Grazia Gonella

Nonlinear Light Scattering and Spectroscopy of Particles and Droplets in Liquids

Nano- and microparticles have optical, structural, and chemical properties that differ from both their building blocks and the bulk materials themselves. These different physical and chemical properties are induced by the high surface-to-volume ratio. As a logical consequence, to understand the properties of nano- and microparticles, it is of fundamental importance to characterize the particle surfaces and their interactions with the surrounding medium. Recent developments of nonlinear light scattering techniques have resulted in a deeper insight of the underlying light-matter interactions. They have shed new light on the molecular mechanism of surface kinetics in solution, properties of interfacial water in contact with hydrophilic and hydrophobic particles and droplets, molecular orientation distribution of molecules at particle surfaces in solution, interfacial structure of surfactants at droplet interfaces, acid-base chemistry on particles in solution, and vesicle structure and transport properties.

Saturation of charge-induced water alignment at model membrane surfaces

Interfacial water alignment at charged membranes saturates via two different mechanisms upon increasing the charge density. The electrical charge of biological membranes and thus the resulting alignment of water molecules in response to this charge are important factors affecting membrane rigidity, transport, and reactivity. We tune the surface charge density by varying lipid composition and investigate the charge-induced alignment of water molecules using surface-specific vibrational spectroscopy and molecular dynamics simulations. At low charge densities, the alignment of water increases proportionally to the charge. However, already at moderate, physiologically relevant charge densities, water alignment starts to saturate despite the increase in the nominal surface charge. The saturation occurs in both the Stern layer, directly at the surface, and in the diffuse layer, yet for distinctly different reasons. Our results show that the soft nature of the lipid interface allows for a marked reduction of the surface potential at high surface charge density via both interfacial molecular rearrangement and permeation of monovalent ions into the interface.

Surface Potential of a Planar Charged Lipid–Water Interface. What Do Vibrating Plate Methods, Second Harmonic and Sum Frequency Measure?

The interfacial electrical potential is an important parameter influencing, for instance, electrochemical reactions and biomolecular interactions at membranes. A deeper understanding of different methods that measure quantities related to the surface potential is thus of great scientific and technological relevance. We use lipid monolayers with varying charge density and thoroughly compare the results of surface potential measurements performed with the vibrating plate capacitor method and second harmonic generation spectroscopy. The two techniques provide very different results as a function of surface charge. Using the molecular information on lipid alkyl chain, lipid headgroup, and interfacial water provided by sum frequency generation spectroscopy, we disentangle the different contributions to the surface potential measured by the different techniques. Our results show that the two distinct approaches are dominated by different molecular moieties and effects. While the shape of the SPOT method response as a function of charge density is dominated by the lipid carbonyl groups, the SHG results contain contributions from the interfacial water molecules, the lipids and hyper-Rayleigh scattering.

Engineering Proteins at Interfaces: From Complementary Characterization to Material Surfaces with Designed Functions

Abstract Once materials come into contact with a biological fluid containing proteins, proteins are generally—whether desired or not—attracted by the materials surface and adsorb onto it. The aim of this Review is to give an overview of the most commonly used characterization methods employed to gain a better understanding of the adsorption processes on either planar or curved surfaces. We continue to illustrate the benefit of combining different methods to different surface geometries of the material. The thus obtained insight ideally paves the way for engineering functional materials that interact with proteins in a predetermined manner.

Comment on "Enhancement of Second-Order Nonlinear-Optical Signals by Optical Stimulation"

In a recent Letter [1], Goodman and Tisdale (GT) report a dramatic enhancement of a second harmonic generation (SHG) signal from a sample, explained by optical stimulation. However, GT’s observations can be simply explained by the well-known heterodyne detection for SHG signals without invoking optical stimulation. Suppose, contrary to the claim of Ref. [1], that the stimulating pulse does not increase the newly generated SHG signal field. In that case, the total intensity Itotal 2ω of the light at frequency 2ω is given by Itotal 2ω ∝ ðu 2ω þ ustim 2ω Þ 1⁄4 ðu 2ωÞ þ ðustim 2ω Þ2 þ 2u 2ωu 2ω , where u 2ω and ustim 2ω are the amplitudes of the SH signal field from the sample and the stimulating field, respectively. Clearly, the interference increases the intensity of the overall SHG field, even in the absence of a stimulated SHG. The presence of the stimulating pulse does, in fact, enhance depletion of the fundamental field, to balance the interference term u 2ωu stim 2ω . Measuring the interference between the field emitted at the sample and a reference field is the essence of heterodyne detection [2–4], requiring a temporal overlap of the signal and stimulating pulses, which explains the signal enhancement only for short time delays [Fig. 1(c) of [1]]. In SHG, the signal field is linearly proportional to the intensity of the fundamental pulse Iω. The stimulating field is proportional to the square root of its intensity Istim 2ω . Thus, the heterodyne term u 2ωu stim 2ω ∝ Iω ffiffiffiffiffiffiffiffi