Optical label-free and model-free probe of the surface potential of nanoscale and microscopic objects in aqueous solution

Abstract

The electrostatic environment of aqueous systems is an essential ingredient for the function of any living system. To understand the electrostatic properties and their molecular foundation in soft, living, and three-dimensional systems, we developed a table-top model-free method to determine the surface potential of nano-and microscopic objects in aqueous solutions. Angle-resolved nonresonant second harmonic (SH) scattering measurements contain enough information to determine the surface potential unambiguously, without making assumptions on the structure of the interfacial region. The scattered SH light that is emitted from both the particle interface and the diffuse double layer can be detected in two different polarization states that have independent scattering patterns. The angular shape and intensity are determined by the surface potential and the second-order surface susceptibility. Calibrating the response with the SH intensity of bulk water, a single, unique surface potential value can be extracted. We demonstrate the method with 80 nm bare oil droplets in water and ∼50 nm dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylserine (DOPS) liposomes at various ionic strengths.

Label-free and charge-sensitive dynamic imaging of lipid membrane hydration on millisecond time scales

“…Candidates for the emission of SH photons are all noncentrosymmetrically structured molecular groups ( 30 , 33 ), that is, lipids or water. Taking into account the number density difference between water and lipids and previous nonresonant SH scattering studies of liposomes ( 34 , 35 ), it is likely that the dominant contribution to the SH intensity will be from water. To test this hypothesis, we added 0.5 mM KCl to the solution.…”

Characterization of the interface of binary mixed DOPC:DOPS liposomes in water: The impact of charge condensation

“…These assumptions were validated in previous studies. 11,13,25 Although we employ the RGD assumption for the linear interaction, the nonlinear interaction contains a correction to account for the change in field amplitude when the optical electromagnetic fields cross the particle/water interface. Dadap et al showed that a linear correction term to the second-order susceptibility is sufficient to correct for the change in the electromagnetic field when it crosses the interface.…”

“…9,10 Polarization-and angle-resolved (AR) SHS were recently demonstrated as a method to obtain a unique value for the surface potential of a particle in aqueous solution. 11 Key to this method is the fact that the nonlinear polarization of water at the second harmonic frequency depends linearly on the ensemble electrostatic field in the solution. This ensemble electrostatic field originates from all charges and partial charges (such as dipoles and quadrupoles) in the solution.…”

Surface-specific vibrational spectroscopy of the water/silica interface: screening and interference

“…However, a second electrolyte concentration-dependent effect has been proposed, based on theoretical considerations and direct observation by second harmonic scattering. 15,23 At very low electrolyte concentrations, the probing depth can be limited by phase matching of the V-SFG probe. Hence, as the penetration depth of the DC field exceeds the coherence length, the signal intensity is expected to be increasingly depleted due to destructive interference between contributions from sufficiently spatially separated non-linear active molecules.…”

“…These values for the w (3) /w (2) ratio seem reasonable compared with previous studies of water at charged surfaces. 20,23 Please note that for simplicity we assume that the charge density is constant for all salt concentrations. Small variation with concentration cannot be excluded as the surface charge density of silica (À0.06 C m À2 o s 0 o À0.02 C m À2 ) 2,4,14,26,27 is typically around the critical surface charge density (s 0,crit ) above which counterion condensation occurs.…”