Probing the colloidal gold nanoparticle/aqueous interface with second harmonic generation

► Freely adsorbing molecules at gold nanoparticle/water interface probed for first time. ► Second harmonic generation (SHG) from organic dye molecules located at interface. ► SHG signal at varying dye concentrations agrees with modified Langmuir model. ► The model fit provides the free energy and maximum site density of adsorption.