Determination of the Surface Charge Density of Colloidal Gold Nanoparticles Using Second Harmonic Generation

Abstract

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 χ (3) technique. The experimental results are fit to the Gouy-Chapman model and to numerical solutions to the spherical PoissonBoltzmann 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 x 10 −3 C/m 2 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.

Determination of Surface Potential and Electrical Double-Layer Structure at the Aqueous Electrolyte-Nanoparticle Interface

“…However, surface potentials obtained by means of the χ ð3Þ nonlinear second harmonic generation (SHG) method deviate substantially from all others (gray solid markers of Fig. 3) [38], including ours-suggesting that the χ ð3Þ SHG method is likely a measure of the electrokinetic (zeta) potential, as recently proposed by Haber and co-workers [43], and not the commonly inferred surface potential.…”

Optical label-free and model-free probe of the surface potential of nanoscale and microscopic objects in aqueous solution

“…It is thus sensitive to the orientational directionality of water molecules in the interfacial region defined as the region from the surface plane to the position where the electrostatic field has decayed to zero [18]. Angle-resolved (AR) nonresonant SHS (AR-SHS) [19] is applicable to a wide variety of hard [20][21][22][23][24] and soft particles systems [25][26][27] and can be used in very dilute solutions and small sample volumes. In absence of chemical effects, the measured intensity depends quadratically on the surface potential [28].…”

Monitoring the Photocleaving Dynamics of Colloidal MicroRNA-Functionalized Gold Nanoparticles Using Second Harmonic Generation

“…The χ (3) term depends on the net polarization of bulk molecules in the solvent, which are induced by the electric field of the nanoparticle surface. 44,45 …”