118 research outputs found
Mean-Field Theory of Water-Water Correlations in Electrolyte Solutions
Long-range ion induced water-water correlations were recently observed in
femtosecond elastic second harmonic scattering experiments of electrolyte
solutions. To further the qualitative understanding of these correlations, we
derive an analytical expression that quantifies ion induced dipole-dipole
correlations in a non-interacting gas of dipoles. This model is a logical
extension of Debye-H\"uckel theory that can be used to qualitatively understand
how the combined electric field of the ions induces correlations in the
orientational distributions of the water molecules in an aqueous solution. The
model agrees with results from molecular dynamics simulations and provides an
important starting point for further theoretical work
Second-Harmonic Scattering as a Probe of Structural Correlations in Liquids
Second-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
Solvent Fluctuations and Nuclear Quantum Effects Modulate the Molecular Hyperpolarizability of Water
Second-Harmonic Scatteringh (SHS) experiments provide a unique approach to
probe non-centrosymmetric environments in aqueous media, from bulk solutions to
interfaces, living cells and tissue. A central assumption made in analyzing SHS
experiments is that the each molecule scatters light according to a constant
molecular hyperpolarizability tensor . 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 . 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
non-linear optical response of water. This finding has the potential to change
the way SHS experiments are interpreted: in particular, isotopic differences
between HO and DO could explain recent second-harmonic scattering
observations. Finally, we show that a simple 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 first step towards quantitative modelling of SHS experiments
Optical Imaging of Surface Chemistry and Dynamics in Confinement
The interfacial structure and dynamics of water in a microscopically confined
geometry is imaged in three dimensions and on millisecond time scales. We
developed a 3D wide-field second harmonic microscope that employs structured
illumination. We image pH induced chemical changes on the curved and confined
inner and outer surfaces of a cylindrical glass micro-capillary immersed in
aqueous solution. The image contrast reports on the orientational order of
interfacial water, induced by charge-dipole interactions between water
molecules and surface charges. The images constitute surface potential maps.
Spatially resolved surface pKa,s values are determined for the silica
deprotonation reaction. Values range from 2.3<pKa,s<10.7, highlighting the
importance of surface heterogeneities. Water molecules that rotate along an
oscillating external electric field are also imaged. With this approach, real
time movies of surface processes that involve flow, heterogeneities and
potentials can be made, which will further developments in electrochemistry,
geology, catalysis, biology, and microtechnology
Ion-induced transient potential fluctuations facilitate pore formation and cation transport through lipid membranes
Unassisted ion transport through lipid membranes plays a crucial role in many
cell functions without which life would not be possible, yet the precise
mechanism behind the process remains unknown due to its molecular complexity.
Here, we demonstrate a direct link between membrane potential fluctuations and
divalent ion transport. High-throughput wide-field second harmonic (SH)
microscopy shows that membrane potential fluctuations are universally found in
lipid bilayer systems. Molecular dynamics simulations reveal that such
variations in membrane potential reduce the free energy cost of transient pore
formation and increase the ion flux across an open pore. These transient pores
can act as conduits for ion transport, which we SH image for a series of
divalent cations (Cu, Ca, Ba, Mg) passing through
GUV membranes. Combining the experimental and computational results, we show
that permeation through pores formed via an ion-induced electrostatic field is
a viable mechanism for unassisted ion transport.Comment: 8 pages, 2 figure
Water-Mediated Ion Pairing: Occurrence and Relevance
We present an overview of the studies of ion pairing in aqueous media of the past decade. In these studies, interactions between ions, and between ions and water, are investigated with relatively novel approaches, including dielectric relaxation spectroscopy, far-infrared (terahertz) absorption spectroscopy, femtosecond mid-infrared spectroscopy, and X-ray spectroscopy and scattering, as well as molecular dynamics simulation methods. With these methods, it is found that ion pairing is not a rare phenomenon only occurring for very particular, strongly interacting cations and anions. Instead, for many salt solutions and their interfaces, the measured and calculated structure and dynamics reveal the presence of a distinct concentration of contact ion pairs (CIPs), solvent shared ion pairs (SIPs), and solvent-separated ion pairs (2SIPs). We discuss the importance of specific ion-pairing interactions between cations like Li+ and Na+ and anionic carboxylate and phosphate groups for the structure and functioning of large (bio)molecular systems
Probing Rotational and Translational Diffusion of Nanodoublers in Living Cells on Microsecond Time Scales
Nonlinear microscopes have seen an increase in popularity in the life sciences due to their molecular and structural specificity, high resolution, large penetration depth, and volumetric imaging capability. Nonetheless, the inherently weak optical signals demand long exposure times for live cell imaging. Here, by modifying the optical layout and illumination parameters, we can follow the rotation and translation of noncentrosymetric crystalline particles, or nanodoublers, with 50 mu s acquisition times in living cells. The rotational diffusion can be derived from variations in the second harmonic intensity that originates from the rotation of the nanodoubler crystal axis. We envisage that by capitalizing on the biocompatibility, functionalizability, stability, and nondestructive optical response of the nanodoublers, novel insights on cellular dynamics are within reach
Liquid-activated quantum emission from native hBN defects for nanofluidic sensing
Nanostructures made of two-dimensional (2D) materials have become the
flagship of nanofluidic discoveries in recent years. By confining liquids down
to a few atomic layers, anomalies in molecular transport and structure have
been revealed. Currently, only indirect and ensemble averaged techniques have
been able to operate in such extreme confinements, as even the smallest
molecular fluorophores are too bulky to penetrate state-of-the-art single-digit
nanofluidic systems. This strong limitation calls for the development of novel
optical approaches allowing for the direct molecular imaging of liquids
confined at the nanoscale. Here, we show that native defects present at the
surface of hexagonal boron nitride (hBN) - a widely used 2D material - can
serve as probes for molecular sensing in liquid, without compromising the
atomic smoothness of their host material. We first demonstrate that native
surface defects are readily activated through interactions with organic
solvents and confirm their quantum emission properties. Vibrational spectra of
the emitters suggest that their activation occurs through the chemisorption of
carbon-bearing liquid molecules onto native hBN defects. The correlated
activation of neighboring defects reveals single-molecule dynamics at the
interface, while defect emission spectra offer a direct readout of the local
dielectric properties of the liquid medium. We then harvest these effects in
atomically smooth slit-shaped van der Waals channels, revealing molecular
dynamics and increasing dielectric order under nanometre-scale confinement.
Liquid-activated native defects in pristine hBN bridge the gap between
solid-state nanophotonics and nanofluidics and open up new avenues for
nanoscale sensing and optofluidics.Comment: 16 pages, 5 figure
High throughput second harmonic imaging for label-free biological applications
Second harmonic generation (SHG) is inherently sensitive to the absence of spatial centrosymmetry, which can render it intrinsically sensitive to interfacial processes, chemical changes and electrochemical responses. Here, we seek to improve the imaging throughput of SHG microscopy by using a wide-field imaging scheme in combination with a medium-range repetition rate amplified near infrared femtosecond laser source and gated detection. The imaging throughput of this configuration is tested by measuring the optical image contrast for different image acquisition times of BaTiO3 nanoparticles in two different wide-field setups and one commercial point-scanning configuration. We find that the second harmonic imaging throughput is improved by 2-3 orders of magnitude compared to point-scan imaging. Capitalizing on this result, we perform low fluence imaging of (parts of) living mammalian neurons in culture. (C) 2014 Optical Society of Americ
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