Surface Charging and Interfacial Water Structure of Amphoteric Colloidal Particles

Colloidal stability and adsorption processes at particle surfaces are controlled by the generation of surface charge and the molecular structure of the resulting electrical double layers. We have applied second-harmonic light scattering (SHS) to address double-layer charging and the orientation of water molecules at surfaces of amphoteric particles in unprecedented detail. For that purpose we have performed the first SHS experiments of neat polystyrene particles with a mixture of sulfate (R–O–SO₃^–) and amino (R–NH₂) surface functional groups. Changing the pH has a dramatic effect on the surface potential which can be tuned from positive to negative values and has allowed us to record changes in SHS signal while the isoelectric point of the particles is being crossed. The SHS signal is a function of two contributions which are directly related to the first molecular layer, its orientation, and to a second electric field induced contribution of the unidirectional field within the interfacial electric double layer. Through additional charge screening experiments we measured the surface charge density for positively as well as for negatively charged particle surfaces and confirmed the isoelectric point, where an increase in ionic strength had little effect on the SHS intensity. Furthermore, we have determined the net orientation of water molecules directly adsorbed to the particle surface from pH-dependent changes in the relative phase of the two SHS contributions

Shedding Light on the Growth of Gold Nanoshells

Nanostructured particles containing noble metals can have highly tunable localized surface plasmon resonances and are therefore of particular interest for numerous applications. Nanoshells comprising a dielectric core and gold or silver shell are a widely researched systems because of the strong dependence of their optical properties on the ratio of core diameter to shell thickness. Although seeded-growth procedures have been developed to produce these particles, the many reported studies show significant variation in the nanoshell morphologies and hence optical properties. In order to establish processes that reproducibly synthesize nanoshells with high optical quality, it is necessary to develop techniques that monitor changes at the core particle surface during shell growth. For that purpose, we have carried out <i>in situ</i> nonlinear second-harmonic scattering (SHS) and linear vis–NIR extinction spectroscopy simultaneously during the seeded growth of gold nanoshells on silica core particles. Our SHS measurements show a striking variation in the nonlinear optical properties of the growing gold nanoshells. In comparison with linear optical measurements and with scanning electron microscopy (SEM) images made of gold nanoshells produced with varying shell completenesses, the SHS signal was observed to reach a peak intensity at a stage prior to shell closure. We attribute this high sensitivity of the SHS signal to the incomplete nanoshell surface morphology to the generation and subsequent degeneration of regions of electric field enhancement at gaps between isolated gold islands, which grow and coalesce. This conclusion is corroborated by finite-difference time-domain simulations of incomplete nanoshells. We suggest that the <i>in situ</i> analytical approach demonstrated here offers significant promise for future activities regarding the in-process optimization of the morphology and optical properties of metal nanoshells and other nanostructured plasmonic particles