic acid and are dialyzed in ultrapure water to remove excess salts and reactants. The second harmonic generation signal from the nanoparticle sample is measured as a function of added sodium chloride and magnesium chloride salt concentrations using the π(3) technique. The experimental results are fit to the Gouy-Chapman model and to numerical solutions to the spherical Poisson-Boltzmann equation that account for the nanoparticle surface curvature, the different salt valences, and ion adsorption to the Stern layer interface. The best fits use the numerical solutions including ion adsorption and determine the initial surface charge density to be (-2.0 ± 0.1) × 10-3 C/m2 at the gold nanoparticle surface, in agreement with electrophoretic mobility measurements. In addition, the sodium ion is observed to adsorb with a higher surface charge density than the magnesium ion. These results demonstrate the important effects of surface curvature and ion adsorption in describing the surface chemistry and surface charge density of colloidal gold nanoparticles in water.

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Journal of Physical Chemistry C

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