Second harmonic scattering multipole analysis of ligand-decorated gold nanoparticles

Abstract

Ligand decoration of noble metallic nanoparticles is often needed for some applications, such as biochemical sensing, catalysis and nanotechnology, and the understanding of its process is of great importance. The second harmonic scattering (SHS) technique with advantages of surface-sensitivity and label-free detection, provides intrinsic information for such a research. In this work, the second harmonic(SH) scattering patterns of two types of ligands (cetyltrimethylammonium chloride and L-cysteine) capped gold nanoparticles (GNPs) with the same radii are measured. Both the intensities and shapes of the SH scattering patterns are changed after the ligand exchange process. In order to explain the pattern changes, the analytic expressions of SH scattering are derived theoretically for a relatively large nanoparticle based on Dadap’s multipolar theory. Considering the derived relationship between the multipole (up to octopole) contributions and the power of the nanosphere radius, the effective size effect is introduced to express the SH scattering signal change for different ligand decorations and well explain the experimental results. This theory provides a new perspective of the SH scattering response to different capping ligands and offers a possible quantitative method to analyze interface physical chemistry for ligands on the surface of nanoparticles.