Solvent fluctuations and nuclear quantum effects modulate the molecular hyperpolarizability of water

Second-harmonic scattering (SHS) experiments provide a unique approach to probe noncentrosymmetric environments in aqueous media, from bulk solutions to interfaces, living cells, and tissue. A central assumption made in analyzing SHS experiments is that each molecule scatters light according to a constant molecular hyperpolarizability tensor β(2). Here, we investigate the dependence of the molecular hyperpolarizability of water on its environment and internal geometric distortions, in order to test the hypothesis of constant β(2). We use quantum chemistry calculations of the hyperpolarizability of a molecule embedded in point-charge environments obtained from simulations of bulk water. We demonstrate that both the heterogeneity of the solvent configurations and the quantum mechanical fluctuations of the molecular geometry introduce large variations in the nonlinear optical response of water. This finding has the potential to change the way SHS experiments are interpreted: In particular, isotopic differences between H2O and D2O could explain recent SHS observations. Finally, we show that a machine-learning framework can predict accurately the fluctuations of the molecular hyperpolarizability. This model accounts for the microscopic inhomogeneity of the solvent and represents a step towards quantitative modeling of SHS experiments.

Second-Harmonic Scattering as a Probe of Structural Correlations in Liquids

Michele Ceriotti, Chung-Wen Liang, Sylvie Roke, Gabriele Tocci, David Mark Wilkins

econd-harmonic scattering experiments of water and other bulk molecular liquids have long been assumed to be insensitive to interactions between the molecules. The measured intensity is generally thought to arise from incoherent scattering due to individual molecules. We introduce a method to compute the second-harmonic scattering pattern of molecular liquids directly from atomistic computer simulations, which takes into account the coherent terms. We apply this approach to large-scale molecular dynamics simulations of liquid water, where we show that nanosecond second-harmonic scattering experiments contain a coherent contribution arising from radial and angular correlations on a length scale of ≲1 nm, much shorter than had been recently hypothesized (Shelton, D. P. J. Chem. Phys. 2014, 141). By combining structural correlations from simulations with experimental data (Shelton, D. P. J. Chem. Phys. 2014, 141), we can also extract an effective molecular hyperpolarizability in the liquid phase. This work demonstrates that second-harmonic scattering experiments and atomistic simulations can be used in synergy to investigate the structure of complex liquids, solutions, and biomembranes, including the intrinsic intermolecular correlations.

Amer Chemical Soc2016

Soft matter probed by nonlinear scattering: self-assembly, interfaces, hydration, and long-range order

Jan Dedic

All organisms are made to a large extent of soft matter - macromolecules such as proteins and polysaccharides, or assemblies of small molecules such as lipids embedded in an aqueous environment. Understanding the role of order in soft matter presents a challenge for experimentalists because such systems are neither completely disordered nor perfectly crystalline. This is especially the case for the aqueous environment itself whose order fluctuates on femto- and pico-second time scales. In this thesis, we attempt to elucidate the presence, extent, changes, and implications of orientational order in several soft matter systems: from polyelectrolyte solutions to lipid membrane interfaces and supramolecular structures. To perform this task, we employ nonlinear optical techniques: second-harmonic and sum-frequency scattering. In the first two chapters, we explore the structure of water in aqueous polyelectrolyte solutions. Polyelectrolytes of high molecular mass are shown to induce orientational order in water that spans hundreds of nanometers. Using an adapted model of second-harmonic scattering from charged particles, we can explain the observed ordering in terms of weakly screened long-range electrostatic interactions that perturb the structure of water via charge-dipole interactions. Upon exchanging ordinary (light) water for heavy water, we observe a significant nuclear quantum effect in the induced order, indicating that bulk water-water correlations are enhanced as well by polyelectrolytes. In the following chapter, we exploit the same nuclear quantum effect in polyelectrolyte solutions to show that the polyelectrolyte-induced orientational order correlates with an anomalous increase in the reduced viscosity. We propose that the viscosity anomaly arises from enhanced order in a solvent which hinders the viscous flow. This observation represents a rare direct link between a microscopic property and a macroscopic observable. In the last two chapters, we change focus from homogeneous solutions to dispersions of liposomes which serve as models of biological membranes. In Chapter 5 we explore the binding of the protein alpha-synuclein to anionic vesicles from the perspective of interfacial water. We show that the adsorption of the protein to a vesicle decreases the order in the interfacial water by 30% but does not affect the surface nor the zeta potential. We propose that the changes in the structure of interfacial water can serve as a label-free probe of protein binding. In the last chapter, we investigate the supramolecular chemistry of inclusion complexes of methyl-Î²-cyclodextrin with various lipids. By combining multiple nonlinear scattering techniques, we show that the complexes self-assemble into micrometer-long directed fibers. Based on measurements of different lipids, we propose that the self-assembly process is driven by hydrogen bonding between the lipid headgroup and cyclodextrin. Lastly, we show that the long-range order of the self-assembled structure is transmitted into the structure of the hydrating water.

