We report ionic strength-dependent phase shifts in second harmonic generation (SHG) signals from charged interfaces that verify a recent model in which dispersion between the fundamental and second harmonic beams modulates observed signal intensities. We show how phase information can be used to unambiguously separate the χ (2) and interfacial potential-dependent χ (3) terms that contribute to the total signal and provide a path to test primitive ion models and mean field theories for the electrical double layer with experiments to which theory must conform. Finally, we demonstrate the new method on supported lipid bilayers and comment on the ability of our new instrument to identify hyper-Rayleigh scattering contributions to common homodyne SHG measurements in reflection geometries.
Non-resonant second harmonic generation phase and amplitude measurements obtained from the silica:water interface at varying pH and 0.5 M ionic strength point to the existence of a nonlinear susceptibility term, which we call " ! (#) , that is associated with a 90° phase shift.
We
report the detection of charge reversal induced by the adsorption
of an aqueous cationic polyelectrolyte, poly(allylamine hydrochloride)
(PAH), to supported lipid bilayers (SLBs) used as idealized model
biological membranes. Through the use of an α-quartz reference crystal, we quantify the total
interfacial potential at the interface in absolute units using heterodyne-detected
second harmonic generation (HD-SHG) as an optical voltmeter. This
quantification is made possible by isolating the phase-shifted potential-dependent
third-order susceptibility from other contributions to the total SHG
response. We detect the sign and magnitude of the surface potential
and the point of charge reversal at buried interfaces without prior
information or complementary data. Isolation of the
second-order susceptibility contribution from the overall SHG response
allows us to directly characterize the Stern and diffuse layers over
single-component SLBs. We apply the method to SLBs formed from three
different zwitterionic lipids having different gel-to-fluid phase
transition temperatures (T
m’s).
We determine whether the surface potential changes with the physical
phase state (gel, transitioning, or fluid) of the SLB. Furthermore,
we incorporate 20% of negatively charged lipids to the zwitterionic
SLB to investigate how the surface potential and the second-order
nonlinear susceptibility χ(2) change with surface
charge.
ThO2 and UO2 nanoparticles synthesized using a COF-5 template exhibit unpassivated surfaces and provide insight into nanoscale properties of actinides.
<div><div><div><p>We report the detection of charge reversal induced by the adsorption of a cationic polyelectrolyte, poly(allylamine) hydrochloride (PAH), to buried supported lipid bilayers (SLBs), used as idealized model biological membranes. Through the use of an α-quartz reference crystal, we quantify the total interfacial potential at the interface in absolute units, using HD-SHG as an optical voltmeter in which the traditional wire leads of a voltmeter have been replaced by photons. This quantification is made possible by isolating from other contributions to the total SHG response the phase-shifted potential-dependent third-order susceptibility. We detect the sign and magnitude of the surface potential and the point of charge reversal at buried interfaces without prior information or complementary data. Isolation of the second-order susceptibility contribution from the overall SHG response allows us to directly characterize the Stern and Diffuse Layers over single-component SLBs formed from three different zwitterionic lipids of different gel-to-fluid phase transition temperatures (Tms). We determine whether the surface potential changes with the physical phase state (gel, transitioning, or fluid) of the SLB and incorporate 20 percent of negatively charged lipids to the zwitterionic SLB to investigate how the surface potential and the</p><p>second-order nonlinear susceptibility chi(2) change with surface charge.</p></div></div></div>
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