Size dependence of second-harmonic scattering from nanoparticles: Disentangling surface and electrostatic...

Area of Science:

• Nanotechnology
• Surface Science
• Physical Chemistry

Background:

• Angle-resolved second-harmonic scattering (AR-SHS) is an optical technique for studying unlabeled nano-particle interfaces in solution.
• AR-SHS signals are modulated by interference between surface and bulk nonlinear contributions, influenced by the electrical double layer.
• Previous work established the mathematical framework for AR-SHS, including ionic strength effects on probing depth.

Purpose of the Study:

• To investigate the influence of particle size on surface and electrostatic form factors in nonlinear scattering.
• To determine the relative contributions of these factors to AR-SHS patterns.
• To experimentally validate the model using silica particles in varying ionic strength solutions.

Main Methods:

• Calculation of size-dependent surface and electrostatic geometric form factors for nonlinear scattering.
• Analysis of the relative contributions of these factors to AR-SHS patterns across different particle sizes.
• Experimental validation using SiO2 nanoparticles in NaCl and NaOH solutions with varied ionic strengths.

Main Results:

• The electrostatic term dominates forward scattering for smaller particles; its relative contribution decreases with increasing particle size.
• Particle surface properties, including surface potential and second-order surface susceptibility, significantly weight the total AR-SHS signal.
• In NaOH solutions, enhanced surface susceptibility for larger particles can overcome electrostatic screening effects at high ionic strengths.

Conclusions:

• This study enhances the understanding of the relationship between AR-SHS patterns and nano-particle surface properties.
• The findings provide a predictive framework for AR-SHS trends across a wide range of particle sizes.
• AR-SHS is confirmed as a valuable tool for characterizing interfacial phenomena in colloidal systems.