EPFL2020

Developing luminescent Brownian probes for near-field investigations of the intracellular environment

Flavio Maurizio Mor

Life sciences have a constantly growing need for novel methodological approaches suitable to investigate the intracellular environment with increased temporal and spatial resolutions. Recently, optical near-field probes, such as laser-irradiated pointed metal, uncoated/metal-coated tapered optical fibers, as well as nano-emitters, such as single molecules or nanoparticles, have attracted increasing attention as key components of high-resolution microscopes. In parallel, super-resolution fluorescence microscopy, operating in far-field regime and overcoming the light diffraction limit, has markedly improved imaging resolution. Although both near- and far-field optical approaches drastically improve image resolution, they still represent numerous drawbacks, such as, technical difficulties concerning probe preparation (near-field) or potential damaging through very high light intensities (far-field). The aforementioned shortcomings of high-resolution optical microscopy can be overcome to a great extent by photonic force microscopy (PFM). PFM employs a strongly focused near-infrared (NIR) laser light to hold a dielectric or metallic particle as a local probe. Such an optical trap enables one to ‘cage’ a mesoscopic particle and track its three-dimensional (3D) thermal fluctuations in the surrounding environment, e.g. a viscous liquid. Therefore, besides offering near-field 3D imaging, PFM reports also on other important quantities concerning local mechanical properties, including force and viscosity as well as dynamical properties of the surrounding medium. This additional information is derived from the careful analysis of the jittery motion (so-called Brownian motion) of the trapped single-particle probe that collides with thermally-activated surrounding molecules. In this thesis, we first study in detail the Brownian motion of a single spherical particle that is confined within the harmonic potential of the optical trap. To this end, we optimize the NIR light path and electronic noise floor of our custom-built PFM set-up for detecting and quantifying resonances in the Brownian motion. These resonances arise from the coupling between the hydrodynamic memory and strong strength of the optical trap. Due to the high sensitivity of the short-time dynamics, the size of the particle can be measured by simultaneous fitting of the velocity autocorrelation function and power spectral density of its thermal fluctuations. In order to choose the best-suitable spherical probe for a given experiment in PFM, computational modeling based on a Matlab toolbox is performed. The generalized Lorenz-Mie theory is computed using the T -matrix method for various experimental conditions, including changes in the size and refractive index of the sphere, as well as different laser polarization states. The axial equilibrium position is examined to predict its location compared with the position of the laser focus. Optical forces acting on the sphere are investigated in 3D to highlight, in particular, the limits of the perfectly harmonic potential assumption. These limits are quantified and discussed. Subsequently, we identify different types of inorganic nano/micro-sized particles that can be trapped by the NIR laser of the PFM and used simultaneously as mechanical and near-field light sources. To this end, we take advantage of the visible light emission from single probes confined in the optical trap and excited by the NIR laser light. The emphasis is on particles revealing nonlinear optical properties. In particular, nonlinear optical field enhancement resulting from excitation of surface plasmon resonance (SPR) on trapped gold particles of 200 nm is demonstrated by measuring the emission of that second-harmonic generation (SHG). Furthermore, we show, under optical trapping conditions, SHG emission from single potassium niobate (KNbO3) nano/micro-sized particles. We also report on the multicolor upconversion luminescence (UCL), at the single particle level, for randomly shaped crystals of sodium yttrium fluoride codoped with ytterbium and erbium, NaYF4:Yb,Er (UCC), when they are held in the optical trap. In parallel, 3D Brownian fluctuations of a single trapped particle are quantified. Moreover, luminescence resonance energy transfer (LRET) between a KNbO3 particle or an UCC and molecules of an organic dye (rose bengal) adsorbed on their surface is highlighted and analyzed. Finally, randomly shaped UCCs and hexagonal nanoplates are characterized in detail at the single particle level. The hexagonal β phase in the polycrystalline and monocrystalline crystals is identified, in both randomly and hexagonally shaped particles by electron microscopy. UCL emission from the trapped crystal is quantified in terms of power and found to be dependent on the laser power density, size and shape of the particle as well as its orientation within the trap. Most importantly, the different colors in the UCL spectrum do not vary with the same trend for different orientations of the randomly or hexagonally shaped crystal upon trapping. This suggests the existence of crystalline anisotropy-dependent UCL. Thereby, we should be able to design the best-suited particle probe for LRET experiments with subwavelength resolution.